U. S. DEPARTMENT OF THE INTERIOR 

HAROLD L. ICKES, Secretary 

BUREAU OF MINES JOHN W. FINCH, Director 

Bulletin 385 

ENGINEERING FACTORS IN THE 

VENTILATION OF METAL MINES 

BY 

c. E. McELROY 

UNITED STATES 

GOVERNMENT PRINTING OFFICE 

WASHINGTON .1935 

"or .... e by the Superintendent of Docwnents, Washington, D. C. - - - - Price 30 cenq 

Introduction ________________ _ 

Purpose and scope of report __ Acknowledgments _________ _ Ventilation surveys __________ _ 

General requirements for metal mines _____________ _ 

Survey methods ___________ _ 

Low-velocity air measure- ments by smoke-cloud methods ________________ _ 

Moderate-velocity air meas- urements by vane-ane- mometer methods ______ _ 

One-point measurements __ _ Approximate traversing methods ______________ _ 

Precise traversing methods_ 

High-velocity air measure- ments by Pitot-tube meth- ods_________ _ ______ _ 

Accuracy of mine air-flow 

measurements ___________ _ 

Temperature observations __ _ Sling psychrometers ______ _ 

Recording thermometers __ _ Rock temperatures _______ _ 

Absolute-pressure measure- ments ________________ _ 

Aneroid barometers ______ _ 

Small absolute-pressure dif- ferences ______________ _ 

Differential-pressure measure- ments ________________ _ 

U-tube water gage _______ _ Manometers ____________ _ Micromanometers _______ _ 

Recording pressure gages __ _ Pressure observations ____ _ Fan-performance data ______ _ Main fans ______________ _ 

Booster fallS ____________ _ 

Auxiliary fan-tubing units __ Air density _______________ _ 

Psychrometric tables and charts __________________ _ 

Cooling-power observations __ Kata--thermoineter _______ _ 

Effective-temperature in- dex __________________ _ 

Survey records ____________ _ 

Flow of air in mine openings __ _ 

Pressure forms and designa- tions ___________________ _ 

Pressure changes __________ _ 

CONTENTS 

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Flow of air in mine openingsContinued. Pressure losses ____________ _ 

Friction pressure losses _____ _ Friction-loss formulas ____ _ Laws of proportion ______ _ 

Friction factors for mine airways _______________ _ 

Friction factors for tubing __ Shock pressure losses _______ _ Shock-loss formula _______ _ 

Data available on shock factors _______________ _ 

Shock factors in terms of 

friction factors and equiv- alent lengths __________ _ 

Flow conditions at bends __ Characteristics and types of bends ________________ _ 

Shock factors for bends in 

airways of uniform area ________________ _ 

Normal bends _________ _ Square bends __________ _ Other bends ___________ _ 

Chart for determining 

shock factors for bends 

in airways of uniform area ________________ _ 

Shock factors for bends in 

airways of nonuniform area ________________ _ 

Bends discharging directly to the atmosphere __ _ Closely spaced bends ___ _ Sinuous, crooked, and 

curved airways ______ _ 

Flow conditions at area changes ______________ _ 

Characteristics and types of area changes __________ _ 

Shock factors for symmetri- cal area changes _____ _ Formulas _____________ _ 

Installation conditiollS __ _ Abrupt expansion ______ _ Gradual expansion _____ _ Abrupt contraction ____ _ Gradual contraction ___ _ 

Contraction followed by gradual expansion ___ _ 

Chart for determining 

shock factors for sym- metrical area changes __ 

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IV CONTENTS 

Flow of air in mine openingsContinued. 

Shock pressure losses-Con. Page 

Shock factors for unsymmetrical area changes_ 64 

Local enlargements of area and dead ends_ _ _ 66 

Doorframes_ _ _ _ _ _ _ _ _ _ _ _ 66 

Regulators_____________ 66 

Timber obstructions_ _ _ _ 66 

Mine cars_ _ _ _ _ _ _ _ _ _ _ _ _ _ 66 

Shielding effect of obstructions____ _ _ _ _ _ _ _ _ 67 

Intermittent obstruc- tions _______________ ._ 67 

Shock losses at splits and 

junctions.______________ 68 

Effect of air-flow conditions on constancy of pressureloss factors and velocity distributions_ _ _ _ _ _ _ _ _ _ _ 68 

Theory of fluid flow _ _ _ _ _ _ _ 68 

Friction factors___________ 69 

Flow through closely packed materiaL _ __ ____ __ _ _ _ __ 69 

Velocity distributions_____ _ 69 

Shock factors_ _ _ _ _ _ _ _ _ _ _ _ 70 

Low-velocity flow in rough airways_____ _ _ _ _ _ _ _ _ _ _ _ 70 

Pressure-quantity relations___ 71 

Resistance factors_ _ _ _ _ _ _ _ 71 

Series flow_______________ 73 

Parallel or split flow _ _ _ _ _ _ _ 73 

Calculation of resistance 

factorsformines________ 74 

Power-quantity relations_ _ _ _ 75 

Charts for determining resistance factors and pressure and power 10s8e8__________ 75 

Economics of air flow _ _ _ _ _ _ _ 80 

Effect of airway conditions on power requirements_ 80 

Size of airways_________ 80 

Character of surface of 

airways______________ 81 

Smooth-lining airways___ 81 

Shape of airways_ _ _ _ _ _ _ 83 

Other factors___ _ _ _ _ _ _ _ _ 83 

Allowable costs for reducing mine resistance_ _ _ _ _ _ _ _ _ 84 

Economic design of mine 

airways_____ _ _ _ _ _ _ _ _ _ 84 

Effect of design on eco- nomic area and total 

yearly costs__________ 86 

Effect of relation between cost factors_ _ _ _ 87 

Economical velocities____ 87 

Effect of quantity on eco- nomic area___________ 88 

Design factors__________ 89 

Natural ventilation___________ 89 

Natural-draft flow conditions_ 89 

Pressures________________ 89 

Temperature changes of un- derground air_ _ _ _______ 90 

Natural ventilation-Con. 

Natural-draft flow conditions-Continued. 

Direction of flow ________ _ 

Quantities of flow ________ _ 

Intensity of natural-draft 

pressures _____________ _ 

Natural-draft measurements_ Calculations ____________ _ 

Application of natural-draft 

measurements _________ _ 

Simple flow circuits ______ _ 

Combined flow circuits ___ _ 

COl!lbiI?-ed and split-flow Clrcmts _______________ _ 

DistributlOn of flow ______ _ Recirculation ____________ _ Shunt-flow circuits _______ _ 

Characteristics of naturaldraft flow _____________ _ 

Control of natural ventila- tion __________________ _ 

Control of natural-draft 

pressures in mechanically ventilated mines _______ _ 

Artificial aids to natural draft_ Stacks __________________ _ 

Furnaces _______________ _ 

Air, water, and steam jets and falling wateL _____ _ Effect of wind _____________ _ 

Convection currents ________ _ Mechanical ventilation _______ _ Centrifugal fans ___________ _ Modern types ___________ _ 

Service designa tions ______ _ Position'designations _____ _ Fan performance __________ _ 

Characteristic curves _____ _ Tests ___________________ _ Laws ___________________ _ 

Pressures and ra tings _____ _ Pressure graphs _______ _ Evase discharge _______ _ 

Pressures underground __ Mechanical efficiency ___ _ 

Application of fans to ventilating systems _________ _ 

Fan as only source of pres- sure __________________ _ 

Fan in series with naturaldraft pressure _________ _ 

Equivalent resistance ____ _ Fans in series ___________ _ Fans in paralleL _________ _ 

Complex combinations of 

pressure sources _______ _ 

Efficiency of fans operating in metal mines ________ _ Selection of fans ___________ _ Size ____________________ _ 

Performance guarautees __ _ 

characteristic-ra tiu charts_ 

Tables of performance ____ _ Types and designations ___ _ Efficiencies ______________ _ 

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Mechanical ventilation-Contd. 

Selection of fans-Contd. 

Effect of blade shape on performance characteris- tics ________________ _ 

Power characteristics ___ _ 

Pressure characteristics __ Efficiency characteristics_ 

Other operating charac- teristics ____________ _ 

Variations in design to ac- commodate conditions of 

installation ___________ _ 

Reversing arrangements- __ Direction of rotation and 

discharge- ____________ _ Drives _________________ _ 

Economic selection of mine fans __________________ _ 

Air-screw fans _____________ _ 

H~g~-l?ressure machines _____ _ AIr mJectors~ _____________ _ 

Pressures generated by mov- ing objects ______________ _ 

Distribution of air currents ____ _ Position of main fan ________ _ 

Use of reversing arrangements- Direction of flow ___________ _ 

Main fans on surface _______ _ Fireproof intake airways ____ _ Fireproof fan installations ___ _ Velocity limitations ________ _ 

Stoppings------------------ Temporary bulkheads ____ _ Deflector brattices _______ _ Permanent bulkheads ____ _ Doors __________________ _ Types ________________ _ 

Airlocks __ ~ ___________ _ 

Automatic doors _______ _ 

Remote-control doors ___ _ 

Fire doors and their con- troL _______________ _ 

Pressures on doors _____ _ Costs ________________ _ 

Leakage----------------- Fan installations _______ _ 

Shaftcompartments ____ _ Adj oining airways ______ _ 

Resistance factors for 

leakage conditions ___ _ 

Resistance factors for doors _______________ _ 

Regulators----- _________ _ crossings- __________ .~ _____ _ 

Effect of mining methods on distribution ___________ _ 

Open-stope methods ______ _ 

Shrinkage-stope methods __ Filled-stope methods _____ _ Top-slice methods ________ _ 

CONTENTS 

Page 

Distribution of air currentsContinued. 

Effect of mining methods on distribution-Continued. 

123 Undercut-caving methods __ 123 Boosterfans _______________ _ 

124 Fan installations _________ _ 

126 Position in openings----- Selection _________________ _ 

126 Auxiliary ventilation _______ _ Double-entry methods ____ _ Use of compressed air _____ _ 127 Local air motion _________ _ 127 Fan-tubing methods ___________ _ 

Development for low-tem128 perature conditions ____ _ 

129 Development for high-temperature conditions _____ _ 

139 Auxiliary booster installa- 130 tions _________________ _ 

132 Development trends at met- 133 al mines ________________ _ 

Fan selection ____________ _ 

135 Iron pipe versus flexible tub136 ing-------------------- 136 Need of lightweight insu137 lated tubing--- ________ _ 

137 Use of beading to stiffen 

138 iron pipe ______________ _ 

139 Installation specifications 

139 and costs for average con- 139 ditions __________________ _ 

139 Distribution records ________ _ 139 Ventilating systems ____________ _ 

140 Protection of life and property 

140 Uncontrolled natural-ventilat- 141 ing systems ______________ _ 

142 Controlled natural-ventilating 144 systems ____________________ _ 

145 Mechanical-ventilating sys- 145 tems __________________ _ 

Quantity requirements ____ _ 146 Pressure req uirements _____ _ 

147 Determining factors in lay- 148 outs __________________ _ 

148 Major,outlines of systeIIU!-__ _ 148 Interventilation of mines_ 

149 Exhaust systems prefer- 149 able ________________ _ 

Location of openings-------- 149 Ideallay-out of airways __ Minor outlines of systems ___ _ 

150 Concentration of work150 ing places ___________ _ 

151 Local sources of heat, 

dust, or gas--------- ____ _ 152 Circulation through caved 153 ground ______________ _ 153 PersonneL ________________ _ 156 Costs ________________________ _ 156 Index __________________ • _____ _ 

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ILLUSTRATIONS 

Fig. Page 

1. Bureau of Mines type of smoke-cloud tube________________________ 5 

2. A, Calibration curve for vane anemometer; B, calibration chart for vane anemometeL_____ _ _ _ _ _ _ _ _ _ _ _ __ _ _ ___ _____ __ _____ __ _ _ __ __ 9 

3. A, Diagram showing path followed with anemometer in the continuous 

approximate method of traversing air-measurement sections of mine 

airways; B, diagram showing position of traverse points in a circular 

air-measurement section for a 5-area 20-point traverse_ _ _ _ _ _ _ _ _ _ _ 11 

4. Pitot tube, with details of ends__________________________________ 15 

5. Bureau of Mines type sling psychrometer and carrying cases_ _ _ _ _ _ _ _ 16 

6. A, Biram-type vane anemometers; B, standard type of small aneroid 

barometer__________________________________________________ 17 

7. Chart for determining weight of moist air and ratio to standard weighL 28 

8. Chart for determining weight of dry air in moist air and volume per pound______________________________________________________ 29 

9. Chart foq determining vapor pressure, dew-point temperature, and' 

relative humidity of moist aiL _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 30 

10. Chart for determining water vapor and total heat in moist aiL______ 31 

11. Effective-temperature chart for men at rest and stripped to the waisL _ 33 

12. Graphs of absolute and relative pressure changes in the same uniformresistance system of airways for three positions of the same pressure- generating source_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 38 

13. Relative effect of spacing of timber sets on friction factor, as estimated 

for average conditions of metal-mine airways_____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 45 

14. A, Sketch showing general conditions of flow at bends in airways; B, sketch defining bend characteristics _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 49 

15. Common types of right-angle bends__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 50 

16. Chart for determining shock factors, or shock pressure losses in terms 

of equivalent velocity pressures, for bends in airways of uniform 

area_______________________________________________________ 53 

17. Sketch showing general conditions of flow and defining characteristics 

of changes in area of airways_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 55 

18. Chart of data for gradual symmetrical expansion at diverging sections 

of airways for included angles of 0° to 60° ______________________ 57 

19. Chart of data for gradual symmetrical contraction at converging sections of airways for included anglfs of 0° to 60° _______________ 58 

20. Flow conditions at changes in are'!. of airways_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 59 

21. Chart for determining shock factors, or shock pressure losses in terms 

of equivalent velocity pressures, for symmetrical changes in area of 

airways____________________________________________________ 65 

22. Line diagram of the main airways of a typical metal mine_ _ _ _ _ _ _ _ _ _ 74 

23. Chart for determining resistance factors for friction pressure losses in 

airways_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ___ _ ____ _ 76 

24. Chart for determining resistance factors for shock pressure losses in 

airways____________________________________________________ 77 

25. A and B, Charts for determining pressure loss and air horsepower from resistance factors and quantities of flow _________________ - - 78 

26. Relative sizes of airways for equal pressure and power requirements: A, Square shaft sections; B, rectangular drift sections____________ 82 

27. Chart for determining economical area for mine airways from design factor and quantity of flow _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 88 

28. Dry-bulb temperature records at surface and in two adjacent timberlined downcast air shafts _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 90 

29. Natural-draft columns for some simple flow circuits_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 94 

30. A, Natural-draft columns for a simple type of combined flow circuit; 

B, natural-draft columns for shunt types of split-flow circuits_ _ _ _ _ 95 

31. A, Natural-draft columns for a multilevel type of split-flow circuit; 

B, pressure-quantity characteristics for natural-draft flow _ _ _ _ _ _ _ _ 96 

32. A, stack on metal-mine shaft; B, stove used with tubing to ventilate 

dead-end prospect adiL _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 100 

33. Multiblade rotors of centrifugal fans of types commonly used for 

ventilating western metal mines_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 101 

34. Characteristic curves of fan performance at constant speed and 

constant air density _______ _ ___ _ _ _ _____ _________ __ _ _ __ _ ___ __ _ _ 106 

VI 
ILLUSTRATIONS VII 

Fig. Page 

35. Graphs of pressure changes in ventilating systems as affected by position of fan with respect to system__________________________ 109 

36. A, Fan acting as only pressure-generating source; B, natural draft 

acting with fan; C, natural draft acting against fan; D, fan characteristic used directly as combined characteristic for natural 

draft acting with fan_ _ _ __ __ ___ __ _ __ _ _ _ ___ __ __ __ ___ _ __ ____ _ _ 112 

37. A, Two fans acting in series; B, solution by trial and error for flow con- ditions for fans acting in parallel on separate airways___________ 116 

38. Two fans acting together in paralleL_____________________________ 117 

39. A, Sketch analysis of flow conditions for fans acting in parallel on separate airways; B, sketch diagram of fans acting in parallel on separate airways_ _ _ _ _ __ _ _ _ ___ __ __ ___ __ _ __ __ __ _ _ _ __ _ ___ _ ___ _ 118 

40. Four types of characteristic-ratio charts of centrifugal-fan performance_ 122 

41. Comparative quantity and power characteristics of mine fans_ _ _____ 124 

42. Comparative pressure and efficiency characteristics of mine fans_ _ _ _ _ 125 

43. Four-gate arrangement for reversing air flow at auxiliary fan-tubing installation_ _ _ _ _ _ _ __ __ ___ ___ __ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ __ _ _ _ __ _ _ _ _ _ __ 128 

44. A, Arrangements for reversing air flow at single-inlet, primarily ex- hausting fan installed on air shaft; B, arrangements for reversing air 

flow at double-inlet, primarily exhausting fan installed on air shaft__ 128 

45. A, Propeller fan installed as main fan on the Moose shaft of the 

Anaconda Copper Mining Co., Butte, Mont.; B, standard-type disk 

fan________________________________________________________ 130 

46. Suggested form of installation for propeller fan____________________ 131 

47. Section of rotary positive-pressure blower_________________________ 132 

48. Venturi type of compressed-air injector used for auxiliary ventilation 

in United Verde mine, Jerome, Ariz ____________________________ . 134 

49. Ventilation door used on motor haulageways in Calumet and Arizona 

mine, Bisbee, Ariz___________________________________________ 142 

50. A and B, Compressed-air-operated ventilation doors, with adjoining man doors, used on haulage drifts in Calumet and Arizona mine, Bisbee, Ariz_____ __ _ _ _ _ _ _ __ _ _________ _ ________ _____ _ _____ ____ 142 

51. Iron-door regulator, with feed screw setting, used in United Verde 

mine, Jerome, Ariz., to proportion amount of air entering each level 

from main-intake air shafL___________________________________ 143 

52. Regulator door used at level intersections on intake air shaft, United 

Verde mine, Jerome, Ariz_____________________________________ 144 

53. Device used in Calumet and Arizona mine, Bisbee, Ariz., for operating ventilation doors by compressed air____________________________ 146 

54. Method of ventilating open stopes: A, Advancing; B, retreating____ 154 

55. Method of ventilating shrinkage stopes: A, Advancing; B, retreating_ 155 

56. A, Method of ventilating horizontal cut-and-fill stopes; B, method of 

ventilating rill cut-aIid-fill stopes _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 157 

57. Two-compartment cribbed raise with timber slide in manway com- partment___________________________________________________ 158 

58. Hurdling method of ventilating top-slice stopes____________________ 158 

59. Ventilation map of 620 grizzly level of Miami mine, Miami, Ariz., May 1929__________________________________________________ 159 

60. Typical underground installation of fan in booster position____ _ _ _ _ _ _ 160 

61. Double-entry methods of advancing into newterritory______________ 162 

62. Fan-tubing installations for ventilating long dead-end openings with 

canvastubing_______________________________________________ 166 

63. Lay-out of fan-tubing auxiliary units for stope ventilation under 

high-temperature small-quantity-of-flow conditions_ _ _ _ _ ___ _ _____ 168 

64. Set of conventional symbols for plotting mine-ventilation data on small-scale level maps_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 175 

65. Diagrammatic sketch showing ventilation system of Calumet and 

Arizona mine, Bisbee, Ariz., October 1929______________________ 176 

66. A, Controlled natural-draft distribution in a small metal mine; 

B, control of natural-draft distribution to prevent reversal of direc- tions of flow _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 179 

67. Plan map of Ahmeek mine, Ahmeek, Mich., showing distribution of 

air currents by natural draft, July 1928_________________________ 180 

68. Sketch indicating the generally desirable features of mechanical-ventilation systems applied to metal mines_________________________ 186 

ENGINEERING FACTORS IN THE VENTILATION OF 

METAL MINES 1 

By G. E. McELROY 2 

INTRODUCTION 

The ventilation of a mine has to do with the coursing of air currents 

through underground openings and may be accomplished by purely 

natural means or by a combination of natural and mechanical means. 

The object of ventilating mines is to provide comfortable, safe, and 

healthful atmospheric conditions at all places where men work or 

travel. 

In all metal mines there is the necessity of removing the fumes of 

explosives used in blasting. In many (and this group includes most of 

the important producing metal mines of the world) there is the necessity of providing air conditions that are comfortable so as to maintain 

a high efficiency of labor. In some there is the need of diluting and 

removing rock dusts dangerous to the health of the miners or inert 

but harmful gases, such as carbon dioxide and nitrogen. In a few 

mines explosive gases, such as methane, or toxic gases, such as hydrogen sulphide, are encountered and require dilution and removal. 

And in all metal mines it may be necessary at times to contend with 

underground fires and the toxic gas carbon monoxide produced by 

them. 

During 1929 and 1930, in connection with a comprehensive study of 

mining methods conducted by the United States Bureau of Mines 

in cooparation with the leading metal-mine operators of the United 

States, the author was enabled to make detailed studies of ventilation 

methods, practices, and costs at a number of large metal mines 

throughout the western United States. This intimate contact with 

the officials directly responsible for the ventilation of the mines revealed that the generally high degree of effectiveness of the systems 

in use was due more to long-continued and expensive development 

through experience than to the application of available engineering 

data. That practice lags behind the development of theory and the 

accumulation of empirical data is due not so much to lack of available 

information as to its slow dissemination. During the last 10 years 

interest has revived in the engineering aspects of metal-mine ventilation, owing primarily to the economic necessity of employing larger 

volumes of air flow to combat the effects of higher-temperature air 

conditions encountered with increase in depth of many of the world's 

largest producing mines. 

1 Work on manuscript completed Mar. 30, 1932. 

'Mining engineer, U. S. Bureau of Mines. 

1 
2 ENGINEERING FACTORS IN VENTILATING METAL MINES 

Sufficient theoretical or empirical material is now available to apply 

to average conditions, but it is scattered through a wide variety of 

technical publications that are ~ot readily available to mine-operating en~eers and is confused by the lack of standards governing the 

umts and the designations used in ventilation literature. 

PURPOSE AND SCOPE OF REPORT 

In the belief that the quickest and cheapest method of obtaimng 

effective ventilation of metal mines with regard to air conditions that 

promote efficiency, health, and safety is through more general application of the engineering factors involved rather than by trial and error 

and that this method of attacking ventilation problems will be facilitated by a simple, concise statement of these factors and the methods 

of using them, an attempt is made in this bulletin to present the subject of metal-mine ventilation to mine operators in a practical way. 

Although the physiological factors of gases, dust, and heat in mine 

air determine both the necessity for ventilation and the results 

achieved by it these subjects are not considered within the scope of 

the present report; and no attempt is made to cover the subject of 

coal-mine ventilation, for although the general principles are the same 

as for metal-mine ventilation the particular applications of these 

principles differ to a considerable degree. An attempt has been made 

to present the material in as nontechnical a form as is possible without 

sacrificing necessary accuracy or completeness, for which reason it is 

thought that it should prove of value to all mine officials and students 

interested in mine ventilation. 

ACKNOWLEDGMENTS 

The author has necessarily drawn on knowledge acquired piecemeal 

from a large variety of sources, including books and periodicals which 

can only be acknowledged en masse and the publications and files of 

the Bureau of Mines. For data on present practice and specific 

conditions he is deeply indebted to mine officials and Bureau mining 

engineers throughout the western United States for their cooperation 

in the underground studies recently made as well as in similar studies 

and specific investigations dating back to 1920. His thanks are also 

due to many manufacturers of ventilating equipment for special data 

freely furnished through correspondence and personal conferences. 

The courtesy of Prof. W. S. Weeks, of the University of California, 

whose book 3 on this subject is a standard reference; of A. S. Richardson; ventilation engineer of the Butte mines of the Anaconda Copper 

Mining Co., which have long served as a pattern for mechanical-ventilation systems applied elsewhere; and of F. Ernest Brackett, in 

critically reviewing the manuscript, is gratefully acknowledged. 

VENTILATION SURVEYS 

GENERAL REQUIREMENTS FOR METAL MINES 

Ventilation is for the most part unseen and unobserved; surprisingly 

little accurate knowledge results from casual observations, and many 

preconceived ideas are upset by the simplest type of systematic obserI Weeks, w. s., Ventilation of Mines: New York, 1926, 221 pp. 
VENTILATION SURVEYS 3 

vations. Systematic and periodic surveys, involving physical measurements, are required for all mines to reveal actual conditions. 

Where hot and humid working conditions are encountered the value of 

systematic surveys is increased so largely that the benefits have been 

recognized by many operators, and surveys are made at regular or 

irregular intervals. Few surveys have been made in naturally ventilated mines, mainly because of a fixed impression that natural ventilation is not subject to control, although a considerable degree of control can be exercised both as to volume and direction of natural-draft 

currents. 

The most direct application of the information gained by a survey 

comes at the time of a mine fire, when it is vitally important to have 

the maximum amount of correct information regarding ventilation 

conditions, particularly the direction of the air currents. Unless a 

regular procedure of ascertaining and recording ventilation conditions 

has been followed the necessary information may not be available 

when and where required. Data so acquired may also be used to 

advantage in preplanning methods of fire control. Probably the next 

most important use of such information is in planning for the betterment of present or future conditions of comfort to improve working 

efficiencies. Other important applications are in connection with fan 

installations or changes in fan installations of mechanical-ventilation 

s:y:stems, chang~s in airways, and control of natural draft. 

Such surveys demand speed rather than absolute accuracy and 

involve only simple measurements; but for the information to be properly recorded on maps and otherwise it is preferable for them to be 

intrusted to a technical engineer who is thoroughly familiar with the 

mine openings. . Individual requirements, of course, govern particular mines as to 

how often surveys should be made and what should be included. 

The essential requirements are that they should be complete enough 

and be made at sufficiently frequent intervals to keep mine officials 

posted so that plans for maximum safety and efficiency may be made. 

In general, the minimum number of surveys to begin with and to 

continue until the seasonal effect of natural draft is accurately gaged 

would be two a year, preferably in the late summer and late winter. 

For shallow, naturally ventilated mines at least four surveys a year 

are required to give the desirable initial information, whereas for very 

deep mines yearly surveys will give enough data after the first year or 

two. Supplementary data are always desirable, however, to ascertain 

the effect of all major changes in distribution systems. 

SURVEY METHODS 

The main requirements of a survey are to ascertain the directions 

of flow and the approximate magnitude of the various air currents. 

N ext to the air-flow information, air and rock temperatures and pressures on doors and- stoppings are most useful records for all mines. 

For mechanically ventilated mines approximate performance data 

covering all ventilating appliances are of primary interest, anq. records 

of the physical characteristics of the airways are required for air-flow 

computations. In hot, humid mines air conditions are of primary 

importance, and records of air temperatures and cooling powers are 

particularly useful. 
4 ENGINEERING FACTORS IN VENTILATING METAL MINES 

. Exact measurements of all physical conditions involved in mine 

ventilation would require skillful technique, and descriptions of all the 

methods that might be used would fill many books. However, refined 

methods are required only for research and exact tests of appliances. 

For ordinary surveys of the complex and ever-changing conditions of 

the ventilation of a mine at relatively infrequent intervals a sufficiently 

exact picture of the conditions obtaining can be determined by use of a 

limited number of approximate methods that require little time and 

more common sense than skill. 

A complete set of instruments required for simple ventilation surveys would cost approximately $100. Average prices for good-grade 

instruments are about as follows: 

Prices of instruments required 

Low-speed vane anemometer, with leather case and shoulder strap______ $35 

Sling psychrometer, with leather case and shoulder strap_______________ 25 

Stopwatch, Ys second, 30-minute register _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 15 

Pitot tube, X-inch O. D., about 15 inches long________________________ 10 

U-tube water gage, with rubber tubing_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 5 

Spare psychrometer thermometer for rock-temperature measuremen ts_ _ _ _ _ 3 Revolution counter _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 3 

25-foot tape in case_____ _ _ ___ __ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ __ _ _ 2 

Aspirator bulb with hard-rubber valves and rubber tubing_____________ 2 

These are minimum requirements for simple surveys. For accurate 

air-density calculations a barometer would be needed for determining 

the absolute pressure, but the latter may be estimated approximately 

from altitude and temperature or estimated closely from the reading 

at some nearby point with which a comparison has been made previously with borrowed instruments. 

For more accurate and more inclusive surveys additional instruments that are desirable are an aneroid barometer, a kata-thermometer, and a sensitive pressure gage. The types and designs of instruments that may be used to advantage for indicating and recording 

ventilation conditions are legion and beyond the scope of this bulletin. 

In the main, they are special instruments satisfying a rather limited 

demand and therefore relatively costly. Practically all can only be 

used under a comparatively small range of conditions, and the number 

required for even limited research or test requirements is large. 

LOW-VELOCITY AIR MEASUREMENTS BY SMOKE-CLOUD METHODS 

For determining the direction and velocity of air currents flowing 

at velocities of less than 150 f. p. m., which observations will ordinarily 

comprise the bulk of the air-flow observations that should be made 

in any metal mine, smoke clouds offer the most readily available and 

practical means. The absolute accuracy of the method, while higher 

than it is commonly rated, is still very low but not lower than conditions demand. More accurate instruments, including special types 

of anemometers and the (dry) kata-thermometer, are available, but 

their use is not justified for mine air-flow conditions except for 

purpose!? of research. 

A most convenient device for generating smoke clouds under mine 

conditions is the smoke-tube device developed 4 by United States 

• Katz, S. H., and Bloomfield, J. J., Sulphur Trioxide Smoke Tubes for Determining Air Currents: 

Rept. of Investigations 2505, Bureau of Mines, July 1923, 2 pp. 
VENTILATION SURVEYS 5 

Bureau of Mines engineers. (See fig. 1.) This device produces a 

dense white cloud which travels with the air currents. Air is 

forced through a glass tube containing pumice stone saturated with anhydrous tin or titanium tetrachloride, preferably 

the latter, as it is less corrosive. A rubber aspirating bulb, with 

hard-rubber valves, is used for forcing aIr through the tube. To 

protect pocket linings from the corrosive 

action of the tube contents and to prevent self-sealing action when the tube is 

not in use the broken tip should be covered with a rubber tip, either a small 

cane tip or a pencil-eraser tip, depending on the size of the tube. The tubes 

may be made up locally as needed or 

may be obtained from one of the mine 

safety-appliance companies. This is one 

of the most useful devices carried by 

mine-ventilation engineers, since it can 

also be used to trace low-velocity air 

currents. A tube of the size shown usually will suffice for the needs of at least 

a full shift underground. 

Other methods of smoke-cloud production are, of course, available. Burning 

fuse has been widely used but is not 

very satisfactory for many situations. 

Apparatus for aspirating liquids and 

finely divided powders are available but 

are at best somewhat messy and usually 

have other disadvantages as to bulk, 

safety, or capacity. 

Rubber tube, 

1 to 2 feet long 

Hard-rubber valve 

Rubber-bulb 

aspirator 

Hard-rubbet valve 

To measure velocities by smoke-cloud 

travel measure or pace off a known distance along as straight and as uniform 

a section as can be found on the airway. Station the smoke-cloud producer 

at one end of this section and an observer with a stopwatch at the other. 

The smoke-cloud producer should call 

the starting time of the cloud production, which is made across the airway 

rather than along it and about halfway between the center and the walls. 

In slow-moving currents the original 

cloud can be reinforced with another, 

or several clouds can be started in 

different positions in the cross-section 

by producing the succeeding clouds FIGURE ~;;;-l~-{l'fo~~ft~~!:es type of 

in line with the advancing cloud first 

produced. The observer should call the finishing time, which 

is taken arbitrarily as that of the passing of the middle of the 

main body of the cloud or clouds. In straight airways of uniform 

cross-sectional area the velocities found about one seventh of the 

height or the width of the airway from the walls approximate the 
6 ENGINEERING FACTORS IN VENTILATING METAL MINES 

average velocity of the section, but smoke clouds generated to travel 

so close to the walls would usually be disturbed by eddy currents 

due to the rough walls. With travel at about the quarter points the 

readings will average about 10 percent high, and determined average 

velocities should be decreased by approximately this amount (multiply 

by 0.9). In irregular airways the distribution of velocities is irregular, 

and the time of travel of clouds started in different positions in the 

cross-section should be averaged. These results will also average 

high and should be decreased arbitrarily about 10 percent. With 

very low velocities of flow through airways where rock and air temperatures differ considerably there are often large differences between 

the velocities in the upper and lower parts of the cross-section, but 

these can be roughly averaged from separate cloud travels. The 

smoke clouds, however, are affected to a relatively larger extent by 

the higher-velocity air streams, and deductions of about 20 percent 

from observed average velocities (multiply by 0.8) are required for 

approximate accuracy. Disturbances due to car, cage, and skip 

travel in nearby airways should be expected :in the vicinity of such 

activities, and an attempt should be made to obtain measurements 

during normal periods of flow only. One of the great advantages of 

smoke-cloud methods ove.r more accurate methods for mine work is 

the fact that the effects of disturbances are readily observed. It is 

also possible to take measurements in airways virtually inaccessible 

or dangerous to enter without safeguards that would take time to 

provide and might interfere unduly with mining activities. 

To determine quantities of flow it is, of course, necessary to know 

the average area of cross-section of the length of airway used for the 

velocity observation. Under ordinary conditions the length is best 

taken as 25 feet. With good light and with velocities near and above 

100 f. p. m. this distance might be increased to 50 feet in a regular 

airway. With very low velocities and where required by the airway 

conditions it might be reduced to 10 or 15 feet. In untimbered rock 

airways average areas based on a few measurements may be in error 

as much as 10 percent, but more refined measurements ordinarily 

will not give the area actually occupied by the flowing current much 

more accurately. Individual determinations of quantities of flow in 

rough-walled airways may be in error as much as 30 percent by smokecloud :methods of measurement, due to combined errors in average 

velocity and average area determinations; but, in general, it will be 

found from check measurements that relative agreement within 10 

percent of more precise air measurements is usually obtained in rapid, 

though careful, work. 

MODERATE-VELOCITY AIR MEASUREMENTS BY VANE-ANEMOMETER 

METHODS 

For measuring velocities of 150 to about 2,000 f. p. m. in mine airways (a range that includes the major air currents of the average 

mine) ordinary commercial types of low-velocity vane anemometers 

are most practical, convenient, and accurate. Other methods of 

accurate measurement of velocities in this range are available but have 

been developed mainly for application to small ducts and have serious 

disadvantages for rapid surveys of mine-ventilation conditions .. 
YENTILATION SURVEYS 7 

The vane anemometer is a small windmill geared to a totalizing 

mechanism through a small clutch, which can be engaged at will. 

The usual type used in American metal mines is the Biram type sho"\yu 

in figure 6, A (p. 17); the common size is about 4 inches in diameter. 

Late :models are provided with a threaded hole through which, by 

means of a small attachment usually provided with the instrument, 

it can be mounted on a small-diameter shaft. For precise work the 

anemometer is mounted on a shaft, but for ordinary rough measurements it is held in the hand as shown. In all cases it is important 

that the dial side face away from the air current. 

Another type of vane anemometer, which is useful in that it can be 

more easily carried around for occasional rough measurements, is 

the watch type, also shown in figure 6, A. 

There is a general impression among mining engineers that anemometers are unreliable, an impression created partly by their own 

attempts to use them improperly and partly by the expressed opinions 

of engineers in other lines of endeavor who are :more familiar with 

methods of measurement applicable to small ducts of uniform crosssection and who have no idea of the variations in :mine-air flow. 

The author has had an extensive and intimate acquaintance with anemometric air measurement in mines, with facilities for comparing 

methods of air measurement, and has found anemometers very reliable 

when properly used. The accuracy of results is affected to a large 

extent by the exact :method of use but not to a much greater degree 

than any other device used under similar circumstances. The anemometer has the important advantages that its readings arelractically independent of air density and (for the range encountere in mines) 

of air temperature and that it is affected but very slightly by nonparallel flow. Accurate measurements of air flow can be obtained 

only by carefully traversing a cross-section at right angles to a straight 

air passage of regular area a considerable distance downstream from 

any disturbance of the flow and from a position outside the airway 

and then applying correct calibration factors to observed readings. 

However, an average accuracy within 5 percent, which is close 

enough for mine-air distribution records, can be obtained with lessrefined methods if calibration and method factors are used and areas 

are carefully determined. 

Anemometer calibrations.-Vane anemometers usually register a 

velocity that is somewhere within lO percent of the true velocity; 

the latter has to be determined by referring to the results of calibration tests. In this country the National Bureau of Standards, 

Washington, D. C., is the recognized authority on anemometers and 

is prepared to calibrate these instruments for a nominal fee. Calibrations are made on a horizontal whirling arm for low velocities, with 

correction for the air swirl set up by the apparatus, and in a horizontal 

wind tunnel for high velocities. The use of a shaped nozzle or orifice 

to give uniform parallel flow at the discharge of a small fan-pipe 

installation would appear to provide a simple and accurate field 

method of calibrating anemometers for both horizontal and. vertical 

flow, but it has not been used, to the author's knowledge. 

Calibrations furnished by the :manufacturer with new instruments 

are usually in the form of tables of plus and minus corrections to be 

made to observed velocities per :minute, as figured from the total 

reading and the total time of operation, and usually agree with Bureau 
8 ENGINEERING FACTORS IN VENTILATING METAL MINES 

of Standards calibrations within 2 percent. The latter are issued in 

the form of a curve, figure 2, A, in which a factor, by which the 

observed velocity must be multiplied to ascertain the true velocity, 

is plotted against the observed velocity. 

It has been found by experiment 5 that the registered velocity of a 

vane anemometer is a straight-line function of the true velocity, as 

shown in figure 2, B, which can be represented by the general equation 

(1) 

where V T is the true velocity, V R is the registered velocity, and A and 

B are constants for the particular anemometer. Apparently, according to calibration curves for a great variety of anemometers, A 

remains constant for anyone instrument, is virtually the same for 

all instruments of a general type, and averages about 30 for the 

common 4-inch, Biram-type, low-velocity (8-blade) instrument. 

The author has records of calibrations in which A ranges from 0, 

for a Rosenmuller special low-velocity instrument, to 60, for a Biramtype, high-velocity (4-blade) instrument. Apparently, also, the value 

of B in the calibration equation is practically constant for anyone 

instrument that is kept fairly clean, well-oiled, and not damaged 

mechanically and varies but a few percent between instruments of 

the same design and make, but varies largely-at least over the range 

0.90 to 1.15-for different designs and makes. It appears possible 

that an instrument could be made with B equal to 1.00, in which 

.case it would only be necessary to add constant A to the registered 

velocity. The influence of design on constant A makes it appear 

probable that an instrument could be designed with values of 1.00 

for both A and B, in which case no calibration would be required. 

The straight-line method of plotting calibration results has a number of advantages. First, the straight line will be a "faired-up" 

average of individual calibration-test results; second, the calibratIOn 

may be based on tests at but two velocities or, if constant A is known 

from previous calibrations, on one (preferably high) velocity only; 

third, the constants for a particular instrument may easily be memorized and applied directly to observed velocities. 

Calibrations are available for horizontal flow only, and it is assumed 

that these hold also for vertical flow upward and downward. Complete 

comparative calibrations to prove this point are desirable but are not 

available. The author has made a few comparative traverses in 

shafts where average velocities were measured by more exact methods 

and found no perceptible discrepancies at moderate to high average 

velocities (1,000 to 2,000 f. p. m.). Comparison of air measurements 

made in horizontal and vertical airways in mines has likewise indicated no larger discrepancies than similar comparisons of horizontalflow measurement. 

The effect of air density on calibrations, according to Ower, is 

negligible, except for very low velocities and large density differences. 

If d is the density where the observation is made and dB the density 

at which the calibration was made (usually about 0.075 pound per 

cubic foot), constant A in the calibration equation should be changed 

to A-J~ to allow for the air-density effect. For the Biram type of 

• Ower, E., Measurement of Air Flows: London, 1927, pp. 101-134. 
VENTILATION SURVEYS 9 

1.20 

1.1 5 

0 

I 

5 

0 

.9 5 

0 I 

250 500 750 1,000 1,250 1,500 

REGISTERED VELOCITY, FEET PER MINUTE 

1,750 2,000 

-.9 

o 

2,00 0 

1,7 50 

00 

0 

I~i/' 

00 

50 

0 

2 

o 250 500 750 -1,000 1,250 1,500 l,751) 2,000 

REGISTERED VELOCITY, VR, FEET PER MINUTE 

FIGURE 2.-A, Calibration curve for vane anemometer; E, calibration chart for vane anemometer. 

122422°--35----2 
10 ENGINEERING FACTORS IN VENTILATING :METAL MINES 

low-velocity anemometer this change results in a difference of but 

approximately 3 f. p. m. as between sea level and points 5,000 feet 

above or below sea leveL 

Ower's tests have also shown that the accuracy of an anemometer 

is affected to but a slight extent when its face is not exactly at right 

angles to the direction of flow. With the face up to 20° off line the 

effect on the accuracy of readings was less than 2 percent, a result 

that is especially significant for mine work where the directions of 

flow are commonly quite variable. 

Method factors.-For rapid mine-ventilation survey work the 

anemometer must be used under conditions differing from those under 

which it was calibrated, and allowances must be made for such 

differences to obtain approximately accurate results. After differences between individual anemometers are eliminated by applying 

calibration factors to the registered velocities, it is necessary to multiply the result by another factor, termed the "method" factor, to 

get the true average velocity over a particular cross-section to allow 

for certain changes between conditions of use and conditions of 

calibration. Important considerations are: Whether average velocities are based on one-point readings or on traverses of the section; 

whether the traverse is made by a precise or by an approximate 

method; whether the anemometer is mounted on a shaft or held by 

hand; whether or not the air stream itself has some abnormal characteristics at the place of measurement; and whether or not the 

anemometer itself causes an appreciable change in the normal conditions of flow. Particular methods of use usually involve a number 

of these considerations to a varving degree, but the author has found 

by experiment that average method factors can be applied for a 

number of common methods of use to give satisfactory accuracy. 

ONE-POINT MEASUREMENTS 

The simplest and fastest method commonly used is that in which the 

average velocity over the cross-section is based on a measurement at 

but one point in the section. With the operator's body out of the 

air current and the anemometer mounted on a shaft, fair results may 

be obtained in straight airways of uniform cross-sectional area. With 

the anemometer held at the center of the section, a method factor of 

0.80 should be used. When it is held about one seventh of the wallto-wall dimension from one wall, or preferably two, the method factor 

is 1.00, but results are not as consistent as for the center position on 

account of the effect of minor irregularities usually present in the wall 

surfaces. However, mine airways that are straight and uniform 

enough to permit accuracy within 5 percent by this method are most 

often shafts carrying the main intake or return air currents, for which 

it is generally desirable to obtain more accurate measurements by 

careful traversing. 

In irregular airways the velocity distributions cannot be foretold 

with any degree of accuracy, and the method is not applicable. 

Actual tests in mine openings under comparable conditions have 

resulted in calculated method factors ranging from 0.65 to 0.90, with 

very little correlation of factors to observable differences in conditions. 

However, one-point measurements of velocity in irregular airways do 

indicate relative changes in quantity of flow, provided they are .made 

at the same point in the same cross-section. 
VENTILATION SURVEYS 11 

APPROXIMATE TRAVERSING METHODS 

Most of the measurements made by anemometers in mines are on 

minor airways where approximate traverse methods, combined with 

the use of method factors, are best suited to the conditions encountered and give sufficient accuracy, that is, within 5 percent on the 

average. Continuous traverses made by moving the anemometer 

slowly through a plane at right angles to the axis of the airway and 

along lines approximately parallel to the walls and not more than 1 

foot apart, as shown in figure 3, A, are recommended. The total 

time should be gaged by the dimensions of the opening and the relative 

accuracy of the results desired; 1 minute for a 30-square-foot opening 

and up to 3 minutes for a lOO-square-foot opening are average figures. 

The speed of traversing should be uniform throughout the section, 

and for this reason it is practically necessary to have an assistant call 

out the 5- or lO-second intervals. Any regulation of speed to time 

should preferably be done when the anemometer is about one seventh 

B 

FIGURE 3.-A, Diagram showing path followed with anemometer in the continuous approximate method 

of traversing air-measurement sections of mine airways; the positions of the anemometer are indicated 

for a-second intervals of a I-minute traverse.. B, Diagram showing position of traverse points in a circ,.lar air-measurement section for a a-area 20-point traverse. 

of the dimensions from the walls, that is, in the zone of approximate 

average velocity. With a little practice it will be found that check 

measurements within 2 percent can be obtained under almost any 

conditions. As an extreme illustration of this point it may be cited 

that the author obtained this degree of relative agreement in 3 sets of 

duplicate traverses of arched tunnel sections 27 feet wide and 20 feet 

high with an anemometer mounted on a 14-foot shaft and operated 

from the top of a 12-foot stepladder. 

For horizontal or slightly inclined openings the anemometer may 

be held by hand at arm's length while the observer moves slowly 

across the section as far as is necessary to complete the traverse. 

The method factors to be used are not very well authenticated but 

have proved satisfactory in the author's experience. They depend 

mainly upon whether or not the traverse is started with the operator's 

body in or out of the current. If started with the operator's body in 

the current, as in unlined rock sections, the first part of the traverse is 

made close to the body in a zone of locally increased velocities, and 
12 ENGINEERING FACTORS IN VENTILATING METAL MINES 

the factor to use is 0.85. If the traverse can be started with the 

operator's body shielded by a timber or a doorframe the factor to use 

is 0.90. This same factor can be used if the operator stands in the 

airway and traverses each half separately, starting each with his 

body at arm's length from the center. These factors are for ordinary 

metal-mine drifts of 35- to 45-square-foot areas. For larger openings 

the ratio of area of body to area of opening is less, and the relative 

effect on method factors is less. For 80-square-foot openings the 

normal factors of 0.85 find 0.90 should be increased to 0.88 and 0.92, 

respectively, and to 0.90 and 0.93, respectively, for openings of 100 to 

120 square feet. 

The fact that an obstruction, such as the operator's body, changes 

the normal velocity distribution of the air stream both alongside and 

for a considerable distance upstream is rarely recognized; only the 

downstream disturbance is commonly considered. To take a special 

case, anemometers are sometimes operated on the end of a short shaft 

by an operator standing downstream and in line with the anemometer 

position. Readings taken in this way will give results probably 5 to 

10 percent-low. Then, again, where separate readings are taken at 

each of a number of properly arranged traverse points the starting 

and stopping of the anemometer, unless some method of remote 

control is used, results in some obstruction of the air currents twice 

for each reading, which will have some effect on the accuracy of the 

observations, depending on the time used for each observation. 

For vertical and steeply inclined airways it is desirable to mount 

the anemometer on a shaft. With the anemometer mounted on a 

shaft and started and stopped from points close to the wall on one 

side of the airway very little interference with the air currents results, 

and no method factors are required. Actually, on account of exaggerated looping in low-velocity areas close to the walls, the results 

in a passage of uniform area will usually average about 2 percent low 

and require a factor of 1.02; but at constrictions, such as doorframes, 

the velocities close to the walls are relatively high, and the results 

will average about 2 percent high and require a method factor of 0.98. 

However, by avoiding excessive looping close to the walls in continuous traverses, the necessity for applying a method factor is avoided. 

PRECISE TRAVERSING METHODS 

Precise traversing methods with anemometers can yield results 

within 2 percent of absolute accuracy but are justifiable only on 

major air currents or in connection with tests of primary fan performance to determine pressure-quantity relations; they should be 

applied only at sections of regular area on straight airways well away 

from disturbing influences. Traverse points should be located-by 

strings, wires, or marks on guideframes-at the centers of approXimately equal areas of the cross-section, which is assumed to be rectangular. Their number depends on the shape of the section and the 

conditions of flow and should not be less than 16 for a square section 

or less than 24 for a section in which one dimension is twice the other. 

Although a maximum spacing of 6 inches is often recommended for 

test work in relatively small ducts a minimum spacing of 1 foot is 

satisfactory in mine airways. If the conditions of distribution are 

such that closer spacing would seem desirable, it is very probable that 
VENTILATION SURVEYS 13 

the conditions of turbulence would prevent a high degree of accuracy 

by any method of measurement. 

Assuming that it is possible to work from without the section or 

from a recess, precise traversing is most practically and conveniently 

carried out under mine conditions by what might be termed "continuouspointtraversing." In this method the anemometer is not started 

and stopped at each traverse position but is started at the first position, is slowly moved to a new position at the end of each 10-, 20-, or 

30-second period, and is stopped at the last position. Traverse 

points may be arranged so that the start and stop positions are on 

the same side of the airway; or the traverse points may be covered 

twice to bring the stop position at the same point as the start position. 

N arrow airways, up to 5 feet in width, can be covered from one side 

with an anemometer attached to a 30-inch shaft with but little 

interference from the operator's arm. Airways up to 10 feet in 

width can be covered in halves, one half from each side, and where 

conditions permit narrow airways can be covered similarly, with 

possibly a slight gain in accuracy. For airways wider than 10 feet it 

is desirable to stretch taut wires across the airway and to use a small 

hook attachment to suspend the anemometer, which is placed as to 

position laterally by means of a shaft, as before, either from one or 

both sides. Pulley and guide arrangements have also been used for 

traversing very wide or very high airways. 

Separate-point traversing methods-that is, methods involving separate velocity observations for each traverse point-are often used 

for precise work but are too slow for ordinary mine work, since at 

least 1- or 2-minute readings are required at each point to minimize 

inaccuracies due to interference with the air currents~n starting and 

stopping the anemometer, and further time is required to change the 

position of the anemometer and secure it in the new position. The 

time intervals may be shortened by using some system of remote 

control to start and stop the instrument, such as a flexible wire in a 

flexible-coil casing, a small air-operated piston, or a small solenoid; 

but anemometers usually are not so equipped, and if the added device 

is at all large a new calibration is required to gage its effect. However, where it is impossible to work from a recess communicating 

with the section some such method is required for precise work; and 

these methods can, of course, be applied from a point without the 

section. Nevertheless, with either continuous-point or separate-point 

traversing, results accurate within about 2 percent can be obtained 

with careful work at good sections without the use of method factors, 

and the slowness of separate-point traversing usually is not justified. 

At doorframes which constrict the area of the airway abruptly 

about 50 percent or more the ordinary number of points do not cover 

the higher-velocity areas close to the walls elosely enough, and a 

method factor of about 0.95 is ordinarily required for 16-point traversing. . 

In order that traverse points on circular sections may represent 

equal areas the circular section is divided into equal areas by concentric circles, as shown in figure 3, B, and traverses are made on two 

diameters perpendicular to each other at points determined by their 

intersection with the alternate circles shown dotted in the figure. 

The full circles are the boundaries of equal areas, and the dotted 

circles are the mean radii of these equal areas. Four points are 
14 ENGINEERING FACTORS IN VENTILATING METAL MINES 

taken for each ring of equal area-that is, 12 for a 3-area traverse and 

20 for a 5-area traverse. The latter is recommended. for precise 

traversing. If D is the diameter of the airway the distances from the 

wall to the points on each diameter will be 0.044 D, 0.146 D, 0.296 D, 

0.704 D, 0.854 D, and 0.956 D for a 3-area 12-point traverse and 

0.026 D, 0.082 D, 0.146 D, 0.226 D, 0.342 D, 0.658 D, 0.774 D, 0.854 

D, 0.918 D, and 0.974 D for a 5-area 20-point traverse. 

With anemometers mounted on a shaft operated on a carefully constructed guideframe relative agreement of successive readings to 0.1 

percent can be obtained under uniform flow conditions, and absolute 

accuracy better than 2 percent can be obtained under the best conditions of mine-air flow. 

HIGH-VELOCITY AIR MEASUREMENTS BY PITOT-TUBE METHODS 

The use of the ordinary vane anemometer in velocities over 2,000 

f. p. m. is liable to crack the jeweled bearings, and at velocities above 

2,500 f. p. m. this result is almost certain. Special high-speed anemometers, usually with half as many vanes as the standard type, are 

made for this work but seldom are required for mine measurements. 

The high velocities that it is most often desirable to measure are 

those thl1t occur inside small pipes or tubing where it is impossible 

to use an anemometer. Here the velocities are high enough to permit 

the use of methods involving simple pressure-measuring instruments; 

and the use of a Pitot tube, which can be inserted through a small 

hole punched in the pipe or tubing, is convenient for obtaining the 

pressures, which can be measured on a U-tube water gage. Since a 

Pitot tube and.a pressure gage are desirable for certain other pressure 

measurements in mine-ventilation surveys, the use of these instruments, rather than a high-speed anemometer, is recommended for this 

work, even though the special high-speed anemometer is more convenien t to use. 

The Pitot tube is a primary standard instrument for measuring 

velocities. Commercial types are generally accurate within 1 percent, 

and types based on recent research can be made to be accurate to 

within 0.1 percent. The usual form, figure 4, is two concentric tubes, 

one within the other, with the end bent at right angles to the shaft. 

The inner tube is open on the end and receives the total pressure of the 

air stream. The outer tube is closed on the end and receives the 

static pressure of the air stream through a number of small pinholes 

set back from the end. The tip is tapered or is made hemispherical 

to reduce interference with the air stream to a minimum. The opposite end of the tube has two connections, one to each tube. The 

difference in pressures obtained with the instrument is the velocity 

pressure at its tip when it is pointed directly against, and parallels, the 

air stream; it is usually determined and expressed in inches of water 

pressure. Velocity pressures vary as the square of the velocity and 

directly as the density of the alr. The constants in the formulas 

connecting velocity with velocity pressure vary slightly with location owing to slight variations in the acceleration due to gravity, but 

sufficiently accurate formulas are: 
VENTILATION SURVEYS 

V=1097.~, or H.=O.OOOOO0831 d VIl, 

where V=velocity, feet per minute; 

H .=velocity pressure, inches of water (at 60° F.); 

d=density of air, pounds per cubic foot. 

At standard air density of 0.075 pound per cubic foot, 

V=400S" H., or H.=0.0000000623 VIl. 

15 

(2) 

(3) 

Velocity pressures ordinarily are so small in mine openings that 

sensitive differential-pressure gages are required to measure them 

accurately. Even with a velocity as high as 2,000 f. p. m. the velocity 

pressure IS only about one fourth inch at standard air density, and 

with a gage reading to hundredths approximate accuracy only is 

obtained. 

Since velocity pressures vary as the square of the velocities the mean 

velocity at a section cannot be obtained accurately from the direct 

FIGURE 4.-Pitot tube, with details of ends. 

mean of the velocity pressures obtained at traverse points but must 

be obtained as the mean of the separately computed velocities or from 

the square of the mean of the square roots of the separate traversepoint pressures. The error in the velocity determined from the direct 

mean of pressures, however, is usually less than 2 percent, and this 

refinement can be neglected in approximate measurements. Ayproximate traverse methods for fan-pipe air measurement are dlscussed in a later part of this section. 

ACCURACY OF MINE AIR·FLOW MEASUREMENTS 

Good relative agreement is, of course, not the same as absolute 

accuracy. In general, rough measurements of velocities and areas 

for minor air distribution will give flow measurements with an average 
16 ENGINEERING FACTORS IN VENTILATING METAL ML.~ES 

accuracy of about 10 percent, and the sum of a group of 10 or more 

measurements will probably be within 5 percent. Careful measurements of velocities in major air courses by approximate traversing 

methods, with the use of the method factors given, combined with 

careful area measurements, will give flow measurements with ail 

average accuracy of about 5 percent, and the sum of a large group 

will probably be within 2 or 3 percent. Considering the normal 

variations in air quantities in mines due to changing natural-draft 

conditions, variations in the total amount of compressed air liberated, 

and the effect of many other uncontrollable variables, these degrees of 

accuracy are precise enough for practical purposes. However, in 

determining fan performance and for certain other purposes, more 

accurate flow measurements may be desirable, and an £,ccuracy within 

about 2 percent can be obtained by precise traversing methods and 

close attention to details. Under mine conditions it is hardly possible 

to obtain accuracy much under 2 percent, even with precise methods 

of air measurement; such work calls for specially skilled technique, 

expensive preparations, and instruments not warranted except for 

research. 

Considering the inaccuracies of most mine air-flow measurements, 

the practice of reporting quantities to the last cubic foot per minute 

is unwarranted. It is suggested that quantities below 20,000 c. f. m. 

be reported to the nearest 500 c. f. m. only and that above 20,000 

c. f. m. even thousands should be considered sufficiently precise. 

In considering the relative agreement of the quantities of flow as 

measured in the various parts of a mine the effect of compressed air 

liberated in the workings and of changes in air density on quantities 

of flow should be taken into account, since either may account for 

differences of at least 5 to 10 percent in almost any mine. 

TEMPERATURE OBSERVATIONS 

Except on intake airways within 1,000 feet of the surface, mine-air 

temperatures change very slowly; readings at 500- to 1,000-foot intervals in shafts and at 1,000- to 2,000-footintervals on working levels, 

combined with readings at. the face of every third or fourth working 

place, give a sufficient indication of air-temperature conditions 

throughout the mine, although additional readings will be required 

to establish the effect of local conditions. Wet- and dry-bulb temperature observations are required for determining relative humidities, 

absolute humidities (water-vapor content), total heat contents, and 

air densities. 

SLING PSYCHROMETERS 

Wet- and dry-bulb mine-air temperatures are conveJ!iently obtained with a sling psychrometer, which is merely an assembly of two 

thermometers mounted on a frame equipped with a pivoted handle 

which permits whirling of the frame to obtain the necessary air movement over the bulb of the wet-bulb thermometer. The Bureau of 

Mines type, figure 5, is in common use in American metal mines, and 

the design includes both an aluminum (a) and a leather (d) case for 

full protection during transportation underground. It carries winch thermometers mounted on a thin aluminum frame with stiffening 

edges. A recent improvement, not as yet incorporated in the stand-
Bureau of MiT'es Bulletin 385 

FIGUltE 5.-Bureau of Mines type sling psychrometer and carrying cases: a, Aluminum case; b, standard 

design; c, water well added to standard design; d, leather carrying case. 
Bullet in 3R5 

1J 

flGPRE 6.-.'1, Biralll-\ y pn vane anCmOlllC\,CrS; n, standard type at small a ncroid barometer. 
VENTILATION SURVEYS 17 

ard design (b), is the addition of a water well across the bottom (c). 

The well can be made of thin brass the width and thickness of the 

frame and three eighths inch deep. The top edge has a one-sixteenthinch slot across the width to admit the tubular wick of the wet bulb, 

preferably of "china silk" about 6 inches long and about the diameter 

of the bulb. The wick is doubled up in the small well and completely 

fills it. A small hole in the back of the well permits filling it with a 

medicine dropper and is closed with a large-head milled-edge screw. 

The use of a dropper is not required if clear water in which the end 

may be immersed is available. A few minutes' immersion saturates 

the wick enough for a day's work in the ordinary damp to wet mine. 

With a saturated wick in the well no trouble has been experienced in 

low-temperature weather, and there is no leakage through the wick 

slot. With free water in the well there might be leakage, and freezing 

of the water might burst the well. 

Thermometers with the scale engraved on the glass are required for 

accurate work. The standard lO-inch psychrometer thermometers, 

graduated from about -20° to + 130° F., are usually accurate within 

0.2°; readings can be estimated in good light to 0.1°, but they usually 

are not read closer than 0.5° under mine conditions. For close readings, involving holding a light close to the instrument, a flashlight 

should be used, and in air currents the operator should stand sidewise to or face the current to prevent the heat of his body from affecting the reading. The frame usually is not at air temperature when 

the reading is started; and whirling, with occasional observations, is 

required until both wet- and dry-bulb thermometer readings are 

constant. The wet-bulb reading should always be made first, as it 

changes quickly when the instrument is brought to rest. It is desirable to record the readings always in the same order, as 60.0-61.5 

for 60° wet bulb and 61.5°, dry bulb. For ventilation purposes the 

wet-bulb temperature is more significant than the relative humidity, 

and it should always be retained in records. 

Obtaining wet-bulb temperatures for temperatures below freezing 

is very slow work, as the evaporation of ice on the wet bulb is very 

slow, and there is an initial heating of the wet-bulb thermometer due 

to the heat release accompanying the change from water to ice on 

the bulb. Persistent whirling will, however, give results; but if the 

ice is evaporated before the true wet-bulb temperature is determined, 

indicated by a rise in the reading, it is necessary to add more water 

to the bulb, let it freeze, and repeat the process. Correct wet-bulb 

temperatures a few degrees below 32° F. may be obtained without 

the wet bulb freezing. 

Ordinary surface drinking water is clean enough for psychrometer 

use, although distilled water is preferable if conveniently obtainable. 

Mine water should not be used, as it often contains a high enough 

salt, or other chemical, content to affect the wet-bulb reading seriously. Dirt gradually accumulates in the wet-bulb covering and 

finally prevents it from absorbing water readily. A new covering is 

then desirable and is easily applied as follows: Cut a piece of muslin 

(from a roller bandage) a little longer than the bulb and wide enough 

to make a good lap; wash it thoroughly and wrap it around the bulb 

while wet; tie the upper end at the narrow groove above the bulb, 

and tie the lower end with a loop started just where the bulb starts to 

round off. As the loop is drawn taut it will slip down over the rounded 
18 ENGINEERING FACTORS IN VENTILATING METAL MINES 

end and draw the muslin taut; the excess muslin below the bulb should 

then be cut off. Any strong thread will do for tying. 

RECORDING THERMOMETERS 

Various instruments for recording wet- and dry-bulb temperatures 

and relative humidities are available. Those for dry-bulb temperatures and relative humidity are relatively simple and cheap. Those 

fOi' wet-bulb temperatures usually are combined wet- and dry-bulb 

recorders for either quiet or moving air and are relatively complex 

and expensive. The ordinary types of wet-bulb instruments require 

water wells and do not operate much below freezing temperatures. 

Maximum and minimum indicating thermometers, which may be 

obtained in a variety of designs, Hre convenient instruments for determining temperature fluctuations on the surface or underground and 

where the fluctuations are quite uniform are useful for approximating 

average temperature conditions over long or short periods. 

ROCK TEMPERATURES 

Rock-temperature observations are commonly made for determining the geothermic gradients or the rate at which the temperature of 

the rock in undisturbed areas increases with increase in depth below 

the surface. Gradients of 20 to 250 feet per degree (F.) have been 

reported, but most metal mines have gradients that fall within the 

range of 50 to 100 feet per degree. 

Although many methods and instrument~ may be used a lO-inch 

psychrometer thermometer mounted in a groove in one face of a 

three-fourths-inch square softwood block makes an easily carried and 

robust instrument. It can be inserted in any ordinary drill hole, 

assumes the rock temperature within about 10 minutes, and changes 

temperature very slowly on being withdrawn from the hole. 

Determination of rock-temperature gradients.-On account of the 

major effect of circulating air currents, water flow, drainage, and many 

other factors it is difficult to get good data in short holes extending 

from mine openings that will show the temperatures in undisturbed 

areas or the virgin-rock temperatures; and many misleading data 

have been published. Lacking precise data on the effect of various 

factors on the accuracy of temperature-gradient determinations, 

experience is the best guide. The author has found that temperature 

observations made in 5- to 6-foot holes in rapidly advancing development headings on the lowest levels of a mine, correlated with Weather 

Bureau or private records of the average mean dry-bulb surface-air 

temperature over a period of years, yield approximately accurate 

gradients. Direct comparisons of observations made on different 

levels underground usually are confined to relatively small temperature and depth differences, with resulting greater effect of small inacc,uracies on deduced gradients. The temperature of the ground 40 to 

50 feet below the surface has been found to average within a degree or 

two of long-time averages of the dry-bulb temperature of surface air. 

