National Bureau of Standards

' ibrary, N. W. BIdg.

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taken from the Library.

'""''^Terformance of a

Coal-Fired Boiler

Converted to Oil

Building Materials and Structures Report BMSlll

United States Department of Commerce

National Bureau of Standards
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Performance of a

Coal-Fired Boiler

Converted to Oil

by Richard S. Dill and Paul R. Achenbach

Building Materials and Structures Report BMSlll

Issued May 20, 1948

For sale by the Superintendent of Documents, U.

Price

S. Government Printing Office, Washington 2 5, D. C. IS cents

Foreword

Many coal-fired boilers are converted to the use of fuel oil each year to obtain

the greater convenience of liquid fuel. Converted coal-fired boilers are usually

not as efficient as boUers originally designed for fuel oil since the efficient combustion of coal requires more combustion space and larger gas passages through

the boiler than does liquid fuel. The tests described in this report were made

to show the advantage of using a precast lightweight combustion chamber

instead of one made of standard firebrick for the oil burner and to evaluate

the increase in capacity obtainable with two types of cast-iron baffles. The

project was part of the research and testing prb^am carried on at the National

Bureau of Standards in cooperation with the National Housing Agency and

its technical staff. E. U. Condon, Director.

Ill

Performance of a Coal-Fired Boiler

Converted to Oil

by Richard S. Dill and Paul R. Achenbach

ABSTRACT

As part of the research program on heating plants for small homes being conducted at the National Bureau of Standards in collaboration with the National Housing Agency and

its technical staff, the performance characteristics of cast-iron boilers, both coal- and oilfired, have been investigated. This report discusses the performance of a cast-iron boiler designed for coal firing after it had been converted to the use of oil by means of a gun-type

burner. It is shown that a refractory firebox of low heat capacity and low thermal conductivity increased the efficiency of the converted boiler several percent, and that baffles which effectively direct the hot combustion gases over the heat-transfer surface increased the capacity of the boiler up to 27 percent.

CONTENTS

Page

Foreword iii I. Introduction 1 II. Specimen and test equipment 2

III. Procedure 4

IV. Test results on combustion chambers 5 V. Boiler performance with baffles 7 VI. Discussion and conclusions 10 VII. References 11

I. Introduction

Many home owoiers would like to use oil fuel for heating because of the greater convenience but do

not wish to discard a coal-fired boiler that is still in good condition. In general, either the gun-type

or the rotary-type oil burner can be installed in a conventional coal-fired boiler or furnace to convert

it to the use of oil fuel. Many boiler manufacturers offer the same boiler for use with either coal or

oil, whereas others design boilers specifically for each fuel. A larger combustion chamber is usually

essential for burning solid fuel to permit complete

combustion of volatile gases, and larger flue-gas

passages are usually provided, chiefly to allow for partial obstruction of the passages by soot and fly ash. A smaller combustion chamber and less excess air are needed for burning oil than for the

combustion of coal and result in higher efficiencies than are usually obtained with a coal-fired boiler. The original reason for the use of refractory

linings in the combustion chambers of oil-burning

boilers was to protect metal parts from the action

of the flames. However, it was soon observed that

hot refractory material in a firebox improved combustion and increased the transfer of heat by radiation from the hot refractory to the heating surface

of the boiler. A refractory firebox made of material ^^^.th low

heat capacity and low conductivity will become

hot more rapidly following a cold start and permit

maintenance of higher temperatures during stead}-

operation of the oil burner than one made of fire- brick. This higher firebox temperature is ex- pected to improve combustion of the fuel and

increase the heat transfer by radiation to the crown

sheet. Moreover, less heat will be lost by conduction through the bottom and sides of the chamber

when the conductance of the refractory is low.

The combustion chamber should be large enough

for the oil spray to burn in suspension, but small

enough to fit the flame so that the refractorj- \\'ill 1
2 BUILDING MATERIALS AND STRUCTURES REPORTS

become hot quickly after a cold start. Turbulence

in the burning gases should also be promoted to provide maximum mixing with the combustion air and stagnant pockets in corners should be avoided

as far as possible.

The results of an investigation by Cross and

Lyman [1] ^ with a pressure-atomizing mechanical

draft burner indicated that the size of the combustion chamber affected the efficiency of a boiler

considerably, but no data in relation to size were

obtained in the present investigation, as the dif- ference of 2 percent in the combustion space of

the two chambers used is negligible.

Metal or refractory ceramic baffle plates are frequently installed in converted boilers to direct

the hot combustion gases more efl'ectively over the

heat-transfer surface. Wlien a gun-type oil burner

is installed in the front of a boiler which has flue passages at the rear, there is a tendency for the

hot gases to pass to the rear of the combustion

chamber and upward to the outlet without having

brushed over the front part of the crown sheet.

Thus, there is, in efl'ect, a dead air space under the

forward part of the crown sheet, and this surface

receives heat chiefly by radiation from below.

By attaching a baffle to the rear wall of the boiler

which extends halfway to the front, the hot gases

are deflected forward and brought in contact with

the entire crown sheet before passing up the rear

flues. Some coal-burning boilers have more than one

set of uptake flues and more cross-sectional flue

area than oil-burning boilers require. If the flues nearer the front of the boiler are closed, all of the

gases will be directed over the .greater area of

heating surface to the flue at the rear. Economizers, which usually consist of additional

water-backed heating surfaces located in the

combustion space or gas passages, are also used to improve the efficiency and capacity of boilers con- verted to oil. They wece not stuclied at this time

as this investigation compared only the results of the use of two combustion chambers of dift'erent material and of somewhat dift'erent form. The

eft'ects of the installation of baffles and of the seal- ing of the clinker and firebox doors to prevent

leakage were also investigated. Arthur H. Senner

has reported the efl'ect of the installation of two

kinds of baffles and fom- kinds of economizers in

round converted cast-iron boilers [2]. An investigation by Cleghorn and Helfinstine [3] shows that, of two boilers made by the same manufacturer, the boiler originally designed for oil fuel averaged 13 percent more efficient than the coalfired boiler that had been converted to oil. The

combustion chamber of the boiler designed for oil was surrounded by water-backed surfaces, wliicli

is the usual practice for some manufacturers but

not for others. Baffles were not used in the conversion boiler during the comparison made by

Cleghorn and Helfinstine. Therefore, the results

of that comparison are not necessarily representative of all boilers of the two types.

