DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, DIRECTOR

BULLETIN 367

THE

SIGNIFICANCE OF DRAFTS IN 

STEAM-BOILER PRACTICE

BY

WALTER T. RAY

AND

HENRY KREISINGER

WASHINGTON

GOVERNMENT PRINTING- OFFICE 

1909

CONTENTS.

Page.

Introduction............................................................. 5

Drafts and their significance............................. t................. 6

Experiments by the Geological Survey at Norfolk, Va., on the pressure drops 

of gases passing through furnaces ....................................... 11

Laboratory experiments relative to the flow of gases through boilers......... 20

Effect of pressure drop through the fuel bed on rate of combustion and on 

evaporation............................................................ 42

Steaming tests by the Geological Survey at St. Louis, Mo.................... 45

Locomotive tests by the Geological Survey and the Seaboard Air Line .Railway at Portsmouth, Va................................................. 5.1.

Summary................................................................ 57

Glossary................................................................. 60

Survey publications on fuel testing ........................................ 61

3
ILLUSTRATIONS.

Page. . 

FIG. 1. Cause of reduced pressure in the base of a high chimney ........... 8

2. Eelation of pressure drops taken over fuel bed, in combustion chamber and in uptake, boiler equipment No. 6....................... 11

3. Eelation of pressure drops through fuel bed and through boiler,

boiler equipment No. 6......................................... "12

4. Pressure drops through boiler equipment No. 5 .................... 14

5. Pressure drops through boiler equipment No. 7.................... 15

6. Diagram showing pressure drops through a hypothetical boiler...... 18

7. Apparatus for studying flow of air through beds of shot............ 21

8. Kelations between pressure drops through lower beds of No. 8 shot

and flow of air................................................... 27

9. Relations between pressure drops through lower beds of No. 4 shot

and flow of air................................................... 28

10. Relations between square roots of pressure drops through lower beds

of No. 8 shot and rates of flow of air............................ 29

11. Relations between thickness of lower beds of No. 8 shot and flow of

air............................................................ 31

12. Relations between thickness of lower beds of No. 4 shot and flow of

air............................................................ 32

13. Relations between total pressure drops and drops through lower beds

of No. 8 shot.................................................. 35

14. Relations between total pressure drops and drops through lower beds

of No. 4 shot.................................................. 36

15. Relations between thickness of lower bed of No. 8 shot and pressure

drops through it............................................... 37

 16. Relations between thickness of lower bed of No. 4 shot and pressure

drops through it............................................... 38

17. Relations between pressure drops through lower beds of No. 8 shot

and flow of air................................................... 40

18. Relations between pressure drops through lower beds of No. 4 shot

and flow of air................................................. 41~

19. Relations between total pressure drops and flow of air for several constant pressure drops through lower beds of No. 8 shot............ 42

20. Relations between total pressure drops and flow of air for several constant pressure drops through lower beds of No. 4 shot............ 43

21. Relations between pressure drops through two boilers and stated

items ......................................................... 46

22. Relations between pressure drops through two fuel beds and stated

items with varying pressure drops through boilers ............... 47

23. Relations between pressure drops through two fuel beds and stated

items with constant pressure drops through boilers .............. 48

24. Relations between pressure drops through two boilers and stated

items......................................................... 49

25. Relation between total pressure drop and rate of combustion and percentage of C02 in flue gases..................................... 50

26. Relation between total pressure drop and horsepower developed .... 55

4
THE SIGNIFICANCE OF DRAFTS IN STEAM-BOILER

PRACTICE.

By WALTER T. RAY and HENRY KREISINGER.

INTRODUCTION.

This preliminary bulletin is the first of a series of two or three 

on the significance of drafts in steam-boiler practice, the succeeding 

bulletins to be along the same lines but of a more advanced character. The conclusions arrived at are tentative, and are the result 

of a study of one of the many problems growing out of the general 

plan of the United States Geological Survey to increase the efficiency 

with which the coals of the country a^re being used. Greater efficiency requires better boiler and furnace design, and means a conserving of the fuel resources of the country.

Some of the work on the succeeding bulletins is already done, and 

it is hoped that opportunity will soon be afforded for making such 

laboratory and large-scale investigations as are clearly desirable as a 

guide for engineers who may attempt to design grates, combustion 

chambers, and boilers adapted to do more work.

The experiments so far made seem to indicate that it is possible to 

double or treble the capacity of a plant without making any radical 

changes in the furnaces and boilers. These increases require about 

double and treble the quantities of air to be put through the fuel 

beds and boilers. It also seems probable that rebaffling the boilers 

will often permit the capacity to be doubled or trebled, while still 

getting more steam than formerly per pound of coal for uses outside 

the boiler room.

These experiments were undertaken with the object of clarifying 

ideas concerning the passage of air through fuel beds and boilers. 

Measured weights of air were passed through two beds of lead shot, 

in series, one of which remained always the same and represented a 

boiler; the other being varied as to size of shot and depth of bed, and 

representing a fuel bed.- Careful observations were made of the 

weight of air passing through the beds per minute. All data were 

plotted in many charts, so as to permit the study of them from several

5
6 DEAFTS IN STEAM-BOILSS PRACTICE.

points of view. A number of laws were deduced bearing on the. relative amounts of power required to force air through fuel beds of 

various thicknesses, composed of various sizes of coal, and through 

boilers of various lengths and areas of gas passages.

An important part of the discussion relates to an increase in the 

capacity of boilers by increasing the amount of power applied to 

pressure and exhausting fans and thus forcing several times as much 

air through the fuel beds and boilers.

It may be possible, as a result of these investigations, to raise the 

rate of working the boiler heating surface to three or even four times 

its present value. Such an. increase would undoubtedly mean new 

designs of grates, stokers, furnaces, and boilers, especially fitted for 

high rates of working. Fan equipments designed to supply three or 

four times as much air under several times the pressure would be 

provided with more efficient engines, thus giving an additional factor 

favoring high-capacity working.

It must be borne in mind, as stated above, that the results are 

tentative. It will cost money to force gases at high speeds through 

fuel beds and boilers, and there will soon be pressing need of such 

quantitative data as will enable the largest possible part of the 

energy imparted by the fans to be advantageously utilized.

We desire to thank our superiors, especially Messrs. L. P. Breckenridge and D. T. Randall, for their helpful criticism and suggestions.

DRAFTS AND THEIR SIGNIFICANCE.

.In the steam-boiler furnace the hot fuel is made to burn by 

passing a current of air through it. This current of air supplies the 

necessary oxygen and carries away the gaseous products of combustion. In order that the boiler may absorb the heat generated by 

combustion and contained in the gases leaving the fuel bed, the 

gases are caused to flow along the heating plates of the boiler and to 

impart heat to them by coming into contact with their surfaces. 

The stronger the current of air through the fuel bed the° higher the 

rate of combustion; and, likewise, the stronger the current of gases 

over the heating surfaces the faster the boiler seems to absorb heat. 

It is thus evident that the motion of the air and of the gases of 

combustion is very essential to the operation of a furnace and boiler. 

The motion of gases is produced by an excess of pressure at any point 

over that at any other point toward which the gases are flowing; the 

greater the pressure difference the higher the velocity of flow. The 

word "draft" is loosely applied sometimes to the motion of gases 

and sometimes to the difference of pressure producing the motion. 

In order to avoid ambiguity and the giving of falge impressions as 

to the cause of motion of gases, the word will ordinarily not be used
DBAFTS AND THEIE SIGNIFICANCE. 7

in this bulletin. A moment's consideration of the likelihood that 

a gas is a discontinuous body will make it clear that there can really 

be no such motive cause as "draft."

There are three ways of producing such differences of pressure as 

will cause gases to flow through the fuel bed and boiler (1) by chimneys, (2) by exhausting fans between boilers and chimneys, and (3) 

by pressure fans supplying air under the grate.

1. The use of high chimneys is most common. They reduce the 

pressures inside their bases so that the greater pressure of the atmosphere outside pushes air through the fuel bed and gases of combustion 

through the boilers into their bases. The reduction of pressures is 

due to the fact that gases expand when heated, in consequence of 

which the chimneys contain smaller weights 'of gases than they would 

if chimneys and gases were cold. We then have the weight of a 

column of hot gases in the chimney pressing against the gases in the 

uptake, and the weight of a column of air of the same dimensions and 

at atmospheric temperature pressing against the air in the ash pit.

Referring to figure 1, let iy.be the weight of the column of hot gases 

in the chimney, and W the weight of a column of outside air of the 

same dimensions; then the difference (d) between the pressures in 

the ash pit and the uptake will be the difference between the weights 

of these two columns when both have unit area of cross section, 

ovd = W w.

The same equation will be obtained if we include in the calculations the weights of the columns of air above the chimney and above 

the aforementioned imaginary column of cold air outside the chimney. Let the weights of these columns be A (see fig. 1); then the total 

weight pressing against the ash-pit door is A +W and the total weight 

pressing against the gases in the uptake is A + w. Therefore the 

difference, d = A + W (A + w) = W - w, as before.

As a result of this difference of pressure the outside air pushes 

the furnace gases into the chimney and through it into the high open 

atmosphere. The air which has been pushed through the fuel bed 

into the furnace is partly modified by the combustion, which produces 

more furnace gases, which are in turn pushed out through the chimney 

into the open air.

It is evident that the hotter the gases in the chimney the more 

they are expanded and the lighter they are, and the greater is the 

difference of gas pressure between the front of the furnace and the 

base of the chimney; also that the higher the chimney the greater 

is the difference of pressure.

The heat energy left in the column of gases escaping from a boiler 

is not entirely wasted, as part of it is used in keeping up the continuous current of gases. By raising the temperature of the gases 

the same difference of pressures between the uptake and the front.
8 DRAFTS IN STEAM-BOILER PRACTICE.

of the furnace can be had by a low chimney as by a high one, although 

such a method would not be economical. Anything which reduces 

the temperature of the gases while in the stack reduces the magnitude of the pressure difference between the chimney base and the 

furnace doors; on this account less coal can be burned and less steam 

made. It follows that a' steel stack without any brick lining on the 

inside is, on account of the high thermal conductivity of the steel,

FIG. 1. Cause ol reduced pressure in the base ot a high chimney.

not as efficient as a brick one/provided the latter is absolutely tight, 

as is seldom or never the case.

2. Another common method of reducing the gas pressure in the uptake of a boiler setting is by an exhaust fan. Usually it is said that 

the exhaust fan sucks or pulls the gases through the boiler; in reality 

no such action can occur. The blades of the fan push the gases out 

of the casing, thereby reducing the pressure in it below that in the
DRAFTS AND THEIR SIGNIFICANCE. 9

furnace; the excess of pressure in the furnace then pushes other gas 

through the boiler into the space of reduced pressure inside the fan 

casing, and so the current continues.

3. Another way of producing an excess of gas pressure under the 

grate over that in the uptake is by connecting a pressure blower or 

fan to the space under the grate, so as to increase the pressure therein 

above that of the atmosphere. In some cases the whole fireroom 

is put under pressure. This method is ordinarily termed the "forceddraft" method, in contradistinction to the "induced-draft" method. 

There is, however, no essential difference between the two.- Occasionally they are used together.

In practice the pressures of the gases in the various parts of the boiler 

setting are measured by instruments called draft gages, the pressures 

being usually expressed in inches of water. The zero points are or 

should be atmospheric pressure. With an exhaust fan between the 

uptake and the stack the pressure anywhere in the boiler setting is 

below that of the atmosphere and is lowest at the edges of the blades 

next the shaft. With a pressure fan applied to the ash pit or fireroom the pressure under the grate is above that of the atmosphere. 

At the top of the stack the pressure is always just atmospheric, and 

at the base of the stack it is sometimes above and sometimes below 

atmospheric.

The drop of gas pressure from one-part of the boiler setting to another varies as some power of the resistance to the flow of gases. 

Thus a great drop from ash pit to furnace indicates a high resistance in 

the fuel bed, and a great drop from the furnace to the uptake indicates 

a high resistance to the flow of gases through the boiler proper. This 

law, which in some respects is similar to Ohm's law, as applied to 

problems involving the electrical resistance of conductors, may be 

stated as follows:

(a) If the resistance to the flow of gases remains constant the pressure drop through any portion of the path of the gases will have a 

constant ratio to the total drop from the ash pit to the uptake. Thus, 

for example, if the pressure drop through the fuel bed is 0.50 inch of 

water when the total drop is 1.0 inch, it will be 1.0 inch if the total drop 

be increased to 2.0 inches, and 2.0 inches if the total drop be increased 

to 4.0 inches of water.

(6) If the total pressure drop from the ash pit to uptake remains 

constant, the pressure drop through any portion of the gas path will 

vary in the same direction as does the resistance to the flow of gases, 

although the magnitudes of the variations may not be in simple proportion. Suppose the total pressure drop is 1.0 inch of water, and the 

drop through the fuel bed is 0.5 inch; now, if the total drop be kept the 

same but the resistance through the fuel bed be increased by doubling
10 DRAFTS IN STEAM-BOILER PRACTICE.

its thickness, the pressure drop through the fuel bed will increase to 

about 0.65 inch of water; or if, for the same total drop, the resistance 

of the fuel bed be increased by quadrupling the thickness, the drop 

through the bed will increase to about 0.80 inch of water.

(c) When the resistance through any portion of the gas path remains constant the weight of gas passing through this portion varies 

with some power of the pressure drop through this portion. Thus, 

since the resistance of that portion of the gas path from over the 

fuel bed to the uptake is ordinarily very nearly constant, the weight 

of gases passing through the furnace and boiler varies as some power 

of the pressure drop between these points. Approximately only, the 

weight-of gases.passing through the boiler is directly proportional to 

the square root of the pressure drop through the boiler or through 

any other portion of the gas path having constant resistance.

Of the above three statements, only the one under a is exactly 

parallel to Ohm's law of the drop of electrical potential through 

conductors of constant resistances.

The relation between drops of pressure and the resistance to the 

flow of gases expressed by the statement under J is similar to Ohm's 

law only in the direction of the variation, not in its magnitude. In 

Ohm's law the relation between the drop of potential through any 

portion of the conductor and the resistance is very simple; that is, 

the drop of potential is directly proportional to the resistance. In 

the case of the flow of gases, however, the analogous relation between 

pressure drop and resistance seems to be either an exponential or a 

logarithmic function. In fact, no definite relation can be stated at 

present because the unit of resistance is not clearly defined.

The statement under c, expressing the relation between the pressure drop and the weight of gas passing through a furnace and boiler, 

comes closer to Ohm's law than that made under I. If the exponent 

of the power were 1.0 the relation would reduce to Ohm's law; that 

is, when drop of pressure is considered as drop of potential and 

the weight of gases flowing per second as the current. .

The preceding statements are deductions from the study of experimental results obtained on large boilers and on small laboratory 

apparatus; the details of the results are given in succeeding portions 

of this bulletin.
EXPERIMENTS AT NORFOLK, VA. 11

EXPERIMENTS BY THE GEOLOGICAL SURVEY AT NORFOLK, VA., 

ON THE PRESSURE DROPS OF GASES PASSING THROUGH 

FURNACES.

Figures 2 to 5 are submitted as practical proof of the statement 

under a, which, asserts that if the resistance to the flow of gases remains constant the pressure drop through any portion of the path of 

the gases will always bear a constant ratio to the total drop.

Pressures were taken in each case almost simultaneously at two 

or more points in the boiler setting. All the observations of any one

B .z

cwea

FUei_eeo

.5 1.0

PRESSURE IN UPTAKE INCHES OF WATER BELOW ATMOSPHERE.