A better base figure would be a virgin-rock temperature close to the 

surface, but access to undisturbed areas usually is impracticable. 

Where the surface is very irregular the correct depth figure is not the 
VENTILATION SURVEYS 19 

vertical depth but the shortest distance between the point of observation and the surface, since the temperature contours of the rocks 

follow the surface contours. 

An allowance of 1 hour is assumed to be ample for the heat of drilling 

a hole and the effect of cool water used in drilling to be dissipated into 

the rock so as not to yield a substantially inaccurate observation. 

Ventilation of the heading, of course, cools the rock, but under ordinary 

circumstances the cooling does not penetrate to the bottom of a 5-foot 

hole during the drilling cycle. High-velocity ventilation of the face 

with air well below rock temperature, a condition usually confined to 

places where the rock temperatures are over 90° F., will cool the rock 

rapidly, and observations should be confined to rapidly advancing faces 

shortly after exposure. The thermometer needs to be left in only long 

enough to be certain that its casing has risen to rock temperature. 

ABSOLUTE-PRESSURE MEASUREMENTS 

Absolute pressures of the atmosphere are required mainly for airdensity determinations, particularly in connection with natural draft, 

fan performance, air sampling, and change-in-quantity calculations. 

For approximate purposes they may be estimated from tables of 

average values for various altitudes and temperatures; 6 but for any 

point the absolute pressure may change as much as 1 to 2 inches of 

mercury during the year, and actual observations are therefore more 

satisfactory for general use. 

The standard apparatus is a laboratory-standard mercurial barometer of the wall type in which the pressure of the atmosphere holds 

the mercury at a certain level in an evacuated tube, and various precautions must be observed both in the construction of the barometer 

and in correcting observed readings to get accurate results. 

ANEROID BAROMETERS 

For mine work the aneroid barometer (fig. 6, B) is commonly used, 

and a small rugged type is preferable. Inside the outer case is an 

evacuated chamber of corrugated metal. Atmospheric pressure tends 

to collapse this metal chamber, which is partly resisted by a spring. 

However, partial collapse occurs, the extent varying with the pressure, and is communicated to a pointer on the face of the instrument 

through levers and a small chain and sprocket. Compensation for 

temperature effect on the lever mechanism is accomplished by making 1 of the levers of 2 or more metals such that lengthening is 

compensated by bending. Very cheap instruments often are not compensated for temperature, although so marked; and most aneroids 

require time for compensating large temperature and pressure 

changes, since it takes time for the operating mechanism to take the 

new temperature, and an effect termed" hysteresis" causes a lag in the 

pressure indication. The most expensive and sensitive types of the 

standard aneroid, which are provided with reading lens and vernier 

reading attachment and are usually quite large-4 or 5 inches in 

diameter as against about 2~~ inches for the simpler instrumentsuffer more frequent changes in calibration from the rough handling 

incident to mine use and from the sudden large pressure changes in 

6 Buffalo Forge Co., Fan Engineering: Buffalo, N. Y., 2d ed., 1925, p. 20. 
20 ENGINEERING FACTORS IN VENTILATING METAL MINES 

vertical shaft transportation than do simpler, more rugged instruments. For accurate work it is essential that aneroids be compared 

frequently with a laboratory-standard wall-type mercurial barometer sl'lch as is usually available at any United States Weather Bureau 

office or physics department of a school or university. 

Average sea-level barometric pressure usually is taken as 29.92 

inches of mercury, and the limits of the standard aneroid ordinarily 

are 20 and 31 inches. For mine work a higher or lower range than 

this sometimes is required, necessitating a special instrument. 

An aneroid barometer occasionally is graduated in feet or other 

units of altitude, since differences of absolute pressures may be used 

for roughly determining differences of altitude; it is then termed an 

"altimeter." The ordinary aneroid usually is graduated in inches of 

mercury on the fixed inner circle and for convenience of use as an 

altimeter is graduated in feet or other linear units on an outer adjustable circle. 

Paulin aneroid.-A new tvpe of sensitive aneroid, perfected in 

recent years by Paulin System, Inc., makes use of a null, or zerogaging, principle of indicating pressures in a manner that eliminates 

virtually all inaccuracies of the standard type, in which the vacuum 

capsule is allowed partly to collapse. In this instrument the top of 

the capsule is brought back to zero position for each reading by a 

spring operated by a slow-motion screw; and by means of two or 

more concentric scales a very fine and accurate reading may be obtained, the readings being made to one one-thousandth of an inch 

mercury compared with one one-hundredth on the standard aneroid. 

SMALL ABSOLUTE-PRESSURE DIFFERENCES 

The measurement 7 of absolute pressures with enough precision to 

permit direct determination of small pressure differences due to flow 

of air in mine openings has always been desirable on account of the 

relative ease of such methods of measurement, but instruments with 

which even approximate results may be obtained have been developed only recently. In addition to the precision aneroids other types 

of instruments are being developed for this particular purpose. One 

type, represented by the Askania Statoscope, makes use of the null 

method in an aneroid-type instrument in which the base pressure 

can be set and small differences from this base pressure measured. 

Another type, of which a number of designs have been used and 

which has been termed a "contrabarometer", is patterned after the 

mercurial barometer but makes use of a second lighter liquid and of 

differences in tubing areas to increase the differences of "ertical 

levels corresponding to small pressure changes. Both types are 

rather bulky in their present forms; but the first appears to have 

greater advantages, as to ease of handling and portability, for mine 

work. 

In determining small pressure differences accurately by absolutepressure methods it is necessary to realize that such instruments 

measure only the absolute static pressure. At any point there is 

some energy in the air in the form of velocity, but it is ordinarily so 

small that it is not considered in ordinary absolute-pressure measure7 Williamson, J. N., Ritson, J. A. S., Cooke, W. E., and Statham, I. C. F., Air·Pressure Surveying in Mines: 'l'rans. lnst. Min. Eng. (London), vol. 81, 1931, pp. 313-33!J. 
VENTILATION SURVEYS 21 

ments. However, it may form a considerable proportion of small 

differences in total absolute pressures, and whether or not it is negligible for a particular condition should always be taken into consideration. 

Where the points concerned in the measurements are at different 

elevations it is necessary to calculate the actual average density of the 

air over the intervening vertical height so that the difi'erence due to 

the normal increase in the pressure of the atmosphere (0.0144 inch of 

water per foot for 0.075 air density) may be separated from the effect 

of air flow. 

DIFFERENTIAL-PRESSURE MEASUREMENTS 

Small pressure differences due to flow of air in mine openings 

usually are measured on a liquid gage by connecting both pressures 

to the gage and measuring the differential height of the liquid supported by the difference in pressure, which is a direct measure of the 

intensity of the pressure difference. Water is commonly used; or, 

if another liquid is used its gravity is determined, and all readings are 

commonly expressed in inches of water. Since the unit weight of 

water changes but very slightly with change of temperature no 

temperature is commonly specified, although for accurate work the 

weight of water at 60° F. is ordinarily used, which is closely equivalent 

to 5.2 pounds per square foot or 0.0361 pound per square inch. 

All common pressure measurements in mine ventilation are differential-pressure measurements, but since the most common measurements are those made to ascertain the difference in pressure between 

the atmosphere the gage is in and some other point at a different 

pressure which is connected to the gage through tubing or pipe one 

leg of the gage is open to the atmosphere and such measurements are 

usually termed merely "pressure" measurements. The term "differential-pressure" measurements is therefore commonly reserved for 

those measurements in which both pressures differ from that at the 

gage position, and both are communicated to it through tubing. 

U·TUBE WATER GAGE 

The most common pressure-difference measuring device for mineventilation surveys, and the only type required under most conditions 

of routine work, is a simple glass U-tube partly filled with water. 

With the tube in a vertical position the difference in level in inches 

between the water levels in the two legs is measured against a scale, 

preferably graduated in inches and tenths. With steady pressures 

the difference in levels can be measured by estimation to the nearest 

one hundredth inch, which is close enough for the constantly changing 

pressure conditions in mines. Commercial types are made with an 

adjustable scale, so that the zero position of the scale can be adjusted 

to the no-pressure-difference position of the water, and the readings 

both ways from the zero are added to obtain the total difference. 

On the assumption that the distances above and below the zero position will be equal, which is only true for absolutely equal bores not 

found in ordinary glass tubing, the scale on some commercial types 

of plain U-tube gage is made double so that only one reading is 

required. This type of gage causes confusion and may be quite 

inaccurate. 
22 ENGINEERING FACTORS IN VENTILATING METAL MINES 

For occasional measurements of the pressure difference between the 

two sides of a door or stopping, which are matters of interest in 

either naturally or mechanically ventilated mines, no great degree of 

accuracy is required, and the readings can be made on a simple 

home-made U-tube. A very simple type for rough work can be eonstructed by mounting a 5-inch U-tube, of one-fourth-inch O. D. 

glass tubing with the legs on about I-inch centers, on one half of a 

piece of white cardboard on which a black-line scale of inches and 

tenths has been ruled. The tube can be secured to the card with 

rubber bands and protected by folding the other half of the card over 

and securing it with another rubber band. The scale should be 

protected from dirt by a piece of flexible, transparent material, held in 

place by mounting tissue of some sort. The water can be left in the 

gage during transportation if the two ends are connected by a piece 

of mbber tubing-one-eighth-inch 1. D. tubing for a tight fit on 

one-fourth-inch O. D. glass tubing. For use, a short length of rubber 

tubing-preferably 2 to 3 feet of heavy-wall tubing to avoid excessive 

kinking and to permit some flexibility in position of the gage for 

reading-is attached to one leg of the U-tube. This is pushed through 

an opening to the other side of the door or stopping against which the 

tube and card are held rigidly and vertically while the difference of 

the two liq uid levels is read. The flexible mounting makes it possible 

to bring one level to an even division of the scale. It is desirable to 

arrange the reading to come about at the height of the eye, and it is 

necessary to take the precaution to have both the open end of the 

U-tube and the open end of the rubber tubing in quiet air, which means 

making sure that neither is opposite a crack or hole where a high 

velocity of flow might exist. 

For measuring fan pressures, however, a larger range and a more 

rigid mounting are desirable for better average accuracy. Commercial types, though cheap and good for fixed positions, usually are too 

heavy and bulky for rapid survey work. A light, easily carried gage 

may be made by mounting a lO-inch glass U-tube, similar to that just 

described, on a 7~- by I~- by ll-inch block of nonwarping wood, with 

each leg wired to the wooden mounting near each end and at the 

middle so that the U-tube is movable vertically but cannot bow away 

from the block. The scale should be ruled accurately on heavy white 

paper after it has been smoothly glued to the block and is thoroughly 

dry. This gage can be protected most easily and carried in a leather 

case similar to that of the Bureau of Mines standard sling psychrometer, with the rubber tubing attached and hanging down outside the 

case. A short extra length of tubing should be carried where differential-pressure readings, such as velocity-pressure readings from a 

Pitot tube, are required. 

MANOMETERS 

Although all differential-pressure instruments are correctly termed 

"manometers" and a water gage is thus a vertical manometer, the 

term is quite generally applied to gages in which one or both legs of 

the U-tube are inclined to obtain more accurate readings of the difference in vertical level of the liquid in the two legs of the gage. These 

are seldom used for routine mine work but are often desirable and are 

used largely for experiment and research. Types with uniform bores 

in both tubes, requiring readings on both legs, are more suitable to 
VENTILATION SURVEYS 23 

work where calibrations are infrequent than types in which the bore 

of one leg, which is vertical, is so large in comparison with the small 

bore of the other leg, which may be vertical but is usually inclined, 

that virtually all of the movement takes place in the small-bore leg 

and the total movement is read on this leg by means of a properly 

graduated scale. The slope of one or both legs may be adjustable, 

and other liquids than water may be used to increase the reading and 

to give better liquid-flow conditions in small-diameter glass tubing. 

Weeks 8 has fully described the construction of a portable, mine-type, 

uniform-bore, adjustable-slope gage which requires calibration only to 

check mechanical accuracy of construction. Many designs of nonuniform-bore instruments, with both fixed slope and range and adjustable slope and range, are commercially available. Because glass 

tubing does not have a uniform bore throughout they require calibration, and types in which the tubing is unsupported or rigidly fixed 

at both ends require frequent recalibration. With proper calibrations 

readings accurate within one or two thousandths of an inch of water 

may be obtained with these instruments, and accuracy within a 

hundredth of an inch is certain without calibration. 

MICROMANOMETERS 

Instruments arranged to make very accurate measurements of 

minute pressure differences commonly are referred to as micromanometers and involve micrometer arrangements and null methods of 

operation to insure accuracy. These are laboratory rather·than field 

instruments and are used as primary standards for calibrating field 

instruments. The simpler types, such as the hook gages, are accurate 

to one one-thousandth inch of water; more complicated types, of 

which the Wahlen gage,9 developed at the University of Illinois and 

largely used in Bureau of Mines ventilation investigations, is a good 

example, can be made accurate to one ten-thousandth inch of water; 

very special types with even greater sensitivity and accuracy can be 

made. 

RECORDING PRESSURE GAGES 

All the instruments so far discussed are indicating gages or manometers. Recording designs are not used to any extent in metal mines 

but are available in a variety of commercial types and undoubtedly 

could be used to advantage. The laws of most coal-mining States 

require recording gages for main-fan pressures, but there are no legal 

requirements of this type for metal mines and none are required. 

Most fans for both coal and metal mines are of the forward-curvedblade type and have such flat pressure characteristics that the record 

of fan pressures, particularly when taken from the inlet side of fans 

in the exhaust position, has little relation to fan performance or mineairway conditions. Records of quantities of flow would be much more 

informative for any fan installation; but records of quantities of flow 

are more difficult to obtain than pressures, and the types of instruments available are few and relatively expensive and require the use 

of a calibration curve to interpret the record. 

8 Weeks, W. S., Ventilation of Mines: New York, 1926, p. 15; A Precise Manometer for Ventilation Measur~ents: Min. and Met., vol. 4, 1923, pp. 19-22. , WIllIard, A. C., Kratz, A. F., and Day, V. S., Investigations of Warm-Air Furnaces and Heating Sys- tems: Univ. of Illinois Exp. Sta. BUll. 120, 1921, pp. 91-98. 
24 ENGINEERING FACTORS IN VENTILATING METAL MINES 

PRESSURE OBSERVATIONS 

Pressure observations often require transmittal of pressures through 

long lengths of tubing and must often be obtained in moving air currents. Any length of horizontal tubing suffices to transmit pressures 

accurately, but where the tubing extends to points at different elevations it is necessary to have the same air density within and without 

the tubing to balance the normal change in absolute pressure that 

accompanies differences in elevation, which means that in vertical or 

inclined openings the tubing should be suspended in the airway for 

,,:hich the measurements are being made and not in a warmer or colder 

airway. 

Mine pressures are always fluctuating, and there is a time lag in 

communication of the fluctuations through the openings of pressure 

instruments and through tubing, which sometimes causes large fluctuations in gage pressures and for good results requires an average to 

be made of anum ber of readings. vvnere both pressures are communicated through long lengths of tubing the gage fluctuations can be 

largely minimized by equalizing the openings of the pressure-obtaining 

instruments and by adjusting the tubing lengths, by trial, to a ratio 

that gives minimum fluctuations. 

To obtain correct static pressures in a moving air stream it is 

necessary to take the pressure lead from a connection to a small 

hole or holes in a surface parallel to the flow and free from projections 

or burrs, for which purpose the static-pressure member of the Pitot 

tube is convenient for survey work. For permanent installations 

larger holes, up to one fourth inch in diameter in side walls or in 

beveled-edge plates, are preferable because they are not so quickly 

clogged. 

FAN-PERFORMANCE DATA 

In mechanically ventilated mines there are usually three types of 

fan installations for which relative performance data are desirable: 

Main fans, booster fans, and auxiliary fan-tubing units. The rapidity 

of change of conditions of normal operation dictates the degree of 

accuracy desirable in routine records, which ranges from about 2 percent for main-fan installations to 5 percent for booster-fan installations and 10 percent for auxiliary fan-tubing installations. 

The relative mechanical efficiency of fan operation-that is, the 

degree to which the operating condition approaches that for which the 

fan was designed-can be determined, usually within about 1 percent, 

from approximate measurements of pressures and quantities by reference to curves showing the manufacturer's test data, but the correlation of pressure measurements to rated catalog or test-curve pressures usually requires some anal vsis, as is discussed in detail in a later 

section on fan pressures and fan ratings. Usually this information 

on relative efficiency suffices for the efficient use of the fans available 

and for the selection of new fans. 

Determinations of absolute mechanical efficiency are difficult to 

make in the field and are liable to be very inaccurate unless supervised 

by someone with a knowledge of the technique necessary for obtaining 

the measurements, for analyzing the results, and, if required, for 

applying the pressure corrections almost always involved in field tests. 

Unavoidable leakages are also a confusing factor in such tests, and 
VENTILATION SURVEYS 25 

since all ,errors are cumulative the final results expressed as absolute 

efficiencies may contain large errors. 

Fan-speed measurements may be made accurately and easily with 

ordinary revolution counters or Veeder counters, and instruments are 

available for determining speeds accurately by measurements requiring only a few seconds. Speed recorders are not always reliable and 

should invariably be checked for accurate work. 

Field measurements of power are difficult to make accurately. Virtually all metal-mine fans are electrically driven, and even though the 

power input to the motor is determined accurately by the mine electrical department the motor efficiency for the exact condition of 

operation must be known and the drive losses estimated or determined separately before the power input to the fan is known. 

In all field tests it should be remembered that fan ratings are based 

on a standard sea-level weight of air or air density, usually taken as 

0.075 pound per cubic foot. Quantities of flow at the fan are independent of air density, but pressures and powers vary directly with it. 

MAIN FANS 

Quantity of flow should be determined by precise traversing methods 

wherever it can be accomplished most conveniently with due regard 

to turbulence and possible leakage. The discharge of surface exhaust 

fans, either with or without tapered (evase) discharge connections, is 

usually so turbulent that measurements at this most conveniently 

accessible point are seldom reliable unless a method factor has been 

determined for the particular type of discharge; and these easily may 

vary from 0.7 to 1 for good types. If measurements are made at a 

distance from the fan, particularly if any great difference of elevation 

is involved, the air density should be determined both at the point 

of measurement and at the fan inlet and the measured quantity 

corrected on an equal-weight basis to the inlet air density. All surface-fan installations should be provided permanently with suitable 

arrangements for getting a good static-pressure reading inside the casing-near the inlet of an exhaust fan or near the discharge of a 

pressure fan-and one free from turbulence effects. A position as 

close to the fan as possible is desirable, but it is even more desirable 

that it be in a section where the flow sensibly parallels the walls of 

the duct. The difference in static pressure between the atmosphere 

and the inlet of an exhaust fan or the discharge of a pressure fan 

should be measured, as well as the area of the section at which it is 

obtained. This last measurement, as well as the density at the fan 

inlet, is required for conversion of the data to standard conditions 

for comparison with rated performance data and determination of 

relative efficiency of operation. 

BOOSTER FANS 

Quantities of flow usually are determined in the vicinity of the fan 

by approximate traversing methods. The fan discharges through a 

stopping which should be provided with an opening through which to 

pass the tubing of the pressure gage. The pressure on the stopping, 

close to both sides of which there is virtually quiet air, is measured 

and may be taken directly as the approximate differential static 

122422°-35-3 
26 ENGINEERING FAC'fORS IN VENTILATING METAL MINES 

pressure produced by the fan. The latter should be based more 

correctly on the static pressures at the fan inlet and outlet, neither 

of which is quite the same, although usually within a tenth of an 

inch of water, as the pressure on the adjacent side of the stopping. 

Corrections for these differences, of which only that on the discharge 

is usually material, can be made by making separate determinations 

of the difference in static pressure between the inlet or discharge 

and the stopping by using the static member of a Pitot tube for 

obtaining the pressure in the moving air current. 

AUXILIARY FAN-TUBING UNITS 

For an auxiliary fan-tubing unit it is desirable, in a survey, to 

determine the approximate quantities both at the end of the line 

and close to the fan to keep informed of the approximate leakage 

condition of the line of tubing or pipe which carries air to or from the 

working place. It is suggested that, for rigid tubing, the end reading 

be taken by holding an anemometer, mounted ona short shaft, at 

the center of the tubing end and applying a method factor of 0.85 

for a discharge end or of 0.7 for an inlet end. With canvas pipe the 

discharge-end condition is such that only the most approximate 

measurements can be made, and it is suggested that the reading be 

taken by holding the anemometer by hand at the center of the 

discharge and applying a method factor of 0.8. 

Both the pressure and quantity in the tubing near the fan can be 

ascertained most conveniently with a small Pitot tube inserted 

through a small hole punched in the tubing. With the static connection only attached to the gage the static pressure in the tubing is 

first measured. Then both connections are attached to the gage, and 

a rough traverse is made on one diameter of the tube with the traverse 

points gaged by a mental picture only of the correct traverse positions 

(see fig. 3, B) accompanied by a rough mental average of the observed 

velocity pressures for later computation, in connection with a calculation of the actual area based on the measured circumference, of 

the average velocity and quantity. The method is admittedly very 

rough but sufficient for accuracy within 10 percent, which is all that 

conditions ordinarily justify. Greater accuracy may be obtained by 

the use of a sensitive gage combined with more attention to the details 

of procedure, such as the use of guideframes for'locating traverse 

points on two perpendicular diameters. 

AIR DENSITY 

The weight of 1 cubic foot of a mixture of air and water vapor, 

commonly designated as "air density", may be found accurately 

enough for ventilating problems from the formula 

1.325 

d=459+t (B-O.378f), 

where d is the weight of 1 cubic foot, pounds; 

t is the dry-bulb temperature, of.; 

B is the barometric pressure, inches of mercury; and 

f is the vapor pressure at the dew point, inches of mercury. 

(4) 
VENTILATION SURVEYS 27 

With the same units the weight of the dry air (da ), in pounds, in 

1 cubic foot of a mixture of air and water vapor may be found from 

1.325 

da=4.59+t (B-f), (:i) 

and the weight of the water vapor (dw), in pounds, in 1 cubic foot. 

approximately from 

].325 0.824 

d W =459+t 0.622 f= 459+t1. (6) 

Slightly different values of the constants in these equations and 

other formulas are given by various authorities; and tables of densities and steam data are also available. The standard weight of 

air for mine-ventilation work is that of dry air at 70° at sea level and 

is 0.07495 pound per cubic foot; 0.075 is sufficiently accurate for 

ventilating purposes. A chart 10 for the approximate determination 

of air densities from (4) and their ratio to standard (0.075) air density 

is given in figure 7, in which the method of procedure is indicated by 

the solution of the following example, with arrows to show the direction of procedure; with a dry-bulb temperature of 93.5°, a wet-bulb 

temperature of 84°, and a barometric pressure of 29.25 inches of 

mercury the weight of moist air is 0.0692 pound per cubic foot and 

its ratio to standard air 0.923. 

For some problems the weight of the air only in 1 cubic foot of the 

mixture of air and vapor, from (5), is desired. This may be obtained 

from figure 8, which is similar in many respects to figure 7 and in 

which the method of procedure is indicated by the same example, 

which shows that the weight of the dry air is 0.0676 pound per cubic 

foot and that 14.79 cubic feet of the mixture' contain 1 pound of 

dry air. 

PSYCHROMETRIC TABLES AND CHARTS 

Tables and charts for determining the physical properties of moist 

air from wet- and dry-bulb temperatures and barometric pressures 

are available. Vapor pressures at saturation (which are independent 

of atmospheric pressure) and relative humidities and dew-point 

temperatures for the range of 23 to 30 inches of mercury, atmospheric 

pressure, may be determined'from wet- and dry-bulb temperatures 

by means of the psychrometric tables 11 of the United States Weather 

Bureau. Data for higher pressures, which may be encountered in 

deep mines, have been published 12 abroad. Although these data, as 

well as absolute humidities (weight of water present in air as vapor) 

and total heat contents, may be determined approximately, according to the scale of presentation, from psychrometric charts, such as 

the Carrier, Hill, and Bulkeley charts, the charts available are limited 

to data for an atmospheric pressure of 29.92 inches of mercury or 

average sea-level pressure. 

10 Based on American Blower Co. Data Sheet 14.5, April 1927. u Marvin, C. F., Psychrometric Tables for Obtaining the Vapor Pressure, Relative Humidity, and 

Temperature of the Dew-Point: U. S. Department of Agriculture, Weather Bureau No. 235, 1915,87 pp_ 12 Jones, J. S., Hygrometry for Deep Mines: Trans. lnst. Min. and Met., vol. 34, pt. 1, 1924-25, pp. :Hi7_ 

Jeppe, C. W. B., Psychrometric Tables, with Special Reference to Deep Mining Ventilation: Jour. Chern. 

Met. and Min. Soc. of South Africa, vol. 31, pp. 88-118, and supple lentary 3D-page pamphlet, October 1930. 
28 ENGINEERING FACTORS IX VENTILATING METAL MINE~ 

Dew-point temperatures are required for air-density calculations 

as well as for the direct determination of absolute and relative humidities and are therefore involved in viTtually all psychrometric determinations. The relation of dew-point temperature to wet-bulb 

depression is quite involved and is a function of the bflrometric presWET-BULB DEPRESSION, I-tw, of. 

/", 10 15 20 25 so 35 40 

, ~ J€i'. IYliI ttJ]tij!/':tttttttj 

65 I I' I , .90 ~ 

1"1] ++ tf' "H-;'!W, Jt11 I:-+. 'Ide ,ii, I f-< 

IIF" t ; 1tf " }pi-l ~ 

fioPtIHY.:J;=;l:±ll~ ~I t~~~' ~l~! IFF! ~ 'ft : ,iT!:!=!' 'Ilr 1~f- m .85 ~ 

, 'C'I ,::p= -il-;!-' j~lJ:iC[jj~tii±±tlttjj 451+ f =t:fi:=", I ' 80 ~ ~.. .+tf'r=; e , • 2:l ~ -J it, .. ~ 

8~;;~~~'~fM*¥·ITif~fR$H~~n4~ k-t=:ft1J=j ~r 

4°1f...Jdl:~"I~,rI~ITIIIIII·7f> ~ 1,;- -Ii k i- ; 'I ,II'~ 

135, I .70 

,h I 

'-t; --rf 

.65 

0.090 0.085 0.080 0.075 0.070 0.065 0.060 0.055 0.050 

WEIGHT OF MOIST AIR, d, POUND PER CUBIC FOOT 

FIGURE 7.-Chart for determining weight of moist air and ratio to standard weight. 

SUI'e and temperatUI'e conditions. It must therefore be determined 

from tables or charts. In figure 9 the Weather Bureau data are 

plotted in the form of a chart, with the values extrapolated to a certain extent to cover a larger range of barometric pressures. It 

serves for the approximate determination of vapor pressures, dew-
VENTILATION SURVEYS 29 

point temperatures, and relative humidities from wet- and dry-bulb 

temperatures and barometric pressures. The method of using the 

chart is indicated by the arrows which trace the solution of the following example: With a wet-bulb temperature of 84°, a dry-bulb temperature of 93.5°, and a barometric pressure of 29.25 inches the 

pressure of the vapor is 1.058 inches, the dew-point temperature 

81.1°, arid the relative humidity 67.4 percent. 

Absolute humidities and total heats, when expressed on a poundsof-dry-air basis, vary with the absolute pressure, but at constant 

pressure depend only on the dew-point and wet-bulb temperatures, 
30 ENGINEERING FACTORS IN VENTILATING METAL MINES 

respectively, since the absolute humidity for any wet- and dry-bulb 

temperature condition is the same as for saturation at the corresponding dew-point temperature, and the total heat for any wet- and drybulb temperature condition is the same as for saturation at the wetbulb temperature. 

FIGURE g.-Chart for determining vapor pressnre. dew·point temperature, and relative 

humidity of moist air. 

With the wet-bulb and dew-point temperatures known, a simple 

chart, figure 10, suffices for determining total heat and vapor contents 

of unit quantities, since these involve direct relations. The method 

of procedure is shown by the continued solution of the same example 

traced by arrows, which shows that there are 47.5 B. t. u. (above 0 F.) 
VENTILATION SURVEYS 31 

a~d 0.0237 pound of water vapor per pound of dry air in the moist 

mr. 

For most mine air-flow conditions (almost saturated air) the pounds 

of the mixture of air and water vapor, found by multiplying the quantity in cubic feet by the weight per cubic foot, or air density, from (4) 

or figure 7, may be used for the pounds of air in the mixture with 

small error. ,The actual pounds of air in the 'mixture is the number 

TOTAL HEAT ABOVE "F. IN MOIST AIR, H., B. T. U. PER POUND OF DRY AIR 20 30 10, 50 60, 70 80 90 '0 

1/;: 1;1 

}I w~ 

.go 

~ -' 

~ ::> 

~" ' y/v ~ 

i fi'! 

:>! 

<>l 

.... :/1, . !//Ij/ ,1:1 

I·~ , 1,1. 

5(~ 

1= 

455:: 

'" 

.. 

o 0.005 0.0 0.015 0.020' 0.025 '0,030 0.0 (40 0.045 0.050 

WEIGHT OF WATER VAPOR IN MOIST AIR, dw, POUND PER POUND OF DRY AIR' 

FIGURE lO.-Chart for determining water vapor and total heat in moist air. 

of cubic feet multiplied by the weight of dry air per cubic foot of the 

mixture, from (5) or figure 9. 

COOLING-POWER OBSERVATIONS 

The main interest in the ventilation of metal mines has always 

been devoted to those mines where the ventilation requirements are 
32 ENGINEERING FACTORS IN VENTILATING METAL MINES 

determined by lack of enough cooling power in the air rather than its 

chemical composition. Until efficiency of working is perceptibly 

reduced through reduced cooling power little attention is ordinarily 

paid to ventilation, and few metal mines would be mechanically 

ventilated were it not for the effect of higher rock temperatures in 

reducing the cooling power of the air and the efficiency of labor. By 

improving the cooling power of the air comfort is improved, and efficiency follows suit. 

The cooling power of the air, or its capacity for cooling the body, 

depends on its dry-bulb temperature, wet-bulb temperature, and rate 

of motion and is the major factor in determining the rate at which 

the waste heat of the body may be transferred to its surroundings, 

although the exact condition of the body also is important in determining the rate. If this rate is too slow or too fast sensations of 

discomfort are experienced which directly affect efficiency of labor. 

KATA-THERMOMETER 

The kat a-thermometer is a large-bulb alcohol thermometer devised 

by Dr. Leonard Hill, of England, to measure its own rate of cooling 

and thus serve as an index of the relative rate of cooling of the body 

under various conditions. The instrument may be used with the 

alcohol bulb bare, in which case "dry-kata" cooling powers are 

obtained; or a wet cloth sack may be fitted over the bulb, and 

"wet-kata" cooling powers are then obtained. The stem of the 

thermometer is etched at the 95° and 100° points; and the time in 

seconds required for the instrument to cool through this temperature 

range, close to bedy temperature, is divided into a factor, etched on 

the stem of the instrument, to obtain its rate of cooling in millicalories per square centimeter per second. The use of these metric 

units has become standard for this instrument, but they can be converted to British thermal units per square foot per hour by multiplying by 13.27. Electrical indicating and recording kata-thermometers may also be obtained. 

The use of the kata-thermometer or similar instruments 13 that may 

be developed for actual determinations of relative cooling powers of 

the air and surroundings on a dry or a wet body is about the only 

way in which actual cooling powers can be determined under mine 

conditions because air motion is almost always an important factor in 

actual cooling rates and is often of such an eddying and turbulent 

nature that no approximation to the equivalent effect of a uniform 

velocity, such as might exist in a pipe or wind tunnel, can be made. 

The actual cooling of the body, in either a dry or a wet state, is 

entirely different in rate from that of the kata-thermometer--roughly 

about 5 to 1~but the instrument serves an excellent purpose in providing relative measurements for all except possibly the most unusual 

conditions. 

Where there is no cooling power, but rather a warming power, the 

relative warming power may also be determined by the kata-thermometer by measuring the time required for the instrument to heat 

through the same temperature range. The frail nature of the present 

instrument and the time consumed by observations have hindered its 

13 Weeks, W. S., A New Instrument for Measuring Cooling Power; the CooIometer: Jour. Ind. Hyg., vol. 13, September 1931, pp. 261-265. 
VENTILATION SURVEYS 33 

general adoption in mine work, but its use is gradually extending in 

the hot mines and, to a very much smaller extent, in other mines. 

EFFECTIVE-TEMPERATURE INDEX 

Studies on human comfort at the research laboratory of the American Society of Heating and Ventilating Engineers and the Bureau of 

Mines at Pittsburgh have determined equivalent conditions of dryand we·~-bulb air temperatures and air motion, with surrounding walls 

at the dry-bulb air temperature, for which persons feel equally comF,GURE ll.-Efl'ective-temperature chart for men at rest and stripped to the waist. 

fortable. The temperature of still, saturated air that produced the 

same sensation of comfort is used as an index for other combinations 

of air temperatures and air motion and termed the" effective temperature." The results of separate series have depended mainly 

upon the amount or lack of clothing worn; several charts of the 

possible combinations are available. For hot mines the results of 

tests made with the subjects stripped to the waist and at rest are 

most applicable to underground conditions and are reproduced, in the 

chart form devised by C. P. Yaglou, as figure 11. The method of 
34 ENGINEERING FACTORS IN VENTILATING METAL MINES 

using the chart is shown by an example, with arrows to indicate the 

direction of procedure. In the example solved the effective temperature (tE ), corresponding to a wet-bulb temperature (tw) of 65°, It drybulb temperature (t) of 80°, and a velocity (V) of 50 f. p. m., is 70°. 

The results of tests on men acclimated to high temperatures and 

working moderately hard while stripped to the waist and covered with 

sweat would be valuable for comparative mine purposes but are not 

available. 

The effective-temperature index offers a method of comparing 

variations in air conditions for average underground conditions 

simply and accurately when the three variables of dry-bulb temperature, wet-bulb temperature, and rate of air motion are known, since 

the walls may be assumed to be at the same temperatura as the drybulb air temperature. But the rate of air motion is often indeterminable, under mine conditions, just where it is desirable to institute 

comparisons, and other than the test conditions of radiation of heat 

and condition of the body may often be met. For these reasons the 

kat a-thermometer type of instrument cannot be equaled for direct 

determinations. The relations of effective-temperature scales and 

kata cooling powers are complex OIl account of the number of variables 

involved and have not been satisfactorily solved, although much work 

has been done on this subject. 

SURVEY RECORDS 

Records of mine-ventilation surveys should be systematic and complete and should be preserved for future reference, since much of the 

detail is such that it cannot be presented in sketch or map form. 

However, as much of the data as possible should be entered on prints 

of the mine maps or on special ventilation maps or diagrams in order 

that it may be used to best advantage. In many cases actual relations are derived only by a study of mapped data. Such a map or 

sketch is required primarily to show the distribution of air currents 

and the positions of devices for producing or controlling the distribution. Special sketch maps, on which the data are entered by means 

of a definite series of symbols, are generally desirable. Since distribution is the important factor in this form of record discussion of 

this phase of the subject is deferred to a later section on distribution 

and control of air currents. 

FLOW OF AIR IN MINE OPENINGS 

Air flowing through ducts, such as mine openings, encounters resistance due to interference of the individual air streams with each other 

caused by the wall surfaces. The result is that part of the kinetic 

(velocity) energy of the flowing stream is converted to heat; and constant renewal of the kinetic energy from the total energy of the flow is 

required to maintain it. The flow therefore acts to cause a continuous decrease in the total energy of the air in the direction of 

flow; and the flow automatically adjusts itself as to quantity so that 

the total energy used on any circuit of connected openings is equivalent to the total energy generated on that circuit. 

The sea of air that surrounds the world has weight, and the pressure 

exerted by this weight increases with depth at a gradually increasing 
FLOW OF AIR IN MINE OPENINGS 35 

rate. The total energy in the air in any air column therefore increases 

with vertical depth; but the increases in separate columns are not 

necessarily the same, and the energy representing the differences in 

these natural increases in energy content is used up by flow through 

openings connecting the columns so as to maintain a condition of 

equilibrium. This naturally generated energy available for producing flow is called" natural draft." Energy may also be added to a 

circuit at any point mechanically, as by a fan, and is then termed 

"mechanical draft." 

PRESSURE FORMS AND DESIGNATIONS 

These energy losses and energy gains involved in air flow are evaluated and measured in terms of pressure energy, even though the actual 

changes in pressure energy do not correctly evaluate the total changes 

in energy content. In a flowing stream part of the energy is in the 

form of velocity or kinetic energy, but this kinetic energy can be 

measured as a pressure exerted only in the direction of flow, as distinguished from a true pressure, which is exerted in all directions. 

The actual energy content of the air at any point in a flowing stream 

is therefore represented correctly only by the sum of the actual 

pressure and the pressure corresponding to the velocity of flow; this 

sum is termed the" absolute total pressure." The true absolute pressure is termed the" absolute static pressure" to identify it similarly. 

Air flow depends on differences of absolute total pressures rather than 

their actual values, and pressure measurements are measurements of 

differences in absolute pressures. In quiet air, such as the atmosphere on the surface, the only pressure exerted is the absolute static 

pressure. In a flowing stream the pressure exerted on the open end of 

a tube pointed against and parallel to the flow is the absolute total 

pressure, whereas the pressure exerted on a hole in a smooth surface 

parallel to the flow is the absolute static pressure, which in practice is 

somewhat difficult to obtain entirely free from velocity effects. The 

difference between the absolute total pressure and absolute static 

pressure at any point is termed its" velocity" pressure .. Although pressure differences must be measured between points 

the differences in average pressures across sections at right angles to 

the flowing stream are chiefly involved in this work. It so happens 

that the absolute static pressure is virtually constant across such a 

section where the flow is sensibly parallel to the walls; but the velocity 

across such a section is not uniform,' and therefore the velocity 

pressures and the absolute total pressures across the section are not 

uniform. Absolute static-pressure differences are therefore the basic 

pressure measurements because easily ascertained. Average velocity 

pressures and average absolute total-pressure differences can be 

obtained by traversing such sections, but in practice such traverses 

are made only as a method of determining average velocities from 

velocity pressures. When the quantity of flow is known the average 

velocity pressure for any section may be calculated accurately enough 

from the average velocity based on the area of the section; and in 

practice these calculated average velocity pressures are used to calculate average absolute total-pressure differences from absolute staticpressure differences. 
36 ENGINEERING FACTORS IN VENTILATING METAL MINES 

The pressure difference commonly measured is that between the 

absolute static pressure at a section in a flowing stream in a duct and 

the absolute static pressure of the atmosphere outside the duct or 

that between the absolute static pressures of two separated quiet 

atmospheres. The result is commonly referred to as the "watergage" pressure, but this is an indefinite expression because various 

pressure differences may be measured on a water gage. The fact 

that a difference of absolute pressures is involved is, however, ignored, 

and the pressure difference is designated as the "static" pressure of 

the air stream in the first case and as the" static-pressure difference" 

between the atmospheres in the second. Where the difference in 

absolute static pressure between different sections in flowing streams 

is desired the difference may be determined directly, or it may be 

determined indirectly from the separate differences from the pressure 

of the atmosphere where it is available; and the same designation of 

"static-pressure difference" is commonly used. The corresponding 

terms for absolute total-pressure differences, usually calculated from 

the foregoing and calculated velocity pressures, are" total pressure" 

and "total-pressure differences." Unless stated to the contrary, 

average total pressures over a section, rather than pressures at points, 

are implied. 

Actual absolute pressures in a flowing stream are not only changed 

by flow but also change with change in elevation due to the increase 

in absolute static pressure that accompanies increase in vertical depth; 

but these changes are automatically allowed for, in the methods of 

measuring pressure differences, by equivalent absolute static-pressure 

differences in the tubing which connects the point to the gage position 

and need not be considered as affecting the flow except through the 

agency of natural draft or unbalanced absolute-pressure changes due 

to differences in air density. 

PRESSURE CHANGES 

The weight flow through an air circuit, in pounds per minute, is 

constant; and if the air density remains constant the quantity of flow 

in cubic feet per minute also remains constant throughout the circuit. 

In a mine-ventilating circuit differences in elevations and temperatures produce small changes in density; and the quantity of flow is 

only approximately constant throughout the circuit, although the 

same number of pounds of air enter the mine as leave it, except for 

actual additions underground, such as compressed air and strata 

gases. With the quantity constant every change in ar.ea of crosssection requires a corresponding change in average velocity and in 

average velocity pressure. Changes in total pressure are affected only 

by energy losses and energy gains, so, assuming no change in total 

energy, changes in area must be compensated by the interchange of 

static aJ?-d velocity pressure; this is actually the oase, although some 

energy IS lost at each change of area. The velocity pressure, for 

constant quantity, changes only with the area of cross-section, but 

the static pressure is influenced by changes in both area and energy. 

In an airway of uniform area of cross-section the velocity distribution remains practically uniform and the average velocity pressure 

likewise, so that the static-pressure changes equal the total-pressure 

changes. Because static pressure is the basic pressure measurement 
FLOW OF AIR IN MINE OPENINGS 37 

and because airways of uniform area are the rule rather than the 

exception in commercial air-flow installations, the whole subject of 

energy losses and energy gains involved in air flow usually will be 

found to be presented on an artificially simple basis of static-pressure 

changes rather than of total-pressure changes, which are directly 

related to energy changes. Velocity pressures are not ignored. but 

are considered by means of various separate conventions determmed 

by the particular conditions, so that the final results are the same to 

those acquainted with the conventions; however, those not aC9.uainted 

with the particular conventions in use are much confused. Smce the 

energy changes can be considered correctly on the basis of tot!1l- pressure changes, this method will be followed throughout this bulletm. 

Wherever the word "pressure" is used without qualification total 

pressure is implied; and static-pressure changes are considered only 

in their true significance of pressure changes brought about by changes 

of both energy and area occupied by the flow. 

Although the change in static pressure in mine openings usually is 

in the same direction and approximately of the same magnitude. as 

the change in total pressure, this is not always true for high-velOCIty 

flow through variable areas of opening, as at mine fans. In fact, 

under certain conditions of enlargement of area the air may be found 

to be fiovving from a lower toa higher absolute static-pressure area. 

Pressure losses, on the basis designated above, are always decreases 

of absolute total pressure in the direction of flow which are caused by 

the flow; and pressure gains always represent increases of absolute 

total pressure generated in the direction of flow. In both cases the 

natural changes in absolute total pressures due to differences in elevation are considered to be balanced. The effect of lack of balance in 

these changes is considered as a generated pressure-natural draft. 

Positive and negative pressures.-It was necessary to qualify the 

foregoing statements with the word "absolute" because, pressure 

differences, which are always positive absolute-pressure differences, 

are virtually always considered with reference to a fixed absolute 

pressure, that of the atmosphere. In this case both total and static 

pressure may be positive or negative, that is, either greater or less 

than the pressure of the adjacent atmosphere; and although the 

amount of the change in absolute total pressure and the direction of 

the change are not affected by the position in the circuit at which the 

pressure-generating source operates, the actual values of the pressures 

relative to a fixed pressure, such as atmospheric pressure, are controlled 

by the position of the pressure-generating source in the circuit, which 

is open to the atmosphere at both ends. Pressure may be generated 

at the entrance, at an intermediate point, or at the exit. For a fan 

these would be designated as the "blower" "booster" and "exhaust" .. " posltlOns~ resl?ectlvely. 

A graph of the pressure changes in the same simple airway circuit 

with three positions of the one pressure-generating source is shown on 

both an absolute and a relative basis in figure 12. It will be noted 

that the pressure losses have been assumed to be uniform throughout, 

that absolute total pressure always decreases in the direction of flow, 

and that, although the pressure-generating source produces the same 

absolute total-pressure gain in the direction of flow at each position, 

the relative magnitude of the total pressure and of the static pressure 

at any section with reference to the atmospheric pressure is determined 
38 ENGINEERING FACTORS IN VENTILATING METAL MINES 

by the position of the pressure-generating source with reference to the 

system as a whole. The total pressure is always zero at the entrance 

to the system and positive and equal to the velocity pressure at discharge to the atmosphere; the static pressure is always zero at a free 

discharge to the atmosphere but negative and equal to the velocity 

pressure at the entrance to the system. Although pressures are considered and plotted ordinarily on this relative basis with respect to 

atmospheric pressure, as shown in the upper parts of the graphs, 

because they are so measured in actual practice for most conditions 

of air flow, it must be kept in mind that pressures so measured are 

concerned merely with pressure changes With respect to atmospheric 

pressure, whereas actual pressure losses and pressure gains are expressed by total-pressure differences that have an absolute-pressure 

origin. This conception of pressure relations is particularly necessary 

in considering the pressure changes at an underground fan, where 

there is no fLxed atmosphere available as a constant-pressure base. 

FIGURE 12.-Graphs of absolute and relative pressure changes in the same nniform-resistance system of 

airways for three positions of the same pressure-generating source. 

In considering the relation of pressure losses to the total-pressure 

differences from the atmosphere, it should be noted that this difference 

decreases in the direction of flow on the pressure side but increases in 

the direction of flow on the exhaust Side of a pressure-generating 

source but that both sides must be considered to evaluate the pressure 

generated. This necessity for considering pressure changes on the 

relative basis of their difference from atmospheric pressure results in 

some confusion but is sanctioned by universal use. 

PRESSURE LOSSES 

Precise evaluation of the pressure losses accompanying mine-air 

flow requires knowledge of the general theories of fluid flow, and for 

purposes of research and experiment it is desirable that all available 

data be correlated by means of a complete theory. But for practical 

application a more simple presentation suffices, restricted to average 

conditions of mine-air flow, since the pressure losses required to be 
FLOW OF AIR IN MINE OPENINGS 39 

calculated occur in comparatively rough airways for flows of moderate 

to high velocities where empirical factors cannot be applied rigidly. 

The main argument revolves about the question of whether the pressure losses can be considered to vary as the square of the average 

velocity of flow. Actually, this condition does not hold exactly for 

all pressure losses in mine airways, particularly for low rates of pressure loss; formulas based on this condition, however, are l'recise 

enough for practical application of empirical data to mine ventilation, 

and the normally small variations that are known to occur, some of 

which are discussed in a later section, can be handled mathematically 

where desired, more easily by small variations in estimated empirical 

factors than by introducing fractional exponents in basic formulas. 

The basic formulas for pressure losses, thus simplified, still appear 

to be the bugaboo in teaching mine ventilation, and many ventilation 

examinations degenerate into mathematical exercises rather than a 

test of ventilation knowledge. As a reaction to this condition continued attempts are made to displace basic formulas with still more 

simplified formulas which, although serving as mathematical aids in 

certain problems, cannot logically replace the basic formulas for 

solving pressure-loss problems. 

Pressure losses may be divided into two separate groups according 

to their mode of origin: "Friction" pressure losses caused by the drag 

of the walls on the air streams, wInch depend primarily on the condition of roughness of the individual wall surfaces, and" shock" 14 pressure losses caused by changes in the area occupied by the air streams, 

which depend primarily on the relative positions of the individual 

wall surfaces WIth respect to the lines of flow. 

All formulas for both friction and shock pressure losses include an 

empirical factor whose value varies with conditions and is determined 

only by actual experiments. These empirical factors are the basic 

data for air-flow calculations; unless approximately correct factors are 

used the calculations are worthless. Yet in spite of the fundamental 

importance of such data, one looks for them in vain in most textbooks 

on mine ventilation. Methods of using these data are given in great 

detail, but few are available. Since the results of Murgue's experiments were published 15 in 1893, a considerable mass of information 

has been made available both on factors for calculating friction pressure losses and on factors for calculating shock pressure losses. However, there has always been a practical preference for a constant 

factor, similar to Atkinson's, that could be used for a mine as a whole, 

regardless of the fact that no such single factor can be used with any 

pretense to accuracy even where the airways involved are roughly 

similar in general type. Attempts to make practical use of such single 

or average factors for a mine as a whole have led to such large discrepancies between theoretical calculations and actual results that the 

practical application of the data available has been very limited until 

recent years; as a matter of fact, rule-of-thumb methods still largely 

govern the field of mine ventilation. 

Friction and shock' pressure losses apparently take place concurrently in almost all types of mine airways and can be arbitrarily 

U The author has selected the term" shock" to designate this group of pressure losses, for which there is no commonly accepted term, after carefully considering such terms as "miscellaneous", "velocity". 

"turbulence", and" impact" .. Murgue, D., Experimental Investigations on the Lo'lS of Head of Air Currents in Underground Work· 

ings: Trans. Am. Inst. Min. Eng., vol. 23, August 1893, pp. 63-112. 
40 ENGINEERING FACTORS IN VENTILATING METAL MINES 

segregated only for the purpose of calculation. Segregation is required because the two types of pressure losses obey different laws. 

Present practice is to calculate the major shock losses separately but 

to include all the minor shock losses in the friction-loss calculation. 

In the discussion presented here shock losses are segregated from 

friction losses as far as seems feasible in practice, but even so some 

minor shock losses are involved in determining the range of pressure 

losses considered as friction losses. The pressure losses in straight 

airways, of as uniform cross-sectional area as is normal for this particular type, are considered as strictly friction pressure losses and 

are so calculated. Any departure from this condition of straight airway of approximately uniform area is considered to involve shock 

losses which may be calculated approximately as friction losses by 

means of certain allowances but which should preferably be calculated separately as shock losses. The relation of calculated pressure 

losses to actual pressure losses in mine airways is only a rough approximation for two reasons: In the case of friction losses there are wellestablished empirical factors, but it is difficult for the individual to 

visualize the exact conditions of application; and in the case of shock 

losses, although the conditions of application are easily visualized, 

there are no well-established empirical factors for many mine-airway 

conditions. :Moreover, for certain types of mine airways the average cross-sectional area of the airway, which has a preponderating influence on pressure losses, is not accurately determinable. 

FRICTION PRESSURE LOSSES 

The friction pressure losses in straight sections of approximately 

uniform area comprise the major part of the pressure losses in almost 

any type of air-ftow system. In mechanically ventilated metal 

mines the friction losses in the main airways often account for 70 to 

90 percent of the total pressure loss sustained in the system, although 

the figure may be much lower for a few mines. They are therefore 

of considerably greater practical importance than the shock losses as 

far as metal mines are concerned. 

:FRICTION-LOSS FORMULAS 

The basic law for the turbulent flow of a fluid through a straight 

duct of uniform cross-section, assuming that the pressure loss varies 

as the square of the velocity, is that first developed for the flow of 

water but later found applicable in general to fluids, either liquid or 

gas: 

flv2 h,=-, 2gr (7) 

where h, is the decrease of pressure, head in feet of fluid flowing; f is an empirical factor based on experiment and depending chiefly on the roughness of the wall surfaces and the system of units used in 

the formula; 

l is length of duct, feet; v is average velocity, feet per second; r is the hydraulic or mean radius, a term used in hydraulics for designating the result of dividing the area of cross-section by the perimeter 

in contact with the flowing fluid; and 

g is the acceleration due to gravity, a figure that varies slightly from place 

to place but is usually taken as constant at 32.2. 
FLOW OF AIR IN MINE OPENINGS 41 

For mine ventilation this formula usually is used in the form: 

p=kS;2, (8) 

where p is decrease in average total pressure, or total pressure loss, pounds per 

square foot; 

k is an empirical factor similar to f and termed "friction factor"; 

S is the rubbing surface, or square feet of wall surface, equal to the perimeter times the length; V. is velocity, feet per minute; and 

A is Cl'oss-sectional area, square feet. 

This formula contains no density term. The latter is included in 

friction factor k, which thus actually depends on the air density considered standard, which is close to 0.075 pound per cubic foot for 

most published values, a value that is assumed as standard throughout this bulletin. For horizontal ducts or airways at or near sea level 

differences from standard density usually are negligible compared 

with the uncertainty regarding the exact value of the friction factor, 

but in metal-mine airways they usually cannot be disregarded; it is 

therefore desirable to retain the density factor. It is also desirable to 

express pressures in the same unit in which they are measured, that 

is, in inches of water. However, the values of k as used in this basic 

formula should not be changed, since many engineers in Englishspeaking countries have these values firmly fixed in mind. A more 

convenient form of the basic formula for mine ventilation is: 

(9) 

where HF is total pressure loss due to friction, inches of water, and equals pX5.2; 

k is the friction factor for a density of 0.075 pound per cubic foot; 

P is perimeter, feet; 

L is length, feet; 

Vis velocity, feet per minute; and 

d is weight of air, pounds per cubic foot. 

Since mine-ventilation engineers usually are concerned with quantities of flow in cubic feet per minute rather than velocities of flow 

and since velocity is equal to quantity divided by area, the basic 

formula may also be written: 

kPLq2 d 

HF = 5.2A3 X 0.075' (10) 

where q=quantity of flow, cubic feet per minute. 

Other forms of friction formulas are in use in other divisions of 

the field of fluid flow, as well as other units for the various factors, as 

dictated by convenience or utility. A factor similar to k appears in 

all these formulas; however, the numerical values differ widely, and 

full knowledge of both formulas and units used is required to transform the friction factors derived for use in one equation to equivalent 

values for use in a different form of the equation, unless the transformations have been made, as in the present case, without disturbing 

the numerical values of the factor. Any constants developing during 

the transformation from the basic formula for fluid flow customarily 

are included in the friction factor, since the latter has no significance 

except as a factor; such a wide variety of values of friction factors for 

similar conditions has resulted that casual comparisons of experimental 

122422°-35--4 
42 ENGINEERING FACTORS IN YIiJNTILATING METAL MINES 

results are impossible. Considerable care is therefore necessary to 

insure that the values used correspond to the formula and exact units 

used in their determination. 

LAWS OF PROPORTION 

The last two forms of the basic formula presented contain in a 

convenient form most of the laws of proportion required for ordinary 

ventilation problems; with all remaining factors constant, the friction 

pressure loss varies directly as the friction factor, perimeter, length, 

density, square of the velocity, and square of the quantity and inversely as the area for constant-velocity conditions or as the cube of 

the area for constant-quantity conditions. 

However, it is seldom necessary in ventilation problems to consider 

a condition where either perimeter or area remains constant while 

the other changes, as this nnplies a concurrent change of shape. The 

normal condition is the consideration of similar shapes, in which case 

a change of area is accompanied by a change in the perimeter. Although the two basic forms so far given are suitable for calculations 

they do not include directly the laws of proportion as to dimensions 

and area for similar shapes of cross-section. 

For similar shapes the formulas show that t.he pressure loss varies 

directly as ~ on a constant.-velocity basis and directly as ~3 on a 

constant-quantity basis. By expressing both P and A in terms of a 

common dimension, 0, it will be found that 

P constant constant 

:4- 0 ..jA and that 

P constant constant 

A3= 0 5 A5P 

The foregoing constants are not the same in each case and differ for 

each different shape of section, but constants do not affect the proportions; therefore the pressure losses for similar shapes vary inversely as the side or square root of t.he area on a constant-velocity 

basis and inversely as the fifth power of the side or five-halves power 

of the area on a constant-quantity basis. 

Laws of proportion, often termed "ventilation laws", can be deduced for any term of the basic equation by transforming the equation 

so that the desired term stands alone on the left; but the result, 

except in particular instances, is more of a mathematical exercise 

than an aid to the understanding of ventilation principles and is 

likely to lead to confusion. It is better, therefore, to memorize the 

basic forms and to make the desired transformation only when 

required. 

FRICTION FACTORS FOR MINE AIRWAYS 

During 1923 and 1924 the author directed a large number of carefully conducted experimental determinations of friction factors for 

mine airways-a few in coal mines and a relatively large number in 

the metal mines of Butte in cooperation with the Anaconda Copper 

Mining Co. Concurrently with the Butte epxeriments, the coal-mme 
FLOW OF AIR IN MINE OPENINGS 43 

experiments were continued in the Bureau's experimental coal mine 

near Pittsburgh under the direction of H. P. Greenwald, and the 

results of these tests, although not published 16 until later, were 

available in reporting the results of the Butte work published 17 in 

1927. Both programs of test work were designed primarily to investigate friction pressure losses, but numerous shock pressure losses 

were determined and other ventilation data obtained incidental to 

the main program. 

Using the Butte results as a "framework" and amplifying by careful consideration of all the then available data as detailed in the report 

of that work, the author and A. S. Richardson developed a table of 

friction factors as applicable to both metal- and coal-mine practice. 

This table is repeated here (table 1), as the data that have since been 

reported 18 indicate no necessity for revising the values there given. 

TABLE I.-Bureau of Mines schedule of friction factors for mine airways 

[Values in italics are recommended as good average values for theoretical computations] 

Values of 1010Xkl 

Straight Sinuous or curved 

Irregularities gj' 'd .0 Slightly Moderately High degree Type of airway of surfaces, '" ~ 0 areas, and 03 alinement ~ il !: .0 » .0 » .0 » ! Xl ~'d ~'d ~'d '" 0 "'''' c !~ 0'd "'''' 0 ""''d 'd ....,...., »'i '" ....,...., 

"'''' »~ ~e "''' »+> "" E ~~ ~" ~t} :Dil ~~ ... '" <=I 1'1 ....,'" ~~ 1'1 1'1 "' ... .1'1 ... '§,2 'dUl .1'1'" 'dt; j ~ '" j .~~ 

~ it; 

.. ...., 0.0 ~ 0.0 0.0 ;:::1;fJ ::so ;::::~ 0 ::so @ ::so rn 0 U1 0 

----- --.------------ {MinimUm .... 10 15 25 20 25 35 25 30 40 35 40 50 Smooth·lined .......... Average ....... 15 , 20 30 25 30 40 30 35 45 40 45 55 

Maximum .... 20 25 35 30 35 45 35 40 50 45 50 60 

Sedimentary rock (or {Minimum .... 30 35 45 40 45 55 45 50 60 55 60 70 

coal) Average ....... 55 60 70 65 70 80 70 75 85 80 85 95 ................. Maximum .... 70 75 85 80 85 95 85 95 100 95 100 110 

Timbered (5·foot cen· {MinimUm .... 80 85 95 90 95 105 95 100 110 105 110 120 

ters) ............•.... Average ....... 95 100 110 105 110 120 110 115 125 120 125 135 

Maximum ..•. 105 110 120 115 120 130 120 125 135 130 135 145 

{MinimUm .•.. 90 95 105 100 105 115 105 110 120 115 120 130 

Igneous rock .•......... Average ....... 145 150 160 155 160 165 160 165 175 170 175 195 

Maximum .•.. 195 200 210 205 210 220 210 215 225 220 225 235 

I 10 in table is equivalent to 0.0000000010; 100, to 0.0000000100. All values of k are for air weighing 0.075 

pound per cubic foot. 

In the construction of this table mine airways were grouped into 

four classes by major differences in type of surfaces, sedimentary 

rock including coal surfaces as of comparable degrees of roughness. 

Each class was then divided into 3 groups as to minimum, ave-rage, 

and maximum degrees of irregularity of surfaces, areas, and alinement 

that occur in practice, and the 12 values assigned to straight, clean 

airways of these 12 general types are not only the basic values upon 

which the table is constructed but also define the range of values for 

16 Greenwald, H. P., and McElroy, G. E., Coal·Mine Ventilation Factors: Bull. 285, Bureau of Mines, 1929, 104 pp. 

11 McElroy, G. E., and Richardson, A. S., Resistance of Metal·Mine Airways: Bull. 261, Bureau of 

Mines, 1927, 145 pp. 

18 Callen, A. C., and Smith, C. M., Measurement of Air Quantities and Energy Losse. in Mine Entries, 

pts. I, II, and III: Univ. lllinois Exp. Sta. Bull. 158, 1926, 77 pp.; BUll. 170, 1927,74 pp.; Bull. 184, 1928, 

59 pp. Smith, C. M., Measurement of Air Quantities and Energy Losses in Mine Entries, pt. IV: Bull. 

199, 1929, 48 pp. 
44 ENGINEERING FACTORS IN VENTILATING METAL MINES 

calculating strictly friction pressure losses. They were determined 

upon mine airways of 20- to 60-square-foot cross-sectional area for 

the most part and at velocities of 500 to 2,000 f. p. m. Average 

spacing of timber sets was about 5 feet center to center,and areas 

were measured inside timbers. Effects of area, velocity, and other 

conditions on the factor, discussed in a later section, are slight and 

within the error of normal selection of factor, which should not be 

more than about 20 percent for general application and possibly 

within 10 percent for those personally acquainted with a variety of 

original test data-degrees of accuracy that are within the practical 

necessities of ordinary application. 

Factors for airways that have wall surfaces of more than one type 

can be derived from these pure-type factors by weighting the factors 

for the types involved according to the proportional parts of the 

perimeter involved. 

In extending the table from the 12 basic friction-factor values to 

values that include the effects of C'Jrvature and degree of obstruction, 

which cause shock pressure losses independent of the degree of roughness of the wall surfaces, strictly correct procedure has yielded to 

convenience, and the probable accuracy of selection is considerably 

less than that of the 12 basic factors. The irregularities of the airways involved, however, are so great compared to the error introduced and the increments used are so sketchily established that this 

procedure is justified for approximate calculations, even though a 

more accurate method is available through separate calculation 

of all shock pressure losses. 

The actual increments to be made to friction factor k to include 

shock losses, as distinct from friction losses, vary directly with the 

ratio of area to perimeter of the airway. The extensions in the table 

are based on a mean value of this ratio for metal-mine airways of 

1.5, for which shock pressure losses expressed in equivalent velocity 

pressure per 100 feet of airway, X, require an increment to k of about 

0.085X. The exact relations are developed in the following general 

section on shock pressure losses, which also includes a description of 

conditions corresponding to the degrees or curvature and obstruction given in the table. 

Effect of spacing of timber sets.-The factors for timber-lined airways are based on the assumption that, when the area used is that 

inside the timber sets, the effect of differences in spacing of sets can 

be considered as the effect of differences in the roughness of the 

surfaces responsible for the friction losses. No distinction is made 

between airways lined on 3 sides and those lined on 4 sides or between 

round and square timber, because the better alinement of the timbers 

in each of the latter cases balances the effect of the smoother floor or 

better edge condition in the former under average conditions, and the 

required friction factors are approximately the same. Although values 

for intermediate spacing are not available for average mine conditions we have the factor 0.0820 for timber sets placed "skin to skin", 

0.07100 for timbering on 5-foot centers, and 0.0880 for 10-foot centers. 

There is a certain spacing that will give a maximum pressure loss, 

because the effect of increasing the spacing is primarily an effect of 

increasing the intensity of the shock loss caused by each set while 

decreasing the number per unit of length. The effect of spacing on 

the friction factor under average conditions of metal-mine airways is 
FLOW OF AIR IN MINE OPENINGS 45 

estimated to be about as indicated in figure 13, although confirmatory 

experimental data are lacking. For spacings greater than 10 feet 

approximate accuracy is obtained only by calculating the shock 

losses due to the timber sets separately, as explained in the following 

section on shock losses. 

FRICTION FACTORS FOR TUBING 

Experimental determinations 19 of friction factors for the types of 

tubing in common use in mines for auxiliary ventilation as attached 

to small fan units give factors that vary slightly with the material, 

area, frequency of joints, and (except for heavy canvas or jute tubing) 

velocity. The variations are small, however, except those between 

the comparatively rough canvas or jute tubing and the relatively 

smooth metal or wooden-stave tubing. That the differences between 

the rigid and nonrigid types are primarily a question .of roughness 

rather than the requirement of mflation of the nonrigid types is 

apparently proved by the fact that the determined factors for tubing 

made of smooth, rubber-covered canvas were considerably lower than 

those obtained on rigid metal and wooden-stave tubing. 