II. Specimen and Test Equipment

The boiler used for the tests was a stock rec- tangular cast-iron boiler with four vertical sections

enclosed in a jacket. It had 34.5 sq ft of heating

surface and a grate area of 3 sq ft. There were two sets of uptake flues at the rear of the boiler, one set between the third and fourth sections, ancl a smaller set between the second and third

sections.

The oil burner was a stock model of the pressureatomizing, mechanical-draft type with a capacity

ranging from 1 to 2 gallons of oil per hour. It had automatic intermittent ignition and the

installation was equipped with the usual ignition

and pressure controls.

A combustion chamber of standard firebrick was

first used in the boiler. This chamber was built in

rectangular shape with interior dimensions 13 in. long, 12 in. wide, and 14 in. deep, in accordance

with the recommendations of the oil-burner

manufacturer. The space between the firebrick

and the sides of the ashpit was filled with an

insulating material. The chamber extended

upward to a point 2 in. above the bottom of the

water legs. Figure 1 shows the dimensions of

the firebrick chamber and its location in relation

to the sides of the ashpit. Firebrick 2}^ in. by

4)^ in. by 9}^ in. were used for this chamber up to the point Avhere the grate-bar supports interfered.

Split brick 1}{ in. thick were used above the

grate level, thus forming the shelf at the upper

edge of the chamber shown in figure 1. The precast combustion chamber was of the

type recommended by the bofler manufacturer

and had a rectangular cross section in a horizontal

plane. The bottom of the chamber was cylindrical in shape with the center line of the cylinder run- ning from front to back of the boiler. The inside

dimensions of the chamber were: width 12 in., length 16 in., and maxirmim depth 13 in. From

the cylindrical bottom, the sidewalls extended vertically upward to a point 2 in. above the bottom

of the water legs of the boiler. Figure 2 shows

the size and shape of the precast chamber and its position in the ashpit.

The volume of the firebrick combustion chamber was 1.26 cu ft and of the precast chamber 1.30 cu ft, exclusive of the space from the top of the

chamber to the crown sheet of the boiler, which

1 Figures in brackets indicate tlie literature references at end ot this paper.
PERFORMANCE OF CONVERTED BOILER

Figure 2. Precast combustion chamber consisting of eight pieces nf refractory fitted together.
4 BUILDING MATERIALS AND STRUCTURES REPORTS

was the same in both cases. The two chambers

were located similarly with respect to the water

legs and to the front and back sections of the boiler. Therefore, the differences in size and location of

the two chambers were negligible.

The crown-sheet baffle was a cast-iron plate,

in. thick, bolted to the back of the boiler through one of the holes normally used for the domestic

hot-water heating coil. This plate extended for- ward in a horizontal plane for a distance of 12 in. from the back of the boiler. It was as wide as the

interior width of the boiler reaching from the back

of the boiler to the joint between the second and

third sections. The plane of the baffle was 6 in. below the crown sheet and 12 in. above the top of

the combustion chamber. A diagrammatic cross section of a cast-iron boiler illustrating the application of the crown-sheet baffle is shown in figure .3. The baffles used in the uptake flues were 2}^- by

6-in. cast-iron plates. They were placed in the

two forward uptakes in the third section entirely

closing the openings. The position of these baffles

is also illustrated in figure 3. The steam risers from the boiler were brought

up fuU size to a level of 30 in. above the top of the

boiler and were connected together into a header.

Pipe connected the header to a steam separator, a gate valve, and a surface condenser. The piping was insulated with 2-in. magnesia pipe covering. The condensate was weighed on a platform

scale graduated in quarter ounces. The drafts

in the firebox and in the smoke pipe were measured

with draft gages graduated in thousandths of an

inch W. G. (water gage). Feed water was introduced into one of the water legs of the boiler

through a %-m. line. The water level in the boiler was maintained at a constant height by a constant-level valve in the feed line. The smoke pipe was connected to the vertical

outlet of the smokehood and was attached to an induced-draft fan. The smokehood and 2

ft of the smoke pipe were insulated with 2 in. of rock wool. An orifice plate inserted in the

insulated portion of the stack served to mix the

WATER LINE

CROWN SHEET

BAFFLE

Figure 3. Diagram of a cast-iron boiler showing position

of baffles. Not drawn to scale.

flue gases and give a more nearly uniform temperature across the area of the smoke pipe. The

flue-gas temperature was measured at the center

of this orifice with a chromel-alumel thermocouple

of 26-gage wire. The stack draft was measured

3 in. below this orifice plate and a tube was inserted

3 in. above the plate for sampling the flue gases. The temperatures of the feed water and the steam

were measured with copper-constantan thermocouples of 30-gage wire. Steam was taken from

the boiler at atmospheric pressure. The oil-supply

tank rested on a platform scale graduated in

quarter ounces. A siphon was used to connect

the oil supply to the oil burner to allow free move- ment of the scale platform. A watt-hour meter

graduated in divisions of 10 watt-hours integrated

the energy used by the burner motor and the

ignition circuit. An Orsat apparatus was used to measure the carbon dioxide content of the flue gases.

