NATIONAL BUREAU OF STANDARDS REPORT

4309

EFFECT OF CHANGES OF ENGINE SPEED ON THE CAPACITY

OP THE 1/3 -TON CARRIER REFRIGERATING UNIT, MODEL D 731 -9 644

by

Minoru Fuji!

C. W. Phillips

P. R. Achenbach

Report to

'Mechanical Engineering Division

Headquarters, Quartermaster Research & Development Command

Natick, Mass,

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003 -20 832 September I3, 1955 U309

EFFECT OF CHANGES OF ENGINE SPEED ON THE CAPACITY

OF THE 1/3-TON CARRIER REFRIGERATING UNIT, MODEL D731-96144

by

Minoru Fujii

C, W, Phillips

P, Ro Achenbach

Heating and Air Conditioning Section

Building Technology Division

to

Mechanical Engineering Division

Headquarters, Quartermaster Research & Development Command

Natick, Mass,

<^s>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, r r in part, is prohibited

unless permission Approvcd foT pubHc relcasc by the

25,D,c. Suchpe Director of the National Institute of eport has been specifically prepared If i Standards and Technology (NIST) eport for its own use. on October 9, 2015.

EFFECT OF CHANGES OF ENGINE SPEED ON THE CAPACITY

OF THE 1/3 -TON CARRIER REFRIGERATING UNIT, MODEL D73l“961;i4

by

Minoru Fuji!, C. W. Phillips and P» R. Achenbach

Abstract

A study was made of the effect of speed change,

ambient temperature, and refrigerator temperature on the net refrigerating capacity of a

plug-type refrigerating unit. Model D 731 ”96l4l4 , manufactured by Carrier Corporation of Syracuse,

New Yorko The test specimen, of nominal l/3 -ton

capacity, was constructed with the compressor

and condenser fan coupled directly to the gasoline engine whereas the evaporator fan was driven

at constant speed by a direct current electric

motor powered by a generator built integrally

with the engine. The results showed that at constant speed the net refrigerating capacity increased linearly with increase in refrigerator

temperature when the ambient temperature was constant, and decreased linearly with increase in

ambient temperature when the refrigerator temperature was constant. A maximum net refrigerating capacity of I4 I 6O Btu/hr was observed at an

engine speed of 2700 rpm. The unit attained

about 93 percent of the nominal l/3-ton rating

at the design engine speed although the engine

speed varied in excess of 10 percent from test

to test under control of the governor. At

speeds near the design speed the capacity of

the unit varied approximately in direct proportion to the Speed.

1, INTRODUCTION

At the request of the Office of the Quartermaster General

studies were made of a l/3 -ton gasoline-engine driven refrigerating unit of the plug type. Model D731"96l44, manufactured

by Carrier Corporation, Syracuse, New York, to determine the

effect of changes of ambient temperature, refrigerator temperature, and engine speed on the net refrigerating capacity.
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Tests were made in the range of ambient temperature from

70°F to 125*^F, and in the range of refrigerator temperature

from -22°P to at design engine speedy and tests were in

the speed range from l800 rpm to 3 OOO rpm with a refrigerator

temperature of 0°F and an ambient temperature of 110°Fo During the series of tests at various speeds the condenser fan

speed varied proprotionally with the engine speed by virtue

of being mounted on the same shaft whereas the evaporator fan

was driven at constant speed because it was driven by a direct

current motor.

2. TEST SPECIMEN

The specimen refrigerating unit 5, Carrier Model D 731 “96l4 l|

was of the plug-type, of nominal one-third ton capacity equip^

ped with an Onan gasoline engine. The unit was designed to

refrigerate a 150 cu. ft. walk-in refrigerator, either demountable or non-sectional, in the field where a source of

electric power was not available for refrigeration purposes.

The non-sectional 150 cu. ft. refrigerator used for capacity

tests with various engine speeds was manufactured by Brown

Industries of Spokane, Washington. The demountable 150 cu,

ft. refrigerator used for capacity tests at various ambient

and refrigerator temperatures was manufactured by U, S.

Thermo Control Company of Minneapolis, Minnesota. Figure

1 shows an exterior view of the refrigeration unit from the

condensing unit end. Figure 2 shows the interior of the

condensing unit with the top and one side panel removed. The

compressor was directly coupled to and powered by a two cylinder' two cycle gasoline engine manufactured by D. W. Ovan

& Sons, Inc. of Minneapolis, Minnesota. The condenser was

air cooled by a five bladed fan attached to the coupling

on the shaft between the compressor and the engine as shown

in Figure 2, Air for the condenser was drawn in at the right

as seen in Figure 2. and discharged around the engine. The

face area of the condenser measured 19 by 20 l/2 Inches, A

part of the built-in starter-generator is shown back of the

air Intake grille in Figure 1.

