NATIONAL BUREAU OF STANDARDS REPORT

2045

. Ventilating System for the Univac Computer

by

0. N. McDorman

Paul R. Achenbach

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Charles Sawyer, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT ms REPORT

1203-30-5790 November 4, 1952 2045

Tests of the Ventilating System

for the Univac Computer

by

0. N. McDorman

Paul R. Achenbaeh

Heating and Air Conditioning Section

Building Technology Division

for

Machine Tabulation Division

Bureau of the Census

The publication, ref

unless permission Is

25, D, C, Such perr

cally prepared If tha

Approved for public release by the

|n part|s prohlb|ted Director of the National Institute o

Standards, Washington Standards and Technology (NIST) eport has been sped fi- on October 9, 2015. >port for Its own use.

The Ventilating System of the Univac Computer

by

O. N. MeDorman

P. R. Achenbach

Abstract

Tests were made of the ventilating; system of

the Univac Computer for the Census Bureau to evaluate the temperatures and ventilation rates of t^ree

complete units when in operation. From these results it was found that with an outside temperature

of 105°F the installation would reauire a minimum

of 20,000 cfm of ventilating air to prevent shutdowns

by the temperature limit devices. The importance of

other factors were evaluated and their influence

on performance was analyzed. Recommendations on

the design and installation of the system at Suitlard,

Maryland, are presented. Precautions against recirculation, duct resistance and discomfort of operating

personnel were included.

After a careful consideration of the several

designs factors it was recommended that the system

be designed for a total of 30,000 cfm of ventilating air. The central computer was to be supplied

with 22,500 cfm and the power supply with 7,500 cfm.

I. INTRODUCTION

In accordance with a request of the Census Bureau

and the Electronic Computer Section of the National Bureau

of Standards tests were made to determine the operating

temperatures and ventilation rates of three computers

manufactured by the Eckert-Mauchly Division of Remington

Rand, Inc. Two units, designed as numbers 1 and 3, were

located at the plant of the manufacturer in Philadelphia,

Pennsylvania. The other unit, number 2, was located

at the Pentagon, Arlington, Virginia. These three units

are the first three commercial models of this computer

produced by the manufacturer.

Some interruptions in the operation of both of the

computer units in Philadelphia had occurred during this

past summer as a result of excessive operating temperatures.

The Heating and Air Conditioning Section of the National

Bureau of Standards was requested to analyze the ventilating systems of these units to determine the cause of the

interruptions and to recommend either a ventilating system

2

or an air conditioning system for unit number 1 when it

is instated in the Census Bureau at Suitland, Maryland

in the near future.

II. DESCRIPTION OF THE UNTVAC COT" Ft! TER

The Uni vac computer consisted of four ma.ior elements:

the central computer cabinet, the power supply, the servomechanism unit, and the control panel. In studying the

ventilating STrstem only the central computer cabinet and

tine power supply were of interest. The central computer

cabinet was a large metal enclosure approximately 14 feet

long, £ feet wide, and 7 feet high enclosing a great many

electronic elements which used electric energy and generated heat. Access to the inside components was provided

by a door on one of the long sides and various groups of

electronic elements were accessible from the outside

through three doors, one above the other, on each of the

13 bays into which the cabinet was divided.

The power supply cabinet measured about 4 by 6 by

7 feet high, was accessible from the outside on three

sides, and contained a door for access to the interior

components on the fourth side. Each of the three bays

contained rectifiers or other heat generating eouinment

arranged in 5 sections, one above the other. Diagrammatic

sketches of the central computer cabinet and power supply

are shown in Fig. 1 with the various bays numbered for

convenience in identifying where certain observations

were made.

THERMOSTATIC PROTECTION

A temperature-sensitive element of the bimetallicdisc type was located in the air stream at the top of

each of the 13 bays in the central computer cabinet and

the 3 bays in the power supply as a protective device

against excessive temperature. These devices were temperature limit switches all connected in series in a control

circuit so that an excessive temperature sensed by any one

of the 16 elements would interrupt the power to both units

until that element was cooled below the tripping temperature and manually reset. These temperature limits, also,

interrupted the power to the ventilating fans for fire

protection. A temperature limit of 135°F had been set as

the maximum safe temperature for the equipment and it was

some of these protective devices that had caused stoppages

of Units 1 and 3 in Philadelphia during hot weather.