III. Procedure

In order to permit ready comparison of the ca- pacity and efficiency of the converted boiler with

the different combustion chambers and baffles, a

fixed set of conditions was substantially reproduced during the first test after each modification

of the boiler. The "fixed" conditions were es- tablished during a preliminary heating period by

adjusting the rate of fuel supply, the draft, and

the air openings into the burner to produce a stack temperature of approximately 600° F, a concentration of 10 percent of carbon dioxide in

the flue gas, and a draft of 0.02 in. W. G. in the

firebox. The heat imparted to the water under

these specified conditions is called the "gross

rated output" or, more briefly, the "capacity" of

the boiler, measured in Btu per hour.

The output of the boiler under each of the test conditions was computed by means of the following formula:

Output= M'(hfi,-\-hf—h^

,

in which

/t/g=heat of vaporization of water at the

measured steam temperature, Btu/lb.

=heat of the liquid of water at the steam

temperature, Btu/lb.
PERFORMANCE OF CONVERTED BOILER 5

hi =heat of the liquid of water at the feed

water temperature, Btu/lb.

water rate, Ib/hr determined by weighing the condensate.

By definition, the heat input to the boiler is the

product of the weight of oil burned per hour and

the heating value of the oil. The direct efficiency

is the ratio of the output, determined by water

rate and enthalpy change, to the input. The input

which results in producing the gross rated output

of heat will be called normal.

When the normal input had been determined,

three sets of observations were made with fuel inputs of approximately 80, 100, and 120 percent

of normal, but with the same firebox draft and with

flue gases of the same composition.

During a warming-up period of at least 90

minutes before each test, the soot was brushed off the heating surfaces, the firebox draft was adjusted, the oil input was regulated, and the air opening on the burner was opened or closed to produce the desired operating conditions. At the

time of starting the test, the oil scale was balanced,

the watt-hour meter was read, and a container was

placed under the condensate outlet. Thereafter,

the following observations were made at 15-

minute intervals during a 2-hr test period: Oil

consumption; steam condensed; electric energy

used ; draft in the firebox and stack ; temperatures

of the feed-water, steam, and flue gas; gage-glass

level; room temperature; barometer; oil pressure on the nozzle ; and carbon dioxide and oxygen content of the flue gases. The rate of oil consumption was adjusted by varying the pressure on the nozzle

within reasonable limits. When greater variation

of input was necessary, a different sized nozzle was

inserted in the burner.

The manufacturer of the oil burner recommended

a No. 3 fuel oil for use in the boiler under test. Two No. 3 fuel oils with specific gravities of 0.863

and 0.871 and total heating values of 19,400 and

19,300 Btu/lb., respectively, were used.

Tests at 80, 100, and 120 percent of gross, rated

output were made with the firebrick combustion

chamber without baffles. For the next series of

experiments, this chamber was replaced by the

precast chamber. Subsequently, the crown-sheet

baffle was used alone with the precast chamber,

and later both the crown-sheet and rear-flue baffles were used with the same chamber for a similar

series of tests. An additional set of three tests was made under the last operating condition, with

fire door, clean-out door, and clinker door sealed

against leakage of air. In all the previous tests no attempt was made to reduce the air leakage

around the doors.

IV. Test Results on Combustion Chambers

The results of the tests of the boiler equipped

with the precast combustion chamber and with

the firebrick combustion chamber without baffles, are plotted in figure 4. It will be noted in this

figure that the gross rated output of the boiler, at a stack temperature of 600° F and 10 percent

of carbon dioxide in the flue gases, was 115,000

Btu/hi- with the precast chamber and 112,000

Btu/hr with the firebrick chamber; whei-eas the

efficiency of the boiler was nearly 5 percent

greater with the precast chamber. The data from

which figure 4 is plotted are summarized in table 1

.

Table 1. Performance of a converted coal-fired boiler" equipped successively with firebrick combustion chamber and precast combustion chatnber

Test number and type of combustion chamber

1. Fire- brick2. Fire- brick3. Fire- brick4. Fire- brick 5. Precast 6. Precast 7. Precast

Grade of oil'-- - . No... 3 3 3 3 3 3 3 Burner nozzle size--- - - - -gal/hr- 1.35 1.35 1.35 1.35 1.65 1.65 1.65 on input rate Ib/hr-- 6.93 8.96 9.00 10. 21 8. 22 9. 12 10. 77 Heating value of oil. . Btu/lb.. 19.400 19, 400 19, 400 19, 400 19, 400 19, 400 19, 400 Heat input rate.-. Btu/hr- 134, 400 173, 900 174, 600 198, 120 159, 400 176, 900 208, 900 Steam condensed. ... ... Ib/hr.- 79. 50 100. 25 102. 25 116. 12 99. 04 109. 86 125. 73 Heat to produce steam. .. .- (hfs+hf-hi), Btu/lb.. 1, 102.2 1, 102.3 1, 099. 6 1, 101. 5 1, 112.8 1,110.2 1, 113.

2

Gross output - Btu/hr-

87, 630 110, 510 112,430 127, 910 110. 210 121,970 139, 960 Direct efficiency - - - percent. 65.

1

63.5 64.3 64.5 69.

1

68.9 66.9

Draft, firebox --- --- -- --in. W. G.. 0. 022 0. 020 0. 021 0. 021 0. 019 0. 021 0.020

Draft, staclc- -- -- - in. W. G.. .017 .017 .020 .020 .010 .011 .015 Fine-gas temperature ... "F- 524 605 603 643 590 607 662 CO2 in flue gas .percent.. 10.1 10.0 9.7 10.0 10.0 10.

