AlllDD ^13001

NATL INST OF STANDARDS & TECH R.I.C.

A1 11 00993009

Kelly, George E/Energy conservation pote TA435 .U58 N0.79 1975 C.3 NBS-PUB-C 1975 NBS BUILDING SCIENCE SERIES 79

V35r

Energy Conservation Potential

of Modular

Gas-Fired Boiler Systems
The Building Science Series

Tho Ikiildinj; Sciciuo Series iJiHsf'minates technical information (ievoloped at the National Bureau of Standards on

i)ui!(iinf; m;ilerials, eomponents, systems, and whole structures. The Series presents research results, test methods, and ]HTforman(f criteria related to the structural and environmental functions and the durahility and safety characteristics of liuildint; elements and systems. These publications, similar in style and content to the NBS Building Materials and Structures Reports (1938-59),

are directed toward the manufacturing, design, construction, and research segments of the building industry, standards organizations, and officials responsible for building codes. The material for this .Series originates principally in the Center for Building Technology of the NBS Institute

for Applied Technology. The publications are divided into three general groups: Building Systems and Processes;

Health, Safety and Comfort; and Structures and Materials. For further information regarding these publications

please contact the Scientific and Professional Liaison Section, Center for Building Technology, Institute for Applied Technology, National Bureau of Standards, Washington, D.C. 20234.

[See mailing list announcement on last |)age |
Energy Conservation Potential of Modular

Gas-Fired Boiler Systems

7

G. E. Kelly and D. A. Didion

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued December 1975
Library of Congress Cataloging in Publication Data

KeUy, George E., 1944- Energy Conservation Potential of Modular Gas-Fired Boiler Systems.

(NBS Building Science Series; 79) Bibliography: p. Supt. of Docs. No.: C 13.29/2:79

1. Boilers—Testing. 2. Boilers—Efficiencies. 3. Energy conservation. 4. Gas as Fuel. I. Didion, D. A., 1937- joint author. II. Title. III. Series: United States. National Bureau

of Standards. Building Science Series; 79. TA435.U58 No. 79 [TH7471] 690'.021s [697'.07] 75-619338

National Bureau of Standards Building Science Series 79

Nat. Bur. Stand. (U.S.), BIdg. Sci. Ser. 79, 54 pages (Dec. 1975) CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.29:2/79). Price $1.15 (Add 25 percent additional for other than U.S. mailing).
Table of Contents

Page

1* Introduction 1

2. Apparatus ^

3. Instrumentation 10

4. Experimental Procedure 14

5. Calculations 17

6. Experimental R esults 22

7. Discussion 23

8. SI Conversion Units 28

9. References 29

10. Appendix A 4 30

11. Appendix B 33

iii

Energy Conservation Potential .of Modular

Gas-Fired Boiler Systems

G. E. Kelly and D. A. Didion

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. Z02 34

The modular concept of boiler operation was examined in a laboratory test of five gas-fired, cast iron, hydronic boilers.

Four of the boilers, each having an input rating of 85, 000 Btu

per hour, were arranged so that they could either be operated

like a single boiler (i. e. , all of the boilers either on or off) or

as a modular installation in which the boilers are sequentially

fired to match the number in operation with the heating load.

The fifth boiler had an input rating of 3 00, 000 Btu per hour and

was operated as a single boiler installation. Efficiency versus

heating load curves were obtained for the single boiler installation, the four small boilers run like a single boiler and the modular installation operated with and without water flowing through

the "idle" modules. These efficiency curves were then used to

theoretically predict the effect of the modular concept and boiler

oversizing on the seasonal efficiency of gas-fired heating plants.

It was found that under certain conditions the use of a gas-fired

modular boiler installation instead of a single large boiler could

result in considerable energy savings.

Keywords: Boiler oversizing; efficiency versus heating load;

modular boilers; modular concept; seasonal efficiency.

1
1. Introduction

Interest in the efficient heating of buildings has been increasing

as this country heads into an energy shortage. One type of heating plant

which has been advocated as being more efficient than many others is the

modular boiler installation. Proponents of modular boilers base their

claim for higher efficiency upon the "modular concept" of equipment

operation. This concept, which has also been applied to compressors,

engines, fans, pumps, etc., is that many advantages can be gained by

employing several small machines (modules) rather than a single large

machine. These advantages include easier installation and repair,

standby capacity (if one of the modules should become inoperative) and

increased system efficiency. It is this potential for increased

efficiency and thus energy conservation of the modular boiler installation that will be considered in this paper.

In the modular boiler heating plant, the modules are sequentially

fired to match the heat output with the heating load. This differs

from a conventional plant employing a single large boiler, which would

normally handle heating loads that are less than the maximum heating

capacity of the boiler by intermittent operation or "cycling".

It is generally believed that the larger the percentage of time off, the

greater are the off-cycle losses. Such an effect will result in a drop

in efficiency of the l--rge boiler as the heating load decreases. If

this drop is very rapid, the seasonal efficiency of the single large

boiler could be considerably lower than its efficiency at maximum load

because a heating system rarely operates at full capacity during the

heating season. In the modular system, on the other hand, only a single

small boiler need cycle, thereby operating at less than full load

efficiency. The other modules that are in operation, the number depending

upon the heating load, will run continuously at their maximum efficiency.

Whether or not the seasonal efficiency of a modular system is indeed

better than the seasonal efficiency of a single large boiler will be

2
determined by how the efficiency of each system behaves as a function of

heating load. The functional dependence of efficiency on load for the

modular system will in turn depend upon whether controls are employed

to prevent water from flowing through the idle modules.

To test the modular concept of boiler operation, five gas-fired,

cast iron, hydronic boilers were obtained from a leading manufacturer.

Four of the boilers had input ratings of 85,000 Btu per hour*, while the

fifth had a rating of 300,000 Btu per hour. The design and const;; ruction

of the large boiler was similar to that of the small units. The boilers

were installed in an experimental setup in which either the single large

boiler or a modular system consisting of the four small boilers could be

tested at various loads. Since the manufacturer of the boilers normally

used the 300,000 Btu unit as the basic module in modular installations,

the modular system tested represented a scaled down version of a modular

heating plant that might be installed in a commercial building. Neglecting

the valving and instrumentation, which are discussed in detail in the

next section, the experimental setup consisted of a closed loop of pipe

through which water, heated by the boiler or boilers being tested, was

circulated. The heating load was simulated by removing hot water from

this loop and replacing it with cold water. By changing the rate at

which this hot water was replaced by cold, different heating loads could

be achieved. The efficiencies of various boiler systems were measured

at different loads and the results plotted to give efficiency versus

load curves. The systems for which this was done are:

1. The modular system with "primary pumping" in which water was allowed to flow through all the modules.

2. The same system as #1, except that the pilot lights were turned

off in the modules that were not in operation (idle modules) at a given

test load.

"Adherence to the International System of Units has not been followed in

the interest of effective communication with the expected readers of

this paper.