- 3 -

VENTILATION

Ventilation air was drawn from the outside throng}

filters, and then forced by centrifugal blowers to a plenum under the floor beneath the central computer and

power supply cabinets. Air from the plenum passed upward through the electronic equipment in the power supply

cabinet and was exhausted outdoors by an overhead duct

system. In the central computer cabinet the air passed

upward through passages between the electron tubes and

other electronic devices and the inside of the exterior

doors on the 13 bays of that cabinet with a small portion

of the air being diverted through the interior of the

cabinet proner. The air stream passed directlv over,

more or less depending on the exact location of certain

elements, the thermostatic limit switch at the top of

each bay. The Pentagon unit, number 2, also employed

an exhaust blower since both the incoming and outgoing

ducts were ouite long and there might be some difficulty

in controlling the air conditioning of that area if a

single blower were used. Units 1 and 3 used separate

blowers and separate duct systems for central computer

and power supply whereas unit 2 used a common supply and

exhaust duct for both components.

At the Philadelphia installations both the air intake

and air discharge ducts terminated in an elbow opening

downward just outside the wall of the building above a

parking lot. The discharge duct was directly above the

intake and the distance between the two was about 10 or

12 feet. At the Pentagon installation the intake and

discharge openings were at ground level about 10 or 12

feet anart facing East and North respectively.

III. TEST APPARATUS AND PROCEDURE

Temperatures were measured by means of a Brown

Electronick Potentiometer Pyrometer Single Point Precision

Indicator. Corper-constantan thermocouples were affixed

with transparent tape around each of the 16 thermal elements.

Any effect of ambient air on the temperature indication

was minimized by having the junction and the adjacent

3 or 4 inches of wire firmly in contact with the metal supporting member. Additional temperature observations were

made of intake and exhaust air for both components. A more

detailed study was made of the temperatures within the power

supply cabinet at the Pentagon installation. A total of

24 thermocouples were suspended in air one inch above selected rectifiers and transformers. A 24-point selector

switch was used to connect anv one thermocouple to the

indicator. Temperature values were recorded usually at

15 minute intervals until a steady state condition was

reached.

- 4

Air velocity measurements were made with a revolving

vane anemometer at all installations. The cross sectional

area of the intake duct was divided into numerous elementary areas of approximately eoual size and the air velocity

was measured at the center of each small area. The air

velocities at each station were averaged mechanically over

a period of one minute which eliminated erroneous readings

due to pulsations. At the Pentagon additional readings

were taken bv the use of a Pitot tube and inclined gauge.

All observations were made directly in the duct system

inasmuch as preliminary measurements indicated that readings taken near the filter banks would be difficult to

evaluate. Values reported were obtained with all inspection

and access doors closed.

TV. TEST RESULTS

Table 1 shows the temperatures observed at the 16

temperature limit devices. The bays are designated in a

clockwise manner, number onp being tc the left of the

main entrance door when entering, as shown in Fig. 1.

Table 2 shows the temperature of the inlet and outlet air,

their difference, the maximum rise in air temperature at

any location, the quantity of ventilating air to the two

components, and their total. Also shown are the calculated

values of heat dissipation.

An examination of Table 1 shows that the temperature

rise in the several bays of the central computer differed

widely. The temperature rise observed in the hottest bay

was about 2-1/2 to 3 times as mreat as that in the coolest bay, and the averare temperature rise for all 13 bays

i^as only about half the maximum temperature rise through

the hottest bay. The three hottest bays in Unit 1 were

7, P and 10; in Unit 2 they were 7, 7 and 9; and in Unit 3

they were 7, $ and 9. An examination of these bays indicated that bays 7 and P contained a considerably greater

number of electron tubes than the others. Thus t

Vi ere was

probably more heat to be dissipated in these bays and at

the same time more obstruction to the air flow.

The temperature rises in the three bavs of the power

supply cabinet were more nearly uniform, than for the

central computer.

No specific reason was established why the average

temperature rise and the maximum temperature rise was greater for Unit 2 than for Unit 1 when the observed total air

circulation rates were nearly the same. However, there

were some leaks in the wooden duct on the intake side of

the blower of Unit 1 beyond the place where the air velocity measurements were made, whereas metal duct with

tight joints was used in Unit 2. Thus the actual air

circulation rate through the central computer of Unit 1

may have been appreciably greater than the observed value.