1

10.0 Wet lluc-gas loss... - .percent . 19.5 22.3 22.5 23. 2 21.7 22.3 23.7 Radiation and incomplete combustion loss . percent 15.4 14.2 13. 2 12.3 9. 2 8.8 9.4 Radiation and incomplete combustion loss Btu/hr. - 20, 720 24, 670 23, 000 24, 380 14, 670 15,550 19, 630 Burner power consumption - -watts 135 179 175 198 190 202 207

' No baffles were used in the boiler for these tests. 775317°—48 2
6 BUILDING MATERIALS AND STRUCTURES REPORTS

ENT 80

70

Q. 60 >O 50 z

LU 40

u

LU 30

U- 800

1

uJ

a:Z5 600

1—<

(X. LU

CL 400

UJ

1- 200

in 0

—o

-H

cP

o Fir H Pr

e Brick ecast Combustion Che mber

"

X

60 80 100 120 140 160 180

GROSS OUTPUT BTU PER HR/IOOO

Figure 4. Coniparison of the -performance of a boiler equipped with firebrick combustion chamber with that of

the same boiler equipped with precast combustion chamber. No baffles were used in either case. For a gross output of 115,000 Btu/hr, figure 4

shows that the boiler efficiency was about 64.2

percent with the firebrick combustion chamber and

69.0 percent with the precast combustion chamber.

The oil consumption rate was 9.26 Ib/hr in the

former case and 8.67 Ib/hr in the latter, representing a fuel saving of 6.3 percent when the precast

chamber was used.

The heat available from the fuel oil consumed

during these tests may be divided into three parts

for the puipose of this discussion. The major or

fii'st part appeared as useful heat in the steam

being delivered from the boiler. The second part

passed up the chimney in the flue gases; this

included the heat carried away by the water vapor

produced by the combustion of hydrogen in the

fuel. The third part was either dissipated from

the boiler setting by radiation, conduction, and

convection (hereafter referred to simply as the

radiation loss), or was represented by fuel that

passed up the chimney without burning. With

10 percent of carbon dioxide in the flue gas and a

stack temperature of 600° F, the second item of

the three above-mentioned is about 22.3 percent of

the heating value of the fuel. Thus, the sum of

the first two items when the firebrick combustion

chamber was used was 85.8 percent in test 2 and

91.2 percent when the precast chamber was used in

test 6, summarized in table 1. Hence, the radiation and incomplete combustion loss in the first case was 14.2 percent and in the second case 8.8

percent of the heat a.vailable in the fuel. The precast chamber became visibly hotter than

the firebriclc combustion chamber during continu- ous operation. The former reached a medium

cherry red color over its entire inside surface,

whereas the latter did not glow visibly at any place

dming the course of the tests. This indicates that

the flow of heat from the area of combustion

through the refractory of the chamber was less for

the precast material than for the firebrick. Evidently, the higher temperature of the precast

chamber considerably increased the radiation of

heat to the boiler, since more heat was liberated

in the combustion chamber in test 6 than in test

2, whereas, the flue-gas temperatures were practically identical during the two tests. In this paper the terms "precast" and "fire- brick" are used to identify the two combustion

chambers. It should be pointed out that the

better residts obtained with the precast chamber

depended on the properties of the material used to construct the combustion chamber, and in particular on its thermal conductivity, heat capacity, and

shape, rather than on the fact that it was of

precast construction.

Figure 4 shows the performance of the boiler

with each combustion chamber for a range of

output from about 20 percent below to 20 percent

above the gross rated output defined under Procedure, chapter III, of this report. Over this

range, with the brick combustion chamber, the

efficiency remained within narrow limits, between

63.5 and 66 percent. Presumably, changes in

radiation and incomplete combustion losses, due to

altering the output, were approximately compensated for by opposite changes in flue-gas losses. Over the same range of output, the efficiency with

the precast chamber varied from about 71 percent

at 88,000 Btu/hr output, obtained by extrapolation, to about 67 percent at 132,000 Btu/hr

output, taken from figure 4. The data in table 1 show that, in general, the

heat lost by radiation and incomplete combustion

expressed in Btu per hour increased as the rate

of fuel consumption increased. The sum of these

two losses was some 8,000 to 9,000 Btu/hr greater

with the firebrick combustion chamber than with

the precast combustion chamber at the same

gross output.
PERFORMANCE OF CONVERTED BOILER 7

V. Boiler Performance with Baffles

A comparison of the results obtained with and

without the vise of baffles shows that the use of the

crown-sheet baffle alone increased the gross rated

output of the boiler about 12 percent, but that it had no effect on the efficiency. The use of both

the crown-sheet and the rear-flue baffles increased

the gross rated output about 27 percent and

increased the efficiency 1 or 2 percent. All of

these tests were made with the precast combustion

chamber installed m the boiler. At a stack temperature of 600° F and with 10 percent carbon

dioxide in the flue gas, the gross rated output of the boiler without either baffle was 115,000

Btu/hr and the efficiency was 69.1 percent. With

only the crown-sheet baffle in use, the gross rated

output was 128,500 Btu/hr and the efficiency was

68.8 percent; with both baffles installed, the gross

rated output was 146,000 Btu/hr and the efficiency was 70.8 percent. The results of the tests made

with the crown-sheet baffle are summarized in

table 2, while those obtained with both baffles are contained in table 3. The results in tables 2 and

3 and tests 5 to 7 in table 1 are plotted in figure 5 to compare the performance of the boiler with

and without baffles.

Table 2. Performance of a converted coal-fired boiler equipped with a crown-sheet baffle

Test number and type oi combustion cliamber

1. Precast 2. Precast 3. Precast 4. Precast

Grade ot oil . „ . Burner nozzle size. . ....

Oil input rate Heating value of oil Heat input rate . ... .. ...

No...

gal/hr__ lb/hr__ Btu/lb..

..Btu/hr..

3 1.65 8.15

19, 400

158, 000

3 1.65

8.2:5

19, 400

159, 600

3

1. 65

10. 27

19, 400

199, 200

3

1. 65

11.51

19, 400

223, 500 Steam condensed. Heat to produce steam

Gross output. . . . ...

Direct eflBciency Draft, firebox ..

Ib/hr..

(hfi-^hf-hi), Btu/lb-

Btu/hr.

.

..percent _ in. W. G.-

98. 00

1, 106. 2

108, 410 68.6 0.020

97. 98

1, 110.7

108, 830

68.

