NATIONAL BUREAU OF STANDARDS REPORT

h33k

CONVERSION OF THE THERMO-KING GASOLINE ENGINE DRIVEN

ONE-TON WAREHOUSE REFRIGERATING UNIT TO ELECTRIC DRIVE

by

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

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NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003-20-Ii832 September 28, 1955 43314

CONVERSION OP THE THERMO -KING GASOLINE ENGINE DRIVEN

ONE-TON WAREHOUSE REFRIGERATING UNIT TO ELECTRIC DRIVE

by

C, W, Phillips

P. R, Achenbach

Heating and Air Conditioning Section

Building Technology Division

To

Mechanical Engineering Division

Headquarters, Quartermaster Research & Development Command

Natick, Masso

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL eUREAU OF STANDARDS

The publication,

unless permlsslo 25,D,C. Such I

cally prepared I

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

or In part, Is prohibited

Standards, Washington

report has been specifir report for its own use.

CONVERSION OF THE THERMO-KING GASOLINE ENGINE DRIVEN

ONE^TON WAREHOUSE REFRIGERATING UNIT TO ELECTRIC DRIVE

by

Co Wo Phillips and P, R„ Achenbach

Abstract

An important item of Army refrigerating equipment,

the nominal 1-ton warehouse refrigerating unit,

was investigated to determine the relative net refrigerating capacity when operated with an electric

motor as compared to operation with a gasoline engine o The ease with which the conversion from gasoline engine drive to electric motor could be made

in the field was studied* Pertinent previous tests

of similar 1-ton gasoline engine driven warehouse

refrigerating units are referenced to substantiate

recommendations for modification of relative fan

speeds* As designed, the conversion kit from gasoline engine drive to electric motor drive, except

for one error in machining size, was able to be installed by two mechanics, without previous experience in making this conversion, in less than eight

hours, including an operational check* The net

refrigerating capacity with electric motor drive

was 5980 Btu/hr as compared to 9720 Btu/hr with

gasoline engine drive, under conditions of 110 P

ambient temperature and 0 P warehouse temperature*

Decreasing the evaporator and condenser fan speeds

by reducing the drive pulley furnished with the

electric motor conversion kit to the same size as

used with the gasoline engine drive is recommended

for an estimated gain of 30^ in net refrigerating

capacity*

1* INTRODUCTION

Among the items of Army refrigerating equipment the 6 OO

cubic foot warehouse with a nominal 1-ton refrigerating unit

has become one of the most important units* Where electric

power is not readily available these refrigerating units are

powered by gasoline engines which start and run as required

under control of the thermostat sensing the warehouse temperature* Such cycling of a gasoline engine requires complicated

control equipment including heavy duty batteries, relays,

„ 2 -

automatic choke^ cranking limit devices, cranking motor, generator, and other controls. Failure of any one of these devices

will render the unit Inoperable, Units can be operated on

cyclic basis by an electric motor with much simpler control

and partly because of this, conversion of the unit to electric

motor operation Is desirable where electric power Is available.

In the development of this Item of Army refrigerating equipment various prototypes have Included combination power plants

with both gasoline engine and electric motor drives. Some of

these combinations were equipped with over-riding clutches

so that either type of drive could be employed without any

mechanical changes, others required belt changes or mechanical

clutch adjustments,

A l“ton warehouse refrigerating unit, Thermo-King Model

MQ-51 , equipped with a Grosley liquid-cooled gasoline engine

was supplied for the performance tests and was converted to

electric motor drive, using a kit manufactured by U, S, Thermo

Control Co., who were also the manufacturers of the ThermoKing unit Involved,

A purpose of the tests was to determine the relative capacity of the l“ton unit driven by the two prime movers. Another

purpose of converting a unit as a part of the investigation was

to see how practical the actual conversion was as a field operation.

2 . DESCRIPTION OF THE TEST ITEMS

The Electric Motor Conversion Kit, manufactured by U, S.

Thermo Control Co.which was further identified as N, B, S.