3
3. The same system as #2, except that "secondary valving" allowed

water to flow only through the modules which were either operating

continuously or cycling.

4. The single large boiler.

5. The modular boilers run together as if they were a single boiler,

i.e. all of the modules either on or off.

System #2 was tested to determine if the pilot lights on the idle

modules amounted to a significant energy waste in the modular system with

"primary pumping" (water passing through all modules). It was possible

that, if this turned out to be the case, the pilot lights in system #1

could be penalizing the scaled down modular system considerably more than

they would an installation made up of larger boilers. The efficiency

versus heating load curves for systems #4 and #5 were compared with those

for system #1, the modular system with "primary pumping", and with system

#3, the modular system with "secondary valving", in order to evaluate the

modular concept of boiler operation.

The data obtained from testing these five gas-fired boilers are con- tained in the section on Experimental Results. The Discussion deals with

the implication of these results and the potential effect of the modular

concept and oversizing on the seasonal efficiency of a gas-fired heating

plant

.

2. Apparatus

Boiler Setup

Schematics of the experimental setup are shown in Figures 1,2 and

3, while Figures 4 and 5 are pictures of the actual installation. As can be seen from the piping diagram in Figure 1, when the single large

boiler was being tested, water could be shut off to the small boilers by

closing valves a,b,Cjd, and e. Similarly when the modular installation

consisting of the four 85,000 Btu/hr modules wa^3 operating, water could

be prevented from circulating through the large boiler by closing

valves f and g. In this paper, a test of the modular system with "primary

4
pump-ing" shall mean that water was continuously circulated through all the

modules. A test of the modular system with "secondary valving" shall

indicate that the valve between the return head,er (a,b,c, or d) and an "idle" boiler (i.e., one which is neither operating continuously or cycling) was closed to prevent water from circulating through the boiler.

In addition, during the secondary valving tests, the pilot lights cm the

idle modules were also turned off. This last configuration was examined

to determine the effect of the losses from the idle boilers on the efficiency

of the modular system.

The supply and return headers were covered with a one-half inch thick

layer of foamed plastic insulation in order to reduce the heat loss. Three

lengths of three-inch pipe were employed to increase the water capacity

of the single boiler test setup and the modular boiler test setup to

approximately 41 and A3 gallons, respectively. This was done so that the

test setup would have approximately the same water capacity as similar

heating plants installed in a real building. The purpose of this was to

obtain the proper on - off cycling rate.

Natural gas was supplied to all five boilers by a single header which

can be seen in Figure 2. A combined pressure regulator - gas valve on each boiler allowed for the reduction of gas pressure from 8 or 9 inches

of water column in the header to the 3 1/2 inches of water column in each

boiler's manifold that was recommended by the manufacturer. This valve

also allowed for manual on-off control of the gas to the pilot light and

manual and automatic on-off control of gas to the main burners. It was

this valve that was manually operated to turn off the gas to the idle

modules in test configurations 2 and 3.

A schematic of the breeching and duct work are shown in Figure 3. The draft diverters were standard factory equipment and were supplied with

the boilers. No mechanical draft inducer was used, although two cast-iron

dampers, and D^, were installed to block out the draft to the system

(the single large boiler or the modular setup) which was not in operation.

5
Heating Load Simulation

The heating load consisted of piping losses between S and R and the

heat removed from the system by dumping hot water at I and replacing it

with cold water at J. (see Figure 1).

Since the maximum heating load which a system can handle equals its

maximum output under steady state conditions, simulating the maximum

heating load for the single boiler or the modular system corresponded to

removing, by the above- means, as much heat as was being transfered to the

circulating water by continuous operation of the single large boiler or the four small boilers, respectively. As will be explained in the section

on experimental procedure, the flow rate was adjusted by means of valve h

to give a 20 F degrees temperature rise to the water passing through the

heating plant under maximum heating load conditions. Thus simulating the

maximum heating load of either the single boiler or modular boiler system

was equivalent to replacing hot water with cold water at such a rate that

the temperature of the circulating water was lowered by 20 F degrees as

it traveled from the end of supply header S to the entrance of the

return header R. This resulted in a constant temperature dlst jrlbution

throughout the system and steady state operation. To achieve this 2'0 F

degree temperature drop, a motorized valve K on the hot water discharge

pipe 1 and an electronic proportional controller were employed. The

signal from a two junction thermopile, which measured the temperature

difference between points L and N, was fed into the controller which adjusted the motorized valve to maintain a given temperature difference

between L and N. As an example, 180° F supply water underwent a temperature drop of about 1.5 F degrees in passing through the piping between

the end of the supply header and point L. Thus the controller would be

adjusted to provide an additional 18.5 F degree decrease between L and N

in order to obtain a total temperature drop between S and R of 20 F degrees.

The primary reason for employing such a complicated control system

however, was not to simulate the maximum heating load, since the application

described above could have been handled using a manually operated valve.

6
The real function of this system was to simulate a heating load when a boiler or boilers was cycling on and off. It was decided to accomplish

this for a load equal to X% of the maximum heating load by replacing

hot water with cold water in such a manner that the temperature of a volume of water entering the return header would bej^— • (20)Jf degrees

lower than its temperature upon leaving the supply headed. This would

have the effect of removing the same amount of heat from each volume of

circulating water and would result in the instantaneous rate at which

heat was extracted from the system (the instantaneous load) being constant

with time. Since the supply temperature V70uld, however, be changing with

time, it was necessary to vary the rate at which hot water would be re- placed by cold to achieve this goal. Thus if the temperature of the water

at L increased (decreased) the controller would cause the motorized valve

to decrease (increase) the rate at which cold water replaced hot water,

since less (more) cold water was now needed to lower the temperature

of a volume of circulating water by the same number of degrees. The

effect of this was that, in simulating a load equal to X% of the maximum

heating load, the temperature of a volume of water would first be decreased

approximately 1.5 F decrees in passing through the pipe between S and L

and then by another I ) (20) - 1.5 _(ioo) F degrees between L and R.

If the above process for simulating the heating load worked exactly

as described, a plot of the temperature of the water entering the return

header against time would have the same profile as a plot of the temperature

of the water leaving the supply header against time, except that the former

would be displaced by the time it took a volume of water to travel from

S to R and would be

[j~YoO~ " ^^^^^ ^ degrees lower in temperature. In actual

practice, this turned out to be approximately the case, although in tests

involving cycling, the instantaneous load did tend to vary slightly with

time in a cyclic manner. The result was a slight flattening out of the

high temperature peaks and a slight filling in of the low temperature

valleys. Figure 6 is a plot of a typical supply and return temperature

and an ideal return temperature corresponding to an instantaneous heating

load which is independent of time.

7
Control

A solid state sequence controller supplied by the manufacturer was ^ used to cycle on and off both the single boiler and the modules in the

modular boiler installation. As used in a normal modular heating plant,

the controller would accept signals from a temperature sensor in the supply

header at S and from an outside temperature sensor. It would then sequentially turn the modules off as the supply temperature increased or sequentially turn the modules on as the supply temperature decreased.