FIG. 2. Pressure drops through two parts of boiler equipment No. 6 when total drop from ash pit to uptake 

is varied and the resistances to theflow of gas are kept constant. The upper curve gives the relation 

between the pressure drop from ash pit to combustion chamber and the total pressure drop from ash pit 

to uptake. The lower curve gives the relation between the pressure drop through grate and fuel bed and 

the total drop from ash pit to uptake.

figure were taken in a total time of only a few minutes, so that the 

resistance of the fuel bed remained substantially constant. The 

resistance to the flow of gases through any boiler is probably very 

closely constant at all times; also, owing to the shortness of the time 

occupied in taking any one set of readings, the fuel bed remained 

very closely of the same temperature and mechanical porosity. It 

is especially worthy of note that the circumstances of the fuel bed
DRAFTS STEAM-BOILER PRACTICE.

being hot and the gases of varying temperature at different times and 

at different points did not curve any of the lines; the fact that they 

were not curved is explicable if we consider that the intrinsic resistance of the fuel bed is probably a function only of the size and shape 

of the air passages through it. It must be constantly borne in mind, 

however, that the amount of air passing through an opening under the 

same pressure difference is greatly affected by the temperature of 

the air.

Tables 1, 2, 3; and 4 contain the original data and computations 

thereon from which were platted figures 2, 3, 4, and 5, respectively.

UJ LO

a:

.NO.I.

..4

NO.3

0 .1 .1 .3 A .5 .6 .7 & .9 1.0 

PRESSURE IN UPTAKE INCHES OF WATER BELOW ATMOSPHERE.

FIG. 3. Pressure drops through two parts of boiler equipment No. 6 when the total drop is varied and 

the resistances to the flow of gas are kept constant. Curve No. 1 gives the ratio of the pressure drop 

through the boiler (from over fuel bed to uptake) to the total pressure drop from ash pit to uptake. 

Curve No. 2 gives the ratio of the pressure drop through the grate and fuel bed to the total drop from ash 

pit to uptake. Curve No. 3 shows the relation between the pressure drop through grate and fuel bed and 

the total pressure drop from ash pit to uptake.

It will be noticed that the lines usually do not go through zero, which 

is because the "draft'' gages were not absolutely correct; but the 

errors are very small.

The data given in Table 1 and plotted in figure 2 were obtained on 

boiler outfit No. 6, which consisted of a hand-fired Heine boiler operated at the time with an exhaust fan. Among the tubes of this
EXPERIMENTS AT NORFOLK, VA. 13

boiler had been inserted one horizontal baffle in such a position as to 

cause the gases to travel forward among the lower two-thirds of the 

tubes and backward among the upper one-third to the point of exit. 

The heating surface of the drum was omitted.

TABLE 1. Data obtained September 30, 1907, on boiler No. 6 by opening and closing 

more or less the damper just beyond the uptake in front of the exhaust fan.

[Pressure in ash pit atmospheric. All pressures arc expressed in equivalents of inches of water.]

1

Pressure 

below atmosphere 

over fuel 

bed.

0.165

.17

.16

.142

.154

.094

. 055

.042

.18

.18

.16

.154

.1.55

.165

.094

2

Pressure

below atmosphere 

in rear of combustion

chamber.

0.8

.8

.76

.68

.7

.43

.26

.21

.83

.82

.73

.72

.71

.76

.43

3

Pressure 

below atmosphere 

in uptake.

1.07

1.07

1.02

.90

.92

.48

.22

.14

1.11

1.10

.95

.94

.92

.99

.49

1

Pressure 

below atmosphere 

over fuel 

bed.

0. 0.54

.048

.037

.178

.178

.17

.172

.168

.17

.029

.067

.102

.142

.159

.182

2

Pressure

below atmosphere 

in rear' of combustion

chamber.

0.25

.24

.17

.79

.79

.78

.79

.78

.72

,13

.29

.45

.61

. .68

.79

8

Pressure 

below atmosphere 

in uptake.

0.21

.2

.11

1.06

1.05

1.04

1.05

1.04

.92

.04

.27

.51

.77

.88

1.06

NOTE. The pressures (drafts) given in this table are not quite correct, because the gages were not accurate. Therefore the ratio of column 2 to column 3, for instance, is not constant, as it should be, especially 

where the pressures are low. For a corrected set of readings on the same boiler outfit see Table-2.

The pressures platted in figure 2 as abscissae were taken about 

2 feet in front of the1 damper located near the entrance of the gases 

into the fan. The ash-pit doors were open to the atmosphere. The 

scales at the left-hand side of the figure and along the bottom are 

correctly named as pressures and not as ''.drafts;" the coordinates 

are expressed in inches of water below atmospheric pressure.

The variations in the gas pressure in the uptake were obtained 

by partially opening and closing the damper located between the 

uptake and the exhaust fan. Headings of pressures at different 

points were taken in rapid succession immediately after each change 

of the damper position. The figure shows that the points fall along 

straight lines as closely as the instruments could be read. The two 

curves indicate that the pressure drops at the three places bear a 

constant relation to one another. This constant relation follows 

because the ratios of the resistances are constant.

The data given in Table 2 (p. 16) and platted in figure 3 were taken 

on the same outfit as the data for figure 2; the gas pressures were varied 

in the same manner. It will be noticed that the slopes of the lower
14 DRAFTS IN STEAM-BOILER PRACTICE.

line of figure 2 and of curve 3 of figure 3 are very nearly the same, 

whence the deduction may be made that the fuel beds in the two 

cases were of very nearly the same resistance to the passage of air. 

Curve 1 of figure 3 gives the ratio'of the pressure drop through the 

boiler (that is, from "over fuel bed" to "uptake") to the total 

pressure drop from the ash pit to the uptake; curve 2 gives the ratio 

of the drop of pressure through the grate and fuel bed to the total 

pressure drop from the ash pit to the uptake. These curves were 

obtained by platting from Table 2 the values of columns 7 and 8, 

respectively, as ordinates and of column 1 as abscissae. Columns

H

Q 3 U)

PRESSURE IN ASH PIT MINUS PRESSURE IN UPTAKE.

FIG. 4. Pressure drops through boiler equipment No. 5. Drop from ash pit to uptake varied. Heine 

boiler with standard baffling and underfeed stoker.

7 and 8 were calculated from the corrected pressures over fuel bed 

given in column 5, which latter are 0.016 lower than the observed 

values given in column 2. The amount of the correction was determined by prolonging curve 3 of figure 3 to the line of zero pressure 

in the uptake; at this line the corresponding pressure drop from ash 

pit to over fuel bed should be also zero; therefore the indicated 

(incorrect) pressure above the fuel bed at the line of zero pressure 

in the uptake was applied as a correction to all the observed readings 

of pressure over the fuel bed. It is noticeable that the ratios given
EXPERIMENTS AT NORFOLK, VA. 15

in columns 7 and 8 are much more uniform than those given in 

column 4; the latter being figured from the uncorrected pressures 

of column 2. .However, both the corrected and the uncorrected 

figures show that the ratio of the drop through the boiler to the total

5 

<

I

O

z 14 

g

fe 

CD \f

O 

O

**

.6 .8 I.O 1.2 1.4 Ifi L8 2.0

PRESSURE IN ASH PIT MINUS PRESSURE IN UPTAKE.

FIG. 5. Pressure drops through boiler equipment No. 7. Drop from ash pit to uptake varied. Babcock 

& Wilcox boiler, semimarine type, equipped with front overfeed stoker.

drop through fuel bed, combustion chamber, and boiler is constant. 

The same constancy pertains to the ratio of the drop of pressure 

through the fuel bed to the total drop. Both curve 1 and curve 2 

signify that the ratio of pressure drops is constant when the respective resistances remain constant.
16 DRAFTS STEAM-BOILER PRACTICE.

TABLE 2. Experiment made September 28, 1907, on boiler No. 6 to determine the ratio 

of the pressure drop through the boiler (that is, from top of fuel bed to uptake) to the 

total pressure drop from the ash pit to the uptake.

[All pressures are stated in equivalents of inches of water.]

Pressure 

in uptake 

below atmosphere.

1

0.70

.56

.36

.15

.14

.71

.77

.79

.85

.94

.21

.14

.68

.79

.88

.90

.92

.82

.62

.27

.17

.88

.29

-.30

.12

.74

.94

.42

.42

.11

.64

.90

.83

.65

.14

.090

.63

.86

Pressure 

over fuel 

bed below 

atmosphere.

2

O.J2

.10

.07

.04

.03

.11

.13

.13

.135

.145

.05

.035

.115

-.13

.14

.15

.15

.135

.11

. 055

.040

.14

.05

.055

.034

, 115

.114

.07

.08

.034

.105

.140

.133

 .110

.030

.030

.110

.150

Drop of 

pressure 

from over 

fuel bed 

to uptake.

3

0.58

.46

.29

.11

.11

.60

.64

.66

.715

.795

.16

.105

.565

.66

.74

.75

..77

, 685

.51

.215

.13

.74

.24

.245

.086

.625

,796

..35

.34

.076

.535

.76

.697

.540

.11

.06

.42

.71

Ratio of 

columns to 

column 1.

4

0. 828

.822

.806

.734

.786

.845

.832

.836

.842

.846

.762

.750

.832

.836

.841

.834

.837

.836

.823

.796

.765

.841

.828

.817

.716

.845

.847

.834

,810

.691

.836

.844

.840

.831

.786

.667

.792

.825

Corrected 

pressure 

over fuel 

bed below 

atmosphere.

5

0.104

.084

.054

.024

.014

.094

.114

.114

.119

.129

.034

.019

.099

.114

.124

.134

.134

.119

.094

.039

.024

.124

.034

.039

.018

.099

.128

.054

.064

.018

.089

.124

.117

.094

.014

.014

.094

.134

Column 1 

minus 

column 5.

6

0.596

.476

.306

.126

.126

.616

.656

.676

.731

.811

.176

.121

.581

.676

.756

-.766

.786

:701

.526

.231

.146

,756

.256

.261

.102

.641

.812

.366

.356

.092

.551

,776

.713

.556

.126

.076

.436

.726

Ratio of 

column 6 to 

column 1.

7

0.852

.851

.850

.840

.900

.868

.852

.856

.860

.862

.839

.865

.855

.856

.859

.851

.854

.855

.849

.855

.859

.859

.883

.870

.850

.866

.864

.872

.848

.836

.861

.862

.859

.856

.900

.845

.823

.844

Ratio of 

column 5 to 

column 1.

8

0.148

.149

.150

.160

.100

.132

.148

.144

.140

.138

.161

.135

.145

.144

.141

.149

.146

. 145

.151

.145

.141

.141

.117

.130

.150

 .134

.136

.128

.152

.164

.139

.138

.141

,144

.100

.155

.177

.156

The data given in Table 3 and platted in figure 4 were obtained on a 

Heine boiler equipped with a Jones underfeed stoker; the boiler was 

baffled in the standard Heine fashion (unlike the Heine boiler from 

which were obtained the data for figs. 2 and 3). The table is divided 

into two parts. Part 1 was obtained by keeping the air pressure in 

the ash pit as nearly constant as feasible and varying the pressure 

(below atmospheric) in the uptake; part 2 was obtained by keeping 

the pressure in the uptake as nearly constant as feasible and varying 

the pressure in the ash pit. All observations are arranged in chronological order.

On inspecting figure 4 it will be noticed immediately that the total 

pressure drop from ash pit to uptake was only a little greater than the 

pressure drop from the ash pit to the space just over the fuel bed. 

For instance, when the total pressure drop was 3.60 inches of water
EXPERIMENTS AT NORFOLK, VA. 17

the drop through the tuyeres and fuel bed was nearly 3.50 inches, 

which left only a little more than 0.1 inch drop through the boiler. 

This latter figure was found to be substantially correct. The drop 

of pressure through the tuyeres alone is several times that through 

the boiler, and this pressure (and fan work) are uselessly absorbed 

unless it be the case that with some coals the high velocity of the 

air leaving the tuyeres serves to keep them clear of clinker and ash.

TABLE 3. Experiment made October 16, 1907, on boiler No. 5.

[Part 1 was obtained by varying the pressure in the uptake while that in the ash pit was kept constant; 

part 2, by varying the pressure in the ash pit while that in the uptake was kept constant. All pressures 

stated in equivalents of inches of water.]

Pressure 

above 

atmosphere in 

ash pit 

(blast).

1

1.5

1.7

1.9

2.2

2.2

2.3

2.3

.5

.45

.42

3. 5

3. 5

3.44

3.4

Pressure 

below 

atmosphere 

over 

fuel bed 

(draft).

2

0.45

.29

-.04

.00 .'32

.39

.43

.35

,01

.39

.53

. II

-.04

.32

.30

Part 1.

Pressure 

below 

atmosphere in uptake 

(draft).

3

0.50

.34

.05

-.02

.38

.46

.52

.34

-.05

.30

.54

.18

-.03

.'14

.43

Pressure 

drop 

through 

grate and 

fuel bed 

equals 

column 1 

plus 

column 2.

4

1.95

1.99

1.86

2.20

2.52

2.69

2.73

.85

.51

.84

.95

3. 61

3.40

3.76

3.70

Pressure 

drop 

through 

grate, 

fuel bed, 

and boiler equals 

column 1 

plus 

columns.

5

2.00

2.04

1.95

2.18

2.58

2.76

2.82

.84

.45

.81 r

.96

3.68

3.47

3.88

3.83

Pressure 

above 

atmosphere in 

ash pit 

(blast).

1

0.9

.9

.4

.9

.9

1.35

1.35

1.85

1.85

3.45

1.08

1.60

1.45

1 25

J. 1

2.0

.9

.8

.8

Pressure 

below 

atmosphere 

over 

fuel bed 

(draft).

2

0.49

.51

.52

.53

. . .

.51

.50

.50

.49

.46

.47

.38

39

48

47

51

5L

52

52

52

53

54

Part 2.

Pressure 

below 

atmosphere in uptane 

(draft).

3

0.54

.55

.54

.55

54

53

54

53

53

53

52

. 55

.54

56

.55

.56

.56

Pressure 

drop 

through 

grate and 

fuel bed 

equals 

column 1 

plus 

column 2.

4

1.39

1.41

1.22

.93

1.41

1.40

1.85

1.84

2.31

2.32

3.88

3.84

2.16

2.07

1.76

1.62

2.52

1.42

1.33

Pressure 

drop 

through 

grate, 

fuel bed, 

and boiler equals 

column 1 

plus 

column 3.

5

1.44

1 45

1.24

.95

1.44

1.43

1.89

1.88

2.38

2.38

4.02

3.98

2. 23

2.14

1.80

2.55

1 41

1.36

1.30

The data given in Table 4 and platted in figure 5 were taken on a 

Babcock & Wilcox boiler of the semimarine type, equipped with a 

Boney stoker, the whole apparatus being designated No. 7 in the

60017 Bull. 367 09 2
J8 DRAFTS IF STEAM-BOILER PRACTICE.

Geological Survey plant. At the time of the experiment the boiler 

was being operated with two fans, an exhaust fan located between 

the uptake and the stack and a pressure fan forcing air into the ash 

pit. The changes in uptake pressure were obtained by opening and 

closing more or less the damper between the uptake and the exhaust

LENGTH OF GAS PASSAGE

.FiG. 6. Pressure drops through a hypothetical boiler and equipment. The arrows represent directions of 

flow of gases. Line ABODE represents pressure drops thrpugh the equipment when a fan is used to 

raise the pressure in the ash pit. Line A'B'C'DE represents the pressure drops through the equipment 

with the same fan running so as to give only half the pressure. Line OB"C"D"E represents the pressure 

drop through the equipment when an exhaust fan is used to reduce the pressure in the uptake, by an 

amount equal to AO, the ash pit being open to the atmosphere. Line A'FGHE represents the pressure 

drop through the equipment when both pressure and exhaust fans are used, but run with half the respective pressure effects of the first and third cases above. Line A'"B'"C"DE represents what would be 

the pressure drop through the equipment if the exhaust fan of the third case above were connected to 

the ash pit and the uptake opened to atmospheric pressure.

fan. For certain reasons only a small range could be covered that 

day, but the points lie as closely as could be expected along a straight 

line passing nearly through zero.