Specimen No, 107 ~ 5m# contained all items required for the

conversion. The items supplied were?

Part No. Nomenclature Q,uantity

1310 Belt, drive, oil piunp 1

9AA2 Belt , fan 2

#10 Cable, power, rubber covered.

I4 wire conductor - 15 ft, long 1

CI498OA-IO Clamp, oil line 1

MQE-D503C Coupling, drive assy. 1

MQE-S517B Cover, relay box 1

QI5-R72IB Grommet, rubber 1

MQE-E503C Motor and base assembly, electric 1

MQE-519B Plate, identification 1

- 3

-

Part No. Nomenclature Quantity

MQE-52OC Plate, Instruction 1

T-205B Puller, drive pulley 1

MQE-D502A Pulley, drive, oil pump 1

MQE-S51OC Pump assembly, oil 1

MQE-E508D Relay box assembly 1

MQE-S522A Strip, nut, oil pump mounting 1

T-206A Wrench, (hx. bar) Compressor

pulley) 1

The electric rnotoPj part of Part No« MQE-E503C above, can

be further identified as?

Westinghouse Life-Line Motor

Type CSP Design B

Frame 28I|

7o5 H,P.

3 Ph - 60 cycle 220/I1I4O volts

20/10 amps/line

Locked KVA Code G

1750 rpm

Ser. #5308

S #ll|l42202

The unit used for the conversion was a Model MQ51 Thermo

King 1-ton warehouse refrigerating unit, originally equipped

with a Crosley liquid-cooled engine* This unit was identified

as N,B.S, Specimen 101-53 » bore manufacturer's Serial No*

QS-200.

Figure 1 shows the Model MQ.51 Thermo King warehouse refrigerating unit used in these tests. It is mounted in operating position in a demountable insulated warehouse. The

wires across the front of the condenser guard are for thermocouples and the hole in the front machine compartment door was

made for test purposes. >fhen used with a gasoline engine a

muffler was attached to the exhaust pipe extending vertically

from the front left top corner.

Figure 2 shows the evaporator side of the test unit mounted

in a warehouse wall. The wire crossing the front of the evaporator guard is a support for thermocouples.

Figure 3 is a front view of a Thermo King 1-ton unit of the

same model as N.B.S. 101, showing the Crosley liquid-cooled gasoline engine, generator-starter assembly and drive coupling. The

two belts from the drive coupling operate the evaporator and condenser fane.

- h

-

Figure I4 Is a side view of the condensing unit section of

the same unit.

Figures 5 ^ front and side views, respectively,

of the primary items involved in the conversion to electric

motor drive. In Figure 5 the square enclosure over the electric

motor contains the transformer-rectifier for the normal directcurrent controls, throttling valve relay, defrost holding relay

and defrost timer. The belt-driven apparatus to the left of

the rectifier enclosure is an oil pump which operates the evaporator air damper. The small wire attached to the front surface of the motor is a thermocouple. In Figure 6 the rectangular enclosure above the motor contains the magneticallyoperated motor relay.

3 . TEST PROCEDURE

The Model MQ51 Thermo King unit, as originally equipped

with the Crosely liquid-cooled gasoline engine was tested to

determine its net refrigerating capacity. The unit was installed in operating position in one wall of a 600 cu. ft. insulated warehouse which was then calibrated for heat loss, A

test of the unit at conditions of 110 F ambient temperature and

0 F warehouse temperature to determine the net refrigerating

capacity was then made. This capacity is the sum of the calibrated heat gain of the warehouse and the heat equivalent of the

electrical energy required to maintain the interior warehouse

temperature at 0 F. during steady operation of the unit. A

Model i| R Thermo King compressor whose capacity had been

measured previously on an N,B,S. calorimeter was substituted

for the original compressor in the unit under test because

its capacity was known. The original Model I4 R Thermo King

compressor had been employed in a series of tests of long duration to determine drfrosting characteristics of this unit

under various methods of control and its condition was not

suitable to simulate new compressor performance.