The evaporator coll was mounted horizontally at the

bottom of the evaporator section of the unit, seen on the

far side of the condensing unit in Figures 1 & 2. The four

bladed evaporator fan was 12 inches in diameter and was

driven by a 12-volt direct-current motor at a speed of 1900

rpm. This motor was located in the center of the vertical

end panel of the cabinet. It drew air in at the bottom of

“ 3

“

the evaporator section through the evaporator coll and forced

the air straight forward into the refrigerator when installed

The overall dimensions of the condensing unit; section

were; width, l\l inches; height, 33 inches; and depth, 25

inches; whereas those of the evaporator section were; width,

28 inches; height, 21 I/I4 inches; and depth, 27 Inches including the wire guard over the evaporator fan,

3, TEST PROCEDURE

Two series of tests were made of the Carrier Model D731~

96i|i| plug-type refrigerating unit. The unit was mounted in

a 150 cuo ft, walk-in refrigerator. The entire system was

operated in a room whose temperature could be controlled at

the desired level. By adjustment of an internal heat load,

the refrigerator temperature was also maintained at the desired level.

For the first series of test, the refrigerating unit

was operated in ambient temperatures of 70°P, 90°F, 110°F

or 125°F during successive periods and the refrigerator

temperature was maintained at 35°^^» ^rad the lowest attain

able value without internal heat load during separate tests

at each ambient temperature. The engine speed was not adjusted manually during the test, but was regulated by a gover

nor. The 15O cu. ft, demountable warehouse employed for this

series of test was manufactured by U, S. Thermo Control Company,

For the second series of test, the refrigerating unit

was operated in an ambient temperature of 110°P and the refrigerator temperature was maintained at 0°F, The engine

speed was regulated by adjusting the governor setting, Seperate tests were made at engine speeds ranging from I8OO to

3000 rpm in increments of 3OO rpm. The non-sectional 150

cu. ft. warehouse employed for this test was manufactured by

Brown Industries,

For both series of tests, observation of temperatures

was made at 30-minute intervals and observations of speeds,

pressures, heat input and fuel consumption were made at

60-mlnute intervals for at least four hours after a steady

state condition was reached.
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Prior to each series of test^ a reverse heat-flow test

was made to determine the rate of heat transfer through the

warehouse wall, which constitutes part of the refrigerating

load. The refrigerating unit was in place but was not operated during this test. The ambient temperature was held at

the warehouse temperature was maintained at 135°F

while observations of temperatures and the amount of heat required to maintain this temperature difference were made at

30-mlnute intervals for at least four hours after equilibrium

was reached.

Calibrated thermocouples, pressure gages, watt hour

meters and other instruments were used for the various measurements.

i^, TEST RESULTS

The heat transmission factors of the 150 cu, ft, warehouses used to test the Carrier l/3-ton refrigerating unit

were found to be 23«1 Btu/hr { °F) for the demountable Thermo

King warehouse and 27<.l Btu/hr ( °F ) for the Brown warehouse.

The net refrigerating capacity of the Carrier l/3-ton

unit. Model D731”96L|.l4, at design engine and compressor speed

is summarized in Table 1 and shown graphically in Figure 3

for a range of ambient temperature and refrigerator temperature, At design conditions of 0°F refrigerator temperature

and 110®F ambient temperature the net refrigerating capacity

was 3710 Btu/hr or about 93 percent of the nominal rating

of one-third ton. At the higher refrigerator temperature of

35°!^ "tB-e net refrigerating capacity was about 88OO Btu/hr

for otherwise similar conditions.

Figure 3 shows that the relation between net refrigerating capacity and refrigerator temperature was approximately

linear for a given ambient temperature. Conversely, the

data in Table 1 can be plotted to show a linear relationship

between net refrigerating capacity and ambient temperature

for a given refrigerator temperature. In Figure 3 the net

refrigerating capacity increased 1^6 Btu/hr per degree rise

in refrigerator temperature on the average for an ambient

temperature of 70°F and the corresponding value was 137«5

Btu/hr C°F) for an ambient temperature of 110°F, The net

refrigerating capacity decreased I4I4O Btu/hr for every 10®F

rise in ambient temperature above 70°F or eight percent of

its capacity at 70°F when the refrigerator temperature was

maintained at 0°F, The corresponding decrease for a refrigerator temperature of 35*^F was 632 Btu/hr for every 10°F

” 5 " rise in ambient temperature above 70°F or percent of

it capacity at ?0°F/

Table 1 shows that the air temperature difference across

the evaporator was approximately proportional to the net refrigerating capacity as would be expected for constant fan

speed and constant air delivery. On the other hand, the air

temperature difference across the condenser did not increase

in proportion to the net refrigerating capacity, indicating

that the heat of compression was lower for lower compression

ratios at any given ambient temperature.