In addition, the controller used the signal from an outside tenperature

sensor to in effect raise the supply temperature setting as the outside

temperature decreased in order to avoid the undesirable effect of having

lower supply temperatures at higher loads.

To avoid the questions of how the heating load was related to the

outside temperature and what effect lowering the outside temperature

had on the supply temperature, all tests on both the modular system and the

single large boiler were run with the cycling unit having approximately

the same on and off supply temperature settings. This was done by bypassing

the outdoor sensor and using a single step on the controller. The normal

temperature control differential was 10 F degrees, with the on-setting at

175°F and the off setting at 185°F. However, the performance of the

controller was such that while fairly repeatable control was achieved

during any single efficiency test, the on and off set points, and to a lesser extent the control differential, tended to vary by +2F degree from

day to day. Thus for a 10 F degree control differential, the cycling

boiler might go on (off) at 173°F (183°F) on one test and on (off) at

177°F (187°F) on another test. In addition, while most tests had a control

differential of approximately 10 F degrees, several tests were run at

control differentials of 8 and 12 F degrees. This day to day variation of • the controller, however, did not seem to have a large effect and repeating

'

a test always gave a result that was within the estimated experimental

error.

8
3. Instrumentation

The temperature rise of the circulating water across the boilers was measured with an eight junction copper-constantan thermopile and a potentiometer-type, strip chart recorder. The range spans used on the re- corder during these tests were 1, 2 and 5 mv and each had an accuracy of

0.25% of span. The accuracy of these spans was verified with a laboratory

type potentiometer prior to the start of testing. The ends of the thermopile were inserted in stainless steel wells located at R and S (see

Figure 1). Each well had an outside diameter of three-eights of an inch,

extended approximately 6.5 inches into the water pipe and was filled with

light machine oil. Right angle bends were located one-half foot upstream

of R and S in order to mix the circulating water and thereby obtain a more

average water temperature at R and S. (The bend upstream of R can be seen

at N in Figure 1; the one upstream of S is, however, not shown in this

schematic.)

The supply and return temperatures at S and R respectively, and the

temperature of the natural gas at the gas meter were also measured using

copper-constantan thermocouples. The signals from these thermopiles were

fed into a multi-pole switch which was used to select which temperature

was to be displayed on a dial-type indicating potentiometer. This instrument was checked prior to the start of testing and was found to be accurate

to 0.5 F degrees in the temperature range of interest. In order to have a continuous record of the supply and return temperature, other thermopiles

at S and R were used in conjunction with an ice bath and a two pen, strip

chart recorder. Although the accuracy of the readings obtained on this

recorder were only 1°F, this was sufficient for monitoring the behavior

of the supply and return temperatures with time.

The meter used to measure the amount of water passing through the

boiler was all/2 inch turbine meter having a linear response in the

flow range from 17.4 to 174 gallons per minute. The flow rates en- countered in the various efficiency tests ranged from 21 to 27 gallons

9
per minute and were thus well wlthip this linear response region.

Magnetically generated voltage pulses from the turbine meter were fed

into a flow totalizer. The meter was factory calibrated and had a reported accuracy over its entire range of +0.U7o and a reported repeatability of +0.1%. Although these figures provided by the manufacturer

will be assumed correct for this experiment, a check was made to

determine if the turbine meter and totalizer were working correctly.

This was done by passing water through the turbine meter and measuring

the length of time required to fill a small tank.

An electric clock was used to measure the duration of a test. The

clock, the chart motors on the two strip chart recorders and flow

totalizer were started and stopped simultaneously by means of a single

switch and several relays.

The meter employed to measure the gas used was an ordinary household gas meter which was donated to NBS by a local gas company. The

meter's register had 0.5 and 5 cubic foot proofing dials and 1,000,

10,000, 100,000 and 1,000,000 cubic foot metering dials. It was calibrated in the Fluid Meter Section of NBS and was found to have the ratio

of actual to indicated volumes shown in Table 1. In all except two

efficiency tests, the amount of gas used by the system being tested was determined by counting the number of revolutions made by the 0.5 and 5 cubic foot proofing dials. In the two exceptions, the tests were run

for the period of time required for the 1,000 cubic foot metering dial

to make one revolution. This latter method, however, required too much

time, especially at low heating loads. The gas pressure at the meter

was measured using a U-tube manometer and an aneroid barometer. The

manometer and the meter had minimum scale divisions of 0.05 inches of

water and 0.02 inches of mercury, respectively. A check was made on the aneroid barometer by comparing it with a mercury barometer; the

former was found to agree with the latter to within 0.05 inches Hg.

10
A MSA Lira Infared Analyzer, Model 300*, was used to measure the

percentage by volume of CO^ in the flue gas from each boiler. The

analyzer was calibrated using sample gases having a known concentration

of C0„. Measurements were taken iust below the draft diverter and the

2 flue gases were passed through a condensate trap and a float-type flow

meter before entering the analyzer. The flow meter assured that the flow

rate remained within the limits recommended by the manufacturer. After

the flue gas had passed through the analyzer it was returned to the

breeching.

The test setup was located in a fully air-conditioned laboratory

which could easily handle the heat lost by the boiler jackets, breeching

and piping. Because of this, the room temperature was measured using

only a single chromel-alumel thermocouple in all tests and a sling

psychrometer in a limited number of tests. The thermocouple was located

approximately three feet from the back of the large boiler and was shielded against radiation. The sling psychrometer was used to measure

the wet and dry bulb temperature at different locations around the test

setup. In all measurements taken with the thermocouple and th^ sling

psychrometer, the dry bulb temperature was always within +4 degrees F

of 79°F and the wet bulb temperature was always within +3 degrees F of

61^F. •

The flue gas temperature of each boiler was measured with a single

chromel-alumel thermocouple having a brazed junction-. It was located

approximately in the center of the flue and about two inches from the

bottom of the draft diverter. The signals from these thermocouples,

along with the signal from the thermocouple used to measure the room

temperature, were fed into a multi-pole switch. This switch was used

to select the temperature which was to be displayed on a dial-type

indicating potentiometer. Prior to the start- of testing, the indicating

*Trade names are used in this report as a means of clear identification

and neither constitute nor imply endorsement by the National Bureau of

Standards

.

11
potentiometer was checked using a more sensitive laboratory-type

potentiometer. The flue gas temperature measured in this manner was

found to be within 2% of the average flue gas temperature as determined

by using a bead-type thermocouple and taking transverse readings across

the duct. [1]

During the period of testing, the higher heating value of the gas

varied -from an absolute maximum of 1033 BTU per standard cubic foot=^ to an absolute minimum of 1004 BTU per standard cubic foot, as detemined

from hourly measurements taken by the local gas company at their central

plant. This agreed well with the value of 1011 BTU per standard cubic

foot which was obtained by the Air Pollution Analysis Section of NBS in

a single test of the gas higher heating value. This test, however,

revealed that the gas^ which came from the NBS mains was exceedingly dry,

having a water vapor content which was less than 0.1% by volume. Because

the energy content per cubic foot of dry gas at standard conditions**

30

is higher by a factor of (~ r—r) than the energy content of a 30-0.517

standard cubic foot of gas [2], the following higher heating value was

used in calculating the efficiency results present in this paper:

30

(1018 + 15) ( 3Q : 0^517 ) = 1036 + 15 (1)

BTU per cubic foot of dry gas at standard conditions.