A more detailed analysis of the temperatures which

existed within the power supply cabinet was made upon

Unit 2. With an entering air temperature of FO°F, temperatures of 130. 0°F and 127. 5°F were recorded one inch above

the rectifiers located in the upper level of the bottom

section in bay No. 3. The highest value observed in bay

No. 1 was 116. 0°F. In the four sections directly above

in bay No. 3 temperatures ofl06.0°F, 107. 0°F, 114»0°F

and 104. 0°F were recorded. Corresponding temperatures

to the left of the entrance door in bay No. 1 were 91.0°F,

111 0 0°F , 101. 0°F, and 103. £°F.

Some recirculation of air between outlet and inlet

was observed in units 1 and 3. By a comparison of the

temperatures of the intake air and air over the parking

lot several feet from the intake and supply openings it

was determined that the inlet air was 7 to 10 decrees

warmer than the outdoor air. Of course, recirculation

would adversely affect the computer performance in hot

weather.

An approximation of the heat absorbed in the various

units can be obtained by using the temperature rise of the

air and the quantity of air circulated. The results of

this computation are shown in Table 2 both as units of

heat and units of electrical power. It was stated bv the

operating personnel that approximatelv 125 KW of power

was used by each unit. The computed values for Units 1

and 3 were appreciably lower than this value and thev were

20 to 30 KW lower than the computed value for Unit 2. The

computed power consumptions for t Ye central computer of

Units 1 and 3 and for the power supple of Unit 1 were appreciably less than the corresponding values for Unit 2.

These variations are attributed to the greater air leakage

into the intake duct systems and the greater outward leakage around the doors or the several bays of the cabinets

for Units 1 and 3 than for Unit 2. Another contributing

factor for the power supply of Unit 1 was the exceptionally small intake opening which would tend to increase the

air leakage even more.

- 6 -

V. DISCUSSION AND CONCLUSIONS

Based on Weather Bureau data for Washington, D.C.,

it was decided that the ventilation rate for the installation at Suit land, Maryland, should be based on a

maximum outdoor temperature of 105°?,, Assuming that the

temperature limit devices in the computer would be set

to shut the unit down at a temperature of 135°F, the

maximum temperature rise that could be tolerated in any

bay would be 30°F.

It is believed that the data obtained on Unit 2

were more accurate than for Units 1 and 3 because the

duct leakage was less, the ducts were longer and a better

measuring station could be found, and more time was avail

able for the observations. Tables 1 and 2 show that a maximum temperature rise of 35.5°F was observed in bay

No. 9 in Unit 2 for an air circulation rate of 14,550

cfm. It can be assumed that the product of the ventilating air rate and the temperature rise would be a constant

That is 14550 x 35.5 = 517,000 = K

Then for a maximum rise of 30°F the required air

circulation rate would be

V = 517.000 = 17000 cfm

30

Similarly for the power supply cabinet

K]_ = 3^50 x 23 = S9000

Vi = 89000 = 3000 cfm

30

These ventilation rates would be the minimum which

would give satisfactory operation during the hottest

weather and they provide no margin of safety. A factor

of safety should be incorporated in the selection of the

design ventilation rates to allow for the following contingencies :

(a) Errors in correctly evaluating duct friction

(b) Excessive dirt on the filters and leaves and

paper on the intake screen

(c) Recirculation between outlet and inlet due to

duct location or wind direction

(d) Loss of air from cabinets due to doors or panel

being left open for inspection or service.

It is recommended, therefore, that the ventilation

rate for the central computer be made about one-third

larger than the minimum value or 22500 cfm.

The decision about how much ventilation air should

be provided for the power supply is confused by the

failure of a number of selenium rectifiers in Unit 2

and the local temperature rise of 50°F observed near

one of these units in Unit 2. Based on the maximum

temperature observed on one of the limit switches in

the power supply of Unit 2, the minimum ventilation

rate would be 3000 cfm as previously shown. However,

considering that the temperature rise of the air near

one of the rectifiers was 50°F and that the rectifier

itself could easily have been 10 or 20 degrees warmer

than the downstream air, rectifier temperatures of the

order of 170°F seem entirely possible for the design

inlet ventilat ing i air temperature of 105°F. The available literature 3 j

21 on selenium rectifiers indicates

that they should be derated as much as 40 percent in

output voltage and current when operated at temperatures on the order of 170°F. By doubling the ventilation rate on the power supply of Unit 2 the temperatures of the hottest rectifier could probably be reduced 20 to 25 degrees. Probably the temperature rise

of this one rectifier could be reduced by a modification

of the air passages inside the power supply to direct

more air over it.