After studying the four charts just discussed it must be admitted 

that such a law as stated under a (p. 9) does exist; that is, with 

constant resistance the pressure drops through any two portions of 

the path of gas flow bear a constant ratio to one another.
EXPERIMENTS AT NORFOLK; VA. 19

We may, then, draw the diagram of figure 6. Let the lengths of the 

gas passages be represented along the line OE and the gas pressures 

at various places by perpendicular distances from this line. Pressures above atmospheric are shown above OE and those below 

atmospheric below it. The highest line, ABODE, represents the 

drop in gas pressure through a fuel bed, combustion chamber, and 

boiler, when the pressure in the ash pit is above that of the atmosphere that is, when, as is commonly said, "forced draft" is used. 

The points B, C, and D represent the gas pressures at the various 

parts of the equipment, as indicated in the figure. Now, suppose 

the blower be slowed down so as to give half the pressure in the ash 

pit, the condition of the fuel bed remaining the same. Then, according to figures 2 to 5, the pressures at all other points will be exactly 

halved.. This case is represented by the line A'B'C'DE.

If the boiler and furnace are operated with an exhaust fan which 

lowers the pressure in the uptake by the same amount as the pressure fan in the first case raises the pressure-in the ash pit, then in 

the diagram DD" equals OA, and the pressure drop through the 

boiler setting is represented by the line OB"C"D"E, of which the 

part OB"C"D" is exactly parallel to line ABCD, when postulating 

that the total pressure drop and the respective resistances are the 

same in the third case as in the first.

TABLE 4. Data obtained October £, 1907, on boiler No. 7, by opening and dosing more 

or less the damper between the uptake and the exhaust fan.

[The pressure fan which fed the air to the ash pit was not manipulated. All pressures are given in 

equivalents of inches of water.]

Pressure in

ash pit above 

atmosphere 

(blast).

1

0.23

.24

.268

.255

.24

.278

.242

.21 c

.265

.205

.20

Pressure in

rear of combustion chamber below 

atmosphere

(draft).

2

1.23

1.22

.94

.96

1.20

1.02

1.17

1.20

.87

1.04

J.24

Pressure in

uptake below 

atmosphere 

(draft).

3

1.93

1.95

1.54

1.55

1.92

1.63

1.87 :i,92

1.38

. .1.67 2. 02 

Absolute pressure 

in ash pit minus

absolute pressure in combustion ' chamber

(thatis, pressure 

drop through 

grate, fuel bed,

and combustion

chamber) equals

column 1 plus

column 2.

4

1.-46

1.46

1.208

1.215

1. 44

1.298

1.412

1.41

1.135

1.245

1.44

Absolute pressure 

in ash pit minus

absolute pressure in uptake

(that is, total

pressure drop 

through grate, 

fuel bed, combustion chamber, and boiler)

equals column 1

plus column 3.

5

2.16-

2. 19

1.808

1.805

2.16

1.908

2.112

2.13

1.645

1. 875

2.22
20 DRAFTS IN STEAM-BOILER PRACTICE.

Line A'FGHE represents the gas pressure drop when both the 

exhaust and pressure fans are used, but run at such slower speeds as 

to give about half the pressures of the first and third cases above. 

 The line EDC"B"'A'" is added to show what the gas pressures 

would be if the exhaust fan of the third case were connected to the 

ash pit and the uptake were opened to the atmosphere. Then (with 

the furnace fire doors closed) the three points B"', C", and D would 

lie exactly as far below the atmospheric line as the corresponding 

points B; C, and D lie above it. ;.

Figure 6 emphasizes the fact that there is no real difference between 

the so-called forced and induced "drafts," provided setting walls, 

doors, etc., are perfectly tight. Under such conditions "forced 

draft" does not crowd hot gases into far corners, nor does "induced 

draft" "suck" or "pull" gases out of them. The volumetric percentage of "dead" corners is exactly the same in either case, provided there are no leaks in or out. "With gases there is no pulling, 

only pushing," is a very truthful phrase. Gases always flow from a 

space of higher pressure to one of lower pressure, so that their absolute pressure entering a boiler must be greater than their absolute 

pressure leaving it. It does not matter whether the difference is 

obtained by raising the pressure in the ash pit above that of the 

atmosphere by the use of a pressure fan or by reducing the pressure 

in the uptake below that of the atmosphere by the use of an exhaust 

fan. So long as the differences of pressures at the two places are 

the same the velocities and the paths of the gases will be very nearly 

the same, inasmuch as under the low pressure differences ordinarily 

used the densities of the gases under the two circumstances are very 

nearly the same. To say that a chimney or exhaust fan sucks or 

pulls the gases through a boiler is as reasonable as to say that the 

condenser of a condensing engine sucks the piston during the latter 

part of the stroke.

LABORATORY EXPERIMENTS RELATIVE TO THE FLOW OF GASES

THROUGH BOILERS.

In the pressure-drop experiments made on large boilers, the 

.results of which are platted in figures 2 to 5, the resistances to the 

flow of gases were kept as nearly constant as possible, and only the 

total pressure drops were varied. It was desirable to make experiments in which the resistance could be varied so as to obtain data 

on which to base the study of the influence of the resistance on the 

pressure drop and 011 the weight of gases flowing through a boiler. 

To make such experiments on large boilers would not only be very 

expensive, but also inaccurate, because on such a large apparatus 

it would be difficult to control the conditions so as to have only one 

or two varying quantities while holding all the rest fixed. Those who
LABORATORY EXPERIMENTS.

have ever tried, to get even relative figures on the resistances of hot coal 

beds, or who have attempted to measure the weight of gases flowing 

through a boiler, will realize how difficult, not to say hopeless, a task 

it would be to make such experiments on a large boiler. For these 

reasons it was decided to make these experiments on a simple laboratory apparatus specially arranged for the purpose.

The apparatus is shown diagrammatically in figure 7. It consists 

mainly of a glass tube which represents the boiler and a sheet-iron 

tank used, for measuring the quantity of air. The glass tube is 1

REGULATING PLUG COCK

WATER SUPPLY 

AIR PASSAGE

THERMOMETER'

UPPER BED OF SHOT 

OF CONSTANT THICKNE

UDWER BED OF SHOT . 

OF VARYING'THICKNESSREGULATING VALVE 

FOR EMPTYING TANK

FIG. 7. Apparatus For studying- flow of air through beds of shot.

inches in internal diameter and 26 inches long. It is connected to 

the measuring tank with an iron pipe, and the pipe is hermetically 

sealed into the neck of the tank. The resistances of the fuel bed and 

of the boiler proper are represented by beds of shot placed in the tube, 

on coarse wire screens. Shot was selected for this purpose because 

of its regularity and uniformity of size and the comparative ease 

with which the aggregation of the spheroids with respect to one 

another can be reproduced. This latter is a very important matter, 

as will be shown later.
22 DfiAFTS IN STiUM-BOTLfiR

Air was made to flow through the glass tube containing the two 

shot beds by filling and emptying the measuring tank with water, 

taken from the water-supply main, the latter supplying water under 

a pressure of about 35 pounds to the square inch. When filling the 

tank the flow of water was regulated with a plug cock in the watersupply pipe, and when emptying it with a quick-opening gate valve 

at the bottom of the tank; both of the valves are shown in the figure. 

By regulating these valves the flow of water into the tank and from 

it could be kept uniform. The pressures above each shot bed were 

measured by U-tube manometers which were connected to the 

middle and to the upper end of the glass tube. A thermometer for 

determining the temperature of the air was inserted into the flowing 

stream of air near the junction of the glass tube with the iron pipe; 

the position of the thermometer is shown in the figure. The measuring tank was provided with a gage glass so that the amount of 

water in the tank could be estimated during the experiment. The 

estimation could be made very accurately, especially when the water 

level reached the narrow neck at the top of the tank, in which the 

water displaced was 1 pound per inch of height. The tank held 636 

pounds of water when full; the volume of the tank was 10.20 cubic 

feet.

Experiments were run with air flowing in either direction through 

the tube and beds of shot. The exact time of starting was noted. 

During the experiment one observer operated the proper valve so as 

to have the air pressures, indicated by the two manometers, as nearly 

constant as possible, the pressure seldom varying as much as 10 per 

cent from the average. Another observer read and recorded the air 

pressures and temperature as often as was convenient. At the close 

of the experiment the exact time was again noted. Thus, knowing 

the duration of the test, the volume of air displaced, and its temperature, the rate of flow of air for each difference of pressure and each 

thickness of shot bed could be computed.

The upper bed of shot was kept at the constant thickness of 0.875 

inch in all experiments. The thickness of the lower bed was varied 

from 0.5 to 4.5 inches, -and a set of 6 to 10 experiments was made with 

each thickness for each of the two sizes of shot, the experiments of 

each set being run at different air pressures. Thus the experiments 

in each set give the influence of air pressure on the flow of gas through 

a constant bed of shot, and the experiments in the different sets give 

the influence of the thickness and the relative resistances on the 

rate of flow. Two sizes of shot were tried in this manner, so as to 

get the effect of the diameter. Several sizes should have been used, 

but more time was not available.

In all 125 reliable tests were made. The first 74 were made with 

the lower bed of shot of commercial size No. 8, which were on the
LABOTtATOBY EXPERIMENTS. 23

average about 0.0974 inch in diameter, and the last 51 tests were 

made on larger shot, known as No. 4, having an average diameter of 

0.125 inch. The upper standard resistance bed was of the smaller 

shot and of the same thickness in all experiments.

Before the 125 reliable tests were run the whole series had been 

completed, but on comparing the worked-up data an inconsistencj7 

was discovered, and the whole series was rejected and all the tests 

were run over. While trying to account for the inconsistency it was 

recalled that in attempting to make the lower bed of shot an even 

number of inches thick in some cases part of the shot had been 

poured out of the tube when the layer happened to be a little too 

thick. When it was thought enough shot had been removed the tube 

was brought back into the vertical position and the bed leveled and 

measured. This process was repeated until the shot bed was of the 

desired thickness. It was found, when the shot bed had been thus 

adjusted by pouring shot out of the tube, that the voids among the 

shot were left much larger and that in consequence air passed through 

much more easily than when the shot was simply poured in and not 

deranged by pouring some of it out. Tests 52, 53, and 54 show the 

effect of the above-mentioned derangement; in these experiments 

exactly the same amount of shot was used as in tests 44 to 51, inclusive, but the aggregation was previously loosened by bringing the 

tube into a horizontal position as if to remove some of the shot, 

whereas in the regular experiments the shot was poured into the 

tube while holding the latter in a vertical position, the tube being 

kept vertical without jarring until the series of tests was completed. 

It will be seen on reference to the table that the derangement of the 

shot as described caused an increase of three-sixteenths of an inch in the 

thickness of the bed, making the depth 3.188 inches; it will also be 

seen that for the same pressure drop much more air flowed through 

the thicker (loosened) bed than through the 3-inch bed.

After having discovered the cause and the effect of the derangement all the experiments were run over, great care being taken to 

pour the shot into the tube from the same height and at the same 

rate. In case too much shot was poured in the first time it was all 

poured out, part of it separated, and the remainder poured back in 

the standard way. This process was repeated until the shot bed 

was of the desired thickness. The beds of tests 1 to 125 inclusive, 

except tests 52, 53, and 54, were prepared in this way.

Table 5 gives the data and computed results of all the reliable 

tests made on the passage of air through beds of shot.

Pressure A was taken above the upper (standard) bed, and pressure 

B between the upper and lower beds. Pressures denoted by + signs 

were obtained when air was forced from the measuring tank through 

the apparatus, and pressures denoted by signs when air was flowing through the apparatus into the tank.
24 DRAFTS IF STEAM-BOILER PRACTICE.

TABLE 5. Experiments on the flow of air through t-wo consecutive beds of shot..

BOTH BEDS OF SHOT REMOVED, LEAVING ONLY THE SCREENS AND THEIR

SUPPORTS.

No. 

of 

test.

1

Length 

of test.

Min.

0.70

Total 

air.

Pounds.

0.738

Air per 

minute.

Pounds.

1. 054

Temperature of 

air.

°F.

05

Pressures 

+ or 

+

Pressure A 

(over 

upper 

bed).

Inches.

0. 19

Pressure B 

(between 

beds).

Indies.

, 0.105

Pressure A 

minus 

pressure B.

0. 085

Square 

root of 

pressure A.

0. 430

Square 

root of 

pressure B .

0. 324

Square

root of 

pressure A 

minus 

pressure B.

0. 201

LOWER BED 0.5 INCH THICK, OF SHOT OF MEAN DIAMETER OF 0.0974 INCH.

2

3

4

5

6

7

8

3.533

3.033

1. 967

1.783

1.333

1.333

.907

0.7C5

.752

.741

.765

.735

.705

.735

0.2165

.248

.377

.429

.552

.574

.760

64

64

64

65

65

65

65

+ 

+

+ '

+

1.34

1.60

3.32

4.68

6.27'

7.10

10.97

0.53

.775

1.49

1.90

2.73

3.10

5.00

0.81

.825

1.83

2.78

3.54

4.00

5.97

1. 157

1.26(1

1.82

2.16

2.502

2.763

3.31

0.727

.880

1.22

1.377

1.651

1.758

2.234

0.90

.907

1.352

1. 666

1.88

2.00

2.442

LOWER BED 0.75 INCH THICK, OF SHOT OF MEAN DIAMETER OF 0.0974 INCH.

9

10

11

12

13

14

15

16

17

18

3.867

2.45 '

1.917

1.917

1.45

1.517

1.467

4.617

1.167

1.10

0.746

.7578

.746

.758

.739

.758

.758

,758

.740

.7275

0.193

.309

.389

.385

.510

.500

.517

.164

.634

.661

68

69

69

69

69

73

74

74

74

70

+

+

+

_

+

+

1.41

3.19

4.51

5.05

7.23

8.47

8.56

1.12

10.97

11.65

0.77

1.51

2.30

2.37

3.63

4.03

4.06

.53

5.47

5.95

0.04 '

1.68

2.21

2.68

3.60

4.44

4.50

. .59

5.50

5.70

1.187

1.785

2.13

2.24

2.69

2.91

2.92

1.06

3.32

3.41

0.877

1.227

1.52

1.54

1.90

2.01

2.01

.73

2.34

2.44

0.80

.1.295

1.485

1. 636

1.90

2.11

2.12

. 769 .

2.34

2.38

LOWER BED 1.125 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.0974 INCH.

19

20

21

22

23

24

25

20

4.533

3.133

2.333

2.250

1.833

1.700

1.550

1.250

0.758

.752

.740

.758

.746

.758

.758

.728

0. 167

.240

.3175

.337

.407

.446

.489

.582

74

74

72

73

73

73

73

71

' +

+

_

+

+

1.43

2.52

3.79

4.64

6.50

7.G1

9.17

11.80

0.80

1.53

2.24

2.59

3.81

4.51

5.46

7.05

0.63

.99

1.55

2.05

2.69

3.10

3.71

4.75

1.195

1.587

1.945

2.15

2.546

2.756

2. 680

3.432

0.893

1.237

1.497

1.61

1.950

2.122

2.335

2.653

0.793

.994

1.243

1.43

1.638

1.758

1.924

2.175

LOWER BED 1.50 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.0974 INCH.