80 

3 4 5 6 7 10 

DISTANCE BETWEEN TIMBER SETS. FEEl' 

FIGURE 13.-Relative effect of spacing of timber sets on friction factor, as estimated for average conditions 

of metal·mine airways. 

For a straight metal or wooden-stave line in good condition an 

average test factor of 0.0815 seems directly applicable, as there is 

very little probable difference between test conditions of installation 

and the best conditions of underground installation. The best of 

the straight canvas or jute installations underground, however, are 

not as straight as test installations, and the constrictions at the 

couplings in general use are more pronounced; the average factors of 

about 0.0820 to 0.0823 obtained in experiments should therefore be 

increased to at least 0.0825 for application to good, straight lines. 

The average conditions under which tubing lines are installed in 

mines do not ,Permit a good, straight line, so that shock losses of 

variable magmtude are imposed on the friction pressure losses, but 

the ratio of area to perimeter (~) is small-0.125 for a 6-inch-diameter 

tubing and 0 .. 50 for a 24-inch tubing-and the increments to k required to allow for them are small. Oonsidering the major difficulty 

of handling leakage losses in computing pressure losses, no great 

10 ·Weeks, W. s., Goss, W. P., and Warren, G. R., Friction Factors in Ventilating Pipe: Eng. and Min. 

Jour., vol. 113, 1922, pp. 1001-1002. McElroy, G. E., and Richardson, A. S., Friction Factors for Fan· 

Piping Used in Mine Ventilation: Rept. of Investigations 2540, Bureau of Mines, October 1923, 15 pp. 
46 ENGINEERING FACTORS IN VENTILATING METAL MINES 

degree of attempted accuracy is justified, and friction factors of 

0.0820 for metal or wooden-stave tubing and 0.0830 for canvas or jute 

tubing are recommended for application to average metal-mine conditions. Pressure losses can be calculated more accurately by computing the shock pressure losses separately and adding them to the 

computed friction pressure losses for straight lines. No allowance is 

necessary for the effect of moderate distortion of flexible tubing, since 

the perimeter remains constant and the change in area that accompanies distortion from a circular to an oval section is very !Small. 

SHOCK PRESSURE LOSSES 

All departures from the condition of a straight airway of uniform 

cross-sectional area result in pressure losses that are here termed 

"shock" pre'>sure losses, because analyses of experimental results 

indicate that they are always due to shock loss resulting from a faster 

stream expanding into a slower stream. Although all shock losses 

may be considered as caused by changes in the area actually occupied 

by the flow, for con venience they are divided in to two general classesthose caused by changes in direction of the airway and those caused 

by changes in cross-sectional area of the airway. 

SHOCK-LOSS FORMULA 

Shock pressure losses have been found to bear a constant ratio to 

the velocity pressure corresponding to tl).e mean velocity of flow, 

absolutely constant for most conditions of area changes and virtually 

constant for bends. They can be generally represented and computed, 

therefore, by means of the general formula, 

Hs=XHv, (11) 

where Hs is total shock pressure loss, inches of water; 

II v is the velocity pressure corresponding to the mean velocity of flow, 

inches of water (approximatelY(4,60oY at standard air density of 

0.075 pound per cubic foot, p. 15); and 

X is an empirical factor or the "shock factor" found by experiment . 

. Since Hs and Hv are always expressed in the same units and 

are equally affected by changes in air density, X is independent of 

both density and the units used, in which respect it differs from the 

friction factor used in formulas for computing friction losses. It is 

the number of velocity pressures equivalent to tlie shock pressure loss. 

When a summation of individual shock losses that occur regularly 

along an airway is desired X is the number per unit of length, for 

which 100 feet will be used throughout this bulletin. Whereas friction and shock factors each relate to but one condition of the airwayrelative roughness of the surfaces and changes in the area occupied by 

the flow, respectively-the conditions for shock loss present an 

almost limitless variety as compared with the conditions of friction 

loss. 

DATA AVAILABLE ON SHOCK FACTORS 

The results of the author's examination and analysis of existing 

data on shock pressure losses at bends and area changes, in terms 

of shock factors applicable to mine-airway conditions, have been 
FLOW OF AIR IN MINE OPENINGS 47 

reported 20 in Infonnation Circular 6663, which fonns the basis for the 

following brief presentation of the subject of shock pressure losses; 

for references, derivation of fonnulas, and more detailed discussion 

the reader is referred to this publication. 

The examination reveals that, although no rational method of 

computing shock factors for bends has been developed, a large number 

of dIrect determinations of factors for bends in ducts of uniform area 

have been made, for which the general relations are consistent enough 

to pennit their general application to bends in mine airways of unifonn 

area. The test data are so inconsistent, however, that their application to nonnal flow conditions in mine airways is subject to a large 

degree of approximation. No rational method of computing factors 

for bends in airways of nonuniform areas and but few directly determined factors are available. 

For symmetrical area changes in mine airways not only is there a 

rational method of computing shock factors for most of the conditions 

encountered, but also the agreement in the data required for tbe computations is good. However, area changes in mine airways are rarely 

symmetrical, and, as in the case of bends between different areas, the 

required shock factors cannot be computed until more data are available on unsymmetrical changes. 

SHOCK FACTORS IN TERMS OF FRICTION FACTORS AND EQUIVALENT 

LENGTHS 

Shock losses are independent of the roughness of the walls and 

therefore cannot be computed correctly as friction losses, although 

methods for obtaining approximate accuracy over a limited range of 

conditions are available. Many of the experimental results on shock 

losses are expressed in terms designed to facilitate their computation 

as friction losses, that is, as increments to the friction factor or as 

equivalent lengths of airway expressed in feet or diameters; formulas 

for expressing the relation between shock factors and these equivalent expressions of shock loss are therefore desirable. 

Shock losses that are regularly intermittent are computed along 

with the friction losses by increasing the friction factor enough to 

make the result of the computation cover the total of both types of 

pressure losses. If kg is the increment to the nonnal friction factor 

required for including a shock pressure loss of X velocity pressures 

per 100 feet, then at standard air density 

k.=O.08324~X. (12) 

The increments to the friction factor thus vary directly as the 

area. d' hi "11 fh 1 . te ratIO, an srnce t s ratIO 18 arge y a matter 0 t e actua 

penme r 

size of the airway the variation in increments for constant shock 

factors of X per 100 feet is large. For a 6-inch circle or square the AlP 

ratio is 0.125, for a 6-foot circle or square it is 1'.5, and for a 20-foot 

circle or square it is 5.0. However, the AlP ratios for mine airways 

"McElroy, G. E., Pressure Losses Due to Bends and Area Changes in Mine Airways: Inf. Circ. 6663, Bureau of Mines, November 1932, 34 pp. 
48 ENGINEERING FACTORS. IN VENTILATING METAL MINES 

of the sizes normally encountered range only from about 1.0 to 2.0, 

with an average of approximately 1.5, for which the increment to the 

friction factor is about 0.085X-the relation used in developing the 

table of friction factors (table 1) from the basic friction factors so as 

to include various degrees of intennittent shock losses. Compared 

to the normal friction factors the increments are relatively small for 

rough-walled mine airways, and the values of X available offer only 

an approximate degree of selection; the possible accuracy of results of 

calculations is therefore not affected seriously by this method of 

computing shock losses if such calculations are limited to the ordinary 

range of sizes of mine airways. 

In computing the total pressure loss from the friction formula 

strictly local shock losses are allowed for by arbitrarily increasing 

the length over the actual length enough to make the result agree 

with the total of both types of pressure loss. The increment in length 

required often is given in terms of the equivalent number of diameters 

rather than directly in feet. If the equivalent feet of airway is 

represented by L, then at standard air density 

L.=O.06324/~X. (13) 

Since the equivalent length in feet varies both directly as the 

area . d' 1 h f' . f h f' . t ratlO an mverse y as t e nctlOn actor t e range 0 mcre- perlIDe er 

ments for constant values of X is so large that approximate accuracy 

of direct application of test data is possible only for a very limited 

range of values of k and A/P. For the limited range of average mineairway conditions the equivalent length may range from about 300X 

to 15X. 

The expression of shock losses in terms of equivalent diameters of 

airway or duct merely means that increment of length Ls is divided by 

diameter D, and at standard air density 

L 8 =O,0781X 

D k ' (14) 

The equivalent number of diameters is thus independent of the 

A;P ratio and, for constant shock factors, varies inversely as the friction factor. The range of direct application of test data is therefore 

limited to corresponding degrees of roughness of airway or duct; but 

for average mine-airway conditions, expressed by friction factors 

ranging from 0.0820 to 0.0720, the equivalent diameters may range 

from about 40X to 4X. 

FLOW CONDITIONS AT BENDS 

Experimental determinations of shock losses at bends indicate that 

the air crowds to the far side of the bend and occupies less than the 

full area available at the departure end of the bend and that the 

excess of pressure loss over that due to friction, as detennined by the 

rubbing surface, is a shock loss due to abrupt one-sided expansion 

from this contracted area to the full area following the bend, as pictured in figure 14, A. The degree of contraction.in the area occu-
FLOW OF AIR IN MINE OPENINGS 49 

pied by the flow at the departure end of the bend apparently is 

determined not only by the position of the walls of the airway throughout the bend but also by the distribution of velocities over the crosssection at the entrance to the bend. The expansion takes place at 

a small angle and thus extends over a considerable length of airway 

downstream of the bend, depending on the degree of contraction preceding the expansion. Where expansion takes place from the contracted stream direct to the atmosphere, as from an elbow on the 

end of a duct, the shock loss is much larger than that for abrupt 

expansion to the airway area only, on account of the greater difference in the velocities involved in the shock loss. Where bends are 

close together the shock loss at the first bend may be affected through 

A 

I 

ABC = i = angle of de1lection 

R . eli t· 15=r= ra us ra 10 

B ~ = a "" aspect ratio 

FIGURE 14.-A, Sketch showing general conditions of flow at bends in airways; B, sketch defining bend 

characteristics. 

lack of complete expansion, in which case the normal velocity distribution at the entrance to the second bend and likewise its degree 

of contraction and shock loss are also affected to some extent. 

In mine airways the area following the bend is rarely the same as 

the area preceding the bend, a condition that affects the degree of 

contraction and the resulting shock loss. 

CHARACTERISTICS AND TYPES OF BENDS 

The inner and outer corners of bends in ducts usually are rounded 

on concentric curves, because it has been found that the shock loss 

decreases as the degree of curvature is increased. The degree of 
50 ENGINEERING FACTORS IN VENTILATING METAL MINES 

curvature is commonly designated by the ratio of the center-line 

radius to the width in the plane of turning, termed the "radius 

ratio", or RID in figure 14, B. Just as it is harder to bend a plank 

on the wide dimension than on the narrow dimension, so it is easier 

as respects shock loss to turn the air flow through the larger dimension of the airway than through the smaller dimension. In rectangular airways the ratio of the side on which turning takes place 

to the other dimension is termed the "aspect ratio", or WID in 

figure 14, B. The" angle" of a bend is the angle through which the 

air flow is deflected, or angle ABO in figure 14, B. 

Right-angle bends in ducts are commonly termed "elbows"; since 

most of the test data available on bend losses relate to elbows the 

L?lP.&[P. 

Normal Square Crowded Inner bevel 

@?~LP Segmental Venturi Blade 

A Double 

J 

1 

Reversed 

FIGURE 15.-Co=on types of right·angIe bends. 

type designations for bends in mine airways follow those used for 

elbows, as illustrated in figure 15. In a "normal" bend (A) the 

outer corner is a radial curve centered on the diagonal, and the inner 

corner is either square or commonly a similar concentric curve. In 

a "square" bend (B) the outer corner is square, and the inner corner may be square or a radial curve centered on the diagonal. 

These are the two common forms. Variants of these forms are: 

The "crowded" bend (0), where the radius of the outer curve is less 

than that of the normal bend; the "inner-bevel" bend (D), where 

the inner curve of the square bend is replaced by a bevel or chord 

sub tending the normal curve; and the "segmental" bend (E) com-
FLOW OF AIR IN MINE OPENINGS 51 

mon to pipe and tubing lines. Special forms used for reducing the 

pressure loss where existing square inner or outer corners cannot be 

displaced are: The "venturi" bend (F), where the area occupied by 

the flow at the entrance is gradually reduced to about two thirds 

at the departure and then gradually expanded to normal beyond the 

bend; and the "blade" bend (G), where short, curved blades are 

installed along the diagonal of either a normal or square bend to 

divide the flow so that the separate sections through which it 

passes have better radius and aspect ratios than the original. Concentric-radius vanes installed in bends also reduce pressure losses 

by dividing the flow so that the separate sections through which it 

passes have different radius and aspect ratios than the bend without 

the vanes. 

Two closely spaced bends in the same plane are termed a "double" 

bend (II) if they deflect the air flow in the same direction or a 

"reversed" bend (1) if they deflect it in opposite directions. Two 

closely spaced bends in different planes are termed a "compound" bend (J). 

SHOCK FACTORS FOR BENDS IN AIRWAYS OF UNIFORM AREA 

Virtually all data available on shock factors for bends in mine 

airways 'refer only to bends in airways of uniform area. Because of 

the variety of conditions that a,ifect the pressure losses the data are 

not complete or consistent, and approximations must serve for the 

present. 

NORMAL BENDS 

Approximate shock factors for normal bends (fig. 15, A) may be 

computed from the formula 

0.25( i)2 X=rVa 90 ' (15) 

where X is the shock factor or shock pressure loss in equivalent 

velocity pressures, r the radius ratio, a the aspect ratio, and i the 

angle of deflection. For right-angle bends (i=90) in rectangular 

airways this reduces to 

X _O.25 - r2..ja' (16) 

and for right-angle bends ill square or round aIrways (i=90 and 

a=l) it reduces to 

X=0.25. r2 (17). 

For the latter case the mmimum value of r is 0.5 for a square inner 

corner, and the maximum value of Xis 1.0. Good results are obtained 

with r= 1.5 (X = 0.11) ; there is little practical advantage in exceeding 

r=2.0 (X=0.06), that is, a center-line radius greater than twice the 

width or diameter. 
52 ENGINEERING FACTORS IN VENTILATING METAL MINES 

SQUARE BENDS 

Approximate shock factors for square 

computed from the formula 

bends (fig. 15. B) may be 

which reduces to 

X = 0.60 (.i.)2 r.Ja 90 ' 

X=0.6~ 

r.,ja 

for a right-angle bend in a rectangular airway and to 

X=0.60 r 

(18) 

(19) 

(20) 

for a right-angle bend in a square or circular airway. 

Bends in mine airways usually are square bends with a square inner 

corner, or r= 0.5, which gives X = 1.20 for a right-angle bend in a 

square airway. Fans are often so connected to multicompartment 

rectangular air shafts that the bend is made on the narrow dimension 

of the shaft where the aspect ratio may be about 0.5 and the shock 

factor 1.7, instead of on the wide dimension where the corresponding 

aspect ratio would be about 2.0 and the shock factor 0.85. 

OTHER BENDS 

Rough approximations of shock factors for other types of bends 

may be obtained from the two sets of formulas given. For the 

crowded bend the formulas for normal bends apply, but the factors 

must be increased arbitrarily; just how much is not definitely known. 

One set of tests indicates that a 40-percent increase is required when 

the outer radius is but three quarters the normal outer radius. The 

inner-bevel bend, often found at mine air shafts, may be treated as a 

square bend with a radius on the inner corner sub tended by the bevel; 

and the segmental bend may be considered a normal bend with inner 

and outer radii circumscribing the segments, but a small increase over 

the factorfor the equivalent normal bend is required to allow for a slight 

" crowding" of the ou ter corner. The common types of venturi bends 

may be considered normal bends having radius ratios of 0.8 to 0.9, 

depending on the details of construction. The factor for a blade bend 

may be taken as the factor for one of the equal sections considered as a 

single normal bend, but a small increase must be allowed for increased 

rubbing surface and edge effect. Factors for bends including radial 

vanes may be approximated by weighting the factors for the various 

sections according to the proportional parts of the area occupied and 

allowing a moderate increase for increased rubbing surface and edge 

effect. The best position for a single radial vane is about one third 

the width from the inner corner. Extensions to radial vanes increase 

the pressure loss slightly under most conditions and should not be 

used. 

CHART FOR DETERMINING SHOCK FACTORS FOR BENDS IN AIRWAYS OF UNIFORM AREA 

Figure 16 is a chart for determining shock factors for shock losses 

due to normal and square bends in mine airways of uniform area; 
FLOW OF AIR IN MINE OPENINGS 53 

this is based on formulas (15) and (18). The method of using the 

chart is shown by the dotted-line solution of an example; the arrows 

indicate the direction of procedure. In the example solved in the 

chart it is shown that when the radius ratio of a normal bend is 0.6, 

the angle of deflection 45°, and the aspect ratio 0.5 the shock factor 

is 0.25. In using the chart it is important to note that the full lines 

for angle of deflection should be used for normal bends and the dash 

lines for square bends. 

... ...; 0> CCl to: ~ .q .. o ~ "'1 -!: , "'! 

. I 

E'.E 

l\: ~ 

I! " 0: 

i!!o '\ ,'\."\ ,1'\ 1,\ ,,",, 

['\ 1,\ IJ ,:-,; 

l!; '" l'\t'\: ~ .1', \.J\. II , 

t":~ l':I ~. 

~ i'--~ ~ ~ J! lfX 

i9,~ ~I"\ R ~ ~ [) '\:'\~ x~~ II 1/ 

1'1. 

\.:. II'\, 

1,\ 

"" £I 

L, 

.1>. I'\. !::'> N 

~ j1!/ 

II ~, II' II 

ffl ~ 

1/ ~~ 'c!§l ~ ~ "'- ~~i'--

v , 1/ 7 / Vv ",. ~~~ aD '. It) ... It) 

SHOCK FACTORS FOR BENDS IN AIRWAYS OF NONUNIFORM AREA 

Except for a few special conditions, no factors have been directly 

determined for average conditions of area change at bends in mine 

airways, and no rational method of computing them has been developed. However, there are data on two conditions that indicate the 

effect of nonuniform areas-bends discharging directly to the atmosphere and closely ,spaced bends-and on intermittently occurring 

bends of sman deflection in irregular airways. 
54 ENGINEERING FACTORS IN VENTILATING METAL MINES 

BENDS DISCHARGING DIRECTLY TO THE ATMOSPHERE 

If a bend discharges directly to the atmosphere, as at an elbow on 

the end of a pressure pipe line, the shock loss is not only much larger 

than the normal loss In an airway but also a constant percent larger, 

variously reported as 50 to 80 percent. With more consistent and. 

more complete data possibly both the degree of contraction and the 

law for shock loss due to one-sided expansion can be developed from 

comparison of similar sets of data and applied for computing shock 

factors for average conditions of nonuniform areas. 

CLOSELY SPACED BENDS 

A double bend (fig. 15, H) has a lower pressure loss than two similar 

single bends, the amount depending on length of offset and type of 

bend involved. This low loss probably is due both to incomplete 

expansion following the first bend and higher velocities than normal 

at the outside of the second bend. 

A reversed bend is reported to have a higher pressure loss than two 

similar single bends under the conditions tested, but the effect probably depends on the relative length of the offset. Incomplete expansion would reduce the loss at the first bend, but the resulting higher 

velocities at the inside corner would increase the loss for the second 

bend. 

A compound bend probably would result in about the same pressure 

loss as two similar single bends, since the decrease in loss at the first 

bend would be largely offset by somewhat higher losses at the second 

bend due to higher velocities along one side; these, however, would 

affect both the inner and outer corners. 

SINUOUS, CROOKED, AND CURVED AIRWAYS 

Mine airways often include numerous small bends and curves involving deflections of 30° or less and are termed "sinuous ", 

"crooked ", or " curved ", as the case may be. Drifts following a vein, 

alternately turning one way and then the other, may be sinuous if 

largely comprising short, curved sections or crooked if comprising 

short, straight sections. "\-Vhere the deflections are approximately 

constant and in the same direction they are" continuous curves" or 

"turns" and are usually very irregular; the individual shock pressure 

losses are so small as to defy calculation, but a very rough approximation may be made of the combined shock losses as follows: 

Slight degree: X-=0.2 per 100 feet; a large-radius curve, a sharp bend of about 

15° deflection, or a wall line close to the center line not oftener than once every 

100 feet; or a small-radius curve, a sharp bend of about 30° deflection, or a wall 

line crossing the center line not oftener than once every 200 feet. 

Moderate degree: X=0.3 per 100 feet; conditions intermediate between slight 

and high degree. 

High degree: X-=0.5 per 100 feet; a continuous large-radius curve, a contin- uous curve of repeated small deflections of 10° to 15° every 20 to 30 feet, bends of 

20° to 30° deflection every 50 to 100 feet, or a wall line crossing the center line 

about every 50 feet. 

The foregoing descriptions are rather meager for lack of comprehensive test data, but they are sufficient to permit of approximate 

c?mputations that will satisfy the variable area conditions of mine 

aIrways. 
FLOW OF AIR IN MINE OPENINGS 55 

FLOW CONDITIONS AT AREA CHANGES 

Experimental determinations of pressure changes and pressure losses 

at changes of area of cross-section in mine airways, whether contraction or expansion, indicate that the excess pressure loss over the normal 

friction loss is a shock loss due to a faster stream expanding into a 

slower stream as determined by the actual areas occupied by the flow 

rather than the areas of the airway. No perceptible shock loss is due 

to the converging of the air stream itself where the flow is contracted, 

but the converging of the air stream causes the flow to contract beyond the edge of the constriction, giving rise to a condition known as 

the" vena contracta" (fig. 17) in which the area of cross-section of the 

air stream is the minimum immediately following the edge of the constriction. For contraction, therefore, the shock loss is that caused by 

expansion from this vena contracta to the full area following the contraction. Where expansion in area only is involved it may be considered as a special condition of general expansion following contracAc 

Ao =c= coefficient of contraction 

1~ = Nc= ratio of contraction (areas) 

AAo = Ne = 1:atio of expansion (areas) e . 

FIGURE 17.-Sketch showing general conditions of flow and defining characteristics of changes in area of 

airways. 

tion, and all area changes may be considered on a single basis as variants of the general conditions illustrated by figure 17. 

CHARACTERISTICS AND TYPES OF AREA CHANGES 

Expansion after contraction involves four areas: Aa, the area 

preceding contraction; Ao, the area at the contraction; Ae, the area 

occupied by the air flow at the vena contracta; and A., the area after 

contraction, to which expansion occurs. Although the shock loss is determined by the relation of area Ae to A. only, area Ae is largely determined by the relation of area Ao to Aa. Shock losses may be expressed 

in terms of the velocity pressure at anyone of these four areas, the 

particular one selected being a matter of convenience in computations; 

both velocity pressures and corresponding shock factors are designated. 

by the same subscripts as those used for the respective areas. The 

magnitude of shock losses expressed in velocity pressures is primarily 

a matter of three area ratios: (1) The ratio of the contracted area to 

the area preceding contraction, or AofAa, which will be designated by 

Ne and termed the" ratio of contraction"; (2) the ratio of the area at 
56 ENGINEERING FACTORS IN VENTILATING METAL MINES 

the vena contracta to the area at the contraction, or A.IAo, which will 

be designated by c and termed the "coefficient of contraction"; (3) 

and the ratio of the contracted area to the area following contraction, 

or AoIAc, which will be designated by Nc and termed the "ratio of 

expansion." . Since area Ac is not directly measurable the value of the coefficient 

of contraction is determined by calculation from experimental determinations of shock losses due to area changes. The fact that it 

has been found to be independent of the ratio of expansion greatly 

broadens the field of application of experimental data. 

In determining shock losses due to area changes the degree of 

symmetry involved in the area change is the main consideration. 

Virtually all of the empirical data available apply only to symmetrical 

changes of area, that is, areas of the same shape centered as respects 

each other; for this condition both contraction and eX.:Qansion of the 

flow affect the whole perimeter of the air stream umformly. It is 

possible to develop formulas for both the coefficient of contraction 

and for shock factors, in terms of the ratios of contraction and expansion, for symmetrical area changes; but their application to unsymmetrical area changes is very uncertain and approximate, for 

although the effect of lack of symmetry on the coefficient of contraction may be estimated there IS no way of estimating the effect of 

unsymmetrical expansion on the shock loss. 

Conditions of expansion and contraction include abrupt expansion 

only, gradual expansion only, abrupt contraction followed by either 

abrupt or gradual.expansion, and gradual contraction followed by 

either abrupt or gradual 'expansion. 

SHOCK FACTORS FOR SYMMETRICAL AREA CHANGES 

FORMULAS 

The shock loss occasionlJd by a faster moving stream expanding 

symmetrically and abruptly to a slower moving stream has been 

found to be equivalent to the velocitYlressure corresponding to the 

difference of the two velocities involve. This law has but a limited 

application in this form, but it can be developed to the following 

general forms for shock factors for abrupt symmetrical area changes 

ill terms of area ratios: 

Xo=(!_N.)2; X.= G-N.y ; and Xa= G-N.y. (21) 

c N.2 N.2 

Where the expansion of the area occupied by the air stream is 

gradual and symmetrical it has been found that the shock loss is a 

constant ratio of that for abrupt symmetrical expansion; the required 

shock factors are X'=yX, (22) 

where X' represents the shock factor for gradual expansion corresponding to the value of X (Xo, Xc, or Xa) for abrupt expansion and 

'Y is an empirical factor based on experiments. Values of'Y vary with 

the included angle or slope of the walls; values of 'Y found by experiment are plotted in figure 18 against the included angle. "Fan 

Engineering" data indicate that angles greater than 60° are rio better 
FLOW OF AIR IN MINE OPENINGS 57 

than abrupt expansion but do not show that the normal angle of symmetrical expansion of air flow is about 7°, a fact indicated by Brigg's 

data and proved by a wealth of corroborating experimental data on 

air flow. Shock losses for gradual expansion depend on N. as well 

as the experimentally determined values of y, and N. is directly related to the slope of the sides and the length in diameters of the 

diverging section. The relations of slope to included angle and of N. to 

length and included angle are also plotted for convenience in figure 18. 

The author has found that a definite relation exists between the 

ratio of contraction and the coefficient of contraction, at least for 

symmetrical conditions of contraction, which may be expressed thus: 

c=-J 1 , Z-ZNc2+N ; 

(23) 

SLOPE 

0 0.1 .2 .3 .4 .5 .6 

LENGTH IN EQUIVALENT DIAMETERS AT INLET 

elJO 2 3 4 5 6 7 8 9 10 

" r I 0 

°0 0.1 .2 .3 .4 .5 .6 .~ I~ .9 LO 

BATIO OF SHOCK LOSS TO SHOCK LOSS FOR ABRUPT EXPANSION 11 

FIGURE l8.-Chart of data for gradual symmetrical expansion at diverging sections of airways for included 

angles of 0° to 60°. 

where Z is an empirical factor, which will be designated as the "contraction factor." Values of Z are particularly affected by the edge 

condition at the constricted area, by the degree of abruptness of contraction, and by the form of the contraction. For free contraction 

to a sharp edge, as at the entrance to a plain open pipe, Z averages 

3.80. For abrupt symmetrical contraction, that is, where a flat 

surface extends from a sharp edge at right angles to the direction of 

flow, Z is about 2.5 for ordinary degrees of sharpness as obtained with 

square-edge plates and square-edge timbers, although the very sharp 

edges of thin orifice j>lates used for air measurement give values of 

2.7 to 2.8. If the edge is rounded the shock loss is greatly reduced, 

but since the edge condition is not susceptible to rigid specification 

this form is not applicable for air measurement; few data are therefore 

available for determining average Z values. For a well-rounded edge 

comparable to that presented by a round timber in a mine airway the 

122422°-31>---5 . 
58 ENGINEERING FACTORS IN VENTILATING METAL MINES 

value of Z is estimated as about 1.5; although definite data are lacking 

it seems probable that the same approximate Z factor of 1.5 can be 

applied where the edge is beveled to about the same degree, that is, 

to the extent that the bevel would be circumscribed by a quadrant of 

a similar circular section. If the contraction in area is gradual instead 

of abrupt the degree of contraction of the flow at the vena contracta 

and likewise the shock loss are reduced. With a bell-mouth entrance 

or a "standard" orifice there is very little contraction of the flow, and 

an average value of Z for the best designs is about 1.05. This form 

can be considered as representing abrupt contraction to a definite 

edge condition and will be referred to hereafter as a "formed" edge. 

For walls converging symmetrically on a uniform slope the only 

data available are for included angles ranging from 10° to 20°. Values 

of Z, plotted against included angles in figure 19, range from 1.57 for 

o 

... 

I 

50 , 

40 

30 

20 

10 

1.00 

SLOPE 

0.1 .2.3 .4 .1> 

LENGTH IN EQUIVALENT DIAMETERS AT INLET 

1 

0 

~ 

~ 

~ 

1.25 

2 3 4 

I 

,. 

1.50 1.75 2.00 

CONTRACTION FACTOR Z 

5 

"/ 

I 

6 

2.50 

FIGURE 19.-Chart of data for gradual symmetrical contraction at convergjnO' oections of airways for 

included angles of 0° to 60°. 

10° to 1.93 for 20°. At 0°, Z would be 1.0, and for abrupt contraction 

of 180° it would be about 2.5. The slope of the curve rndicates that 

included angles of convergence greater than 60° are no better as regards shock loss than abrupt contraction. 

The coefficient of contraction for gradual contraction depends not 

only on the experimentally determined value of Z but on the value of 

No also, and No is directly related to the slope of the sides and the 

length in diameters of the converging section. The relations of slope 

to included angle and of No to length and included angle are also 

plotted, for convenience, in figure 19. 
FLOW OF AIR IN MINE OPENINGS 59 

INSTALLATION CONDITIONS 

Installation conditions may be considered in three groups (fig. 20) 

for the purpose of presenting data on shock factors: (1) Entrance to 

an airway, (2) discharge from an airway to the atmosphere, and (3) 

within an airway. 

However, a discussion based on the conditions of expansion and 

contraction is a more convenient method of presenting the various 

shock factors than this grouping. . Table 2 shows in condensed form the shock factors corresponding 

to the generally applicable contraction factors of 2.5 for a square edge, 

1.5 for a rounded or beveled edge, and 1.05 for a formed edge, where 

~-=---= 

-I--

Normal 

E 

~-===--=;:==------:: ~--=-=-=== 

Abrupt 

expansion 

K 

·'~I .::-c=--"=-_=-"""_~_-

/,-C:=::- .:::c. 