*A standard cubic foot of gas is 1 cubic foot of gas saturated with water

vapor and measured at a temperature of 60° F and a pressure of 30 inches

of Mercury [2 ]

.

**Standard conditions are defined to be a temperature of 60°F and a pressure of 30 inches of Mercury.

12
4. Experimental Procedure

Before the first efficiency test was performed, the gas regulator

on each boiler was adjusted so that the heat input to each boiler was within 2% of its rated input and the pressure regulator on the cold

water feed line was adjusted to obtain a water pressure inside the

boilers of approximately 35 psig. In addition, since the manufacturer

recommended that the water passing through each boiler have a 20 F

degree temperature rise under steady state operation, the flow rate of

the circulating water was adjusted to give this recommended temperature

rise in the modular system under maximum heating load simulation. This was accomplished by allowing the four small modules to run continuously

and adjusting valve h and the electronic proportional controller ( and

thus the motorized valve K) until: (1) a steady state supply temperature

of 180°F was achieved and (2) the thermopile across the heating plant

indicated a temperature difference of 20 F degrees. Without changing

valve h, the single boiler system was then run with the proportional

controller adjusted to also give a steady state supply temperature

of about 180°F. It was found that the larger pressure drop across the

single large boiler reduced the flow rate. This was, however, exactly

compensated t>y the decreased heating capacity of the single boiler to

again give a 20 F degree temperature rise to the water passing through

the single boiler. Thus a single setting of valve h resulted in roughly

a 20 F degree temperature rise in both the modular boiler system and the

single boiler system at their respective maximum heating levels. This

valve setting was therefore maintained throughout the entire testing

period.

The experimental procedure, employed in determining the efficiency

of the various systems at different heating loads is most easily

described by dividing the tests into two categories: steady state tests

and non-steady state or cycling tests.

13
steady State Tests

In the steady state tests, the simulated heating load was such that - the boiler or boilers which were operating ran continuously at their rated

input. The result was that the temperature rise through the heating plant was exactly cancelled by the temperature drop caused by the simulated

heating load and thus the temperature distribution throughout the system

remained constant. This occurred lat the maximum heating load for each of

the five configurations discussed in the introduction. In addition, it

occurred at approximately three-quarters, one-half and one-quarter of the

maximum heating load for configurations 1, 2 and 3; that is, it occurred

whenever the heating load exactly cancelled the heat input from the steady

state operation of three modules, two modules and one module, respectively.

In preparing for a steady state test, the boiler or boilers that

were to be run at their maximum heating capacity, were turned on and the

supply temperature was allowed to reach approximately 180°F. The

electronic proportional controller was then adjusted so that the simulated

heating load equalled the heat output of the heating plant and a constant

supply temperature between 180°F and 183°F was achieved. The system was

then run for at least an hour in order to reach equilibrium. During

this period, fine adjustments were made, when necessary, to the controller

to keep the supply temperature within the above range. When the supply

temperature no longer appeared to be changing, its value was noted and

the test was begun. During the test, the system was required to maintain the supply temperature within 3 F degree of this initial value,

without further adjustment of the electronic controller. In the

majority of these tests, the supply temperature actually varied less

than one F degree.

The shortest steady state test lasted 53 minutes, while the majority

of them ran for a period of 1 hour. The supply and return temperatures,

the temperature rise across the boilers, the duration of the test, the

amount of water passing through the boilers during the test, the barometric

pressure, the gauge pressure and temperature of the gas at the gas meter,

and the volume of gas used as indicated by the gas meter were measured

14
in accordance with the discussion in the Instrumentation Section. This

information was used to determine the efficiency of the boiler con- figuration by the calorimetric method [1]. A discussion of this method

and the calculations involved are given in the next section. In addition,

the" stack temperatures, the percentage of CO^ in the flue gas, the

room's dry bulb temperature, and, in some cases, the room's wet bulb

temperature were also measured as described in the section on Instrumentation. Although this was normally done for the purpose of monitoring

the installation, information obtained on two tests was used as a check

on the experimental procedure. This was accomplished by calculating

the stack losses for the modular system and the single large boiler,

when both were being run at their respective maximum heating loads.

By using these losses plus estimated jacket losses to perform heat

balances [3], an independent determination of the maximum efficiency

for each system was made which was then compared with the efficiency

determined by the calorimetric method. The calculations involved in

determining the efficiency of each system by the heat balance method

are given in Appendix A.

Non-Steady State Tests

Whenever the output of the heating plant was not instantaneously

cancelled by the heating load, the supply temperature varied with time,

and cycling occurred . In systems 1, 2 and 3, a single module would

cycle on and off while in systems 4 and 5, the single large boiler

and all four of the modules, respectively, would cycle.

In tests involving cycling or non-steady state operation, the boiler

or boilers that were to be tested at a specific load were run until the

supply temperature reached 180°F. The electronic proportional controller

was then adjusted to give the desired heating load and the system was allowed to operate until a repeating pattern was observed in the curve giving the temperature rise between R and S with time. This warmup

period was usually at least one hour. After it was completed, the clock,

recorders, and flow totalizer were started and the efficiency test begun

as the cycling boiler came on (or went off). During the test, the amount

15
of gas used, as indicated by the gas meter, was determined as discussed

in the Instrumentation Section. With the exception of two long tests

which were run until 1000 cubic feet of gas was consumed, the efficiency

tests lasted between 53 minutes and 111 minutes. The number of cycles

completed by the cycling boiler during the different tests, varied

from 3 to 15. A test was ended by turning off the clock, recorders and

flow totalizer when the cycling boiler again came on (or went off) , after

having completed a certain number of cycles. At the start of each test

and just before the end of each test, the barometric pressure, the gauge

pressure of the gas at the gas meter and the temperature of the gas at

the meter were measured. These were then averaged to obtain a mean value

of barometric pressure, gauge pressure and gas temperature for each test.

This information, along with the results from the gas meter, was used to

determine the number of standard cubic feet of gas used during a test.

This calculation, as well as the calculations involved in determining the

efficiency of a given configuration at a specific load, are discussed in

the next section.

The room temperature, stack temperatures and percentage of in

the flue gas of each boiler were also measured. This information, however,

was only used to monitor the behavior of the installation. The supply

temperature and return temperature were continuously recorded throughout

the test by means of the two pen recorder mentioned in the section on Instrumentation.