Table 1 Indicates that the superheat of the refrigerant

vapor at the thermal expansion value bulb decreased as the

refrigerator temperature decreased. The superheat at the

expansion value bulb averaged about 0.5°F at the lowest

attainable refrigerator temperature, 2»l\°F at a refrigerator

temperature of 0°F, and 9-9°F at a refrigerator temperature

of A. comparison of the temperature at the compressor

inlet and the saturated suction temperature in tests 1 and

in Table 1 suggests that liquid refrigerant could have been

entering the compressor at times during these two tests. The

observed values of net refrigerating capacity for these two

tests, as plotted in Figure 3j however, do not appear to deviate significantly from expected trends.

The engine and compressor speed was not constant during

the tests represented by Figure 3 at various ambient and refrigerator temperatures even though the governor was permitted

to control the speed during all tests. The average speed

ranged from a low value of 193^ ^*0^ the three tests in an

ambient temperature of 90°F to a high value of 217l| rpm for

the three tests in an ambient temperature of 110°F. This is

a variation of about 11 percent. Considering these variations

in speed and their effect on the net refrigerating capacity

the curves for ambient temperatures of 70°F, 90°F, and 125°F

in Figure 3 should be displaced to the left a little with

the greatest shift occurring in the curve for an ambient temperature of 90°P to make the results truly comparative with

those shown for an ambient temperature of 110°F.

The results of the capacity tests for a range of engine

and compressor speeds from 1800 rpm to 3 OOO rpm are summarized

in Table 2 and shown graphically in Figures [| & 5» All of

these tests were made with an ambient temperature of 110°F

and a refrigerator temperature of 0°F,

» 6 -

Figure 5 shows that the net refrigerating capacity increased progressively as the speed was increased from l800

rpm to 2700 rpm but not proportionally over the entire range,

to a maximum value of about I|l60 Btu/hr. At an engine speed

of 3000 rpm the capacity decreased to about 37OO Btu/hr. In

this case the decrease of capacity at 3OOO rpm cannot be attributed to an increase in evaporator fan power because the

evaporator fan was driven at constant speed for all of the

tests. For this unit the shape of the capacity curve in

Fig. 4 reflects the effect of the small increase in compression ratio, the change in value action, and the change

in discharge refrigerant temperature as the engine and

compressor speed was increased.

Table 2 shows that the discharge pressure increased a

little as the speed was increased whereas the suction pressure decreased gradually as the speed was increased from

1800 rpm to 2I4OO rpm. These trends caused a small Increase

in compression ratio up to a speed of 2400 rpm followed by

a gradual decrease in compression ratio at higher speeds.

The air temperature difference across the condenser also decreased slowly for speeds above 2400 rpm as shown in Table

2. The data observed during the tests also showed that the

temperature of the refrigerant at the compressor discharge

decreased for speeds above 2400 rpm. All of these results indicate that the pumping capacity of the compressor was

leveling off or decreasing at speeds above 2400 rpm.

On the other hand Table 2 shows that the suction gas

temperature was 1.6°F in test ^ at an engine speed of 3OOO

rpm whereas the corresponding temperatures ranged from 39°F

to 57°F for the tests at lower speeds. The low temperature

at the compressor suction combined with the low value of

superheat at the thermal expansion valve bulb in test 5

suggests that the expansion value may have been permitting

liquid refrigerant to approach the compressor suction in

this test. If this was occurring, it would tend to lower

the useful refrigerating capacity in the refrigerator.

Thus the operating conditions are not exactly comparable

between test 5 £^nd the remainder of the tests even though

they are representative of the performance capabilities

of the unit under the speed conditions selected for the

investigation. That is, the net refrigerating capacity of

the unit at a speed of 2,000 rpm might have been higher if

the expansion val vs had controlled the superheat in the

same manner as for lower speeds.
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Pigiire 5 shows the relation between fuel consumption

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as the capacity of the unit levelled off.

5 . DISCUSSION AND CONCLUSIONS

The tests of the Model D731“96i|ij Carrier l/3 -ton refrigerating unit showed that its capacity at rated speed was

about 93 percent of the 1 /3 -ton rating at an ambient temperature of 110 °F and a refrigerator temperature of 0°F,

Figure I4 shows that the nominal rating of I4 OOO Btu/hr could

be attained at an engine speed of 2 I4 OO rpm and that the maximum capacity of I4 I 6O Btu/hr was attained at an engine speed

of 2700 rpm.