/ Iii Flush 

I 

~~~~ I Constricted 

G 

~---=:==~-=-:-:: 

=~~-:=.:::== 

Gradual 

expansion 

L 

Entrance to airways 

Discharge from airways 

~---:-,I-

~~.·t~"~~ 

Abrupt 

contraction 

M 

~~-E:::-== Orifice constriction 

N 

§~~~- ~~~E--'S ~~~~~ ::=-~..:: :<f!7iZ72« 2-

Expansion 

following 

contraction 

o 

Gradual 

contraction p 

Expansion 

following 

contraction Q 

Standard venturi 

R 

Within airways 

FIGURE 20.-Flow conditions at changes in area of airways. 

Formed venturi 

S 

No and N. have fixed values of 1 or ° or are equal to each other. The 

table also gives simplified forms of the general formulas applicable to 

these special conditions. ' ABRUPT EXPANSION 

Abrupt expansion that is not preceded by contraction of How 

presents a special case of application of the general formulas which 

provide for such contraction, in which the value of No, Z, and 0 is 1.0. 

There are only two common conditions of installation: In the airway 

-(fig. 20, K), where N. may have any value between 0 and 1; and at 

discharge into the atmosphere (fig. 20, E), where N. is 0 and Xa is 1. 
TABLE 2.-Shock factors or shock pressure losses, in terms of equivalent velocity pressures, for symmetrical area changes of airways for common 0':1 

conditions of abrupt 1 contraction and expansion in which the ratios of contraction (Nc) or expansion (N,) are 1,0, or equal to each other 0 

Constant ratio and special formulas 

N,=1 

X.= X,= (1-N.), 

X.= (~.-1)' 

N,=O 

x'=G-N'~' C ' "i-Nt 

X.=~ 

XIl=co 

c=.J£ Z 

N.=1 

X.=X,= 0-1)' 

0-1)' X.=---w;rN.-O 

I Xo~ 

1 

X·=CJNoi 

X.=CD 

Variable ratio of contraction (N,) or expansion (N.) , 1 I 0.9 I 0.8 I 0.7 , 0.6 I 0.5 I 0.4 I 0.3 , 0.2 , 0.1 , 0 

Contrac- Shock factor based on area before contraction (or expansion), Xa; on area at contraction, Xo; and on area Condition of area change tion 

factor, Z following expansIon (or contraction), X. 

---

Abrupt expansion without (1.0) Xa=XG 0 0.010 0.040 0.090 0.160 0.250 0.360 0.490 0.640 0.810 1.0 preceding contraction. X. 0 .012 .063 .184 .445 1.00 2.25 4.99 16.0 81. 0 co 

Wall contraction to: Square edge _______________ 2.50 X, .340 .462 .608 .774 .960 1.17 1. 39 1. 64 1. 90 2.19 2.50 X. .340 .569 .953 1. 58 2.67 4.66 8.70 IS. 2 47.6 219 co Round edge _______________ '1. 50 X, .050 .105 .180 .275 .389 .524 .679 . S54 1. 05 1.26 1.50 X. .050 .130 .280 .561 LOS 2.10 4.24 9.49 26.2 126 co 

Formed edge ______________ 1. 05 X, .001 .016 .051 .106 .181 2.76 .391 .526 .681 .856 1. 05 

Wall contraction to: Square edge _______ ., _______ 2.50 X. 0 ,022 .091 .220 .446 .839 1. 59 3.21 7.92 33,2 co 

X,=X. 0 ,018 ,058 . lOS .161 .210 ,254 .289 .317 ,332 ,338 Round edge _______________ '1. 50 X. 0 ,003 .012 .030 .061 .120 ,230 .471 1.18 4.96 co 

Xo=X, 0 .002 .007 .015 .022 .030 ,037 .042 .047 .050 ,050 Formed edge ______________ 1. 05 X. 0 -----._- ----.--- -------- .001 ,001 ,003 .005 ,015 .062 co 

Xo=XfJ 0 .-.----. -------- ----.--- --.---.- ------.- -------- ---------- .001 .001 .001 Obstruction with: Square edge _______________ ' 5. 00 X, 0 .132 .495 1.13 2.19 4,00 7,41 H,8 36.0 151 co 

XO=X6 0 .107 .317 .554 .787 -1. 00 1.19 1.33 1.44 • 1. 51 1. 53 Round edge _______________ ' 3.00 X. 0 .038 .152 .361 .723 1. 35 2,54 5.14 12.5 52,7 co 

X,=X. 0 .031 ,097 .177 .260 ,338 .406 .462 .501 .527 ,536 Formed edge ______________ ' 2.10 X. 0 ,012 .051 .127 .258 .503 .936 1. 91 4.71 19,8 co 

Xo=X. 0 .010 ,033 .062 .093 ,126 .150 .171 .188 .198 .202 

Wall contraction to: Square edge _______________ 2.50 X. 1 1. 59 2,41 3,60 5,45 8.50 14.1 26.3 61.1 249 co 

X, 1 1.29 1. 54 1.77 1. 96 2,13 2,26 2.36 2.44 2,49 2,50 Round edge _______________ ' 1. 50 X. 1 1. 35 1.84 2,56 3.67 5,50 8,88 16.2 37.0 150 co 

X, 1 1.10 1.18 1.26 1.32 1. 38 1.42 1. 45 1. 48 1. 50 1.50 Formed edge ______________ 1.05 X. I 1. 25 1. 59 2,09 2,86 4.15 6.51 11.6 26,3 105 co 

X, I 1.01 1.02 1.03 1.03 1.04 1.04 1.04 1.05 1.05 1. 06 

t:;j 

S 

z 

t:;j 

t:;j 

~ Z 

1;:1 

~ Ll 

~ 

55 

1-1 

Z 

~ 

§ 

1;:1 

~ 

~ 

~ t" 

rJ), 

~ 
N.=N,(=N) r Wall contraction to: 

X.= G-N) , Square edge _______________ 2.50 Xa=X, 0 .068 .304 .80

X. 0 .055 .195 .39

Xa=X·-G-N)' 

Round edge _______________ ' 1.50 Xa=X8 0 .027 .128 .36

X. 0 .021 .082 .17

~ 

Formed edge ______________ 1.05 Xa=X, 0 .014 .068 .19

X. 0 .011 .044 .09

Obstruction with: Square edge _______________ ' 5.00 Xa=X8 0 .224 .909 2.23 

X. 0 .182 .582 1.0

Round edge _______________ 23.00 Xa=Xe 0 .093 .409 1. 0

X. 0 .075 .262 .52

Formed edge ______________ ' 2.10 Xa=Xe 0 .049 .227 .6

X. 0 .040 .146 .30

1 For gradual contraction interpolate in table for value of Z from fig. 19. For gradual expansion, with or withou

• Tentative approximations ouly 
62 ENGINEERING FACTORS IN VENTILATING METAL MINES 

GRADUAL EXPANSION 

Gradual expansion that is not preceded by contraction of flow also 

presents a special case of application of the general formulas which 

provide for such contraction, since the value of Ne, Z, and c is 1.0; 

but the shock factor, equation (22), is y times the factor for abrupt 

expansion, and y is derived from figure 18 or a similar source. The 

most common condition of installation is that within the airway 

(fig. 20, L). This form, the diverging connection or transformation, 

finds an important application in reducing pressure losses in mine 

airways by substituting gradual expansion for abrupt expansion. The 

same type of construction may be used at the discharge end of the 

airway (fig. 20, F); but gradual expansion is then followed by abrupt 

expansion, each with a separately determined factor, although the 

two factors can be combined into a single factor in terms of the normal 

area of the airway or of the area at discharge. This form is also of 

great practical importance in reducing the pressure losses at the discharge from mine-airway systems. As applied to a fan discharging to 

the atmosphere it is termed an "evase" discharge, the main purpose 

of which is to reduce the velocity at the place where abrupt expansion 

(to zero velocity) occurs. For practical purposes it is rarely economical to use a length that will make the discharge area more than 4 

times the area at the entrance to the diverging section, that is, to 

make Ne less than 0.25. 

ABRUPT CONTRACTION 

Abrupt contraction followed by abrupt expansion is a common 

cause of pressure loss in mine airways and embraces all three common 

conditions of installation. This fact is not readily apparent unless one 

realizes that the areas actually occupied by the flow rather than airway 

areas are the determining conditions. The common picture of abrupt 

contraction is that where a smaller section follows a larger section 

with abrupt transition in areas, as in figure 20, l.1, where N e= 1. 

An unrestricted entrance presents a quite similar form (fig. 20, A 

and B) if the area preceding contraction is considered as expanded 

indefinitely. In addition to the special condition of N.= 1 is the 

special condition of Ne=O. Figure 20, A, illustrates the special condition of free contraction to an edge, as at the entrance to an exhaust 

pipe line, for which Z=3.8, rather than the common condition of 

contraction against or guided by a surface. The flush surface entrance 

(fig. 20, B) represents the common entrance form for mine airways, 

where X.=0.34 for the ordinary square edge, Xe=0.05 for a rounded 

or beveled edge having a radius or bevel approximately equal to the 

wall thickness of a lined mine airway, and X.=0.0006 for a formed 

edge. 

A restricted entrance (fig. 20, 0) has only the one special condition 

Ne=O; it follows that the coefficient of contraction c, equation (23), 

is 0.633 for a square edge (Z=2.5), 0.817 for a round edge (Z=1.5), 

and 0.976 for a formed edge (Z= 1.05). Abrupt constrictions at discharge from a duct to the atmosphere (fig. 20, G) have only the special 

condition N.=O. Both of these forms are frequently used in testing 

fans, usually with orifice plates whose edge condition can be closely 

approximated by Z=2.5 (square edge). 

Flow through a small hole in the thin wall of a large chamber 

(fig. 20, I) may be considered a special condition of constricted dis-
FLOW OF AIR IN MINE OPENINGS 63 

charge. Both N c and N. are so small that they can be considered to 

be 0, and Xo=Z. The values of Xo for a square edge, round edge, and 

formed edge are, therefore, 2.5, 1.5, and 1.05, respectively, for mineairway applications. The values usually given range from about 2.7 

to 2.8 and are for orifice plates with very sharp edge conditions. 

A constriction in an airway of uniform area (fig. 20, N) has only the 

special condition Nc=N. and represents the set-up for air measurement by orifice plates in ducts and constrictions in mine airways. 

However, the more usual condition of actual application to a constriction in mine airways, such as a regulator, involves unequal areas 

and presents the general condition of abrupt contraction followed by 

abrupt expansion (fig. 20, 0). General formulas (21) and 23) are 

required for both, and tables of values or graphic charts are desirable 

to obviate computations. 

GRADUAL CONTRACTION 

Where gradual contraction is followed by abrupt expansion from 

the vena contracta to the area following contraction of the flow the 

shock factors are less than those for- corresponding conditions of abrupt 

contraction because of the effect of the angle of convergence on the 

value of Z, which may be determined roughly for the common edge 

condition at the junction of surfaces from figure 19. The special condition of contraction at a uniform rate against a formed surface, 

pictured for particular conditions of installation in figure 20, B, 0, I, 

and N, has a constant value of Z= 1.05 and may therefore be considered abrupt contraction with a special edge condition, which is here 

termed the "formed edge." 

The common form of gradual contraction is that in which a section 

of the airway converges in the direction of flow and the transition of 

areas is gradual. The common picture is that of a larger section converging on a uniform slope of sides to a smaller section, as in figure 

20, P, for which N. is 1 and Z is derived from figure 19 or a similar 

source. In-mine airways a constriction that has been eased off on the 

approach side only might take the form shown in figure 20, Q, for 

which N. is less than 1. 

A converging discharge from an airway (fig. 20, II) has only the 

special condition N.= 0. A converging section used as the discharge 

from a chamber (fig. 20, J) 01' as the entrance to an airway (fig. 20, D) 

actually combines abrupt contraction with gradual contraction. The 

latter can be determined separately, but N. is indeterminate for the 

abrupt-contraction condition at the entrance. The shock loss accompanying abrupt contraction at the entrance is reduced rapidly as the 

included angle of the converging section is increased, but separate 

graphs of shock factors are required for each combination of conditions 

of installation. N one are given here, as these particular forms of 

installation of converging sections have little practical application to 

mine airways. . 

CONTRACTION FOLLOWED BY GRADUAL EXPANSION 

Either abrupt or gradual contraction may be followed by gradual 

expansion, in which case the normal factors for abrupt expansion are -multiplied by y (fig. 18) according to equation (22). Such combinations are seldom met in practice, except that of gradual contraction 
64 ENGINEERING FACTORS IN VENTILATING METAL MINES 

followed by gradual expansion. This combination permits a large 

change of velocities and pressures with but a small shock loss and is 

therefore extremely useful in fluid-measurement methods based on 

pressure changes. The most common form is the standard venturi 

(fig. 20, R), although equally good results may be obtained with the 

special venturi (fig. 20, S) in which the converging section is replaced 

by the formed surface, for which Z may be taken as 1.05. Shock 

factors for the resulting shock loss, which primarily determines the 

cost of this form of air measurement for obtaining a continuous 

record, may be computed from general formulas (21), (22), and (23) 

by the use of appropriate values of y from figures 18 and, for the standard venturi type, of Z from figure 1 9 or similar sources. 

CHART FOR DETERMINING SHOCK FACTORS FOR SYMMETRICAL AREA CHANGES 

A chart for determining shock factors for the multitude of conditions of symmetrical contraction and expansion of the air stream that 

may be encountered in practice is presented in figure 21. It has 4 

sections, marked A, B, 0, and D, corresponding to the 4 possible 

steps in computations, all of which mayor may not be required for a 

particular condition. 

In A the intersection of the proper value of Z with the curve for 

Nc determines the value of c from equation (23). In B the intersection of the value of c with the curve for Ne determines the value of 

Xo from equation (21). Corresponding values of Xe and Xa are directly related to Xo through the relations of velocity pressures to 

area ratios; thus Xe=~o2' and Xa=-NXoz' Therefore in 0 the interlv. c 

section of the proper value of Xo with the line for N. determines the 

value of Xe; similarly, its intersection with the line for Nc determines 

the value of Xa' In D the intersection of the proper value of X o, X., 

or Xa for abrupt expansion with the proper line for y (fig. 19) for the 

condition of gradual expansion determines the corresponding value of 

Xo', X.', or Xu' for gradual expansion. Seetion D is not required, of 

course, for conditions of abrupt expansion; nor is seetion A required 

for conditions of expansion only, without preceding contraction, 

since c= 1. In the latter case Xa=Xo, and X o for c= 1 is easily 

determined as the intercept of the Ne curve with the Xo scale in 

section B. 

A continuous method of solving examples is indicated on the chart 

by the solution of a particular example; arrows show the direction of 

procedure. The example represents a condition of gradual expansion 

following abrupt contraction, where Z=2.5, N c=0.25, N e=0.35, and 

y=0.3. The solution shows that the shock factor corresponding to 

the area after expansion (X.') is 3.5, and that corresponding to the 

area before contraction (Xa') is 6.9. Ordinarily, one would not be 

interested in X o', bl,lt it could also be determined through section D 

if desired. The intermediate values c, X o, X., and Xa are shown to 

be 0.645, 1.44, 11.8, and 23.1, respectively. 

SHOCK FACTORS FOR UNSYMMETRICAL AREA CHANGES 

Obstructions in mine airways, whether along the walls or out in 

the air stream, cause more or less abrupt contraction and expansion of 

the air stream, which are not symmetrieal, and consequent shock losses 
FLOW OF AIR IN MINE OPENINGS 65 

due to changes in the area occupied by the flow. Contraction may 

take place on one or more sides of rectangular openings or only along 

the edges of obstructions in the airway, and an obstruction may be 

moving relative to the air flow and in the same or opposite direction. 

The few data that have been presented on symmetrical contraction 

and expansion, mainly on the few types involved in air measurement 

"dec aQ... ~..-4 

Ox NOLLOVlLLNOO :LV V3HV HOi! HOlOViI }!OOHS ... '" .. Z H010VII N01LOVlLLNOO ... "'= Q 

by pressure methods, do not begin to cover the field of shock losses in 

mine airways due to area changes and can serve only as a basis for 

very rough approximations until more data on the effect of unsymmetrical area changes on contraction factors and shock loss are 

available. 
66 ENGINEERING FACTORS IN VENTILATING METAL MINES 

LOCAL ENLARGEMENTS OF AREA AND DEAD ENDS 

In mine airways there are many local enlargements of one dimension of the airway over a limited length, such as dead-end stations off 

shafts, local widenings at sidetracks in drifts, and local increases in 

height, for which the shock loss is negligible because the expansion of 

the area occupied by the flow is small and over but a limited length. 

Where the enlarged section is relatively long shock losses result from 

expansion and contraction, but they are counteracted by the decreased friction loss occasioned by the lower velocity of flow in the 

intermediate section, with the net result that the change in pressure 

loss over that of an equal length of normal cross-sectional area is 

usually negligible. The total pressure losses for sections including 

such enlargements may therefore be taken as equivalent to the 

friction losses for equal lengths of normal area. 

DOORFRAMES 

Doorframes at open doors on mine airways are a typical form of 

abrupt nonsymmetrical contraction, with contraction and expansion 

on but three sides of an off-center rectangle. Until substantiated 

methods and values are available it is recommended that the formulas 

for symmetrical expansion be used with Z=2.5 for a square-timber 

frame only and Z=2.0 for the same frame with the door open. 

REGULATORS 

A common device used to control the distribution of air currents in 

mines is a small opening in a stopping that can be regulated, usually 

by a sliding shutter, to reduce the flow on one circuit in order to 

improve the flow on other circuits. It is simply a device for causing 

a desired loss of pressure and amounts to installing a square-edge 

orifice (fig. 20, N) in the airway. A value of 2.5 for Z is probably 

high enough for a symmetrically placed regulator opening, but the 

value of Z that should be used for different degrees of unsymmetrical 

placing, particularly where the opening is in a stopping parallel to 

the main flow, as at a split or junction, is unknown. 

TIMBER OBSTRUCTIONS 

A few Bureau tests indicate that shock factors for timbers across 

mine airways may be approximated roughly from the formulas for 

symmetrical expansion by doubling the contraction factor for the 

edge condition involved, that is, using Z=5.0 for a square edge. Lagging laid across the center of vertical shaft compartments gave an 

average Z value of 5.2, and center posts of roughly squared timber 

in a coal-mine entry gave an average Z value of 5.3. Contraction of 

the flow caused by obstructions of this type takes place only along 

the perimeter of the obstruction and not along the walls. 

MINE CARS 

A mine car or trip of cars causes a similar type of contraction of 

the air stream, but in this case the obstruction is so long that three 

pressure losses are involved: (1) Abrupt contraction, similar to the 

preceding case, except that the expansion following contraction takes 

place in the constricted area; (2) increase in friction pressure loss due 

to flow for a certain length of airway in the constricted section rather 
FLOW OF AIR IN MINE OPENINGS 67 

than in the full area of airway; and (3) abrupt expansion from the 

constricted area to the full area of the airway. 

The Bureau's data for pressure losses due to mine cars in a c6111-

mine entry indicate that shock factors for the pressure loss caused 

by the front of the car may be approximated roughly from the 

formulas for symmetrical area changes by doubling the normal value 

of Z for the edge condition involved, as for timber obstructions, that 

is, using a Z value of 5 for all square-edge obstructioI;lS in the airway 

and 3 for round-edge obstructions. 

The friction loss due to high velocities through the constricted area. 

may be large enough to justify precise calculations, but for ordinary 

purposes it can be taken as equivalent (within 5 percent for the 

f .. di·) 2-N. th al average range 0 nune-aIl'Way con Nons to ~ tunes e norm 

loss for the same length of unobstructed airway, where N is the ratio 

of constricted area to normal area. The shock loss due to abrupt. 

expansion at the rear end of the car or trip of cars may be approximated by using shock factors for symmetrical expansion. 

SHIELDING EFFECT OF OBSTRUCTIONS 

Tests on mine cars have also revealed the effect of closely spaced 

obstructions to flow, which is commonly termed the "shielding 

effect." When the obstructions to flow are so closely spaced that the 

flow does not travel far enough downstream to expand fully before 

encountering another obstruction the shock loss is diminished to that 

corresponding to a higher value of Ne than the actual areas would 

indicate. Tests on cars in a coal-mine entry showed that the cars 

had to be spaced about 30 fe~t center to center (about 25 feet clear 

distance between cars) before the pressure loss due to a group of cars 

was equivalent to that for one car multiplied by the number of cars. 

Where the cars form a closely spaced train-the normal conditionthere is but 1 front shock loss and 1 rear shock loss, and the intermediate cars contribute only the increased friction loss in the constricted airway proportional to the total length they occupy coupled 

together. 

INTERMITTENT OBSTRUCTIONS 

Mine airways often contain intermittent obstructions which change 

the area of cross-section and cause individual shock losses so small 

that they defy computation; it is then more convenient to determine 

directly the average total shock loss per unit of length, which may be 

approximated roughly as follows: 

Slight degree: X =0.1 per 100 feet; trolley box, water box, large :flanged pipe, 

occasional small falls of roof, occasional small crossbars, hangers, props, etc. 

Moderate degree: X =0.3 per 100 feet; large fan pipes or tubings, occasional 

large crossbars or heavy hangers, frequent small falls, occasional constrictions. 

etc. 

High degree: X = 1.0 per 100 feet; combinations of obstructions given above, 

large falls, occasional storage piles of timber or pipe, closely set crossbars, props or constrictions, etc. 

These values are for average areas determined by subtracting the 

area occupied by the obstruction where the latter is continuo~s and 

of appreciable size. For very high degrees of obstruction occaSIOnally 

found in mine airways it is impossible to fix even approximate limits, 

as the normal characteristics become unimportant when compared 

with the characteristics of the obstructions; each case must therefore 
68 ENGINEERING FACTORS IN VENTILATING METAL MINES 

be computed by analyzing its separate features and calculating 

separately the friction and shock losses involved. 

SHOCK LOSSES AT SPLITS AND JUNCTIONS 

Splits or junctions-air currents leaving or entering the main 

current-involve shock losses due to changes in both direction and 

area. The few test data available indicate that for splits or leaving 

currents the losses are comparable to bend losses, except that the 

velocity pressure of the partial current or current in the split should 

be used. Where minor air currents enter the main stream, however, 

both reason and test data indicate that the normal deflection shock 

losses for the entering stream are increased by its interference with 

the air stream of the main current. Exact quantitative data are 

lacking, but since the effects are know to be of considerable magnitude 

it seems prudent to use X factors for normal conditions, based on the 

velocity pressure of the entering stream, multiplied by at least 1.5, 

or an allowance of 50 percent. 

Evidently shock losses at splits and junctions are largely determined 

by the proportional divisions of the total quantity of flow, which 

depend on the total friction and shock losses in the branch circuits as 

well as the shock losses at entrance and departure in each. Problems 

of calculating free distribution are therefore difficult. Approximate 

solutions require a reasonably close estimate of the proportional divisions of the flow, and accurate solutions are reached only through a 

process of trial and error. 

Virtually all splits and junctions in mine airways present irregular 

conditions, such as nonuniform areas at bends and unsymmetrical 

area changes, for which no strictly rational methods of computation 

have been developed; although a few shock factors for special combinations have been determined directly, few data for these important 

types of pressure losses in mine airways are available. 

EFFECT OF AIR-FLOW CONDITIONS ON CONSTANCY OF PRESSURE-LOSS 

FACTORS AND VELOCITY DISTRIBUTIONS 

In the preceding discussion both friction and shock factors have been 

assumed to be constant; that is, pressure losses have been considered 

independent of flow conditions, except as represented by the formulas 

given, although reference has been made to the fact that slight variations in these factors were actually required to compensate for variations in flow conditions. Velocity distributions and other air-flow 

conditions have been found to vary in a similar manner, and the various sets of circumstances are apparently related. The variations are 

so small as to be a negligible factor in pressure calculations for such 

erratic and irregular conditions as those presented in mine airways, 

but they must be considered in accurate experimental work, in gaging 

the application of experimental data to conditions markedly different 

from the conditions of the experiment, and in their application under 

special flow conditions. 

THEORY OF FLUID FLOW 

These variations are ade<l,uately explained by a theory of fluid flow 

developed by English scientIsts, in which the variations are correlated 

with a factor termed the "Reynolds Criterion" of flow conditions; 
FLOW OF AIR IN MINE OPENINGS 69 

good correlation has been obtained for both fluids and gases. This 

theory shows that the flow conditions of any fluid, including air, are 

the same for equal values of the Criterion for similar passages having 

the same relative roughness or same relative degree of disturbance of 

flow. The Criterion is the product of the diameter, velocity, and 

density divided by the viscosity. 

FRICTION FACTORS 

Many available friction-factor data for mine airways have been so 

correlated by the author in a previous paper 21 which shows that, 

although there are three distinct zones of fluid flow-viscous in which 

the factor decreases as the Criterion increases, unstable in which the 

factor increases rapidly as the Criterion increases, and turbulent in 

which the factor for low rates of :pressure loss decreases slightly as the 

Criterion increases but in which It is virtually constant for high rates 

of pressure loss-fortunately mine-air flow falls in the turbulent zone 

and mainly involves high rates of loss. 

For smooth-lined airways there is a perceptible change of factor in 

the mine air-flow range, but test work has demonstrated that minor 

changes in type of wall surfaces have such a large effect on friction 

factors that changes in flow conditions are relatively insignificant~ 

For rough-lined airways there is no perceptible change of factor in this 

range. 

In applying friction factors developed in small airways to relatively 

large airways with the same absolute conditions of roughness of surfaces the reduction in relative roughness that accompanies an increase 

of area must be taken into account, as well as the fact that friction 

factors may represent both friction and shock losses. For example, 

the author has found that with the same absolute roughness the friction factors for 400-square-foot railroad tunnels were about 25 percent 

less than for straight 40-square-foot metal-mine airways for both 

timbered and rock sections; and factors for smooth pipe were reduced 

10 percent for a 4 to 1 area ratio. Similar and approximately equal 

ratIOS of friction factors have been reported for a range of sizes of 

rough-lined (corroded) pipe. 

FLOW THROUGH CLOSELY PACKED MATERIAL 

The critical velocity or velocity at which turbulent flow changes to 

unstable flow is about 22/D f. p. m., where D is the diameter in feet; 

such velocities are insignificant in mine airways. Velocities of flow 

which would bring the flow conditions into the viscous or the unstable 

zone practically occur only where air leaks through closely packed 

rna terial such as finely broken ore and waste fill; here the pressure losses 

are so small and the distribution so complicated that mathematical 

analysis appears to be futile. 

VELOCITY DISTRIBUTIONS 

In normal flow through straight airways or ducts of uniform crosssectional area the distribution of velocities is regular throughout the 

cross-section; the velocity at the center is much higher than the average and grades down to relatively low velocities close to the wall. 

The velocity for any position in the section, as averaged over an 

"McElroy, G. E., MIne·Air Flow: Trans. Am. Inst. Min. and Met. Eng., vol. 74,1926, pp. 297-311. 
70 ENGINEERING FACTORS IN VENTILATING METAL MINES 

a.ppreciable period of time, bears a constant ratio to the mean velocity 

of flow; the mean velocity can therefore be determined from the velocity at but one point in the stream. The ratio, however, has been 

found to vary with the Reynolds Criterion and correlates with the 

variations found in friction factors; that is, the rate of change is perceptible in smooth ducts but is practically nil in very rough airways. 

Since the other components of the Criterion are usually constant the 

change may be correlated against velocity only and, over the limited 

range of most test or experimental work, will plot as a straight line on 

logarithmic-coordinate paper. The ratio of mean velocity to center 

velocity is the position constant frequently used; for straight sections 

of uniform area with undisturbed flow in the turbulent zone it 

approaches a constant value of 0.8122 at high values of the Criterion 

for smooth pipe but has this constant value of 0.81 for rough airways 

where the friction factors are constant. The positions of points of 

average velocity have been calculated on the bases that the distribution 

of velocities is a parabolic curve with the axis at the center of the airway and that the distribution curve is a logarithmic curve, a point 

still unsettled. 

However, a perfectly straight duct with absolutely uniform area of 

cross-section is rarely met even in test work, and since position constants are affected by minor deviations from this condition they 

should be determined in situ. In test work on ducts and airways 

the author has determined center-position constants ranging from 

0.75 to 1.0, although the general average is about 0.80. Widenings 

give low values, and constrictions give high values. Velocity distributions are also largely affected by conditions that disturb the flow 

and by temperature differences between the air and the walls. 

Warmer air gives higher velocities in the upper part of the section 

and colder air, higher velocities in the lower part of the section. 

With very small flows this effect may be so pronounced that velocity 

of flow is perceptible only along the roof or along the floor according 

to whether the air is warmer or colder than the rock. 

SHOCK FACTORS 

Shock factors have not been correlated by the Reynolds Criterion, 

but it seems probable that they can be so correlated, as the variations 

noted in test work agree closely with the variations in friction factors 

and in position constants of velocity distributions. For low rates of 

pressure loss where velocity of flow was practically the only variable 

a definite change of factor with velocity has been noted, whereas for 

high rates of pressure loss the change of factor has been practically 

nil. Similarly, appreciable decrease in the factor accompanied large 

increases in size of an elbow designed to have a low rate of pressure 

loss. Enough information on exact test conditions is not yet available to justify an attempt to correlate the data on shock factors, nor 

is the subject important as respects mine-airway pressure losses. 

LOW-VELOCITY FLOW IN ROUGH AIRWAYS 

In correlating friction factors by the Reynolds Criterion the curve 

for certain English tests on an 8- by 8-foot timbered airway showed a 

rapid increase in friction factor with decrease in the value of the 

"Stanton, T. E., Friction: Longman's, Green & Co., 1923, p. 51. 
FLOW OF AIR IN MINE OPENINGS 71 

Criterion, even though tests on a small-scale model of this airway 

showed an almost constant factor. F. Ernest Brackett suggested 

that the effect experienced was a velocity-cascade effect and that at 

the lower velocities of flow used in the tests, which extended as low as 

15 f. p. m., the flow expanded into the space between timbers.23 The 

author made a few observations with smoke in timbered airways to 

check this assumption and found that for velocities above about 250 

f. p. m. the angle of expansion of the flow appeared to be constant at 

an included angle of about 7°, but as the average velocity of flow was 

decreased from 250 f. p. m. the angle of expansion increased to such 

an extent that at 50 f. p. m. the edge of the current struck the lagging 

behind 10-inch square timbers at about the midpoint between sets on 

5-foot centers. A few friction-factor tests 24 made later in timbered 

and very rough rock airways confirmed this result-the friction-factor 

values increased rapidly as velocities were decreased below about 250 

f. p. m. This result is significant but would rarely enter into pressureloss calculations. It shows that the normal angle of expanSIOn of air 

is not uniform but increases at velocities below a certain minimum. 

Both friction and shock factors are affected at these low velocities 

because, although shock loss only is directly involved, friction factors 

for rough surfaces, especially for timber lining, are based largely on 

included shock losses. These results also probably explain the 

"kink" in the curves obtained from tests 25 of pressure-volume relations at some English coal mines; the tests revealed a sudden change 

in the slope of the curve expressing the relation at low volumes of 

flow. 

PRESSURE-QUANTITY RELATIONS 

For fixed or constant conditions of an airway or system of airways 

the relation of pressure to quantity of flow is constant and can be 

simply expressed both as a mathematical aid in solving problems of 

combined pressure losses and for the direct determination and 

application of pressure-quantity relations without reference to such 

details as dimensions and other particulars of airway conditions. 

Many such expressions have been and are in use-"equivalent orifice", "mine temperament", "pressure potential", "inherence", 

"blast area ", "Atkinson ", "Murge", and "Guibal "-the values of 

the resulting factors depending both on the form of the expression and 

the units used. 

RESISTANCE FACTORS 

Confusion is obviated by using but one form for pressure-quantity 

relations; the most common form is H=Rq2, that is, the pressure (lI) 

equals a constant (R) times the square of the quantity (q) for constant conditions. This form was proposed by Atkinson about 1850 

as a mathematical aid in solving problems of combined airways and 

by Shaw in England in 1890 and Rateau in France in 1891 for direct 

expression of the pressure-quantity relation for mines and sections of 

mmes. 

In this form R may be considered a unit of specific resistance comparable to the ohm of electrical resistance; its value will depend on the 

'8 McElroy, G. E., Mine-Air Flow: Trans. Am. Inst. Min. and Met. Eng., vol. 74, 1926; dlscussion, p.309 . .. McElroy, G. E., Ventilation Experiments in the Moffat Tunnel: Eng. News-Record, Dec. 15, 1927. II Briggs, H., Williamson, J. N., Penman, J. S., and Hyde, H., Experiments Concerning the Relationship between Pressure and Air Volumes in Mines: Trans. Inst. Min. Eng. (London), vol. 73,1926-27, pp. 41-73. 
72 ENGINEERING FACTORS IN VENTILATING METAL MINES 

units used for pressure, quantity, and density, although the latter 

term does not appear in the formula. Since pressures vary directly 

as the air density and the formula contains no density term, a standard 

density is implied. In mine-ventilation practice pressures are 

determined and expressed in inches of water and quantities in cubic 

feet per minute; 0.075 pound per cubic foot is the standard for air 

density. The use of cubic feet per minute as the unit of quantity 

would result in extremely small values of R for mine circuits; for 

convenience in computing and handling pressure-quantity relations 

for mine airways the author therefore uses 100,000 c. f. m. as the 

quantity unit. Values of R are then inconveniently large for fantubing calculations, but these are a minor phase of mine-ventilation 

calculations. 

The standard formula for pressure-quantity relations used throughout the remaining part of this bulletin as an aid in discussing and 

evaluating airway conditions and mine and fan relations is: 

H 

H=RQ2 or R=Q2' (24) 

where H is total pressure, inches of water, for an air density of 0.075 pound pel: 

cubic foot; 

Q is quantity of flow, 100,000 c. f. m.; and 

R is a factor of specific resistance, whieh will be referred to as "resistance 

factor." 

R is equivalent to the pressure required for a flow of 100,000 cubic 

feet of air weighing 0.075 pound per cubic foot. The value of R is 

unity (1) when 1 inch of water pressure is required to pass 100,000 

c. f. m. of standard air. With few exceptions all calculations can be 

carried out on a standard air-density basis. The complete formula is 

HX9.~75=RQ2, but it is more convenient to make the calculations 

on a standard density basis and make the correction for density either 

at the beginning or end of the calculation, since many problems of 

distribution may be solved on a standard air-density basis without 

considering actual air densities. 

Calculation of resistance factors.-Values of R may be determined 

by tests or calculated separately for friction and shock losses. Other 

designations for pressure-quantity relations may also be expressed in 

terms of equivalent values of R. The basic formula (10) for friction 

pressure loss may be written: 

KPLq2 X 0.075 

Rf=Hf = 5.2A3 0.075 

q 2 ( q )2 100,000 

where K=k (100,000)2=kX 1010. 

k (100,000) 2 PL KPL 

5.2A 3 5.2A s' (25) 

The basic formula (11) for shock pressure loss may be transposed 

from a velocity base to a quantity base through equation (3) and the 

relation V=~: 

R H. XH. XO.07623V2 

8Q2 Q2 Q2 

XO.07623q2 

A2 623X 

A2 (26) 
FLOW OF AIR IN MINE OPENINGS 73 

For resistance in terms of equivalent orifice 0, in square feet, from 

O O.000388q R 1,510. F . . fA ki 'R ..JI1 ' = ~ or resIstance III terms 0 t nson s A, a 

similar unit in quite common use in England-that resistance which 

requires a pressure of 1 pound per square foot to pass a quantity of 

1,000 cubic feet per second of 0.075 air-R=0.535 RA • 

The large unit for quantity simplifies the calculation of friction 

losses, since k multiplied by 100,0002 is k times 1010, a result that 

requires moving the decimal point 10 places to the right; for k=0.0820, 

kX 1010 becomes 20, and for k=0.07100, kX 1010 becomes 100, figures 

that are more easily remembered and handled than actual values of k. 

The same holds for shock losses; 623 is more easily remembered and 

handled than 0.07623. 

SERIES FLOW 

The combined R for any individual section of the flow system is 

found by adding the individual R's for the particular friction and 

shock-loss conditions involved in the section. Similarly, where the 

total quantity of flow passes through a single circuit or system the 

resistance of the system (R) is simply the sum of the separate resistances (Rt, R2, Ra ••• ) of its separate parts, and 

(27) 

PARALLEL OR SPLIT FLOW 

Where the total quantity flows through two or more airways in 

parallel whose individual resistance factors are RI, R2, Ra ... the 

equivalent series resistance (R) encountered by the total flow may 

be derived as follows: Total quantity (Q) is the sum of the separate 

quantities (QI, Q2, Qa ... ), and the pressure difference (II) between 

the beginning and end of each split or path in parallel is the same; 

that is, 

Q=-/i, 

Q=Ql+Q2+Qa+ . .. and Ql=..;tl;, etc. 

-fJ=..;tl;+/l;+~+ . .. and ). 

1 1 1 1 

..jJl= ..j7I;+ ..jR2+ ..jR/· 

Therefore 

(28) 

The distribution of quantities will vary directly with the JR value 

of the different splits; this value may be termed their "conduct. 

ance factor" (0). Since R=~' 0= .J~. 

122422°-3&--6 
74 ENGINEERING FACTORS IN VENTILATING METAL MINES 

CALCULATION OF RESISTANCE FACTORS FOR MINES 

The resistance factor (R) for a mine or for a complete ventilating 

circuit is the equivalent series resistance for a system of airways 

arranged in series and parallel. For any large metal mine the airway 

system usually comprises long lengths, through which all the air flows, 

and an intricate network of cross-connected splits and branches com-

-

-

-

-

-

Abandoned 

workings 

caved or 

sealed 

-

-

-

plicated by leakage paths; 

exact mathematical solutions for R are impracticable, if not impossible. 

However, the main airways and main branches 

usually constitute 90 percent or more of the total 

- I resistance to flow, and 

f approximate factors of 

sufficient accuracy can 

usually be obtained by 

analyzing the more important members of the 

network into a series of 

branching splits and adding the equivalent series 

resistance for the split 

flow to that for the series 

flow. 

-

-

-

-

-

- The main purpose of 

such calculations, as regards metal mines, is to 

determine, in conjunction 

with whatever naturaldraft components may be 

acting in series with it, 

the duty of a fan installation for mechanical ventilation; for most metal 

mines a sufficiently accurate figure for assuring 

fan operation close to 

maximum efficiency may 

be derived by considering 

only the main airways of 

FIGURE 22.-Line diagram of the main airways of a typical metal mine. 

the system and not the 

confused and in trica te 

network of minor airways 

offered by a working zone of stopes or other openings. . An analytical sketch of the main airways of the usual metalmme 

will generally resemble a ladder with the upper rungs missing, as 

shown in figure 22. In this case there is only one place to start the 
FLOW OF AIR IN MINE OPENINGS 75 

calculation-at the bottom split. The equivalent series resistance 

ur resultant for this split is then added to the values of R for any 

airways in series with it, usually a section of a shaft and the resultant 

R for a number of raises; this combined R for the group of airways is 

then considered as one leg of the next split above. In this way the 

resultant for a larger and larger group of airways is obtained as leg 

after leg or rung after rung is considered, until finally a single value of 

R represents all flow on and below the first open level; this value of R 

added to the values of R for both upcast and downcast shaft sections 

above this level gives the R value for the system as a whole. Such 

it calculation requires many arbitrary decisions as to the effective 

length of airways to avoid the mathematical difficulties involved in 

handling networks, but a large latitude is allowed in the ordinary 

metal-mine airway system because of the generally small ratio of the 

series resultant for the split flow to the total resistance. 

POWER-QUANTITY RELATIONS 

The horsepower required to pass air through airways at a definite 

pressure is equal to the total pressure loss in pounds per square foot 

multiplied by the quantity in cubic feet per minute, which is the rate 

at which work is being done in foot-pounds per minute, divided by 

33,000, the number of foot-pounds per minute equivalent to a horsepower. Since pressure in inches of water (at 60° F.) times 5.2 is equal 

to pressure in pounds per square foot, 

5.2Hq Hq Horsepower (hp.) =33000=6350' , , (29) 

RQ2X100,000Q But H=RQ2 and q=100,000Q, so hp. = 6350 15.75 RQS. (30) , 

This is the value for the air horsepower at standard air density. At 

other densities the pressure changes directly with the density ratio 

and the horsepower likewise. Since the process of applying and using 

power is never 100 percent efficient the actual power supplied to the 

prime mover of a ventilating appliance is always larger than the air 

horsepower, over-all efficiencies commonly ranging from 40 to 60 

percent. The "brake horsepower" or actual power requirement is 

th f air horsepower 

ere ore ffi' ( . )' e Clency as ratlO 

CHARTS FOR DETERMINING RESISTANCE FACTORS AND PRESSURE AND 

POWER LOSSES 

Tables for determining pressure losses in mine airways are available 26 but are not very satisfactory; since so many variables with 

different rates of variation are involved, concise tables cannot be 

readily interpolated, and tables complete enough to reduce the errors 

of interpolation to a reasonable limit would be very bulky and laborious to compute. 

Figures 23,24, and 25 were developed for the rapid determination 

of resistance factors, pressure losses, and air horsepower from dimen-

,. Fitch, Thomas W., Jr., Mine Resistance: West Virginia Coal Min. Inst., June 1910. Reprinted by A,merican Blower Co. as Pamphlet 297. 

Weeks, ·W, S., Ventilation of Mines: New York, 1926, pp. 60-61. 
76 ENGINEERING FACTORS IN VENTILATING METAL MINES 

SHAPE RATIO, FH 

1 Z 3 

LENGTH OF SECTION, L, FEET 100 1,000 

SHAPE RATIO, Fll 

P 

FH= 3.55 v,;r 

0 1.00 

1 0 1.03 

{:J 1.09 

~ 1 

10 1.13 

~ 1 f 1.50 1.15 . 1 

i .Q~ 1.16 

.9 1 

.!! en 10 1.20 '" iii 2 

8- 191.30 

.a c:::::J 1.41 .. 4 oS 

g 

:e -a 10] 1.60 E C) 1 1 

~ t \Q 1.69 

1

11111-10'000 ~ i lCIIJ 1.95 g;i .. 1'1 

S~{E1.97 as. 11 

~ .~ zIT]] 2.07 

gj.!! I I I 

r"l :§' 10Il] 2.2S IS: :l I I I 1 

Z ~ 2rTTTn 2.39 o 'r'trW 

IDt'tzt:lzl.uJ -1,000 E 1,000-100 

~ ~ I>: g!-

~ 

10-1 

10 100 

.1 1 

AREA OF CROSS'SECTION, A, SQUARE FEET 

FIGURE 23.-Chart for determining resistance factors for friction pressure losses in airways • 

• NOTE.-Use corresponding scales of A and P and of A and R; k X 10'0 may be scaled only along the line 

mdicated. 
FLOW OF AIR IN MINE OPENINGS 77 

10-100,000 9 

8 

7 

6 ('" 

4 

3 

2~~ l\~I\\ R\ 

1-10,000 9 

8 

6 t-~ 6 

4 

3 

2 t-.~ ~ 

1 • 9 

7 

6 l\l,l 

6 (\~ 

4 

3 

2 II ~~ ~ 

• 1-1,000 

§ 

I 

~ l\\' 

9 

8 

{§ 

"" .01-100 

t2 

6 

4 i 3 

2 II ~ I 9 

8 

7 

6 

6 

4 

3 

2 

.0001-1.0 

8 

7 

6 3466789 3456789 

10 100 1,000 .1 1 10 AREA OF CROS&SECTION, A, SQUARE FEET 

FIGURE 24.-Chart for determining resistance factors for shock pressure losses in airway!!. 

NOTE.-Use corresponding scales of A and R; X may be scaled along any vertical "25" or "79" line. Moving the decimal point in X requires a similar movement of the decimal point in R. 
78 ENGINEERING FACTORS IN VENTILATING METAL MINES 

sions, friction factors, and shock factors. They are based on a standard 

air density of 0.075 pound per cubic foot, but figure 25 includes a 

subsidiary chart for converting pressure and powers from this standard 

air density to actual air density, or vice versa. On account of the 

small scale used they serve only for very approximate solutions, but 

these are often all that the precision of the available data justifies. 

The charts also provide a rapid means of roughly checking more precise computations. 

The general procedure in using the charts is to find the resistance 

factors for separate friction pressure losses from figure 23 and for 

separate shock losses from figure 24. These are then combined 

according to the principles of series or parallel flow, as the case may 

be, and the total resistance factor for the section of airway or system 

of airways, together with the known or assumed quantity of flow, are 

used tQ determine the pressure loss and air horsepower from figure 25. 

The latter chart can also be used to determine values of R corresponding to .J~and 0 (equivalent orifice). As a matteroffact, a large variety 

of air-flow problems may be solved by the use of these three charts. 

The friction chart (fig. 23) provides for the direct determination of 

the resistance factor (RFlOCc) for the friction loss in a 100-foot length 

of circular section and multiplication of this value by both a shape 

ratio and a length ratio to give the resistance factor (RF ) for friction 

loss in a section of any shape or length. The length ratio is a simple 

proportion and requires no explanation. The shape ratio is also a 

question of simple proportion, as will be shown, but the proportion 

must be calculated, determined separately through the "shape-ratio" 

scale in the lower part of the chart, or estimated from the values given 

along the right margin. 

The shock chart (fig. 24) is plotted directly from equation (26) and 

the pressure and power chart (fig. 25), from equations (24) and (30). 

Derivation of shape ratio, FR.-The influence of shape of section on 

friction pressure losses and resistance factors representing these losses 

is shown in general formulas (10) and (25) by ~3' Since both P 

(perimeter) and A (area) are determined by the same dimensions their 

ratio may be determined in terms of area only for different shapes. 

By expressing both perimeter and area in terms of the same d;men-

. . '11 b f d h f . ul h P constant If F slOns It WI e oun t at or any partlc ar s ape A3= A5/2 . 

represents the constant then 

P F=~-;{ (31) 

F may be termed the "shape factor" since it determines the relative 

value of different shapes with respect to resistance factors and thus 

to pressure and power requirements. For a circle F equals 3.55; for a 

square, 4.00; for a rectangle proportioned 1~ to 1,4.08; for a 2 to 1 

rectangle, 4.24; and for a 3 to 1 rectangle, 4.62. For irregular shapes 

F may be determined from direct measurements of perimeters and 

areas through equation (31). Where the flow is through similar sections, whether contiguous to each other or not, as in shafts divided 

into compartments or in airways in parallel, the shape factor for the 

combination will be found equal to F..jN, where F is the shape factor 
Bureau ot Mines lIulletln 38.5 

PRESSURE OR HORSEPOWER AT 0.075 AIR DENSITY ' .. 15 2 3 4 5 6 7 8 9 10 15 20 ~ u .q·a / ~&'lJ~ '/V 

~, 1..,( / / / L ./ V' V 1/ 

/ A ... j / / / V V /I / 

. 8;> G gf 'C?9/ :/ / ... 1/ / / ~ I<L' lL. ~ L L / 1/ 

V;: !AI /. / L~ /. /. / ~ L L ~ ~ /' V 1/ 1/ / f"'.r .. (1 ~ l~ l"-!f P ~q~ ~f9" ~ 2 ~ ~ [!(>g ~ '\f '0;>/ V / . / V V 1/ // j V ~ V V V /: ~ /1(H=~t>f / ~ V V /V / V V V / 

'QY/ ~/ t-' '" l7 ~ V 1/ V,/ /1""'/ ,. I>"' 7' 7 "'[7 V V V V V 1/ V/, V 

/fv / V / v, V V~ ~ [}' / V V / L ~ ~V V' ~ L lL V L V / 

V1/1v V V V~ ~ lY V V V/ V / V / l,L ~~ Iv V / / V / L / V V 'O<t L L L V L 

0.5 .6 .7 .8 .9, 1 1.5 2 3 4 5 6 7 8 9 10 

0.01 

PRESSURE 'OR HORSEPOWER AT ACTUAL AIRDENSITY 

A 

EQUIVALENT ORIFICE; 0, SQUARE FEET 

(From intersection of R with"O line") 

10 100 

CONDUCTANCE FACTOR, JR. 

(From intersection of R with ':JR line'') 

.1 1.0 

/ / V 

V L 1/ 

L V 

/ 

/ V 

/ 

23456 '789 23456789 2345 

5 'X '/.fJ LN 1/ /' I'f... I'A 'f.. tN f/l fY. I'f... y., ('L f.i'J 

4 f..i 01/)( 1i-ll'iJ:'A 'J r')(V IN 'Jl .,.., ~ f1 I/)( Ii Il/J; :)S v, "III W J',IIM lI'{..l'.1tJ. I{ I/'il)i[l, U.rx 

, (-,. ') I)<Il'I flit ~Yl..1fl 1'1 J'v!) VI v.~ IO<It> Vlf :) Cf 1'Y b</ (f)11{~ '-L )'alb ~[li~ .LLirL 

't~ 0'11' K'/)< ~ N It'l«r IlIJ rf) r? V 1.!'( (iIl'QKlJ(l1 01K r,( r III J tL2 fl:tlti IIllfl Q II'ri ~~ ~~ l.L 

3~k7K(;i1 ~ [!j ~ ~raC' II!; L(/ ~ ~ t;0 ~~V, (i()fh ~ll) X flL0ff2lQ ~ K ~ fi ~~ D<)SVY ~~ ri< 

2~~!j] !J k' ~'~ f'., feN\)< ~ ') ~ [j D ~~D ~~ 0 k2 ~~I9JlU, ~1/11 ~ k5 ~~~ ~~ fie ~ ~ f"'l 1](; fY.. '" (7'. t'. INIA IJ'A 'Y.. ,,, If: IJ tall"> ~ 

001 2 3 456789 23406789 

10;000 100,000 

2 3 4 5 

1,000 

QUANTITY OF FLOW. q;'".C'UBIC FEET PER MINUTE 

FIGURE '25,-A and B, Charts for determining pressure loss and air horsepower from resistance factors and quantities of flow. 

N OTE.-R may be scaled along any vertical" 635" line; hp. may be scaled along any vertical" 1" line, Moviug the decimal paint 2 places to the right in R requires moving it 

1 place to tbe left in .IE and in O. 

122422°-35. (Face p. 78.) 

FLOW OF AIR IN MINE OPENINGS 79 

for the single compartment or airway and N is the number of compartments or airways in parallel. If all conditions other than shape 

are constant re,;istance factors and pressure and power losses will vary 

directly as the shape factors. This expedient has been used in developing the friction chart (fig. 23), which is plotted for a circular shape, 

for which shape factor F is 3.55. Corrections are therefore required 

for other shapes according to the ratio of their shape factor to 3.55. 

If this shape ratio is designated by FR then 

(32) 

The transposed form of equation (25) used in plotting the friJtion 

chart is thus 

R kXIOIOXPXL kXlOIOX3.55XlOOXF X..!::..-- } F 5.2A8 5.2XA6/2 R 100-

(33) 

kXIQIOX68.2 XF X..!::..-

A6/2 R 100 

Use of charts.-The methods of using the charts are best shown by 

the solution of a particular example, which is indicated by arrows on 

the three charts: 

Example.-Find the pressure and power required to pass 22,000 

c. f. m. of air weighing 0.064 pound per cubic foot through a 550-foot 

length of a 5- by 6.5-foot rectangular timbered airway, which has 

timbers in average alinement on 5-foot centers but is slightly sinuous 

and includes one right-angle bend. 

Solution.-The area (A) of cross-section is 32.5 square feet and the 

perimeter (P) 23 feet. These determine the shape ratio (FR ) (lower 

part of fig. 23) as 1.14. For a straight airway the friction factor 

(kX10 10 ) is estimated to be 100, which, for an area of 32.5 square 

feet, determines the resistance factor for 100 feet of similar circular 

section (RF1OOC) as 1.18. To correct for shape proceed parallel to 

the 45° diagonal ruling from this value on the scale at the left to intersection with a vertical through FR=1.14 from the top scale and then 

horizontally to the left scale. This procedure multiplies 1.18 by 1.14 

and gives 1.35 as the resistance factor for 100 feet of straight section 

(RFlOO). 

Before correcting for actual length it is desirable to determine the 

resistance factor for the shock loss per 100 feet due to the sinuosity of 

the airway. For a slightly sinuous airway the shock factor for 100 

feet (XlOO ) is taken as 0.2, and for an area of 32.5 square feet the corresponding resistance factor (RSlOO ) is 0.118 per 100 feet (fig. 24). 

The total resistance factor for friction and shock for 100 feet of 

sinuous airway (RlOO ) is then 1.35+0.12 or 1.47. 

Enter figure 23 on the left with this value and proceed parallel to the 

45° diagonal ruling to intersection with the vertical through L=550 

and then horizontally to the scale on the right (or left). This procedure multiplies 1.47 by 5.5 and gives 8.1 as the resistance factor 

(RA ) for the sinuous airway . . If the shock factor for the particular bend conditions (XB ) is estimated as 1.1 the resistance factor for the bend (RB ) is 0.65 for an 

area of 32.5 square feet (fig. 24). The total resistance factor (R) 

for the airway is then 8.1+0.65 or 8.7. 
80 ENGINEERING FACTORS IN VENTILATING METAL MINES 

In figure 25 the resistance factor of 8.7 determines the pressure 

(HO•075 ) as 0.42 inch and the air horsepower (hP.o.on) as 1.46 for a 

flow (q) of 22,000 c. f. m. of air weighing 0.075 pound per cubic foot. 

Reduced to the actual weight of 0.064 pound per cubic foot through 

the subsidiary chart at the top of figure 25, the final results show that 

a pressure (HO•064 ) of 0.36 inch of water would be required to pass 

22,000 c. f. m. through this airway and that the air horsepower 

required (hp.o.o64) would be 1.25 hp. 

In this same figure the intersections of R=8.7 with the" ;11 line" 

and the" 0 line" determine the condutance factor (;11) as 0.34 and 

the equivalent orifice (0) as 13.2 on the upper scales. The ;11 

value would be used if this airway were considered as one of a nUID ber 

in parallel; it is actually a coefficient of conductance similar to O. 

Either may thus be used to determine the corresponding R for either 

single or multiple airways, provided they have been determined for a 

standard weight of air of 0.075 pound per cubic foot. 

ECONOMICS OF AIR FLOW 

The economics of air flow is mainly a question of balancing capital 

charges against power charges. Either can, of course, be lowered 

by increasing the other, but a proper balance between the two is the 

essential requirement for minimum total costs. 

Power is determined primarily by quantity, since power varies as 

the cube of the quantity. If quantity is fixed the power varies 

directly as the resistance, or value of R for the circuit, which may be 

modified in many ways by changes in the physical characteristics of 

the system of airways involved. Increase in quantity may be 

obtained through increase in power consumption only, but the rate 

of increase is so rapid that large increases in quantity of flow without 

change of the resistance of the airway system often result in impracticable power increases; then the only practical solution is a reduction 

of the resistance through physical changes in the airways or in t.he 

way they are used. 

EFFECT OF AIRWAY CONDITIONS ON POWER REQUIREMENTS 

Since the power requirements vary directly with the resistance, 

which is represented by R for the airway or system of airways, the 

effect of the various physical conditions of the airways on the power 

requirements for const.ant quantities of flow can be readily examined 

through the formulas for R. 

SIZE OF AIRWAYS 

For airways of similar shape the value of R, and therefore the power 

requirements for friction loss for constant quantities, varies inversely 

as the five-halves power of the area and, since the change in area is 

twice the change in a similar dimension, as the fifth power of similar 

dimensions. For an area ratio of 2 to 1 the power ratio is 1 to 5.66; 

whereas for a dimension ratio of 2 to 1 the power ratio is 1 to 32. 
FLOW OF AIR IN MINE OPENINGS 81 

On account of this preponderating effect of size of airways on power 

requirements due to friction pressure losses the provision of airways 

of ample size, whether single or multiple, is a primary consideration 

in obtaining a maximum ventilating effect in either naturally or 

mechanically ventilated mines and in realizing minimum power 

requirements in the latter. In practice this means large main airways, through which the total flow or large portions of it pass, and 

splitting of the flow in the workings so that the combined area through 

which the total fldw passes is as large as is consistent with the general 

plan of the mine workings and full utilization of the velocity of the 

air current. 

The preponderating effect of both area and quantity of flow on 

friction-loss power requirements and power costs is illustrated in 

table 3, which shows a ratio of power and power costs of about 21 to 1 

as between a 4- by 6-foot and an 8- by 10-foot airway and of 64 ~o 1 

as between flows of 25,000 and 100,000 c. f. m. Other assumptIOns 

for length, friction factor, over-all mechanical efficiency, and unit 

cost of power would, of course, alter the actual figures for power 

and power costs but would not change the ratios. 

The effect of size is almost as pronounced for shock losses as for 

friction losses; the value of R in this case, however, varies as the 

square of the area or the fourth power of similar dimensions instead 

of the five-halves and fifth powers, respectively. 

TABLE a.-Comparative power costs due to friction pressure losses for 1 ,ODD-foot 

lengths of straight timber-lined airways 

Yearly power cost at 60 per· 

Area of Resist· Air horsepower for- cent over·aIl efficiency and at 

ancefac· 1 cent per kilowatt·hourforSize of airway (feet) airway tor (R) (square forK= feet) 0.0 7 1 25,000 50,000 100,000 25,000 50,000 100,000 c. f. m. c. f. m. c.f.m. c.f.m. c. f. m. c.f.m. 

--------------------- 4 by6 ________________ 24 27.82 6.85 54.8 438 $746 $5,965 $47,716 5 by7 ________________ 35 10.77 2.65 21.2 170 289 2,308 18,472 6by8 ________________ 48 4.87 1. 20 9.6 77 131 1,044 8,353 7by9 ________________ 63 2.46 .61 4.8 39 66 528 4,219 8 by lO _______________ 80 1.35 .33 2.7 21 36 290 2,316 

CHARACTER OF SURFACE OF AIRWAYS 

The effect of roughness of the walls of the airway on resistance and 

power requirements is measured by direct comparison of the corresponding friction factors. As between the normal minimum and 

maximum conditions found in metal-mine airways, there is a range 

of about 1 to 10. If the pressme is constant the quantity of flow 

will vary as the square root of the ratio of the corresponding friction 

factors, a relation easily established by transposing the. basic formula 

for friction pressure losses. The comparative sizes of different types 

of mine airways required to pass equal volumes of air with equal 

friction pressure losses are shown graphically in figure 26. 

SMOOTH-LINING AIRWAYS 

In most metal mines the major part of the total resistance to flow 

and thus the major consumption of power occurs in the main intake 

and return airways as these are commonly limited in size and carry 
82 ENGINEERING FACTORS IN VENTILATING METAL MINES 

.S9· diamete 

Equivalent shaft areas 

Smooth-lined .. ~::'.~ .. ~.:.:':':::r.17.31 sq. ft. 

Smooth lined (circular) 16.53 sq. ft. Sedimentary rock~26.88 sq. ft. Timbered. __ .-=_32.94 sq. ft. Igneous rock __ 38.76 sq. ft. 

FIGURE 26.-Relative sizes of airways for equal pressure and power requirements: A, Square shaft sections: 

B, rectangular drift sections. 
FLOW OF AIR IN MINE OPENINGS 83 

the total air current. Where these airways are types that have high 

factors of frictional resistance, such as timbered airways, the quantity 

of flow can be greatly increased by smooth-lining the airways. If all 

of the airways of a mine were changed from timber-lined to smoothlined airways and the pressure remained constant the air flow would 

be increased about 2.2 times; a similar change confined to the main 

intakes and returns ordinarily would at least double the quantity of 

flow. In a naturally ventilated mine the increased power requirements would be supplied gratis. In a mechanically ventilated mine 

some of the increase in power requirements usually would be supplied 

gratis by the accompanying natural draft, and the speed of the fan 

could be reduced to bring the fan-power requirement back to its 

original value without material sacrifice of quantity or the quantity 

back to its original value with a large reduction in fan-power requirements. 

SHAPE OF AIRWAYS 

Compared to other factors shape of airway has a relatively slight 

influence on resistance and power requirements. Its effect on friction 

resistance is measured directly by shape factor F. If area and type 

of surface are fixed shape assumes some importance. The ratio of 

perimeter to area is the minimum for a circular shape, which is 

therefore the most advantageous shape as respects friction losses. 

A regular octagon has almost the same shape factor as a -circle, and 

since this shape is better adapted to timber linings it is being used 

to an increasing extent in the metal :mines of this country, particularly 

at Butte, Mont. If the relative resistance for a circular shape is 

designated as 1 then the values for other shapes are given by the 

shape ratios of figure 23. A range of approximately 1 to 2 for normal 

mine-airway shapes therefore offers some scope for obtaining minimum 

resistance to flow through attention to shape of airway, which is 

primarily a matter of original design rather than of alterations. 

The resistance due to shock pressure losses is also affected to some 

extent by the shape of the airway, which affects the aspect ratio of 

bends and the degree of symmetry at area changes. 

OTHER FACTORS 

Resistance to flow and thus the power requirements are deter:mined 

by both friction pressure losses and shock pressure losses, and any 

method of reducing either of these reduces the power requirements 

for constant quantities. 

Friction pressure losses vary directly with the length; although 

length is usually a fixed factor it often can be adjusted to the mininmm in planning a:mine lay-out, or it can be reduced effectively by 

changing the existing lay-out of the distribution system. 

Losses due to bends and changes of area can be reduced to the 

minimum by careful planning, proper alterations, and attention to 

mining details, such as removing obstructions from air courses. The 

exact procedure for any particular case is governed by the velocity 

of flow, since shock pressure loss is a function of the velocity of flow 

as determined by the quantity and area. 
84 ENGINEERING FACTORS IN VENTILATING METAL MINES 

ALLOWABLE COSTS FOR REDUCING MINE RESISTANCE 

Small main airways containing necessary bends and obstructions 

are economic conditions that must be met in metal-mine ventilation 

and offer a large field for improvements of a payable nature in all 

systematically ventilated mines. Most changes in metal-mine ventilation systems are made to increase the quantity of flow, and expense, 

although important, is secondary. Low-velocity flow involves so 

little power that costly changes in airway conditions or modifications 

of mining procedure cannot be economically justified. With highvelocity flow such matters as providing large-radius bends and removing or stream-lining obstructions require careful investigation although 

on account of the comparatively short time element involved in most 

metal-mining installations somewhat crude and relatively cheap 

installations are often more economical than elaborate and expensive 

types designed to reduce the pressure loss and power cost to the minimum. In judging the economic advantage of a change in airway conditions with respect to power requirements the saving in power costs 

should at least balance the capital charges-interest and amortization-of the investment required. In roughly estimating the advantage of a change it is convenient to remember that 100,000 c. f. m. at 

I-inch pressure costs $1,000 per year at one half cent per kilowatt-hour 

with an over-all efficiency of the fan and drive of 51.5 percent. Costs 

for other quantities, pressures, and unit costs are then a matter of 

direct proportion. Velocity pressures at standard air conditions are 

approximately 1 inch at 4,000 f. p. m., one fourth inch at 2,000 f. p. m., 

and one sixteenth inch at 1,000 f. p. m. The amount that might 

economically be spent for each dollar of power saved is then primarily 

a question of the useful life of the installation, which is largely uncertain and therefore justifies approximate solutions only. Any increase 

in maintenance charges should, of course, be deducted from the 

power-cost saving; similarly, any decreases should be added. 

ECONOMIC DESIGN OF MINE AIRWAYS 

Although changes in existing airway and fan-installation conditions 

offer the most common opportunities for effecting economical operation of mine-ventilating systems the largest possibilities for obtaining 

this result lie in the original design of the main airways. Most of the 

openings so used are originally designed for other operating purposes. 

Natural conditions, suoh as heavy ground, often seriously limit the 

size of opening that can be maintained without excessive expense; 

and various operating factors, such as velocity limitations on traveling roads, often dictate size requirements in excess of those for 

economy in ventilation only. 

Increasing the area of an airway increases the cost for excavation 

and lining almost proportionately but effects a very rapid decrease 

in power requirements. Balancing these two factors of expense to 

give the minimum total yearly operating cost for ventilation is 

mainly a matter of proper original design of the main airways of the 

mine, since the large volumes handled through these parts of the system involve large power costs and thus present opportunities for 

effecting large savings. 

Total yearly costs for coursing air through mine airways are primarily a question of unit costs, quantity of flow, and type and size 
FLOW OF AIR IN MINE OPENINGS 85 

of airway. All but the latter usually are fixed by the conditions 

obtaining, and design centers on the selection of a type and size of 

airway that will give minimum yearly costs for the existing conditions. 

This calls for mathematical analyses involving a dozen or more 

variables that differ not only in range of values met in practice but 

also in rates of variation. The' subject is too involved for adequate 

discussion here; it has been separately treated in a recent paper,27 in 

which formulas and charts are presented for the determination of the 

elements of design where the conditions of service can be roughly 

approximated. The mathematical analyses made therein lead to the 

following conclusions: 

1. The area for maximum economy is determined both by the 

absolute value of a dozen or more separate factors and by the relative value of the 2 groups-capital-cost factors and power-cost factors-into which they naturally fall; the relation between these 2 

groups largely determines the area required for economy, which might 

vary as much as 4 to 1 for probable variations in practice. 

2. Of the separate factors involved quantity of flow is the most 

important in that it affects the economic area at a rate three times 

that of all of the other separate variables. Range of variation in 

probable values of the other separate factors determines their importance as affecting economic area. Character of wall surfaces and 

ratio of unit cost of lining to unit cost of excavation are the two other 

separate factors of major importance. Unit cost of excavation, unit 

cost of power, shape, thickness of lining, service life, rates of interest, 

and mechanical efficiency of fans have but minor effects on economic 

area, decreasing in importance in the order given. 

3. Where operating conditions limit the size of airways and require 

larger or smaller areas than the area for maximum economy the approximate cost of the limiting condition may be determined by the 

methods of mathematical analysis for economic area and cost. 

4. When the area of either airway or fan-tubing line is that for 

maximum economy the yearly power cost is about 40 percent of the 

yearly capital cost if friction pressure losses only are involved. Assuming, as an average figure for metal mines, that the yearly capital 

charge is 20 percent of the original cost of construction, then the 

yearly power cost should be 8 to 10 percent of the original total cost, 

depending on the ratio of shock loss to friction loss. 

The results of many calculations for assumed conditions lead to 

the following deductions: 

1. Total costs per unit quantity of flow may vary as much as 20 

to 1 for probable variations in the values of the variables involved, 

particularly unit costs. 

2. Under conditions of comparable unit costs a probable range of 

variation of about 4 to 1 in total costs per unit quantity of flow, as 

affected by design, seems possible. 

3. Shape of airway has only a minor effect on total costs and 

assumes importance only when the other major factQrs of design are 

fixed. On a constant-factor basis circular airways have a slight 

advantage over single-compartment rectangular airways, and the 

latter have a slight advantage over multicompartment airways. 

" McElroy, G. E., Economic Size of Metal-Mine Airways: Inf. eire. 6585, Bureau of Mines, 1932,21 pp.; Trans. Am. Inst. Min. and Met. Eng., vol. 102, 1932, pp. 59-85. 
86 ENGINEERING FACTORS IN VENTILATING METAL MINES 

EFFECT OF DESIGN ON ECONOMIC AREA AND TOTAL YEARLY COSTS 

In judging the economic value of different types of openings for 

use as airways the total yearly operating cost-power cost plus 

capital charge-is a proper criterion, provided the values assigned the 

various variables agree with average practice. The selection of a 

representative set of values is largely a matter of personal judgment 

and experience and is therefore subject to controversy. However, in 

order to determine approximately the effect of various variables on 

area and costs, the author has assumed sets of roughly corresponding 

values for the various variables and has calculated 3 groups of area 

and cost data for 100,000 c. f. m. through 1,000 feet of airway; the 

results are tabulated in table 4. In the first group various types of 

airways are compared for assumed average values of the variables. 

They show that, where feasible, unlined airways cost about half as 

much as lined airways; that circular airways have a small advantage 

over rectangular airways; and that single compartments have only a 

slight advantage over multicompartment airways. In determining 

the relative value of several types of lined airways, where the unit 

cost of excavation is constant, the relative unit costs assumed for 

lining practically control the results obtained. The figures therefore 

hold only for the relative values shown in the table and, as assumed, 

indicate slight advantages for thin-board lining inside open-timber 

framing over solid cribbing, for the latter over concrete lining, and for 

concrete lining over open-timber lining. Small changes in the 

assumed unit cost of lining in each instance, however, would change 

the results slightly, but the differences are so small that in any 

particular case they might easily be offset by safety, operating, or 

maintenance advantages or disadvantages. 

TABLE 4.-Design factor Z for common types of metal-mine airways for selected 

values 1 of the controlling factors, economic size for flow of 100,000 c. f. m., and 

yearly costs per 1,000 linear feet of airway 

... 

'" For tlow of 100,000 c. f. m. (q) .9 

"" 

.~Z' I 

~ Annual expense Type of airway and lining (ratio of .., ~ "<!Z- S longer to shorter side of rectangles ~ ==~ ,; ~ .;. per 1,000 feet of 

1.25 in all cases) and relative -" 0 ofg$ airway 

" 0._ ",- :!l value of factors <S ~.g "t .~~ :6 <S 0 

Ol 0" '03 t' 'd" :3 " Ol " .~ ... 

"i ,,0' '6 ...,"" "'- 'd_ 

" =E ",,e 0 'i5.:! ~'" 

~ ... p A r;.. A ~ 0" "'''' ~8 ~g ------- ------

Average value of factors: 

Unlined circular, in sedimentary 0.0'6 ------ 1.440 74.6 9.75 diameter ____ 1. 341'$5. 595 $2,238 $7, S33 rock. 

Unlined rectangular, in sedi- .0'6 ------ 1. 493 77.3 7.87 by 9.83.. ____ 1.293 5, SOO 2,320 8,120 

mentary rock. 

Unlined circular, in igneous rock_ .0'15 ------ 1.871 96.9 11.11 diameter ___ 1.032 7,267 2,907 10,174 

Unlined rectangular, in igneous .0715 ------ 1.940 100.5 8.97 by 11.21.. ___ .995 7,535 3,014 10,549 

rock. 

Rectangular, open-timber, .0'2 $0.60 .768 39.8 5.64 by 7.05 ______ 2.515 10,198 4,079 14,277 

smooth-lined. 

Rectangular, solid timber-crib .082 .SO .742 38.4 5.54 by 6.93.. ____ 2.604 11,119 4,447 15,566 

lining. Circular, concrete Iining _________ .0'2 1.50 .670 34.7 6.65 diameter ____ 2.881 12,601 5,041 17,642 

Rectangular, concrete Iining .. _____ .082 1.50 .674 34.9 5.28 by 6.61.. ____ 2.866 14,131 5,653 19,784 

1 Low, average, and high values of factors not listed are as follows: Over-all mechanical efficiency, 55, 

60, and 65 percent; lining thickness in percent of longer dimension or diameter, 15, 20, and 25; capital return (interest and amortization), 10, 15, and 25 percent; unit cost of power, $50, $100, and $150 .l!er horsepoweryear (0.77, 1.53, and 2.31 cents per kilowatt-hour); and unit cost of excavation, $0.25, $0.50, and $0.75 

per cubic foot ($6 75, $13.50, and $20.25 per cubic yard). 
FLOW OF AIR IN MINE OPENINGS 87 

TABLE 4.-Design factor Z for common types of metal-mine airways for selected 

values of the controlling factors, economic size for flow of 100,000 c. f. m., and 

yearly costs per 1,000 linear feet of airway-Continued 

... 

" For flow of 100,000 c. f. m. (q) 8 

.~~ I ~ S Annual expense Type of airway and lining (ratio of ... 

... ' ~.s ~ "i~ g per 1,000 feet of longer to shorter side of rectangles .£ .: ," p. airway 1.25 in all cases) and relative " 'Cl.S j "'~ l!l value offactors $l ~'§ ~f :c!-

" 0" "''' 

:1 

b ~ " .~ " 3~ ... "a' ·S ~~ .,~ 

" 'S .,~ oS ·a~ .... '" '1:: " ... '" ",.<:I 00 0 0 

I'< P A I'< A »- 0" 1'-<" 8" 

-- Average value of factors-Contd. 3 rectangular 

concrete Iiuing. compartments, 0.0'2 1. 50 0.831 43.1 3-3.4 by 4.2 ...... 2.322 14,468 5,787 20,255 

Rectangular, open·timber lining. . 0'9 .50 1. 203 62.3 7.06 by 8.82 ...... 1. 606 14,945 5,978 20,923 3 rectangular compartments, • 0'9 .50 1.470 76.2 3-4.5 by 5.61.. ... 1.313 15,650 6,260 21,910 

open·timber lining. Unlined rectangular airway in ig' neous rock: 

Values of factors selected to give: Low capital costs-low power .0'9 ------ 1.840 costs. 95.3 8.74 by 10.92 .••.. 1.050 2,382 953 3,335 

Low capital costs-high power costs . . 0721 ------ 3.365 174.3 11.81 by 14.76 .... .574 4,357 1,743 6,100 

. Average capital costs-average .0715 ------ 1.940 100.5 8.97 by 11.21.. ... .995 7,535 3,014 10,549 

power costs. High capital costs-low power costs. .0'9 ------ 1.035 53.6 6.55 by 8.19 ...... 1.867 10,045 4,018 14, 063 

High capital costs-high power .0721 ------ 1.892 98.0 8.85 by 11.07 .•.. 1.021 18,373 7,349 25,722 costs. 3 rectangular compartments with 

open·timber lining: 3 compartments Values of factors selected to give: each: 

Low capital costs-low power .0'8 .25 1. 648 85.4 4.77 by 5.97 .... 1.171 4, 824 1,930 6,754 costs. 

Low capital costs-high power .0710 .25 2.522 130.7 5.90 by 7.38 .... .765 7,385 2,954 10,339 costs. 

Average capital costs-average .0'9 .50 1.470 76.2 4.51 by 5.63 .... 1. 313 15,650 6,268 21,910 

power costs. High capital costs-low power .0'8 .75 .835 43.2 3.40 by 4.24 .... 2.313 26,431 10,573 37,004 costs. 

High capital costs-high power costs. .0710 .75 1. 277 66.2 4.20 by 5.25 .... 1.511 40,442 16,177 56,619 

EFFECT OF RELATION BETWEEN COST FACTORS 

A much larger variation in possible total yearly costs is shown in 

the second and third groups of calculated data in table 4, where the 

effect of varying relations' between the factors that influence power 

cost and those that affect capital charges is shown for 2 widely different types of airways under 5 different combinations of power- and 

capital-cost factors. Here the personal estimate of maximum, minimum, and average values of factors affects only the range of the 

results. Unit costs largely determine the total yearly costs, but the 

area for maximum economy is largely a question of balance between 

the two sets of factors. Unit costs for ventilation, although necessarily including many other items, cannot help but vary accordingly 

and therefore do not furnish, by themselves, any direct basis for 

judging the degree of economical planning and operation of the ventilating system. 

ECONOMICAL VELOCITIES 

Velocities for maximum economy are also shown in table 4. They 

are subject to the same variations as the areas for maximum economy, 

but in the reverse direction. The economical velocity therefore also 

depends on the particular conditions obtaining and cannot be judged 

offhand from a small set of sample values. 
88 ENGINEERING FACTORS IN VENTILATING METAL MINES 

EFFECT OF QUANTITY ON ECONOMIC AREA 

If the effect of all variables other than quantity are combined in a 

single factor, Z, which is here termed the "design factor", the relation 

400 

)00 

200 

100 

90 

so 

10 

IiO 

50 

~ 40 

"......... 

~~JO .-.:;;- N 

, 

< 20 

1.0 

.9 . 

• 8 

.7 

.6 

.5 

'0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0 G 7 8 9 10 

DESIGN FACTOR,' Z 

FIGURE 27.-Chart for determining economical area for mine airways from design factor and quantity oftlow. 

between economic area Am in square feet and quantity of flow q in 

cubic feet per minute may be expressed by 

( q )!! A .. =Z 1,000 7, 

(34) 
NATURAL VENTILATION 89 

and the effect of quantity on area may be gaged by reference to figure 

27, which is a chart expressing this relation. The solution of an 

example-Am=85.4 square feet, when Z= 1.65 and q= 100,000 

c. f. m.-is shown in dotted lines on the chart and indicates the method 

of procedure. 

DESIGN FACTORS 

Although design factors are determined by a large number of variables those listed in the first group of table 4 can be used as a rough 

approximation for general practice wherever there is a semblance 

of a balanced relation between power-cost factors and capital-cost 

factors. This is the normal condition; at small mines both sets of 

cost factors are likely to be high; at moderate-size mines, medium; 

and at large mines, relatively low. 

NATURAL VENTILATION 

The pressure differences required to cause flow-pressure gains in a 

flow circuit as opposed to pressure losses-may be generated by 

natural or mechanical forces. The most common natural force for 

generating pressure differences is that due to unequal densities of air 

columns in and adjacent to the mine openings. The flow resulting 

from this type of pressure generation is termed "natural ventilation" 

and the pressures generated, "natural-draft pressures." Other natural 

forces, such as surface winds and falling water, may be utilized to 

cause flow but are specially designated in each instance; the general 

terms refer, by common usage, to flow produced and pressures created 

by air-density differences. Pressures so generated also act to cause 

complete flow circuits in undivided dead-end openings, but such air 

currents are designated as "convection currents" and will be discussed 

later. 

Most small metal mines and many large ones are ventilated entirely 

by natural means, and natural-draft pressures play an important 

part in the ventilation of mechanically ventilated mines, the degree 

of importance increasing with depth and the rate at which the virgin 

temperature of the rock mass increases with depth. 

NATURAL-DRAFT FLOW CONDITIONS 

PRESSURES 

Natural-draft pressures are differences in total weights of air 

columns of unit cross-section for the same differences of elevation or 

vertical height. The differences in vapor contents or average absolute pressures of any two columns under consideration is small, so 

that the important factors in natural draft are temperature differences and vertical distances. 

The columns concerned in natural draft extend into the atmosphere 

as high as temperature or pressure differences exist, but those for 