5. Calculations

When the calorimetric method is used to determine the efficiency of

a hot water boiler installation, the efficiency may be defined as heat

transferred to the circulating water during the testing period (Qq^|-)

divided by the energy input to the installation in the same period of

time (Q. ). xn

In the case of steady state operation, the only constraint on the

testing period is that it be long enough to obtain an accurate measurement

of Q^^j. and Qj^^' In addition, since the supply temperature, return

16
temperature and flow rate are constant with time, the heat transferred

to the circulating water may be written:

Q = M "C AT , (2) out * where C is the average thermal capacity of the water as it passed through

the boiler installation, M is the total mass of water flowing through the

boiler or boilers during the testing period, and AT is the temperature

rise of the water. Since the thermal capacity of water is, for all

practica.1 purposes, linear over the range of water temperature encountered

in this study, one can write:

C = C (T) , where C(T) is the thermal capacity of water at the temperature T, and — _ supply temperature + return temperature

2 is the average temperature of the water as it passes through the boiler

installation. Substituting the above equation into (2), we obtain:

Q ^ = M C(T) AT (3) out

This equation was used to determine the heat transferred to the water

during the steady state tests.

With tests involving non-steady state operation, Q is more diffiout

cult to determine. This is true even if the heating system is cycling in

a repeating pattern (i.e., with a cyclic supply and xeturn temperature

as illustrated in Figure 6), which was the case in 'the non-steady state

efficiency tests. Again using the fact that the thermal capacity of

water is very nearly linear over the range of temperatures encountered

in the tests, the heat tranf erred to the circulating water during the

testing period is:

0*^f /Tp(t) + T (t p cl-^ -± \ |jr^(t +r) - T^(t)| dt (4)

to

Q ^ = M

out

17
where M is the mass flow rate and is a constant for any one test, ^ is time required for a mass of water to flow from R to S,

T (t) is the temperature of the water at R at the time t,

R

T (t) is the temperature of the water at S at the time t,

t is the time at the start of the test^ and

o t^ is the time at the end of the test.

Equation (4) can be simplified by observing that while

D

Tg (t +T) - (t)^

varies over a wide range,

C

changes very little. In fact, in every test

was always within + 0.4% of C(T), where T can now be defined as:

f ° J t

in equation (4) is replaced by C(T), only a very small error is introduced

(see Appendix B) and a considerable simplification is achieved. Making

this substitution,

t

J Tg(t + r)dt - J

Tj^(t)dtJ (6)

t

which can be transposed to:

c-MC(T)

r P f t + r p

t -

T (t')dt' + V T (tOdt' - - %(t)dt (7)

^4o4t S

J

S J^^ R J

-t

18
If the test period is tak^n to be the time between the occurrence of two

identical events in two separate cycles.

+ r pt^ + T

Ts(t)dt =1 Tg(t)dt (8)

because of the cyclic nature of the supply temperature. Equation (7)

then becomes:

Qout :^>iC(T)| ^[T^(t) - Ti^(t)Jdt (9)

- MC(T) (10a)

1 r

t f

where At . --_.J

[T^ (t) - T^Ct) ]dt (10^)

and use is made of the fact that (K) (t^.- t ) = M.

f o Equation (10a) , which is very similar to (3) , was used to determine Q^^^

in all efficiency tests involving cycling. In almost all cases, the

test was begun when the cycling boiler(s) came on and ended several

cycles later when the same boiler(s) again cycled on. The quantities

T and AT were found by evaluating the integrals in (5) and (10b) by

determining the area under the curves: .

T^(t) + Tg(t +f

)

2 and (T (t) - T (t)), respectively, and then dividing by the time (t - t b R to

The curves were obtained from the strip chart recorders and the area mider them was approximately determined by counting squares. •

19
The energy input during the testing period (Q^^^) w^s found by

determining the number of cubic feet of gas at standard conditions usad

during the test and multiplying this by the higher heating value in (1)

.

The number of cubic feet of gas at standard conditions that were burned

were found by using the ideal gas law to relate the volume of gas which

the meter indicated was used to the volume the gas would occupy at

where is the volume of gas at standard conditions which was used,

V is the volume of gas used according to the meter in cubic feet.

R is the appropriate ratio of actual to indicated volume (see Table 1)

T is the gas temperature at the meter in degrees R, and

m

is the gas pressure at the meter in inches of Hg.

In the steady state tests, the instantaneous heating load equaled

the rate at which heat was transferred to the water by the boiler or boilers

being tested. This was not, however, true in the non-steady state

efficiency tests. In these tests, which involved a repeating pattern of

operation, the heat removed from the water equaled the heat transferred

to the water only over a period of time equal to one or more complete

cycles. Although the rate at which heat was removed was fairly constant

with time, there was a tendency for it to have a slight cyclic fluctuation,

as mentioned in the section on apparatus. This was primarily because

the electronic controller was often a little slow in responding to a very rapid change in the supply temperature, such as occurred when a cycling boiler first came on. As a result of this, an average heating

load, <£>(t)>, was defined, which is given by:

s tandard condit ions

.

The formula used was:

(11)

m

1 Q <X(t)> H X(t)dt = out (12)

(tf - to) (tf - to)

20
where £(t) is the instantaneous heating load at time t, and the last

equality follows from the fact that the heat removed during the test

f^f

1 l(t)dt must equal the heat output, Q ^, since (t^ - t ) is the

I out r o

t o time required for several complete cycles. Since equation (12) is true

for the steady-state tests as well, <5,(t)>will be used in plotting the

results for both cycling and non-cycling tests.

6. Experimental Results

The experimental results which were obtained by the calorimetric

method are shown in Figure 7,8, and 9. The dotted lines in these figures

represent an extrapolation of the data down to zero load. The curves

are not normalized since both the single boiler and the modular system

had approximately the same maximum efficiency. Figure 7 gives the

efficiency versus load curves for systems 1 and 2. As mentioned in

the Introduction, system 2 was tested to determine if the pilot lights

were penalizing the scaled down modular system in system /^l more

than they would an installation made up of larger boilers. This

turned out not to be the case since the two curves in Figure 7 are nearly Identical.

Figure 8 contains a plot of efficiency versus heating load for

system //3, i.e. the modular system with secondary valving. In this

test, the flue losses were minimized by shutting off the water and gas

to the modules which were neither operating continuously or cycling

(idle modules). The unusual drop in the curves at a heating load

approximately equal to 25% of the maximum heating load, is due to the

fact that when the load on the second module is small, the flue losses,

jacket losses and lower efficiency of this module tend to decrease

the efficiency of the system. Similar although smaller drops in efficiency can be expected to occur when the heating load is approximately

equal to 50% and 75% of the maximum heating load. These were not,

however, experimentally investigated because the decreases in efficiency

were expected to be less than the experimental error. In addition, these

discontinuities will become smaller as the number of modules in an

21
installation increase. The efficiency versus load curves for system //4,

the single large boiler, and system #5, the modular system run like a single boiler, are shown in Figure 9. There was very little difference

in the results obtained for these two systems.

