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National Bureau of Standards, was undertaken with the assistance of the Central Housing Committee, an informal organization of governmental agencies concerned with

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Federal Housing Administration.

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United States Housing Authority.

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Albert G. Bear,

Veterans' Administration.

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Federal Home Loan Bank Board (Federal Loan Agency).

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Public Buildings Administration (Federal Works Agency)

.

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[For list of BMS publications and how to purchase, see cover page III]
UNITED STATES DEPARTMENT OE COMMERCE • Harry L. Hopkins, Secretary

NATIONAL BUREAU OF STANDARDS • Lyman J. Briggs, Director

BUILDING MATERIALS

and STRUCTURES

REPORT BMS54

Effect of Soot on the Rating of an Oil-Fired

Heating Boiler

by

RICHARD S. DILL a?td PAUL R. ACHENBACH

ISSUED JULY 18, 1940

The National Bureau of Standards is a fact-finding organization;

it does not "approve" any particular material or method of construction. The technical findings in this series of reports are to be construed accordingly.

UNITED STATES GOVERNMENT PRINTING OFFICE • WASHINGTON

FOR SALE BY THE SUPERINTENDENT OF DOCUMENTS, WASHINGTON, D. C. • PRICE lO

1940

CENTS
Foreword

This paper calls attention to an important factor which should be considered in the

adjustment or testing of heating boilers or furnaces on the basis of a constant flue-gas

temperature and in the design of chimneys. The paper describes the large effect on

flue-gas temperature which can be expected from a comparatively light deposit of soot on heating surfaces. Some assurance is offered to users of heating boilers and furnaces

that an increased flue-gas temperature due to soot deposition does not necessarily signify a great decrease in capacity or efficiency. Chimneys and smoke pipe should be able

to withstand the relatively high temperatures that may occur if there is an extraordinary amount of soot deposited on heating surfaces during some period of improper

combustion.

Lyman J. Bkiggs, Director.

[n]
Effect of Soot on the Rating of an Oil-Fired Heating Boiler

by RICHARD s. DILL and Paul r, achenbach

CONTENTS

Page

Foreword ii

I. Introduction 1 II. Test specimen and equipment 1

Page

III. Test procedure 2

IV. Results 2

V. Discussion of results 3

ABSTRACT

It is shown that the rating of a boiler, obtained by a

test method based on constant flue-gas temperature,

can be seriously affected by soot on the heating surfaces, although the soot may be too slight in amount to affect greatly either the capacity or the efficiency at a constant

firing rate. At constant firing rate the soot has a large

effect on the flue-gas temperature.

I. INTRODUCTION

During trials of a proposed method for testing

and rating oil-fired domestic steam boilers, some discrepancies appeared in the results which

could not be readily accounted for; and, al- though the fact that the soot deposits were tliin suggested at first that the effects noted were

not to be attributed to soot, a series of runs was

made, the results of which indicated that the

soot did cause the discrepancies.

The test method under consideration, in

common with some other methods proposed in

the past, provides for testing boilers on a basis

of constant flue-gas temperature and carbon

dioxide content. Information on the effect of soot on the

capacity and efficiency of coal-fired boilers is available from other work.'- In the presei^t

work, the point of interest is that a soot deposit

causing a small decrease in efficiency results

' p. Nichols and C. E. Augustine, Effect of Soot on Heat Transmission

in Small Boilers, U. S. Bureau of Mines, R. I. 3272.

in a relatively large increase in flue-gas temperature. Then, when the flue-gas temperature is decreased to the point chosen for the test, by

duninishing the firing rate, the delivery of the

boiler is also diminisbed. In this way the

delivery of the boiler, on which tlie rating is based, is affected.

II. TEST SPECIMEN AND EQUIPMENT

The boiler tested was a jacketed rectangular

cast-iron boiler of four sections with a nominal

rating of 280 ft- EDR.^ The boiler and the

gun-type oil burner used were made by the same manufacturer. Boiler and burner were

erected for the tests in accord with the manufacturer's instructions, and testing equipment was applied in compliance with the test method

under consideration. A potentiometer and

thermocouple system were used for temperature measurements. An. automatic float valve was provided to control the feed water. OU

consumption was measured by weigbt, and the

carbon dioxide in the flue gas was measured

with the Orsat apparatus. The thermostat

and safety controls for the oil burner were

installed in the conventional maimer. The

fuel was No. 2 fuel oil of 0.861 specific gravity

and 19,500 Btu/lb higher heating value. Boiler

output was determined from condensate weight.