1

0. 021

125. 50

1, 105. 9

138, 790 69.6

0. 020

139. 25

1, 105.

1

153, 890

68.8 0.020

Draft, stack .. . Flue-gas temperature. . . CO2 in flue gas . Wet flue-gas loss. . Radiation and incomplete combustion loss.- .

in. W. G.. °r

..percent . . ..percent-.

... . percent..

.012

557 10.1 21.0

10.4

.011

571 9.9

21.3 10.6

.019

603 10.2

22.3

8.1

.025

641 10.0

23.3 7.9

Radiation and incomplete combustion loss. ... . _ Burner power consumption- Btu/hr--

-. watts--

16, 420

195

16, 910

193

16, 120

208

17, 640

226

Table 3. Performance of a converted coal-fired boiler equipped with crown-sheet baffle and rear-flue baffles

Test number and type of combustion chamber

1. Precast 2. Precast 3. Precast 4. Precast 5. Precast 6. Precast 7. Precast 8. Precast

Grade of oil No... 3 3 3 3 3 3 3 3 Burner nozzle size- - -. . .-gal/hr-- 1.00 1.35 1.35 1.65 2. 00 1.65 1.65 2.00

Oil input rate--- -- Ib/hr-- 5. 56 7.09 8.31 10. 60 10. 92 11.03 12.69 15.41 Heating value of oil. Btu/lb.. 19, 300 19. 300 19, 300 19, 300 19, .300 19, 300 19, 300 19, 300 Heat input rate - - - . Btu/hr. - 107, 120 136,910 160, 440 204, 000 210, 780 212, 900 244, 860 297, 335 Steam condensed. . Ib/hr-

67. 87 87. 05 102. 41 129. 71 132. 29 131.81 149. 27 183. 0.; Heat to produce steam -..(ft/g+ft/--fti), Btu/lb -

1, 107. 2 1,117.1 1,117.5 1, 116.4 1, 118.8 1,119.1 1, 120. 6 1, 108. 2

Gross output. ... . Btu/hr -- 75, 150 97,240 114, 400 144, 810 148, 110 147, 510 167. 270 202, 500 Direct efficiency percent- 70.

1

71.0 71.2 70.6 70.

1

69.2 68.3 6S. 1 Draft, firebox in. W. G-- 0. 020 0. 021 0. 021 0. 1121 0.022 0. 021 0.020 0.03O

Draft, staoli in. W. G.. .008 .023 .010 .021 . '122 .021 .026 .051 Flue-gas temperature . "F-- 408 495 525 599 603 606 644 703 C02 in flue gas.. .. . percent 9.9 9.8 10.0 9.8 9.9 10.0 9.9 9.9 Wet flue-gas loss percent.. 16.7 19.0 19.8 22. 5 22. 5 22.4 23.5 25.7 Radiation and incomplete combustion loss.. percent.. 13.2 10.0 9.0 6.9 7.4 8.4 8.2 6. 2 Radiation and incomplete combustion loss Btu/hr.. 14, 150 13, 690 14, 440 14, 110 16, 600 17, 890 20. 040 18, 420 Burner power consumption ... .. watts.

141 146 143 135 208 217 231 163

Figure 5 shows the oil input, stack temperature, and efficiency of the boiler for a range of gross output from about 90,000 to 170,000 Btii/hr:

when using the crown-sheet baffle only, when using

both baffles, and when using no baffle, all with the

precast combustion chamber. The curves indi- cate that, at some typical intermediate gross out- put such as 130,000 Btu/hr, the use of both baffles

reduced the stack temperature from 635° to 565° F,

reduced the fuel consumption from 10.0 to 9.5

Ib/hr, and increased the efficiency from 68.2 to

70.5 percent; for this gross output, the baffles effected a fuel saving of 5 percent. , Figure 6 affords a comparision of the performance of the boiler equipped with the precast combustion chamber and both baffles with that of tlie
8 BUILDING MATERIALS AND STRUCTURES REPORTS

11 80

70

Q. 60 >o 50 Z

UJ 40 O

EFFl 30

800

1

1 1 i

_>

I

<

UJ

ft, 400

s:UJ

)-

:^ 200 O

to 0

1 2

10

_1 8

1-D

a. 6 z

4

_jo

^ —t;i—=—

H

o No Baf Crown

Tie

Sheet B

•1 aoffie nd Reor

-

• Flue B<affles

80 100 120 140 160 180 GROSS OUTPUT BTU PER HR/iOOO

"

Figure 5. Effect of using the crown-sheet baffle alone and

in conjunction with the rear-flue baffles in a boiler equipped

with precast combustion chamber. same boiler equipped with the firebrick combustion

chamber with no baffles. The gross rated output was increased from 112,000 to 146,000 Btu/hr or

30.3 percent by installation of the baffles and the

precast combustion chamber. Again compai'ing

the performance at f30,000 Btu/hr gross output,

the curves show that the precast chamber and

baffles reduced the stack tem.perature from 655°

to 565° F, reduced the oil consumption rate 9.5 percent from 10.5 to 9.5 Ib/hr, and increased the

efficiency from 64.1 to 70.5 percent.

All of the tests previously discussed were con- ducted without sealing the fire door, the clinkf'r door, and the clean-out door against leakage of air into the combustion chamber. Inasmuch as these

doors are frequently sealed in oil-burning instal- lations, tests were made to determine what effect this leakage had on the performance of the boiler. Figure 7 shows the characteristics of the boiler

for a range from approximately 80 to 120 percent

of gross rated output with the doors sealed when

the precast firebox and both baffles were used.