Following the test under the gasoline engine drive, the

gasoline engine was removed and the electric motor drive

conversion kit was installed for a comparison test. The instructions in a manual entitled ^Thermo-King (Model MQ51~E)

Instruction Manual and Parts List With Instructions for Conversion from Model MQ-51 Gasoline to Model MQ-^l Electric”

were followed in making the conversion. The steps listed in

the manual were as follows;

” 5

“ 1,

Disconnect battery cables from batteries. Leave

negative cable attached to engine. Leave positive cable

attached to unit.

2. Remove batter>ies from compartment in bottom of unit.

Clean the battery compartment.

3 . Open petcock under water pump and drain coolant into

a container.

I],. Disconnect radiator hoses at end of extension tubes

fastened to bulkhead. Leave hoses connected to engine.

5 . Disconnect engine exhaust hose at pipe under condenser coil. Leave exhaust hose connected to engine.

6 . Disconnect engine terminal board with wire harness.

7. Disconnect all wires connecting engine to bulkhead

terminal board, at board.

8 . Disconnect two flexible oil lines at engine.

9o Disconnect positive lead to starter-generator . Disconnect this cable at both ends, otherwise it will be '^hot"

and may cause a short circuit,

10. Disconnect fuel line at fuel pump,

11. Loosen clamps and remove carburetor air cleaner.

Store air cleaner with engine for possible future use,

12. Remove center post from between two front doors.

13 . Remove four bolts fastening engine to side flanges

of power tray. Remove two bolts at open end of tray.

II4

. Remove engine from unit, carefully sliding splined

shaft out of splined hole in drive coupling.

15 * Loosen bolts holding condenser fan housing and

bracket assembly to flanges on condenser coll. Slide fan and

bracket assembly downward in adjustment slots.

16 . Remove fan belts from unit and store with engine for

possible future use.

17. Remove socket head cap screw holding coupling to

tapered end of compressor shaft » Save this screw for later

use in making the conversion#

18. Using special puller supplied with conversion kit,

remove drive coupling from compressor shaft#

19. Install electric motor assembly in power tray through

front opening of unit. Slide assembly to right as far as possible to allow clearance for Installation of new drive coupling

on compressor shaft# Be sure that keyway in coupling fits

over key in shaft.

20# Place socket head cap screw (referred to in para.l) on

end of special extension wrench and insert in coupling to fasten

drive coupling to compressor shaft#

21# Place oil pump drive belt over shaft of electric motor,

and new set of longer fan belts over drive pulley and fan pulley.

22# Slide electric motor toward compressor, carefully inserting motor shaft into drive coupling. See that shaft key

is in place# Move motor to the left until holes in motor base

line up with matching holes in power tray# Install and tighten

four bolts and two bolts.

23 . Tighten two socket head screws in split collar on motor

shaft#

2l|o Loosen bolts holding oil pump assembly# Remove assembly and connect two flexible hydraulic oil lines. Fasten oil

line clamp under corner screw of oil pump cover.

25 . Install oil piimp assembly on bracket and place belt

over pulleys# Loosen set screw in oil pump drive pulley and

align the two pulleys# To adjust belt tensions raise oil

pump assembly and tighten two bolts. Allow 3/k" slack in

belt midway between two pulleys. Tighten set screw in drive

pulley,

26# Remove oil filler plug and fill oil pump reservoir

with one quart of SAE 10-W oil# Install and tighten oil filler

plug.

27 . Remove relay housing cover. Slip rubber grommet over

wire harness. Fit grommet into slot in relay housing. Connect

terminal board to terminal board on transformer. Install a brass nut on each active binding post. Install housing cover.

28. Connect positive battery cable to brass post on linestarter box. It may be necessary to spread the cable clamp

before installing.

29 « Adjust tension of fan belts? move condenser fan

housing upward in slotted holes and tighten adjustment screws.

Allow 3/I4” to 1

‘* slack in belts midway between pulleys.