Draft in the firebox was mamtainecl at 0.02-in.

water gage.

2 1 ft2 EDR is defined as the ability to emit 2-10 Btu/hr.

237508°—40 [1]
III. TEST PROCEDURE

The procedure during these tests consisted

essentially in operating the boiler under the

"stand-by" condition for several days and of

observing the capacity, fuel consumption, etc., under test conditions once each day. Two

series of runs were made, one at 10 percent of

carbon dioxide and the other at 8 percent of carbon dioxide. During both series, the flue- gas temperature was adjusted to 600° F by

setting the filing rate prior to each capacity

determination. The setting then remained

fixed mitil the next determination. The runs

at 10 percent of carbon dioxide covered a period

of 8 days and those at 8 percent of carbon

dioxide, a period of 5 days.

At the end of each series, a run was made

with the firing rate reset at the original value.

The heating surfaces were then cleaned, and

the performance of the boiler was observed to be substantially the same as it was before the

deposition of soot began, which indicates that

the results noted were caused by the soot.

IV. RESULTS

The results of the tests, shown in figures 1 and 2, indicate that soot can have a considerable

eft'ect on the rating of a boiler when such rating

is based on a hmiting flue-gas temperature,

although tlie soot may be too slight in amount

to seriously aftect either the efficiency or the

capacity at constant firing rate. Tlie results of the runs at 10 percent of

carbon dioxide indicate that the effect of each

day's deposition of soot is less than that of the

preceding day's deposition and that the effect of the deposit approaches an upper Hmit with

time. Tliis efi'ect is not clearly indicated by

the results of the runs at 8 percent of carbon

dioxide. The possibiHty of an error in the

observation made on the second day in this

series suggests itself, since it seems most

reasonable that the function shoidd be a curve such as that so strongly indicated by the

results of the first series. In the series of runs at 10 percent of carbon

cUoxide, the gross output of the boiler decreased

from 136,000 to 70,850 Btu/hr, at the constant

stack temperature of 600° F, owing to soot deposited during 8 days at the stand-by condition. With this soot deposit and the initial firing rate, the output was 127,400 Btu/hr and

the efficiency was 63 percent mstead of the

initial 67 percent, whereas the flue-gas temperature was 790° F instead of the initial 600° F. The soot deposited during the 8 days was

estimated to be about ^2 in. thick. A sample

averaged approxunately 10 mg/in.^ of the heating surface from which it was scraped. These

figures are not considered sufficiently precise to be a basis for calculations.

In the series of tests at 8 percent of carbon

dioxide, the output of the boiler decreased from

115,400 Btu/hr to 84,230 Btu/hr owmg to soot deposited during 5 days. With this soot deposit and at the initial firmg rate, the output

140

130

m 120

110

^ 100

0.(-o 90

80

< >i

EFf ICIEN

i 2

ITPUl

3 4 5 6 7 ELAPSED TIME (DAYS)

100

90

80

70

60 >z

LlI 50 o

40

10

Figure 1.

Effect of soot on boiler out- put at constant stack temperature

{600° F).

Soot was deposited by continuous operation at stand-by condition. Deposition of soot and

daily observation of output were made with

10 percent o f carbon dioxide in flue gas. Points with subscript "1" were obtained with initial

011 rate with soot on heating surfaces. Points with subscript "2" were obtained with initial

oil rate after cleaning soot from heating surfaces.

[2]
Figure 2.

Effect of soot on boiler out- put at constant stack temperature

(600° F).

Soot deposited by continuous operation at stand-by condition. Deposition of soot and

daily observation made with 8 percent of car- bon dioxide in flue gas. Points with subscript "1" were obtained with soot on surfaces but

with initial oil rate. 31-m

140

130

20

110

b 100 aO 90

80

» i FICIE

-i NCY' d "\0U TPUT

was 103,630 Btu/lir with an efficiency of 60

percent instead of the initial 65 percent and the

stack temperature was 680° F instead of the

initial 600° F. No satisfactory method of specifying or measuring the amount of soot in a thin deposit on heating surfaces has been suggested. Soot

is important primarily on account of its effect on heat transfer; and, since soot can vary in

nature, a density determination, based on a measurement or estimate of thickness and

weight deposited per imit area is of doubtful

value. No attempt was made to measure the

daily deposition of soot, but as a practicable

expedient the effects of successive layers of soot

deposited during successive 24-hr rmis at constant conditions were observed.