Considering the sensitivity of the selercium rectifiers to high temperatures it is recommended that 7500

cfm of ventilating air be provided for the power supply.

Iri our opinion, the cooling of the Univac computers

can be done adequately at less cost and without introducing the mechanical problems of refrigeration equipment

by the use of the proper quantities of ventilating air.

The difficulties with the present installation stem primarily from insufficient ventilating air, inadequate duct

system, and recirculation for Units 1 and 3, and from insufficient ventilating air and dirty intake screen and

filters on Unit 2.

a. Metal Rectifiers, by H„ K.Henisch (1949 ) Oxford

University Press, London.

b. Selenium Rectifiers, by S . Nicie jewski , Radio and

Television News, Oct. 1952.

3

The following factors should be given careful consideration in the design of the ventilating system for

the installation at Suit! and

(a) Care must be taken to prevent the heated

exhaust air from reentering the air intake.

Possible solutions would be to terminate

the supply and exhaust ducts on oprosite

sides of a wing of the building or face

the discharge and intake openings in opposite directions. The intake should

probably be upwind from the exhaust for

the prevailing summer wind direction.

(b) The intake for ventilation air should be

high enough above ground level so it will

not collect leaves , scrap paper , and trash.

This was an important source of trouble

on Unit 2 at the Pentagon.

(c) A reserve supply of air filters should be

kept on hand and adeouate maintenance of

filters should be assured. The blowers

should be selected to deliver the required

ventilating air with a reasonable dirt load

on the filters (say 0.5 inch water gage

pressure drop across the filters).

(d) Duct restrictions and ISO degree bends

should be avoided in the intake and exhaust

ducts

(e) A check test to determine the amount of air

provided should be made after installation

is completed and at stated Intervals during

use of the computers.

Consideration should be given to the installation of

a l6»point temperature recorder for a continuous record of

the temperatures at important locations to warn of impending shutdowns before they occur. Such an instrument would

cost approximately $2,000.

A manual reset of the control circuit on the ventilating fans that would permit starting the fans to cool the

TJnivac after it was determined that no fire existed in the

equipment would shorten shutdown periods that did occur for

any reason. An interlocking device should be used on this

manual reset so the main control circuit would again he the

nrimary control when the computer was in operation.

It was shown in Table 1 that several of the bays

in the central computer operated at temperatures well

above the average. Designers of the Univac computer

should give consideration to regrouping of the electric

components to attain more uniform heat °-eneration in the

several bays or provide more ventilating air for the

hotter bays by baffles or scoops in the supply plenum

or redesign of the air passages in these bays. Considerably less ventilating air could be used if the heat dissipation in the several bays were balanced.

Cabinet doors with less air leakage would greatly

aid in promoting the comfort of the operating personnel

especially if a single blower is used to force air through

the ventilating system. Locating the control panel and

the servomechanism unit in a room adjacent to the central

computer cabinet and the power supply cabinet would greatly enhance the comfort of the operators and reduce the

amount of comfort air conditioning required if that is

contemplated

Table 1

TEMPERATURES OF PROTECTIVE DEVICES

ON

UNIVAC COMPUTERS, °F

Bay Central Computer Power Supply

Unit No. Unit No.

No. 1 2 3 1 2 3

1 97.9 103.9 99.0 92.0 97.1 91.1

2 97.2 107.3 99.8 92.2 103.1 92.1

3 96.0 101.0 100.2 96.2 102.2 96.0

4 96.5 102.0 100.0

5 98.3 102.9 92.0

6 101.0 106.5 103.4

7 113.3 113.0 110.0

2 102.1 114.0 106.0

9 102.0 115.5 104.9

10 102.2 106.2 91.9

11 104.0 107.1 92.0

12 95.0 100.0 97.9

13 96.1 95.0 99.0

Average 104.6 105.3 100.2 93.5 100. 8 93.1

Inlet

Air Temp. 22.0 20.0 25.0 22.0 20. 0 25.0

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DIMENSIONAL



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