The indications are that sealing the doors did not

alter the efficiency for the gross output of 120,000

Btu/hr and increased it only one percent for a

gross output of 170,000 Btu/hr. Sealing the

doors appreciably increased the gross rated output

from 146,000 to 152,000 Btu/hr and the oil con- sumption was increased 0.42 Ib/hr. Under the

method of control used for the tests, sealing the

doors would be expected to increase the primary

air by an amount equal to the leakage at the doors

which should increase the completeness of combustion. It should also prevent interference with

the heat transfer by an envelope of relatively cool

air leaking in around the hot combustion gases. Therefore, an increase in efficiency and heat transfer would be expected from sealing the doors.

Figure 7 was plotted from the results of tests 1 to

3, inclusive, shown in table 4.

o

cm

LlJ >

a:

I—<OH

UJ

a.

80

70

60

50

40

30

800

600

400

n— —a cP— Cl

200

<J

i<

</>

X

n Fire O Prec

Brick C

ast

jmbustior

" Chomber, no Be

, Boff

)ffles es

60 80 100 120 140 160 180 GROSS OUTPUT BTU PER HR/iOOO

Figure 6. Comparison of the performance of a boiler equipped with firebrick combustion chamber and no baffles with that of the same boiler equipped with precast combustion chamber, crown-sheet, and rear-flue baffle.
Table 4.

PERFORMANCE OF CONVERTED BOILER

Performance of a converted coal-fired boiler^ with fire door and cleanout door sealed

9

Test number and type of combustion chamber

1. Precast 2. Precast 3. Precast

Grade of oil. No... 3 3 3 Burner nozzle size... ... gal/lir.. 1.35 1.65 1.65

Oil iTir^nf" 9. 00 11. 14 12. 85 Heating value of oil Btu/lb.. 19, 300 19, .300 19, 300 Heat input rate ....... Btu/hr.. 173, 700 215,020 248, 030 Steam condensed.. Ib/hr.. 111. 27 139. 30 155. 72 Heat to produce steam (h/,+hf-hi), Btu/lb.. 1,098.6 1,098.3 1,099.6 Gross output . . Btu/hr.. 122, 240 153,000 171, 230 Direct efficiency . ._ . ... --percent-. 70.3 71.1 69.0 Draft, firebox . in. W. G.. 0. 021 0.020 0.019

Draft, stack

...

... . . ..-.in. W. G.. .010 .019 .023 Flue-gas temperature . . °F. 538 598 646 CO2 in flue gas. . . ... percent.. 10.0 10.

1

10.0 Wet flue-gas loss . .percent-. 20.5 22.3 23.4 Radiation and incomplete combustion loss. ... . _.

. .. percent-- 9 2 6.6 7.6 Radiation and incomplete combustion loss ... . Btu/hr.. 15, 980 14, 200 18, 830 Burner power consumption . ... watts.. 145 140 151

a Both crown-sheet and rear-flue baffles were used for these tests.

I-5 80 ,

, ,

\ 1 r—

O

^ 70 =^ o

UJ

Q. 60

O 50 ^ 40

O

80 100 120 140 160 180 200

GROSS OUTPUT BTU PER HR/iOOO

FiGUEE 7. Performance of a boiler eqvip-ped with precast combustion chamber, crown-sheet, and rear-flue baffles when

the fire door, clinker door, and clean-out door are sealed.

Figure 8 shows the performance of the converted

boiler using the precast combustion chamber with

both baffles for a range of gross output from 75,000

to 200,000 Btu/hr. The maximum efficiency of

the combination boiler-burner unit occurred at a

gross output of about 120,000 Btu/hr. However^

the efficiency remained above 68 percent for thc^

40 80 120 160 200 240

GROSS OUTPUT BTU PER HR/iOOO

Figure 8. Performance of a boiler for heat output ranging

from 75,000 to 200,000 Btulhr.

Boiler equipped with precast combustion chamber, crown-sheet, and rearflue baffle.
10 BUILDING MATERIALS AND STRUCTURES REPORTS

entire range of output shown. The lowest oil input of 5.6 Ib/hr was obtained with a 1.00-gal/hr

nozzle under a pressure of 105 lb/in. ^ and the

highest oil input of 15.5 Ib/hr was obtained with

a 2.00-gal/lir nozzle under a pressure of 145 Ib/in^.

Some soot was deposited on the heating surfaces

at the lower firing rates when the 1.00-gal/hr and

1.35-gal/hr nozzles were used, but at the higher

firmg rates no soot was visible on the heating

surfaces at the end of the test. Figure 8 was

plotted from the results of tests 1 to 8, inclusive,

shown in table 3.

VI. Discussion and Conclusions

The results of this work are considered to establish the general possibility of improving the

efficiency and of increasing the ratings or capacities

of boilers after conversion from the use of coal to

oil, by means of baffles in the flue passages and

combustion chamber, by means of precast combustion chambers of special shape and low-density

material, and by sealing the fire door to prevent

leakage.

The results of the particular tests reported

herein show that an improvement in efficiency of 5 percent and an increase in capacity of 2}^ percent

are possible by the use of a typical precast combustion chamber in a typical cast-iron coal-fired

heating boiler. Boiler capacity or rating has been

defined as the heat output when the firing rate is regulated to maintain the stack temperature at 600° F, a concentration of 10 percent of carbon

dioxide in the flue gas, and a draft of 0.02 in. W. G. in the firebox. On this basis, the results

also show that boiler rating or capacity can be

raised 27 percent and the efficiency about 2 per- cent by the use of a crown-sheet baffle and rearflue baffles. These results are attributed to higher

temperatures in the combustion chamber, to the

greater turbulence of the flue gases, and to more

effective circulation of the combustion gases over

the heat-transfer surfaces, all caused by the baffles. Sealing the flre doors against air leakage in- creased the bofler capacity or rating about 4

percent but did not measurably improve the

efficiency.

Boilers differ in their proportions; therefore, the same design of baffle or of combustion chamber is not applicable to every boiler. To convert a given

bofler from coal- to ofl-firing, the user should apply

to the bofler manufacturer or to some other competent source for mformation and for the necessary

parts.