27

28

2930'

31

32

33

34

35

5.450

4.400

2.950

2.550

2.233

2. 050

1. 733

1. 767

1.417

0. 7515

.758

.746

. 7575

.758

. 7575

.733

.758

.734

0.138

.172

.253

.297

.339

.369

.422

.429

.517

70

70

69

69

69

69

67

68

68

+

_

+

_

' +

+

1.30

1.86

3.56

4.66

5.93

7.07

8.21

9.75

11.90

0.91

1.18

2.33

2.93

3.72

4.44

5.38

6.16

7.85

0.45

.08

1.23

1.73

2.21

2.64

2.83

3.59

4.05

1. 165

1.363

1.886

2. 157

2.433

2. 660

2. 863

3.120

3.447

0.953

1.080

1. 525

1.710

1. 927

2. 106

2.317

2.480

2.800

0. 670

.825

1.108'

1.314

1. 485

1. 623

1. 681

1.893

2.012

LOWER BED 2.25 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.0974 INCH.

36

37

38

39

40

41

42

43

6.367

4.217

3.500

2.900

2.450

2.050

2.067

1.667

0. 754

.7335

.737

.758

.754

.737

.755

.730

0.1185

.173

.211

.261

.308

.359

.365

.438

70

67

70

67

69

68

08

69

_

+

+

+

+

1.37

2.51

3.49

4.85

6.60

8.20

9.02

11.76

1.00

1.92

2.66

3.57

4.87

6.26

6.67

9.00

0.37

.59

.83

1.28

1.73

1.94

2.35

2.76

1.170

1.583

1.866

2.200

2.567

2.860

3.01

3.426

1.00

1.384

1.630

1.888

2.205

2.501

2.58

3.000

0.608

.768

.910

1.131

1.314

1.392

1.532

1.060
LABORATORY EXPERIMENTS. 25

TAIVIYR 5. .Experiments on thejloin of air through two e,onse.c.utivc. beds of shot Gont'cl. 

LOWER BED 3 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.0974 INCH.

No. 

of 

test.

44

45

40

47

48

49

50

51

Length 

.of test.

Min.

8.133

5.407

4. 217

3. 333

2.417

2.300

2.100

1.833

Total 

air.

Pounds.

0.751

.763

.751

.703

.732

. 703

.703

.733

Air per 

minute.

Pounds.

0.0923

. 1394

.178

.229

.303

.332

.303

.400

Temperature of 

air.

°F.

05

00

05

05

00

00

00

GO

Pressures 

+ or 

+

_

 +

+

+

Pressure A 

(over 

upper 

bed).

.Inches.

1. 19

2.11

3. 22

4.74

7. 54

8.73

10.70

11.90

Pressure B 

(between 

beds).

Inches.

0. 975

].(57

2.58

3.75

0. 135

0.85

8.25

9.00

Pressure A 

minus 

pressure B.

2.15

.44

.04

.99

1.405

1.88

2.51

2.24

Square 

root; of 

pressure A.

1. 091

1. 451

1. 793

2. 170

2.744

2. 952

3.277

3.447

Square 

root of 

pressure B.

0.980

1.292

1. 000

1.935

2.474

2. 010

 2.870

3. 105

Square

root of 

pressure A 

minus 

pressure B.

0. 403

. 003

. 800

. 994

1. 184

1. 370

1.583

1.495

LOWER BED 3.10 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.0974 INCH.

The same identical shot balls were used for tests 44 to 51, inclusive, as for tests 52 to 54, inclusive; but

before making the latter three the lower bed of shot was loosened by turning down and up several times, 

and the loosening was accompanied by an increase in the total thickness from 3 inehes to 3.10 inches.

52

53

54

5.233

2.433

1.86G

0.744

.744

.744

0. 142

.306

.399

07

07

67

+

L

1.827

5.75

9.05

1.327

4.36

7.32

0.500

1.39

2.33

1.351

2. 390

.3.104

1. 151

2.080

2.704

0.707

1.178

1.525

LOWER BED 3.75 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.0974 INCH.

55

56

57

58

59

60

61

62

63

64

9.033

7.333

5. 900

4. 634

3.950

2. 734

2.617

2.383

2.250

2.020

0.765

.705

.765

.759

.765

.759

.765

.747

.705

.741

0. 0794

.104

.128

. 164

.194

.278

.292

. 313

.340

.307

64

64

64

64

64

63

64

64

64

64

+

+

+

+

+

1.13

1. 560

2.150

3.140

4.230

7.320

8. 130

9.080

10.50

11. 70

0.949

1.330

1.820

2. 690

3.480

6.340

6. 660

7.700

8.79

9.84

0. 181

.230

.330

.450

.750

.980

1.470

1.380

1.71

1.86

1.062

1.248

1.465

1.770

2.055

2.704

2.850

3.008

3.24

3.417

0.973

1.151

1.348

1.638

1.863

2.516

2.579

2. 774

2. 963

3.135

0.425

.429

.574'

. 671

. 805

. 989

1.212

1.1.74

1. 307

1.363

LOWER BED 4.50 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.0974 INCH.

05

60

07

68

09

70

71

72

73

74

10. 700

7.333

5.333

4.150

3.350

3.020

2.917

2. 550

2.450

2. 350

0.765

.759

.765

.759

.765

.759

.759

. 765

.765

. 759

0. 0715

.1035

.1435

.183

.228

.2513

.200

.300

.312

.323

64

64

64

64

64

64

64

64

04

63

_

+

+

+

+

+

1.040

1.85

2.87

4.42

0.28

7.20

7.52

10.53

10. 50

11.08

0.890

1.05

2.74

3.83

5.39

0.30

0. 03

8.85

9.00

9.70 '

0.150

.20

.13

.59

.89

.90

.89

1.68

1.50

1.38

1.020

1. 360

1.092

2.101

2.505

2. 685

2. 740 '

3.244

3. 246

3.326

0.942

1.283

1. 654

1.956

2.322

2.508

2. 574

2.974

3. 000

3.113

0.387

.447

.360

.767

.942

.947

.942

1.295

1.248

1.174

LOWER BED 1 INCH THICK, OF SHOT OF MEAN DIAMETER OF 0.125 INCH.

75

76

77

78

79

80

81

82

83

4.500

3.383

2.583

2.133

1.783

1.500

1.483

1.200

1.117

0. 766

.755

. 766

.754

.766

.738

.767

.736

.754

0.170

.223

.297

.3534

.429

.492

.517

.613

.675

63

62

62

62

63

64

63

64

64

+

+

-t-

+

+

1. 055

1.525

2.61

3.49

4.99

6.42

7.68

9.03

11.55

0.422

.710

1. 155

1.58

2.16

2.88

3.38

4.13

5.37

0. 633

. 815

1. 455

1.91

2.83

3.54

4.30

4.90

0.18

1.027

1. 234

1.614

1.867

2.234

2.533

2.770

3.004

3.396

0.649

.842

1.074

1.256

1.468

1.696

1.837

2.032

2.317

0. 745

.902

1.200

1.382

1.081

1.880

2.073

2.214

2.485
2fi DRAFTS IN STEAM-BOILER PRACTICE.

TABLE 5. .Experiments on the flow of air through two consecutive beds of shot Cont'd. 

LOWER BED 1.50 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.125 INCH.

No. 

of 

test.

84

85

86

87

88

89

90

91

Length 

of test.

Min.

4.183

2.567

2.200

1.667

1.683

1 450

1. 350

1.167

Total 

air.

Pounds.

0.757

.760

.762

.757

.762

.762

.732

.744

Air per minute.

Pounds.

0.181

.296

.346

.454

.4525

.525

.542

.637

Temperature of 

air.

°F.

65

66

66

66

66

66

66

66

Pressures 

+ or-

+

+

_

+

+

Pressure A 

(over 

upper 

bed).

Inches.

1.400

3.240

4.180

6.310

7.190

9.180

9.400

11. 750

Pressure B (between 

beds).

Inches.

0.806

1.710

2.190

3.540

3.840

4.910

5.220

6.520

Pressure A 

minus 

pressure B.

0.594

1.530

1.990

2.770

3.350

4.270

4.180

5.230

Square 

root of 

pressure A.

1. 182

1.798

2.044

2.510

2.680

3.028

3.064

5.426

Square 

root of 

p ressure B.

0.897

1.307

1.478

1.880

1.958

2.214

2.284

2.551

Square

root of 

pressure A 

minus 

pressure B.

0.770

1.236

1.409

1.662

l.*28

2.065

2.043

2.285

LOWER BED 2 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.125 INCH.

92

93

94

95

96

97

98

99

5.950

3.233

2.583

2.177

1.867

1.617

1.600

1.216

0. 762

.744

.762

.750

.762

.738

.762

.732

0.128

.230

.295

.354

.408

.456

.476

.602

66

66

66

66

66

66

66

66

+

+

+

+

1.294

2.830

3.900

5.500

6.960

8.300

9. 350

.11.900

0.761

1.810

2.350

3.450

4.290

5.230

5.720

7. §30'

0.533

1.020

1.550

2.050

2.670

3.070

3.630

4.370

1.137

1.681

1.973

2.344

2.636

2. 880

3. 055

3.447

0.872

1.344

1.532

1. 856

2.070

2. 285

2.390

2.743

0.730

1.010

1.244

1. 430

1.632

1.750

1.903

2.088

LOWER BED 3 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.125 INCH.

100

101

102

103

104

105

106

107

6.417

4.750

0.762

.762

3.550 .756

2.917 j .762

2.333

2.133

1.866

1.550

.738

.762

.762

.744

0.119

160

.213

.261

.316

.357

.408

.480

66

66

66

66

66

66

66

66

+

+

_

+

' +

1.055

1.733

2.800

4.050

5.570

6.930

9.100

11. 560

0.811

1.214

2.070

2.810

4.090

4.870

6.450

8.440

0.244

.519

.730

1.240

1.4SO

2.0CO

2.650

3.120

1.027

1.317

1.673

2.012

2.360

2. 631

3.015

3.398

0.900

1.102

1.439

1.676

2.023

2.206

2.538

2.904

0.494

.720

.854

1.113

1. 21.6

1. 438

1.627

1.765

LOWER BED 4 INCHES THICK, OF SHOT OF MEAN DIAMETER OF 0.125 INCH.

108

109

no

111

112

113

114

115

116

117

118

9.000

5.750

4.133

3.300

2. 4G7

2.330

2.217

2.170

2.000

1.833

1.667

0.753

.750

.747

.753

.742

.754

.736

.753

.753

.736

.730

; 0.0837

.1305

.181

.228

.301

.324

72 i -

72

71

71

71

72

. 332 | 72

.347

.377

.402

.438

72

72

72

71

+

+

+

+

+

+

0.797

. 1.532

2.670

4.010

6.13G

6.970

7. 360

7.890

 9. 950

9.920

11. 800

0.597

1.185

2.060

2.980

4.760

5. 210

5.740

5.970

7. 500

7.820

9.100

0.200

.347

.604

1. 030

1.376

1.760

1 620

1.920

2.450

2.100

2.700

0.893

1.237

1.633

2.005

2.477

2.640

2.710

2.808

3. 154

3. ISO

3.434

0.772

1.088

1.437

1.725

2.180

2. 281

2.395

2.443

2.740

2.796

3. 015

0.447

.589

.777

1.015

1.172

1.326

1.272

1.385

1.565

1.448

1.642

NO LOWER BED OF SHOT, ONLY THE STANDARD UPPER BED.

119

120

121

122

123

124

125

2.967

2. 167

1.583

1.367

1. 283

1.050

.800

0.742

. 754

.725

.754

,754

.718

.718

0.250

.348

.458

.551

.587

.683

.898

71

71

71

71

71

71

71

+

+

+

1.021

2.044

3.004

3.840

4.530

. 5. 250

8.940

1.021

2.044

3.004

3.840

4.530

5.250

8.940

1.011

1.429

1.732

1.958

2.128

2.292

2.990

1. Oil

1.429

1.732

1.958

2.128

2.292

2.990
LABORATORY EXPERIMENTS,

Figures 8 to 20, inclusive, show graphically the interrelations between the items computed from the test data.

Figures 8 and 9 show the relations between the pressure drops 

through the lower beds of shot and the weights of air passing through 

the apparatus, for shot sizes No. 8 and No. 4, respectively. Each 

curve gives these relations for a constant thickness of the lower shot 

bed, as indicated on each figure. Points platted as hollow circles 

are for tests with air flowing from the measuring tank, and points 

indicated by solid black circles are for tests with the air flowing in

3 4 56 7 6

PRESSURE DROP THROUGH LOWER BED OF SHOT INCHES OF WATER,

PIG. 8. Relations between the pressure drops through lower beds of No, 8 shot and rate of flow of air through 

apparatus. Each curve gives the relation for one thickness of the lower bed, as indicated.

the opposite direction. In shape these curves look like parabolas 

of the form yz =px, which suggests the approximate law that the 

weight of air varies directly as the square root of the pressure drop. 

To test this law, the weights^-of air were platted with the square roots 

of the pressure drops. Figure 10 gives the curves thus obtained 

from experiments in which the lower bed was of No. 8 shot. It 

appears that for small pressure drops the law holds fairly well, but 

that for large drops the rate of flow of air is somewhat larger than the 

law suggests.

Figures 11 and 12 were platted to show how the thickness of the 

lower shot bed affects the rate of flow of air through the apparatus
28 DRAFTS TN STEAM-BOILER PRACTICE.

*

at constant pressure drops. The curves show that as the thickness 

of the lower beds of shot increases, the weight of air passing through 

the apparatus decreases, Very rapidly at first, but as greater thicknesses are reached the rate of decrease gradually diminishes. All 

curves are asymptotic to the line of zero weight of air when the 

thickness of the bed of shot is infinite.

Figures 11 and 12 seem to bring out one principle rather strongly,

which is that for constant weight of air the ratio of the thickness of

. the lower shot bed to the pressure drop through the latter is constant.

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I 2 3 4 56 78

PRESSURE DROP THROUGH LOWER BED OF SHOT INCHES OF WATER.

FiG. 9. Relations between pressure drops through lower beds of No. 4 shot and rate of flow of air through

apparatus.

Thus, for example, using figure 11 and taking the constant weight of 

air passing through the apparatus as 0.2 pound per minute, the 

pressure drop is about 1.0 inch of water when the lower bed is 1.0 

inch thick,' 2.0 inches when the 'lower bed is 2.0 inches thick, 3.0 

inches when the lower bed is 3.0 inches thick, and so on. Again, 

let us take the weight of air passing through the apparatus as constant at 0.45 pound per minute, then we have through the 0.5-inch 

shot bed a drop of 2 inches of water, through the 1.0-inch bed a drop 

of 4 inches, and through the 1.5-inch bed a drop of 6 inches, giving 

a constant ratio of the thickness of the shot bed to the pressure drop 

'of about 0.25. Table 6 gives these ratios for the No. 8 shot for each
LABOKATOKY EXPERIMENTS. 29

inch of pressure drop and for every 0.5-inch thickness of the shot 

bed. In each horizontal row the line headed by A gives the thickness of the lower bed of shot, and the line headed by B gives the ratio 

of the thickness of the bed to the pressure drop through it. Thus 

in the first horizontal row'the first figure in the line headed by A is 

1.55, and the first figure in the line headed by B and given as 1.55 is 

the ratio of 1.55 to 1.0, the latter value being the pressure drop at

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1.0 1.5 <LQ H.5 3.O 3k5 4.0 4 5

SQUARE ROOTS OF PRESSURE DROPS THROUGH LOWER BED OF SHOT INCHES OF WATER.