30. Install center post between doors,

31. Remove wiring dlagram-instruc tion plate, and identification plate from unit. Replace with new plates covering

electric motor operation. Store old plates with engine for

possible future use,

32. Connect motor lead cable to source of 220 volt, 60

cycle, 3 phase electric current. Connect lead so that compressor rotates in a clockwise direction when viewed from the motor

end. Turn unit on momentarily to check, direction of rotation.

If compressor turns counter-clockwise, reverse the position of

any two power leads in the connector cable.

Two laboratory mechanics without previous experience in

making this conversion were assigned the job of making the

actual conversion. No difficulty was experienced until Step

22 . At this point is was discovered that the bore of the

drive coupling (Part No. MQE-D503C ) on the motor end was

1 1/8’* while the motor shaft was 1 I/I4". The coupling assembly

was sent to U. S. Thermo Control Co, for exchange or modification and was returned in about a week. No further complications in making the conversion were encountered, and, with

correction for the time lost because of the incorrect sizing

of the drive coupling components, the installation was easily

made by two mechanics in less than eight hours, including

a check for operation. All parts, including clamps and other

miscellaneous hardware, necessary for the job were provided

in the kit.

After Installation of the electric motor drive and accessories, the unit was again operated in the calibrated warehouse

and the net refrigerating capacity determined. The power input to the motor, nominally rated at 7 l/2 h.p, was measured.

» 8 -

In each of the comparative capacity tests, the unit was

operated for more than twenty four hours and the data reported

is that observed during a stable period of six hours at or near

the end of the operating period,

i|o TEST RESULTS

The results of the two comparative tests of net refrigerating capacity, first with the unit powered by a Crosley liquidcooled gasoline engine, and second, with a Westinghouse electric

motor are presented under Tests 1 and 2 in Table 1 , This table shows a net refrigerating capacity of 9720

Btu/hr with the gasoline engine drive in Test 1 and ^980 Btu/hr

with the electric motor drive in Test 2 , The primary difference in operating characteristics between the two tests was the speed of the compressor. In both

cases it was direct-driven with the gasoline engine driving

the compressor at a speed of 2I4I8 rpm whereas the electric

motor drove the compressor at I7O3 rpm. The belt-drive assembly for the condenser and evaporator fans furnished with the

electric motor conversion was sized to operate the fans at a speed about 150 rpm lower than that observed when the gasoline

engine was driving the compressor at 2 i|00 rpm. For example,

when the compressor was driven at 2418 rpm by the gasoline

engine, the condenser and evaporator fans operated at I6OI rpm

and 1379 rpm, respectively. When the compressor was driven at

1703 rpm by the electric motor, the fan speeds were II45O rpm

and 12143 rpm, respectively. That is, in Table 1 the net refrigerating capacity with the electric motor drive was 61 , 5^ of

that observed with the gasoline engine drive whereas the speed

of the compressor with electric drive was 70,14^ of the that

observed with the gasoline engine drive. Correspondingly, the

evaporator and condenser fan speeds with electric drive were

about 90^ of those observed with gasoline engine drive.

Also given in Table 1 are the results of capacity tests

of two other 1 -ton Thermo King gasoline engine driven warehouse

refrigerating units. In Test 3 another 1 -ton unit of the same

model number was operated with a compressor speed of 1330 rpm

with fan speeds In proportion as determined by the pulley ratios.

In this test the net refrigerating capacity was 79,2^ of that

observed in Test 1 whereas the compressor and fan speeds were

about 55^ of those in Test 1 .
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In Test I4 the same compressor as that used in Tests 1 and

2 produced a net refrigerating capacity of 10,100 Btu/hr at a speed of 2291 rpm when used in a third 1 -ton warehouse refrigerating of the same modelo A calorimeter test of compressor A,

the one used for Tests 1 , 2 and 4, showed that its gross refrigerating capacity was 13,700 Btu/hr under the same operating

conditions as those shown in Test I|<>

Tests 3 h in Table 1 were not a part of the present

comparison, but they provide data which will be used to help

explain under Discussion and Conclusions the disproportionate

decrease in capacity in Test 2 as compared to Test 1 when the

compressor speed was decreased as a result of conversion to

electric drive.