V. DISCUSSION OF RESULTS

Assume a plant of 100,000 Btu/hr capacity

in which the efficiency changes from 65 to 64

percent on account of the deposition of soot. The heat liberated in the combustion chamber when the boiler was clean w^ould be

100,000

0.65 153,800 Btu/hr.

After the efficiency changed, the heat delivered to the house would be

0.64X153,800 =98,430 Btu/lir.

The difference, 100,000-98,430 = 1,570 Btu/hr,

would appear largely as additional heat in the

100

90

80 zo

70 ccLU

Q- 60 >o

z

50

UJo

u.u. 40 LU

2 3 4 5 6 7 ELAPSED TIME (DAYS)

10

flue gas. Assummg the heat value of the fuel

to be 20,000 Btu/lb and 20 lb of flue gas per

pound of fuel, we have

153,800X20 = 154 lb of flue gas per hour.

20,000

The specific heat of the flue gas may be taken

as 0.25 Btu/lb deg.

The increase in flue-gas temperature due to the 1 -percent change in efficiency would then be

1,570

154X0.25

= 41° F.

If the initial flue-gas temperature were 600°

F, the flue-gas temperature with sufficient soot on the heating surfaces to change the

efficiency 1 percent might be as high as 640° F. A change of 41 degrees F in flue-gas temperature would probably be unimportant, but

greater changes than 1 percent in efficiency

due to soot are likely to occur in service and

to result in greater changes in flue-gas temperature. If the relation is linear, a change in

efficiency from 65 to 60 percent would result in a 205 degree F change of flue-gas temperature, so that, at constant firing rate, flue-gas

temperature may become excessive due to a

soot deposit capable of causing a small change

in efficiency.

The results of the tests at 10 percent of

carbon dioxide show that sufficient soot to reduce the efficiency from 67.3 to 63.0 percent

at constant firmg rate was sufficient to reduce

the output from 136,137 Btu/lir to 70,840

[3]
Btu/lir at constant flue-gas temperature. This

amounts to 136,140-70,840

15,200 Btu/lu- for each 67.3-63.0

percent of efficiency,

or

15,200

136,140 X100=11.15 percent of capacity for

eacli percent of efficiency.

For the tests at 8 percent of carbon dioxide,

the same figures are 111,540-8-^,230

6,826 Btu/lu- for each 64.0-60.0

percent of efficiency,

or

6,826

jjY'^^^X 100 = 6.12 percent of capacity for each percent of efficiency.

If, after a boiler has been in service for a

time, a layer of soot gathers on the heating

surfaces, the flue-gas temperature will be

higher than it was when the boiler was new and

clean. Then, if an attendant or inspector resets the oil rate to maintam the initial flue-gas

temperature, measured with a thermometer,

in order to comply with some specification or

safety regulation, the capacity of the boiler

may be so seriously reduced that it cannot heat

the house satisfactorily.

An increase in flue-gas temperature, whether

due to a soot deposit or some other cause, will result in an increased draft, and this may affect the operation of the heating plant. For those

types of oil burners which rely upon blowers

for draft, only a small or neghgible effect on

operation would be expected, but the effect on

the operation of those types of water heaters

and heating devices which rely upon chimney

draft miglit be important. Natural draft oil- burning devices are sometimes connected to

existing chimneys which do not furnish sufficient draft. The effect of the soot deposit is, in such

cases, in the right direction to increase the

draft, and this may be a reason why the soot

deposit ceases to increase after attaining a certam tliickness. In these tests, draft was obtained by means of an induced-draft fan; and, since the draft was

maintained at 0.02-in. water gage, data on the

effect of temperature change on chimney draft were not obtained.

It is realized that a boiler-burner combination will generate and deposit less soot under some operating conditions than under others,

that some combinations may be in general

better than others in this respect, and that some

combinations, if properly adjusted, may not

generate or deposit perceptible soot. Nevertheless, these observations indicate that it would

be w^ell for cliinmeys intended for use with oil- burning furnaces or boilers to be designed to

%\'ithstand reasonably high temperatures, since

such high temperatiires may be reached on

account of soot deposited during some period of

maladj ustment

.

This work emphasizes the importance, during

tests of a boiler by any method based on a constant flue-gas temperature, of assurmg

either that no soot exists on the heatmg sur- faces or that the soot deposit has readied an

equilibrium condition by operating the boiler

under the test condition for a time preceding

the observations.

o
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