The efficiency versus load curves for systems #1, #3 and #4 and

#5 are compared in Figure 10. It can be seen that the curve obtained for

the modular system with primary pumping is almost identical to the curve

for the single boiler and the curve for the modular system run as a single boiler. There is however considerable difference between the re- sults from these three systems and those from the modular system with

secondary valving. The latter had a higher efficiency over the entire

range of heating loads tested.

The efficiencies of the single boiler and modular boiler installations

at their maximum heating loads were respectively found to be .765 and

.775 by the heat balance method described in Appendix A. This agrees

well with the efficiencies which were obtained at the same loads by the

calorimetric method (see Appendix A) .

'

7. Discussion

Since nearly identical efficiency versus heating load curves were

obtained for the modular system with primary pumping (system #1) and

the modular system run like a single boiler (system #5), the modular

concept of boiler operation proved no better, in our test setup with

the water flowing through the idle boilers, than the conventional method

of operation (i.e. all on or all off). This result is also expected to

hold for an installation containing more than four of the 85,000 BTU/hr

gas-fired modules tested. The reason for this is that while the losses,

at a given percentage of the maximum load, in a modular installation with

primary pumping increase as the number of modules in the installation

increase, the heat output tends to rise proportionally. For example,

>at approximately 25% of the maximum load, a modular installation with

primary pumping containing 16 boilers will have losses from twelve idle

modules and output from four operating ones. This is a ratio of three

22
idle modules per operating one, which is the same as that found in our test setup at 25% of the maximum heating load.

A comparison of the efficiency versus lodd curves for system #1

and system //4 indicates that, for this boiler design and this size range,

there was practically no difference in efficiency between the modular

system with primary pumping and the single boiler tested. Whether or not a modular installation containing a larger number of 85,000 BTU/hr

gas-fired units and having primary pumping would be better than a single

large boiler would depend on the efficiency versus load curve of the

single large boiler choosen for comparison. If the efficiency of this

single boiler dropped slower than the efficiency of the 85,000 BTU/hr

module as the percentage of the maximum heating load decreased, the

single boiler would be more efficient. If it dropped faster, the modular

system with primary pumping would be more efficient. In the latter case,

however, the increase in system efficiency is not attributable to the

modular concept since operating the modular system like a single boiler

would achieve the same result.

In our test setup, the modular concept of boiler operation resulted

in an improved system efficiency when the modules operated independently

of each other. This occurred in system //3, the modular system with

secondary valving, since in this case the idle boiler losses were minimized

and did not greatly affect the efficiency of the operating units. The

purpose of testing this system was to simulate the employment of self

igniters, automatic valves, and pumps, which would light the gas on a module and circulate water through it only when the output from that

module was needed. The heating system could be arranged as in the NBS

test setup or as shown in Figure 11. The latter arrangement is usually

referred to as "primary-secondary pumping" [4,5], and differs from system

#3 in that; 1) the boilers, in our experimental setup were in series with

the primary water loop whereas in Figure 11 they are in parallel with

the primary loop, 2) there was only a single pump in the NBS test setup

and consequently the flow rate through the operating modules increased

as the water to more and more idle boilers was cut off, and 3) in system

23
#3 the pilot lights on the cycling module operated continuously and the

water circulated through such a boiler even when the unit was off. It

is, however, felt that these differences will not have an appreciable

effect on efficiency and therefore the tests results from system #3 are also representative of an installation employing primary-secondary

pumping.

As discussed under Experimental Results, the efficiency versus load

curve obtained from system //3 is higher than the curve obtained from the

single bpiler (system #4) and the modular system run like a single boiler

(system #5). In the region where data was obtained the difference in

system efficiencies varied from about 1% at 60% 'of the maximum load to

between 10% and 14% at 10% of the maximum load (see Figure 10). To

illustrate how this might affect the seasonal efficiency of each system,

test data from a report [6] published by the Engineering Experiment

Station, The Ohio State University, Columbus has been used. These data,

which are shown in Table 2, give the percentage of time that a given

heating load (or cooling load) existed in the Legal Aids building and

the Devonshire School building during the testing period. Both of

these buildings were located in Columbus, Ohio. A plot of their load

distribution function, f(2-), is contained in Figure 12, where f(l)Mi

is the fraction of time the load wa^ between I and £ + A£. These graphs

show that the boiler in the Legal Aids building was fairly well sized

for the maximum required heating load and that the one in the Devonshire

School building was slightly oversized. The load pattern of the latter

building was rather unique in that, while heating loads up to 90% of the

boiler capacity were occasionally required, during 68.8% of the testing

period the heating load was between 0.0 and 30%. In contrast, the Legal

Aids building had a heating load between 30 and 60% of its boiler's

capacity for 50.8% of its testing period.

Although the boilers in both buildings had a considerably larger

heating capacity than our modular boiler installation, it was assumed

that the curves in Figure 10, for the modular system with secondary

valving and the modular system run like a single boiler, are respectively

24
representative of:

1) a modular system with primary-secondary pumping containing

four large gas-fired modules and 2) a single gas-fired boiler system,

either of which might be installed in each of these two buildings. The

seasonal efficiencies of these two hypothetical systems were calculated

by multiplying the efficiency of each system at a given heating load,

by the percentage of time at that heating load, summing over all heating

loads and dividing by the total percentage of time that a heating requirement existed. Five cases were investigated: 1) both heating systems

.installed in the Legal Aids building and both having the same capacity

as the existing boiler in that building, 2) both systems installed in

the Legal Aids building but both having been oversized by 50%, 3) both

systems installed in the Legal Aids building but both having been oversized by 100%, 4) both systems installed in the Legal Aids building but

both having been oversized by 200%, and 5) both heating systems installed

in the Devonshire School building and both having the same capacity as the existing boiler in that building. Table 3 summarizes the results

obtained. For the Legal Aids building, the difference in seasonal

efficiency between the two systems ranged from approximately 4% when the

systems were well sized to roughly 10% when the systems were 200% oversized. It should be noted in the latter case, however if many more

smaller modules had been used in the modular installations, its seasonal

efficiency could probably have been increased from 65.5% to between

72 and 75%. This would have resulted in a difference in seasonal

efficiency between the modular boiler system with primary-secondary

pumping and the single boiler of between 16 and 19%. For the Devonshire

School building, the two systems differed in seasonal efficiency by

approximately 9%. Again if more modules had been used, this difference

could have been increased to between 11 and 15%. Comparing the calculated

seasonal efficiencies in Table 3 for cases 1 and 5, it is apparent that

the skewed load pattern in the Devonshire building caused about a 10%

lower seasonal efficiency for the single boiler and about a 5% lower

seasonal efficiency for the modular system containing four modules.