Figure I4 shows that the capacity of the unit Increased

from 3300 Btu/hr at a speed of 193^ to 3750 Btu/hr at

a speed of 217ll rpm or about 12 percent of the higher capacity. This is the variation observed in average speed of

the test specimen when controlled by the governor during the

tests at various ambient temperatures. That is, the data

in Figure I| indicates that the curve plotted in Figure 3

for an ambient temperature of 90°F should be shifted to

show capacities about 12 percent higher in order to make it

comparable with the plotted data in F“'igure 3 ambient

temperature of 110°F. Corresponding changes in the positions

of the curves in Figure 3 For ambient temperatures of 70°F

and 125 *^ could be worked out on the same basis.

The data observed at various refrigerator temperatures

and ambient temperatures indicated that the relation between

unit capacity and either ambient temperature or refrigerator

temperature was linear when the other remained constant.

ON

Test Number

Net Refrigerating Capac

Engine Speed (a)

Avg, Refrigerator Temp.

Avg. Ambient Temp, 1

Discharge Pressure

Suction Pressure

Compression Ratio

Gasoline Consumption Ra

Suction Gas Temp.

Sat. Suction Tem.p.

Superheat at Expansion

Bulb

i:

Temperature of Air Ente

Condenser

Temperature of Air Leav

Condenser i

Air Temp. Difference Ac

Condenser

Temperature of Air Ente

Evaporator

Tempterature of Air Lea

Evaporator

Air Temp. Difference Ac:

Evaporator 1

(a) Compressor and Cond

was constant at 190

(b) Vacuum, in Hg.
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TABLE 1

EFFECT OF REFRIGERATOR AND AKBIENT TEMPERATURE

ON THE CAPACITY OF THE CARRIER REFRIGERATING UNIT D73I-9644

Test Number

Net Refrlgeratln/5 Capacity, Btu/hr

Engine Speed (a) RPM

Avg. Refrigerator Temp. °F

Avg. Ambient Temp. °F

Discharge Pressure psig

Suction Pressure "

Compression Ratio

Gasoline Consumption Rate Ib/hr

Suction Gas Temp. °F

Sat. Suction Temp. °F

Superheat at Expansion Valve

Bulb °F

Temperature of Air Entering

Condenser °F

Temperature of Air Leaving

Condenser °F

Air Temp. Difference Across

Condenser °F

Temperature of Air Entering

Evaporator ®F

Tempterature of Air Leaving

Evaporator °F

Air Temp. Difference Across

Evaporator °F

(a) Compressor and Condenser Fan s was constant at 1900 rpm

(b) Vacuum, in Hg.

1 2 3 4 5

2170

2019

-24.1

69.5

5470

2020

0.1

69.8

11320

1954

34.7

69.0

2410

1957

-15.1

88.9

4650

1939

0.2

90.1

86

4.0(b)

7.92

3.8

-24.8

-28

93

0.8

6.95

3.6

29.9

-19

109

12.0

4.63

4.0

48.5

5

114 123

0.4(b) 2.8

8.88 7.87

3.8 3.6

-17.1 24.8

-22 -14

0.3 3.3 11.6 0.3 1.1

72.2 72.0 71.1 90.1 91.7

75.2 77.3 79.9 93.8 96.9

3.0 5.3 8.8 3.7 5.2

-23.3 1.2 35.7 -14.5 0.8

-24.7 -4.3 24.0 -16.2 -4.0

1.6 5.5 11.7 1.7 4.8

were identical with engine speed.

6 7 8 9 10

10050 2770 3710 8790 2920

1913 2198 2208 2115 2165

35.8 -8.5 -0.1 35.9 -1.1

90.3 ' 110'r7 09.9 110.2 124.9

147 160 156 188 191

13.6 1.8 2.8 15.1 3.8

5.71 10.59 9.76 6.80 11.11

3.4 3.3 3.6 2.9 3.8

54.0 24.3 51.0 65.7 57.0

8 -17 -14 11 -12

9.1 0.9 3.4 9.1 1.7

93.4 112.4 109.4 112.6 126.1

102.0 116.7 14.3 121.2 130.6

8.6 4.3 4.9 8.6 4.5

36.5 -7.7 1.6 37.4 -0.5

25.2 -9.7 --g-,8 27.5 -3.0

11.3 2.0 4.4 9.9 2.5

rator Fan speed

11

74S0

20S2

35.1

126.8

226

16.6

7.69

4.3

65.1

13

9.7

129.7

138.4

8.7

36.2

27.5

8.7

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THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project

or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of

Standards, Washington 25, D. C.
NBS