surface openings relatively close together are assumed to be (and 

usua¥y are) balanced above the level of the uppermost surface 

openmg. 

Columns of equal height must always be considered, whether for 

the mine as a whole, a part of a mine, or a separate circuit under122422°-35-7 
90 ENGINEERING FACTORS IN VENTILATING METAL MINES 

~round. Natural draft acts between all connected openings involvlllg vertical differences; and each separate circuit, although it may 

overlap with others, develops its own natural-draft pressure, which is 

used up by the pressure losses accompanying flow. Part of the 

columns considered may be underground and part in surface air. 

Except for a few special cases, such as a mountainous region like the 

Coeur d'Alene district of Idaho, the surface components or those 

parts of the total pressure generated by the columns between the 

elevations of surface openings are seldom important because they 

usually act over insignificant vertical distances compared to the 

underground components. 

TEMPERATURE CHANGES OF UNDERGROUND AIR 

Although the surface components usually are small the temperature 

of the surface air is highly important, since it usually has a range 

50 

-30 

14 15 16 17 i 18 

JANUARY 1927 

Main shaft_2,5OQ 

19 20 21 22 

FIGURE 28.-Dry-bulb temperature records at surface and in two adjacent timber-lined downcast air 

shafts; average velocity, 750 to 850 f. p. m. in main shaft and 1,500 to 1,700 f. p. m. in air shaft. 

throughout the year of at least 100° F. and determines the temperature at which air enters the underground openings_ The temperature of air flowing underground is changed more or less rapidly by 

contact with the rock mass (see fig. 28), which acts as a huge heat 

interchanger. The effect of surface-air temperatures on undergroundair temperatures is primarily a question of velocity of movement as 

governing the time of contact. With very low entrance velocities 

the effect of variations in surface-air temperatures may be im-perceptible only a few hundred feet from the surface, whereas with highvelocity flow, common to mechanical systems, a yearly difference of 
NATURAL VENTILATION 91 

as much as one tenth of the surface variation may be noted as far as 

3,000 feet below the surface. 

DIRECTION OF FLOW 

In general, air flows from the colder to the warmer column. In 

vertical or inclined openings the colder column sinks, while the warmer 

column rises; thus the opening containing the colder air is a downcast 

and that containing the warmer air an upcast. 

Variations in surface-air temperature produce seasonal changes in 

the temperature of underground columns, confined largely to the upper 

parts of the downcast airways, which are large enough normally to 

reverse the flow seasonally wherever the surface components are large 

or the mines comparatively shallow. In deep mines surface-air 

changes usually are not sufficient to reverse the flow, except through 

cross-connections near the surface; the directions of the main flows 

remain constant the year round, although the seasonal effect is 

reflected in the variation in quantities of flow. As depth increases 

the seasonal effect is diminished because it is exerted over smaller 

proportional parts of the total length of the air columns involved. 

Where conditions are such that reversal occurs, ventilation of the 

mine or parts of it may be sluggish or entirely lacking for hours, days, 

or weeks. There are two such periods-late spring and late fall. 

Often the flow 'will be reversed daily over a considerable period 

on account of the difference between day and night surface-antemperatures. 

QUANTITIES OF FLOW 

As the temperature differences usually are small and variable so 

also are the natural-draft pressures and the natural-draft flows, 

although where the openings are large and numerous the small pressures generated often produce ample flows even in large mines. 

Virgin mine-rock temperatures increase with depth and approximate 

the average yearly temperature of the air close to the surface. Mineair temperatures, which are largely affected by rock temperatures, 

therefore average higher than surface-air temperatures, and larger 

natural drafts are produced when the surface temperature is below 

its average for the year than when it is above the average. In a 

naturally ventilated mine the circulation is better in the winter than 

in the summer, and in a mechanically ventilated mine the naturrudraft pressure acting in series with the primary fan circulation is 

much larger in winter than in summer. 

INTENSITY OF NATURAL-DRAFT PRESSURES 

In a shallow mine the natural-draft pressure affecting the total 

flow usually is a few hundredths to a few tenths of an inch of water, 

whereas in a deep mechanically ventilated mine it may range from 

about an inch in the summer to as much as 3 inches of water in the 

winter. 

In a tunnel the 2 columns above the 2 portals may have different 

temperatures up to the height of the highest intervening surface, and 

the natural-draft pressures generated, although extremely vanable, 

may be quite large at times. Absolute-pressure measurements at 

the portal positions of the 6-mile Moffat Tunnel in Colorado showed, 
92' ENGINEERING FACTORS IN VENTILATING METAL MINES 

over a period of a year, maximum differences of more than 5 inches of 

water in winter and 2 inches of water in the opposite direction during 

the summer season, accompanied by faster rates of variation than 

those .prevamng in mines; both columns were affected by rapidly 

changmg surface-air temperatures on opposite sides of a high mountain. 

NATURAL-DRAFT MEASUREMENTS 

Differences in total weights of columns of air between equal elevations by definition equal the natural-draft pressure. If a stopping is 

erected at any point in an airway circuit so that it stops the flow 

entirely the natural-draft pressure may be measured as the difference 

of pressure on the two sides of the stopping. If all the conditions that 

control the weights of the air columns lue the same when the stopping 

is removed and flow resumed the pressure measured is the pressure 

causing that flow. In many instances the conditions obtaining are 

sufficiently exact to permit good approximations, since underground 

temperatures change very slowly, and the measurements may be made 

when surface temperatures are also changing slowly. It is often 

impracticable, however, to stop the flow of a mine-ventilating system, 

in which case the natural-draft pressure can be calculated from observations of temperatures and absolute pressures, usually with greater 

accuracy. 

CALCULATIONS 

The weight of 1 cubic foot of moist air may be calculated from 

formula (4) or determined from figure 7. Temperature conditions are 

so variable in mines that rigid mathematical solutions for average 

te~peratures and absolute pressures are impracticable. Temperature and flow-pressure changes generally tend to make the densitychange curve virtually a straight line, so for practical application it is 

accurate enough to determine the average density of each column from 

weighted averages of temperatures and absolute pressures for separate 

sections of the column, subtract the average densities to find the 

average difference in weight in pounds per foot of column, and multiply 

by the number of feet of column to obtain the natural-draft pressure. 

The result is pressure in pounds per square foot, which may be converted to inches of water by dividing by 5.2. Results by this method 

can be accurate to within 2 or 3 percent. It is important to note, 

however, that the average temperature of a variable-temperature 

column is not one half the sum of the top and bottom temperatures; 

0. fair average for a deep-mine downcast column can be obtained from 

0. few closely spaced observations neal" the surface, coupled with rather 

widely spaced observations below the point of maximum surfacetemperature effect. As upcast temperatures usually change very 

slowly, a good average can be obtained from a few widely spaced 

observations in the absence of large leakage conditions. Absolutepressure determinations at the same elevation just below the centers 

of the two columns under consideration are sufficiently close to the 

average for good approximations. 

The effect of differences in absolute pressure and vapor contents of 

air columns on the natural-draft pressures varies, of course, with the 

conditions but is rarely more than 5 percent for each in a naturally 

ventilated mine or more than 10 percent for each in a mechanically 

ventilated, deep, hot mine. For rapid estimates of such a highly 
NATURAL VENTILATION 93 

variable quantity the effect of variations in average absolute pressures 

and vapor contents in the columns may be neglected and the following formula employed for calculating the approximate naturaldraft pressure directly: 

(35) 

where Hn is the natural-draft pressure, inches of water; 

B is the average absolute pressure, inches of mercury, obtained approximately by direct measurement near the center of the columns or from a measurement at any elevation corrected for difference in elevation 

from the center at the rate of 1 inch change per 1,000 feet or more closely if the rate of change is known; 

L is the vertical height of the air columns, feet; and 

Tl and T2 are the average absolute temperatures of the columns" OK, 

obtained as weighted means according to lengths in the C8IIIle d 

variable temperatures. 

The intensity of natural-draft pressures may be roughly estimated 

as about 0.03 inch of water for each 10° F. difference in average temperatures for each 100 feet difference in vertical elevation at standard 

air density. 

The fact that the small difference in elevation between the 2 ends 

of a connection between vertical or inclined openings forms part of 1 

of the columns under consideration may be ignored for almost horizontal connections, which have an allowance for grade only, without 

perceptible error. 

APPLICATION OF NATURAL-DRAFT MEASUREMENTS 

N atural-dra,ft measurements are particularly useful when changes 

in the ventilating system in either a naturally or a mechanically 

ventilated mine are desired, in which case a more or less close approximation of the resulting changes in quantities and distribution of the 

flow may be made, depending on the complexity of the distribution 

system. The pressure losses accompanying flow are the same 

whether the pressures causing the flow are generated naturally, 

mechanically, or by both agencies. The flow conditions, however, 

are determined by the positions of the pressure-generating sources in 

the flow system. The resistances of separate circuits and separaoo 

airways may be determined in any event, and the flow may then be 

calculated according to the ,Pressure types and positions. In changing from natural to mechamcal ventilation it is particularly desirable. 

to determine the resistances 28 to flow by measurement of naturaldraft pressures and flows as a check on calculated values; and in 

attempting to increase quantities of flow in a mechanically ventilated 

mine it is important to know the intensities of the natural drafts 

that may operate with or against the fan pressures in causing the 

primary flow, so that fan speeds and fan horsepowers may be calculated correctly for the new duties, as the application of the so-called 

"fan laws" only (see following section) is not a complete solution. 

" McElroy, G. E., and Richardson, A. S., Determining the Air-Flow Resistance of a Small Shaft MiDD 

by Natural Draft: Rept. of Investigations 2890, Bureau of Mines, September 1928, 15 pp. 
94 ENGINEERING FACTORS IN VENTILATING METAL MINES 

SIMPLE FLOW CIRCUITS 

For a tunnel through a hill, the simplest case, the columns to be 

considered, as shown in figure 29, A, extend from the lowest portal 

to the top of the intervening surface. Where the vertical distance is 

small the portal temperatures and the absolute pressure at either 

port.al may be considered a.s the average temperatures and pressures 

of the columns. Here the draft is effected entirely by surface components, since both legs of the columns involved are almost wholly 

in surface air. Where the vertical distance is large and the portals 

at a distance from each other the data for density calculations are 

impracticable to obtain, and the best procedure is to measure the 

absolute pressures at both the portals, add the calculated weight of 

the column in the tunnel to the pressure at the upper portal, and subtract tIllS result from the pressure reading at the lower portal. If the 

result is minus, the air will flow from the upper to the lower portal; 

if plus, from lower to upper. Quantities of flow can be computed 

readily if the resistances in series are known or can be estimated. 

-;..;0-",""",,-- ----n-Ii 

II 

1/ 

1/ 

II 

II 

'I 

II 

,r~I======~T~un=De71======~~ 

" Adit and over·th~hi1l shaft 

B 

Adit and sidehill shaft:. 

-'1----- ---- 'I 

I, 

" 

Sidehill shafts 

FIG'C'RE 29.-Xatural-draft columns for some simple flo~,v circuits. 

Without att.empting to show small differences for grade, figure 29 

shows the columns that act to create the natural-draft pressure differences in some simple flow circuits: A, for a tunnel; B, for an adit and 

sidehill shaft; C, for an adit on one side of a hlll and a shaft on the 

opposite side; and D, two shafts on sloping ground. If the surfaceair temperatures were always the same as the underground-air temperatures no flow would result. With the surface temperatures 

colder than the average nUne-air temperatures flow would be in the 

adit and up the shaft for Band C and down the shorter shaft and up 

the longer shaft for D. With the surface-air temperature hlgher 

than the average mine-air temperature the flow would be just the 

reverse, or out the adits and shorter shaft. 

COMBINED FLOW CIRCuITS 

Figure 30, A, represents a simple type of combined circuit. The 

main flow through the adit and the upper part of the inclined shaft 

is produced by the difference between columns a and b, whereas the 
NATURAL VENTILATION 95 

flow through the inside loop is produced by columns c and d and is 

independent of the main flow. If columns a and b were balanced so 

that no flow entered air would still circulate around on the inside 

loop according to the pressure generated by columns c and d. With 

circulation on the outer circuit the proportion of intake air circulating 

on the inner circuit would depend on the conditions at the junction 

of the adit and shaft, particularly the temperatures of the air currents 

there. The usual result would be a rather large proportion of intake 

air circulating on the inside circuit. However, an inherent disadvantage of natural ventilation is that flow conditions are controlled 

only by the pressures generated, and recirculated air satisfies the conditions just as well as intake air. With a bypass for the air flow on 

the adit at the junction with the inclined shaft the pressure producing 

flow in the then single circuit would be the difference between columns 

a+c and b+d, a lay-out that would generally result in much better 

ventilation. 

COMBINED AND SPLIT-FLOW CIRCUITS 

Most metal-mine lay-outs are a combination of combined circuits 

and overlapping or split-floW circuits. Calculations of distribution 

',' 

" II 

,I 

II 

II " 

II 

II 

B 

FIGUBE 3O.-A, Natural-draft columns for a simple type of combined flow circuit; B, natural-draft columns 

for shunt types of split·flow circuits. 

then require the solving of simultaneous algebraic equations embracing as many equations as unlmowns. 

The most common type of metal-mine lay-out of airways is a 

multilevel system similar to that sketched roughly in figure 31, A. 

Here there are four overlapping circuits (acb, adb, aeb, and alb) 

through each of which the sum of the pressure losses occasioned by 

the necessarily varying quantities in each part of each circuit must 

balance the pressure generated by differences in weights of the columns 

(Cl and C2, dl and d2, el and e2, and 11 and 12, respectively) above the 

bottom of each circuit. Calculation of the distribution for this mine, 

when the separate components of natural-draft pressures and resistance factors for the separate airway sections are lmown, wo~~d involve 

5 unlmowns-total quantity of flow and separate quantItIes across 

each of the 4 horizontal connections. Five algebraic equations would 

therefore be required; 1 can be obtained by equating quantities of 

flow-the total flow is the sum of the 4 cross-flows-and the other 4 

can be obtained by equating the natural-draft pressure for each 

circuit to the sum of the pressure losses otthe circuit. The solution 
96 ENGINEERING FACTORS IN VENTILATING METAL MINES 

for quantities is, of course, tedious, as are all mathematical solutions 

of distribution for actual airway systems; in practice it is often necessary to omit much detail from consideration and confine the mathematics to the main airway system only. 

DISTRIBUTION OF FLOW 

With mechanically generated pressure acting at a single point in 

the airway system the solutions for distribution are comparatively 

simple, since combined resistance factors may be computed for the 

mine, or section of a mine, as a whole, and a direct relation for total 

quantity of flow and fan pressure may be obtained. However, in 

the naturally ventilated mine just discussed the distribution corref 

I 

c 

I c 1 

I d I 

I e I 

[ f I 

u A L 

Nafur~l-draft characteristic 

T 

~ 

] 

L or\.-=~--------------~I--- ---'Quantity--- ~ 

B 

FIGURE 31.-A, Natural-draft columns for a multilevel type of split-flow circuit; B, pressure-quantity characteristics for natural-draft flow. 

sponds to that effected by 4 fans acting in parallel, 1 on each crossconnection, each operating at a pressure equivalent to that generated 

by natural draft above that level; in general, natural-draft distribution must be considered as similar to that produced by a large number 

of fans, 1 on every connection between vertical or inclined openings. 

Since in all mechanically ventilated mines natural-draft pressures 

are also generated throughout the flow circuit calculation of the 

exact distribution would require the same type of solution as required 

for natural draft alone, with the fan pressure added to the circuits 

in whicn the fan pressure .is generated; in this case the natural-draft 

pressures act as a group of fans operating in parallel with each other 
NATURAL VENTILATION 97 

but in series with the mine fan. In practice, detailed distributions 

are not calculated, but a procedure is followed that assures fan 

operation close to maximum efficiency and approximate equality of 

quantities and major distribution to that desired. The minor 

distribution is usually a matter of ever-changing detail and as such 

is solved as accurately as possible by a process of trial and error, 

guided by experience. The effect of natural-draft pressures on fan 

performances is discussed in a later section. 

RECIRCULATION 

The separate components of natural draft do not necessarily 

generate pressures that cause flows in the same direction; often the 

pressures oppose each other. Temperature changes are more in 

evidence near the surface than farther underground, and often the 

pressures generated above the upper cross-connection (see fig. 31, A) 

are in the opposite direction to those generated below that elevation, 

with the result that the air is recirculated at times from the return 

side to the intake side. This, of course, is a seasonal effect but 

often reaches large proportions in a naturally ventilated mine, even 

though the resulting· temperature changes act automatically to 

reduce the magnitude of such effects. A. more common result is 

that certain open connections between airways, particularly those in 

the upper part of a mine, carry large flows normally, yet contain 

stagnant air over long periods and are often filled with fog. 

The intensity of flow or quantity in circulation depends on the 

relation of pressure gains and pressure losses; if the airways and 

temperature conditions of an underground circuit are such that a 

certain amount will circulate, that certain quantity will be circulated 

regardless of the source of the air. Where the openings to the 

surface are small compared to the underground openings, as in many 

metal mines, large quantities of air circulate underground, although 

only a mere dribble of air may be entering and leaving the mine . 

. SHUNT-FLOW CIRCUITS 

A. common condition in metal mines is parallel flow through 2 

branches of a split, which has but 1 branch on a horizontal plane, 

with pressure generated by natural draft on the inclined or vertical 

branch. This might be termed a "shunt circuit", since one branch 

leaves and returns to a main airway and may be either above or 

below it. Temperature conditions in a mine are usually such that 

the location of the shunt circuit-whether above or below the levelis very important, since the temperature of the air in a main airway 

is usually lower than the rock in the vicinity, and feeble flows leaving 

a main flow tend to increase in temperature quite rapidly. 

Consider the conditions represented by figure 30, B. Flow of a 

certain intensity passes direct from A to B under the action of pressures generated naturally or mechanically on the main circuit. 

Under uniform temperature conditions, that is, without local generation of natural-draft pressure on the shunt branch, the flow through 

the branch would be that required to give the same pressure loss 

from A to B. But when flow occurs the general result is a warming 

of the air as it passes through the shunt circuit, which if above the 
98 ENGINEERING FACTORS IN VENTILATING METAL MINES 

level generates natural pressure opposing the direction of flow, 

whereas if below the level generates pressure acting with the direction of flow. For this reason circuits below the level are relatively 

easy to ventilate, whereas those above the level are relatively hard 

to ventilate; for good ventilation in workings above a throughairway it is necessary to cut an opening to the level above, increase 

the resistance of the main airway between the two openings of the 

shunt circuit, or brattice off the main airway entirely, thereby making 

the shunt circuit part of the main-airway circuit. The first remedy 

decreases the total resistance to flow, whereas the latter two increase 

it and thereby reduce the total flow on the main airway. The particular remedy would be governed by local conditions and the flow 

required in the shunt. In general, the most satisfactory ventilation 

conditions for overhand stoping result from mining systems that 

include an opening to the level above. 

Effect of gas-air mixtures.-Small percentages of gases in mine air 

do not affect natural-draft pressures appreciably through density 

differences, but the large percentages sometimes encountered locally 

in mines may affect them and thus the efficacy of the methods of 

dispersing local accumulations. Light gases, such as methane, act 

like heated air and are more difficult to remove from shunt circuits 

above a level than below it, whereas heavy gases, such as carbon 

dioxide, act like cold air and are more difficult to remove from shunt 

circuits below the level. Where one column is air and the other a 

methane-air mixture high natural-draft pressures may be generated 

by relatively short columns. 

CHARACTERISTICS OF NATURAL-DRAFT FLOW 

Although natural-draft pressures are not at all constant, particularly 

where large surface components are involved, they may be considered 

approximately constant with respect to changes in quantities of flow 

produced other than by changes in natural drafts, such as a change in 

fan speed. Such changes in quantities have but a minor effect in 

changing air temperatures over a short period. Under such conditions natural draft generally is considered a constant pressure-generating source; that is, the pressure remains constant while the quantity 

changes. An increase in quantity, with the pressure constant, results 

in an increase in power proportional to the increase of quantity; in a 

mechanically ventilated mine, therefore, the power supplied gratis by 

natural draft acting with the fan or that required by adverse natural 

draft changes approximately as the quantities of flow. The pressurequantity characteristic for naturally generated pressures is therefore 

a straight line, whereas that for mine-airway pressure losses is a parabolic curve expressing the relation of pressure varying as the square 

of the quantity. If both are plotted to scale, as in figure 31, B, the 

resulting quantity of flow will be indicated by the crossing of the two 

curves, termed" characteristics", as at point a. 

CONTROL OF NATURAL VENTILATION 

The main difficulty with natural ventilation is the relative lack of 

control over direction of flow, particularly during a mine fire, which 
NATURAL VENTILATION 99 

may alter the natural drafts and directions of flow in a very short time. 

Even in mechanically ventilated mines a fire in a shaft creates such 

powerful natural drafts as often to reverse the flow against the normal action of the fan or fans. . Natural ventilation is not subject to the same degree of control as 

mechanical ventilation and is so little understood by the average 

operator of a naturally ventilated mine that few attempts are ever 

made to alter existing conditions unless some particularly disagreeable 

or inconvenient operating trouble results. Poor ventilation often is 

accepted as a matter of course when a few simple changes in airway 

conditions or their grouping would effect radical improvements. 

Where the openings are large and numerous or where the surface 

components of the natural-draft columns are large ventilation is 

usually so good that control is unnecessary, except at the time of a 

mine fire. Where there is timber decay reversal of the direction of 

flow and recirculated flow often carry the spores of decay all over the 

mine and largely increase costs of support. 

Airway circuits can be changed by opening or closing connections, 

and air flows thus diverted change the temperature of the air and 

affect rock temperatures. Such changes in temperature may result 

from direct mingling of air currents or from velocity changes as affecting the time of contact with the rock. In certain instances the air 

temperatures may be changed by adding heat locally. Operating 

shafts are generally natural upcasts because of the little extra heat 

normally generated in them. 

Where there is no extraneous heat production to control the direction of flow the airway in a vertical or inclined circuit that has the 

smaller cross-section usually will be found to be the upcast, because 

of the smaller temperature changes that accompany the highervelocity flow. Once any shaft becomes an upcast or downcast, it 

tends to maintain that direction of flow since the downcast air averages colder than the rock mass and the upcast air warmer. For this 

reason the direction of. flow sometimes may be mechanically changed 

in a shaft long enough to change the natural direction of flow permanently, particularly where the surface component is small and the 

summer heating effect on the intake air can be confined to a short 

vertical distance from the surface through low-velocity flow. However, surface-air temperature changes often reverse the direction of 

flow. If the surface columns are large such reversals cannot be 

prevented, but if they are small the tendency toward reversal usually 

can be checked by proper manipulation of the air currents so as to 

change the existing distribution of temperatures and natural-draft 

generation. 

If the surface components are large and the flow is reversed seasonally attempts sometimes are made to determine the temperature of 

the surface air at which reversal will take place. Where natural 

draft is due almost wholly to surface components and the flow circuits 

underground are quite stable a close determination (within 1° or 2°) 

may possibly be made; however, where large underground components 

are also involved reversal is the effect of average temperatures over 

a period of time rather than of instantaneous surface temperatures, 

and variations of as much as 10° in the surface-air temperature at the 

time of reversal may be noted. 
100 ENGINEERING FACTORS IN VENTILATING METAL MINES 

CONTROL OF NATURAL-DRAFT PRESSURES IN MECHANICALLY VENTILATED 

MINES 

Ip mechanically ventilated mines also many opportunities-which 

are often neglected-are presented for improving natural-draft 

pressures, such as preventing leakage of warm return air into a cold 

intake and deflecting warm currents from local heating sources direct 

to return airways. In high-velocity downcasts the air usually is 

colder at depth than the surrounding rock but often is warmed 

considerably by circulating at low velocities in dead-end openings 

off the shaft. ' In a deep, mechanically ventilated mine the naturaldraft pressure always operates with the fan pressure; conserving the 

temperature differences between upcast and downcast not only increases the horsepower supplied gratis by natural draft but also 

improves circulations in. the lower part of the mine by increasing the 

natural pressures generated there. 

In mechanically ventilated mines many sections that are off what 

might be termed the" fan-pressure loop" or main airway circuit are 

ventilated mainly by natural draft, because the natural-draft pressures 

are much larger than the acting fan-pressure components; and seasonal 

reversals are common in parts of fan-ventilated mines. Many of the 

somewhat perplexing effects noted in the distribution of air in 

mechanically ventilated mines are purely natural-draft effects, but 

as the average metal-mine operator considers natural draft merely a 

question of differences of surface elevations he is at a loss to account 

for them. A few of the more common conditions encountered are: 

The extension of a multicompartment downcast shaft for several 

hundred feet below the lowest connection is usually well-ventilated by 

natural draft, and provisions for auxiliary fan-pipe ventilation are 

generally discarded; the compartment in a downcast shaft that 

contains warm-pipe columns often has little or no velocity of flow and 

sometimes has up cast flow even though the flow in the adjoining 

compartments, to which there are many openings, is downcast at 

high velocities; shafts between the main-upcdst and main-downcast 

shafts often carry but little flow, and a constant direction of flow is 

maintained in many instances only by occasionally applying emergency measures which may fail to prevent reversal of flow; and that 

part of a downcast shaft that extends above an intake adit may 

reverse with the seasons. 

ARTIFICIAL AIDS TO NATURAL DRAFT 

STACKS 

A number of methods are available for controlling or increasing 

natural-draft circulation and providing circulation in special instances when natural or mechanical circulation is absent. One of the 

simplest ways of increasing the natural-draft pressure is to increase the. 

length of the acting columns by an actual increase in length of one of 

the pair, as by adding a stack or chimney to the return-air shaft. 

Several examples of these, similar to that shown in figure 32, A, may 

still be seen in the metal-mining districts of this country. Their efficacy depends on the relative increase in length of the acting columns; 

as installed in deep mines, they rarely increase the flows perceptibly 

and are practically useless. Attached to a line of tubing extending 
Hnrean of Mine~ Bulletin 3RIi 

A B 

F]( lH; ;';2.- A, Siaek on metal-mine shaft; B, stove used with tubing to ventilate dead-end pro cet adit. 
Bllre:1Il ofMine~ Bulletin 385 

FJ U t;ltE 33.- Multiblade rotors of centrifugal fans of types commonly used for ventilating western metal 

mines: a, American Sirocco (forward-curved blades), American Blower Co.; b, American High Speed 

(bac;kward-curved blades), American Blower Co .; c, Safeload (backward-curved blades), Jeffrey Manu- facturing Cu.; d, Stepped Multiblade (stepped forward-curved blades), Jeffrey Manufacturing Co. 
NATURAL VENTILATION 101 

to the face of a dead-end opening, they might at times give quite 

effective circulation, but if in surface air there would be seasonal 

reversals and quiescent periods. Some stacks are attached to fantubing pressure units, used to ventilate moderately deep prospect 

shaft openings affected by strata gas, as an emergency aid to natural 

draft should power or fan fail; with stacks about 20 feet high and 

shafts 500 to 1,000 feet deep it is doubtful if the stack is of any 

material assistance once the tubing becomes upcast, as it practically 

always will in time, but in winter the stack probably reduces to some 

extent the time required to reverse the flow in the tubing. 

FURNACES 

Before the introduction of fans, with their relatively high efficiency 

in the use of energy, furnaces were often used to increase the temperature of the upcast artificially where fuel was cheap, as at coal 

mines, and stacks were sometimes added to the top of the u~t 

air shaft to increase the pressure generated. The furnace was usually 

placed at 01' near the bottom of the up cast shaft. The method has 

many elements of danger and has been discarded in favor of fan 

ventilation. 

A variation of this method is occasionally used to ventilate de'adend prospect openings in metal-mine districts not easily accessible 

or supplied with power. This consists of a small stove, usually of the 

Selby or cone type, with a short stack (fig. 32, B), from which a line of 

tubing extends to the faces of underground openings 500 to 1,000 feet 

away. Wood is burned in the stove after a face is blasted underground, 

and enough draft is produced by the short stack to clear the workings 

of blasting gases in a few hours; operations that would be confined to 

1 shift with diffusion alone acting to disperse the blasting fumes are 

thus able to work 2 shifts and enjoy air less contaminated with powder 

fumes than would otherwise be the case. 

AIR, WATER, AND STEAM JETS AND FALLING WATER 

Air, water, and steam jets often are employed to provide circulation 

of air and may be used under certain circumstances to control naturaldraft circulation. They act naturally by changing air temperatures 

and thus natural drafts; they also act mechanically in that the 

mechanically produced velocity of discharge is equivalent to generation of pressure. They usually are pointed in the direction of flow in 

order to act with, rather than against, the other pressures causing 

flow and are occasionally useful in an emergency in mine ventilation, 

particularly in the control of direction of natural-draft flow, as at 

times a very small additional force will prevent reversal of flow. 

Falling water may act in several ways to help or hinder flow. The 

temperature of the water may alter the temperature of the air and 

thus change the natural drafts. The velocity of fall creates energy 

which, through the resistance of the air to its passage, generates pres;. 

sure in the direction of fall; if the water falls in a pipe the same generation of pressure may serve to entrain air mechanically and carry it to 

lower levels, as in the once-used but now discarded method of the 

trompe. All such ventilation methods were important means of 

ventilating dead-end prospect openings and even mines before the 
102 ENGINEERING FACTORS IN VENTILATING METAL MINES 

introduction of mechanical ventilation, which is so much more efficient 

that there is no comparison. They are now useful as ~mergency 

measures an~ for the ventilation of out-of-the-way prospect openings; 

as such, theIr importance and possible uses are so limited that no 

attempt will be made to present mathematical treatment. 

EFFECT OF WIND 

Wind is energy in the form of velocity and represents pressure 

exerted in the direction of flow. The actual effect of surface winds 

on the ventilation of mine openings depends largely on how the directions of movement coincide. Direction of surface wind with respect 

to direction of mine openings not only determines the component of 

the wind pressure causing flow but also affects the pressure losses 

during flow. The effects are so variable and usually so small compared to natural draft and mechanical draft that they are important 

only in the absence of these sources of pressure generation. Crosswinds at air outlets create pressure losses that affect the flow by adding a deflection of the air stream at the point of discharge. Deflectors 

may be used on discharges to direct the discharge in the direction of 

the wind and make use of the suction effect of the latter, as illustrated 

by many types of roof ventilators applied to small buildings. Winds 

denote a lack of equality of absolute pressures on the surface and often 

are given full credit for ventilation of tunnels through hills on which 

their effect may be negligible. Where surface winds are steady and 

strong, deflectors might be added to mine-air inlets or outlets to 

advantage to obtain the maximum useful effect of wind pressure, but 

such devices are cumbersome when applied to large openings and, 

unless properly made with a large-radiu~ turn, would add a constant 

pressure loss greater than the intermittent loss they were designed to 

obviate. 

CONVECTION CURRENTS 

Natural-draft currents induced by air-density differences due to 

temperature differences are termed "convection currents" when the 

air circulates in a dead-end opening without separate passages for 

intake and return. Such currents are responsible for the distribution 

of warm air from a warm-air outlet or from heated surfaces to all 

open parts of a room; they are also responsible for the good ventilation 

of relatively short-end openings in a mine. 

The temperature of the air flow passing the end of a horizontal 

dead-end opening usually is at least a few degrees different from that 

of the rock surfaces that control the air temperature in the dead end, 

and the small natural pressure generated by this temperature difference acting over the height of the airway must be used up as pressure 

losses occurring during flow. The result is a circulation without 

definite boundaries; the warmer air circulates along the roof and the 

colder air along the floor. If the temperature differences between 

the two ends of a dead-end opening are large, as where a cold intake 

current passes the open end of a development drift in hot rock, the 

flows along roof and floor sometimes reach velocities of 100 to 200 

f. p~ m. at the open end but gradually diminish toward the face, 

partly on account of temperature changes but largely on account of 

the change in elevation due to grade, since it has been observed that 

the bottom of the upper flow is a level surface. This latter condition, 
MECHANICAL VENTILATION 103 

evidenced by the level lines on the walls marking the extent of the 

wetted surfaces where the return current along the roof cools and 

deposits condensed water on the walls, apparently limits the distance 

to which such currents can extend as the point at which the roof has 

the same elevation as the floor at the open end. The temperature 

of the air passing the open end, not its quantity, is the controlling 

factor in such circulation. As an extreme example of ventilation of 

this type, the temperature of the air at the face of a 5- by 7 -foot crosscut in 115° F. rock, 120 feet from an airway carrying air saturated at 

66°, was 90° saturated, and the velocities of the opposing flows along 

the roof and the floor, although not measured, were very perceptible. 

In another mine, where a cold intake current at 42° passed the open 

end of a 1,500-foot development crosscut in 85° rock, the lower level 

of the return current along the roof was marked by a level line intersecting the floor line at a point about 1,000 feet in. 

Dead-end vertical or inclined openings show convection currents 

less often, but they are usually present to some degree, particularly 

where water runs down the walls or where only part of the cross-section 

at the open end is in contact with a markedly different air temperature 

than the remaining part of the section. 

MECHANICAL VENTILATION 

As natural ventilation of mines is generally inadequate and unreliable and artificial aids to natural drafts inefficient and not always 

applicable, miners have been interested since early times in the 

development of machines for creating the pressure differences required 

for adequate and controllable flow of air through mine openings; most 

of the machines now in use .for this purpose are based on designs 

originally developed for mine ventilation. All of the more ancient 

machines were of the positive displacement type-bellows, rotating 

paddle wheels, and reciprocating compressors. They had small 

capacities and were used in conjunction with box conduits extending 

from the surface to the working places. Their application to circulating air through mine openings resulted in such huge, cumbrous forms 

that other types of mechanical ventilators were sought. These 

generally took the form of a rotating drum on the rim of which different forms of plates were mounted; this type developed into the form 

now known as the centrifugal fan. Although other types of ventilators, such as air screws and rotary displacement blowers, are used to a 

limited extent in mine ventilation, almost all of the large machines as 

well as the great majority of the smaller ones in use are centrifugal 

fans. Mechanical ventilation therefore refers almost always to the 

production of air currents in mines by pressure differences produced 

at particular points in the air circuits by centrifugal fans, acting in 

conjunction with natural-draft pressures generated throughout the 

air circuits. Most American coal mines and approximately half of 

the metal mines are ventilated mechanically. The performance of 

centrifugal fans should therefore be a subject of major interest to mine 

operating officials, yet the essentials are known to comparatively few 

of them, misconceptions are prevalent owing to a rather confused 

literature, and sales representatives are not always familiar enough 

with basic facts to combat arguments. Arguments between operators 

and manufacturers regarding fan performance are common, due more 
104 ENGINEERING FACTORS IN VENTILATING METAL MINES 

to the operator's misunderstanding of fan performance than to 

misrepresen'tation by the manufacturer. 

CENTRIFUGAL FANS 

The centrifu~al fan is a low-pressure large-quantity air pump. It 

acts by producmg a difference in absolute total pressure between its 

inlet and outlet. This pressure difference is generated by blades set 

in a drum-shaped wheel rotating in a scroll-shaped housing. Rotation 

of the blades, according to present conceptions, generates static pressure as tension is generated in a string by which an object is whirled 

through the air, and velocity pressure develops at the blade tips as a 

result of the centrifugal and rotative velocities of the air passing 

through the fan; and part of the velocity pressure is converted concurrently to static pressure in the scroll-shaped housing. The full 

application of knowledge of air flow resulting from recent aeronautical studies probably will modify the concept of pressure development 

by centrifugal fans. The mine operator or mine-ventilating engineer 

is not concerned with how the pressure is developed; his major concern, under present-day conditions, is in its proper utilization, a subject complex enough to engage his best efforts. When fans were constructed on the job to rule-or-thumb design each operator necessarily 

interested himself in design, but with efficient designs manufactured 

by specialists available his primary interest should be in correctly 

ascertaining the duty of the fan when applied to his particular conditions, so that efficient service will result. 

MODERN TYPES 

Virtually all modern designs of centrifugal fans are of the multiblade type; that is, the rotors or v:heds have a large number of comparatively shallow blades and differ mainly with respect to shape and 

position of the blades. Although the development of types having 

radial, forward-curved, backward-curved, and composite-curved 

blades has advanced simultaneously in recent years the forwardcurved multiblade type, on account of its much greater capacity for 

equal size and speed, has been the favorite for both coal- and metalmine ventilation. All of the fans installed for primary ventilation 

of metal mines in this country, except a few very recent installations, 

have been of this type. Recent developments in centrifugal fans have 

been governed mainly by commercial rather than mine application; 

most standard designs are therefore made for direct attachment to 

ducts, with no arrangement for gradual expansion in area except that 

provided in the scroll-shaped housing. For fans made to discharge 

direct to the atmosphere a discharge section of gradually enlarging 

area, termed an "evase", is usually attached to the standard design, 

although this feature is incorporated in some designs. 

Within the last 5 years certain advantages of the backward-curved 

multiblade type for mine ventilation have come to be more generally 

recognized, with the result that there has been a spurt in the development of designs of this type. A few of these are now in service for 

primary ventilation of coal mines, a few others have been considered 

and possibly installed for primary ventilation of metal mines, and 

a large number of the smaller sizes are in use for secondary and 

auxiliary service in both. 
MECHANICAL VENTILATION 105 

Most of the mechanically ventilated metal mines in this country are 

widely separated in the West, and there is no great incentive for the 

manufacturers of the East to compete for their fan busin-ess, except 

where several mines are grouped. As a result, the number of types 

represented in main-fan service is virtually confined to three makes, 

with rotors of the general designs shown in figure 33. A few other 

makes and types of fans are represented by those of smaller capacity 

used in secondary service and still more in the small units used for 

auxiliary service. 

SERVICE DESIGNATIONS 

The three common designations of type of service required of a fau 

installation relate to the common division of the air currents of the 

distribution system into main, secondary, and auxiliary circuits. 

Thus, a fan installed to handle the entire flow of the mine or of one of 

the maj or circuits is a "main" fan, one installed to aid distribution in 

a section of the workings only is termed a "secondary" fan, and one 

used with tubing to ventilate dead-end openings is an !' auxiliary" fan. 

Metal-mine distribution systems are sometimes so complex that the 

distinction between primary and secondary fans is difficult when the 

fans are operating in series-parallel combinations on separate intakes 

or returns, and as secondary fans usually operate in the "booster" 

position they are referred to more ~ommonly as "booster" fans. 

POSITION DESIGNATIONS 

The position of the fan with reference to the particular point in the 

airway circuit at which it generates a pressure difference determines 

the familiar trade designations-considered by many to include differences in design or at least in performance-such as "pressure fan" or 

"blower" at the intake end, "exhaust fan" or "suction fan" at the 

discharge end, and "booster fan" for intermediate positions. The 

position of the fan in a circuit determines the difference in pressure 

between any particular section of the circuit and the adjacent atmosphere but, within the limits of accuracy involved in the application 

of fans, does not affect the pressure generated 29 by the fan or, provided 

the direction of flow is the same, the pressure losses due to flow. 

These pressure relations are discussed in a later section. 

FAN PERFORMANCE 

As shown, the pressure-quantity relation for pressure loss in an 

airway or a system of airways may be expressed by the simple equation H=RQ2 accurately enough for practical application. When R 

is known plotting the characteristic of the airway or the curve showing this relation of pressure loss to quantity of flow is simple; but in 

the case of the fan a variable amount of the pressure actually generated is used as pressure loss due to flow through the fan casing, and 

there is no simple mathematical relation between the net pressure 

generated and the quantity of flow. The pressure-quantity relation 

for a fan must therefore be determined by actual tests, and the results 

of the tests should be plotted in the form of a curve. This curve is 

termed a "characteristic" curve of the fan. 

"McElroy, G. E., and Richardson. A. S., Experiments on Mine-Fan Performance: Tech. Paper 447, Bureau of Mines, 1929, 61 pp. 

122422°-3&-8 
106 ENGINEERING FACTORS IN VENTILATING METAL MINES 

CHARACTERISTIC CURVES 

Although the relation of total pressure difference generated to 

quantity of flow is the basic relation that determines what will happen 

when the fan is applied to the airway system and is thus the basic 

characteristic of the fan a number of other conditions that relate to 

the particular performance of the fan are interesting-changes in 

mechanical efficiency, power requirements, and relative proportions 

of pressure components, static and velocity, that accompany changes 

in this basic relation. These are also determined experimentally 

and, when plotted to correspond to the pressure-quantity characteristic, likewise specify the performance of the fan. Since these are 

also termed "characteristic" curves the separate curves are commonly 

designated according to their origin, and the basic characteristic is 

termed the" fan" or, more commonly, the" total-pressure" characteristic. Such a simple set, specifying the performance of a particular 

°u~~------L-------~------i----Q~~------~O 

QUANTITY 

FIGURE 34.-Characteristic curves of fan performance at constant speed and constant air density. 

fan at a certain constant speed, is shown in figure 34. The particular 

point, such as a on the total-pressure curve, at which the fan will 

operate is determined by the similar total-pressure curve for the 

system on which it operates, and this point in turn determines the 

other conditions of performance of the fan. Point of operation a 

(fig. 34) determines the scale values of the quantity as Q and the total 

pressure as H. Since characteristic curves for other conditions of 

performance are plotted to the same quantity base, their scale values 

are determined by the intersection of a vertical line through a with 

their respective curves, which determines the efficiency as E, the 

power as P, and the rated static pressure as S. 

TESTS 

Characteristic performance curves for fans usually are determined 

by laboratory fan tests conducted according to an exact procedure 
MECHANICAL VENTILATION 107 

such as that laid down in the Standard Test Code of the National 

Association of Fan Manufacturers and the American Society of 

Heating and Ventilating Engineers.3o Five sets of measurements are 

required-speed, density, quantity, pressure, and power. To cover 

the full range of performance these measurements must be repeated, 

with the speed practically cons.tant, for eight or more different con"': 

ditions of resistance to the ventilating system. In the laboratory 

tests a short duct having the same dimensions as the discharge or 

inlet connection of the fan is added to the fan, and the resistance to 

flow is varied from the fully closed to the wide-open condition by 

varying the amount of resistance added at the discharge or inlet 

end of the test duct. Characteristic curves or other performance 

data are not always available for the fans in use at a mine, and it is 

often desirable to test fans in place to determine the approximate 

characteristics or, more commonly, to obtain data on the operating 

condition and on the mine resistance. 

LAWS 

It would obviously be impracticable for manufacturers to determine 

the characteristic performance curves of all types of fans for all sizes 

and for all possible speeds by tests. Fortunately, this procedure is 

not required, since the changes in performance have been found by 

experiment to obey certain laws with enough accuracy to permIt 

their general use over a large range of variations in conditions of flow. 

For a fan operating alone on a given ventilation system the following rules govern the performance of the fan and are useful in correcting 

test results to constant-speed, standard-density conditions: 

1. The quantity of flow varies directly as the fan speed and is independent of 

the air density. 

2. The pressures produced by the fan vary directly as the square of the fan 

speed and directly as the air density. 

3. The power input to a fan varies directly as the cube of the fan speed and 

directly as the air density. 

4. The mechanical efficiency of a fan is independent of the fan speed and of 

the air density. 

The performance of similar fans, that is, fans made to the same' 

design with all dimensions proportional, is based on the fact that 

within a limited range of sizes fans operated to have the same linear 

velocity at the tips of the blades, referred to ordinarily as the" tip 

speed", will produce equal pressures when the quantities of flow vary 

as the square of the outside diameters of thejan wheels or rotors. 

Fan-performance characteristics for other speeds and sizes can 

therefore be computed from one set of constant-speed tests against 

variable resistance conditions. Actually, a very slight increase in 

efficiency accompanies increase in speed-so small that it is ignored. 

Also, fan performance differs slightly, depending on whether the fan 

inlet takes air at zero velocity, as in the blower position, or whether 

it takes air having a definite velocity, as in the exhaust or booster 

positions, since the shock pressure loss in the fan entrance is larger for 

the first condition. The difference is so small, however, that it can 

be neglected with coned or rounded inlets. A slight increase in efficiency also accompanies increase in size of fan; this is sufficiently 

30 Journal of American Society of Heating and Ventilating Engineers, Standard Code for the Testing of Centrifugal and Disk Fans: Vol. 29, 1923, pp. 371-380 and 438. 
108 ENGINEERING FACTORS IN VENTILATING METAL MINES 

marked for the smaller sizes as to require separate characteristics 

for separate size groups. The larger sizes used for main-fan service usually can be considered as in the same size group without 

appreciable error. 

PRESSURES AND RATINGS 

The subject of fan pressures and fan ratings is confusing because 

the pressure differences measured directly against a tmospheric pressure 

do not correspond directly to the total-pressure difference generated 

by the fan. In an attempt to avoid confusion most manufacturers 

mte their fans on a static-pressure basis, which is equivalent to rating 

the fan for a particular condition of installation seldom encountered 

in mine-ventilation practice except in auxiliary service-fan in blower 

position discharging into a duct of uniform area the same as that of 

the fan-discharge connection. When the air density at the fan inlet 

is standard the static pressure in such a duct at the fan discharge is the 

pressure rating given to the fan. The velocity pressure at the discharge of the fan under these conditions is equivalent to the shock 

pressure loss at the discharge end of a duct of lmiform area and is 

therefore disregarded, as it is not available for overcoming the pressure losses in the duct. Present concepts of fan pressures and fan 

ratings are really based on this concept of a simple duct system; to 

take care of the widely different conditions met in practice various 

artificial concepts are introduced so that accurate results may be 

obtained by those conversant with their origin and relation to basic 

facts, while the avera.ge run-of-mine operator is left in a fog. Fan 

performance has therefore come to be based almost entirely on staticpressure measurements, which have no direct relation to fan performances but which, in general, approximate them closely enough to 

obvinte serious errors in the great majority of cases, inasmuch as the 

velocity pressures involved in the usual mine-fan installation are 

small. Attempts to obtain accurate results by artificial concepts are 

morc difficult, in the author's opinion, than a procedure based on the 

facts of pressure gains and pressure losses (p. 35); the latter course is 

therefore recommended and the only one outlined in any detail 

throughout this bulletin. 

A static-pressure rating of a fan refers to a definite area and operating position. Ordinarily, it is based on the area of the discharge 

connection provided and the blower position. Fans for use only in 

the exhaust position are sometimes given static-pressure ratings based 

on the area of the inlet connection; in this case, although the ratings 

may refer to a fan with free discharge, they usually refer to a fan 

equipped with an evase discharge section as an integral part of the 

standard design. PRESSURE GRAPHS 

An easy way to avoid confusion in specific cases, particularly in 

correlating field test results to fan ratings, is to make a graph of the 

pressure changes. Working from atmospheric pressure as a base but 

remembering that all pressure changes are changes in absolute pressures, one must consider five general facts to avoid confusion: (1) The 

difference between the average total pressure at the inlet and discharge 

of a fan is the only constant value for all operating positions and the 

value to be used in determining the rated static pressure, which customarily is one discharge velocity pressure less than this value; (2) the 
MECHANICAL VENTILATION 109 

average total pressure at any cross-section of the ventilating system 

is the algebraic sum of two mutually convertible components, static 

pressure and average velocity pressure, whose relative proportions 

depend only on the area of the cross-section; (3) average total pressures and static pressures may be positive or negative referred to the 

atmospheric pressure as a base, but velocity pressure must always be 

considered a positive pressure; (4) the absolute value of the average 

total pressure always decreases in the direction of flow; and (5) a fan 

always increases the absolute value of the average total pressure in 

the direction of flow. 

The relations of fan and ventilating-system pressures to atmospheric pressure are shown graphically in figure 35, A, B, and 0, for 

three different positions of the fan in the same ventilating system, 

assuming uniform resistance conditions and velocities throughout the 

system. In each position the total pressure developed by the fan is 

the same, and the fan static-pressure rating is equivalent to the fan 

total pressure minus the velocity pressure at the discharge; that is, it 

FIGURE 35.-Graphs of pressure changes iu ventilating systems as affected by position of fan with 

respect to system. 

is based on the discharge area of the fan operating in the blower position. For any point in the ventilating system static pressure (S. P.) 

is equal to the algebraic difference between the total pressure (T. P.) 

and velocity pressure (V. P.). 

As shown in fiS'ure 35, B, the negative total pressure at the inlet for 

the exhaust/osItion is equal to the static-pressure rating (blower 

position) an , as long as the fan discharges to the atmosphere, will 

remain constant regardless of changes in area of the duct at the inlet. 

It is therefore advocated and used as an alternative rating for a fan 

in the exhaust position. One-point total-pressure measurements can 

be made, but the average total pressure over the section is that 

involved and can be obtained directly only by a traverse and indirectly from the measured static pressure and the calculated mean 

velocity pressure. However, some investigators consider one-point 

total-pressure measurements correct, whereas others are concerned 

with determining a position in the duct that will give the average 

total pressure. A position that will give a good approximation of the 
110 ENGINEERING FACTORS IN VENTILATING METAL MINES 

average total pressure can be determined for conditions of uniform 

area but can rarely be applied at mine-fan installations. Although 

the actual values of the pressures are the same the use of two such 

totally different pressures as rating pressures for different positions 

leads to confusion, which is increased by the fact that still another 

method of determining the rating must be used for a fan in the booster 

position. Facts are therefore more reliable: The position of operation 

is immaterial; the true performance is the total pressure difference 

generated; the rated static pressure is an arbitrarily determined 

figure that has a direct relation to the pressure generated; and fan 

performance is not necessarily directly related to the pressures measured in the ducts adjacent to tho fan but must be determined from 

measurements made according to position of fan, duct areas, and, if 

they are involved, corrections for pressure losses between points of 

measurement and the inlet and discharge of the fan. 

EV ASE DISCHARGE 

The effect of an evase, a section of gradually expanding area added 

to the discharge of the fan proper, on a fan operating in the exhaust 

position is shown graphically in figure 35, D. The evase reduces the 

total resistance of the ventilating system by reducing the shock pressure loss of the flow system on discharge to the atmosphere. If the 

evase is considered a part of the fan then the true total pressure 

rating of the fan is reduced by the amount of pressure lost in the 

evase and the calculated efficiency of the fan likewise, although the 

pressure produced by it is thus used more efficiently. The evase 

discharge section is sometimes a component part of the fan but 

usually is attached to standard fans by either the manufacturer or 

the mine operator, and performance must be correlated with the 

ratings for standard fans without evase sections. For this reason the 

evase section is more properly considered a part of the ventilating 

system. The burden of using the velocity component efficiently is 

thus thrown on the operator, who can best provide the proper airway 

and duct condition. Special types of fans, however, may have such 

a normally high velocity of discharge tluLt efficient use of the velocity 

component is a practical necessity. The evase is then made a component part of the fan design, and the fan is always so rated. 

PRESSURES UNDERGROUND 

Fans operating underground are always in the booster position, 

with resistance to flow on both inlet and discharge sides. Auxiliary 

fans connected to tubing have only a negligible resistance to flow on 

the side open to the mine airways, so this resistance is not considered; 

the position is considered as determined by the resistance to flow in 

the tubing only, and the adjacent atmosphere is used as the pressure 

base. Where the flow for a fan in the booster position underground 

is through mine airways, rather than separate ducts or tubing, the 

graph of the pressure changes is the same as that shown in figure 

35, 0, but the atmospheric pressure is not available as a base or 

datum pressure. The conditions are the same as for a fan on the 

surface, but the pressures must be measured entirely within the ducts 

of the ventilating system. With the atmospheric pressure line omitted the graph is actually a graph of the absolute-pressure changes, 

and the basic measurement is the difference in static pressure CD. S. 
MECHANICAL VENTILATION 111 

P.) between a point close to the inlet, such as X, and a point close tv 

the discharge, such as Y. If the quantity of flow and the areas at 

X, Y, fan inlet and fan outlet are known, velocity pressures can be 

computed and the graph followed to determine the fan total pressure 

and the rated static pressure. 

MECHANICAL EFFICIENCY 

All of the energy supplied to a fan does not result in useful work. 

A certain proportion, which varies with the resistance against which 

the fan operates, is required for flow through the fan itself; the rest 

represents useful work, the horsepower in the air. The useful work 

per unit of time divided by power input for the same unit of time is 

the mechanical efficiency of the fan. The useful work is obtained 

from the total pressure of the fan and the quantity of flow. The 

term "mechanical efficiency" in data on fan performance refers to 

efficiency based on the total pressure, even though the pressure rating 

of the fan is given on a static-pressure basis. Occasionally, for convenience of application with rated static pressures, an efficiency 

termed the "s ta tic efficiency", or efficiency calculated from the rated 

static pressure, is given. This bears the same ratio to efficiency that 

rated static pressure does to total pressure and can thus be used 

directly with rated static pressures to calculate power requirements. 

APPLICATION OF FANS TO VENTILATING SYSTEMS 

Fans may act singly or in combination with other pressure-generating sources such as natural draft or other fans. In a metal-mine 

ventilating system there are natural-draft pressures acting in series 

with or against the fan pressure in most instances, and often there are 

two or more fans acting in series, parallel, or series-parallel combinations with each other and natural-draft pressures. 

When a fan is the only pressure source acting on a ventilation system 

the duty required of it is constant and is controlled entirely by the 

resistance of the system; but when a fan operates in combination with 

other pressure-generating sources all combine to overcome the 

resistances of the system, and the duty of a fan operating at a particular point in the system is determined by the position and intensity 

of the other pressure sources acting with it to produce the flow. 

Under such circumstances the duty of the fan is not constant but 

variable, as determined by the proportion it contributes to the total 

pressure generated throughout the system which varies with fan 

speeds and natural-draft pressures. 

FAN AS ONLY SOURCE OF PRESSURE 

A fan running at constant speed can produce only the combinations 

of pressure and quantity of flow indicated on its characteristic curve 

for that speed as determined directly by tests or indirectly by application of fan laws to test results. The pressure losses in the ventilating 

system vary with the quantity of flow, and the combinations that 

may exist are indicated by the characteristic curve of the system. 

Since the pressure generated must equal the pressure losses only one 

combination of pressure and quantity will satisfy both fan and 

system characteristics; this combination is indicated graphically by 
112 ENGINEERING FACTORS IN VENTILATING METAL MINES 

the intersection of fan and system characteristics at a in figure 36, A, 

which determines the pressure as H and the quantity as Q. 

FAN IN SERIES WITH NATURAL-DRAFT PRESSURE 

Except in auxiliary service, metal-mine fans rarely are the only 

source of pressure. Components of natural-draft pressures always 

act either with or against fan pressures. If of small intensity they 

may be neglected in determining the fan duty, but in a deep mine 

they are an important factor in determining the quantity of flow and 

the duty of the fan. If all workings are on one level, or practically 

so, there is but one component of natural draft-that developed between the highest surface opening and the level of the workingsand this pressure acts in series with a fan handling the total flow. 

U~-----+'Q----~~ 

QUANTITY 

'A 

FIGURE 3B.-A, Fan acting as only pressure-generating ,ource; E, natural draft acting with fan; G, natura draft acLing against fan; D, fan characteristic used directly as combined characteristic for natural draft 

acting with fan. 

In a multilevel mine, however, all components of natural-draft pressures do not act in series with fan pressures; rather, they act as a 

group of constant-pressure (pressure constant while quantity changes) 

fans operating in parallel with each other, the resultant of the group 

operating in series with fans on the main flow_ Although problems 

of distribution require a different treatment, as previously discussed 

in the section on natural draft, the resultant of the components acting 

in series with a main fan may be considered as approximately that 

developed between the highest surface opening and the uppermost 

cross-connection that carries an appreciable part of the total flow. 

Natural-draft pressures developed below the uppermost main crossconnection have little effect on the duty of a fan on the total-flow 
MECHANICAL VENTILATION 113 

part of the circuit but act mainly to change the distribution below this 

level from what it would be with a fan acting alone on the same 

system. 

The conditions for natural draft acting in series with a fan are 

shown in figure 36, B. The flow is determined by the combined pressure characteristic of both fan and natural draft; this is obtained by 

adding the pressures for equal quantities and plotting the results 

against the respective quantities. The flow conditions are indicated 

by the intersection of the combined pressure characteristic and the 

system characteristic at a, which determines the total pressure as H 

and the quantity as Q. The latter determines the operating position 

of the fan for this condition, as at b on its charactenstic, and the fan 

pressure as HFb • 

With natural-draft pressure HN acting alone on the system the 

intersection of the natural-draft and the system characteristics at c 

would indicate the resulting quantity of flow as QN. With the fan 

acting alone on the system the intersection of its characteristic with 

that of the system at d would indicate its operating position and 

determine the pressure as HFrl and quantity of flow as QF. As respects 

the duty of the fan, the practical result of natural draft acting with 

the fan is in effect a decrease in the resistance of the system and an 

increase in quantity of flow over what would be produced by the fan 

acting alone. 

The conditions for natural draft acting against the fan are shown 

in figure 36, O. The combined characteristic obtained by subtracting 

the natural-draft pressure from the fan pressure for the same quantities of flow crosses the system characteristic at a and detennines 

the pressure and quantity of flow as Hand Q, respectively. The 

latter determines the operating position on the fan characteristic at b 

and the fan pressure as HFb • 

With natural draft HN acting alone the characteristics cross at c 

and indicate that the quantity would be QN and, although not so 

indicated by the graph, the flow would be in the opposite direction. 

With the fan acting alone the characteristics cross at d and indicate 

that the flow would be QF and the fan pressure HFrl. The duty of the 

fan is changed, through the action of natural draft, from d to b. As 

respects the duty of the fan, the practical result of natural draft 

acting against the fan is in effect an increase in the resistance of the 

system and a consequent reduction in quantity of flow. 

Since the combined pressure characteristic for fan and natural draft 

changes with every change in natural draft, even though the fan is 

run at constant speed, it follows that its intersection with the constant mine characteristic will change positions and a difference in 

duty of the fan will result. Fan duty is therefore determined not 

only by the mine resistance but also by the varying intensities of 

natural-draft pressures. If the limits of the variations in naturaldraft pressures acting in the same or opposite directions are known 

combined characteristics can be laid off for each condition and the 

range of variation in fan duty determined by the quantity intercepts 

on. the fan characteristic, the quantities being determined by the 

intersections of the combined pressure characteristics with the system 

characteristic. 

A more convenient method is to plot the same system characteristics 

directly on the fan-characteristic-curve sheet to separate pressure 
114 ENGINEERING FACTORS IN VENTILATING METAL MINES 

bases determined by laying off the natural-draft pressures to the same 

scale. The corresponding conditions of fan performance are then 

determined directly by the intercepts of the separate system curves 

and the fan curve, since the latter, if considered with reference to the 

proper pressure base, is equivalent to the combined pressure curve 

of figure 36, B or O. The 3 points of operation of the same constantspeed fan on the same mine system, which result under 3 conditionsfan operating alone, minimum natural draft acting with the fan, and 

maximum natural draft acting with the fan-are indicated in figure 

36, D, by the 3 intersections a, b, and c on the fan characteristic. 

EQUIVALENT RESISTANCE 

It is important to realize that when pressures are acting in combination on all or part of the flow in a system of airways the characteristics of a fan operating at a particular point are governed by 

the fan laws, but the operating point on the characteristic determined 

for a particular set of fan-operating conditions is determined not only 

by the pressure-quantity relation or resistance of the system, but also 

by the other pressures acting on the flow. Since the major point of 

interest in a mine-ventilation system is the pressure-quantity relation 

required at the main fan this relation often is referred to incorrectly 

as the mine resistance, whereas there is no common designation for 

this particular relation. It has been termed the" equivalent resistance" against which the fan must operate, which seems to define it 

well enough; this designation will therefore be used. 

Since the equivalent resistance or pressure-quantity relation for 

a particular pressure-generating source in a mine-airway system 

acted upon by pressures in combination is determined partly by 

pressure losses which follow a definite law and partly by pressure 

gains which do not follow a definite law it can be determined only 

by direct test or by graphic methods. When the characteristics of 

the pressure gains and pressure losses are known graphic methods of 

solution may be used. 'When these are unknown the pressurequantity relations at a fan run at different speeds may be obtained 

and plotted as the characteristic of the equivalent resistance. Then 

the particular point of operation of the same or a different fan at 

the same point for the same conditions may be obtained by using 

this equivalent-resistance characteristic as a system characteristic 

and solving graphically for the point of operation as though the 

fan were the only pressure source. 

FANS IN SERIES 

Although generation of all the mechanically applied pressure of 

a mine-ventilating system or a duct system at one point in the 

system is generally cheaper, a more efficient distribution and better 

operating conditions may sometimes be obtained by genera.ting 

pressure at more than one point, particularly where the resistance 

to flow is abnormally high or certain leakage circuits are unavoidable. 

Some type of air pump can be purchased for any pressure-generating 

duty; but centrifugal fans, the common type used in mine ventilation, have a limiting tip speed beyond which they cannot be operated 

safely. Theoretically, the pressure and volume can be increased 
MECHANICAL VENTILATION 115 

indefinitely by increasing the speed, but practically the speed, as 

well as pressures and quantities, is limited. 

Fans operating in series are those operating on the same flow 

circuit, each handling the total flow of the circuit and each generating part of the total pressure required to balance the pressure 

losses of the flow circuit. Any number of fans can be so used. 

Solutions for only two fans in series will be illustrated; but the same 

methods are, of course, applicable to any number or to combinations 

of fans in series with natural-draft pressures. 

Since the pressure-quantity relations for fan performance are 

empirical graphic methods are required. As in the case of fans 

operating in combination with natural-draft pressures, the primary 

condition is that the sum of the pressures generated by the fans 

must equal the pressure losses for the particular quantity of flow. 

The flow conditions are determined by the system characteristic and 

the combined pressure characteristic of the fans. The latter is obtained by plotting the sum of the pressures for the same quantities. 

Figure 37, A, shows the separate and combined characteristics for 

two fans (A and B) acting in series on the system whose characteristic 

is shown. The resulting flow conditions are determined by the intersection of the system and combined fan characteristics at c, and the 

indicated system pressure and quantity of flow are Hand Q, respectively. Since each fan must pass this quantity the operating 

position for each fan at the speed for which its characteristic has been 

drawn is indicated by the intersection of the vertical through this 

quantity line with its characteristic-at d for fan A and at e for fan 

B--:which determines the separate fan pressures as HA and HB • 

If the mine characteristic does not intersect the combined characteristic of the two fans then the larger-capacity fan will handle 

more air alone and the flow conditions require more air to pass 

through the smaller-capacity fan than it will handle alone when 

operating wide open or against no external resistance. In this 

case the pressure losses in the smaller fan are greater than the pressure it generates, and instead of representing a pressure gain in 

the circuit the fan becomes a pressure loss even though it actually 

continues to generate pressure. The condition has no practical 

importance, but experiments 31 indicate that it can be solved by 

roughly extending the characteristic of the smaller fan for increased 

quantities and negative pressures and using this extrapolated section to determine the combined pressure characteristic for quantities 

in excess of the capacity of the smaller fan. Practically, a change 

of fan speeds is indicated if both fans are to produce a useful effect, 

and the major point to be determined would be whether the speeds 

could be adjusted so that each fan would operate at close to maximum 

efficiency. 

Unless the two fans are properly selected for the work to be done 

they will not operate together at maximum efficiency. Where one 

fan, however, is operating inefficiently because designed for a lower 

resistance than that encountered, whether due to changes in resistance or misapplication, a second fan may be installed to operate in 

series with it at any point on the total-flow part of the system so 

selected that both fans will operate at or near maximum efficiency. 

31 McElroy, G. E.,.and Richardson, A. S., Experiments on Mine·Fan Performance: Tech. Paper 447, 

Bureau of Mines, 1929, p. 19. 
116 ENGINEERING FACTORS IN VENTILATING METAL MINES 

Whether or not this arrangement is advisable depends on the 

economic factors involved-whether the gain through increased 

efficiency is greater than the cost of the second installation and 

whether the same increase in efficiency might be better attained by 

replacing the single fan with another designed to operate at maximum efficiency under existing conditions. 

Q 

QUANTITY 

A 

FIGURE 37.-A, Two Cans acting in series; B, solution by trial and error Cor flow conditions Cor Cans acting in parallel on separate airways. 

FANS IN PARALLEL 

To take full advantage of the lay-out of airways available for 

a mine-ventilating system, it is sometimes desirable to operate fans 

in parallel, that is, to have a number of fans together handle the 

total flow. Fans may also be used in parallel where space is limited, 

or to provide for efficient operation within certain limits of change 
MECHANICAL VENTILATION 117 

in resistance, or to increase the efficiency of a fan that by' itself 

would operate against a resistance much lower than that for which 

it was designed. 

Fans on same airway.-Fans may be placed together so that they 

operate on the same airway, or they may be on separate airways. 

In the first case they act together directly on the total flow; and a 

simple graphic solution, similar to that for fans in series, is available. 

The primary condition is that the pressure generated by each must 

equal the pressure loss sustained by the total quantity they produce 

at that pressure. The combined pressure characteristic of fans acting 

in parallel at the same point in the system is obtained by plotting 

the sum of the quantities handled at the same pressure against the 

pressures. The intersection of the system and combined fan characteristic determines the pressure; and, since the pressure must be the 

same for both fans, the pressure determines the relative quantities 

that each will contribute. In figure 38 the combined characteristic 

for the two fans acting together in parallel, each with the constantspeed characteristic shown, intersects the system characteristic at 

point c, which determines the pressure of both the flow and the fans 

as H and the total quantity of flow as Q. The constant-pressure line 

o'~~~------~~--~~------------~ 

FIGURE 38.-Two fans acting together in parallel. 

H intersects the individual fan characteristics at d and e and determines the separate quantities handled by each as QA and QB. 

If the mine characteristic does not intersect the combined characteristic then the higher-pressure fan will handle more air alone than 

the two fans together, and if an attempt is made to operate them in 