FIG. 10. Relations between the square roots of the pressure drops through the lower beds of No. 8 shot 

and the rates of flow of air through the apparatus. Points taken from figure 8.

the head of the vertical column. In the same manner the second 

figure of the line headed by A is 3.15; and 3.15 divided by 2.0, the 

pressure drop given at the head of the second vertical column, is 

1.58,' which is given in the horizontal line headed by B as the ratio 

of 3.15 to 2.0. The constancy of the ratios given in the table in each 

of the horizontal lines headed by B is rather striking, and is apparently more than an accident, because the same constancy of the same 

ratio is found to. exist with the No. 4 shot, as shown in Table 7.
30 DRAFTS IN STEAM-BOILER PRACTICE.

TABLE 6. Computations showing the-constancy of the ratio of the thickness of the lower bed 

of shot to the pressure drop through the lower bed required to deliver a given amount of air 

per minute for tests in which the lower bed was of shot of mean diameter of 0.0974 inch.

[Linesheaded A give thickness of lower bed' in inches; B lines give ratio of thickness to pressure drop.]

Air per 

minute.

Pounds. 

0.15

.20 

.25 

.30 

.35 

.40 

.45 

.50 

.55 

.63 

.65

Line.

f A

t B

1 A

i B 

f A 

t B 

1 A 

\ B

u- u 

{*/ A 

i B U

{B 

U

Pressure drops, in inches of water, through lower bed of shot.

1.

1.55 

1.55 

1.03 

1.03 

.70 

.70 

.49 

.49

2.

3.15 

1.58 

1.99 

.99 

1.39 

.69 

1.04 

.52 

.77 

.38 

.61 

.30 

.49 

.24

3.

. 3.03 

1.01 

2.07 

.69 

1.51 

.50 

1.17 

.39 

.93 

.31 

.75 

.25 

.63 

.21 

.53 

.18

4.

4.17 

1.04 

2.84 

.71 

2.05 

.51 

1.55 

.39 

1.24 

.31 

1.01 

.25 

.84 

.21 

.71 .18' 

.62 

.15 

.52 

.13

5.

3.63

2^55 

.51 

1.95 

.39 

1.53 

.31 

1.25 

.25 

1.04 

.21 

.88 

.18 

.76 

.15 

.65 

.13

6.

4.44 

.74 

3.15 

.53 

2.29 

.28 

1.83 

.30 

1.48 

.25 

1.24 

.21 

1.C6 

.18 

.90 

.15 

.76 

.13

7.

3.73 

.53 

2.77 

.40 

2.13 

.30 

1.71 

.24 

1.41 

.20 

1.20 

.17 

1.03 

.15 

.88 

.13

TABLE 7. Computations showing the constancy of the ratio of the thickness of the lower 

bed of shot to the pressure drop through the lower bed required to deliver a given amount of 

air per minute for tests in which lower bed luas of shot of mean diameter of0.125 inch.

[Linesheaded A give thickness of lower bed in inches; B lines give ratio of thicknessto pressure drop.]

Air per minute.

Pounds. 

0.20

.25 

.30 

.35 

.40 

.45 

.50 

.55 

.60 

.65

Line.

u

f A 

1 B

IB

U 

U 

U

i!

1 B 

f A 

\ B 

f A

VB

Pressure drops, in inches of water, through lower bed of shot.

1.

l.Gf) 

1.05 

1.16 

1. 16

2.

3.25 

1.62 

2.18 

1.09 

1.63 

.81 

1.28 

.64 

1.01 

.50

1

3.

3.37 

1.12 

2.45 

.82 

1.87 

.62 

1.50 

.50 

1.23 

.41 

1.00 

.33

. 4.

3.30 

.82 

2.47 

.62 

1.96 

.49 

1.62 

.40 

1.36 

.34 

1.24 

.31 

.95 

.24

5.

3.10 

.62 

2.43 

.49 

2.01 

.40 

1.69 

.34 

1.43 

.29 

1.22 

24 

1.03 

.21

6.

3.85 

.64 

2.94 

.49 

2.40 

40 

2.01 

.33 

1.72 

.29 

1.48 

.25 

1.28. 

.21

7.

3.44 

.49 

2.77 

.40 

2.32 

.33 

2.00 

.29 

1.74 

.25 

1.51 

.22

8.-

4.09 

.51 

3.14 

.39 

2.65 

.33 

2.28 

.28 

1.97 

.25

9.

 - |

3.55 

.40 

2.93 

.33

]' K A CTIC A r, 1) EI > UCT10 N S.

Practical deductions from the above principle can be made as 

follows:

Other tilings remaining the same, if the thickness of the fuel .bed be 

doubled the work done by the fan must be about doubled in order 

that the same weight of air shall pass through the fuel bed per minute.
LABORATORY EXPERIMENTS. 31

Since the absolute pressure of the air remains the same within 1 or 2 

per cent, the volume of the air remains nearly the same in all cases; 

hence nearly the'same volume of air is displaced against double the 

pressure by the expenditure of double the work. The same reasoning 

applies whether the thickness of the fuel bed is doubled or quadrupled. 

Further, by likening the bed of shot to the tubes in a water-tube 

boiler, if the length of the path of the gases among the tubes be 

doubled while other things remain the same, the fan work required 

to put the same weight of gases through the boiler is doubled, and so

2. 3 

THICKNESS OF LOWER BED OF SHOT INCHES.

FIG. 11. Relations between the thickness of the lower beds of No. 8 shot and the rates of flow of air through 

the apparatus. Each curve shows the relation for one constant pressure drop through the lower bod.

on. This is an important principle, as affecting the arrangement of 

the heating surface of a boiler. The principles of heat transmission 

require, for high efficiency in heat absorption, long and narrow gas 

passages, and the fan work required perhaps increases about directly 

with the length of the gas passage through the boiler. Fortunately 

the steam used by a fan is a small portion of that generated, so that 

there seems to be a chance to improve the economy of steam-boiler 

operation by judicious use in boiler design of the above principle as 

applied to both fuel bed and boiler.

By following any vertical line in figure 11 it is seen that the pressure 

drop increases faster than the weight of air, although it does not
32 DBAFTS IN STEAM-BOILEE PEACTICE.

increase quite as the square of the latter. On the average, for constant thickness of the shot bed, to double the weight of air requires 

that the pressure drop be increased about 3.25 times; to treble it the 

drop must be increased about 6.00 times. If a similar law holds good 

for hot gases and in steam boilers, the following practical deduction 

can be made:

Suppose that it is desired to double or treble the capacity of a plant 

without making any changes in the furnaces and boilers. Such 

increases of capacity would require about double and treble the quan-

'is 2.0 as 3.0

THICKNESS OF LOWER BED OF SHOT INCHES.

4.0

FiG.12. Relations between thethickness of the lower beds of No. -1 shot and the rates of flow of air through 

the apparatus. Each curve shows the relation for one constant pressure drop through the lower bed, 

as indicated on the chart.

tities of air .to be put through the fuel beds and boilers. As already 

stated, in order to force through such greater amounts of air the total 

pressure drops would have to be, respectively, about 3.25 and 6.00 

times as large as in the original case; so that in the case of doubled 

capacity it would be necessary to force twice the weight (volume) of 

gas against 3.25 times more pressure, which would make the total 

work required from the' fan 2 (volumes) X 3.25 (pressure) = 6.50 times 

as large as in the original case; and in the case of trebled capacity 

three times the weight (volume) of air forced against 6.00 times 

more pressure would make the total work required from the fan
LABORATORY EXPERIMENTS. 33

3 (volumes) X 6.00 (pressure) = 18.0 times as great as in the original 

case of ordinary capacity.

If at 100 per cent of rated capacity the plant consumes 2 per cent 

of the total steam generated in running fans, then when doubling the 

capacity the steam consumption by the fans will be

 2 per cent X 6. 5 

and when trebling the capacity the steam consumption by the fans 

will be

2 per cent X 18 _ £ , . N =12 per cent 3 (capacity) L

of the total steam generated.

It is apparently undesirable to attempt more than a doubling or 

perhaps trebling of the rate of working of most boilers already installed, unless more efficient fans are designed and used, as they well 

may be. The mechanical efficiency of most fans now obtainable is 

probably well under 50 per cent in most cases; some English makes 

of mine- ventilating fans are up in the seventies, and it is quite likely 

the reason fans in general are so inefficient is that purchasers demand 

nothing better or are not willing to pay a somewhat higher price. An 

increase in grate area will keep the power consumption of the fans 

from rising as fast as the above calculation indicates. Rebaffling 

the boilers will often permit the capacity to be doubled or even trebled 

while still getting more steam than formerly per pound of coal for 

uses outside the boiler room. It is always to be remembered that the 

work expended on the gases in driving them through the fuel bed and 

boiler, is partly recovered as heat by the boiler. But the most hope 

of all lies in the designing of new types of boilers specially fitted for 

high rates of working.

It is well to bear in mind that fan equipments designed to supply 

three or four times as much air under several times the pressure would 

also be provided with more efficient engines, which is an additional 

factor favoring high-capacity working.

In the cases 'discussed above of doubling or trebling boiler capacity, 

somewhat thicker fuel beds would be required, but some locomotive 

tests given in a succeeding portion of this bulletin show that even then 

the work required to force the air through the fuel and boiler does not 

increase any faster than the above calculation indicates. Provided 

that the boiler proper were such that its efficiency as a heat absorber 

did not drop much on increasing its rate of working, and that the fuel 

were such as not easily to be carried up from the bed, such an increase 

in steam production would be a very sound step commercially. The

. 60017 Bull. 367 09 3
34 DRAFTS IN STEAM-BOILER PRACTICE.

waste heat of the steam from the fans could be largely recovered in 

heating the feed water, inasmuch as the auxiliaries of the modern 

turbine power plant do not ordinarily supply enough steam for this 

purpose. Best of all, the first cost of the plant would be increased 

very little, while it would supply two, three, or four times the customary amount of steam. With new boilers properly designed this great 

increase in capacity need be at no sacrifice in equivalent evaporation.

It may well be asked whether the rate of combustion would keep 

up with the increased supply of air. At present there are no accurate 

experimental data at hand to prove that it would, but everyday 

observations of blast furnaces and blacksmith's forges indicate that 

the faster the air is blown through the fuel bed the higher is the rate 

of combustion, the temperature remaining fully as high or increasing. 

Of course, when increasing the pressure drop through the fuel bed, 

and consequently the supply of air, the supply of fuel and the thickness of its bed must be regulated and carefully watched. At present 

too thick a fuel bed is often carried with a comparatively low draft, 

in the attempt to obtain a high temperature or to reduce the chances 

of the formation of holes in the fire. The first attempt is questionable as to its soundness and the second is made by lazy firemen. Too 

thick a fuel bed places needless resistance in the path of the gases. In 

all attempts to force up the capacity of boilers care must be taken not 

to have the velocity of air through the fuel bed high enough to carry 

up fine coal; resort can usually be had to more grate area.

On following any curve of figures 11 and 12 from left to right, it is 

evident that the weight of air flowing through the apparatus per minute decreases rapidly at first with increasing thickness of the bed of 

shot, and then more and more slowly. If it is justifiable to reason 

from beds of shot to water-tube boilers of the cross-flow type (like the 

Babcock £ Wilcox), and perhaps also to the parallel-flow type (like

the Heine), it is probable that doubling the height (number of rows 

of tubes) of the first type and the length of the tubes of the second 

type will not reduce the weight of gases flowing through the boiler to 

anything like half, if the pressure drop through the boiler proper 

remains unchanged.

According to the curves given for the shot, increasing the bed to 

four times the thickness in certain cases will not quite reduce the 

weight of gases flowing to one-half; increasing the thickness to nine 

times will not quite reduce the weight of gases flowing to one-third. 

This fortunate relation actually seems to hold more or less in the case 

of small laboratory boilers. In a forthcoming bulletin on heat transmission into boilers, now nearly finished by the present authors, are 

given data from about 250 tests on small laboratory multitubular
LABOBATOBY EXPERIMENTS. 35

boilers fed with air heated electrically. Some interesting results from 

two boilers of ten tubes each are tabulated below. <=

Tests from two laboratory multitubular boilers fed 'with air heated electrically.

Internal diameter of tubes...... . .....................................

Length of tubes.... . .................................................

Air passing through boilers per second under a pressure drop of 

Ten inches of water at an initial temperature of 700° .F. .........

........do....

.....pounds..

Boiler 

No. 1.

0. 175

8.28

.0090 

.0073

Boiler 

No. 3.

0. 175

16. 125

.0088 

.0067

. The longer tubes absorbed about 10 per cent more of the heat 

available to them from each pound of air than did the shorter ones, 

and only about 8 per cent less air passed through them, under the same 

pressure drop. Therefore the longer tubes actually absorbed as

mi]

Ul1-

u. 

o

DROP PRESSURE THROUGH LOWER BEDSHOT OFINCHE M 0>A ^UCP 

ich heat total as did the shorter ones, from less air. Taking as 100

s

. <t "

4

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56789 

TOTAL PRESSURE DROP INCHES OF WATER.

FIG. 13. Relations between total pressure drops and drops of pressure through lower beds of No.- 8 shot.

per cent the weight of air passing through the shorter tubes, the 92 

per cent of air passing through the longer ones is higher than one 

might expect on the basis of the experiments with shot. One possible explanation is that a considerable part of the resistance to the 

passage of air through the boiler tubes lies at the entrances to the 

tubes in some vena-contracta effect.
36 DRAFTS IN STEAM-BOILER PRACTICE.

Figures 13 and 14 show the relation between the total drop through 

botn beds of shot and the drop through the lower bed only. The 

curves are as nearly straight lines as can be expected, indicating 

that the ratio of the two pressures is constant and confirming the 

statement under a (p. 9), the same statement having already been 

confirmed by experiments on large boilers.

Figures 15 and 16 show how the pressure drop through the lower bed 

increases with the thickness of the bed when the total drop remains 

constant. Each of the curves gives this relation for a constant total 

drop, as indicated in the figures. Each curve is asymptotic to the

I"

t0. O 

£2

*5 £*' 

34 5 6 7 P 9 10 II 12

TOTAL PRESSURE DROP INCHES OF WATER. 

FIG. 14. Relatious between total pressure drops and pressure drops through lower beds of No. 4 shot.

corresponding pressure drop through the lower bed of shot, showing 

that the latter drop approaches the total drop as the thickness of the 

lower bed increases. The two pressures would be equal if the thickness of the lower bed were infinite. A similar statement is applied 

elsewhere in this bulletin (see p. 41) to a steam boiler and its fuel bed. 

Figures 17 and 18 show the relation between the pressure drop 

through the lower shot bed and the weight of air passing through 

the apparatus. Each curve gives the relation for a constant total 

drop through both beds, as indicated on the figure. The curves 

show that the weight of air passing through the apparatus decreases 

as the pressure drop through the lower bed approaches the total
LABORATORY EXPERIMENTS. 37

drop, and that the weight becomes zero when the two drops are equal. 

The points at the extreme left were determined by leaving the lower 

bed of shot out in one set of experiments.

Figures 19 and 20 give the relation between the total drop through 

the apparatus and the weight of air passing through the latter per 

minute. Each of the curves gives this relation for a constant pressure drop through the lower bed of shot. The curves are drawn 

starting from a total pressure drop equal to that through the lower 

bed. This is done because if there should be no pressure drop 

through the upper bed of shot, no air would pass through it and

 is 2 zs 3 ;» 4

THICKNESS OF LOWER BED OF SHOT INCHES. 

FIG. 15. Relations between the thickness oi the lower bed of No. 8 shot and the pressure drops through it.

therefore none through the apparatus. The curves show that when 

the drop through the lower bed remains constant the rate of flow of 

air through the apparatus increases with the total pressure drop. 