In addition to the use of baffles, bofler capacity can also be increased and the efficiency improved

slightly by adding sections to a cast-iron boiler,

thereby increasing the heating surface. The problem may arise as to whether it is more economical

to attain an increase in bofler capacity by the use

of baffles, by the instaflation of additional sections

in a cast-iron boiler, or by the use of a new and

larger bofler. It is anticipated that, of the three

methods of increasmg boiler capacity, the use of

baffles wiU prove to be the best expedient, as no

change in piping or other arrangements would be

entafled.

A summary of the results obtained on the con- verted boiler, by using two dift'erent combustion

chambers and one or two sets of baffles, with the

fuel-air mixture set to produce 10 percent carbon

dioxide in the flue gas, a stack temperature of

600° F, and 0.02-in. W. G. draft in the firebox, is shown in table 5.

Table 5. Summary of boiler performance for jive test conditions

[Carbon dioxide 10 percent, stack temperature 600° F, firebox draft 0.02-in. W. G.]

Combustion chamber Baffles Gross

rated output

Effi- ciencyOil input

Firebricls: None...

Btu/hr

112,000

115,000

128, 500

146, 000

» 152, 000

Percent

64.3

69.1 68.8

69. 5 70.4

Lblhr

9. 05

8. 72 9.70

10. 78 11.20

Precast do Do Crown-sheet . Do

Do

Crown-sheet and rear-flue. . . do .

a Fire door, clean-out door, and clinker door sealed. Although all of the tests reported were made

with the oil burner operating continuously, it is expected that the type of combustion chamber and

baffles used would have similar effects on the

efficiency and capacity of the bofler with intermittent operation. Investigations by L. E. Seeley

and E. J. Tavanlar [4] have shown that oil-burning

boilers are less efficient when operating intermittently than when operating continuously, especially when the ratio of operating time to idle time

is small. A lightweight combustion chamber,

such as the precast specimen, should provide a

greater improvement in efficiency for intermittent

operation tlian for steady operation because it becomes hot more rapidly. Also, the present tests were made with a flue-gas carbon dioxide content

of 10 percent to correspond witli the requirement of

tiie applicable commercial standard, CS75-42 [5]. The pi'obability is that the lightw^eight combustion

chamber woifld show a stfll greater advantage if smoke production rather than carbon dioxide in the

flue gas were made a limitation, since the higher

surface temperature of a lightweight combustion

chamber should favor better combustion.
PERFORMANCE OF CONVERTED BOILER 11

VII. References

[1] R. C. Cross and W. R. Lyman, A Study of Oil-fired Heating Boilers. Heating and Ventilating 28, 45

(Oct. 1931).

[2] Arthur H. Senner, A Study of the Oil Burner as Applied to Domestic Heating. U. S. Dept. Agri. Tech. Bui. 109.

[3] M. P. Cleghorn and R. J. Helfinstine, Performance of Pressure-type Oil Burners. Iowa Eng. Exp. Sta.

Bttl. 151, (1941).

[4] L. E. Seeley and E. J. Tavanlar, Study of Performance

Characteristics of Oil Burners and Low- pressure Heating Boilers. ASHVE Trans. 37, 517 (1931)

[5] Automatic Mechanical Draft Oil Burners Designed

for Domestic Installations, Bur. of Standards, U. S. Dept. of Commerce, CS75-42.

U. S. GOVERNMENT PRINTING OFFICE: 1948

BUILDING MATERIALS AND STRUCTURES REPORTS

[Continued from cover page ii] BMS32 Structural Properties of Two Brick-Concrete-Block Wall Constructions and a Concrete-Block Wall Construction Sponsored by the National Concrete Masonry

Association 15($ BMS33 Plastic Calking Materials 150 BMS34 Performance Test of Floor Coverings for Use in Low-Cost Housing: Part 1 15jS BMS35 Stability of Sheathing Papers as Determined by Accelerated Aging * BMS36 Structural Properties of Wood-Frame Wall, Partition, Floor, and Roof Constructions with

"Red Stripe" Lath Sponsored by The Weston Paper and Manufacturing Co 10^ BMS37 Structural Properties of "Palisade Homes" Constructions for Walls, Partitions, and

Floors Sponsored by Palisade Homes * BMS38 Structural Properties of Two "Dunstone" Wall Constructions Sponsored by the W. E. Dunn Manufacturing Co 10j5 BMS39 Structural Properties of a Wall Construction of "Pfeifer Units" Sponsored by the Wisconsin Units Co 100 BMS40 Structural Properties of a Wall Construction of "Knap Concrete Wall Units" Sponsored

by Knap America, Inc 100 BMS41 Effect of Heating and Cooling on the Permeability of Masonry Walls * BMS42 Structural Properties of Wood-Frame Wall and Partition Constructions With "Celotex"

Insulating Boards Sponsored by The Celotex Corporation 150 BMS43 Performance Test of Floor Coverings for Use in Low-Cost Housing: Part 2 150 BMS44 Surface Treatment of Steel Prior to Painting 100 BMS45 Air Infiltration Through Windows 150 BMS46 Structural Properties of "Scot-Bilt" Prefabricated Sheet-Steel Constructions for Walls,

Floors, and Roofs Sponsored by The Globe -Wernicke Co 100 BMS47 Structural Properties of Prefabricated Wood-Frame Constructions for Walls, Partitions,

and Floors Sponsored by American Houses, Inc 100 BMS48 Structural Properties of "Precision-Built" Frame Wall and Partition Constructions

Sponsored by the Homasote Co 150 BMS49 Metallic Roofing for Low-Cost House Construction 150 BMS50 Stability of Fiber Building Boards as Determined by Accelerated Aging 100 BMS51 Structural Properties of "Tilecrete Type A" Floor Construction Sponsored by the Tile- crete Co 100 BMS52 Effect of Ceiling Insulation Upon Summer Comfort 100 BMS53 Structural Properties of a Masonry Wall Construction of "Munlock Dry Wall Brick"