25
The above calculations indicate that employing a gas-fired modular

boiler installation with primary-secondary pumping instead of a single

large boiler could result in considerable energy savings - especially

when there is a chance that the system might be oversized or the building

to be heated has a load distribution function whose mean value occurs at

a small percentage of the maximum heating requirement. It must be

emphasized however, that this example does not recommend or condone the

installation of oversized heating plants, but only illustrates that in

the unfortunate case where oversizing does occur its effect is less

detrimental to the modular system with primary-secondary pumping than

to the single boiler system.

Acknowledgements

The author would like to express his appreciation to Mr. James D. Allen

for his assistance in reducing data and in performing several of the

efficiency tests. The help of Messrs. Walter M. Ellis and John W. Grimes

in setting up the instrumentation and the support and encouragement

during this investigation of Dr. James E. Hill are gratefully acknowledged.

26
8. SI Conversion Units

In view of the present accepted practice in this country for buildin

technology, common U.S. units of measurement have been used throughout

this paper. In recognition of the position of the United States as a signatory to the General Conference on Weights and Measures, which gave

official status to the metric SI system of units in 1960, assistance

is given to the reader interested in making use of the coherent system

of SI units by giving conversion factors applicable to U.S. units used

in this paper.

1 inch = 0.0254 meter (exactly)

1 inch of water at 60°F = 248.8 pascal

1 psi = 6894.1 pascal

1 cubic foot = 0.02832 cubic meter

1 gallon = 0.003785 meter"^

1 BTU = 1055 joule

1 pound = 0.4536 kilograms

Temperature, °C = (°F - 32) 1/8 (exactly)

27
9. References

USA Standard for Gas-Fired Steam and Hot Water Boilers, USAS

Z21.13, 1967.

Gas Engineers' Handbook edited by S. H. Graf (McGraw-Hill, New

York, 1934).

A. J. Johnson and G. H. Auth, Fuels and Combustion Handbook

(McGraw-Hill, New York, 1951).

Sotirious Contos, Economics of Primary - Secondary Pumping of

Modular Heating Plants, ASHRAE Journal, December 1972, p. 34.

C. G. Segeler, Match Heating Loads With Modular Boilers, Power,

June 1972, p. 102.

A Study of Commercial Buildings Heated with Gas, Report EES 269x,

Engineering Experiment Station, The Ohio State University, Columb

(Project Supervisor: Charles F. Sepsy, Department of Mechanical

Engineering)

.

28
10. Appendix A

In this appendix, the steady state efficiencies of the single large

boiler and the modular system, operating at their respective maximum

heating load, are determined by performing heat balances on the two

systems. The heat balance method sets the heat input per cubic foot of

gas at standard conditions equal to the heat output per cubic foot of

gas at standard conditions. Since the latter consists of useful heat

(heat transferred to the circulating water) and losses, the following

equation is obtained:

useful heat output per cubic ft. of gas at standard conditions =

= higher heating value of gas at standard conditions - losses per cubic

of gas at standard conditions. (Al)

Dividing both sides of (Al) by the higher heating value of the gas, the

efficiency of the heating plant is found to be:

. , Losses per cubic ft. of gas at standard conditions efficiency = 1 - r-r-r r——: ^ —^—^ Tn~- (A2) higher heating value of gas at standard conditions

The losses in a boiler may be broken down as follows [3]:

' 1. dry flue gas loss,

2. loss due to evaporation of water formed from burning

hydrogen in the fuel,

3. radiation and convection losses from the boiler jacket,

4. loss due to heating the moisture in the entering

combustion air,

5. loss due to moisture in fuel,

6. loss due to CO and other combustibles in the flue gases,

7. loss due to mconsumed carbon in refuse.

In our tests, the major losses were items 1, 2 and 3. The losses from

item 4 will be neglected here, since for a dry bulb temperature of 79°F

and a wet bulb temperature of 61°F, the amount of water vapor which is

present in the air has only a very small effect on the efficiency. The

loss due to the presence of water vapor in the fuel need not be con29
sidered because, as mentioned in the section on Instrumentation, the gas was extremely dry. In addition, gas burns very clean and produces little

CO when there is excess air present, as was the case. Thus the losses

from items 6 and 7 are also neglegible and will not be considered.

Although the single large boiler and the modular system will be

discussed separately, the following information is common to both systems

and will be used in determining the losses from items 1, 2 and 3:

a. the heating value of the gas is 1036 BTU per cu ft of gas at

standard conditions,

b. the rooms' dry bulb temperature is approximately 79°F,

c. approximately 0.10 lb of water is formed upon burning one cubic ft of gas at standard conditions,

d. radiation and convection losses from the boiler jacket for a modular heating plant are approximately 1.5% of the energy

input [4].

The uncertainity associated with these values and the flue gas temperatures

and CO^ concentrations to be given below will not be considered.

The Single Large Boiler

In the steady state test of the single large boiler, the flue gas

temperature was found to be 525° F and the percent by volume of in

the flue gas was 8.8%. From Figures 13 and 14, which are nomographs

obtained from reference [3], we find that the dry flue gas loss and loss

due to hydrogen in the fuel are respectively 102 and 125 BTU per cubic

foot of gas at standard conditions. The loss due to radiation and

convection from the boiler jacket equals (.015) (1036) or 16 BTU per

cubic foot of gas at standard condition consumed. Inserting these losses

into equation (A2) we obtain:

T (102 + 125 + 16) „ efficiency = 1 - ttttt = 0.765

10 36

This efficiency is in excellent agreement with the value of .761 which

was obtained by the calorimetric method.

30
The Modular System

In the steady state test of the modular system, modules 1, 2, and 3 were found to have a flue gas temperature of approximately 425°F and

a percent by volume of in their flue gas of 7.6%. Module 4, however, was slightly less efficient, having a flue gas temperature of 462°F and

a concentration of in its flue gas of 6.9% by volume. From Figures

13 and 14 we find that the dry flue loss and loss due to hydrogen in

the gas were respectively 92 and 120 BTU per cubic foot of gas at standard

conditions for modules 1, 2 and 3. For module 4, these losses were

respectively 110 and 122 BTU per cubic foot of gas at standard conditions.

As in the case of the single large boiler, the loss due to radiation and

convection from each module jacket will be taken to be 16 BTU per cubic

foot of gas at standard conditions. Inserting the losses for each module

into (A2), we find for modules 1, 2 and 3:

• 1 (92 + 120 + 16) ^ efficiency = 1 - 1036 0.780

and for module 4;

• 1 (110 + 122 + 16) _ efficiency = 1 - — , = 0.761

10 Jo

The average efficiency for the modular system as obtained by the heat

balance method is then:

[3(0.780) + 0.761] _ Q^^^^

4

The steady state efficiency which was obtained by the calorimetric method

for the modular system was .759. Considering that there is an error associated with both methods, these two efficiency values are felt to be

in fairly good agreement. The difference in efficiencies obtained by the

two methods could easily be the result of underestimating the jacket loss

from the small modules in the heat balance method.