The points for the curve starting from 0 were obtained in a set of 

tests run with no lower bed.

From figure 19 the following practical deductions can be drawn: 

As the lower bed of shot represents the fuel bed and the upper one 

the boiler, by analogy we can reason from the showing of the curves 

that with the same pressure over the fire the weight of gas passing 

through the boiler will increase with the total pressure drop. Also, 

by considerations drawn from following any vertical line in figure 19,
38 DEAFTS IN STEAM-BOILER PRACTICE.

we may reason that with the total pressure drop constant more gas 

will flow through the boiler as we cross the- curves of lower drops 

through the fuel bed. This is contrary to what is ordinarily expected 

by men in charge of boiler rooms. They reason that the higher the 

pressure drop through the fuel bed.the greater the amount of air 

which will pass through and consequently the higher the rate of 

combustion; the faultiness of this reasoning lies in its failure to 

consider also the total pressure drop from ash pit to stack.

L5 2.0 .25 30

THICKNESS OF LOWER BED OF SHOT INCHES.

4.0 45

FIG. 16. Relations between the thickness of the lower bed of No. 4 shot and the pressure drops through it.

RELATION BETWEEN MEAN DIAMETER OP SHOT AND WEIGHT OF AIR PASSING THROUGH

THE APPARATUS.

4

Table 8 gives a comparison of the weights of air passing through 

the beds of the two sizes of shot. Any horizontal row gives the 

weight of air and the ratio for the same pressure drops, but for 

various thicknesses of fuel bed, and any vertical column gives the 

weights and ratios for the same thickness of beds and various pressure drops. Each horizontal line headed by A gives the weights of 

air passing through four beds of No. 8 shot; each horizontal line 

headed by B gives the weights of air passing through beds of No. 4 

shot of the same respective thicknesses; figures in the horizontal 

lines headed by C are the ratios of the two above weights; and figures 

in the horizontal lines headed by D give the percentage of the varia-
LABORATORY EXPERIMENTS. 89

tion of each ratio from the average of all the ratios. As shown in 

the horizontal lines headed by C and D, the ratios of the two weights 

are nearly constant for all pressures and bed thicknesses, suggesting 

that the weight of air is a direct function of the diameter of the shot. 

The average of the ratios of the weights of air is 0.756, and the ratio 

of the diameters of the large and small shot sizes is 0.779. The two 

ratios are about the same; whether this is a coincidence is not known; 

more experiments are needed with several sizes of shot. a

TABLE 8. Comparison of pounds of air passing through beds of small and large shot

under the same conditions.

Pressure

drops

through 

lower

beds of

shot.

1

4

w

6

7

Line.

f A

B

C

D

A

B

C

D

A

B

C

D

A

B

C

D

f A

1 B

1 C

ID

f A

1 B

1 C

[Bf AA 

1 B

1 C

D

Thickness in inches, of lower beds of shot.

1.

0.202

.273

.740

-2.0

.306

.402

.7(51

 + .7

.385

.500

.770

+2.0

.452

.585

.772

+2.2 '

.510

.660

.773

+2.3

.566

.730

.775

+2.6

2.

0.123

.180

.683

-9.7

.199

 .262

.700

+ .6

.256

.335

.764

+1.2

.305

.394

.774

+2.5

.344

.452

.760

'+ .6

.380

.501

.758

+ .3

.412

.5oO

.749

- .8

3.

0.098

.140

.700

-7.3

154

.211

.729

-3.4

.200

.268

.747

-1.2

.241

.315

.765

+1.3

.274

.356

.769

+1.8

.306

.396

.773

+2.4

.336

.430

.782

+3.6

4.

0.081

.118

.687

-9.1

' .128

.178

.719

-4.8

.168

.226

.743

-1.6

.204

.271

.752

- .4

.236

.312

.757

+ .2

.264

.344

.767

+1.6

.288

.374

.770

+1.9

The practical deduction from the above, as regards the sizes of 

coal burned under a steam boiler, is that the weight of air passing 

through a fuel bed increases with the size of the coal, which is in 

accord with everyday observation. In what way the weight of air 

depends on the size of coal can not be ascertained from experiments 

with shot, and even if this relation were determined for one coal 

such a determination would not be applicable to other coals of 

different natures. For instance, the shape of the pieces of coal 

probably has some influence on the flow of air through the bed, and 

it is a known fact that the pieces of western bituminous coals are in 

angular shapes, while those of the soft eastern bituminous coals are

a It is interesting to note here that the percentage of voids among spheres is the same whatever their 

size, if sizes are not mixed. As a check on this conclusion weighings were made of a glass flask filled 

successively with No. 8 and No. 4 shot; after deducting the weight of the flask the contents weighed the 

. same in the two cases within about 1 per cent.
40 DRAFTS IN STEAM-BOILER PRACTICE.

rounded. The resistance of the coal bed on the grate would also 

depend somewhat on the manner in which the coal is put onto the 

grate. Thus the bed will have a different resistance when the coal 

is shoveled on in hand firing than when it is gradually fed in along 

with a very gentle agitation, as by a chain-grate stoker, or when fed 

in and moved along with considerable agitation, as by stokers of the 

overfeed and the underfeed type. Then, too, the caking of most 

coals in hot furnaces is constantly changing the resistance to the 

passage of air.

From the results of the preceding-experiments it may be deduced 

that, since the gas is the carrier of heat, less heat is delivered to the

I 2 3

PRESSURE DROP THROUGH LOWER BED OF SHOT INCHES OF WATER.

FlG. 17. Relations between pressure drops through the lower beds of No. 8 shot and rate of flow of air 

through apparatus for various constant total drops.

boiler when the resistance through the fuel bed increases, the. initial 

temperature of the gas and total pressure drop remaining the same. 

With any steam-generating apparatus the resistance to the flow 

of gases from the fuel bed to the uptake is very nearly constant, and 

perhaps is affected only by the quantity of soot and ashes on the 

heating surface. This constant resistance is fixed when the setting 

is put up and the baffles are inserted in the boiler. The only resistance which varies through a considerable range is that of the fuel bed. 

An unusually high pressure drop through the grate and fuel bed is an 

indication of unduly high resistance.
LABORATORY EXPERIMENTS. 41

The pressure drop through the fuel bed is often looked upon as so 

much "draft" available for the burning of coal; and it is widely 

believed that with any one apparatus the higher the drop through the 

fuel bed (the "draft" in the furnace), regardless of the total pressure 

drop, the higher the rate of combustion and the higher the capacity 

developed.

An application of the principles developed by the experiments on 

shot beds shows that this almost prevalent belief results from incomplete views of things. The pressure drop through the fuel bed is 

rather an indication of the resistance which the fuel bed and grate 

offer to the passage of air, and in judging the rate of working this

» 23456769

PRESSURE DROP THROUGH LOWER BED OF SHOT INCHES OF WATER.

FIG. 18. Curves showing the relations between pressure drops through lower beds of No. 4 shot and rate 

of flow of air through apparatus for various constant total pressure drops.

drop should always be considered in connection with the total drop 

from ash pit to uptake. With the same total pressure drop the drop 

through the fuel bed can be made to equal any quantity from nearly 

zero to the total drop by changing the resistance of the fuel bed only. 

If the air passages of the grate and all other openings into the furnace 

were entirely stopped, the pressure drop from ash pit to furnace 

would be equal to the total pressure drop. In other words, if the 

resistance to the flow of air from ash pit to furnace were made infinite 

the pressure above the grate would be the same as in the uptake. 

On the other hand, with the ash-pit doors fully open arid with a bare
42 DRAFTS IN STEAM-BOILER PRACTICE".

grate having large air passages, the pressure in the furnace would be 

close to the pressure in the ash pit, or to that of the outside atmosphere; that is, by making the resistance from ash pit to furnace 

nearly zero the drop of pressure through the grate would be nearly 

zero. By similar reasoning it could be shown that by decreasing 

the resistance to the flow of gas from over the fuel bed to the uptake, 

the pressure drop between these two points could be decreased and 

that between the ash pit and furnace increased. However, the 

resistance from furnace to uptake is fixed by the design and construeTOTAL PRESSURE DROP INCHES OF WATER.

FIG. 10. Relations between total pressure drops and rate of flow of air through apparatus for several constant 

pressure drops through lower beds ol No."8 shot.

tion of the boiler, and therefore can not be changed by manipulation 

or by the quantity of the coal; for this reason it is not given more 

detailed consideration in this bulletin.

EFFECT OF PRESSURE DROP THROUGH THE FUEL BED ON RATE 

OF COMBUSTION AND ON EVAPORATION.

Since for any one steam-generating apparatus the resistance to the 

flow of gas between the furnace and the uptake is practically constant, it is reasonable to expect that for a greater difference in pressures between these two places a greater weight of gases will pass 

through the boiler, provided that the temperature of the gases 

remains unchanged. The passage of a greater weight of gases at the
EFFECT ON HATE OF COMBUSTION AND ON EVAPORATION. 43

same temperature is the accompaniment of a higher rate of combustion 

and the cause of a more rapid production of steam. Thus we may 

logically expect that the rate of combustion and the capacity of the 

boiler will vary somewhat with the difference of pressures between 

the furnace and the uptake.

The resistance which the fuel bed offers to the flow of air varies 

with the thickness of the fire, the size of the coal, and probably with 

the moisture of the coal and the nature and quantity of ash on the 

grate. It would hardly be reasonable to expect increased rate of

3 4 56789 

TOTAL PRESSURE DROP INCHES OF WATER. .

FIG. 20. Relations between total pressure drops and rale of flow of air through apparatus for several 

constant pressure drops through lower beds of Mo. 4 shot.

combustion and higher capacity when one or more of these factors 

increases the resistance of the fuel bed, although the pressure drop 

from ash pit to furnace be thereby increased. The resistance caused 

by these factors can be changed by manipulation of the fuel bed.

The resistance of the grate is constant and should be small, because 

the energy required to move air through it is uselessly expended. 

It follows that the percentage of air spaces in the grate should be as 

high as mechanical construction and factor of safety allow. The 

pressure drop of air through the grate should be as nearly zero as 

possible. The power applied to the fans should be utilized as completely as practicable in two ways only (a) in scrubbing ashes and
44 DKAFTS IN STEAM-BOILER PRACTICE.

gases of combustion off the solid pieces of fuel, and (b) in scrubbing 

the soot and cooled gases off the water-heating surface.

A pressure ("draft") gage connected to the space above the fuel 

bed gives useful information as to the condition of the fire to a fireman who understands the significance of its indications. Thus, after 

cleaning a fire, if the pressure drop from ash pit to fuel bed is too small 

the fireman may be sure that there are holes in the fire, or that it is 

too thin; or, if the pressure drop is too great, it is probable that the 

fire is too thick. A gradual increase of the drop through the fuel bed, 

after a fire has burned for some time (in case of hand firing especially), is an indication of the accumulation of clinker next the grate. 

Of course, a drop through the fuel bed is adjudged high or low only 

after considering it in connection with the total drop through the 

whole apparatus. It may happen, in the same apparatus, with the 

same coal from the same bin, and with the same total pressure drop, 

that much less coal is burned and a smaller amount of steam produced on one day than on another, although the pressure drop through 

the fuel bed is higher on the day of smaller steam production. The 

fireman may wonder why this is. The explanation may be drawn 

out thus from the preceding paragraphs: When coal is taken out of 

the side and bottom of a bin, the larger pieces tend to flow out first, 

leaving the smaller pieces and dust in the far corners, which stay there 

to the last, until all the coarser coal has been burned. When burning 

the finer coal, the resistance to the passage of air through the fuel bed 

is greater, and this greater resistance causes a higher pressure drop 

that is, a higher "draft" above the fire; simultaneously the smaller 

air supply results in a lower rate of combustion and a smaller steam 

production.

Ohm's law is much better understood among electrical engineers 

than are the above-stated laws of gas flow among- steam-boiler

engineers; probably the reason is that in electrical measurements 

the units of drop of potential, resistance, and rate of flow of energy 

are all named, and the existence of names makes measurements much 

easier than in the case of the analogous quantities in problems relating 

to the flow of gases.., The well-defined electrical units enable their 

users to think much more accurately than do the men who have 

inherited, from times when science was hardly existent, such vague 

ideas as that of "draft."

The following statements are general:

(a) If in any one steam-generating apparatus the total pressure 

drop and the temperature of furnace gases remain constant, the weight 

of gases passing through the apparatus per unit of time varies inversely as the total resistance. Since in the same apparatus only the 

resistance of the fuel bed is changeable, it may be said that under the
STEAMING TESTS AT ST. LOUIS, MO. 45

above circumstances the weight of gases passing through the apparatus per unit of time varies inversely as the resistance of the fuel bed; 

therefore it varies also inversely as some power of the pressure drop 

through the fuel bed.

 (Z>) In any one apparatus the resistance to the flow of gases through 

the combustion chamber and boiler remains constant, and therefore 

with constant temperature of the gases the weight passing, through 

per unit of time is proportional to some power of the pressure drop 

through the combustion chamber and boiler.

STEAMING TESTS BY THE GEOLOGICAL SURVEY AT ST. LOUIS, MO.

As a test of the soundness of the reasoning in the preceding paragraphs, tests 140 to 434, inclusive, made at the Geological Survey's 

fuel-testing plant at St. Louis, Mo., were classified in various groupings 

according to gaseous pressure drops ("drafts"), and the results were 

platted in the charts reproduced as figures 21 to 24.

The boilers were of the standard hand-fired Heine type, not fitted 

with pressure or exhaust fans, but with steel chimneys. The two 

boilers and their settings were exactly alike."

The pressures in the furnace were taken in the middle of the length 

of the grate and about 20 inches above its surface. The pressure in 

the uptake was taken in the hood at the base of the stack, about 

2 feet below the damper and 3 feet above the point of exit of the gases 

from the boiler proper.

The points given in the charts are not individual tests, but each is 

o an arithmetic average of a number of tests. The figures in any 

small circle indicate the number of tests averaged to get the points 

lying on that particular vertical line.

The efficiency 72* used in these charts is the ratio of the heat 

absorbed by the boiler to the heat of the combustible ascending from 

the grate. The designation 72* refers to that item of the Government 

reports of the steaming tests made at the Survey's.fuel-testing plant. 6

Curve 1 of figure 21 is the product of pounds of dry coal burned per 

square foot of grate per hour and pounds of dry chimney gases per 

pound of "combustible." 0 Considering that a large number of tests 

were used in the compilation of this chart, the product just mentioned is very nearly proportional to the weight of gases passing 

through the boiler per hour. The indication of curve 1 is that this 

latter weight increases steadily with the increase of drop of pressure 

through the boiler. Curves 2 and 3 show that the rate of combustion and the capacity rise decidedly with this difference of pressures.

oFor detailed description see Bull. U. S. Geol. Survey No. 325, 1907, p. 173- 

b For discussion see Bull. U. S. Oeol. Survey No. 325,1007, p. 137.

c in this bulletin quotation marks inclose the word "combustible" when it is loosely used to mean "coal 

free from moisture and ash."
46 DRAFTS IN" STEAM-BOILER PRACTICE.

Curve 4 is rather irregular, however; the general indication seems 

to be that more air is used to .burn 1 pound of "combustible" as the 

difference of pressures becomes larger. This increase in the amount 

of air is partly due to increased leakage through the boiler setting.

jj a

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DROP OF PRESSURE FROM FURNACE TO UPTAKE INCHES OF WATER.