parallel the higher-pressure fan will blow air back through the lowerpressure fan. Intersection at a point that indicates a higher pressure 

than the lower-pressure fan attains at any point on its characteristic 

indicates the same condition of reversed flow through the smallerpressure fan. Fans that have steeply sloping pressure characteristics 

act together well in parallel; those that have characteristics combining 

a comparatively flat part and a steeply sloping part must be operated 

on the steeply sloping part, or performance range, at a sacrifice of 

efficiency to prevent sudden changes in the equivalent resistance of 

the system, causing the higher-pressure fan to take all of the load 

with the possible result of burning up a motor. With forward-curvedblade fans so installed various precautions are taken to guard against 

operating trouble, such as the use of fans exactly alike operated at 

the same speed or from a common drive shaft, arrangements for 

speed regulation on one or both fans, and/or an "equalizing" tube 

connection between the fan inlets or the fan discharges. 
118 ENGINEERING FACTORS IN VENTILATING METAL MINES 

Fans on separate airways.-The more common installation of fans 

operating in parallel is that in which the fans are installed in separate 

airways. Where the resistances of the separate airways on which the 

fans are installed are comparatively large very little operating trouble 

is experienced, but it is difficult to determine the equivalent resistance 

against which fans so installed must operate for a fixed speed. The 

characteristics of the separate airways must be considered, as well as 

that of the system beyond the point of junction of the separate flows. 

Flow conditions and fan performances must be solved graphically by 

a process of trial and error. 

Where fans in parallel are the only sources of pressure on the flow 

circuit the distribution may be indicated by fans discharging through 

separate branches into a common duct, as shown in figure 39, A, for 

two fans, although the actual conditions represented might be quite 

different. If the actual flow conditions were as sketched in figure 

39, B, the series resistance b+c+d would be equivalent to A, that for 

e equivalent to B, and that for a+J equivalent to O. The primary 

condition is that the quantities of flow in the two branches must be 

such as to make the differences between the pressure generated by 

each fan and that used in flow through its branch equal to each other 

and equal to that required to pass the total flow through the rest of 

the system. Both fan 

FanB 

c 

A 

Fan A 

bcd=A} characteristics and 

! I a+i~g offig.39.t I a the three mine-section 

FanB 

. - characteristics are 

plotted on the same Fan A lb sheet to scale, as L--=<~B~--' shown in figure 37, 

FiGURE 39.-A, Sketch analysis of flow conditions for fans acting in 

parallel on separate airways; B, sketch diagram of fans acting in 

parallel on separate airways. 

B, and the flow conditions are solved 

graphically by successive trials. A total quantity of flow, Qe, is assumed; the intersection of this quantity line with system characteristic 0 determines 

the pressure (He) that would be required in the total-flow or 0 section 

of the system. The balance of each fan pressure is available for flow 

through its respective branch; the intersections of curves similar to 

the fan characteristics, but with the fan pressures reduced by the 

constant amount He, with system characteristics A and B of the 

branches determine the quantities of flow through the branches as 

QA and QB and the pressures required as HA and HB, under the 

assumed condition of total quantity. If the combined quantities 

total less than the total quantity assumed a smaller total quantity 

would be assumed in the succeeding trial; if they add up to more than 

the total assumed a larger total quantity would be assumed in the 

next trial. When the quantities check, the intersections of the 

quantities through branches QA and QB with the characteristics of 

their respective fans indicate the equivalent-resistance conditions 

against which the fans must operate and the corresponding pressures 

as HFA and HFB. • 

COMPLEX COMBINATIONS OF PRESSURE SOURCES 

In large metal mines fans may operate at various points in the system in conjunction with natural-draft pressures. In some mines the 
MECHANICAL VENTILATION 119 

lay-outs may be resolved into equivalent simple flow systems, but in 

many the various fan pressures act on but part of the flow in series, 

parallel, and series-parallel combinations with each other and natural 

drafts; many leakage circuits are involved with resultant changes in 

quantities of flow in addition to those caused by density changes and 

the addition of compressed air underground. Solutions are therefore 

complex and necessarily approximate. The important condition 

is that the pressure losses on any complete circuit, regardless of changes 

in quantity, are equal to the pressures generated Oil that circuit, 

whether by natural draft or by fans. Where the pressure-generating 

source may be considered to have a constant-pressure characteristic, 

as for natural draft and for certain limited ranges of operation of 

forward-curved-blade centrifugal fans, solutions may be aided by 

mathematics. 

EFFICIENCY OF FANS OPERATING IN METAL MINES 

If the conditions under which a fan operates agree in all respects 

with those for which it was designed the fan will operate at maximum 

efficiency; if the operating conditions do not meet these requirements 

the fan will operate at less than maximum efficiency. Comparison 

of the actual mechanical efficiency of a fan during operation with the 

maximum efficiency possible with that fan gives the relative efficiency 

of the installation. 

Regardless of the degree of complexity of flow systems, most of the 

main fans used in western metal mines operate within 5 percent of 

maximum efficiency, largely owing to the fact that centrifugal fans 

operate against a wide range of resistance conditions with but little 

decrease from maximum efficiency. In many instances it is partly 

due to the fact that manufacturers' representatives assisted in determining the duty for the original installation and that the change in 

mine resistance, on account of the preponderating influence of the 

resistance of the main airways, has been small. Fan speeds, and even 

the fans themselves, are changed occasionally, but in the great majority of instances the fans have been kept at relatively high efficiency 

by occasional extensive changes in the airways, a method often employed when the operator's guess as to the required duty of the fan 

falls wide of the mark. 

Secondary or "booster" installations in the same group of mines 

generally operate at average efficiencies 5 to 10 percent under maximum efficiencies, partly because conditions change rapidly but mainly 

because the operating or equivalent-resistance condition is more 

difficult to determine than for main-fan service. Auxiliary fan installations present the same difficulties but to a higher degree; they 

operate at average efficiencies ranging from 10 to 20 percent below 

maximum, although for this and booster service results are more 

important than mechamcal efficiency. 

SELECTION OF FANS 

As shown in the preceding section, the performance of a fan in a 

ventilating system is determined by its characteristic (total-pressure) 

curve and the mine characteristic, if acting alone on the system, and 

by its characteristic curve and the characteristic of the equivalent 
120 ENGINEERING FACTORS IN VENTILATING METAL MINES 

resistance, if acting in combination with other pressure sources. Characteristic curves of fans are a matter of design and are controlled by 

the manufacturer. The characteristic curve of a mine is a matter of 

the lay-out of the ventilating system and is controlled by the mine 

operator. 

SIZE 

Manufacturers do not. make special fans to fit conditions; rather, 

they make graduated series of fans of increasing size, with all dimensions in proportion, of a limited number of standard designs or types. 

Special fans-those whose proportional dimensions are not the same 

as the standard-are occasionally made to suit speed limitations or 

fixed speed conditions and usually involve merely a change in the 

proportion of width of wheel and casing to diameter of wheel. For 

use in the exhaust position on mine shafts an evase discharge section 

usually is attached to the standard design; some standard designs 

are altered slightly to include an evase discharge, and a few types 

developed particularly for use in the exhaust position include it as 

standard design. 

The relative performance conditions for different-size fans of the 

same design practically amount to this-anyone fan will operate at 

maximum efficiency against only one resistance condition, but a size 

can be selected that will operate at or close to maximum efficiency 

against any particular condition of resistance. The primary condition for satisfactory and efficient operation is that the size of the fan 

must fit the mine or its position in the mine. The lower the resistance 

the larger the fan required; the higher the resistance the smaller the 

fan required. Resistance to flow, which practically means size of 

airways, and not quantity of flow determines the size of fan. Quantity of flow for an existing installation can be increased only by increasing the fan speed or reducing the resistance to flow. The fan 

speed is limited by mechanical stress to a safe maximum tip speed for 

the particular design. Since, with a fixed resistance, the power requirements of fans increase as the cube of the fan speed and quantity, 

the quantity usually can be increased more cheaply by reducing the 

resistance than by increasing the fan speed. With the same fan speed 

the increase in power requirements is then closely proportional to the 

increase in quantity, but the power may be made normal by reducing 

the fan speed at a sacrifice of part of the quantity increase. Notwithstanding these facts, which have been given wide publicity, the 

average mine operator usually attempts to increase the quantity by 

replacing a small fan with a larger one. With fans of the same design the resulting difference in quantity of flow depends largely on 

the relative tip speeds of the fan wheels; so the new quantity may be 

more or less than the old. The usual result is disappointing, and 

eventually the increase in quantity desired is obtained by using more 

power or by changing the airways. 

PERFORMANCE GUARANTEES 

Determination of the range in duties or range in resistance conditions against which the fan must work is the primary concern of the 

mine operator. Having determined the requirements, he submits 

them to one or more manufacturers who quote proposals covering a 
MECHANICAL VENTILATION 121 

limited number of sizes and types from which to make an economic 

selection on the basis of satisfactory performance over the life of the 

installation at a minimum total cost for installation and operation. 

The proposals of the manufacturers can be held to offer only fans 

guaranteed to operate, within the limit of the safe tip speeds, at certain efficiencies over a given range of resistance conditions without 

developing mechanical defects. If the actual resistance conditions 

differ from those specified or if the resistance conditions change after 

the fan is installed, the manufacturer is not responsible for the resulting 

lower efficiency of operation. 

CHARACTERISTIC-RATIO CHARTS 

From the results of constant-speed tests against variable resistances 

with a single size of fan the manufacturer has enough data to determine the operating characteristics of a group of similar fans for any 

speed. Such separate characteristics for constant speed and definite 

size (see fig. 34) or for the same size of fan at a number of speeds 

answer the common requirements of application, but for general purposes of fan selection to meet particular requirements a more general 

method of representing performance is required. The basic method 

used is that of plotting the separate characteristics as ratios and percentages rather than actual values; these may take a variety of forms. 

In almost any form they present a concise conception of the relative 

changes in performance for the particular design, but their practical 

use for fan selection involves considerable mathematical ability, some 

degree of experience in using the particular type involved, and often 

some basic starting data not marked on the chart but usually given as 

supplementary data. Some types of ratio charts are simpler and 

require fewer computations than others, but no really simple and 

complete type has been developed to the author's knowledge. What 

is needed is a chart from which, starting with quantity and pressure 

data, one could determine fan size, speed, power, and efficiency. 

Figure 40 shows four types of characteristic-ratio charts, of which 

type B represents an approach to these requirements. The ratio-ofopening type (A), now generally discarde4, is based 32 on the pressure 

losses at orifice plates used in tests; since these may have a wide range 

of variations the base is variable rather than constant as between 

different manufacturers using orifice plates of different types for their 

tests. 

TABLES OF PERFORMANCE 

Since the more concise method of presenting fan-performance data 

in the form of ratio charts is practically limited to the use of specialists 

and manufacturers' representatives the more bulky method of presenting these data in tables is used for catalog and general use. The 

tables, in general, are presented in three different forms. In one type 

only the rated performances-performances for combinations of pressure and quantity that permit the fan to operate at maximum efficiency-are given. A single table usually represents a complete line 

of sizes of the same type and design. In the second and third types 

a separate table shows the performance for each size over a limited 

" McElroy, G. E., Ratio of Opening of Fan Performance in Terms of Direct Pressure-Quantity Relations: 

lour. Am. Soc. Heat. and Vent. Eng., vol. 34, 1928, pp. 783-793. 

122422°-35--9 
122 ENGINEERING FACTORS IN VENTILATING METAL MINES 

range of high-efficiency operation: In one type of table the rated 

performances are indicated by bold-face type; in the other they must 

be deduced mathematicallv. Efficiencies are not given directly but 

may be ca.lculated from the power, pressure, and quantity data and 

the rated performance thus obtained. Data for performances intermediate between those given ma.y be determined approximately by 

interpolation and accurately by calculations based on the fan laws. 

A sufficiently simple type of graphic-computation chart, could one 

be developed, would be a great improvement over catalog tables for 

general use. 

, v .. ba ~do out tar a , 

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QUANT/TY, PERC~NT OF MAXIMUM 

QUANTITY, CUBIC FEET PER MINUTE B 

140 .. 

1~ ~ 

e-o lOO 

'" 

~80 <> 

40 

20'ur;S=;~~~;S~~1~~ f ~ 40 60 80 100 1~ 140 160 180 ~ QUANTITY, PERCENT RATED QUANTiTY D 

FIGURE 40.-Four types of characteristic-ratio charts of centrifugal-fan performance. 

TYPES AND DESIGNA'l'IONS 

Centrifugal-fan designs differ mainly in the design of blade of the 

wheel or rotor, particularly its shape wi.th respect to the direction of 

rotation. The shape of the blades practically determines the relative 

performance characteristics but is seldom mentioned in catalog data 

and cannot always be inferred from trade designations. Blade 

shapes fall into four general classes-radial, forward-curved, backward-curved, and combination. The type characteristics differ 

widely; those for the latter class are similar to those of the basic 

shape it roost nearly approaches in design. Most manufacturers 
MECHANICAL VENTILATION 123 

offer designs embodying the three basic shapes as suitable for various 

types of service, but for many years the most popular type of fan for 

metal-mine ventilation has been the multiblade, forward-curvedblade centrifugal. Radial-blade types, often termed "paddle-wheel" 

or "steel-plate" (an early trade name) types, usually have a few deep 

blades and are now applied only to auxiliary service in American metal 

mines. Most forward-curved and backward-curved types have a 

large number of closely spaced blades of shallow depth, but a few 

designs have in addition a small number of deep blades. 

Although the blade shape usually determines both the operating 

characteristics of the design and its trade designation a large number 

of common designations also are used to indicate variations of basic 

designs and methods of installation, which will be discussed later. 

EFFICIENCIES 

The comparative mechanical efficiency of the various types is fairly 

well established by consensus of opinion. Virtually all designs can 

be made to give equally high efficiencies if cost is disregarded, but the 

designs now offered to meet competitive commercial conditions have 

the following comparative ranges of maximum efficiency: Radial 

blade, 50 to 60 percent; forward-curved blade and straight-side backward-curved blade, 60 to 70 percent; and backward-curved blade with 

coned sides (now apparently limited as to maximum diameter by 

mechanical design), 70 to 80 percent. These are actual or total-pressure efficiencies. Efficencies based on static :pressures average about 

10 percent lower. In general, the lower effiCIencies apply to smaller 

sizes and the higher efficiencies to larger sizes. The maXlIllum variation for designs of the same type apparently is limited to about 5 

percent. Maximum efficiencies are being gradually increased by 

refinements in design, and improvements are rapidly incorporated in 

most competitive designs. 

EFFECT OF BLADE SHAPE ON PERFORMANCE CHARACTERISTICS 

The general performance characteristics of centrifugal fans attributed to blade shape are illustrated by the five sets of characteristicratio curves of figures 41 and 42. These are for purposes of illustration only and represent types rather than particular designs. A form 

different from those in common use is employed to bring out more 

definitely the relation of performance characteristics to flow-resistance 

characteristics, since these are the two factors that must be coordinated 

in application. 

POWER CHARACTERISTICS 

Five sets of characteristics are shown-power, total pressure, rated 

static pressure, quantity, and efficiency. The dominating char- acte~istic for mine apphcation is that of power .. Since the power 

reqUIrements of the backward-curved type are practICally constant at 

or lower than th.e ~ated p~wer it is said t? have a "nonoverloading" 

power characterIstIc and IS therefore SUItable where the resistance 

conditions fluctuate over a large range. Both radial-blade and forward-curved-blade types have rising power characteristics; they are 

therefore subject to large overloads if the resistance is greatly reduced, 

as by short circuits due to doors left open or to tubing disconnected 
124 ENGINEERING FACTORS IN VENTILATING METAL MINES 

from an auxiliary fan. These types require motors of excess capacity, 

up to 50 percent, to guard against possible overloads from a fluctuating 

resistance. 

PRESSURE CHARACTERISTICS 

Pressure characteristics of fans probably have received the most 

attention, as they are related more directly to blade shape. They 

determine the suitability of the type for operation in parallel and for 

special application where constant pressure or constant quantity rather 

than constant efficiency is required. The backward-curved-blade 

200 

80 "-

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3 466789 2 3456789 

10 100 

PERCENT OF VALUE AT MAXIMUM EFFICIENCY 1,000 

RESISTANCE 

FiGURE 41.-Comparative quantity and power characteristics of mine fans. 

type has a very steep total-pressure characteristic and is therefore 

suitable for operation in parallel at maximum efficiency. The forward-curved-blade type has a very flat total-pressure characteristic 

near maximum efficiency and requires a large sacrifice in efficiency for 

operation of the sloping part of the characteristic, as required for 

successful operation in parallel. The radial-blade type has a gently 

sloping characteristic and requires but a small sacrifice in efficiency for 

parallel operation. Pressure characteristics may be largely modified 
MECHANICAL VENTILATION 125 

by the design of the housing, as instanced by the design of certain 

forward-curved-blade types developed for auxiliary service which have 

a steeply sloping pressure characteristic similar to that indicated here 

for the backward-curved-blade type. Forward-curved-blade types 

are good for constant-pressure requirements and backward-curvedblade types for approximately constant-quantity requirements. 

80 

"' -- 4 k;- ;-

II 

00 " 1-' 

.. I80 .~~ . ~ 

",01~~ ." 200 

60 ~ ~ W' , , 1 V , 

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40 '/ 3456789 3456789 

10 100 1,000 

PERCENT OF VALUE AT MAXIMUM EFFICIENCY 

RESISTANCE 

FIGURE 42.-Comparative pressure and efficiency characteristics of mine fans. 

The question, so frequently discussed in coal-mining periodicals, 

of inferring changes in mine resistance produced by changes in airway 

conditions from observed or recorded fan pressures is easily solved by 

inspection of the pressure characteristics. The "water gage" for the 

fan in the blower position follows the rated static-pressure characteristic, and increases or decreases in water gage generally indicate 
126 ENGINEERING FACTORS IN VENTILATING METAL MINES 

similar changes in resistance without necessarily indicating the degree 

of change. The water gage of a fan in the exhaust position, the more 

common position of mine fans, follows the total-pressure characteristic; 

the change in pressure generally indicates a similar change in mineairway and resistance conditions, except for the forward-curved-blade 

type; the characteristic for this is so flat over the usual operating range 

that no inferences as to airway changes can be deduced from pressure 

changes. The large majority of coal- and metal-mine fans are forwardcurved-blade types operating in the exhaust position on the flat part 

of the pressure characteristic where the water-gage records serve no 

useful purpose other than to gage the constancy of the fun speed. 

The most useful single record of fan performance is that of quantity 

of flow; unfortunately, it is more difficult and costly to obtain than 

a pressure record. 

EFFICIENCY CHARACTERISTICS 

Economy of operation is not only a question of maximum efficiency 

but also of range of resistance conditions that can be accommodated 

at high efficiency. The flatter the efficiency characteristic in the 

vicinity of the maximum the more suitable the type for operation 

against changing resistance. The efficiency characteristics shown 

indicate that, in general, the backward-curved-blade and radial-blade 

types have a slight advantage over the forward-curved-blade type. 

OTHER OPERATING CHARACTERISTICS 

The characteristic charts shown do not reveal the speed and other 

characteristics directly related to mechanical performance. For the 

same resistance and flow conditions the speed of rotation of the 

backward-curved-blade type is almost double that of the forwardcurved-blade typo, with the speed of the radial-blade type intermediate. The speed of the ordinary disk fan, seldom used for metal-mine 

ventilation, is a bout twice that required of the backward-curved-blade 

centrifugal. High speed is a definite advantage in direct-connected 

motor-driven units, since high-speed motors cost much less than lowspeed motors. 

Quietness of operation, although an important factor in some commercial applications, ordinarily is relatively unimportant in mine 

ventilation. Although the conversion of velocity pressure to static 

pressure in the housing is mainly responsible for noise, speed of operation and details of design of both fan and connecting ducts also contribute. In general, the forward-curved-blade types are the noisiest 

and the backward-curved-blade types the quietest. 

The uniformity of the intensity of the noise indicates the constancy 

of performance. Cases of unstable performance, such as would 

require multiple characteristics rather than single characteristics to 

indicate the performance, are very rare and, to the author's knowledge, 

confined to particular installations of the forward-curved-blade 

types only, probably because of the high degree of velocity-pressure 

conversion in the housing of this type. The inference is that such 

rare cases of unstable performance are caused by unstable turbulence 

effects at the inlet due to "wild" turbulence or excessive shock 

pressure losses in the inlet passages. 
MECHANICAL VENTILATION 127 

VARIATIONS IN DESIGN TO ACCOMMODATE CONDITIONS OF INSTALLATION 

Basic designs usually are offered in a variety of forms differing in 

number of inlets, width, degree of completeness of housing supplied 

by manufacturer, direction of rotation, and direction of discharge. 

The basic design of standard single-width fan may have single or 

double inlets; that is, the air can enter from one or both sides. A 

double-width fan always has 2 inlets and is virtually 2 single-width 

fans joined with the rotors on a common shaft. Double-width fans 

are popular as main fans for mines, particularly for underground 

installations, because both fan and total installation costs generally 

are lower for large-capacity units than single-width installations. 

The latter generally give lower total costs for small capacities and for 

sidehill surface locations. The double-width fan handles just about 

twice as much air at the same pressure as the standard single-width 

fan of the same design, and in the absence of separate characteristics 

those for the single-width fan may be used with the quantity doubled. 

Fans of special width occasionally are used for special conditions of 

operation not conveniently handled by standard single- and doublewidth designs. 

The standard fan calls for a complete metal housing and is then 

termed "full-housed." The operator constructs the foundations 

according to plans supplied with the fan; since a building to house 

the drive and ducts to connect the fan with the mine airways must 

also be erected he sometimes prefers to build part of the fan housing 

of the same or more permanent materials. The fan manufacturer 

will supply fans and necessary plans for completing the housing 

accordi.'1gly; the fan is "half-housed" if the operator constructs the 

lower half or "three-quarter" housed if the operator constructs the 

lower quarter. Various advantages, including reduction of leakage at 

fan connections, thus accrue to the operator. Where the casing 

extends below the ground level it is particularly important that means 

for draining the pit thus formed be provided. 

REVERSING ARRANGEMENTS 

Centrifugal fans cannot be reversed by reversing the direction of 

rotation; special provision must be made in the fan housing and the 

connecting ducts to reverse the direction of flow through the ventilating system while the direction of flow through the fan remains 

constant. The necessary ducts must be provided and equipped with 

suitable doors, if the ducts are large, or dampers, if tubing is used, 

so that in one case the mine air is directed to the inlet of the fan which 

discharges to the atmosphere, whereas in the other the atmosphere is 

connected to the inlet and the discharge to the mine. The general 

arrangement for tubing and dampers is shown in figure 43. A common design for a single-inlet fan is shown in figure 44, A, and for a 

double-inlet fan in figure 44, B. Reversing in these instances alters 

the direction of discharge (consequently, the fan characteristic) and 

the resistance to flow through the ducts that connect the mine with 

the fan. Designs are therefore selected for the better-performance 

conditions for both fan and duct connections and designated as 

"primarily blowing" or "primarily exhausting" reversible. In 

general, provision of the reversible feature usually involves larger 

leakages, higher resistances, and higher costs than for nonreversible 
128 ENGINEERING FACTORS IN VENTILATING METAL MINES 

installations. The extent of these differences can, of course, be 

largely controlled by proper design. The cost of the reversing feature 

for the average main-fan surface installation is usually 10 to 20 percent of the total cost of the installation, but when the reversing 

feature is included in an underground installation the total cost may 

easily be double that of a nonreversible lay-out, for which reason the 

reversing feature has been applied to but one underground fan in the 

West, to the author's knowledge-the main fan of the United Verde 

mine in Arizona. The Central Eureka mine of the Mother Lode in 

FIGURE 43.-Four-gate arrangement for reversing air flow at auxiliary fan-tubing installation. 

r 

F,GURE 44.-A, Arrangements for reversing air flow at single-inlet, primarily exhausting fan installed on air shaft; B, arrangements for reversing air flow at double-inlet, primarily exhausting fan installed on air 

shaft. 

California, however, accomplishes the same result with an installation 

of two separate fans, operated one at a time and automatically 

interconnected to reverse the air currents by remote control. 

DIRECTION OF ROTATION AND DISCHARGE 

In fitting the fan to conditions at the point of installation it is usually desirable to fix the position and direction of discharge with respect to the airways or duct connections. This merely involves the 
MECHANICAL VENTILATION 129 

rotation of the casing about the fan shaft and does not affect the performance of the fan but only the details of supporting the wheel and 

the housing and applying the drive to the shaft. The direction of 

discharge is designated by the relation of the center line of the discharge to the fan shaft when the fan is viewed from the drive side 

which, in the case of a single-inlet fan, is always taken to be the side 

opposite the inlet, regardless of the actual position of the drive. 

If in this position the normal direction of rotation of the wheel is 

clockwise the discharge is clockwise and if counterclockwise the discharge is counterclockwise. Clockwise is right-hand and counterclockwise left-hand rotation, but these designations are not used in 

this connection. If the line of discharge is horizontal and above the 

shaft the discharge is top horizontal and if below the shaft, bottom 

horizontal. If the line of discharge is vertical it is either upblast or 

downblast according to whether above or below the shaft, respectively. 

If the discharge is in any other direction it is angular discharge and 

specified in degrees as top or bottom angular, up or down discharge, 

as the case may be. 

DRIVES 

A drive mechanism of almost any type may be used to drive the 

fan shaft. In the smaller sizes the shaft bearings usually are supported 

on the housing, but in the larger sizes, particularly those used as 

main fans, the shaft and wheel are supported on pedestal bearings 

independent of the housing. When foundations settle, as they often 

do around mine workings, the wheel and casing are thrown out of 

alinement and the performance is seriously affected. 

Constant-speed alternating-current motors generally are used for 

driving mine fans either through direct or belt connection. With 

belt connection the speed can be changed by changing the pulleys. 

Variable-speed motors are advantageous for certain installations but 

are seldom used on account of the higher cost. Direct-connected 

units are used almost exclusively for auxiliary service and occasionally 

for secondary or booster service. The speeds thus obtainable are 

limited to the few that are available as determined by the phase and 

cycles of the electric current. Three-phase, 60-cycle current predominates in western metal mines, but 3-phase, 25-cycle current is used in 

a few mines. On account of the saving in space of installation shortcenter V-belt drives are gaining favor over the common belt drive in 

almost universal use, particularly as the latter often gives considerable operating trouble in underground service. Squirrel-cage induction motors are the common type in use, although synchronous 

motors are sometimes used on large units for power-factor correction 

and other types of motors for multiple-speed control. 

For auxiliary service fans of several types are available for direct 

drive by compressed air through a turbine attached to the fan wheel, 

and small compressed-air motors are also obtainable. These drives 

are less efficient than motor drives but are useful on failure of power 

supply, as at the time of a mine fire, and for handling temporary conditions at a distance from the power supply, since compressed air is 

generally available throughout a metal mine. 
130 ENGINEERING FACTORS IN VENTILATING METAL MINES 

ECONOMIC SELECTION OF MINE FANS 

The selection of a fan for maximum efficiency in the use of power is 

primarily a matter of fan size-selection of a size that most closely 

fits the resistance encountered. The primary object of a fan installation, however, is to obtain the required flow at a minimum total cost 

per year of operation, and total cost is not only a question of power 

but also of capital charges based on a fair return on, and replacement 

of, the money invested. The total cost of a simple main-fan installation usually is at least 3 and may be 10 or more times that of the fan 

alone. Although the cost of the fan increases with size-at the approximate rate of $500 per foot increase in diameter of wheel for single 

widths and $1,000 per foot increase for double widths for main-fan 

sizes-the increase in total costs and capital charges is not in proportion to the increase in size. To make an economical selection from 

the fans offered for a particular duty, with different possible arrangements and necessary connecting ducts and foundations, total costs 

comprising both power and capital charges must be computed. Details of differences in mechanical construction may indicate probable 

differences in maintenance charges, which must also be considered. 

Generally, a fan for large capacity must be selected to fit the resistance conditions as closely as the sizes available permit or may even 

demand a special width at a slightly higher first cost. But where the 

capacity required is moderate to small a size smaller than that 

demanded for maximum efficiency of use of power often gives a smaller 

over-all cost even though it operates at less than maximum efficiency 

on the resistance conditions existing. Proper provision for the future 

operation of a fan on an increasing or decreasing resistance also demands consideration where the conditions can be forecast or roughly 

estimated. 

AIR-SCREW FANS 

Air-screw fans, that is, disk and propeller fans, have found limited 

use in mine ventilation. Many of the ordinary disk type with short 

cylindrical casing are used in booster service in coal mines, but very 

few are found in metal mines. A special type of single-stage propeller 

fan, the Coppus, which includes a propeller, guide vanes to take the 

"spin" out of the air, and an evase discharge section, has found large 

use in secondary and auxiliary service in the metal mines of this 

country; both single-propeller and multiple-propeller fans now under 

development may be expected to become strong competitors of the 

centrifugal fan for all mine service. One recent installation, which is 

also the heaviest-duty fan 33 in American metal-mine service, is shown 

in figure 45, A. It is a single-propeller design of a new nonuniformpitch (Schmidt) type, which, like the Coppus, has vanes to take the 

spin out of the air and a good evase for velocity-pressure conversion. 

This fan is rated at 300,000 c. f. m. at 9 inches static pressure for an 

air density of 0.057 pound per cubic foot (equivalent to almost 12 

inches at standard density of 0.075), is built to withstand pressures 

up to 40 inches, and is reported to have a maximum mechanical 

efficiency of 83.5 percent. 

13 Richardson, A. S., Propeller·Type Mine Fan at the Moose Shaft, Butte, Mont.: Trans. Am. Inst. 

Min. and Met. Eng., February 1932, vol. 102, pp. 86-96. 
Bureau of Mines Bulletin 385 

B 

F IGURE 45.-A, Propeller fu n installed as main fan on the Moose shaft of the Anaconda Copper Mining 

Co .• Butte, Mont.; H, standard-type disk fan. 

MECHANICAL VENTILATION 131 

The ordinary-type disk fan (fig. 45, B), as made for mine service, 

has about the same range of maximum efficiencies as the radial-blade 

centrifugal (50 to 60 percent). Air-screw fans obey the same laws of 

performance as centrifugals and are selected in the same way from 

characteristic curves or performance tables. The characteristic 

curves of the standard disk types, as shown in figures 41 and 42, 

differ mainly from centrifugals in that the power decreases as the 

resistance decreases and quantity of flow increases; that is, disk fans 

require the maximum power when delivering no air and the minimum when delivering the most air. The reverse is true for centrifugals. They are otherwise high-speed fans with steeply sloping 

pressure characteristics and adapted to parallel operation. Pressures 

developed at available speeds are low and do not exceed 1 to 2 inches 

of water, depending on the design. Special disk and propeller types, 

however, by converting velocity pressure at discharge into static 

pressure, can work at high efficiency against the high pressures 

required in metal-mine service and have a very flat power and 

efficiency characteristic. 

The experiments of F. A. Steart 34 in Australia have shown that 

multistage air screws, in this case multiple, 2-blade, airplane propellers 

mounted on a common shaft, not only can develop high pressures at 

. 7° diverging 

-

FIGURE 46.-Suggested form of installation for propeller fan. 

efficiencies comparable to centrifugal efficiencies but may be adjusted 

as to number and pitch of blades to maintain maximum efficiency 

against changing resistance conditions, a very valuable characteristic. 

Several large-capacity installations of this type have been made in 

England and South Africa; development should be rapid on account 

of simplicity, ease of reversal, and ease of adjustment. Adjustment 

possibly may be facilitated by applying the control mechanism developed for an experimental type of airplane motor, in which, according 

to news accounts, the pitch of the blades is adjusted from the cockpit. 

An air screw is so much more efficient near the tips of the blades 

in moving air that a partial vacuum tends to develop back of the 

blades near the center and induces reversed flow, which radically 

lowers the efficiency. In most designs of disk and propeller fans this 

reversed flow is greatly reduced by blanking off the central part of 

the wheel, although this apparently is not required with multiplepropeller types. As a result of their studies of air flow in wind tunnels and of propeller design Caldwell and Fales 35 propose the design 

sketched in figure 46 for arranging a single propeller and housing to 

34 Steart, F. A., The Application of Air Screws to Mine Ventilation: Trans. Inst. Min. Eng. (Londoll), 

vol. 68, 1924-25, pp. 310-321. '6 CaldWell, F. W'o and Fales, E. N., High-Efficiency Air Flow: Jour. Am. Soc. Heat. and Vent. Eng., May 1920, p. 412. 
132 ENGINEERING FACTORS IN VENTILATING METAL MINES 

obtain high pressures at high efficiency. Steart's original experiments 

with 2-blade propellers showed average maximum efficiencies of 60 to 

65 percent; with more blades and better housing design these figures 

possibly have been, or at least will be, surpassed. 

On account of their relative cheapness disk types of air-screw fans 

have been so misapplied to mine ventilation that the air-screw type, 

in general, is more or less in disrepute for this service. However, 

where they can be applied the installation is so much cheaper and 

simpler than for a centrifugal that efficiency can be sacrificed to some 

exten t if necessary. 

HIGH-PRESSURE MAcmNES 

High pressures are only required in mine ventilation for auxiliary 

units ventilating dead-end openings through long lines of tubing. 

The general tendency at metal mines is to use low-pressure fans installed in series rather than a single high-pressure machine for highresistance conditions. Thus no specialized units are required and 

Discharge 

FIGURE 47.-Section of rotary positive-pressure blower (P. H. & E. M. Roots Co.). 

both pressure differences and leakage conditions are comparable 

with those experienced with shorter lines and single-fan installations. 

High-pressure machines capable of producing pressure differences 

measured in pounds per square inch (1 pound per square inch=27.7 

inches water) rather than inches of water are especially applicable to 

the ventilation of long tunnels through small-diameter pipe during 

the driving period. Positive-pressure blowers have generally been 

used for this service, for which centrifugal compressors are also applicable. Positive-pressure blowers, represented by the Roots blower 

(fig. 47) are rotating displacement compressors which trap air between 

rotating impellers and force it through the system regardless of 

the pressure required, except for a small amount that leaks 

back from discharge to intake. The amount of leakage, termed 

"slip", depends only on the pressure and impeller clearances. Cen-
MECHANICAL VENTILATION 133 

trifugal compressors act similarly to centrifugal fans and have similar characteristics. Both types of machines have a range of efficiencies comparable to centrifugal fans. The compressors require 

additional ducts for reversing the flow, whereas the flow through the 

blowers can be reversed merely by reversing the direction of rotation of the impellers provided the necessary clearances have been 

incorporated in the design. 

The pressure changes are so large with high-pressure machines 

that the flow relations cannot be considered on a constant-density, 

constant-quantity basis, as has been done throughout this bulletm 

for low-pressure mine flow. Different methods of computing pressure drops and machine performance are required that take into 

account the changes in density and quantity that accompanY'pressure 

change. Although the same general method of determimng performance by correlating system characteristic to machine characteristic is applicable, both characteristics vary with the position of the 

machine in the flow system and a more involved procedure is therefore required to determine performance under specific conditions. 

This procedure has been ably stated by Professor Weeks,a6 and since 

it has little application to metal-mine ventilation and might introduce 

an element of confusion were it set out here no discussion of the 

theory and 'Practice of high-pressure ventilation will be presented. 

AIR INJECTORS 

Air jets, injectors, ejectors, and similar devices for producing air 

flow have found limited application in metal-mine ventilation. They 

are easily installed and cheaply constructed but waste so much power 

that they are used only in temporary or emergency auxiliary service. 

They depend on the conversion of the velOCIty pressure of a highvelocity jet, of what actually amounts to a separate flow system, into 

useful pressure to 'produce flow in the system where desired. In mine 

work the motivatmg system is usually a high-pressure, low-quantity 

system-compressed air discharging through a small-diameter nozzle 

-but it may be a low-pressure, large-quantity system similar in 

general aspects to the "Saccardo" system of tunnel ventilation discussed later. Air jets and injectors may take many forms-from 

nozzles designed to screw onto a compressed-air hose to well-designed venturi types, such as that shown in figure 48. The latter 

are used to a large extent in German and South African mines for 

auxiliary ventilation and have been used in a few of the metal mines 

of this country since their introduction at the United Verde mine 

several years ago. Simpler types, such as a jet discharging on the 

axis of a uniform-diameter pipe or on the axis of single or multiple 

cones for air-motion effects at dead-end faces, have also been used to 

a limited extent in American metal mines. 

The over-all efficiency of pressure generation is the ratio of the useful 

work done on the main air-flow system to that required in the activating flow system. As Weeks a7 shows, the efficiency with which the 

energy of the jet is utilized in common designs is very low, and the overall efficiency of both flow systems is still lower . Using compressed air in 

36 Weeks, W. S., Theory and Practice in the Ventilation of Long Tunnels: Eng. and Min. Jour .. Nov. 17, 1928, pp. 779-781. . II Weeks, W. S., Ventilation of Mines: New York, 1926, p. 141. 
134 ENGINEERING FACTORS IN VENTILATING METAL MINES 

the "Modder" design, after which most home-made types are patterned, Weeks obtained a maximum jet efficiency of 7.3 percent and an 

over-all efficiency, based on an average figure of 45 percent for efficiency of compressed-air production, of 3.3 percent. HIgher efficiencies 

have been reported for special designs, but the corresponding maxiTImms probably are not over 15 and 7 percent, respectively. The latter 

figure should be compared with the average of about 60 for centrifugal 

fans in determining the status of injectors. However, they are useful 

emergency and temporary installations and will operate under conditions that would ruin a fan and motor in short order. 

Performances for injector designs may be approximated only roughly 

by theory 38 and must therefore be actually determined by tests 

against a range of resistance conditions, as in the case of fans. Similar 

characteristic curves of performance may be used in solving problems 

ASSEMBLY 

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NOZZLE 

I! 

BRACKET 

Ii I ~ ~ F,GURE 4B.-Yenturi type of compressed-air injector used for auxiliary ventilation in United Verde mine, 

Jerome, Ariz. 

of application. Curves are required for each orifice size and orifice 

pressure with compressed-air jets and correspond roughly to the 

separate fan characteristics required for separate speeds and sizes but 

to the author's knowledge without the benefit of a set of laws similar 

to the" fan laws" to facilitate the determination of a multitude of 

characteristics from one set of tests or the use of characteristic-ratio 

curves. 

Saccardo system.-Although the Saccardo system of injector ventilation was developed for ventilating tunnels and has been used exclusively for this purpose, it has a possible use under special conditions 

in metal-mine ventilation, that is, where operating advantages, such 

as those resulting from elimination of an air lock on a busy haulageway 

or shaft, would offset the relative inefficiency of power application. 

In this system air flow from a fan is led through ducts to a small 

as Colliery Engineering (London), Ejector Ventilation: Vol. 3, November 1926, pp. 474-475. 
MECHANICAL VENTILATION 135 

opening or nozzle, where it discharges at high velocity into the main 

airway system in the direction of flow. A large part of the velocity 

pressure at the discharge of the nozzle is converted to static pressure in 

the airway system. The total pressure usefully applied to the system 

is the difference between the velocity pressure at the nozzle discharge 

and the pressure loss due to shock loss attending the change of area 

as between the nozzle discharge area and the area of the airway into 

which it discharges. The performance 39 varies with the ratio of 

nozzle area to airway area, direction of discharge with reference to the 

axis of the airway, and design of nozzle; like fans, each design will 

operate at maximum efficiency against but one resistance condition 

and at lower efficiencies against all others. The maximum possible 

efficiency in the use of jet energy, with abrupt expansion over almost 

half of the perimeter, can hardly exceed about 50 percent, which 

means a probable maximum over-all efficiency of the system of about 

30 percent; lower values may be expected in actual practice, since the 

duct pressure losses from the fan to the nozzle are not included in this 

figure. The pressure losses in the ducts carrying the air to the nozzle 

increase at the same rate as the velocity pressure at the nozzle and 

limit the application of the system to relatively low-resistance 

ventilating systems. 

PRESSURES GENERATED BY MOVING OBJECTS 

Still objects in airways obstruct the flow and cause pressure losses, 

but when they are moving in an airway they not only act as obstructions to flow but also generate pressure in the direction of their movement. When they are moving in the same direction but at a slower 

rate than the average velocity of flow, the pressure losses exceed the 

pressures generated, and the net results are pressure losses similar to 

those experienced with fans which, under the action of other pressure 

sources, are forced to pass more air than they normally handle against 

no resistance. "When objects, such as cars, skips, or cages, move 

through an airway at a faster rate than the average velocity of flow 

but in the same direction, the pressures generated exceed the pressure 

losses, and the net pressures generated act with or without other 

pressures to create aIr flow. "When the movement is opposed to the 

direction of flow, the pressure generated is opposed to that causing the 

flow and is then equivalent to a pressure loss, as when natural-draft 

pressure is opposed to fan pressure. Theoretical calculations of 

pressures generated by moving objects are of doubtful and incomplete 

application if based on relative velocities and empirical data resulting 

from pressure-loss determinations for still objects as obstructions in an 

airway. It seems probable that characteristic curves of the net 

pressures generated by moving objects could be devloped from actual 

tests in which all factors were taken into account but to the author's 

knowledge no authentic data are available. 

Large-volume circulations often are reversed temporarily by rapid 

movement of skips and cages in shafts and trains of cars on horizontal 

airways, where the moving object almost fills the airway or where the 

pressure inducing the circulation is small, as in natural-draft ventilation. In an open-timbered shaft the movement of skips or cages 

" Bulletin of the International Railway COnl:l"ess, The Ventilation of Tunnels on Saccardo's System: Eng. ed., vol. 8, April 1899, PP. 477-495. 
136 ENGINEERING FACTORS IN VENTILATING METAL MINES 

operating in balance affects the air flow only slightly, since opposing 

pressures are generated and there is always plenty of clearance for 

the air flow in adjoining compartments. Where the air flow is in 

separately lined compartments with small clearance the piston effect 

is large, although usually limited considerably by openings between 

compartments at frequent intervals. As a rule, these are provided 

to facilitate repair of the shaft, but they also limit the piston effect of 

cage and skip travel and thus reduce interference with normal 

distribution. 

DISTRIBUTION OF AIR CURRENTS 

Natural distribution of air currents in mine openings is practically 

always inefficient and unsatisfactory; although circulations on the 

main openings may be good very little air is circulated through the 

working places where it is most needed. Some places receive more 

air than they need and others none at all. For effective and efficient 

distribution both direction and quantity of flow must be controlled. 

General directions of flow are determined by the points in the 

system of connected openings at which the pressures are generated 

and the general quantities of flow by the intensity of the pressures 

generated. Both direction and amount, however, can be controlled 

not only by regulating the position and intensity of the pressures 

generated but also by artificially regulating or modifying the natural 

resistances to flow of the mine openings or, if necessary, by providing 

artificial airways. Resistance to flow on an airway may be increased 

by tight stoppings so as to stop the flow entirely, or it may be increased by loose stoppings to restrict the flow to leakage, or it may be 

increased a definite amount by regulators to provide only the desired 

amount of flow rather than the larger amount that would occur 

without regulation. Resistances to flow may be decreased by all the 

various methods of changing the particular physical conditions of 

the airway, or an equivalent result may be obtained by local generation 

of pressure on an airway circuit, as by a secondary or booster fan. 

Where there are no separate intake and return airwa;ys, as in dead-end 

development openings, a single airway may be diVIded by a wall or 

brattice to make two separate airways, or an artificial airway may be 

constructed within the original airway, as in auxiliary ventilation by 

fan-tubing units, or at the junction of airways to permit air currents 

to pass each other without mingling. 

Where the natural or artificial features of the distribution system 

are not tight, leakage not only increases the power required to produce 

the desired circulation but increases the hazards at the time of fire 

through lack of control. If the leakage in the circuit does not go 

direct from the system to the atmosphere but is recirculated underwound the fire hazards and likewise the waste of power are greatly 

mcreased. 

POSITION OF MAIN FAN 

Directions of flow, which largely determine the air conditions in 

operating zones, conditions of leakage and recirculation, and control 

of distribution both normally and in the event of underground fire 

are primarily determined by the position of the main fan or fans. 

The demands of service and safety sometimes coincide but usually are 

antagonistic either in whole or in part. With 2 surface openings 
DISTRIBUTION OF AIR CURRENTS 137 

available, 1 in active use for operating purposes and the other an 

inactive air shaft, primary consIderation of the ventilation problem 

involves 3 decisions-whether the general direction should be fixed 

or subject to reversal in emergency, what should be the normal 

direction with respect to the operating opening, and whether the fan 

should be on the surface or underground. 

Actual conditions control the final decisions on these controversial 

subjects, but in most instances a reasonable regard for the safety of 

life and property requires a reversible fan, primarily exhausting, 

installed on the surface at the nonoperating opening. This makes 

the operating opening, which the men use in going to and from their 

working places and to which they naturally go at the time of an 

emergency, an intake opening and one most likely to provide a safe 

exit as well as a safe base for attacking the cause of the emergency. 

USE OF REVERSING ARRANGEMENTS 

The reversing arrangement should be installed regardless of whether 

the necessity for it can be foreseen or not. It is primarily an insurance 

against unforseen happenings and as such has proved its worth in a 

number of instances. Lack of reversing arrangements, on the other 

hand, has been held responsible by competent observers for much of 

the loss of life and property in certain of our greatest mine-fire 

disasters. Whether or not to reverse a fan during a mine fire is 

usually a serious decision-one to be made only by the highest 

officials immediately available after due consideration of all factors 

involved; the reversing arrangements should therefore be locked 

unless reliable attendants are employed. Time is an important 

element, and since time is· required to ascertain the necessity of 

reversal and to carry out the operation it is important not only to 

insure smooth operation by occasional trials of the mechanism but 

also to plan other methods of control to insure safe exit of the men 

and to prevent the fire from gaining headway rapidly, such as installing fire doors and poss bly slowing down or stopping the fan or fans. 

DIRECTION OF FLOW 

The important consideration in determining direction of flow is that 

the usual means of exit should be on intake air; this usually means 

that the operating opening should be an intake. Other means of 

obtaining the same result are, of course, just as effective, such as 

making the operating opening a fresh-air outlet, or upcast in the case 

of a shaft, or putting it under pressure with intake air. In the average 

naturally ventilated mine the directions of flow cannot be controlled 

easily; in many in which the general directions are constant the operating shaft normally becomes an upcast, although this tendency is 

not as marked now as when steam pumps were commonly used underground. Where possible the flow should be reversed to ~ke the 

operating opening an intake. 

In cold climates one of the chief objections to the use of operating 

openings as intakes is the formation of ice in the opening and the 

freezing of water lines. Most of these difficulties can be largely overcome at moderate expense which, even in addition to the discomfort 

produced under certam operating positions, does not begin to balance 

122422°-35---10 
138 ENGINEERING FACTORS IN VENTILATING METAL MINES 

the sacrifice in general safety conditions that would otherwise result. 

Because of the effect of rapidly changing temperature and moisture 

conditions on timber and rock, intake openings are naturally subject 

to higher maintenance charges near the entrance than the returns with 

their uniform air conditions, but return air currents not only may be 

highly corrosive to equipment but also make for wet, muddy, hot, 

foggy, and smoky combinations of air conditions, which do not promote efficient operation in a zone of great activity. 

'Where large quantities of dust are raised on or adjacent to the 

opening that should for safety be used as an intake, airway provision 

should be made, of course, to preven t the dust from entering the intake 

currents; such special conditions make every mine a separate problem 

for which only general results can be indicated with any degree of 

exactitude. In a large mine many conflicting features are involved, 

and the system should be planned to take care of as many of them as 

possible in the order of their importance, first, as respects safety and, 

second, with regard to service. 

MAIN FANS ON SURFACE 

In general, main fans should be installed on the surface where they 

are subject to ma:\.imum control. A fan underground may be put out 

of commission entirely during a fire; about the only control assured 

is that of cutting off the power. Some operating conditions, however, 

may make it desirable to install the main fan underground for normal 

operation, in which case a surface fan may be installed as a stand-by 

in case of a fire underground or separate power sources may be connected to the fan and remote control provided. In some lay-outs an 

underground installation may actually be more accessible under ordinary conditions than a surface installation, but it is alwa;rs wise 

policy to provide surface equipment for emergency service. Mille fires 

occur so often and are so extremely costly that all the insurance 

possible is not too much. 

Where nIl surface openings are required for operations, underground 

installation is the only solution usually considered in American metal 

mines, but in English coal mines surface insta.llations involving operation of cages through air locks or double-door arrangements are common practice. These arrangements are relatively expensive where 

rapid hoisting through shafts is involved, but the operation of main 

fans underground is often much more expensive through waste of power 

than is generally realized. 

In surface installations the greatest pressure differences between 

intakes and returns occur in the upper part of the mine where the 

stoppings ordinarily can be kept in good condition on account of their 

infrequent use. In underground installations the greatest pressure 

differences are on stoppings and doors in an active zone where they 

are used frequently and are difficult to keep tight. Surface installations usually result in lower-resistance air-flow systems than underground installations, as the latter usually must be placed so that the i~ow is confined to les;s than the full number of airways available. 

Underground installations usually require air locks on main haulage 

levels that might otherwise be left open and practically always result 

in considerable recirculation, a dangerous factor in case of mine fire 

and an undesirable condition under any circumstances. 
DISTRIBUTION OF AIR CURRENTS 139 

FIREPROOF INTAKE AIRWAYS 

Intake airways generally range from damp to dry and present a 

fire hazard against which it is difficult or impossible to protect the men 

underground, except through fireproof construction; this should, if 

feasible, be combined with smooth-lining for efficiency. Even with 

fireproof construction on the intakes, they cannot be regarded as so 

absolutely fireproof as to dispense with the reversible feature of the 

fan installation; burning materials in the openings or on stations may 

give off enough gases to create a dangerous condition. Fireproof construction is also desirable in main return airways, but as these are 

generally wet throughout such construction is mainly a question of 

insuring against property loss and interference with the continuity of 

operations. 

FIREPROOF FAN INSTALLATIONS 

Fan installations are in the same category as intake openings and 

should by all means be made of fireproof construction, as is the case 

for main fans at most metal mines. Operators are less scrupulous 

regarding secondary and auxiliary fan installations; these often present considerable fire hazards, particularly the forward-curved-blade 

centrifugals, which under certain circumstances overload and burn 

out their motors. Even the backward-curved-blade centrifugals with 

their constant-power characteristic are not entirely safe on this score, 

as without proper instruction the first arrivals may be set up to run 

backward or the reverse of their normal direction of rotation, in which 

event they put a large overload on the motor and may burn it out. 

VELOCITY LIMITATIONS 

High velocities often are uncomfortable or disagreeable but usually 

can be avoided except on the main airways. All airways having 

velocities above 500 f. p. m. should be lighted electrically. Carbide 

lights can withstand velocities up to 1,000 f. p. m. but only with difficulty. Reliance should not be placed on the lighting service in emergencies; high-velocity sections of emergency escapeways should be 

provided with regulators for reducing the velocities temporarily to 

permit safe travel With carbide lights. Dust at shift-changing 

periods and smoke at blasting periods often can be controlled by 

regulating the speed of the fan temporarily to reduce velocities of 

flow, by stopping the fan, or by using regulators on the main flow or 

on local air currents. 

STOPPINGS 

Virtually all devices used to control the distribution of air currents 

by introducing an obstruction to resist the flow or by using artificial 

division walls in the airway are included in the general term "stoppings", since the materials and conditions of installations are similar. 

TEMPORARY BULKHEADS 

A tight stopping placed across an airway to cut off the flow is 

generally termed a "bulkhead" in metal-mine practice. Temporary 

bulkheads are seldom used in metal mines except to check air currents 

during a fire until more permanent types can be erected. They may 
140 ENGINEERING FACTORS IN VENTILATING METAL MINES 

be made of a variety of materials and are of various designs, depending 

on conditions. A skeleton frame covered with light boards is the 

common construction in metal mines because other materials usually 

are not readily available. It ordinarily is made tight by stuffing- or 

covering the openings with clay, rags, or pieces of old canvas tubmg; 

in rough rock airways any high degree of tlg"htness is practically impossible except with clay. Brattice cloth IS commonly used in coal 

mines 40 for temporary stoppings because they can be rapidly and 

cheaply constructed with it, but it is rarely used in metal mines because blasting concussions are stronger and because considerable 

labor is required to insure a good fit and minimum leakage against 

rough rock walls. Tarred roofing paper is probably a better material 

in the absence of blasting concussions, as it can be torn and wedged 

against the rock to make a good fit without the use of a multitude of 

cleats. 

DEFLECTOR BRATTICES 

Stoppings constructed without regard to tightness for deflecting air 

currents from their normal path are usually termed "deflector brattices" in metal mines or simply "brattices" in coal mines. Placed 

across an airway in a coal mine, they often serve to "hurdle" the air 

into a high spot in the roof in which explosive gas would otherwise 

collect; whereas, when so placed in a metal mine their most common 

use is to "hurdle" the air through a working place above or below the 

airway. Deflector brattices have been tned in metal mines for 

increasing the average velocity through working zones by limiting the 

space occupied by the flow. They usually are of rough wood construction. Canvas brattices are not successful in metal-mine stopes 

on account of the damage done by blasting concussions. Deflector 

brattices are used partieularly in hot metal mines to prevent convection currents from idle dead ends entering a cooler intake airway, 

as such currents cause mUltiple small-temperature increases in the 

cooler air current. 

PERMANENT BULKHEADS 

Permanent bulkheads are used to only a limited extent in metal 

mines. Caved or fill material in openings is generally relied upon to 

prevent the flow of air through abandoned workings; stoppings are 

used in but few mines for this purpose. Ventilation systems and 

safety conditions would, in general, be improved by the use of permanent bulkheads, but in the average mine they are seldom used 

except for sealing off mine-fire areas. The common type in solid 

ground is a rough-board stopping covered with gunite-a thin 

coating of a sand-cement mixture applied with compressed air--over 

expanded metal lath or, less frequently, wooden lath or chicken wire. 

Although absolutely tight when first erected, it develops minor leaks 

from blasting concussions and ground pressures and requires periodic 

inspection and repair. Where a good permanent job is required for 

sealing fire areas over a long period it is now more common practice to 

use concrete wall bulkheads with or without light reinforcement. 

Such bulkheads usually are more satisfactory and, over a period of 

years, nearly always cheaper in total cost. 

" Rice, G. S., and Owings, C. W., The Use of Brattice Cloth in Coal'Mines: Rapt. 0 IInvestlgatioDS 2872, Bureau of Mines, May 1928, 6 pp. 
DISTRIl3UTION OF' AIR CURRENTS 141 

Comparative costs of various types of tight permanent stoppings 

are given in Bulletin 99.4' The average cost of a gunited stopping 

for an ordinary isolated 5- by 7-foot rock airway is about $15 or 

approximately 40 cents per square foot. Larger isolated installations 

can be made for about 30 cents per square foot and large jobs of continuous guniting of airway surfaces for about 20 cents per square foot. 

Wood-fiber plaster, the common material for sealing board stoppings 

in coal-mine practice, is seldom used in metal mines, but lime-sandcement mortars have proved satisfactory where there are no blasting 

concussions. When used in highly humid atmospheres the natural 

clays found in some mines retain enough moisture to make a very 

effective sealing material against heavy concussions and ground 

pressures. 

Where ground pressures are particularly severe even reinforced 

concrete stoppings crack and become displaced, and repairs are difficult and costly. Stop pings of timber packs or of timber and rock 

packs are especially suitable for this condition, as they crush into such 

a tight mass with increasing ground settlement that passage of air is 

not only prevented but the timber is preserved thereby. 

Brick and concrete blocks are used only to a limited extent for 

permanent stoppings in metal mines because they cannot withstand 

ground pressure. Rough-board stoppings are never tight, even when 

battens are used, and should be covered with sealing material. 

Tongue-and-groove boards can be used to give tight constl1lction but 

are actually no better than battened rough boards unless the edges 

are sealed to the rock. 

DOORS 

Where passage through a stopping is required only under exceptional conditions the stopping may be provided with a manhole or 

small hole with a removable cover, but where frequent passage for 

men and materials is required a door must be installed in the stopping. 

The door usually is almost as large as the stopping, and the installation 

of both door and frame is commonly termed simply a "door." Doors 

are used more extensively in metal mines for controlling distribution 

of air currents than in coal mines because, although the same difficulties of obtaining satisfactory service are presented, the dangers 

attending the use of doors in mines generating explosive gas are almost always absent. They are therefore used to a large extent in 

metal mines where stoppings would be used in coal mines, and transportation and travel are thereby facilitated. 

Keeping ventilation doors shut in metal min~s is one of the most 

troublesome operating features of any complicated ventilation system. 

The discipline required seems to most under officials at mines to be 

entirely out of proportion to the results obtained, and pressure must 

usually be brought to bear on them by higher officials conversant with 

the advantages of such installations to obtain even fair results. 

Under ordinary operation conditions the effects of large door leakages 

and of leaving doors open are not always readily apparent, and a 

detailed ventilation survey is sometimes required to establish their 

full significance. Mine fires often bring these matters forcibly to the 

attention of the operating force, especially when the fire gases permeate the whole mine before doors are closed and leakages reduced to 

41 Williams, R. Y., Mine-Ventilation Stoppings: Bull. 99, Bureau of Mines, 1915, 25 pp. 
142 ENGINEERING FACTORS IN VENTILATING METAL MINES 

the minimum. The most successful and efficient ventilation systems 

are those that rely the least on doors, but scattered working places 

often require the use of a large number, particularly where the main 

fans are placed underground. 

A mine door is more or less temporary, yet for most conditions 

should be as tight as possible. It receives rough use, requires frequent repairs, and allows rather large leakage through openings of 

apparently insignificant size. The only good door as regards leakage 

is an absolutely tight one with an absolutely tight frame, as there is 

difficulty enough in holding the leakage occasioned by the fit of door 

to frame to the minimum. Good sill blocks are also essential. and 

cltnvas strips should be used at least along the sill where the leakage 

Note: Spaee between frame and rock surfaces filled with concrete' 

A 

SECTION AA 

FIGURE 49.-Ventilation door used on motor haulageways in Calumet and Arizona mine, Bisbee, Ariz. 

. . due to necessary clearance is usually large. Unless a little leakage 

through the door is required to keep the air clear and retard decay of 

timber, canvas strips or the equivalent of weather-stripping should 

be used on all edges. Drainage ditches without water seals and 

excessively large openings for pipes and trolley wires are common 

causes of excessive leakage through doors. 

TYPES 

The most common type of door in the metal mines of this country 

is a single, vertically hung, unpainted door made of 2 plies of 1-inch 

boards with cloth or paper between the plies. (See figs. 49 and 50.) 

One ply is placed horizontally and the other vertically. The plies 
Bureau of Mines Bulletin 385 

FIGURE 50.- A amI IJ, C'ompressed-air-operated ventilation doors, with adjoining man doors, used on haulage drifts in Calumet and Arizona mine, Bisbee, Ariz. 
Bureau of Mines nlllletin 385 

FIGURE .5I.- Iron-door regul ator, with feed screw setting, llsed in Un iten Verde mine, Jerome, Ariz., to proportion amount of air entering eaeh l.lVf' 1 from main-intake air shaft. 
DISTRIBUTION OF AIR CURRENTS 143 

are held together by clinched nails; reinforcement strips are sometimes 

used but add to the weight without improving the construction. 

The combination of horizontal and vertical plies seems to resist warping better than diagonal plies and usually is found where the best door 

has been developed through trial of many designs. The door is 

hung on a frame of heavy timbers by 2 or 3 heavy strap-iron hinges 

extending the full width of the door; it can sWing in but one direction 

and closes against the frame rather than in a jamb. In solid ground 

a jamb might give a better fit and less leakage, but doors are usually 

placed in ground subject to slight movement, and any settlement or 

movement of the frame would bind the door if set in a jamb. This 

method of flush closing requires only a facing strip on the hinge side of 

the frame and simplifies construction. Concrete sills sometimes are 

used but are hard to keep in repair. Wooden sill blocks, secured to 

the track tie or to a sill member of the frame, are more common; they 

provide clearance at the track and project 2 to 3 inches above the 

rails to serve as stop blocks. 

Most doorframes are made of 6- by 8- to 8- by lO-inch timbers 

with the space to the rock walls filled with concrete or boarded over 

and gunited. Doors are wmally provided with a heavy counterweight 

to facilitate opening and to keep the door closed under all pressure 

conditions. Printed cards often are tacked to the door to designate 

the normal position, whether closed or open. 

Many other types of doors and frames are in use. Where leakage 

is not important single-ply doors made of canvas nailed to a crossbraced wooden frame have served this purpose, but they add to the 

fire hazard, warp readily, and seldom are used because more substantial types require fewer repairs and result in fewer interruptions 

to ventilation. Double doors are seldom used, even. on the larger 

airways. Virtually all doors and frames, except those for fire control, 

are made of wood although, where corrosion conditions permit, light 

sheet-metal doors in iron frames 42 would be as effective at comparable 

costs. Iron doors set in iron frames embedded in a concrete bulkhead 

are commonly used for fire doors on timber-lined openings and are 

similar in construction to the regulator door shown in figures 51 and 52. 

These usually ,are provided with latches, wedges, or clamps, rather 

than counterweights, to insure their remaining tightly closed under 

all pressure conditions. 

In relatively few designs is the frame set on a batter to insure the 

automatic closing of the door. This feature, apparently standard for 

coal mines, is seldom used in metal mines, except where the door is 

hung centrally from a timber set on a batter as respects the plane of 

the frame; the door can thus swing in either direction with automatic 

return to the normal closed position. A heavy bumper block, iron 

spring, or other arrangement is usually fastened to each face of the 

door so that it may be bumped open. These designs allow large 

leakages and are restricted to low-pressure conditions. An extremely 

sturdy door is required to withstand bumping by mine cars even for 

a very short time. . Two doors swinging in opposite directions are sometimes attached 

to opposite sides of the same frame to allow reversal of direction of 

air flow. In many Michigan copper mines emergency fire doors are 

.. Glaeser, O. A., New Type All·Metal Ventilation Door: Eng. and Min. Jour., vol. 1<'2, 1931,!t. 366. 
144 ENGINEERING FACTORS IN VENTILATING METAL MINES 

hung horizontally and are normally supported in the open position 

against the roof on buttons attached to short rod hangers. Occasionally a sliding or roller-curtain type is adapted to a local condition. 

AIR LOCKS 

For positions where the pressure difference is high or where the 

door is open often or long enough to disturb ventilation seriously 

doors should be placed in pairs to form an air lock, so that only one 

door of the pair is open at a time. The use of air locks also reduces 

4" by 4" by r angle 4' 6" long 

r:~r~~::==~~:::fi~-r .. !::I 

] 

ELEVATION 

}----4'6" I Pinion 

Nut 

SECTION 

SCREW BAR 

FIGURE 52,-Regulator door used at level intersections on intake air shaft, United Verde mine, Jerome, Ariz, 

leakage to the minimum and enables the doors to be readily opened 

and closed when the pressure difference is high. They find most 

frequent use on' motor haulageways where the doors must be far 

enough apart to accommodate the maximum length of train with 

ample clearance. Good positions for air locks are not always available 

if the openings have been made regardless of ventilation plans, and 

it is sometimes necessary to accommodate the length of the train to 

the length of the air lock. 
DISTRIBUTION OF AIR CURRENTS 145 

AUTOMATIC DOORS 

Automatic control of doors on motor haulageways is desirable; many 

types of automatic doors, controlled by movement of the cars, as by 

their weight on a false rail, by engaging a bell-crank lever attached 

through pulley and rope to the door, by engaging studs on an endless 

chain revolving on pulleys parallel to the track, or by engaging spring 

bows parallel to the track which are pushed out and straightened by 

the side of the cars, are either commercially available or designed 

locally and put into service. A rail-operated type, used largely in coal 

mines, is giving good service in the few metal mines in which it is used; 

in general, however, the metal-mine operator has found that his needs 

are served just as well and at less expense by remote-control doors. 

REMOTE-CONTROL DOORS 

Where opening doors by hand would be especially inconvenient and 

would waste time, a remote-control compressed-air mechanism usually 

ia installed in western metal mines. These are made locally and differ 

mainly in size of air cylinder. The typical design illustrated in figure 

53 operates the doors shown in figures 49 and 50. This design includes 

a small hand-operated side door for the use ·of the men-a desirable 

safety feature, as numerous accidents have been caused by single-door 

installations although many other devices, such as glass panels, 

warning lights, and compressed-air-operated whistles, are used to 

prevent them. The door is opened by a compressed-air piston operating in a pipe cylinder and is closed by a heavy counterweight when 

the pressure is released. The valve on the compressed-air line is 

controlled from the moving train through levers attached by long 

wire ropes to a single three-way valve. If there is no side door additional levers attached to the common wire-rope system are installed 

a few feet on either side of the door for the use of the men in passing. 

In curved drifts simple pipe rollers are provided to reduce friction, 

The wire ropes stretch and must be adjusted frequently through 

turnbuckles provided for this purpose. The pressure of the compressed air must be adjusted through an independent supply valve 

to conform with the air-pressure conditions to give free operation 

without too violent action. These doors require considerable attention but have proved very satisfactory as applied to a wide range of 

metal-mining conditions. 

Some sinular designs operate by water pressure; these appear to 

have the advanta~es of smoother operation and lower maintenance 

charges but reqUIre corrosion-resistant cylinders at a higher first 

cost, and disposal of the water is often a problem. The compressedair valve of a few designs is operated by solenoid switches, the levers 

of which are thrown by the motorman in passing. The wire-rope 

control is satisfactory up to about 200 feet but has too much stretch 

for long-distance control. For exceptionally long trains the United 

Verde mine installs separate three-way, direct-throw, compressed"-air 

valves at the operating positions, which are connected direct to the 

cylinder at the door; air pressure thus serves both to open and close 

the door. 
146 ENGINEERING FACTORS IN VENTILATING METAL MINES 

FIRE DOORS AND THEIR CONTROL 

In case of fire it is desirable to reduce the velocity of certain ventilating currents immediately without changing the directions of flow. 

About the only sure way to accomplish this under all conditions is to 

be able to shut off all main airways with doors. These are normally 

blocked open and used only in case of fire; they are therefore termed 

'___ _ _ Center 1in~ gf...,tr"'a"'ck'--__ 

PLAN 

r x2"x2'6" lever 

1--------5'6"---------01 

DETAIL OF AIR CYLINDER 

FIGURE 53.-Device used in Calumet and Arizona mine, Bisbee, Ariz., for operating ventilation doors by 

compressed air. 

"fire doors" to distinguish them from ventilation doors, which they 

resemble in all respects, except that those installed in timbered sections for this purpose usually are of fire-proof or at least fire-resistant 

construction. 

The best position for the main group of fire doors is on all the open 

airWays off the main or operating shafts; many metal mines are so 
DISTRIBUTmN OF AIR CURRENTS 147 