Sponsored by the Munlock Engineering Co 100 BMS54 Effect of Soot on the Rating of an Oil-Fired Heating Boiler 100 BMS55 Effects of Wetting and Drying on the Permeability of Masonry Walls 100 BMS56 A Survey of Humidities in Residences 100 BMS57 Roofing in the United States—Results of a Questionnaire * BMS58 Strength of Soft-Soldered Joints in Copper Tubing . 100 BMS59 Properties of Adhesives for Floor Coverings 100 BMS60 Strength, Absorption, and Resistance to Laboratory Freezing and Thawing of Building

Bricks Produced in the United States 150 BMS61 Structural Properties of Two Nonreinforced Monolithic Concrete Wall Constructions 100 BMS62 Structural Properties of a Precast Joist Concrete Floor Construction Sponsored by the Portland Cement Association 100 BMS63 Moisture Condensation in Building Walls 150 BMS64 Solar Heating of Various Surfaces : 100 BMS65 Methods of Estimating Loads in Plumbing Systems 15e BMS66 Plumbing Manual * BMS67 Structural Properties of "Mu-Steel" Prefabricated Sheet-Steel Constructions for Walls,

Partitions, Floor, and Roofs Sponsored by Herman A. Mugler 150 BMS68 Performance Test for Floor Coverings for Use in Low-Cost Housing: Part 3 200

BMS69 Stability of Fiber Sheathing Boards as Determined by Accelerated Aging 100 BMS70 Asphalt-Prepared Roll Roofings and Shingles 200

BMS71 Fire Tests of Wood- and Metal-Framed Partitions 200 BMS72 Structural Properties of "Precision-Built, Jr." Prefabricated Wood-Frame Wall Construction Sponsored by the Homasote Co 10c BMS73 Indentation Characteristics of Floor Coverings 100 BMS74 Structural and Heat-Transfer Properties of "U. S. S. Panelbilt" Prefabricated SheetSteel Constructions for Walls, Partitions, and Roofs Sponsored by the Tennessee

Coal, Iron & Railroad Co 150 BMS75 Survey of Roofing Materials in the North Central States 150 BMS76 Effect of Outdoor Exposure on tke Water Permeability of Masonry Walls 150 BMS77 Properties and Performance of Fiber Tile Boards lOe BMS78 Structural, Heat-Transfer, and Water-Permeability Properties of Five Earth-Wall Constructions 250 BMS79 Water-Distributing Systems for Buildings 200

BMS80 Performance Test of Floor Coverings for Use in Low-Cost Housing: Part 4 150 BMS81 Field Inspectors' Check List for Building Constructions (cloth cover, 5 x 7>^ inches) 200 Out of print. [List continued on cover page iv]
BUILDING MATERIALS AND STRUCTURES REPORTS

f Continued from cover page iii) BMS82 Water Permeability of Walls Built of Masonry Units 20^ BMS83 Strength of Sleeve Joints in Copper Tubing Made With Various Lead-Base Solders 10^ BMS84 Survey of Roofing Materials in the South Central States 15^ BMS85 Dimensional Changes of Floor Coverings with Changes in Relative Humidity and Temperature 10(S BMS86 Structural, Heat-Transfer, and Water-Permeability Properties of "Speedbrik" Wall Construction Sponsored by the General Shale Products Corporation 15^ BMS87 AjMethod for Developing Specifications for Building Construction—Report of Subcommittee on Specifications of the Central Housing Committee on Research, Design,

and Construction 15^ BMS88 Recommended Building Code Requirements for New Dwelling Construction with Special

Reference to War Housing * BMS89 Structural Properties of "Precision-Built, Jr." (Second Construction) Prefabricated

Wood-Frame Wall Construction Sponsored by the Homasote Co 15^ BMS90 Structural Properties of "PHC" Prefabricated Wood-Frame Constructions for Walls,

Floors, and Roofs Sponsored by the PHC Housing Corporation 15f5 BMS91 A Glossary of Housing Terms 15^ BMS92 Fire-Resistance Classifications of Building Constructions 30ji BMS93 Accumulation of Moisture in Walls of Frame Construction During Winter Exposure..- 10^ BMS94 Water Permeability and Weathering Resistance of Stucco-Faced, Gunite-Faced, and

"Knap Concrete-Unit" Walls 10^ BMS95 Tests of Cement-Water Paints and Other Waterproofings for Unit-Masonry Walls 25^

BMS96 Properties of a Porous Concrete of Cement and Uniform-Sized Gravel 10^ BMS97 Experimental Dry-Wall Construction with Fiber Insulating Board 10{S

BMS98 Physical Properties of Terrazzo Aggregates 15<5 BMS99 Structural and Heat-Transfer Properties of "Multiple Box-Girder Plywood Panels" for Walls, Floors, and Roofs 15^ BMSlOO Relative Slipperiness of Floor and Deck Surfaces 10^ BMSlOl Strength and Resistance to Corrosion of Ties for Cavity Walls 10^ BMS102 Painting Steel 10^ BMS103 Measurements of Heat Losses From Slab Floors lOfS BMS104 Structural Properties of Prefabricated Plywood Lightweight Constructions for Walls,

Partitions, Floors, and Roofs Sponsored by the Douglas Fir Plywood Association.. 25^ BMS105 Paint Manual $1. 00

BMS106 Laboratory Observations of Condensation in Wall Specimens.* * BMSlO? Building Code Requirements for New Dwelling Construction 20^

BMS108 Temperature Distribution in a Test Bungalow With Various Heating Devices 10^ BMS109 Strength of Houses: Application of Engineering Principles to Structural Design $1. 60

BMSllO Paints for Exterior Masonry Walls. 15^ BMSlll Performance of a Coal-Fired Boiler Converted to Oil 15^

* Out of print.