FIG. 21. Relations between pressure drop through two boilers and several items. Curve 1, pounds of 

coal burned per square foot of grate area per hour times the weight of dry chimney gases per pound of 

"combustible;" curve 2, pounds of dry coal burned per square foot of grate area per hour; curve 3, percentage of rated capacity developed; curve 4, pounds of dry chimney gases per pound of "combustible;" 

curve 5, over-all efficiency of grate, furnace, and boiler that is, from coal to steam (efficiency 72*). In 

platting these curves all tests were used, the classification being simply on the basis of pressure drop 

through the boiler.

It may also be the case that among the tests with the higher pressure 

differences coals high in ash or slow-burning coals predominate, in 

which case high pressure differences were necessary to obtain rating, 

and consequently more air entered the furnace through cracks and 

holes in the fuel bed.
STEAMING TESTS AT ST. LOUIS, MO. 47

Curve 5 shows that the efficiency drops slightly as the difference 

of pressures increases. This drop pf efficiency can be partly 

accounted for on the ground that the rate of combustion and the 

capacity increase. As the former increases more gases leave the

.05 .10 .12 .14. .16 .18 .2Q

PRESSURE IN FURNACE INCHES OF WATER.

.24 .26

FIG. 22. Relations between pressure drops through two fuel beds and several items, with varying pressure drops through boilers. Curve 1, pounds of dry coal burned per hour per square foot of grate 

surface; curve 2, total drop of gas pressure from furnace to uptake; curve 3, rated capacity of boiler 

developed, per cent; curve 4, pounds of dry chimney gases per pound of "combustible;" curve 5, 

over-all efficiency (72*), per cent. In platting these curves all tests were used, the classification being 

simply on the basis of the pressure drop through the fuel bed.

fuel bed, with the result that they must pass faster through the combustion space and be afforded less time in which to burn; hence the 

combustion is less complete. Also, with higher capacity, in order 

that more heat may be conducted through the heating plates the 

temperature difference between th6 dry and wet surfaces must be
48 DRAFTS IN STEAM-BOILER PRACTICE.

greater. As the temperature of the boiler water .remains the same, that 

of the wet surface of the heating plate can not be lowered. The only 

way, then, by which the temperature difference can be increased is by 

raising the temperature of the dry surface. It is this dry surface 

which receives heat from the gases, and therefore the latter leave the 

surface hotter when its temperature is higher. The net result is a

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PRESSURE DROP THROUGH FUEL BED INCHES OF WATER.

FIG. 23. Relations between pressure drops through two fuel beds and several items, with constant pressure drop through boilers. Curve 1, pounds of dry coal burned per hour per square foot of grate area; 

curve 2, percentage of rated capacity developed; curve 3, pounds of dry chimney gases per pound of 

"combustible;" curve 4, over-all efficiency of furnace and boiler (efficiency 72*),per cent. In platting 

these curves only those tests were used for which the gas pressure drop through the boiler was 0.35 inch 

of water.

slightly lower proportionate heat absorption by the boiler when the 

capacity is higher.

The classification of figure 22 is based on the pressure drop through 

the fuel bed. According to curves t and 3, it would seem that the 

rate of combustion and the capacity increase with the pressure drop 

through the fuel bed. However, if these two curves be compared
STEAMING TESTS AT ST. LOUIS, MO. 49

with curve 2, it becomes evident that the rise in the rate of combustion and the capacity is due to the increasing difference between 

pressures in the uptake and over the fuel bed. The points of curves 

1, 2, and 3 rise and fall together. Curve 4 shows that in general 

the weight of air used per pound of "combustible" increases with the

.55

DROP OF PRESSURE FROM FURNACE TO UPTAKE INCHES OF WATER.

FIG. 24, Relations between pressure drops through two boilers and several items. Curve 1, product of 

pounds of dry coal burned per hour per square foot of grate area and pounds of dry chimney gases per 

pound of " combustible; 1 ' curve 2, pounds of dry coal burned per hour per square foot of grate area; 

curve 3, pounds of dry chimney gases per pound of " combustible;" curve 4, percentage rated capacity 

of boiler developed; curve 5, over-all efficiency (72*), per cent. In platting these curves only those tests 

were used for which the gas pressure drop through the fuel bed equaled 0.12 and 0.13 inch of water

pressure drop through the fuel bed, which increase indicates that no 

economy is gained by raising the resistance of the fuel bed. Curve 5 

shows a decided drop in efficiency when the pressure drop through 

the fuel bed increases. There are probably several causes for this 

decrease in efficiency, the principal ones being the rise in the rate 

600.1.7 Bull. 367 09 4
50 DRAFTS IN STEAM-BOILEE PEACTICE.

of combustion and the capacity and the higher percentages of ash in 

the coals of the tests on the right. It has already been explained in 

connection with figure 21 why the efficiency decreases when the rate 

of combustion and capacity increase.

Figure 23 was worked up to show the effect of drop of pressure 

through the fuel bed alone on the evaporation, without the influence 

of the pressure drop through the boiler. In compiling this chart only 

those tests were used for which this latter pressure drop was about 

0.35 inch of water. Curve 1 shows that as the pressure in the furnace 

decreases (that is, as the "draft" increases) the rate of combustion

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LARGE BRIQUETS 

SMALL BRIQUETS

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TOTAL PRESSURE DROP INCHES OF WATER.

FIG. 2.5. Relation, in locomotive tests, between total pressure drop and rate of combustion (lower curve) 

and percentage of C02in the flue gases (upper curve). The upper curve indicatesthat thenumber of 

poundsof air per pound of coal did not increase with the pressure drop. The points are for individual tests.

remains nearly constant. Curve 2 shows a decided drop in capacity,' 

which drop, and also the drop in efficiency, can be explained by the 

indication of curve 3, which shows that as the pressure drop through 

the fuel bed increases more air is required to burn 1 pound of '' combustible," whence results a lower temperature of the gases and a 

greater loss of heat up the stack and a lower efficiency. The low 

temperature of the gases reduces their volume so much that, even if 

their weight passing through per second increases, their velocity 

remains the same, or may even decrease, which explains the apparent 

contradiction with statements made in previous paragraphs.
LOCOMOTIVE TESTS AT PORTSMOUTH, VA. 51

Figure 24 is similar to figure 21. In platting this chart only those 

tests were used for which the average pressure drop through the fuel 

bed was 0.12 and 0.13 inch of water, so that the pressure in the furnace may'be considered a constant amount below the atmosphere. 

In general this chart agrees with figure 21.

The conclusion which can be drawn from the four charts based on 

classification of tests according to gas pressures in the boiler setting 

is that in any one apparatus the rate of combustion and the capacity 

increase with the difference of pressures in the uptake and the furnace, and not with the pressure drop through the fuel bed alone. Or, 

stated otherwise, an unduly high pressure drop through the fuel bed 

is an indication of bad conditions therein, in that the flow of air 

through it is difficult.

From the charts just discussed the inference must not be drawn 

that large pressure drops ("drafts") are apt to be accompanied by

low efficiencies; in the above cases the coincidences are a consequence 

of the equipments used, and in some cases of certain properties of the 

coals burned.

LOCOMOTIVE TESTS BY THE GEOLOGICAL SURVEY AND THE 

SEABOARD AIR LINE RAILWAY AT PORTSMOUTH, VA.

In order to demonstrate further some of the principles developed 

or advocated in the preceding pages, such data and results as bear 

directly on the subject of this bulletin were extracted from 14 locomotive tests and compiled into Table 9. These tests were made with 

run-of-mine coal, with large square briquets, 6| by 4f by 3 inches, 

and with small oval briquets, 3 J inches in diameter by 2 inches thick. 

Both styles of briquets were made expressly for these tests by the Geological Survey at its Norfolk testing plant, from part of the run-ofmine coal. 0 The object of the tests was to determine whether the 

use of briquets in a steam locomotive would increase the economy 

and capacity sufficiently to pay for the briqnetting of the coal, and 

whether one kind of briquet possessed any advantages over the other. 

All tests were made with the locomotive standing in the yards, and 

the steam was discharged into the steam main of a stationary boiler 

plant.

The locomotive was a new one of the 10-wheel freight type. The 

fire box was 104 inches long and 41 inches wide. The height of the 

crown sheet above the grate was about 55 inches. The fire box contained a brick arch inclosing and supported by five 2 £-inch water tubes. 

A rocking grate of the finger type supported the fuel. The boiler 

contained 328 tubes, each 2 inches in internal diameter and 14 feet

a For analysis of coal used, see Bulletin No. 302, Mine sampling and chemical analyses of coals tested 

at Norfolk, Va.,in 1907, byJ. S. Burrows. A detailed account of the tests will bo. found in a bulletin, 

now in course of publication, by W. T. Hay and H. Kreisinger.
52 DEAFTS IN STEAM-BOILER PEACTICE.

If inches long. The pressure drop, or draft, was produced as usual 

by the small steam jet or by discharging steam directly from the 

boiler- through the exhaust nozzle; when using the latter method the 

cylinder valves were removed and the discharge of the steam producing the pressure drop was regulated by the main throttle.

The most significant feature of the results of these tests as applied 

to the subject of this bulletin is that the rate of combustion keeps up 

with the total pressure drop, as might be predicted from the relation 

between the total pressure drop and the rate of flow of air, obtained 

from experiments on the flow of air through beds of shot. This feature is brought out very prominently by the lower curve of figure 25. 

It shows that for a certain range the rate of combustion increases 

with the total drop, and that the increase in the rate of combustion 

is faster than the Jaw of its proportionality to the square root of the 

total drop would suggest. Thus, at a total drop of 3 inches of 

water the rate of combustion is 45 pounds of coal per square foot of 

grate surface per hour; at a total pressure drop of 9 inches of water it 

is 90 pounds, which is twice as much as in the first instance, whereas 

if the square-root rule held the rate of combustion would be only the 

square root of 9-^3 = 1.73 times as large, or only about 76 pounds.

The points of the upper curve are somewhat irregular, but in 

general they show that the air supply per pound of coal remains 

about constant, or, rather, that the rate of combustion keeps up 

with the air supply.

Strictly speaking, the lower curve of figure 25 shows the rate of 

disappearance of coal from the grate and not the rate of combustion, because part of the coal was not gasified but went up the stack 

in the form of sparks. However, the amount of sparks seldom 

exceeded 5 per cent of the coal fired, so that the curve would not be 

changed very much if correction were made for the coal lost in sparks.

It may be interesting to compare the indication of figure 25 with 

that of figure 11 and with the discussion of the latter beginning on 

page 31. It is stated there that if a given boiler uses up 2 per cent 

of the total steam generated in operating the fan when the boiler 

is working at 100 per cent of its rated capacity, the consumption of 

steam by the fan will rise to 6.5 per cent of the steam generated if 

the capacity of the boiler be doubled, and to 12 per cent if the 

capacity be trebled. Turning now to figure 25, let us assume that 

the capacity of the boiler increases directly with the nominal rate 

of combustion. Starting with the point at the total pressure drop 

of 2 inches, where the rate of combustion is 37 pounds, the following 

will be the pressure drops and relative amounts of work required 

of the fan for doubled and trebled rates of combustion and capacities. ' The amount of work done by the fan in each case is figured
LOCOMOTIVE TESTS AT PORTSMOUTH,, VA. 53

from the relative volume of air and the pressure it is moved against 

by the formula "Work of fan is proportional to volume X pressure." 

Pressure drops are taken from the curve of figure 25.

TABLE 9. Data and results of locomotive tests made at the Seaboard Air Line Railway

shops at Portsmouth, Va.

No. 

of 

test.

1 

2 

3 

4

5

0 

7 

 8 

9 

10 

11 

12 

13 

14

Kind of fuel.

Run of mine.... 

.....do..........

.....do.......... 

.....do..........

Large briquets.. 

.....do.......... 

.....do.......... 

.....do.......... 

Run of mine". . 

Small briquets..

.....do..........

Run of mine.... 

Small briquets &

Pressures (drafts), 

inches of water.

1

S 

a

%

0.77 

2.30 

2.27 

.59 

1.25 

2. 63 

5.48 

7.70 

7.30 

2.11 

8.10 

5.07 

5.50 

11.8

*+H

O

 s . 

gj

|5

<

0.61 

1.86 

1.86 

.53 

1.00 

1.96 

3.69 

4.10 

4.54 

1.59 

5.48 

3.69 

3.89 

8.74

1

Ui 

0)

> 

O

0.48 

1.10 

1.12 

.41 

.70 

1.37 

2.54 

3.01 

2.65 

.56 

2.73 

1.95 

2.41 

3.10

 d 

$ "3

Q)

°

0.39 

1.05 

.87 

.33 

.57 

1.03 

1.95

Weight fired ofcoalas burned hourper 

foot ofgrate. square 

Us. 

25.5 

36.5 

39.4 

19.2 

27.9 

45.0 

50.3 

80.2 

78.1 

39.8 

82.4 

61.1 

63.0 

114.0

Equivalent evapora- tion pound of per

dry coal.

7,6s. 

11.91 

11.44 

10.81 

11. 52 

11. 24 

11. 10 

10. 93 

10.18 

8.66 

11.21 

10.53 

10.68 

9.86 

8.88

<x> >

0)

73

t "O1

ow

251. 8 

343.0 

349.0 

183.0 

2GO.O 

410.0 

£04.0 

672.0 

542.0 

374.0 

723.0 

548.0 

501.0 

827.0

a 

o

0 .

»,a o <*>

CD -l^ O

l« 

1

11.45 

11. 90 

11.40 

11.10 

11. 9G 

12.45 

11.50 

12.05 

10. 10 

13. 87 

13. 57 

11.78 

12.20 

11.15

Per Oi incent offlue

gas.

6.92 

7.00 

7.49 

7.84 

7.07 

5.87 

7.08 

0.93 

8.49 

4.75 

4.49 

6.74 

6.94 

7.52

Per ofCOcent influe

gas.

0.23 

.1 

.23 

.14 

.22 

.17 

.15 

.13 

.25 

.20 

.30 

.05 

.20

Per ofNinflue cent2 

gas.

81. 63 

80.81 

80.95 

80.83 

80.83 

81. 46 

81.25 

80.87 

81.22 

81. 13 

81.91 

81, 18 

80.81 

81.13

Average thickness of

fire.

In. 

8 

1! 

12 

0 

15 

15 

18 

16 

10 

7 

12 

8 

15 

16

« Coal wet and mostly slack. b About 30 per cent of briquet had crumbled.

Pressure drops and relative amounts of ivorlc required of fan for doubled and trebled rates 

of combustion and capacities.

Rated capacity.

Per cent.

100

200

300

Rate of 

combustion.

37

74

111

Pressure 

drop, 

taken from 

curves.

2.00

6.85

11.65

Total steam 

consumed 

by fans.

Per cent.

o2.00

66.85

b 11.65

Relative 

total work 

required 

from fan.

1.00

6.85

17.50

a Assumed: 6 Calculated.

In the above comparison the rate of combustion was taken, rather 

than the horsepower developed by the locomotive boiler, because the 

consideration of the former is more nearly parallel to the experiments 

on the flow of air through beds of shot.

Figure 26 shows how the-boiler horsepower developed by the locomotive increases with, the total pressure drop from ash pit to stack. 

The points for the three styles of fuel do not fall so nicely in one line 

as they do in figure 25. Evidently the combustion was not as good with 

run-of-mine coal as it was with briquets, and for that reason the capacity does -not increase as fast with rising total pressure drop. The location of the points is such as to make it advisable to pass three curves 

through them, one for each style of fueL The highest point of the
54 DRAFTS IN STEAM-BOILER PRACTICE.

small briquet curve is apparently too low; before making these tests 

the briquets had been transferred several times from one tender to 

another, so that about 35 per cent of them had crumbled before the 

test. This condition undoubtedly pulls the point down toward the 

run-of-mine curve. The slower increase in boiler horsepower of the 

curve for run-of-mine coal is largely due to the fact that the coal contained a high percentage of slack which was carried by the strong 

current of gases through the furnace and boiler and out through the 

stack. In the calculations this loss is charged against the heatabsorbing ability of the boiler, even though it does not increase the 

rate of steam production in any way, and even .though it absorbs fan 

work in being moved along.