provided, at least in theory. In practice, the connections most 

recently opened often have no doors; the advantage of previous 

installations is thus largely nullified, as a chain is no stronger than 

its weakest link, and the time required to close off such openings is 

just the time that such a system is designed to save. 

In general, the fire doors are placed in position, and the operating 

force is relied on to use them in an emergency to cut off the flow 

from the fire area and thus retard the spread of the fire and fire gases. 

Realizing the importance of the time element, a number of mines 

have installed remote control on the sets of doors adjacent to particular shafts, so that all may be closed from one or more positions, 

particularly from the surface. Water pressure, compressed air, and 

electrically operated mechanisms have been installed. They take the 

form of a latch, which holds the door open normally against the pull 

of a counterweight and which is drawn back when it is desired to 

close the door. The latch is withdrawn from a distance by either 

applying or removing current or pressure. Under mine conditions 

continual inspection and trial of such mechanisms are required to 

insure their successful operation when wanted; failures through 

ground movement and corrosion have been common enough to retard 

the spread of this very useful idea. The more successful types apparently are those that rely on water or air pressure to keep the latch 

in position, as failure, which is indicated by the closing of the door, 

is soon noted. Stopping the fans does not stop the air currents in a 

mine, and stich a set of remote-control fire doors on operating shafts 

is one of the most effective means of handling an underground fire 

with minimum loss of time, which in most instances means minimum 

loss of life and property. 

PRESSURES ON DOORS 

Pressures on single doors should not greatly exceed 1 inch of w,ater, 

because of the difficulty of opening them against higher pressures, 

even with pry-bar arrangements. For higher pressures an air lock is 

desirable, 3.8 this distributes the pressure difference over both doors; 

or more than two doors may be used for this purpose. In practice, 

pressures on air locks are seldom above 2 inches of water; where the 

pressure difference is higher passage through the doors is difficult 

and possibly dangerous unless they are equipped with supplementary 

small doors through which a man can crawl in an emergency. The 

small door can be opened easily, even though the pressure is high, 

and its opening equalizes the pressure on both sides of the large door; 

the large door may then be opened and closed easily. After the first 

door of the air lock is passed and both large and supplementary doors 

closed the process is repeated at the second or succeeding doors of 

the air lock. Doors under high pressure may also be equipped with 

mechanically operated devices, such as compressed-air or waterpressure cylinders; such installations are desirable on frequently 

used doors that operate against pressures greater than one half inch 

of water. To facilitate pressure-difference determinations at doors 

during surveys and as a check on leakage, a ~-inch-diameter hole 

through either door or frame should be provided; the same idea 

should also be applied to stoppings, particularly at undergroundand surface-fan installations. The leakage through such a hole is 
148 ENGINEERING FACTORS IN VEN'I1ILATING METAL MINES 

insignificant-the resistance factor is about 45,000,000, and the 

leakage at a pressure of 1 inch is only about 5 c. f. m. 

COSTS 

A few data collected in 1928 in Arizona copper mines indicate that 

costs for substantially constructed and installed doors and frames with 

about a 4.5- by 6.5-foot opening are about as follows: Replacing 

door only, $25; door in concrete frame, $50; door in concrete frame, 

air-operated with remote control, $175; air-operated door 'Vvith adjoining narrow man door, $225; and water-operated door with 

adjoining narrow man door, $250. An automatic door with adjoining man door costs approximately $400. A less substantial wooden 

door and frame of average size can be installed for $15 to $25. 

LEAKAGE 

Leakage at stoppings, particularly doors in active sections and at 

fan installations, is a common cause of inefficient distribution of air 

in mines; it results in poor ventilation of working places, waste of 

power, and where the leakage is recirculated increases the hazard 

during an underground fire. Actual tests in a large number of coal 

mines show that in the average mine not more than 30 to 50 percent 

of the air handled by surface fans reaches the active workings. A verage conditions in metal mines, on account of the general differences 

in distribution lay-outs, are somewhat better, probably ranging from 

50 to 60 percent for the average mine and lay-out to about 85 percent for the best. An allowance of about 15 percent for unavoidable 

leakage is ·the minimum that is ordinarily practicable under the best 

lay-out. If all main fans are underground and close to active workings it is almost impossible to avoid considerable recirculation; 

ordinarily, such fans handle 1% to 3 times the amount of air that 

actually enters the mine. In a warm or hot mine increased velocities usually improve comfort; in general, however, recirculation of air 

currents results in increased air temperatures .and decreased comfort. 

Leakage and recirculation can be largely avoided by a properly 

planned lay-out, substantial construction, and close attention to 

details. Individual leakages are often insignificant and difficult to 

detect, but the cumulative effect of small leakages is by no means 

insignificant; it is therefore imperative to use airtight construction 

wherever possible, as unavoidable leakages will cripple the system 

enough without loading it with avoidable leakages. 

FAN INSTALLATIONS 

Pressure differences are highest at fan installations, and leakage is 

particularly important-more important, in fact, than the average 

construction crew realizes. Steel-plate housing and connections, even 

though carefully erected by a reliable crew, are rarely tight. Leakage 

conditions vary, particularly with the amount of surface exposure and 

the number of doors. At pressures of 3 to 4 inches of water the leakage at a single-inlet nonreversible fan with short duct connections may 

be as low as 1,000 to 2,000 c. f. m., but that at the usuallarge,reversihIe, double-inlet installation offset from a shaft collar usually is as 

high as 3,000 to 10,000 c. f. m., depending on the use of air locks on 

openings and the proportion and surface exposure of steel plate to 
DISTRIBUTION OF AIR CURRENTS 149 

concrete or other airtight construction used in the housing and connections. Wooden ducts, even though gunited, generally are poor 

construction as regards leakage and add to the fire hazard. 

Fans often are installed at shaft collars built up of waste rock from 

sinking operations. Unless the shaft is lined to solid rock with concrete or its equivalent such a rock fill will "leak like a sieve", and the 

fan installation will be a failure or at least require remedial measures, 

in the way of sealing the shaft walls, that are not only expensive but 

usually effective over but short periods. Concrete linings sealed to 

solid rock will show a high first cost but will practically always show 

a lower final total cost at main-fan installations. 

SHAFT COMPARTMENTS 

The practice of carrying main intake and return currents in 

adjoining shaft compartments, although common in coal mines, is 

comparatively rare in metal mines. In theory leakage can be held to 

the minimum by airtight construction, but in practice such lay-outs 

have proved very inefficient on account of leakages. They add tC'< 

the .danger in case of fire and are properly condemned by mine safety 

engIneers. 

ADJOINING AIRWAYS 

The practice of carrying main intake and return currents in parallel 

airways separated by a short distance and connected by stopped-off 

openings at frequent intervals, although common in coal mines, is 

also comparatively rare in metal mines. In general, surface openings 

at metal mines are considerable distances apart; but where two are 

close together the general practice is to use them as parallel airways 

with free intercommunication and, if no other surface opening is 

available, to drive a new opening, usually a raise, or raise to an adit 

solely for ventilation. With surface-fan installations the pressures 

on the stoppings and doors separating parallel airways usually are 

high, and leakages are difficult to control, particularly if operations 

require frequent passage of men and materials from one to the other. 

RESISTANCE FACTORS FOR LEAKAGE CONDITIONS 

A leakage path is simply a path parallel to the main flow of which 

it is a j>art, and resistances to flow are encountered on this path just 

as well as on airways. Leakage paths usually involve flow through 

small orifices at such velocities that the pressure loss conforms to the 

square law, or the same pressure-quantity relation as that for airways. Where flow through finely packed material is involved the 

velocities may be so low that the quantity varies almost directly as 

the pressure (see p. 69), but this condition is relatively unimportant 

in metal-mine ventilation and may be ignored. 

Resistance factors for leakage conditions are relatively high, 

indicating relatively high resistance to flow. If a stopping is so 

tight that no flow can occur, regardless of the pressure, the resistance 

factor R is infinity; but some air practically always leaks through 

stop pings, and actual values of R, though large for tight stoppingsr 

decrease rapidly as leakage increases. Atone end of the resistance scale 

are tight stoppings for which the leakages are measured in units 

and tens of cubic feet and the resistance factors in millions or hundreds 

of thousands i these leakages are so small that they may be ignored. At 
150 ENGINEERING FACTORS IN VENTILATING METAL MINES 

the other end of the resistance scale are regulators or stop pings constructed or used to provide a definite resistance to flow; the resistance 

factors for these are generally less than 10 (see p. 151). Between 

these extremes are hosts of leakage conditions, such as those represented by doors, joints in fan housings and connections, and tubing 

connections, for which the resistance factors may vary with time and 

pressure, but for which approximate values may be obtained. Heretofore, leakages have been either ignored in mathematical solutions 

or handled by a system of rough allowances. Actually, they can be 

put on a comparative basis through the use of resistance factors, and 

mathematical solutions are thereby facilitated. Leakages often 

cannot be measured, but where pressure and volume measurements 

can be made they are not only much more reliable than visual inspection for determining the actual condition but also serve as a base for 

estimating conditions where the leakage, although an important 

factor, cannot be measured directly. 

RESISTANCE FACTORS FOR DOORS 

The higher the resistance factor for a door the better it serves its 

purpose, except where a small leakage is desirable, in which case the 

door acts as a regulator. A good door and frame without any special 

precautions against leakage should not pass more than 3,000 c. f. m. 

under a pressure of 1 inch of water. The corresponding resistance 

factor is about 1,100. Under many conditions such leakage is permissible, but where many doors are involved or where a door is 

seldom used this leakage is larger than desirable. With the same door 

weather-stripped with pieces of heavy canvas, particularly at the 

bottom where most of the leakage occurs, the leakage might be 

reduced to 500 c. f. m. at a pressure of 1 inch; the corresponding 

resistance factor is 40,000. Old belting or similar materials may also 

be used as weather-stripping to reduce leakage. Doors that do not 

have tight frames-those with wooden frames stuffed with rags or 

with tight frames carried only to the lagging instead of the rock 

surfaces-may have resistance factors as low as 300; where other 

major leakages occur concurrently, as through lack of sill blocks, 

unsealed drainage ditches, holes for pipes, and excessively large 

trolley-wire openings, the author has obtained factors as low as 30 

for doors that at a casual glance would be considered pretty fair 

installations. 

REGULATORS 

Regulators are stoppings in which the size of the opening purposely 

provided is adjustable, so that the resistance offered to flow may be 

adjusted, in series with the normal resistance of the airway or section, 

to allow only the quantity of flow desired. The position for a regulator should always be selected with regard to creating minimum 

pressure differences on sooppings-on the intake side of the circuit 

in a pressure system and on the return side in an exhaust system. 

Regulators are used extensively in coal mines but rarely in the average metal mine. A few metal mines, however, use specially designed 

regulators (see figs. 54 and 55). Most metal mines could use them to 

advantage, particularly for controlling distribution through stopes, 

as often 1 stope of a series between 2 levels will take most of the air 

flow on account of its relative position and size of openings. Opera-
DISTRIBUTION OF AIR CURRENTS 151 

tors realize the conditions, but many have abandoned the use of 

regulators on stopes because of the difficulty of keeping them in place 

on active fill raises and manways. 

The operator of the average metal mine wants all the air he can get 

to a certain working zone or practically none at all. Distribution, 

therefore, usually is regulated by reducing the resistance to flow rather 

than increasing it and thereby reducing the total quantity in circulation. Regulation by adding resistance has been confined mainly to 

inactive openings where a little flow is sufficient for the work in 

progress or is required to prevent falls or excessive decay of timber. 

Usually a door is fastened open, blocks being placed at both the cap 

and sill under high-pressure conditions to prevent undes~red warping 

of the door. Many metal mines have used regulator slide openings 

in doors that, after installation, were found to pass more air than 

desired with the slide closed. 

Since a regulator is merely an orifice in an airway pressure losses 

and resistance factors can be calculated for constant conditions from 

data on orifice pressure losses (see p. 66). Few data are available, 

except for symmetrical square-edge orifices; although the author has 

developed a chart 43 for determining the area for particular conditions 

of use of this type, in general practice the conditions of possible use are 

more or less unknown at the time of installation, and flexibility in use 

requires that a large opening be provided, which can be adjusted in 

service to give the desired results more quickly and accurately than 

the adjustments could be calculated. Even though enough data on 

pressure losses were available for the particular type and conditions 

of installation, it would usually be wise to make a larger opening than 

required to provide for unforeseen demands, particularly in metal 

mines, as they rarely can control distribution with such precision as 

coal mines. The approximate area of a regulator opening, in square 

feet, for average mine conditions where the opening required is not 

more than about 20 percent of the area of the airway is: 

A 1,510 rea=~, (36) 

where R is the resistance factor for the resistance added by the 

installation. 

CROSSINGS 

Crossing of air currents in metal mines is rarely necessary because 

the vertical openings are in different planes or the surface openings 

are more or less regularly spaced where the workings are all in one 

plane; where it is required a few doors or bulkheads are usually all 

that is necessary. Elaborate structures, such as the overcasts and 

undercasts used in coal mines, are seldom encountered. Coal-mine 

practice could sometimes be followed to advantage; but with few 

exceptions the only separate structure used is a pipe carrying a minor 

return across or through an intake current with which, on account of 

high temperature, possible gas content, or possible fire hazard, it is 

considered undesirable for it to be mixed. The pipe ordinarily 

extends between 2 doors or stoppings erected in a return airway, 1 on 

either side of an intake airway. 

IS Weeks, W. S., The Air-Current Regulator: Trans. Am. lost. Min. and Met. Eng., vol. 76, 1928, p. 145. 
152 ENGINEERING FACTORS IN VENTILATING METAL MINES 

EFFECT OF MINING METHODS ON DISTRIBUTION 

The exact conditions of distribution at working places where the ore 

is extracted depend largely on the details of the mining method, both 

as to position and type of openings and time sequence of driving 

openings. In general, the larger and more numerous the openings 

the better the air conditions at working places, and the larger the 

opening the farther it can be driven as a dead end with comparable 

air conditions. However, the primary necessity for good ventilation 

is two or more openings to all working chambers, usually termed 

"stopes" in metal mines, so that through-circulations may be 

established. Quantity of flow usually is less important than velocity 

of flow, especially for removing blasting smoke to the main flow, where 

it may be diluted, or for providing air-motion cooling effect in warm 

atmospheres. Quantity of flow is increased as openings are increased 

in number and size, but velocity of flow is increased by restricting the 

area of cross-section of the openings. The openings should therefore 

be the maximum size at all other parts of the circuits and the minimum size at working places. Exactly the opposite condition normally 

obtains with all mining methods, since the working places are large 

openings and the connections to them relatively small. The velocities 

of flow in working places are thus relatively low, but they can be 

increased if desirable by proper attention to details of size of openings 

used in mining or by a change of mining methods. This feature 

usually requires little attention because even very low velocities on 

through-circuits, together with diffusion and convection currents in 

openings off through-connections, usually suffice to move blasting 

smoke out of the working places in the interval between 1- and 2-shift 

operations j but with continuous operation, blasting during the shift, 

high-temperature conditions, or gas conditions it cannot be neglected. 

,Vhere the working places and their local connections are all on 

the same horizontal plane the positions of the connections to a working place are not particularly important if there are two or more, but 

where the working places are on an inclined plane, as in metal mining, 

the relative positions of the connections with respect to differences in 

elevation are important on account of natural-draft and convectioncurrent effects. Two or more openings to the same airway rarely 

produce the desired conditions, except occasionally where the connections are from an airway to a working chamber below it. The 

essential requirement is that the connections be made to airways 

above and below the working opening, so that ascending throughcircuits are possible and maximum advantage is taken of naturaldraft pressures. The next essential requirement in utilizing available 

flow is that the position of the through-openings be offset with respect 

to the working opening so that the actual working place is not a long 

dead end off a through-opening. 

In general, systems of mining that normally result in large openings 

and large quantities of flow at low velocities are used on ore deposits 

that have low rock-temperature gradients and normal air vitiation 

only, for which the general conditions of low-velocity distribution 

give satisfactory results. Systems of mining that normally result in 

small openings and small quantities of flow are usually applied to 

deposits that require relatively restricted openings at working places 

owing to the necessity of back-filling openings, which automatically 
DISTRIBUTION OF AIR CURRENTS 153 

increases the relative velocities of flow through working places. 

Increased velocities are usually required in mines of this type by 

such natural conditions as high rock-temperature gradients and gases 

resulting from oxidation or coming from the rock strata. 

OPEN-STOPE METHODS 

Ground that can be worked by open-stope mining methods usually 

is easily ventilated,' as the requirements are low and many large 

openings usually can be incorporated in the mining method without 

perceptibly increasing costs. The essential requirement is at least 

one opening from each large stope or working place to the level above, 

as indicated in figure 54, A. Velocities of air travel are usually 

extremely low, but when blasting is confined to the end of the shift 

the products of general blasting have only a short distance to travel 

before reaching inactive openings through which their time of passage 

is relatively immaterial. 

Depths of working sufficient to encounter high temperatures usually 

also require changes in mining methods, as from advancing stoping 

to retreating stoping, and openings cave more readily, automatically 

confining the flows to active openings where the requirements gradually increase because of higher temperatures. Recently caved ground 

adjacent to active openings usually is relatively open as respects air 

flow and constitutes a good second opening of low resistance for the 

retreating working faces, as indicated in figure 54, B. 

At considerable depth the returns may be depended upon to rise 

to more open caved ground above and eventually reach the upcast 

main opening. Even in ground that eventually becomes quite compact considerable air usually will flow through caved sections but will 

be concentrated along the edges of pillars and solid ground. Eventually, with increased depth it may be necessary to provide special 

return airways connected to the caved ground or to partly maintained 

openings through it at strategic horizons, as at every third level where 

the retreat is made in groups of three levels. 

SHRINKAGE-STOPE METHODS 

Ground that can be worked by temporary-fill methods, such as 

shrinkage-stope methods, is also easily ventilated, since the openings 

usually embrace at least one at the bottom and another at the top 

of the stope. The working opening is also restricted, and with blasting confined to the end of the shift the small quantities and low 

velocities of the normal air flow result in good air conditions, sometimes good enough to J?ermit occasional blasting of large slabs during 

the shift without detrunent to the general air conditions, although 

the blasting of boulders generally is confined to the half-hour lunch 

period or end of the shift. Safety requires at least two openings to 

each stope. Good ventilation demands that the openings connect 

both to the level below and the level above, the usual practice (fig. 

55} A). In continuous shrinkage stoping the development openings 

driven for chutes in advance of active stoping sections usually provide 

the necessary bottom opening, but even where the chutes have a shallow depth of coarsely broken ore enough air will seep through a small 

number of such chutes to ventilate the stope adequately under many 

conditions. However, where a floor pillar is left under the level above, 

122422°--3&-11 
154 ENGINEERING FACTORS IN VENTILATING METAL MINES 

it is necessary to pierce the pillar at intervals to avoid long and poorly 

ventilated dead-end sections in the stope. Shringage stopes driven 

without an opening to an upper level usually are poorly ventilated. 

StoppiD", only if required by 

conditiOi'" 

r=~===:::=J< . 7 ""---- Small openings 

near shaft" 

I 

I ••• j!"!' .... " ....... -. 

A 

B 

Some .. tum through oaved workings direct to .".,. •• ohaft 

FIGURE M.-Method of ventilating open stopes: A, Advancing; B, retreating. 

particularly where the rock in the stope is warmer than the air on 

the airway to which it has two or more connections. When the wall 

rock of the stope is colder than the air on the airway natural-draft 
DISTRIBUTION OF AIR CURRENTS 155 

currents which may be quite effective are set up. Ore pulled from 

chutes draws some air from the airway into the stope by displacement, 

but this is a slow process for a large stope opening although it is a 

bOOn above active zone replaeed by Otopping. 

Air upcast along sbaft piUa. to more open abandoned workings above connected to upcast .haft 

c=::J ':"""6.~ q ,~~~;~.D.:,r-~~~ 

JI 

]I~_~~~ 

l

- . .>--'u' ...... ~ air for ,actions retreating 

towards upcast abafts through 

conneeted development drifts B 

FIGURE 55.-Method of ventilating shrinkage stopes: A, Advancing; B, retreating. 

fairly effective means of ventilation at the tops of manways adjoining 

chute compartments through which large quantities of ore are drawn, 
156 ENGINEERING FACTORS IN VENTILATING METAL MINES 

as f?r chute-c(:mtrol stations at the tops of dead-end raises in undercutcavrng operatIOns . 

. Often the ventilation of shrinkage stopes that do not connect by 

raIses. to the upper levels could be ~asily improved by erecting a loose 

curtam or. canvas door on t.he draWIng level between the end openings 

and hurdlmg some of the aIr through the stope. (See J>. 140.) This 

method is seldom used in metal mines but could be used to advantage . . Retreating methods of shrinkage stoping on moderate slopes proVide exceptionally good ventilation conditions (fig. 55, B), as the 

working chamber is a relatively confined space with large openings to 

both the level below and the level above and the slowly caving 

ground back of the broken ore makes a low-resistance return. If 

desired, the flow can be confined to the stopes by temporary stoppings 

or doors inside the last active drawing chute. 

FILLED-STOPE METHODS 

With most of the mining methods that require filling soon after 

extraction the natural lay-outs for mining are favorable for ventilation of stope openings, since openings from the level below and the 

level above are required. However, the ground conditions are likely 

to be such that the airways of the main circulations are relatively 

small, and likewise the quantities, even though mechanical ventilation 

is used to augment them. The larger compartments of the inclined 

openings are generally full of ore or fill so that the actual openings 

for circulation depend on the manway compartments being kept open. 

Usually two manways are kept open for safety at all times. In the 

early stage of stope operations both manways may lead off the lower 

level, and in the later stages the only manways maintained may lead 

to the upper level. Again, the one raise manway may lead directly 

from level to level; the stopes must then depend largely on eddy 

currents off this manway for ventilation. Good ventilation usually 

requires that the details of the method be laid out with ventilation 

in mind so as to have ascentional flow with the air coursed through the 

actual working area, as shown in figure 56, A. Manway design is particularly important, as the free area provided largely determines the 

amount of air flow. A good design used in many western metal 

mines is shown in figure 57. With increase in velocity requirements, 

as in high-temperature ground., the size of the working openings 

must be kept to the minimum; filling must therefore follow extraction 

closely, regardless of the actual 'necessity of the fill for immediate 

support. Rill methods of fill stoping generally fulfill this requirement naturally and favor good ventilation, as indicated in figure 

56, B. Retreating methods of rill stoping are particularly good with 

respect to facility with which the stopes may be ventilated. 

TOP-SLICE METHODS 

Top-slicing methods, in general, provide about the poorest ventilation conditions-dead-end workings with abnormally high ventilation 

requirements due to the heat generated by the decaying crushed 

timber in the mat. Good results can be obtained only by paying considerable attention to details of ventilation in laying out the openings, 

and special openings for ventilation must often be incorporated in 

the system. Three general methods are applicable, according to the 

particular conditions-hurdling from the level below by means of a 
DISTRIBUTION OF AIR CURRENTS 157 

well-arranged system of stoppings and doors; connecting at each 

floor in succession to a side raise in an adjoining section developed 

for mining by this or a different method but connected with the 

return airway; and connecting from the open space above a light 

mat and fill to return airways. About 1920 the Miami mine (Miami, 

A 

B 

FIGURE 56.-A, Method of ventilating horizontal cut·and·fillstopes; B, method of ventilating rill cut .... nd· fill stopes. 

Ariz.) used a ventilation level of closely spaced openings provided 

with a multitude of doors and stoppings for hurdling air through 

closely spaced raises to extensive top-slice operations by means of a 

mechanical ventilating system and obtained excellent results that 
158 ENGINEERING FACTORS IN VENTILATING METAL MINES 

permitted continuous blasting in the top-slice stopes. In general, 

the hurdling method (fig. 58) is applicable to the common conditions 

encountered, whereas the other two are applicable only to special 

~ ~ !l .JIf'i -..-- --~ 

n V/~ 

~ l\ ~ 10 ----:~ 

~ ,A ro I :<2::~-"'::;-

~ f'i ;,=-

~ ~ I'l R= =::> ~I \ 

to 

IS" "- -==- ~ '0 

z=-..~ 

E::;S ==:- ~-~ 

-c <-' =-::>.? '--- - '/ 

ELEVATION AND SECTION 

10' r ... 6"1-----4' 6"I+-L 

Timbe 

slide r_f-

• 16" 6exibl tube 

r:-' 

I"XIO"I .. &,~ 

~2'+ ~1+16"-

2"xS" 

.J: 2"xlO" 

~)( ~~ 12"xlO" 

~...J 

Manway 

S"x6"x5' &, 

"' ~ 

&, ~:o 1 ,~ J I'" 

1 

I 

&, 

Chute PLAN 

FIGURE 57.-Two-compartment cribbed raise with timber slide in manway compartment. 

Manwayaad 

supplyCOJD-o 

partment 

6upply fringe 

drift 

j" L_--Hi--n Chute raises full of ore 

Haulage crosscut Door 

FIGVRE 58.-Hurdling method of ventilating top-slice stapes. 

Mainhaulage 

drift 

conditions of regularly advancing or retreating methods or of mining 

small sections or pillars in ground that stands well. 
Bureau of MiQes 

@ 

~r=====~~~~~~~~~~~~~~~~~~~~f7~~~~~~==~~~~~13~ 

m.m Completed stope 

~ Producing stope 

LEGEND 

~ Developing stope-Iste stage 

V /,'1, ~ Dev~loping stope-early stage 

Note+ Complete stope outlines not shown 

~ We~and dry-bulb air temperatures 

~ Direction of horizontal air 80w 

"5JiliO" Quantity of flow in cubic feet per minute 

'N No air flow 

0.2'" 

==;t= Ventilation door and pressure in inches of water 

==!== GUDited wooden brattice 

===t= Canvas brattice 

~Caved 

Direction and quantity of vertical air flow 

Air upcast to level 

Air upcast from level 

Air dO\VDCast to level 

Air downcast from level 

122422°-35, (Face p. 159.) 

To 595 

. IF ' 21jO Scale of feet . 

FIGURE 59_-Ventilation map of 620 grizzly Jevel of Miami mine, Miami, Ariz., May 1929_ 

Bulletin 385 

DISTRIBUTION OF AIR CURRENTS 159 

UNDERCUT-CAVING METHODS 

The undercut-caving methods used so extensively in Arizona and 

Nevada require the driving of many relatively short dead ends in 

development that are difficult to ventilate; fair ventilation is obtained, 

however, by limiting operations in certain classes of working places 

to one shift. All these mines have mechanical systems that provide 

for rapid clearing of the grizzly drifts, where almost continuous blasting is required, and other active operating places. With extended 

experience on certain types of ground the need for continuous blasting 

is gradually decreased as methods for obtaining more uniform crushing 

of the ore are developed. The chief requirement for the ventilation 

of active operations seems to be that the main returns be connected 

direct to the active grizzly drifts and that there be no direct connections between the main returns and the main-haulage level; the latter 

should be connected only to the intake airways. The general plan 

is that the intake currents supplied to the main-haulage level rise to 

the grizzly level through the raises of new development only and pass 

to the active drawing drifts and from them to fringe drifts connected 

to the returns. The difficulty of keeping the raises in drawn and 

drawing sections full of waste or ore makes control difficult; if all but 

the active drawing drifts of the grizzly level are closed at the fringe 

drifts by doors or stoppings fair control may be obtained for certain 

lay-outs. The general lay-out, however, is often such that simple 

methods are not applicable, and the Miami mine in 1929 was considering including a, special ventilation level for a new section so that 

the ventilation could be controlled more accurately. Figure 59 shows 

a ventilation map of a typical grizzly level of an undercut-caving 

operation. 

Development for this system of mining involves many openings 

without upper connections, unless these can be provided by connecting 

to old prospect openings, which are often present but irregularly 

distributed. The numerous openings between the undercut and 

grizzly levels usually induce small convection and natural-draft currents which supply enough circulation to permit one-shift operation; 

but, in general, the conditions in boundary caving shrinkage stopes 

and drifts are poor unless a well-planned system of eonnected openings 

is incorporated in the general system with ventilation in mind. 

BOOSTER FANS 

The development of metal-mine openings is often so irregular, on 

account of the variations in occurrences of ore bodies, that the resistances of the separate districts or splits into which the flow may be 

divided vary widely. Certain districts may offer such high resistance 

to flow that they naturally receive a small percentage of the total 

flow. A larger flow may be obtained (1) by reducing the resistance 

of the split by actual changes in the physical characteristics of the 

airways, as by smooth-lining, removing obstructions, easing sharp 

bends, and increasing areas of cross-section; (2) by increasmg the 

resistance of the other sections until the desired proportional flow is 

obtained in each and then increasing the speeds of the main fans until 

the total flow, which was reduced by the added resistances, is brought 

up to the desired quantity; or (3) by generating pressure directly on 

the high-resistance split, as by a fan. Under average metal-mine 
160 ENGINEERING FACTORS IN VENTILATING METAL MINES 

conditions the third method is generally the cheapest and most effective, and secondary fans operating in the booster position (booster 

fans) are used freely. They regulate the flow by reducing the equivalent resistance of the section with respect to the main-flow system. 

FAN INSTALLATIONS 

Booster fans present practically the same conditions as any fans 

underground but usually are installed in active workings where provision must be made for the passing of men and ore trains without 

disturbing the flow. Where two openings are available the fan is 

placed in one with discharge through a tight bulkhead, and an air 

lock is placed on the other. Where there is only one opening at the 

desired location a run-around drift is sometimes driyen to take the 

fan installation; where the ground stands well it generally is cheaper 

to widen the drift oyer a sufficient length and provide an airlocked 

passage alongside the fan installation. Figure 60 shows a typical 

lay-out of the latter type, which could be improved somewhat by 

carrying the full width of widening to the end of the air lock on the 

discharge side. Where the fan required is small and passage requirements are limited to single cars the fan is often mounted against the 

Compressed-air .. operated doors 

Drift 

1T==;:r===c="'-:o:F,~~;;;;e;~'iT"'""""i5=-c-"""RL. Wire·mesh brattice - Locked door /';/~-­

~ 

Air lock 

Q 5 10 I, .. 1,., ,., 

Scale in fcet 

FIGURE 6O.-Typical underground i"stallation of fan in booster position. 

wall at a short air lock on a drift or crosscut, and the discharge is 

carried through the air lock by a short length of pipe having about 

the same area as the fan-discharge connection. Provisions for minimum resistance to flow are rarely given much thought at booster 

installations; the resistance added to the section by the fan installation is often nearly equal to the natural resistance of the section. 

Leakage at booster-fan installations is also neglected in most metal 

mines and is often a high proportion of the total duty of the fan. 

Likewise the necessity for fireproof installations is not generally 

realized, and serious fire hazards result. 

POSITION IN OPENINGS 

The position of the booster fan is largely controlled by the lay-out 

of the openings available. Minimum resistance to flow' and minimum leakage of return currents to intakes are the major factors to 

be considered. Minimum resistance requires that the total flow, 

necessarily confined to 1 airway at the fan location, be limited to 1 

airway over as short a distance as possible and that maximum use 

be made of the airway areas available. Minimum leakage of return 
DISTRIBUTION OF AIR CURRENTS 161 

air to intakes gene1'iIlly requires that the booster fan be on the same 

side of the ventilated openings as the main fan, that is, on the return 

side with a surface exhaust fan and on, the intake side with a surface 

pressure fan. Minimum leakage also requires that the fan position 

be in relatively inactive territory, but this Condition must often be 

subordinated to the others mentioned. 

The requirements listed also apply, in general, to selection of the 

position of a main fan in series with another surface fan except that, 

if possible, the fan in series should be installed on a surface opening 

for maximum control in case of fire. Seldom, however, is more than 

one surface position available, and the driving of separate openings 

solely for ventilation is often beyond economic consideration. 

SELECTION 

Determination of the duty of a booster fan by the operator is 

usually a rough estimate based on previous experience, or a selection 

is made from the fans on hand; the actual duties of the installation 

often are highly variable because of train movements, door-controlled 

distribution, and changes in number and conditions of openings. It 

is therefore desirable to install a type having a nonoverloading power 

characteristic and provide for speed changes. The general selection 

is a constant-speed motor with a short-center-type drive; speeds are 

changed by changing pulleys. Multispeed motors would be desirable 

under most conditions but have not been used for this purpose. 

For fairly constant resistance conditions the approximate resistance 

of the split circuit with the fan installed may be calculated, and 

after allowance for normal leakage the approximate duty of the fan 

may be set at this figure. The actual duty of the fan is affected also 

by its relation to the pressures generated on the main-flow circuits; 

for most installations, however, this effect may be disregarded, and 

the approximate duty may be determined accurately enough with 

reference to its own split. This statement, of course, does not apply 

to fans operating underground in series with a surface fan and handling all or a large proportion of the total flow. These are often referred to as booster fans because they boost the pressure on the main 

flow, but they are not strictly booster fans in the sense in which the 

term is used here. 

Whereas main fans usually operate against pressure-quantity relations expressed by resistance factors of less than 10, the equivalent 

factors for booster-fan operation generally fall in the range of 20 to 

200; this fact explains in large part the average negligible effect of 

main-fan operation on the duties required of booster fans. 

AUXILIARY VENTILATION 

The general designation" auxiliary ventilation" may be applied to 

all methods of ventilating unconnected openings, which are commonly 

termed "blind" or "dead -end" openings, although in practice the 

use of this term has been limited to fan-tubing methods of auxiliary 

ventilation. The length of dead-end opening to be ventilated is of 

primary concern; it is desirable therefore first to discuss the effect of 

methods of extending openings into new territ'Ory, as required for 

prospecting and development. in limiting the length of dead-end 

openings. 
162 ENGINEERING FACTORS IN VENTILATING METAL MINES 

DOUBLE-ENTRY METHODS 

The general method of extending workings into new territory in 

coal mines, as developed by experience for maximum control of 

explosive gas given off by the strata and later incorporated in State 

laws, is by the double-entry method (upper part fig. 61) of driving 

parallel openings in coal, connecting these by cross-openings at 

uniform intervals as they are driven, and sealing all but the last crossconnection. One of the parallel openings is used as an intake and 

the other as a return airway, and air is circulated close to the advancing faces, which are extended but short distances as single openings. 

The distance, usually specified by law, is seldom over 100 feet for the 

opening, including the cross-connection. Multiple-entry methods or 

methods using some multiple of two parallel openings are also used 

in coal mines to provide larger air-carrying capacities and to avoid 

the disadvantages of relying on the tightness of numerous stoppings 

separating main-intake and return air currents. The openings are 

driven in pairs by double-entry methods, but the separate pairs are 

connected only as required for operating purposes and, when connected to the general ventilating system, become two airways in 

parallel. 

Intake Breekthroughs on 50'''' lOO-foot centoro 

s:::r~,::~:;,~~ Retum'--=----=---cc""O;-:-AL ... MIN='E;;:(;;;PIan;:-:-;-)--=--=Bra~t?tice..:=..:;used~for=...lcommoD 

condition of aplPaiveIntake==-==I'r-==Retums 10 ac~e .Iopee Bbove;:==:!1 _____ _ 

~ ~ F -tabiDg iDRaIJatiollll_for - irregularly .paced but .. Idom .;:"'1 COJIditiollll of length. heat. _ closer thaD 500 feet _ dWlt,or strata .... occurrIIICIt 

Intake - . - - - ------- METAL MINE (Elevation) 

FIGURE 61.-Double-entry methods of advancing Into new territory. 

The general methods of developing new territory in metal mines 

are also equivalent to double-entry methods of advancing (lower 

part fig. 61), but the exceptions are more numerous owing to the 

irregular occurrence of the ore. Although the details of application 

differ widely from coal-mine practice the general method that proves 

most satisfactory as respects ventilation is to drive two or more 

parallel openings, usually drifts on an inclined vein, 100 to 150 feet 

apart and to connect them at intervals with a cross-connection, 

usually a raise driven from a lower level to an upper level. 

Occasionally, the parallel openings may be at the same elevation, 

as in driving long tunnels by the double-entry method to facilitate 

both haulage and ventilation or in driving a heading parallel to a vein 

in the rock for haulage coincident with the driving of a drift on 

the vein. 

Explosive gas rarely is encountered at metal-mine faces, and compressed-air exhaust from drills is usually available for auxiliary ventilation and for clearing faces of blasting gases; the cost of driving 

cross-connections in rock or ore is high, and the disposal of waste 

rock from development often is inconvenient and costly. For these 

reasons, there is no point in limiting the length of dead-end faces 

ouly insofar as to avoid extra expense for ventilation. The average 
DISTRIBUTION OF AIR CURRENTS 163 

distance between cross-connections in development in metal mines is 

about 500 feet and the average length of dead-end advancing faces 

about 100 feet longer than the cross-connection spacing. Closer 

connections usually are required for extraction of ore but usually are 

driven after development. The positions of cross-connections are 

largely limited by the occurrence of the ore, as it generally is desirable 

to drive in ore or to prospect favorable ground. Where there is no 

other opening to connect to in prospecting new ground or the cost of 

driving the connection would be very costly openings are often driven 

as dead ends up to 2,000 or 3,000 feet without artificial ventilation; 

but, in general, fan-tubing installations are installed for lengths in 

excess of 1,000 feet, although the necessity for artificial ventilation is 

always comparative, being governed by the exact conditi~ns that 

obtam. The common coal-mine practice of ventilating dead ends by 

bratticing or dividing the opening into a separate intake and return 

passage by a division wall is rarely applicable to metal-mine openings 

because the intensity of the blasting concussions would require heavy 

walls with pressure-release panels. Metal-mine drifts are usually too 

small and space too much at a premium to even consider this method. 

The average metal-mine operator scrutinizes costs closely, particularly the extra costs of driving connections before they are actually 

needed in the sequence of operations of ore extraction. However, 

the indirect effects of ventilation conditions on costs of driving and 

extraction generally are given too little consideration, because they 

are largely unknown for many combinations of conditions and can 

only be roughly approximated by experience. Costs of driving as 

affected by time, on the other hand, generally can be estimated within 

close limi ts and na turally are considered more carefully, with the resul t 

tha t many opportunities for reducing final costs by driving connections 

in time to get maximum use of them with respect to ventilation are 

neglected. This statement applies especially to the "hot" mines 

where opening up new territory for free circulation of the ventilating 

currents as lV-uch as a year or two in advance of stoping operations 

yields large dividends III decreased stoping costs resulting from the 

cooling of large blocks of ground before extraction. Operators, in 

general, are converted with difficulty to mining methods involving 

the free use of raise connections, but once such methods are adopted 

they are seldom discarded, regardless of changes in mining conditions. 

On paper the extra costs of such methods loom large, but in practice 

it is generally found that final costs are lower, without regard to the 

humanitarian feature of providing more healthful and safe working atmospheres. 

USE OF COMPRESSED AIR 

The fact that compressed air is used generally as the motive force 

for operating drilling and other appliances at metal-mine faces is a 

powerful factor in obtaining better air conditions than would otherwise obtain. It combines both motive force and considerable 

auxiliary ventilation and for this reason will always be difficult to 

displace with other types of power for general utility in metal mines. 

In the absence of compressed air at a dead-end working face or 

during periods of nonoperation of appliances using the compressed 

air diffusion and convection currects only are available for ventilating 

the face. Under ordinary circumstances their action is very slow but 
164 ENGINEERING FACTORS IN VENTILATING METAL MINES 

may permit one-shift operation of short dead ends-not over 380 

feet-without serious discomfort if air currents are circulating through 

the adjacent openings. Diffusion alone is a very slow process, but 

its action is increased by movement of men and cars and by local 

convection currents produced by the heat of men, animals, and lights 

and by the heat liberated by blasting. Under low-temperature 

conditions the convection currents that act between the dead end and 

the adjoining airway, although very feeble, discharge blasting gases 

to the main air circuits and prevent more than a small rise in the 

carbon dioxide content of the air due to the presence of men, animals, 

and lights. If there is no circulation in the adjoining airWay or connection the air conditions throughout a whole section or mine may 

become very bad owing to the gradual accumulation of blasting gases 

diffused into the atmosphere; the air conditions in the dead ends, 

where diffusion at any particular time is not complete, may become 

extremely bad. 

If compressed air is used intermittently at the face for operating 

drills conditions at the face, regardless of length, and throughout 

relatively short dead ends-under 500 feet-are generally good in 

the absence of high temperatures, strata gases, and dust. They 

would not be good for some hours following a blast were it not for 

the fact that it is general practice to blow a relatively large quantity 

of compressed air close to the face for 15 minutes to an hour or more 

after a blast. The blowing usually is sufficient to remove the blasting gascs from the dead end entirely or to force them out the opening 

to some distance from the face, where trammers and others pass only 

occasionally and are exposed to the diffused gases for short periods 

only. Such exposure seldom produces noticeable physiological 

effects, although in long headings-over 500 feet-a general feeling 

of depression or a slight headache may result. In places the muck 

may be so fine that it traps blasting gase8, which are released gradually 

as mucking proceeds; if no drills are operating at a face that has 

been blown out after the blast the gases may be present in such 

harmful concentrations as to cause severe headache. This condition is more pronounced when scrapers and mechanical mucking 

machines are used on fine muck, because of the more rapid release 

of gases; mechanical mucking sometimes requires fan-tubing ventilation where hand-mucking would not. 

Air conditions at advancing dead-end faces are best when drilling 

and mucking progress concurrently on the same shift; usually this 

is the first change made to increase comfort under rising temperature conditions. A drill exhaust averages about 100 c. f. m. and 

usually is 10° to 20° colder than the surrounding air; it therefore 

affects comfort at warm rock faces markedly. In addition to producing air motion where it does the most good a drill operating at 

the face of a small heading (5 by 7 feet) cools the air at warm rock 

faces usually 3° to 5°; two or more drills operating in a larger heading 

will give the same result. 

There is a tendency, though not among mine operators, to consider the use of compressed air for auxiliary ventilation wasteful 

under all conditions except that for actual drilling. This, of course, 

is not true, for although compressed air is costly final cost is a matter 

of unit cost and a host of other variables, particularly length of 

time of use and installation costs. With no cost for installation 
DISTRIBUTION OF AIR CURRENTS 165 

and short period of use undoubtedly auxiliary ventilation by compressed air is the cheapest form available for a comparatively large 

range of use. The difficulty is that habits of wasteful use gradually 

grow in the ordinary mine with attempts to ameliorate discomfort 

due to increase in air temperatures through increase of rock temperatures accompanying increasing depth. Although compressed air 

may be used economically for short periods to combat local and 

temporary discomfort due to air temperature or to unusual gas, 

smoke, or dust occurrence, its continuous use for such purposes 

generally is extremely wasteful, costly, and a detriment to the primary purpose of operating drilling and other appliances on account 

of the low pressures that result. 

The effect of compressed air as used to improve comfort at deadend working faces in warm rock is almostly strictly limited to the 

air motion it produces, since the temperature of the compressed 

air at free discharge from a hose is seldom more than 5° less than 

that of the air under average conditions. The air motion is desirable 

for improving comfort and the efficiency of the miners but can be 

more cheaply provided than by direct discharge from a high-pressure 

pipe or hose. Injector devices that use only a small quantity of 

compressed air but entrain and give motion to many times this 

quantity of free air have been developed for producing local air 

motion in deaq. ends and in other working places where the quality 

of the air is sufficiently good. Many mines have tried such devices 

but, in general, their success has been limited by the general lack 

of cooperation given to such efforts by the miners, and the devices 

have been discarded. . 

LOCAL AIR MOTION 

In addition to the injector devices tried at dead-end faces many 

mines have tried various devices for improving comfort by increasing 

the air motion locally in warm working places where the general 

ventilating currents provided air of good quality. Injectors of types 

similar to,those tried in development faces and various designs of 

compressed-air-operated and electrically operated fans have been 

used. The objection to most devices has been the miner's complaint 

about their noise, particularly the shrill and penetrating noise of 

exceptionally high-speed operation; in general, the devices have met 

with so little success compared with the discipline required to effect 

their continued use that they are either disabled or discarded after 

a short time, regardless of the actual benefits of increased comfort 

and efficiency: in workin~ 

Most miners will stand considerable discomfort before they will 

exert themselves to alleviate conditions; this fact greatly retards the 

introduction of devices for auxiliary ventilation, since the supervision 

and discipline required for the successful operation of a new devicenew to that particular mine-are out of all proportion to the benefits 

to the management, at least in its initial stages. The miner, as a rule, 

is the one who would derive the greatest benefit but is the last to 

encourage efforts for improving his own comfort. Turning on a compressed-air hose to induce circulation of air is so simple that it is the 

common recourse of the miner unless prevented by severe discipline. 

Where discipline is lacking or ineffective in a warm mine the waste of 

compressed air by "blowers" is a serious problem to the management 
166 ENGINEERING FACTORS IN VENTILATING METAL MINES 

FAN-TUBING METHODS 

DEVELOPMENT FOR LOW-TEMPERATURE CONDITIONS 

Fan-tubing ventilation methods are very old; in fact, they are men- tion~d ~y Agricola (seventeenth century) as a common means of ve~tllatmg long tunnels, although the motive force usually was bellows ~hlCh.forced or withdrew air through wooden-box conduits. In time, 

Iron pIpes largely replaced the wooden boxes, since wooden-box conduits generally are unsatisfactory as regards leakage and weight, and 

fans replaced the bellows. With the introduction of compressed air 

longer dead ends could be driven without recourse to fan-tubing 

methods, and such. methods were limited in actual use to exceptionally long openings, seldom under 1,000 feet, or to strata-gas or hightemperature conditions. 

In general, the main consideration under low-temperature conditions 

is removal of blasting smoke and gases at a point near the face to keep 

the opening clear of the regularly spaced zones of smoke and fog that 

develop in long headings ventilated by compressed air only. Exhaust 

units are commonly used; that is, the fan is placed at the portal end 

of the tubing and draws air in through the mine opening and out 

through the tubing. This is a logical installation under low-temperature conditions, particularly where much dust is produced by drilling 

with dry drills; these, however, have been largely displaced by wet 

drills during the last 10 years. The general practice of moving the 

L targe..diam~ ~!1DV88 tubing 

; - -oW Face b ..... er ~ Main blower . 

FIGURE 62.-Fan-tubing installations for ventilating long dead-end openings with canvas tubing. 

blasting gases from the face to the end of the tubing, which usually is 

kept 50 to 150 feet from the face to avoid mechanical injury during 

blasting, is to blow a sufficient amount of compressed air at the face 

immediately after the blast, the pressure and amount being regulated 

to give the desired speed of movement. Sometimes the exhauster is 

shut down after the blasting gases have been removed, but the more 

general practice is to keep it operating, often at reduced speed, to 

maintain clear and constant air conditions and to remove the invisible 

rock dust produced at the face. Where air motion increases the comfort of the miners the direction of air travel is usually reversed, after 

the blasting gases have been handled. 

In headings of large cross-section the amount of compressed air 

required to move the blasting gases back to the tubing end becomes 

important; it is often cheaper to reduce the amount required by using 

a supplementary blower with a short length of small-diameter tubing 

to bear the brunt of mechanical injury involved in keeping the circulation close to the face. Under moderate temperature conditions this 

arrangement is especially desirable on account of the air motion produced but is effective only where the tubing end is kept within about 

25 feet of the face. Canvas tubing cannot be used with exhausters, 

but the installation shown in figure 62 is sometimes used; the fan 

is placed inside the opening and blows toward the portal. At long 

intervals it is moved closer to the face, and ventilation between the 
DISTRIBUTION OF AIR CURRENTS 167 

fan and the face is maintained with a supplementary blower fan of 

smaller capacity; this is placed just outside the main fan and blows 

air through canvas tubing of smaller diameter to a point close to the 

face. After a blast compressed air is blown to force the gases back 

to the point where they come under the action of the small blower 

and are carried rapidly back to the inlet of the large blower, which 

discharges them through its tubing to the portal. Results are not 

quite as good as those obtained with a main exhausting unit, as leakage 

results in some recirculation of blasting smoke in the opening outside 

the main-fan position; this, however, occurs only over a short interval 

in an inactive zone. Canvas tubing, which may be standard for other 

fan-tubing installations, is thus used throughout. 

DEVELOPMENT FOR HIGH-TEMPERATURE CONDITIONS 

The highest development of fan-tubing methods of auxiliary ventilation has been in connection with the high-temperature conditions 

generally found in deep mines. Here the greatest factor is air movement required for comfort. Although a few mines have attempted to 

use exhaust units for these conditions, following development under 

low-temperature conditions, the general choice has been blowing units 

to obtain the advantage of maximum air movement at the working 

face. 

The first western metal mines to encounter high-temperature conditions were those of the Comstock Lode at Virginia City, Nev.; their 

experiences in driving dead ends under exceptionally high-temperature 

conditions probably will remain for a long time to come the world 

record for mining under difficult conditions. Starting about 1868, 

these mines used many fan-tubing units comprising positive pressure 

blowers and iron tubing with bell joints. Eliot Lord 44 mentions the 

use of canvas tubing, but later accounts mention only iron pipe. 

The next large district to encounter difficulties from heat on a 

large scale was Butte, Mont.; here the use of commercial types of 

small centrifugal fans to blow air through small sizes of galvanized 

iron pipe started on a comparatively large scale about 1915. The 

average size of pipe was gradually increased, and special narrow widths 

of standard commercial designs of forward-curved-blade centrifugal 

fans were adapted to this service. The need for a more flexible type 

of tubing than that afforded by iron pipe under average Butte conditions led to the development of types of canvas tubing in which 

Butte operators took a leading part. For many years Butte has 

been the proving ground for new ideas in fans and tubing for fantubing installations, although the use of fan-tubing methods has 

spread to large numbers of metal mines and is making rapid headway 

in coal mines even against opposition 45 based on the natural dangers 

inherent in the use of this method under explosive-gas conditions. 

For high-temperature conditions the best installation is a nonreversible blower unit, as local cooling by movement of air and rock 

cooling by volume of flow are required continuously and the presence 

of diffused blasting gases and smoke in the opening back of the face 

.. Lord, Eliot, Comstock Mining and Miners: U. S. Geol. Survey Mon. 4, 1883. p. 339. 

" "In the interest of safety, the Bureau of Mines, Department of the Interior, recommends that auxiliary fans or blowers should not be used in coal mines as a substitute for methods of regular and continuous cours- ing of the air to every face of the mine." (Decision 4, Mine Safety Board.) 
168 ENGINEERING FACTORS IN VENTILATING METAL MINES 

is comparatively unimportant. Under most explosive-gas conditions 

a continuous nonreversible blower installation or its equivalent is also 

desirable, as the velocity at discharge is required to diffuse the gas 

rapidly as it is liberated at the advancing face; even a short-time 

reversal has proved a dangerous expedient where explosive gas is 

generated, as the end of the tubing never extends entirely to the face 

and an unventilated pocket is formed in which the gas accumulates 

during the period when the fan is operated as an exhauster, often 

with fatal results if any source of ignition is present on the opening 

traversed by the concentrated gas cloud when blowing is resumed. 

However, similar gas clouds may be formed by large gas "feeders" 

back of the face, and unless such flows can be safely diffused by hurdles 

a continuous nonreversible exhaust installation, with small branches 

leading to high pockets in the vicinity of the feeders, is safer. 

AUXILIARY BOOSTER INSTALLATIONS 

The development of methods of using fan-tubing installations to 

combat particularly difficult high-temperature conditions has led to 

many interesting types of installations. One developed independently 

at several rines, where natural conditions prevented or were thought 

to prevent an increase of main circulations that would effectively 

Upcaat raise 

FIGURE 63.-Lay-out of fan-tubing auxiliary units for stope ventilation under high-temperature smallquantity-of-f1ow conditions. 

combat the high-temperature conditions of working places, is shown 

in figure 63. The main fans are used only to supply air to an intake 

shaft, which is used as a plenum chamber, from which fan-tubing 

installations draw air and, discharging through air locks, blow it 

through tubing to all working faces; thence it moves at extremely low 

velocities to the return shaft. The costs per unit of quantity circulated are high, and air conditions throughout all parts of the Inine 

except the working faces and the intake shaft-throughout the more 

inactive parts-are very uncomfortable; air conditions in active 

zones, however, are much better than they would be if throughcirculations were allowed and dead ends were ventilated with fantubing units installed at the nearest point of supply, which would be 

on a warm, slow-moving current. The method is particularly adapted 

to Inining methods involving many dead-end working places in hightemperature rock and their ventilation with a liInited supply of air. 

For a small Inine, under certain conditions, the method may be 

econoInical, but for a large mine with large ore bodies its final economy 

is questionable. In many rines the increased cost of operation due 

to unfavorable working conditions would be more than sufficient to 

amortize a heavy investment in ventilation openings. 
DISTRIBUTION OF AIR CURRENTS 169 

DEVELOPMENT TRENDS AT METAL MINES 

Tha history and development of fan-tubing methods in any 

particular metal mine follow certain definite trends and encounter 

the same difficulties. In a warm mine the use of fan-tubing units for 

auxiliary ventilation generally precedes primary mechanical ventilation by a number of years, such units being used to ventilate local 

headings or stopes where the need for mechanical assistance to natural 

circulation first becomes manifest. The original installations are 

made on a basis of first cost rather than results and usually comprise 

small fan units and small-diameter tubing, 2.5- to 5.0-hp. fans and 

6-. to8-inch tubing; and tubing ends are kept about 150 feet from the 

face to avoid mechanical injury to the tubing. As the air conditions 

become more uncomfortable the tubing installations are made with 

more regard to tightness, and finally larger tubings and fans are 

employed; but the tubing ends are still not carried close to the faces 

even though the air conditions there have become quite uncomfortable. 

Oarrying the tubing lines close to the faces involves the dismantling 

and erection of 50 to 150 feet of tubing every time the face is blasted. 

If the fan is in operation during the blasting period the damage 

extends to greater distances, particularly with canvas tubing; the 

operator has therefore learned to have the fans shut. down at this 

time. 

As the air conditions at the face become more uncomfortable the 

miners can be persuaded to keep the ends of the tubing lines close to 

the face, within 25 feet at the maximum to benefit appreciably from 

the air motion, but they rarely take down enough tubing before blasting 

unless severe disciplinary measures are used; although the expense is 

reduced by the use of old tubing or smaller-diameter tubing near the 

end the cost of replacement per foot of advance in hard rock is often 

high and may easily exceed $1 per foot. Generally, operators go to 

considerable expense in providing conditions that permit good ventilation at development faces, both to promote comfort and working 

efficiency, only to have their efforts nullified by the indifference of the 

miners, who become acclimated tol high-temperature conditions and, 

up to a certain limit, prefer uncomfortable conditions to the labor and 

bother of keeping the tubing in proper use. The entire benefit of the 

installation is not lost, of course, if the end of the tubing is not ke:(lt 

close to the face, as blasting gases are rapidly discharged to the mam 

circulation, rock-dust concentrations are diluted, and comfort conditions are improved along the tubing; and convection currents between 

the discharge and the face act more or less feebly to accomplish the 

same results on a smaller scale in this more active zone. The rock 

walls are thus gradually cooled and average temperature conditions 

improved throughout the dead-end opening. The major benefit of 

motion of air in promoting comfort and efficiency is lost, however, when 

the tubing end is not within 25 feet of the face. Operators realize this 

fact, but the remedy requires service at so many scattered points at 

about the same time that the delegation of this service to special 

employees usually is not considered feasible or economical. 

When the temperature conditions rise above the equivalent of 900 

saturated still air (900 Effective Temperature) the miners can easily be 

persuaded to make effective use of fan-tubing installations, but withou t 

rigid discipline the cost for replacements is always high. 

122422°-35--12 
170 ENGINEERING FACTORS IN VENTILATING METAL MINES 

FAN SELECTION 

The majority of the fans employed for fan-tubing auxiliary service are 

standard high-speed types developed for industrial use; some are 

suitable, but many are not. The average conditions of fan-tubing 

installations in metal mines demand a low-quantity high-pressure fan; 

a number of types have been developed for this service in the last 10 

years. The most recent type and apparently one of the most suitable is the direct-connected, high-speed, backward-curved-blade 

centrifugal. In addition to light weight through use of high-speed 

motors these types have a nonoverloading power characteristic and 

cover a wide range of resistance conditions with little change iri. efficiency and quantity of flow. Special types of single-propeller fans 

have similar advantages, in addition to the advantage of axial flow, 

but are larger for equal pressures. Since they must be used against 

slowly changing resistances in most eases it is possible that variablepitch, multiple-propeller types will be developed for this class of 

service as well as for main-fan and booster service. 

In the average installation the leakages that develop at joints and 

elsewhere make it necessary to use a unit that will handle ayproximat ely twice as much air as is required at the end of the line. If 

resistance factors for average conditions of leakage can be estimated 

for the actual conditions of service or determined by tests on an average 

installation the equivalent resistanee of the line may be closely calculated by considering the leakage as an airway in parallel with the flow 

through the tubing from the point of leakage to the point of discharge. 

The leakage for individual connections may range from 0 to 100 

c. f. m. at pressures up to 6 inches of water; where the sections themselves are not tight owing to type of construction, deterioration, or 

damage the leakage per section, including a connection made underground, may range up to 500 c. f. m. 

Virtually all auxiliary fans are driven by direct connection to 

motors, usually a.-c. induetion motors, although d.-c. motors occasionally are used to permit supply from trolley wires of electric haulage. A 

few compressed-air-operated types are available, but economy demands that they be used only for temporary service in remote places 

or for emergency service, particularly fire-fighting. Compressed-air inje~tors are also available or can be made up easily for emergency 

servIce. 

Auxiliary installations usually have a comparatively short life, a 

month to a year at anyone location; for this reason the fan is often 

supported with light timber, which presents considerable fire hazard. 

In most metal mines the fans are operated singly as blowers. They 

are seldom used in the exhaust position, except in combination with a 

face blower on exceptionally long development headings, but are sometimes used in the booster position when a good natural position for the 

fan is not at the same point as the in take air desired. Where the length 

of the tubing is too great for a single fan of the types available the fans 

are used in series with either iron pipe or flexible tubing. A highrressure fan or a positive pressure blower could be used, but ordinarily 

the amount of such work is limited and the tubing available is not provided with tight enough joints to give good service under the highpressure conditions that would result from the use of a single fan. When 

fans are used in series the maximum pressure differences between tubing and atmosphere are diminished, and leakage is reduced. Difficul-
DISTRIBUTION OF AIR CURRENTS 171 

ties are experienced in the use of flexible tubing in series blower lines, 

since negative pressures must be avoided. If all the resistances, 

including leakages, are accurately known the positions of the fans can 

be calculated accurately. Experience has shown that little trouble 

will develop if the second fan is placed one third the length of the line 

from the fan at the inlet. More than two fans are seldom used on 

canvas lines, but equivalent positions can be selected where more are 

used. 

Selection of an auxiliary fan involves the same principles as for 

main-fan selection. Speeds, however, usually are limited to the constant speeds of the motors available, which vary for alternating current with the phase and cycles of the current; only a limited munber 

of speeds are therefore available. Careful selection demands that the 

pressure-quantity characteristics of the fans be available for the 

available speeds and that characteristics of different types, lengths, 

and diameters of tubing be plotted to the same scale with the use of . equivalent resistance factors that include average leakage effects 

rather than theoretical resistance factors based only on type, diameter, and length. If the proper material is available selection of a 

suitable fan for a particular piece of work is merely a matter of graphically determining where characteristics cross. 

IRON PIPE VERSUS FLEXIBLE TUBING 

Although many different materials may be used for tubing lines and 

are used for comparatively permanent installations where great length 

demands maximum conditions of tightness of both tubing and joints 

flexibility in use is the predominant characteristic for comparatively 

short and temporary installations used for auxiliary ventilation in 

metal mines. Canvas and jute tubings, therefore, have largely replaced galvanized-iron pipe for mine installations; and the iron pipe 

used (actually zinc-coated sheet steel) commonly has stove-pipe 

joints. Considerable discussion centers on the comparative economies of using iron pipe with wrapped joints or flexible tubing with 

patented couplings. Both are commercially available or can be made 

locally in different weights of material. Comparable weights cost 

about the same per foot for equal diameters; that is, lightweight flexible tubing costs about the same per foot as lightweight pipe of the 

same diameter for mine service. Because iron pipe is handled more 

frequently and more roughly in mines it is made of material averaging 

a gage or two heavier than for commercial work on surface installations. It costs about three times as much as flexible tubing to install 

and has other disadvantages as respects handling, storage, and ease 

with which the ends are deformed. Although canvas tubing will last 

as long as iron pipe under very favorable conditions-freedom from 

fungus, acid mine water, and other conditions of deterioration against 

which its chemical treatment may not prove effective-it generally 

lasts about one third as long and under certain air conditions encountered in mines will last such a short time that its use is almost impossible. Circumstances should decide the most suitable type. For the 

irregular, crooked openings of the average metal mine the advantages 

of flexible tubing outweigh its disadvanatges, and it will be found to 

be the material in common use. For straight and more permanent 

installations iron pipe appears to have enough advantages to warrant 

its use; it has a normally lower resistance to flow, in the ratio of about 
172 ENGINEERING FACTORS IN VENTILATING METAL MINES 

3 to 5, can be made more nearly airtight than canvas tubing with the 

couplings now available, and for the same mechanical wear can be 

kept within about half the distance from the face during blasting as 

that required for flexible tubing 'under average hard-rock conditions. 

Iron pipe is not very satisfactory where many bends are required, not 

only because the bend is difficult to make but also because the pressure 

waves from a blast, by causing a vacuum inside the pipe and pressure 

outside, occasionally flatten the pipe at a bend unless a heavier gage 

is used than for the straight sections. 

NEED OF LIGHTWEIGHT INSULATED TUBING 

The most important improvement in fan-tubing practice would be 

the development of a lightweight, insulated, flexible tubing. The 

major use of such installations in metal mines would be to combat 

high-temperature conditions in long dead-end openings. Uninsulated short tubings are very effective for this service, but effectiveness 

decreases rapidly with length of tubing, as they are in the low-velocity 

return current from the face, which is heated by the rock and in turn 

heats the air in the tubing. Larger tubings and quantities of flow 

obviate this decrease to some extent; but in dead-end development 

openings over 1,000 feet long the dry-bulb temperature of the air discharged from the tubing is often virtually the same as the rock temperature, and although the wet-bulb temperature at discharge, on 

account of heating of the air in the tubing without access to moisture, 

may be considerably lower it generally has relatively little effect 

except very close to the discharge. The effectiveness of the installation is therefore limited almost strictly to velocity effects. The abstraction of heat from the rock continues, of course, but the relative 

effect is less because of the large surface exposure, and the effect on 

face conditions is small because it is spread along the opening. Conditions at the face could be improved by intensifying the abstraction 

of heat through the use of insulated tubing, for comfort at the face is 

the objective rather than precooling of the rock; the latter can be 

accomplished more effectively in the territory where it will serve a 

useful purpose by large-quantity flow under low pressure after connections have been made. 

Actual costs of developing ground in hot rock are not so much a 

question of cost of fan-tubing methods of ventilation as of the most 

effective use of double-entry or equivalent methods of opening up 

ground so as to reduce the length of openings necessarily driven as 

dead ends. 

USE OF BEADING TO STIFFEN IRON PIPE 

In connection with the use of iron pipe deformation of the pipe, 

particularly the ends, always has been a troublesome factor. This 

is largely being eliminated at progressive mines by a bead or semicircular circumferential projection rolled close to two opposite edges 

in the separate sheets used in making up the pipe. These have a 

radius of X to 72 inch and are 2 to 3 inches from the ends; without 

increasing the weight of the pipe they increase its resistance to deformation enormously. In addition to the cover of tarred cloth ordinarily used to prevent leakage at assembly joints the Magma mine, 

Superior, Ariz., covers the wrapped joint with a two-piece bolted 
DISTRIBUTION OF AIR CURRENTS 173 

clamp which has two corresponding beads properly spaced. This 

makes an exceptionally rigid and tight line. 

INSTALLATION SPECIFICATIONS AND COSTS FOR AVERAGE CONDITIONS 

For average conditions of metal-mine installations, where the basic 

factor is improvement of comfort conditions at working places and 

consequent increase of working efficiency, suitable equipment may be 

approximated roughly for 30- to 50-square-foot openings in warm rock 

(85° to 90°): 8-inch tubing with a fan operated by a 3-hp. motor at a 

speed to give about 1,000 c. f. m. at 4 to 5 inches pressure for lengths 

up to 250 feet; 12-inch tubing with a fan operated by a 5-hp. motor 

at a speed to give about 2,000 c. f. m. at 5 to 6 inches pressure for 

lengths up to 500 feet; 16-inch tubing with a fan operated by a 10-hp. 

motor at a speed to give about 3,000 c. f. m. at 6 to 8 inches pressure 

for lengths up to 1,000 feet; and similar fans in series for lengths much 

in excess of 1,000 feet. If the tubing lines are in average condition 

about 40 to 70 percent of the air handled by the fan will be discharged 

at or near working faces. 

For higher-temperature rock and larger openings larger quantities 

of flow are desirable and can be obtained with larger tubing and largercapacity fans (quantity rather than pressure, since the benefit of 

increased pressures would generally be lost through increased leakages). For very high temperature conditions openings large enough· 

to permit motor haulage and mechanical mucking are usually the most 

economical and are often required for future ventilation needs. Where 

the size of opening is restricted the use of tubing lines in parallel 

often solves space difficulties. 

Cost of fans, motors, foundations, cable extensions, auxiliary electrical apparatus, and installation labor vary greatly; in general, the 

total cost of the fan installations mentioned, in place and ready for 

operation, will range from about $500 for the smaller sizes to $1,000 for 

the larger sizes. Tubing costs vary with time, quality, weight of 

material, length of sections (number of couplings), and quantity 

requirements. An average weight of 12-inch tubing, in 10-foot 

lengths for iron tubing or 50-foot lengths for flexible tubing, will 

cost approximately $0.50 per linear foot in small quantities. Costs 

for other sizes vary about as the material content, that is, roughly 

as the diameter. For flexible tubing the cost per foot is about 10 

percent lower for 100-foot sections and 10 percent higher for 25-foot 

sections. 

The major factor in obtaining satisfactory and economical results 

with fan-tubing installations is a good crew of specialists for installing, 

maintaining, and repairing tubing lines. Under average conditions 

the cost of installing flexible tubing is about one third that of iron 

tubing. Approximate average figures are about 2 cents a linear foot 

for flexible tubing and 6 cents a foot for iron pipe. 

Costs per cubic foot of air delivered at or near the working place 

by fan-tubing methods vary widely, but a rough average figure for 

total cost--capital return, power cost, and maintenance-is about 

0.2 cent per 1,000 cubic feet, which compares with rough averages of 

about 2.0 cents for compressed air released at the face and 0.05 cent 

for main-fan circulation. 
174 ENGINEERING FACTORS IN VENTILATING METAL MINES 

DISTRIBUTION RECORDS 

Maps showing the main features of the air-distribution system, as 

determined at regular intervals, are desirable at all metal mines, both 

for intelligent planning of the ventilation for normal operation and 

for guidance in fire emergencies. Where the lay-out and ventilation 

requirements are simple a simple plan or section map suffices-one 