The small variations in the ratio between fan speed and

compressor speed observed in Tests 1 , 3 and 4 in Table 1 are

thought to be due to belt slippage.

All components of the electric conversion kit functioned

as intended. The oil pump developed sufficient pressure to

operate the evaporator air damper. All controls normally operated by 24 volts DC functioned satisfactorily with the transformer-rectifier furnished.

5 . DISCUSSION AND CONCLUSIONS

As stated in the introduction the comparison tests of the

two methods of driving a 1-ton Thermo-King warehouse refrigerating unit were made to determines

1. Ability to make the conversion from gasoline engine

drive to electric motor drive as a field operation,

2, Net refrigerating capacity for each method of drive.

Two laboratory mechanics, neglecting the time delay caused

by the improper size of the drive coupling assembly, were able

to make the conversion including operation of the unit for

punposes of check within eight hours. Since neither of the

mechanics had previously performed this conversion it appears

practical as a field operation. No opening of refrigerant

lines was required, therefore, a knowledge of the assembly of

refrigeration circuits was not a prerequisite. The step by

step instructions provided in the manual adequately described

each required operation and the kit contained every item required to make the complete conversion.
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- 10

The net refrigerating capacity of the 1 -ton warehouse refrigerating unit was reduced 38,5^ tiy virtue of the conversion

to electric drive whereas the speed reduction of the compressor

was only 29 « 6^ and the speed reduction of the condenser and

evaporator fans was only 10^.

In previous tests made of a l/3-ton gasoline engine driven

warehouse refrigerating unit, it was observed that the heating

effect of the evaporator fan was a primary contributing factor

to the reduction of the net refrigerating capacity of the unit

as the speed was increased. In that series of tests, it was

shown that when the speed of compressor and fans were increased

proportionally a balance point was reached where the increase in

compressor capacity was exactly offset by the increase in evaporator fan power because the compressor capacity increased proportionally with speed whereas the evaporator fan power increased

as the cube of its speed. At speeds above that corresponding to

this balance point the net refrigerating capacity decreased.

As stated earlier, a calorimeter test was made of the Model

1} R Thermo-King compressor A, (Serial No. 2138-x) used in test

1, 2 and I4 summarized in Table 1 of this report. The capacity

of the compressor itself operating under the same conditions

as those summarized in Test I4 was 13,700 Btu/hr at 2299 rpm,

while the net refrigerating capacity of the unit was 10,100

Btu/hr at 2291 rpm as shown in Test I4, The heating effect of

the evaporator fan was a major part of the difference between

the gross compressor capacity and the net refrigerating capacity of the unit. Assuming that the heating effect of the fan

contributed all of the difference observed between gross compressor capacity and net unit refrigerating capacity, and a study of the data for the tests involved Indicated that this

was essentially true, then a value of 38OO Btu/hr is indicated

as the energy released by the evaporator fan for a speed of

122k rpm as shown in Test I4. The energy released by the evaporator fan was computed for Tests 1, 2 and 3 in Table 1

assuming that it varied as the cube of the speed. The gross

refrigerating capacity was computed for each test in Table 1

assuming that it was equal to the sum of the net refrigerating

capacity and the evaporator fan power, as it was shown by test

to be in Test I4. A comparison of the values for computed gross compressor

capacity, computed heating effect of the evaporator fan, and

observed net refrigerating effect for the four tests summarized in Table 1 indicate the following conclusions^

11

1 . The gross compressor capacity increased steadily

with speed in the range from I33O rpm to 2I4I8 rpm.