According to the curves of figure 26 the increase in total pressure 

drop required to develop twice a given horsepower would be as follows:

Increase in total pressure drop required to develop twice a given horsepower.

SMALL BRIQUETS.

Case 1 ..........................................

Case 2. .........................................

Total pressure 

drop in inches 

of water.

2.0

8.5

Boiler horsepower developed.

365

730

Ratio of pressure drops of cases 2 and l=8.5-=-2^4.25 

LARGE BRIQUETS.

2.0

7.7

332

664

Ratio of pressure drops of cases 2 and \ = 7.7-^2.0=3.85.

In order, then, to double the capacity the total pressure drop

must be multiplied by 4.25 when burning these particular small 

briquets, and by 3.85 when burning these particular large briquets, 

in this particular furnace. With the run-of-mine coal it would be 

necessary to multiply the total pressure drop by about 10.2 to obtain 

the same increase. It is evident that run-of-mine coal containing a 

high percentage of slack is not fit to use for high capacities with high 

pressure drops through the fuel bed, for, reasons already stated, one of 

them being the carrying up of fine coal unburned. Fuels having 

pieces of uniform size, such as briquets, are better suited for use in 

this manner.

The average of the ratios of the pressure drops in the two above 

cases for the two styles of briquets is about 4, or 22 . Taking into consideration also the fact that at such higher pressures at least twice 

the weight of gas must be put through the fuel bed and boiler, the
LOCOMOTIVE TESTS AT PORTSMOUTH, VA. 55

ratio of the work required from the fan in the above two cases will be 

as given in the last column of the table below:

Case 1 ..............

Case 2..............

Relative 

capacity.

100

200

Relative 

pressure 

drop.

2

2X 92

Relative 

weight 

of gas 

(volume;.

1

9

Relative 

work required of 

fan.

2

Relative

work required of 

fan, considering 

case 1 as

unity. .

8 or 23

Therefore, to double the capacity of an existing boiler (without 

rebaffling or otherwise changing it) the work of the fan must be 

multiplied by 8 that is', by 23 . On studying figure 26 it would

§

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o SMALL BRIQUETS 

 LARGE BRIQUETS 

 RUN-OF-MINE COAL

,-^

TOTAL PRESSURE DROP INCHES OF WATER.

FIG. 20. Relation, in locomotive tests, between the total pressure drop and the boiler horsepower developed. The points are individual tests on run-of-mine coal, large square briq uets .and small oval briquets.

seem that with a well-designed furnace and with a fuel of uniform 

size the same relation as above would hold for a case of trebled 

capacity, as given below:

Relative 

capacity.

300

Relative 

pressure 

drop.

2X3"

Relative 

weight 

of gas 

(volume).

3

Relative 

work required of 

fan.

54

Relative

work required of 

fan, considering 

case 1 as

unity.

27 or 33
56 DRAFTS TN STEAM-BOILER PRACTICE.

The relative work in the three above cases is obtained by multiplying together the relative weights of the gases (volumes) and the 

relative pressures against which the volumes are displaced, which 

procedure is in accordance with the well-known formula, "Work is 

proportional to volume X pressure."

The figures in, the preceding three cases state that the work 

required of the fan varies as the cube of the capacity developed by 

the boiler. This relation is found to be approximately correct, at 

least for the tests of figures 25 and 26. With boiler outfits designed 

for high capacity the work of the fan might be far less than indicated above, while with some of the existing outfits it would be more.

A caution is in place here. The attempt must not be made to put 

more air through existing boilers by running the fans a great deal 

faster, because the power consumed will increase far faster than the 

above calculations estimate. New fans and engines must usually 

be installed of sufficiently larger size to supply the larger quantities 

of air at as high an efficiency, if not higher.

As has already been suggested, one way of reducing the work 

required from the fan in the case of doubling the capacity of the 

boiler is to increase the grate surface, so as to avoid a high increase 

of pressure drop through the fuel bed, increasing materially only 

the pressure drop through the boiler proper. A low pressure drop 

through the fuel bed would also insure better combustion of the fine 

particles of coal which would be carried out of the stack unburned 

if high gas velocities through the fuel bed were employed, the high 

velocities being obtained by high pressure drops. This last method 

is being successfully used by H. G. Stott and W. S. Findley, of the 

Interborough Rapid Transit Company, New York City. They have 

recently installed an extra Honey stoker under the rear end of each 

of several Babcock & Wilcox boilers, with the result that the amount.

of steam produced was nearly doubled, the combined efficiency of 

the boiler and furnace dropping only about 3 per cent. A complete 

description of the outfits and the results is given in a paper read by 

Walter S. Findley, jr., before the American Institute of Electrical 

Engineers in December, 1907. In this case the pressure drop through 

the fuel bed was the same as with -the single stoker, or perhaps 

decreased slightly. Of course, the pressure drop through the boiler 

proper increased considerably. An electrical engineer would say 

that the above experimenters put two fuel beds in parallel and with 

the same potential drop obtained twice "the current (weight of gases). 

The same result could have been obtained by thickening somewhat 

the fuel bed on the single stoker and increasing the pressure drop 

through it, in which case the electrical engineer would say that the 

experimenters put two fuel beds in series and by increasing the drop 

of potential obtained twice the current (twice the weight of gases).
SUMMARY. 57

The method of increasing the grate area is a promising one because 

it requires less work from the fans; it is especially to be preferred 

in those cases where there is a high percentage of slack in the coal, 

as already explained.

The figures and principles derived from the experiments and tests 

presented in this bulletin may not be applicable directly to special 

problems; they suggest methods by which each problem can be 

studied and its successful solution brought about. Further experiments with laboratory apparatus as well as with hot fuel beds are 

desirable before more accurate figures can- be given. The Geological 

Survey contemplates the making of such experiments in the near 

future, the results to be worked up and published in the next bulletin 

on "Drafts."

SUMMARY.

The substance of this bulletin may be summarized as follows:

If the resistance to the flow of gas remains unchanged, the pressure drop through any portion of the gas passage bears a constant 

ratio to the total pressure drop.

If the total pressure drop from, ash pit to uptake remains constant, 

the pressure drop through any portion of the gas passage varies with 

the resistance to the flow of gas through that particular portion. 

Thus with a constant total drop from the ash pit to the uptake the 

pressure drop through the grate and fuel bed varies with the resistance of the grate and fuel bed, the variation being in the same direction but not of proportional magnitude.

If the resistance. of any portion of the gas passage remains constant, the weight of gas passing through that portion varies as some 

power of the pressure drop through it. The index of the power 

is approximately 2, so that in order to double the weight of gas 

passing through, the pressure drop must be quadrupled, and in order 

to treble it the pressure drop must be made nine times as great.

The above three statements are general for all conditions and for 

all combinations of "forced" and "induced" "draft."

There is really no difference between "forced" and "induced" 

"draft," provided the setting is tight. The volumetric percentage 

of '-'dead" corners in a boiler is the same in either case.

"With gases there is'no pulling, only pushing." Gases travel 

from places of higher pressure to those of lower, the motion of the 

gas occurring because of its expansion.

The term "draft" is indefinite, being applied sometimes to the 

motion of gases and sometimes to the cause of the motion that is, 

to the pressure difference.
58 DEAFTS IN STEAM-BOILER PRACTICE.

Experiments shedding light on the flow of gases through boilers 

were made in a laboratory by passing air through two beds of lead 

shot. The results of the experiments are expressed in the following 

brief paragraphs:

The rate of flow of air through a bed of shot of constant thickness is 

approximately proportional to the square root of the pressure drop 

through the bed.

As the thickness of the beds of shot increase, the weight of air passing through per minute decreases very rapidly at first, then more and 

more slowly.

To put air at the same rate through twice as thick a bed of shot 

requires twice the pressure difference between the two sides of the bed 

and twice the work must be expended.

If the height (number of rows of tubes) of a cross-flow type of watertube boiler be doubled, or if the length of the tubes of. a parallelflow type of water-tube boiler be doubled, or if the thickness of a 

fuel bed be doubled, it is likely that the work required of a fan will 

be about doubled if the same weight of gas is to be passed through 

per minute.

If two or three times the usual amount of gases be forced through 

a stoker and boiler so as to produce nearly two or three times as 

much steam, the total work expended on the fans will be increased 

perhaps 8 and 27 times, respectively (23 and 33 ). Suppose that it 

takes 60 pounds of steam per hour to run the fan when developing 

100 boiler horsepower in a certain boiler, to develop 200 horsepower 

in the same boiler a fan of the same efficiency would use 60 X 8 = 480 

pounds of steam, and to develop 300 horsepower a third fan of the 

same efficiency would use 60X27 = 1,620 pounds. Expressed in 

percentage, the consumption of steam in the three cases would be as 

follows, taking as a boiler horsepower 30 pounds of steam produced 

under 70 pounds gage pressure from feed water at 100° F.:

Fan consumption of steam as a percentage of total steam, generation.

Boiler

horsepower.

100

200

300

Total

steam generated

per hour.

Pounds.

30X100-3,000

30x200-0,000

30X300-9,000

Total

steam consumed per

hour by

fans.

Pounds.

60

480

1,620

Percentage of

steam used by

fans.

60:3,000- 2

480:6,000- 8

1,620:9,000-18

This greater proportionate expenditure of steam would probably be 

saved in other operating costs, besides which there would be the 

great advantage of working the investment two or three times as hard.
SUMMARY. 59

There is, however, a limit beyond which it would not pay to increase the total pressure drop in order to raise the capacity of a 

boiler. Assume that the work applied to the fan would continue to 

increase as in the above three cases, starting the same at 2 per cent. 

With higher capacities the steam consumption of the fans would be 

as given below:

Boiler 

horsepower.

]00

200

300

400

Percentage 

of steam 

used by 

fan.

2

8

18

32

Boiler 

horsepower.

500

GOO

700

800

Percentage 

of steam 

used by 

fan.

50

72

98

128

This table shows that it would be impossible to multiply the 

capacity by more than 7. Beyond this the fan would take more steam 

than the boiler would make. At 5 times the original capacity the 

fan would take about half the steam. Doubling, trebling, and perhaps with larger grate areas even quadrupling the rate of working a 

boiler seems to be sound commercial practice. Furthermore, by 

carefully designing a fan the above steam consumption could nearly 

be cut in half. This we must admit if we study efficiency tests made 

on fans of various makes by trustworthy experimenters, who have 

shown that the efficiency of a blower from the shaft to air delivered 

lies ordinarily between 10 and 50 per cent.

Increasing the length of a multitubular boiler and leaving the 

internal diameter of the tubes the same seems in some cases to increase 

slightly the total amount of heat absorbed when the boiler is working under the same pressure (draft) difference between the two ends 

of the tubes, although the weight of air passing through is less.

A large pressure drop through the fuel bed (or a high draft above 

the fire) does not necessarily mean that the rate of combustion is 

high; it is likely to mean the opposite. High pressure drop through 

the fuel bed always means high resistance in it to the flow of gas.

The pressure drop through the fuel bed, or the draft over the fire, 

is significant only when considered in connection with the total 

pressure drop.

The capacity of a boiler increases with the difference between the 

pressures in the uptake and over the fire.

With constant total pressure drop (total draft) the capacity of a 

boiler decreases as the pressure drop through the fuel bed increases.
60 DEAFTR IN RTEAM-BOTLER PBACTTCE.

GLOSSARY.

ASH PIT. Used in this bulletin to mean the space under the grate through which air 

must pass before entering the grate and fuel bed.

COMBUSTIBLE. A loose name and misnomer for the substance properly described as 

"coal free from moisture, sulphur, and ash." In this bulletin it means "coal free 

from moisture and ash."

DRAFT. This term is sometimes applied to the motion of gases and sometimes to. the 

difference of pressures producing the motion. To speak of draft being absorbed 

by sharp turns can not mean anything else than that the velocity of the gases is 

reduced. To speak of a draft of 1 inch of water can not mean anything else than 

that the difference between the pressures at two points is sufficient to support a 

column of water 1 inch high. The vague term draft probably originated from the 

mistaken idea that something was pulling or drawing the gas. See pages 6-9 for 

arguments that there is really no such motive power effective in the movement of 

gases.

EXHAUST FAN. Commonly called induced-draft fan.

FUEL BED. The mass of burning fuel on a grate.

HOOD. See Uptake.

PRESSURE DROP. The pressure drop through any portion of a gas passage is the 

difference between the pressures at the two ends of the portion. It is equal to the 

difference between "drafts" at the two points.

Total pressure drop is the difference between the pressures at the two extreme 

ends of a gas passage, including all the component portions.

PRESSURE FAN. Commonly called forced-draft fan.

UPTAKE, sometimes called HOOD. The conical passage connecting the place where 

the gases-leave the boiler proper with the flue leading to the chimney. The pressures taken in the uptake were obtained within 2 or 3 feet of the last tubes of the 

boiler proper.
SURVEY PUBLICATIONS ON FUEL TESTING.

The following publications, except those to which a price is affixed, 

can be obtained free by applying to the Director, Geological Survey, 

Washington, D. C. The priced publications can be purchased from 

the Superintendent of Documents, Government Printing Office, 

Washington, D. C.

BULLETIN 261. Preliminary report on the operations of the coal-testing plant of the 

United States Geological Survey at the Louisiana Purchase Exposition, in St. Louis, 

Mo., 1904; E. W. Parker, J. A. Holmes, M. R. Campbell, committee in charge. 1905. 

172 pp. 10 cents.

PKOFESSION'AL PAPER 48. Report on the operations of the coal-testing plant of tllG 

United States Geological Survey at the Louisiana Purchase Exposition, St. Louis, 

Mo., 1904; E. W. Parker, J. A. Holmes, M. R. Campbell, committee in charge. 1906. 

In three parts. 1492 pp., 13 pis. $1.50.

BULLETIN 290. Preliminary report on the operations of the fuel-testing plant of the 

United States Geological Survey at St. Louis, Mo., 1905, by J. A. Holmes. 1906. 

240 pp. 20 cents.

BULLETIN 323. Experimental work conducted in the chemical laboratory of the 

United States fuel-testing plant at St. Louis, Mo., January 1, 1905, to July 31, 1906, 

by N. W. Lord. 1907. 49 pp.

BULLETIN 325. A study of four hundred steaming tests, made at the fuel-testing 

plant, St. Louis, Mo., 1904, 1905, and 1906, by L. P. Breckenridge. 1907. 196 pp.

BULLETIN 332. Report of the United States fuel-testing plant at St. Louis, Mo., 

January 1, 1906, to June 30, 1907; J. A. Holmes, in charge. 1908. 299 pp.

BULLETIN 334. The burning of coal without smoke in boiler plants; a preliminary 

report, by D. T. Randall. 1908. 26 pp.

BULLETIN 336. Washing and coking tests of coal and cupola tests of coke, by Richard Moldenke, A. W. Belden, and G. R. Delarnater. 1908. 76 pp.

BULLETIN 339. The purchase of coal under Government and commercial specifications on the basis of its heating value, with analyses of coal delivered under Government contracts, by D. T. Randall. 1908. 27 pp.

BULLETIN 343. Binders for coal briquets, by J. E. Mills. 1908. 56 pp.

BULLETIN 363. Comparative tests of run-of-mine and briquetted coal, by W. F. M. 

Goss. 1908. 57 pp.

BULLETIN 366. Tests of coal and briquets as fuel for house-heating boilers, by 

D. T. Randall. 1908. 44 .pp.

61

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