showing directions and quantities of flow, position of control devices, 

and a few temperatures. For more complex lay-outs of openings and 

ventilation requirements intelligent mapping of the distribution 

system requires more detail and possibly a more diagrammatic type 

of map. 

Attempts to record ventilation data without the use of symbols 

usually result in loss of desirable data or unintelligible data. Detailed 

data for complex ventilating systems ordinarily are recorded on plan 

maps made to a scale of 100 to 200 feet to the inch. Uniform groups 

of symbols are desirable, so that those officials who use the maps 

infrequently are not confused in interpreting them. The set used by 

the author (fig. 64) has proved practical and useful and provides for 

all but very special conditions. 

For a large mine or one having a complex system of openings some 

sort of schematic drawing or section may be required to show the 

main features of the system free from confusing detail, so that broad 

analyses of conditions may be made simply and easily. Such drawings, however, are seldom made except for purposes of publication, 

since the average mine official relies on his mental picture of the ramifications of the mine openings. A number of diagrammatic sketches 

representing very complex systems of mine openings may be found 

in a recent Bureau publication.46 For slope workings intersected by 

vertical shafts simple isometric forms are particularly applicable. 

For more complex systems of openings more involved isometric forms, 

such as that shown in figure 65, have a particular field of use. Such 

maps are to be preferred to separate plan maps or simpler types of 

diagrammatic sketches, since they permit a complete record of the 

distribution on one map from which continuous analyses of the system 

may be made. 

Certain observations, such as temperatures, quantities, and fanperformance data, often are made at regular intervals and involve too 

much detail for recording on the ventilation maps. Loose-leaf notebooks with separate pages for each section where observations are 

made regularly provide a convenient form of recording observations, 

although in circumstances where these data are particularly important 

in analyzing and controlling the distribution it is desirable to transfer 

the records to a card form which is always accessible on the surface. 

VENTILATING SYSTEMS 

Any set of connected openings and the forces and appliances that 

cause flow of air through them may be considered a ventilating system, 

whether or not the flow is controlled in any way through systematic 

effort. The theory and practice of separate features of ventilating 

systems have been discussed in preceding sections; combinations of 

applications found in actual practice will now be discussed. Virtually 

.. McElroy, G. E., Ventilation of the Large Copper Mines of Ariwna: Bull. 330, Bureau of Minllli, 1930, 142 pp. 
VENTILATING SYSTEMS 175 

every metal mine differs from every other metal mine in the combinations of openings, pressure sources, and natural ground conditions 

presented. Few points of resemblance are found in comparing 

different mines, even where the ore deposits are quite similar; the 

major points of resemblance are found in comparing details or the 

separate parts. 

The normal procedure at metal mines is to accept the natural flow 

through the openings made as required for mining operations; this 

o 

(!) 

® 

® 

UQ9" 

Direction of horizontal air flow 

Air upcast from level 

Air I).pc~st to level 

Air downcast from level 

Air downcast to level 

Through vertical air flow 

=:;;r;;:;;:;;:= Quantity of air flow in cubic feet per minute 

'5;tOO Closed door 

,4 Door used as regulator 

===......J~j:== Open door 

: Door frame only 

Canvas deflector brattice 

===1F== Wooden deflector brattice 

o . 

.4 .~.4' A I 0 

Air-tightbrattice or bulkhead 

Pressure on stopping in inches of water 

Open for air flow but not passable 

Caved ground 

Booster fan or fan in booster. position 

Auxiliary fan unit 

~ ~ Overcast, undercast or oth!l1" air crossing 

=:;6;:2-:;;;7;:2= Wet and dry-bulb air temperatures, OF. 

:::::lR.T.-65 Rock temperature, OF. 

,rA.S.-72 A' I . . d b Ir samp e posItIon an num er 

c;::.. r=\ ~ Type of cross-section and area in square feet 

£]Q} t.llilJ ~ 

k = 110 Estimated value of friction factor 

FIGURE 64.-Set of conventional symbols for plotting mine-ventilation data on small-scale level maps. 

results in an uncontrolled natural-ventilation system. As the workings increase in extent, particularly with depth, air conditions normally grow progressively worse, and eventually some control of air 

flow is required; natural-draft currents may be controlled, resulting 

in a controlled natural-ventilation system, or the flows may be both 

controlled and augmented by mechanical means, resulting in a mechanical ventilating system. 
176 ENGINEERING FACTORS IN VENTILATING METAL MINES 

PROTECTION OF LIFE AND PROPERTY 

Uncontrolled systems of ventilation present natural hazards in case 

of fire; mechanical systems, although they may be installed with 

particular attention to the fire hazard, naturally increase the normal 

hazard through increased velocities of flow and added electrical installations. Consideration of ventilating systems is therefore inextricably 

tied to consideration of protection of men and property in case of fire; 

while many mines may be considered to have but superficial fire 

hazards yet there are few large metal mines in the world whose 

history does not include a disastrous or at least costly underground 

fire, regardless of the presence or absence of natural hazards, such as 

sulphides, which are liable to spontaneous combustion on crushing 

or caving. Moreover, there are few metal mines, either large or small, 

that have operated more than 10 years and have not experienced a 

fire in a surface structure which, under special conditions of occurrence, 

might have imperiled a mine opening. 

Ventilation and fire protection should therefore be considered together. The difficulty is, of course, tha·t whereas the one does not 

necessarily involve expense the other does. Most metal mines have 

fairly satisfactory air conditions, although where the natural conditions of temperature and vitiation are severe satisfactory circulation 

will be due largely to increase in use of mechanical installations dating 

back not more than 2 decades; these will be found to be coping more or 

less successfully with adverse conditions, according to individual 

natural conditions and operating personnel. 

Recirculation.-The status of metal mines with respect to fire protection is not so satisfactory. Air currents that are satisfactory as 

regards air conditions will often recirculate to such an extent as to 

create a veritable firetrap in case of fire underground; the difficulty 

is that the condition of recirculation is not always manifest and is 

disclosed only by a survey of the distribution system. Such a condition can exist in mechanically ventilated mines but is more likely 

to be present in naturally ventilated mines, as the very existence of 

the mechanical system is evidence that considerable thought has been 

devoted to the distribution system. The common set of physical 

conditions responsible for recirculation in workings on inclined 

deposits is as follows: There are two zones of relatively large and 

numerous openings, one near the surface and another at depth in 

the zone of active operations; but hetween these zones and between 

the upper zone and the surface the openings are relatively small and 

few in number. 'With the natural drafts ordinarily present comparatively large volumes may be circulated in a deep mine, but large 

proportions of the upcast currents short-circuit through the upper 

open zone to the downcast airways; the proportion of intake air from 

the surface is determined by the relative resistances to flow as between 

the upper open zone and the connections from it to the surface. Where 

the latter are very small the amount of surface-air intake may be 

negligible compared to the quantities circulating in the active zone 

of operations; in case of an underground fire the smoke and gases 

from the return side of a fire anywhere in the mine soon reach the 

active zone. As a matter of fact, the warning agents of smoke and 

odor may be largely removed from the fire gases through condensation 

in the upper zone before they reach the active zone of operations, and 
Bureau of Mineg 

Legend 

.........!IL.-Main fan instaliation 

- -Direction of air flow 

12,()()() Quantity of flow in cubic feet per minute 

:: .. :1r.: Ore zones active areas cross-hatched 

- -No open or direct connection 

Scale of feet A 

c 

Sketch plan showi

to which active 

FIGURE 65.-Diagrammatic sketch showing ventilation systBrn of Calurne1 and Arizona mine, Bisbee, Ariz. , October 1929: A, Isometric projection of three intersecting vei'ticai plane

position of three planes of projection, 

VENTILATING SYSTEMS 177 

the men may be overcome by carbon monoxide before they have any 

evidence of a fire in the mine. The situation is more dangerous than 

most metal-mine operators realize, and for this reason all metal 

mines should have controlled circulation, preferably a mechanical 

ventilating system. Because of its importance the subject will be 

stressed further by citing a few examples of recirculation recently 

found in some large, well-known, and otherwise progressive metal 

mines, in which the air conditions were so satisfactory that the 

ordinary account of the mining methods would dismiss ventilation 

with the statement "The mine is naturally well-ventilated." 

Examples of recirculation.-In a large gold mine the total quantity 

of air leaving the mine was found to be but little larger than the 

estimated amount of compressed air liberated underground, and no 

surface intakes were found; yet the circulations underground were 

relatively large. Investigation revealed that the main-intake airway, 

a raise from old workings to the surface, had been tightly bulkheaded 

about 6 months before to prevent "high-grading", and its effect on 

the ventilation of the mine had not been considered or noticed. 

The circulation was practically a closed circuit between the lower 

active zone and a fairly open and abandoned zone near the surface, 

and only the compressed-air supply maintained the chemical purity 

of the air against the minimum vitiating agents of men, animals, 

lights, and blasting at the end of the single shift worked. 

In another large gold mine the main-intake and main-return shafts 

were driven in the same vertical plane, one vertical and the other 

inclined, and crossed above the active zone of operations. They 

were entirely open at the intersection, with the result that much of 

the surface intake air never reached the workings and a large proportion of the return air was being recirculated by natural-draft pressures; some control of natural-draft circulations was attempted in 

this mine, but only to eliminate fog at points where it interfered 

with operations. 

In a large copper mine the total quantities entering and leaving the 

mine were found to be large, and only a detailed survey revealed the 

fact that the total circulations in the extensive active zones of operation were about twice as large as the surface returns. Actually, 

about half the total returns in upcast shafts were crossing comparatively open zones close to the surface and short-circuiting direct to 

the downcast shafts. 

In another large copper mine mechanically ventilated by underground fans a survey of the distribution had just been begun but was 

discontinued when a small fire broke out underground. After the 

fire was under control and all returns but a minor one from the fire 

zone were stopped off operations were resumed in a section adjacent 

to the main-downcast shaft. Within a few hours the men complained of headaches and were withdrawn. Several collapsed at the 

surface, and about 20 showed symptoms of moderately severe carbon 

monoxide poisoning. The mine maps were then consulted, and leakage paths from the fire zone via caved workings were traced to the 

main return and from the main return, as door and stopping leakages, 

to the main intake above the zone of operations. Although the 

leakage at one door or stopping may be negligible the total for a 

large number is often a surprisingly large amount, as it was found to 

be in this particular case. 
178 ENGINEERING FACTORS IN VENTILATING METAL MINES 

UNCONTROLLED NATURAL-VENTILATING SYSTEMS 

Most small metal mines make not even the simplest systematic 

effort to control natural flows. These are free, and as long as they 

provide fair to good air conditiol1s most of the time to most working 

places they are accepted" as is"; in fact, many operators of naturally 

ventilated mines, whether small or large, have only the vaguest idea 

of what causes air to flow in their openings or the reason for dead 

ends clearing up if given enough time and often have a very incorrect 

or incomplete knowledge of the paths traveled by air currents. This 

lack of knowledge often has but little effect on air conditions, as there 

is a general attempt to make the intensity of operations conform to 

the average intensity of the ventilating currents, but it does have 

considerable bearing on what might happen in the event of a fire 

underground or at a surface opening. 

Under favorable natural conditions, particularly where the mining 

methods involve large and numerous openings, as in open-stope 

methods of mining, metal mines have been carried to great depths by 

natural ventilation, both with but more generally without systematic 

attempts to control circulation; several groups of large mines are still 

so ventilated. Mines in mountainous country generally are wellventilated by natural draft down to the horizon" of the lowest surface 

connection and can be well-ventilated for a considerable depth below 

this level if two openings are available and some attention is paid to 

distribution; but reversal of direction with the seasons can be prevented only by mechanical installations. . 

CONTROLLED NATURAL-VENTILATING SYSTEMS 

Even where the general system may be considered uncontrolled 

natural ventilation, separate parts or features may be controlled to 

some extent. The mining method in use, although not applied with 

regard to ventilation features, may have been developed in part to 

insure good ventilation through a process of trial and error. When 

conditions become exceptionally bad in any district of a shallow mine 

an opening generally is driven to the surface, in ore if possible, as it 

has been found that such procedure will in most instances improve 

circulation. Exceptionally long dead ends or shorter ones in hot 

rock may be ventilated by auxiliary fan-tubing units. 

Often circulation can be both improved and controlled by erecting 

a few doors and bulkheads, as indicated for a typical small metalmine lay-out in figure 66. The important consideration with respect 

to ventilation conditions in large mines is that the workings be divided 

into sections by natural or artificial barriers extending from the surface 

to the top of the active zone. The air currents are thus forced to cross 

from downcast to upcast sections through the active zone of operations, 

and little recirculation is permitted. Both ventilation and fire-protection conditions are then better than with uncontrolled cross-flow 

in abandoned openings above the active zone. 

Such a system is well-illustrated by the copper mines of Michigan, 

which are well-ventilated by natural drafts at depths of 3,000 to 6,000 

feet vertically and 4,000 to 9,000 feet on the dip below a practically 

level surface. In this district the deeper mines have reached the point 

where mechanical ventilation is desirable on account of high-temperature conditions, but some' of them are so well-ventilated through 
VENTILATING SYSTEMS 179 

oontrol exercised by natural conditions or by erection of vertical lines 

of stoppings along shafts for fire protection that reproduction solely 

by mechanical means would be almost impossible. An example of 

remarkable ventilation through unintentional control resulting from 

barrier pillars separating properties is the Ahmeek mine of this group, 

for which the distribution in the active zone is shown in figure 67. 

These are retreating open-stope workings with relatively large and 

numerous openings on a low-grade bedded native-copper deposit 

~Directions of flow with control 

=--== Directions of flow without control 

~ote: DiBtributions shown are for winter conditions 

A 

Inactive 

r~===f=~~~F;'- _W:{.inKe. 

I Active 

stopes 

I 

--+ 

Development 

B 

-&-Door 

~Regulator 

--+-- StoppiDI 

FIGURE 66.-A, Controlled natural-draft distribution in a small metal mine; B, control of natural-drart 

distribution to prevent reversal of directions of flow. 

dipping about 35°. Details of the mine and ventilation conditions 

may be found in a separate Bureau publication.47 The recirculation 

indicated at the bottom of the wedge of Mohawk ground was due to 

lack of connections through the lens of poor ground below this point. 

It was later reduced considerably and the total quantities of flow 

increased by driving connections through this section at all lower 

levels. 

Examples of good ventilation resulting from intentional control 

designed for fire protection rather than ventilation are the Osceola 

.7McElroy, G. E., Natural Ventilation in the Michigan Copper Mines: Tech. Paper 516, Bureau of 

Mines, 1932, 40 pp. 
4-

'-'-;-::O:-O;:;:'7:.~:-; 5 

MOHAWK 

o 

" 

" 

' .. '0 \ 

q.~ 

- -" ~-' ~i !'~ - '" j"'" = 1\\~y=" 'J.~ 

'" ,,-'" .~," 

"'~.;~~--:. • .;;- DJlfEC7/ONDI'"FI.OWANDQVANTITYIIVCIJ6/CFEt:7P£IPNI"Zur A: 

~ ~ ,- " 

'" 'S:.. AC71VE S7Q"/N<iAREAS '/: 

.. ' '

" N/ DOUGLASS 

FIGURE 67.-Plan map of Ahmeek mine, Ahmeek, Mich., showing distribution of 
VENTILATING SYSTEMS 181 

and Isle Royale mines 47 of the same group working on ore of similar 

occurrence. Both mines are divided into downcast and upcast sections 

separated by barriers of lines of bulkheads along slope shafts from the 

surface to the active zone and by fire doors down through the active 

zone .to the bottom connection. In the Isle Royale the lines of 

bulkheads and doors are on both sides of the slope shafts, whereas in 

the Osceola they are on but one side. With such a lay-out the air 

currents can be practically shut off, as far as shaft flow is concerned, 

if there is a fire underground. In general, 1 line of stoppings per slope 

shaft is sufficient for low-temperature conditions, but 2 are desirable 

on the downcast shafts for high-temperature conditions even though 

the total quantities of flow are decreased thereby. 

The same general scheme of dividing a mine into separate sections 

can be applied in various lay-outs for naturally ventilated mines; 

control for fire protection is as complete as with nonreversible mechanical installations. However, the reversible feature provided with 

mechanical installations is not possible, and quantities of flow, 

although they may be increased somewhat by attention to temperature 

and airway conditions, cannot be controlled accurately. 

Control of seasonal reversals.-Seasonal reversal of natural-ventilation flows is undesirable as regards ven tila tion, opera tion, and safety. 

Smaller average flows dependable in direction are preferable to the 

alternate feast-and-famine conditions of uncontrolled reversed flow; 

with the general abandonment of the use of steam pumps underground 

flow reversals are more common than formerly. Reversals are reduced considerably by eliminating large differences in surface elevation between inlets and outlets, by coursing intake downcast air at low 

velocities close to the surface so that surface-air-temperature variations affect only a relatively small part of the downcast air column, 

and by coursing the up cast air at relatively high velocities to maintain relatively high temperatures in the upcast column. Application of these methods to the general lay-out of openings of a mediumsize mine in mountainous country is illustrated in figure 65, B. The 

necessity for the air lock shown on the main haulage adit would 

depend on circumstances; the resistance of a long adit might be so 

great compared with the resistance of the downcast raise system that 

the small inflow during the winter and outflow during the summer 

would not be a disadvantage. The outer door would be installed in 

any event for fire protection and in cold climates could be closed to 

minimize freezing troubles in winter. 

MECHANICAL-VENTILATING SYSTEMS 

When the results obtained through controlled natural ventilation 

are inadequate or unsatisfactory for intensive operations the common recourse is to increase the quantities of flow through mechanical 

installations. These are advisable for virtually all metal mines, 

regardless of whether they are normally used, to provide the reversible 

direction-of-flow feature for emergency use and ill many instances for 

use at intervals when surface temperatures limit the natural drafts. 

Consideration of mechanical-ventilation systems is nearly always 

a question of making the system conform with a set of existing air-

., McElroy, G. E., Natural Ventilation in the Michigan Copper Mines: Tech. Paper 516, Bureau of Mines, 

1932,40 pp. 
182 ENGINEERING FACTORS IN VENTILATING METAL MINES 

ways and their possible extensions. Rarely are the outlines of the 

deposit known and the lay-out of openings planned in advance. The 

usual metal mine manages with natural draft until the air conditions 

get so unsatisfactory that more air and better distribution are required. In most metal mines the primary cause of the change is the 

increase in air temperatures encountered with depth; in undercutcaving operations it is the necessity for quickly removing the dust, 

smoke, and gases resulting from continuous blasting of boulders on 

grizzly levels; at some mines it is the desire for better control of air 

currents when there is an underground fire; and at a few it is difficulty 

with strata gases. Minor objectives are: Reduction of the amount 

of compressed air used for aIr-motion cooling, reduction of timber 

decay, and elimination of fog. 

In installing a mechanical system in a mine the two main factors to 

be considered are quantity of flow and lay-out of openings. 

QUANTITY REQUIREMENTS 

In a natural-ventilation system quantities of flow are taken as 

they come, or if deficient some attempt is made to increase them. In 

a mechanical system increase in quantity over natural flows is obtained 

only at considerable expense, which increases rapidly with increase in 

quantity whether the latter is obtained by reducing the resistance or 

by increasing the pressure generated. It is therefore desirable to 

limit quantities to those that will give satisfactory conditions. In 

practice, quantity requirements cannot be calculated accurately 

and are not controlled by law, as in coal mines. Determination of 

quantities is largely empirical, differs for each mine, and usually is the 

decision of responsible officials, based on observations under naturaldraft conditions or with temporary equipment. Probably the best 

general statements of requirements are those based on quantity per 

man underground and the general types of mine conditions. Data 

on quantities per man underground on the largest shift for a number of 

large metal mines may be found in table 5 (p. 192). 

If air conditions are normal and general blasting is confined to the 

end of each shift 50 c. f. m. per man ordinarily would be satisfactory for 

operating concentrated working places two shifts with a long interval 

between the shifts, but if the working places are scattered widely 

requirements would approach 100 c. f. m. If operations are concentrated and blasting intermittent, as in undercut-caving operations, a 

velocity of about 100 c. f. m. is desirable on grizzly drifts for rapid 

removal of blasting smoke, dust, and gases; general requirements are 

200 to 300 c. f. m. per man. Extensive timbering, moderate production of gas, such as usually accompanies work in sulphide ores, or 

moderately high temperatures would individually increase requirements 50 to 100 c. f. m. per man. Except under extreme hightemperature conditions, the requirements will seldom exceed 500 

c. f. m. per man, but under such conditions as much as 1,000 c. f. m. 

per man might be required for satisfactory results. However, such 

wide variations in mine conditions and corresponding quantity requirements are possible that considerable play of individual jud~­

ment is involved, and the quantities necessary to give a satisfactory aIr 

supply are not only approached by trial steps under many conditions 

but are also subject to a continual though gradual change. They are 
VENTILATING SYSTEMS 183 

also limited to a large extent by the lay-out of openings and the 

expense of increasing quantities by new airways or modifications of 

existing airways, so there must also be a continual compromise 

between expense and the acceptable standard of satisfactory air conditions. In general, the mines that need the most air are those where 

the ground requires fill methods of mining and where other conditions 

obtain, such as difficulty of maintaining openings, that result in 

relatively few, small openings unless the details of the lay-out required by t~e mining methods a~e arranged ~th regard. to ventilatIOn to prOVIde more and larger "aIrways. Mmmg operatIOns usually 

are facilitated to some extent by having more large openings, and the 

excess over absolute mining requirements need not be charged entirely to the ventilation system. 

PRESSURE REQUIREMENTS 

Where the quantities of flow are set at certain approximate figures 

the pressure requirements to give these flows are determined largely 

by the lay-out of the openings. If the quantities are constant the 

paid-for power requirements will vary directly with the excess of the 

pressure requirements over the natural-draft pressures; it is therefore 

desirable to maintain high natural-draft pressures. Where the lay-out 

of openings is constant the pressure requirements vary with the 

methods of use, and an endeavor should be made to obtain a balance 

between the resistance conditions of the major parts of the system, 

such as the main intakes and the main returns, even though multiplefan systems thereby result. Pressure requirements are fundamentally 

a question of the resistance to flow of separate sections of the airway 

system; it is particularly desirable to use all the airways available 

and, where these would lead to excessively high pressures, either to 

add to their number or size or otherwise reduce the resistance to flow. 

Sections of restricted area relative to the rest of the system, or 

"bottlenecks", would account for an abnormal proportion of the 

pressure requirements of the system and should be avoided by all 

means. After the main openings have been fixed the lay~out of 

minor openings as required by different mining methods may have an 

important effect on the pressure requirements; these openings, however, usually are so numerous that the pressure requirements are 

practically determined by the lay-out of the main airways. The 

exact lay-out of openings in working zones determines more particularly whether or not the air supplied by the system reaches the actual 

working places. 

Fans may be installed at one or more points in the system; one 

usually is enough for a small mine. In workings that are extensive 

laterally fans are often used in parallel on separate openings, but in 

those that are extensive in depth fans are often used in series on or 

adjacent to the same opening. In very large mines, where the 

workings are extensive both laterally and in depth, both parallel and 

series combinations may be found. The variations depend on the 

variations in lay-out of openings although ventilation by: a single main 

fan is always desirable if it can be arranged economically. 

Fan pressures are limited by maximum safe tip speeds to maximums 

of 5 to 10 inches of water for large centrifugal designs. The actual 

pressures used for main fans in metal-mine ventilation usually fall 
184 ENGINEERING FACTORS IN VENTILATING METAL MINES 

within 3 to 5 inches of water. Higher pressures generally are avoided 

on account of leakage, difficulty of passage through high-pressure 

doors or air locks, and possible mechanical troubles at higher fan 

speeds. Under favorable conditions pressures up to 9 inches have 

been used. Equivalent resistance factors (p. 114) for main-fan operation at large metal mines rarely exceed 10; the average is about 5-

comparable to operation to give 100,000 c. f. m. at 5 inches pressure. 

Small mines require small quantities, for which the econoIillcal sizes 

of airways are smaller and the equivalent resistance factors might 

be as high as 20 to 30 for single-fan systems. 

DETERMINING FACTORS IN LAY-OUTS 

In determining the lay-out or method of mechanical ventilation 

many factors must be considered, particularly those pertaining to 

safety, convenience of operation, and efficiency in the use of power, 

both for the present and the future. Usually the general features of 

the system can be determined on a nontechnical basis, as maximum 

use of existing openings with minimum interference with existing 

mones of operation is always the primary object. Few airways can 

be driyen expressly for ventilation because of their high cost and 

relatiyely short life; rather, an attempt should always be made to 

utilize existing openings required by active mining operations or 

resulting from past operations and to limit the necessity of maintaining openings for ventilation only to the minimum. 

MAJOR O"C"TLINES OF SYSTEMS 

The number, size, condition, use, and position of surface openings, 

such as shafts und adits, should determine the major outlines of the 

system. The desired features to be incorporated have been discussed 

separately and include: (1) Utilization of openings for transportation 

of men into and out of the mine as intakes or fresh-air outlets' (2) 

utilization of all available surface openings or enough to give iowresistance systems; (3) coursing of intakes directly to the bottom levels 

of active zones, whence they ascend through active zones to connections to the main outlets; (4) limiting distances to be traveled to the 

minimum; (5) maintaining a balanced resistance relation between the 

main-intake airways and the main-outlet airways and between the 

separate sections of main airways; (6) reducing obstructions, such as 

doors and air locks, on active operating openings to the minimum; 

(7) avoidance of leakage, recirculation, and fog; (8) and circulation of 

air from active zones to caved ground rather than from caved ground 

to active workings. 

INTERVENTILATION OF MINES 

Interventilation of adjoining mines is rarely desirable, as cooperation of separate operating staffs on ventilation matters is not dependable. Where the mines belong to the same organization questions of 

economy usually dictate interventilation, in which case maximum 

provision for safety demands that the ventilation of the mines in 

any interventilated group be under the control of one official. Where 

the ownership or control of adjoining mines is separate interventilation 

should be considered only as a temporary expedient, that is, only 

until the openings necessary for a separate system can be driven. 
VENTILATING SYSTEMS 185 

Isolated sections sometimes can be ventilated conveniently only 

through a shaft on an adjoining property, in which case it is advisable 

to enter into a lease for full control of the shaft for the necessary 

period. Connections between adjoining mines are, of course, desirable 

as emergency escapeways, particularly for deep mines with hoist 

service in but one shaft; but such connections normally should be 

shut off with fire-door air locks which may be fastened, if considered 

desirable, with seals that can be broken easily in an emergency. 

EXHAUST SYSTEMS PREFERABLE 

Sometimes natural conditions, such as sealed fire areas or broken 

ground along the line of an adjacent mine ventilated by a pressure 

system, seem to favor a pressure system, whereas other factors may 

demand that the general system be an exhaust system. Such a 

conflict cannot always be remedied satisfactorily, but often a booster 

fan underground will supply the pressure conditions required in 

limited sections, while the major circulation is by the exhaust system. 

Most metal mines can be ventilated satisfactorily-if safety is given 

the prominent position it deserves-by exhaust systems, with the 

fans on the surface at minor shafts or adits. Air is drawn in through 

the operating openings (which are assumed to be used also for transporting the men), is kept practically intact until it reaches the lower 

active levels, and is then allowed to upcast through the active workings to the minor openings through which it is drawn to the surface. 

Although a bulkhead suffices for a nonoperating shaft and a door or 

bonnet for a shaft that is used infrequently a surface-fan installation 

at an operating shaft requires some type of air lock to permit intensive 

use of the shaft. This condition is common in English coal mines 48 

and is solved most effectively by the use of elaborate, automatically 

operated air-lock installations, which are satisfactory but undoubtedly 

very expensive. If no inactive opening is available for a main-fan 

installation the general procedure at American metal mines is to 

drive a system of raises to the surface or to drive a surface adit from 

the top of the active zone or from a good connection.to this point; 

this practice by no means has been confined to relatively shallow 

mines, since raise systems of 1,000 to 2,000 feet are quite common. 

Similar raise systems are also used to continue openings provided 

by inactive shafts to lower horizons and often are driven in comparatively shallow mines to avoid driving long connections to existing 

openings, which, of course, is the logical procedure. Such raise 

systems usually are more expensive than air-lock systems on shaft 

tops but prevent operating incQnveniences and permit a better 

selection as to position-features that compensate for the difference 

in cost. The cost of raise systems through ground that does not 

require support o.r maintenance is relatively small where the existing openin~ provide numerous points of attack, as the cost of driving 

raises 18 governed largely by the length of the single sections. A 

raise system paralleling a mam upcast airway is a desirable method of 

obtaining the equivalent of an increase in area of the main up cast, 

since the up cast airway at a deep, hot mine may offer very disagree- able working conditions . 

• 8 Haddock, M. H., MineVentilation andVentiiators: London and Philadelphia, 1924, pp. 156-160. 

122422°-3&--13 
186 ENGINEERING FACTORS IN VENTILATING METAL MINES 

LOCATION OF OPENINGS 

To decrease the distance traveled, provide safety, and reduce the 

chances for leakage the intake and outlet openings, in general, should 

be so arranged that the active workings lie mainly on the line between 

them. If active workings extend in two or more directions from a 

downcast operating shaft an upcast shaft situated near the limit of the 

active workings in each direction would be desirable. If the shafts 

are spaced on a line at more or less regular intervals, as in working an 

extensive vein or lode, alternate shafts should be upcasts and downcasts, respectively, in order to distribute the resistances to flow and 

eliminate dead areas or those that receive little flow. This arrangement is particularly good where the openings in active zones are comparatively small; where these are large compared with the shafts dead 

areas rarely develop. Two shafts close together generally are more 

satisfactory when they carry air in the same direction because extensive stoppings are not required on the connections between them 

Air shaft Operating shaft Air shaft , ~ , 

-D--Door 

--+-Bulkhead 

FIGURE 68.-Sketch indicating the generally desirable features of mechanical-ventilation systems applied to metal mines. 

leakage is avoided, and the distance traveled by the air is also usually 

the minimum. 

Openings well located and properly selected for use as upcasts or 

dowDt:lasts are a large factor in insuring a simple and effective ventilating system, but the major point is always to use every available 

opening that can be utilized to advantage in building up the system. 

IDEAL LAY·OUT OF AIRWAYS 

The lay-outs of metal-mine openings to which mechanical ventilat.ion has been applied usually are so irregular that the main features 

of the system are obscured, in diagrammatic sketches designed to 

.show the outlines of the system, by the necessity of including important details peculiar to the particular mine. Diagrams of actual 

systems therefore do not illustrate desirable major features as well as 

an ideal lay-out, suc~ as is shown in figure 68, assumed for this pur-
VENTILATING SYSTEMS 187 

pose. This system illustrates the generally desirable features that 

may be incorporated in most actual systems. Surface exhaust fans 

with reversible housings installed· on outlying and relatively small 

inactive shafts in the hanging wall of a steeply dipping vein draw air 

in through the more centrally located and relatively large operating 

shaft to the bottom levels, through which it passes to active stopes 

and ascends through them to less active workings connected to the 

up cast shafts. The eight desirable conditions (see p. 184) previously 

listed as the objectives for major features are admirably met, and a 

safe and efficient system should result. 

MINOR OUTLINES OF SYSTEMS 

The minor outlines of ventilation systems are controlled largely by 

the individual characteristics that distinguish one mine from another, 

even though quite similar ore bodies are worked. Some of the more 

important are: Degree of concentration of operations, as determined 

by occurrence of ore, changes in economic conditions, and mining 

methods; position of active areas, as determined by occurrence of ore 

and sequence of operations; relative ease or difficulty of maintaining 

openings, as determined by ground conditions and methods of support; 

position of local sources of heat, as from large electrical installations 

or crushed sulphide stopes, of cold air, as from large compressed-airoperated appliances, of dust, as from crushers or rotary dumps, or of 

strata gases, as from fissures in limestone areas; position of sealed 

fire areas or areas liable to spontan~ous ignition; condition of abandoned and inaccessible workings, such as unsealable openings between 

shafts near the surface and unfilled raises in filled stopes; and the precooling requirements of zones of warm rock. All these and many 

more special conditions may require certain modifications of the major 

outlines laid out for the system as a whole, but if the major objectives 

and their relative importance are kept in mind in planning the lay-out 

an effective system should result. 

CONCENTRATION OF WORKING PLACES 

Probably the greatest factor in insuring satisfactory air conditions 

at low cost is the degree to which active mining operations are concentrated in as small a territory as possible. At times the nature of 

the ore occurrence and other conditions make it impossible to concentrate operations in a relatively small zone. Concentrated working zones are especially desirable where the operations are in warm 

rock, as at depth, and good :management usually will dictate a policy of cleaning up levels completely so that they can be abandoned 

in regular order as the depth increases, thus maintaining a uniform 

depth of active zone. Sections not commercial at the time of temporary abandonment should be sealed thoroughly, as maintenance of 

support can be preserved both by comlpete se&ling and by thorough 

ven tila tion. The in-between condition of insufficien t ven tila tion leads 

to the most rapid deterioration of timber support and the most extensive sloughing of rock walls. 

Extensive operations above the active zone of first mining, such as 

reworking of gob bed areas, leasing operations on scattered pillars and 

lean zones, and retreating pillar extraction, should have separate intakes and, preferably, separate returns; the circuit should be entirely 

separated from the main circuit below it by continuous barrier pil-
188 ENGINEERING FACTORS IN VENTILATING METAL MINES 

lars, natural or artificial, as by bulkheading all raise openings in 

floor pillars or by completely filling all but main openings from one 

level to the level above. In fairly good. ground a shallow depth of 

fill on timber bulkheads can be used to reduce the expense of reopening the barrier, but in heavy ground raises that are completely filled 

so as to exclude air completely usually can be opened more cheaply 

because they require fewer renewals of timber support. Insistence of 

mill operators on uniform grades of ore output from sections of nonuniform grade of occurrence complicates matters and is a large indirect factor in preventing the concentration of active workings. 

LOCAL SOURCES OF HEAT, DUST, OR GAS 

The proper handling of local sources of large amounts of heat, 

dust, Or gas and of any other condition that interferes with the normal arrangement of an otherwise desirable system is often troublesome and occasionally very expensive. A separate split of the intake 

air usually must be diverted direct to an outlet not used for the transportation of men, or an entirely separate air circuit must be formed 

to include the local condition. For example, in the United Verde 

mine a circulation of 60,000 c. f. m. was planned (1930) for an underground generator and hoist installation, and in the Village Deep 

mine a split of 75,000 c. f. ro. was allocated to a similar installation. 

In the Calumet & Hecla Conglomerate mine two large pumping stations on a return shaft each have a circulation of 30,000 c. f. ro. At 

the Ray undercut-caving mine a separate concrete-lined shaft with a 

surface exhaust fan connects with 3 rotary durops on the ore-hoisting 

shaft and handles over 40,000 c. f. m. Here the horizontal crosscuts 

from dumps to air shaft testify to the necessity of the installation, as 

they roust be shoveled out at short intervals to prevent choking the 

system. At the somewhat similar :Morenci mine, where large cars 

are dumped in transit over large pockets at the ore shaft, a raise 

driven from a drainage adit up to the shaft stations provides a separate natural-draft circuit for this shaft, which can be reinforced in 

case of inadequate or reversed flow by installing a small fan on the 

drainage adit. 

Pump or pipe shafts-shafts devoted solely to pipe columns connected to underground pump stations-are good natural upcasts and 

should always be used as such if possible. vVhere required, they 

usually are close to another shaft, and it is coroparatively simple in 

most cases to make the adjoining shaft a downcast and split off 

enough air from it to ventilate pump stations and other sources of 

heat, dust, and gas thoroughly, with the returns all in the purop 

shaft. Auxiliary fan-pipe installations are an effective roeans of carrying the return from a local source to a relatively distant return 

airway. 

Where the doors and air locks required for an effective method of 

ventilating the local source of excessive heat, dust, or gas would interfere to an unreasonable extent with intensive operations the pressure 

requirements might be supplied economically by a relatively inefficient injector method, such as the "Saccardo" (see p. 134), that does 

not require obstructions in the airways; this method has been applied 

to railroad and vehicular tunnels. 

According to those who have had extensive experience with such 

conditions, sealed fire zones are best handled by putting the sealed 
VENTILATING SYSTEMS 189 

area under enough pressure to overcome any reverse natural drafts 

and providing a small separate split to the main return or a small 

separate return from a point above the fire zone. 

CIRCULA.TION THROUGH CA.VED GROUND 

Another difficulty, which appears in many forms, in making ventilation systems effective after they have been carefully planned and 

installed, is circulation through filled or caved ground. The most 

common form is leakage through the fill where the shaft collar has 

been built up above the natural ground level from the waste rock 

resulting from sinking operations. A concrete lining from the collar 

to solid rock or to tight material, such as clay, is required for a good 

job. 

In caved ground the circulation depends mainly on the way the 

ground caves. Even if the proportion of fine material is large enough 

to prevent circulation through the mass itself, the spaces along the 

hanging wall and adjacent to pillars or solid ground may be large 

enough to permit relatively large flows. Enough air can circulate up 

through 15 to 20 feet of coarsely broken ore to ventilate a shrinkage 

stope adequately, and enough has been known to pass up through a 

200-foot fill of moderately coarse rock in a raise of large cross-section 

to ventilate a small stope. In the retreating systems of mining employed in the Michigan copper mines air circulates very freely through 

the caved areas adjacent to active stoping operations, so that such 

caved zones are equivalent to low-resistance return airways. 

Although, in general, it is not desirable to have intake air circulations pass fro:m. caved ground (with or without crushed timber) to 

working places, the degree of undesirability of the method depends on 

circumstances and seldom is important enough to justify the sacrifice 

of major objectives or the use of pressure rather than exhaust systems. 

Where the cover is shallow, holes and fissures serve as good intakes, 

since oxidation and timber decay are the minimum and the intakes 

are often in very favorable positions to give short lengths of travel. 

Where the cover is thick enough to prevent free intake of air the 

amount intaking is usually so small that even with easily oxidized 

materials and decaying timber in the caved ground its effect is 

negligible if the general circulations are of satisfactory volumes. 

Large circulations enter only through fissures in which the velocities 

are so high that the time of contact with oxidizing material and decaying timber is too short to affect air conditions seriously. 

Under certain combinations of conditions a pressure method of ventilation may appear desirable to insure that the circulations will be 

from working places to and through caved ground, particularly in 

top-slicing under a heavy timber mat and 100 to 300 feet of overburden. In certain sections pressure could be maintained in a general 

system of exhaust ventilation by means of a booster-fan installation. 

If the whole mine is involved it is possible to use a general system of 

pressure ventilation and still attain the desirable major objectives 

lis'ted-particularly that openings used for transporting the men carry 

intake air. This usually requires entrance through an air lock, a 

main fan underground on the intake side, or the use of the shaft for 

transporting men as an intake-air upcast. All these methods usually 

involve additional inconvenience to mining operations or considerable 

additional expense over a straight surface exhaust system. 
190 ENGINEERING FACTORS IN VENTILATING METAL MINES 

Air circulation through loose material, such as the broken ore in 

raises, often adds to the problem of distribution, as a shallow depth of 

coarsely broken ore in a chute has no value as a stopping and makes 

an uncertain regulator. In many metal mines broken ore in chute 

raises must be relied upon for controlling the minor features of the 

distribution but proves very unreliable in practice. The men filling 

the raises can be relied on for a close record of the material put in the 

raise, even where the ore runs freely through a grizzly, as in undereutcaving methods; but almost superhuman discipline is required to 

prevent main-level motor crews from running chutes empty, regardless 

of orders to the contrary. 

PERSONNEL 

Ventilation at the ordinary metal mine is generally everybody's 

business and consequently no one's responsibility. Under favorable 

natural conditions no great harm results if the lines of responsibility 

and action are strictly defin8d when a fire occurs. However, in case 

of fire the loss of life or property damage may be large through ignorance of the air-distribution system. Under unfavorable conditions 

ventilation becomes an important fa(~tor in operations, and good 

results are obtained only where lines of authority are strictly drawn. 

In goneml, ventilation matters should be confined to three classes of 

officials-a trained crew to make the actual installations of distribution features, a ventilation foreman to supervise such installations and 

record the distrihution, and a high official with authority to initiate 

major changes and control the major features. Even though the 

need for the installation of distrihution features is intermittent, it is 

desirable to have them all installed by the same men, who can thus 

become proficient, particularly whpre dimensions, parts, and other 

features of installations are standardized. 

In a small mine the mine foreman 01' an under official, such as a 

shift boss, may be required to act as the ventilation foreman, but in 

a large mine ventilation authority should be independent of the general operating force-the individual vested with this duty should 

report direct to the ranking operating official. The" ventilation" 

foreman, so termed here to distinguish him, might devote but a 

minor part of his time to ventilation and the remainder to other duties, 

or one man might serve a number of mines where conditions are favorable. Since he has little to do with the major features of the system, 

except in an advisory capacity, technical ability and a knowledge of 

ventilation theory, though desirable, arc not so important as operating experience, intimate knowledge of the mine workings, and ability 

to gain the cooperation of the operating officials; but where the 

natural conditions are such as to make good ventilation a matter of 

primary importance in operations a high degree of technical ability 

and knowledge of ventilation theory are required to reduce the load 

on the higher responsible official. 

The requisite technical ability and knowledge of ventilation theory 

for installing and extending ventilation systems should reside in one 

higher official selected to hayc sole responsibility for the ventilation 

system. In order that ventilation may be fairly represented, the 

higher the rank of this official the better, since decisions may involve 

large sums of money. In a moderate-size mine only one man in the 

organization is eligible-the superintendent or manager. In a large 

organization one man often is better fitted for the work, through 
VENTILATING SYSTEMS 191 

interest and knowledge of ventilation theory, than the others. Although the chief mining engineer, if there is one in the organization, 

is the usual selection, the chief electrical engineer or the chief mechanical engineer would often prove a wiser one. For a particularly large 

mine or group of mines operating under difficult conditions a mining 

engineer specializing in ventilation would be the natural selection for 

this position, with or without other duties of minor importance. 

COSTS 

Ventilation costs depend largely on mine conditions and quantity 

requirements and will necessarily vary over wide limits for different 

operations and conditions, regardless of the degree of economy practiced. (See Economic Size of Airways.) The large expenses come at 

irregular intervals, and as costs ordinarily are charged as they accrue 

the year-to-year totals for the same mine also are subject to large 

variations. Moreover, costs are largely determined by the particular 

bookkeeping methods employed. The ordinary ventilation system 

usually falls heir to an elaborate airway system already paid for and 

is charged only with extensions and maintenance of openings used for 

airways; but certain high-cost jobs, such as a new ail' shaft, properly 

chargeable to ventilation often are carried in separate accounts and 

are never entered or prorated on the ventilation account. Whether 

or not a proportionate chnrge is made for compressed air used for 

ventilation has an all-important effect on the cost data. Few mines 

make any charge to ventilation for compressed air; most of those that 

do, use such ·an unreasonable basis that the figures have no relation to 

facts. 

Compiled cost data for two groups of mines, taken from recent 

Bureau publications 49 in which they are presented in more detail, are 

given in table 5. 

This table includes average and unit-cost data for the 5-year period 

1924-28 and pertinent operating and performance data for approximate conditions existing at the times the mines were visited. With 

one or two exceptions the cost data include a normal number of highcost construction items, such as occur at irregular intervals, but contain no prorated charges, such as might be made for compressed air, 

for maintenance of openings used for a dual purpose including ventilation or for high-cost construction preceding the 5-year period. Few 

of the items have any real significance, and the' significant items serve 

only to define the probable range that may be met in practice. For 

these large mines yearly mean total costs show a range of approximately $12,000 to $98,000, equivalent to 0.32 to 18.7 cents per ton. 

of ore mined, 0.02 to 0.27 cent per pound of metal produced, and 0.8 

to 11 cents per 100,000 cubic feet of ventilating air. If proportionate 

charges are made for compressed air and first cost and maintenance 

of all airways are spread over their estimated life the true costs for 

yentilation. for the mines listed would range up to about double those 

given for the low-cost mines and up to 50 percent higher for the 

high-cost mines. 

" McElroy, G. E., Ventilation of the Large Copper Mines of Arizona: Bull. 330, Bureau of Mines, 1930" 

142 pp.; Mine Ventilation in the Coeur d'Alene Mining District: Ini. Circ. 6382, Bureau of Mines, 1931. 

38pp. 
192 ENGINEERING FACTORS IN VENTILATING METAL MINES 

TABLE 5.-Ventilation cost data for two groups of large metal mines 

Operating data, 1929 Performance data, 1929 Cost data, 1924-28 

'0 ol g~ "'0 .s.S '" ol 0 ~ol " :0" ~t ;E Mean :0 '" a:o .S s ol g yearly unit 

'" 0 " _:0 .~ 0 

ol .E .:" :s ... '" ~:= OlOl S!;l, s ... - "0 §.~ cost o'~ :; """ Mine .E til" :pEi ... :; " .E ~~ ~.<:l :;" "" 0" 

Metal, -" ""'" .$t "'" ""ol i >: 8- S~ ""ol ""g tl ... ... - " percent "'~ g"" _:0 ~" if§ ... 0 "f ",0 

ol"" "" " ti '0 ... "" 'si ~~ ~~ ~'O ""~ :"§! tl :o~ ,,'" 0 I» .021 "" .... " "" "" ~- .~ s .~-e ~ " :a~ ol ~ ",0 

~ S "" ~ " ~ol m&s " :0 ~.~ 

"§~ '§ ~.;2 0 ol" " Olo ... 0.0 0 8 Z 8 U"" u Il: >" ~ u u " 0- --------- ------------------- MorencL _______ 5,000 2.0 Cu __ 350 235 168,000 715. 1,040 0.045 2.0 $17,647 2.5 1.2 0.035 Ray ____________ 10,000 1.3 Cu __ 800 400 162,000 405 1,430 .Oll 2.7 11,752 .8 .39 .0l7 In~pir~tion _____ 23,000 1.1 Cu __ 1,760 810 165,000 205 1,440 .005 3.2 16,535 2.0 .32 . 019 M18mL ________ 13,500 .9 Cu ___ 725 470 105,000 225 470 .020 16.6 27,567 4.5 .85 .052 Magma _________ 800 7.0 Cu __ 400 200 156,000 780 390 .68 23.7 44,601 6.0 18.7 .156 Old Dominion __ 1,300 2.7 Cu __ 600 300 180,0001 600 900 .32 14.0 58,625 7.9 17.0 .266 Copper Queen: Porphyry ___ 2,200 2.1 Cu __ 295 150 { .016 } 30.0 41, 018 6.8 { 6.2 .179 Limestone __ 850 4.3 Cu __ 500 250 }100, 000

1 

250 540 .24 9.5 .092 Calumet and 1,750 5.0 Cu __ 680 450 140,000 310 470 .26 13.3 60,335 7.8 12.5 .128 Arizona. United Verde ___ 2,400 4.0 Cu __ 900 500 350,000 700 780 .13 35 97,899 4.6 11.8 .147 United Verde 2,200 9.0 Cu __ 450 330 90,000 270 670 .11 26 33,963 3.7 16.3 .078 Extension. Pilares __________ 2,800: 2.5 Cu __ 1,200 600 240,000 400 1,020 .11 35 18,174 1.6 2.3 .044 Bunker Hill & 1,40°1

10.01 Ph __ 5S0 540 40,000 75 500 .06 21 11,656 6 2.6 ------ Sullivan. 

Morning ________ 1 100J9.11 Ph __ } 4501 305 40,000 130 570 .12 22 21,040 11 7.4 .022 , 1\6.0Zn-_ 

3001270 Hecla ___________ 8501

10.01 Pb __ 70,000 260 470 .19 7 12,865 6 4.0 .023 
INDEX 

Page 

Absolute pressure, instruments for measur- ing ______________________________ 19-20 

Air, absolute pressure, measurement_________ 19 

cooling power, measurement_______________ 32 density, formula___________________________ 26 

dry, weight, determmation________________ 2!l 

moist, dry air in, weight and volume, chart_ 29 physical properties ______ ' ________________ 27-31 charts _________________________________ 30,31 

weight, calculation______________________ 92 chart_ _ _ _______________ __ __ _ _ _ _ _ _____ __ 23 

underground, temperature, factors con- trolling______________________ ____ 90 

Air, water, and steam jets, as aid to natural 

draft, value______________________ 101 

Air columns, direction of flow _______________ 91 

natural-draft, diagrams _________________ 94,95,96 

quantities of flow, calculation____________ 94 temperature determinations_ ____________ 92 

Air currents, distribution, controL ________ 136-174 effect ofmiuing methods ______________ 1520-159 recirculation, cause________________________ 176 examples_ _ __ _ __ __ ______________ _________ 177 

fire hazard_______________________________ 176 

seasonal reversal, controL__________________ 181 

velocity, controL__________________________ 146 limitations______ _ _ _ _ ______________ ______ 139 

Air flow, at bends in airways, conditions_ _ _ _ 48 diagram_______________ ________________ 49 

direction, controL_________________________ 137 economics, discussion ______________________ 8!Hl9 

in airways, economical velocities, table____ 87 

resistance, measuremenL________________ 93 

velocity distributions____________________ 69 in mine openings, discussion _______________ 34-89 regulation_ __ __ __________________________ 159 

natural-draft, characteristics_______________ 98 distribution_ _ _ __________________________ 96 

recirculation_____ _ _ __ __ __ __ __ ________ ___ _ 97 

natural-draft measurements, application___ 93 

Air-flow conditions, effect on pressure-loss fac- tors and velocity distributions ___ 68-71 

natural-draft, factors controlliug ___________ 89-92 

Air horsepower, determination, charL_______ 78 Air injectors, efficiency _ _____________________ 133 

performance, determination_______________ 134 types__ _ _ _ _ __ __ _ __ _ _ __ ___ ___________________ 133 

use in metal mines_________________________ 133 Air locks, use_____ ___________________________ 144 

Air measnrement, accuracy__________________ 15 

high-velocity, pitot-tube methods_________ 14 instruments, prices________________________ 4 

low-velocity, smoke-cloud methods________ 4-6 

moderate-velocity, vane-anemometer meth- ods__ _ _ _ _ _ _ _ __ _ _ __ _ _ _ __ _ __ _ __ _ _ _ _ 6-14 

Airways, area changes. See Area changes. character of surface, effect on resistance and 

power requirements_____________ 81 cost of operating, table_____________________ 85 design, discussion __________________________ 84-89 

effect on area and costs__________________ 86 table______ ____________________________ 86 

factors governing _________________________ 84,89 table_ _ ______ ____ ______________________ 86 

igneous-rock, friction factors, table_________ 43 intake, fireproof constrnction _____ ~________ 139 

lay-out, diagrams _______________________ 94,95,96 

main, diagram_____________________________ 74 

phys!cal conditions, effect on power re- qUirements____________________ __________ 80 

qnantity of flow, effect on ecomonic area___ 88 charL ____ •••••••••• _________________ 88 

Page 

Airways, relative sizes, for equal pressure and 

power requirements, diagram____ 82 resistance, reduction, costs_________________ 84 

resistance factors, calculation______________ 71 charts ___ . ____________________________ 76,77 

sedimentary-rock, friction factors, table____ 43 

shape, effect on resistnace and power re- qulrements______________________ 83 

size, effect on power requirements and 

power costs______________________ 81 table_ _______________________________ 81 

smooth-lined, friction factors, table________ 43 

smooth-lining, effect on resistance and 

power requirements_____________ 81 

timbered, friction factors, table____________ 43 types ___________________________________ 94,95,97 

relative operating costs, table____________ 86 

American Blower Co., work cited___________ 27 

American Society of Reating and Ventilating , Engineers, work ________________ 33,107 

Anaconda COPPBr Mining Co., cooperation with.______ ____ ____ ______________ 42 

Anemometer. See Vane anemometer. Aneroid barometer, description______________ 19 view _____ _ _ __ _ __ _ __ __ _ _____ ______________ __ 17 

Area changes, in airways, air-flow conditions_ 55 diagrams ______________________________ 55,59 

characteristics_ __________ __ ___ ______ _____ 55 diagram_ _ ___ ____________ __ ______ __ _ ___ 55 

types________ ___ ____ ___ __ ___ ___ ___ __ _____ 55 

symmetrical, in airways, shock factors, charL__ ______ __ ___ ______________ 65 

discussion ___________________________ 56-64 

charL _ _ _ ________ ___ _____ ______ _____ _ 65 

formulas______ __ ____ _ __ ___ ___________ 56 

shock pressure losses, charts ____________ 57,58 

unsymmetrical, in airways, shock factors, discussion _______________________________ 64-67 

Auxiliary booster installations, for high- temperature conditiom, cosL______________ 168 diagram _ _ _ __ ___ _ _ ___ _ _ _ _______________ __ 168 

Auxiliary fans, position, calculation__________ 171 selection____ _ _________ _____________________ 170 types__ _ _ _ _________ ___ _____________________ 170 

use_ _ __ __ _ ______________ ___________________ 170 

Auxiliary installations, leakage______________ 170 life_ _ _ _ _ _ _ _____ __ ___ __ _ __ __ ________________ 170 

performance data__________________________ 26 use of flexible tubing_______________________ 171 use of iron pipe____________________________ 171 

Auxiliary ventilation, definition_____________ 161 discussion _______________________________ 161-166 

installation specifications and costs____ _ _ _ _ _ 173 , methods, description _______ . ____ , ________ 1620-165 use ______________________________________ 1620-165 

Bends in airways, air-flow conditions________ 48 characteristics, diagram ___________________ ,_ 49 

shock factors, chart________________________ 53 discussion _______________________________ 51-54 formulas _________________________________ 51,52 

shock pressure losses, computation _ _ _ _ _ _ _ _ 51 

ty~gram::::::::::::::::::::::::::::::::: 49, g6 Biram-type vane anemometer, view_________ 17 Bloomfield, J. J., work cited _________ -_______ 4 

Booster fans, duty, determination___________ 161 installation_ ___ _ __ __ _ __ __ __ __ __ _________ _ __ 160 diagram_ _ __ _ __ __ __ _ __ __ _ _ _ ____ ___ ___ __ __ 160 

performance data__________________________ 1~~ 

~el!~:fo~_~~~~~~~~~~~r~l~~~~:::::::::::::::: 161 

Brackett, F, Ernest, acknowledgment_______ 2,7, 

193 
194 INDEX 

Page Briggs, R., work cited_______________________ 71 

Buffalo Forge Co., work cited_______________ 19 

Bulkheads, permanent, costs________________ 141 

types ____ -----------------_______________ 140 use in metal mines_______________________ 140 

temporary, construction___________________ 140 _ use in metal mines_______________________ 139 

Bureau of Mines, ventilation survey _________ 1 work_ _ _ __ _ _ _ _ _ __ __ _ ____ _ _ _ __ _ __ _____ ___ ___ 33 

Bureau of Mines type sling psychrometer, description_ _____________________ 16 

view _______ _ ___ ___ _ _ _______ _ __ _ __ __ _ __ _ __ _ _ 16 

Bureau of Mines type smoke-cloud tube, diagram ________________________ _ 

Caldwell, F. W., work cited_________________ 131 

Callen, A. C., work cited____________________ 43 

Centrifugal fans, acting against natural draft, duty, diagrams__________________ 112 

asonlysourceofpressure, duty _______ ______ 111 diagrams___ __________________________ 112 

blade sbape, effect on performance char- acteristics _____________________ 123-126 description_ ____ _______________________ 101 

direction of rotation and discharge__ _ _ _ _ _ _ _ _ 128 efficiency ________________________ 111,119,123,125 

in series wi t h natural-draft pressure, duty _ _ 112 

diagrfLills ____ _______ ________ ________ _ ~_ 112 

multiblade rotors, views_ _ _ _______________ 101 

position, effect ou pressure changes, graphs__ 109 

position and service designations_ __________ 105 

types_____ ___ __ _ __ _______ ______________ ____ 104 

vdth evase discharge, effect on ventilating system_ _ _ _________ ___ _ _ _ _ ____ _ __ 110 

Characteristic-ratio charts, for determining fan performancB ___________ 122,124,125 

Colliery Engineering (London), work cited__ 134 Compressed air, for auxiliary ventilation, advantages_ _____________________ 164 cosL _ _ _ ____ _____ __ ______________________ lIl5 

waste________ __ ___ _________________ ___ ___ 164 

Cooke, W. E., work cited__ _ _ _______________ 20 

Coppus fan, use in metal mines______________ 130 

Cribbed raise, with timber slide in mammy, diagram_________________ ________ 15S 

Cut-and-fill stopes, ventilating methods, diugrams_ _______________ ________ 157 

Day, V. S., workcited_______________________ 23 

Dead-end openings, airconditions____ _______ _ lfi4 devices for improving_ _______ ____________ 165 

\'entilation, costs__________________________ lIl3 

methods___ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ __ __ _ _ _ _ 161-1116 

re1uirements_ _________________________ _ _ 163 

Deflector braWces, use______________________ UO Differential pressure, measnrement __________ 21-24 instru~ents _____________________________ 21-23 

Doors, automatic, useinmetalmines_________ 145 construction__________________________ _ _ ___ 142 cosL _ _ ____________________________________ 148 

disadvantages_ _ ___________________________ 141 

fire, controL_______________________________ 145 

leakage, reduction_________________________ H2 

pressures on, controL______________________ 14i 

remote-control, de'ICription________________ 145 resistance factors_ _ _ _ _________ _____ ________ 150 types___ _ __ _ ____ __ __ ______________ __ ________ 142 

use in meta] mines________________ _ ________ 141 

ventiJation, device for operating, diagram_ _ 146 figures ____________________________ 142,143,141 

Doorframe,s, in airways, shock factors________ 66 

Double-entry methods, of advancing into new territory, description_______ 162 diagram_ __________ _ _ __ _ _ _ _ _ _____ __ ___ ___ 162 

Effective temperature, definition____________ 33 chart- __ _ _ _ ___ _ _ _ _ ___ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ __ _ _ _ 33 

Equivalent resistance, definition_____________ 114 

Evase, attached to fan, effect on ventilating system_ __ _______________________ 110 

Fales, E. N_, work cited_____________________ 131 

Fans, air-screw, description__________________ 131 efficiency __ __ __________________ _ _ _ ___ __ _ _ 131 

selection_________ __ _________ ___ ____ ___ __ _ 131 

use in mille ventilation__________________ 130 

application to ventilating system, discus- sion ___________________________ 111-119 

disk, view_________________________________ 130 

in parallel, diagrams _____________________ 117,118 duty ____________________ ________________ 116 

Page 

Fans, in series, diagram______________________ 116 duty -_ __________________________________ 115 

main, performance data___________________ 25 

position, conditions controlling__________ 137 

surface installations, advantages_________ 138 

performance, charts _____________ 106,122,124,125 datR-- ___________________________ 24-26,10,5-111 

guarantees________________ __ ___ __ __ _ __ _ __ 120 laws__ _ _____ _ __ __ ___________________ _____ 107 

tables, form~_____________________________ 121 

power characteristics_ _____________________ 123 curves___ __ _ _ _ __ __ ______________ _________ 124 

pressure characteristics____________________ 124 curves__ __ ___ __ __ __ ________________ __ ___ _ 126 

pressure ratings __________________________ 108-111 

propeller, figures _________________________ 130,131 

quantity characteristics, curves____________ 124 reversing arrangements, provision _______ 127,137 tliagrams_ _ _ _ _ __ __ _ _ __ _ _ _ ___ _ ___ _____ __ _ _ 128 

selection ___________________ ,_ __ ___ ___ _ _ __ 119-130 

size requirements, deterrnination__________ 120 

types and designations____________________ 122 

variations in design to accommodate con- ditions ofinstallation____________ 127 

See also Auxiliary fans; Booster fans; Centrifugal fans. 

Fan drives, types____________________________ 129 

Fan installations, fireproofconstruction______ 139 

leakage conditions_________________________ 148 

types_ _ _ __ _ __ __ __ __ _ _ _ _ _ _ _ __ _______ _______ _ 24 

Fan-tubing installations, abuse______________ Wi) cost _ _ _ _ ___________ ________________________ 169 

diagrams ________________________________ 166,168 

types____ ______________________ ____________ 167 

Fan-tubing methods, development trends at metal mines_____________________ 169 

for high-temperature conditions, develop- menL___________________________ 167 

for low-temperature conditions, develop- ment- ___________________________ 160 

Filled-stope mining, ventilation require- ments_ ________ __________________ 156 

Fire doors, controL__________________________ 146 

Fitrh, 'rhomas W., Jr., work cited___________ 7.) 

Fluid flow, theory___________________________ 68 

Friction factors, for airways, Bureau of Mines schedule_____________________ ____ 43 

for tnbing_________________________________ 45 

relation to air-flow conditions______________ 69 Friction pressure losses, formulas ____________ 40,41 

IH\vS ofproportion________________ 42 

Furnaces, as aid to natural draft, danger_____ 101 

meeser, O. A., work cited___________________ 143 Goss, work cited_____________________________ 45 

Greenwald, R. J'., work cited_______________ 43 

Haddock, M. R., work cited_ _ ______________ 185 Rig h-pressure machines, efficiency ____ _ _ __ _ _ _ 133 typo,__ _ _ _ _____ _ _______ __ _ _ _ _ _ __ _____ ______ 132 

use in metal mines_________________________ 132 

Hill, Leonard, Dr., work____________________ 32 

Hyue, fl., work cited________________________ 71 

Igneous-rock airways, friction factors, table__ 43 

Insulated tubing, for fan-tubing installations, need________________ _ ____ ___ ___ __ 172 

Intern~tional Railway Congress, work cited_ 135 

Interventilation of mines, disadvantages_____ 184 

Iron pipe, for fan-tubing installations, com- parison with flexible tubing______ 171 use ofbeading___________________________ 172 

Jeppe, C. W. B., work cited_________________ 27 

Jones, J. S., work cited______________________ 27 

Kata-thermometer, for determining cooling 

power of air, description_________ 32 

Katz, S. R., work cited______________________ 4 

Kratz, A. F., work cited_____________________ 23 

Leakage, in distribution system, causes______ 148 controL _ _ _______________________________ 148 

resistance factors _________________ C'______ 149 

tests, resnlts_____________________________ 148 

Lord, Eliot, work cited______________________ 167 

Manometers, accuracy_______________________ 23 description_ _ __ _ _ ____ __ __ ______ ____ _ _ __ __ _ _ 22 

Marvin, C. F., work cited___________________ 27 McElroy, G. E., work cited ________________ 43,45, 

47,69,71,85,93,105,115,174.179,181,191 

Mechanical-ventilating systems, circulation 

through caved ground, methods_ 189 
INDEX 195 

Page 

Mechanicai-ventiJating systems, concentra- tion of working places, as aid to good air conditions______________ 187 desirable features, diagram_ _______________ 186 determining factors__ ___ ___________________ 184 

exhaust, description_______________________ 185 

features to beincorporated_________________ 184 

major outlines, characteristics controlling_ 184-187 

minor outlines, characteristics control- ling ____________________________ 187-190 

objectives_ _ _ __ _ __ __ _ _ _ _ ______________ __ ___ 182 

pressure requirements_ _ ___________________ 183 

quantityrequirements_____________________ 182 

sources otheat, dust, or gas, handling_ _ ____ 188 Miami mine, ventilation map_ _ _____________ 159 

Micromanometers, accuracy_________________ 23 

Mines, interventilation, disadvantages_ _____ 184 

resistance factors, calculation ________ ~_____ 74 

Mine cars, in airways, shock fitctors_________ 66 Mine openings, arrangemenL________________ 186 

Mine Safety Board, work cited______________ 167 

Mining methods, effect on distribution of air currents· _______________________ 152-159 

Murgue, D., workcited______________________ 39 

Natural draft, artificialaids_________________ 100 

N atural-draftmeasurements, application___ _ _ 93 Natural-draftpressure,formula______________ 93 

Natural ventilation, controL_________________ 98 discussion ________________________________ 89-103 

Natural-ventilating systems, controlled, air 

and fire-protection conditions______________ 178 diagrams ______________________________ 179,180 

examples_ ___ __ _ _ _ __ _ _ __ _ __ __ __ _ __ __ ___ __ 179 

uncontrolled, air conditions________________ 178 

Obstructions, intermittent, in airways, shock 

losses, determination ___________ _ 

Open stopes, ventilating method, diagram ___ _ Open-stope mining, ventilation requirements_ Ower, E., work cited _______________________ _ Owings, C. W., work cited _________________ _ 

Paulin aneroid barometer,description _______ _ Penman, J. S., workcited ___________________ _ Pitot tube, diagram ________________________ _ use __________________________ ___ __________ _ 

67 

154 

153 

8 

140 

20 

71 

15 

14 Pitot-tube me~,!ods, for measuring air ve- lOCItIes_ _ _ _______________________ 14 

Powerrequirements,reduction_______________ 83 relation to airway conditions _______________ 80-89 

Power-quantity relations, for airways, for- mula ___________________________ _ 

Pressure, of air ,measurement ________________ _ Pressures, calculation _______________________ _ 

75 

19 

35 generation, by moving objects, effect on air flow _ ____ ______ ________________ 135 

natural-draft, control in mechanically vendiSCUSSi;~a:~~ _llli~~~ ~ ~:::::::::::::::::: effect of gas-air mixtures ________________ _ f ormula _________________________________ _ intensity _______________________________ _ measurement ___________________________ _ 

Pressure changes, discussion ________________ _ graph _____________________________________ _ 

Pressure forms and designations, discussion_ Pressure losses, caloulation _________________ _ types ________ ~. ___________________________ _ 

See also Friction pr8S!3ure losses; Shock pressure losses. 

Pressure-quantity characteristics, for natural100 

89 

98 

93 

91 

92 

36 

38 

35 

38 

39 

draft flow, diagram______________ 96 

Pressure-quantity relations, discussion ______ 71-75 formula_______ ___ __ _ __ ___ __ _________ _____ __ 72 

for fan, determination_ _ ___________________ 114 

Psychrometer. See Sling psychrometer. 

Psychrometric tables and charts, for determining physical properties of 

moist air ________________________ 27-31 

Reciroulation of air currents. See Air cur- rents. Recordingpressure gages, use in metal mines_ 23 Regulators, position_________________________ 150 

pressure losses, calculation_________________ 151 

resistance factors, calculation______________ 151 shock factors, calculation_ _ ________________ 66 use in metal mines ________________________ 66,150 

Resistance, or airways, reduction, costs_______ 84 

Resistance factors, for airways, caloulation____ 71 charts _______________ • _________________ 76,77 

Page 

Resistance factors, for complete ventilating cirCUit, calculation_________________________ 74 purpose_________________________ ___ ______ 74 

for doors_ ____ _____________ _______ __________ 150 

for leakage conditions_ _ ____________________ 150 

for regulators, calculation_ _ ________________ 151 

Reynold's Criteron, defioition_______________ e8 Rice, G. S., work cited______________________ 140 

Richardson, A. S., work cited_______________ 2, 

43,45,93, 105, 115, 121, 130 Ritson, J. A. S., work cited__________________ 20 Rock temperatures, determination___________ 18 Roots blower, diagram______________________ 132 

Roots, P. H. & E. M., Co., acknowledgment. 132 

Saccardo system of injector ventilation, efficiency _______________________ _ nse in metal mines ________________________ _ 

Schmidt fan, propeller, efficiency ___________ _ 

Sedimentary-rock airways, friction factors, 

135 . 

134 

130 

table_ _ _ _ ___ _____________________ 43 

Shape ratio, derivation_ _ ____________________ 78 

Shock factors, data ayailable_________________ 46 

for bends in airways, determination, chart _ 53 formulas _________________________________ 51,52 

for symmetrical area changes in airways, determination, chart_____________ 65 discussion _______________________________ 51Hl4 

formulas___ _ _ _ _ __ _ __ _ __ _ __ __ _ _ _ _ _ _ __ _ _ _ _ _ 56 table_ _ _ _ _____________________ ___________ 60 

for unsymmetrical /U"8a changes in airways, discussion _______________________ 64-67 

in terms of friction factors and equivalent lengths, computation ___________ _ relation to air-fl9w conditions _____________ _ 

Shock pressure losses, at splits and junctions in airways, determination ______ _ for area chang~ll in airways, discussion ____ _ for bends in airways, determination _______ _ 

for symmetrical area changes in airways, determination, charL ___________ _ formulas ________________________________ _ table ___________________________________ _ formula ___________________________________ _ 

Shrinkage stopes, ventilating method, dia48 

70 

68 

55 

54 

65 

56 

60 

46 

gram_ _ __________________________ 155 

Shrinkage-stope mining, ventilation require- ments __________________________ _ 

Sling psychrometer, description _____________ _ use _______________________________________ _ Smith, C. M., work cited ___________________ _ 

SmoklHlloud methods, for measuring air ve- locities, procedure _______________ _ 

Smooth-lined airways, friction factors, table __ Stack, as aid to natural draft, efficacy ________ _ on ]netal~minesbaft, view _________________ _ Stanton, T. E., work cited __________________ _ Steart, F. A., work cited ____________________ _ 

Stove, used with tubing to ventilate dead-end 

153 

16 

16 

43 

5 

43 

100 

100 

70 

131 

adit, view________________________ 109 

Statham, I. C. F., work cited_________________ 20 Stoppings, types, description ______________ 139-147 use in metal mines. ______________________ 139-14" 

Temperature, mine-air, comparison with sur- face-air temperatnre, graph______ 9U determination ___________________________ 1&-1~ 

factors controlling____ _ _ _ _ _ ___ __ __ ________ 90 

Thermometers, for determining mine-air tem- peratures ________________________ 17,18 

Timber obstructions, in airways, shock fac- tors _____________________________ _ 

Timber sets, spacing, effect on friction factors_ graph _________________________________ _ 

Timbered airways, friction factors, table ___ _ 

Top-slice mining, ventilation requirements __ 

Top-slice stopes, ventilating method, dia66 

44 

45 

43 

156 

gram_ ___________________________ 158 

Underout caving, ventilation requirements __ . U-tube water gage, construction ____________ _ description _______________________________ _ 

Vane anemometer, acouracy ________________ _ Biram type, view ________________________ _ calibration ________________________________ _ 

charts __________________________________ _ 

method of traversing, diagram ____________ _ U86 _______________________________________ _ 

Vane-anemometer methods, for measuring air 

159 

22 

21 

7 

17 

7 

9 

11 

6 

velocities, discussion _____________ 1(}-14 procedure _____________ ~ _____ ~ _________ __ &-14 diagram_ ____ _ _ __ __ _ _ _ __ __ __ ___ ________ 11 
196 INDEX 

Page Page 

Velocities, economical, In airways, table_ _ ___ 86 

Velocity pressures, formula__________________ 15 

measurement______________________________ 14 

Ventilation systems, danger to life and propdiscussion ~~~~::::::::::::::::::::::::::: -i 74--t ~~ Ventilation, Bureau of Mines work__________ 1 

Ventilation costs, factors controlling_________ 191 table_ _ _ ___________________ ________________ Ig2 

Ventilation data, methods ofrecording______ 174 

symbols for plotting_______________________ 175 

Ventilation laws, formulas___________________ 42 

Ventilation personnel, duties________________ 190 

Ventilation surveys, instruments required, prices____________________________ 4 

methods_______ ___ _________________________ 3 

records_ _ _ _ _ _ _ _ _ _ _ _ _ _ _____ _ _ _ _ _ __ _ _ _ _ _ _ __ _ _ 34 

requirements_ _ _ _ _ _ _ __ _ __ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ 3 

Ventilation system, of Calumet and Arizona 

mine, diagram___________________ 176 

tire protection_ ____________________________ 176 

See also Mechanical-ventilating systems; Natural-ventilating systems. Ventilators, mechanical, types_______________ 103 

Venturi-type air injector, diagram___________ 134 

Warren, G. R., work cited_ _________________ 45 

Water, as aid to natural draft, value_________ 102 

Water vapor, in air, determination__________ 27 

Weeks, W_S.,workcited ___ 2,23,32,45,75,133,151 Willard, A. C., work cited__________________ 23 

Williams, R. Y., work cited_________________ 141 

Williamson, J. N., work cited _______________ 20,71 

Wind, effect on mine ventilation____________ 102 

Yaglou, C. P., acknowledgment_____________ 33 

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