31
a:iroT~r;'^ sa: ; 11. Appendix B

Error Analysis

The maximum possible uncertainty associated with the efficiency

result obtained for configuration 1 through 5 can be estimated by examining

the potential errors in Q ^ and Q, for non-steady state operation at

out in ^ small loads

.

The energy transferred to the circulating water, in case of a non- steady state test, is given by equation (10a). This equation may be

rewritten as:

-0;b;vv. Yd Q^^^ = (1 + 0.004)MC(T)AT , (Bl)

making use of the fact that -fiC^X "^^ ^(^. u"^7 .)J

^^^^ approximated by

(1. + .004)C(T) in order to obtain equation (10a) (see section on Calculations). Since the error involved in determining the total mass of water

flowing through the boiler or boilers during the test period is primarily

due to the inaccuracy of the turbine meter and the totalizer, the error

in M is +0.5%. The error in C(T), which results from the uncertainty

in T, is very small and may be neglected. The error in the value of

AT arises from three sources: the error associated with the thermopile

used to measure the temperature difference, the inaccuracy of the recording device, and the error resulting from counting the boxes under the curve (T (t) - T (t)) in order to evaluate the integral in (10b). The

o R

maximum error from the thermopile* is estimated to be +1.6%. The accuracy

of the recorder used in conjunction with the thermopile is +0.25% of span.

Since a full span was rarely used, a better estimate is +0.5°/,. Estimating

the error resulting from counting the boxes under the curve (T (t) - (T (t))

b R

is exceedingly difficult, since overestimating the area in one region

will compensate for underestimating it in another. It will be assumed

*The question of whether there is an error resulting from measuring the

average temperature of a circulating mass of water using a thermopile

inserted in two stainless steel wells has not been considered because of

the extreme complexity of this problem.

32
that this process contributed an error of +0.5%. Combining these errors,

equation (Bl) may be written:

Q = (1. + 0.004 4- 0.005 + 0.016 + 0.005 + 0.005)MC(T) AT

out — — — — —

= (1. + 0.035)MC(T)AT (B2)

where it is understood that M, C(T) , and AT are the measured values.

The error in Q^^> the energy input during a test, is the sum of errors

in the number of cubic feet of gas at standard conditions used during the

test (Vg) and the potential error in the higher heating value of the gas

(see equation (1)). Examining equation (11), we find that the uncertainty

in is due to any error which might arise in determining by reading

the gas meter, the uncertainty in R and any measurement errors associated

with T and p . m m

The uncertainty in Vm is estimated to be approximately 0.1% in the

worst case. The values of R, which are given in Table 1, will be assumed

to be sufficiently accurate so that they may be disregarded as a source

of error. The temperature T and pressure p have respective maximum

m m

errors of approximately 0.2% and 0.3%. Combining these results, is

found to be known to within + 0.6%. The higher heating value of the gas

is known to within + 1.5% if we assume that the gas used in any one test

could have had a heating value anywhere between the maximum and minimum

values reported by the gas company during the period of testing. The

error in Q. is thus found to be + 2.1%. in —

Since efficiency is defined as the ratio Q /Q , the maximum

out ^in error in the measured efficiencies is (+ 3.5% + 2.1%) or + 5.6%. As an example, system #5 had an efficiency of 0.66 at 8% load. Multiplying

0.66 by 5.6%, we find that the efficiency of this system at 8% of the

maximum heating load could have been higher or lower than the measured

result by a maximum value of 0.04 (i.e. efficiency = 0.66 + 0.04).

It should be understood, however, that the error obtained in this manner

is to be considered the maximum possible error. The actual error is

expected to be considerably smaller than this, due to the high probability

that the various errors will not be additive.

33
34
35
36
gure 4. Front view of experimental setup.

37
38
3dniva3di/\)3i

39
.80

.20

.10

.00

System 1 System 2

J L

10 20 30 40 50 60 70 80 90 100

HEATING LOAD <A(t)> (% OF MAXIMUM HEATING LOAD)

Figure 7. Efficiency versus heating load curves

for systems 1 and 2.

40
.80

.70

.60

.50

3

o

.40

>-O

z

UJo

Lu .30

U.UJ

.20

.10

00

10 20 30 40 50 60 70 80 90 100

HEATING LOAD <-ft(t) > (% OF MAXIMUM HEATING LOAD)

Figure 8. Efficiency versus heating load for

system 3.

41
.80

.70

.60 —

.50 _ //

U. .30

Ui

.40 —f/

I

I

.20

.10

00

I10

System 4

System 5

20 30 40 50 60 70 80 90 ICQ

HEATING LOAD <Sl{i)> (% OF MAXIMUM HEATING LOAD)

Figure 9. Efficiency versus heating load curves

for systems 4 and 5.

42
HEATING LOAD <-l(t) > {% OF MAXIMUM HEATING LOAD)

Figure 10, A comparison of efficiency versus heating

load curves for systems 1, 3, 4, and 5.

43
44
H

I—I <

<:u

OSw

t—I o

pqXOo(—I <O<I

0;u

•i-i

0)0>Q

0) 0

•i-i 0 u0

cn • n um0• H U

4-> (J ill a 0 1—1

.1-1 ^ CQ

^1 CO

(-> TJ

CO • i-< .1-1 T3 <

1—

H

OJ

OjO 0 OJ 1—1

nd • 1-1 -M

OJ

<u ,^ • r-l 0) M uil 0

•4-> 0 1—1 0

I—1 0

(J < C/2

1—1

0)

(T) *

45
A

3"

4-

5-

o

a>Q.-6

CVJ o

o

7-

8-

9-

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50
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. iniUI.K A 1 ION OK Kl-I'OK 1 NO. NBS-BSS-79

2. Gov't A( ( (-,., ion No.

3. Recipient's Acces.sion No.

4. TITLP: AND SUBriTl.i;

Energy Conservation Potential of Modular Gas-Fired Boiler

Systems

5. Publication Date December 1975

6. Performing Organization <^.ode

G. E. Kelly and D. A. Didion

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 462 6401

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as 9.

13. Type of Report & Period Covered Final

14, Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 75-619338

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The modular concept of boiler operation was examined in a laboratory test of five

gas-fired, cast iron, hydronic boilers. Four of the boilers, each having an input

rating of 85,000 Btu per hour, were arranged so that they could either be operated

like a single boiler (i.e., all of the boilers either on or off) or as a modular installation in which the boilers are sequentially fired to match the number in operation

with the heating load. The fifth boiler had an input rating of 3 00, 000 Btu per hour

and was operated as a single boiler installation. Efficiency vs. heating load curves were obtained for the single boiler installation, the four small boilers run like a single boiler and the modular installation operated with and without water flowing

through the "idle" modules. These efficiency curves were then used to theoretically

predict the effect of the modular concept and boiler oversizing on the seasonal efficiency of gas-fired heating plants. It was found that under certain conditions the use of a gas-fired modular boiler installation instead of a single large boiler could

result in considerable energy savings.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Boiler oversizing; efficiency vs. heating load; modular boilers; modular concept;

seasonal efficiency.

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