2. The heating effect of the evaporator fan increased

rapidly with speed. There was more than a fourfold Increase

in the heating effect for the range of fan speed from 71^14 rpm

to 122k rpm and more than a six fold Increase in the heating

effect for range of fan speed from 7^4^ rpm to 1379 rpm,

3. The relatively high evaporator fan speed and corre=

spending high heating effect of this fan in Test 2 probably

accounted for the disproportlonally large decrease in net

refrigerating capacity observed as a result of conversion

from gasoline engine drive to electric drive,

I4. The fourfold difference in the heating effect of

the evaporator fan between Tests 2 and 3 probably explains

why a higher net refrigerating capacity was observed in Test

3 with a compressor speed of 1339 than was observed in

Test 2 with a higher compressor speed.

The results in Table 1 Indicate that the minimum fan speed

for effective operation, at least for the evaporator fan,

should be determined by suitable Investigation for the Model

MQ-51 Thermo “King unit, either gasoline engine or electric

drive, because of the significant effect the heating load of

the evaporator fan has on the net cooling capacity of the unit

in relation to the gross capacity of the compressor. Pending

such an investigation it is recommended that the drive assembly for the evaporator and condenser fans furnished with the

electric motor drive conversion kit be made the same size as

is normally used with the gasoline engine drive. Based on the

relative compressor and fan speeds observed in Test 1, the fan

speed for the condenser and evaporator fans, respectively,

when the compressor speed is reduced to 1703 rpm wou"* d be 1129

rpm and 972 rpm. with such a drive assembly. At 972 rpm the

heating effect of the evaporator fan would be (972 )® 36OO =

, {i23in®

1760 Btu/hr. Assuming the gross computed compressor capacity

at 1703 r>pm compressor speed in test 2 would remain constant

if the fan speeds were so reduced the net capacity would be

approximately 96I|0 - 176O = 788O Btu/hr, an increase of 1900

Btu/hr or 32^ sis compared with Test 2. It is, of course, recognized that the reduction in fan speed must be considered

in the light of proper air circulation vjithln the refrigerated

space, but the possibility of more than 39^ Increase in net

refrigerating capacity warrants consideration.
• 4 TJ
TABLE 1

COMPARISON OF PERFORMANCE OF THREE MQ^l THERMO-KING

UNITS WITH ELECTRIC MOTOR OR GASOLINE ENGINE DRIVE

Test 1 Test 2 Test 3 Test l4 NBS Specimen Number of Unit NBS #101 NBS #101 NBS #110 NBS. #1014

Prime Mover Gasoline Electric Gasoline Gasoline

Engine Motor Engine Engine

Thermo King Compressor Used A A B A

Net Refrigerating Capacity,

Btu/hr 9720 5980 7700 10,100

Ambient Temperature, °F 110.9 110.1 110.6 109.6

Warehouse Temperature, °F 0.6 2,8 1.1 -0.8

Compressor Speed, rpm 2l|l8 1703 1330 2291

Condenser Fan Speed, rpm 1601 1450 86I4 1471

Evaporator Fan Speed, rpm 1379 1243 7U4 1234

Discharge Pressure, psig I8I4.5 160 183 177

Suction Pressure, psig 2.1 3«9 - 0.6

Temperature of Air Entering

Condenser, °F 116.7 111.0 120

Temperature of Air Leaving

Condenser, °F 127,6 119.0 127,6

Temperature of Air Entering

Evaporator, 2,0 3o3 2.2

Temper ature of Air Leaving

Evaporator, °F -lo 6 0.9 ”3«2 _ Entering Evaporator Surface

Temperature, °F -8.3 -3.8 -10.9

Evaporator Surface Temperature, Midpoint, ^ -6.7 -2.7 -11.0

Evaporator Surface Temperature at T.E,V. Bulb, °F -5,8 -lc5 -L|.2

Computed Heating Effect of

Evaporator Fan, Btu/hr 5020 3660 810 3600

Computed Compressor Gross

Capacity, Btu/hr 11)740 961|0 8510 13700 a)

Compressor Speed Ratio 1.05 0.74 0.58 1

Gasoline Consumption, Ibs/hr 5.0 X » » Electric Power Input, watts X 5294 X X

a) This value is a calorimeter measurement for compressor A.
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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 hy 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. Repor.ts 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