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NATIONAL BUREAU OF STANDARDS REPORT

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. AIR DELIVERY PERFORMANCE OF A HERMAN NELSON UNIT VENTILATOR

UNDER ISOTHERMAL CONDITIONS

by

Joseph C. Davis

Mechanical Systems Section

Report to

Government

of

District of Columbia

U. $. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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

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July 1957 to June 1960 (Includes Titles of Papers Published in Outside Journals 1950 to 1959) ($2.25); avail- able from the Superintendent of Documents, Government Printing Office, Washington 25, D. C.
NATIONAL ByHEJktJ @F STANDARDS REPORT

MBS PROJECT

1003-30-10630 -September 20, 1962 7714

AIR DELIVERY PERFORMANCE OF A HERMAN NELSON UNIT VENTILATOR

UNDER ISOTHERMAL CONDITIONS

by

Joseph C. Davis

Mechanical Systems Section

Building Research Division

to

Government

of

District of Columbia

IMPORTANT NOTICE

NATIONAL BUREAU OF STA ur 1 u

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.?

88 a“°un

.t, "g d^ument*

intended for use within the G Approved for public release by the satiy published it is subjected

to additional evaluation and re Director of the National Institute of ^production, or open-literature

listing of this Report, either if CtanHarrk and Terhnnlopv fNISTT

00

l* obta

.,ned m wrlfmg

the Office of the Director, Nat ^tandards and 1 echnology ( IN lb 1 ) !uch permission is not needed,

however, by the Government i on October 9, 2015. repared if that agency wishes

to reproduce additional copies

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

AIR DELIVERY PERFORMANCE OF A HERMAN NELSON UNIT VENTILATOR

UNDER ISOTHERMAL CONDITIONS

by

Joseph C. Davis

1 . INTRODUCTION

In accordance with a request from Mr. Norman G. Smith,

Engineer of Materials and Research, Bureau of Design

Engineering and Research, Department of Highways and Traffic

Government of the District of Columbia, tests were made on a Herman Nelson unit ventilator. These tests were performed - in compliance with specifications supplied by the Government

of the District of Columbia.

The unit ventilator under test was selected, by

representatives of the District Government from a number of

the same models which were being installed In one of the

District schools. The test specimen was identified as

Model No. QWPGJXG 4341-26 and serial No. 373194. Only tests

pertaining to air delivery were performed; no evaluation

was made of its heating performance. Figure 1 is a front

view of the unit ventilator tested.

The air delivery from the unit to the school room can

be 100 per cent fresh air , 100 per cent recirculated air or

any proportion of fresh air to recirculated air. between

these amounts. During normal operation, the proportion is

regulated by automatic control of dampers, but during

testing, the dampers were set manually at selected positions

to maintain fixed proportions of fresh air to recirculated

air.

The name-plate voltage was 115 volts. The supply voltage

to the permanent split-capacitor motor driving the four

centrifugal blowers, was adjustable by the use of taps on

a transformer. In order to attain a minimum air delivery

rate of 1000 cfm when the impressed voltage on the transformer

was 115 volts. It was necessary to use the transformer tap

delivering the maximum voltage to the motor. The Impressed

voltage on the transformer was maintained at the constant

value of 115 volts by means of a variac for all tests.
- 2-

2. METHODS OP TESTING

Air delivery from the supply grille of the unit was

measured by means of a pitot tube. The pitot tube measurements were made at 70 equally-spaced positions in the crosssectional area of a duct having the same cross sectional

area as the gross area of the supply grille and a length of

2 feet. The duct was attached at an angle conforming to

the angle of discharge of the air as determined during one

of the tests. No other duct was employed during the specification

test.

The frictional resistance encountered by the moving air

In the duct of 2 feet length at the discharge grille was not - appreciable. Since the duct was fixed at an angle conforming

to the angle of discharge* there was little effect due to

air- Impingement on the walls. Further, reference to the

Guide for 1961 of the American Society of Heating,

Refrigerating and Air Conditioning Engineers, will show that

at an air velocity of 500 fpm and for a duct of this length

and of 6 Inch diameter, the pressure drop due to resistance

is only about 0.002 inches W.G. Comparative air flow

measurements made using the unit ventilator, this duct, and

a duct at the fresh air grille also showed that the

pressure drop was not appreciable. The duct at the discharge

grille and at the fresh-air grille can be seen In Figure 3.

Figure 2 shows the pitot tube in position for measurement at the top of the duct at the discharge grille.

Measurements were made with the impact opening of the pitot

tube in the cross-sectional plane at the end of the duct.

There were no significant differences in results when

measurements were made with the Impact opening and the

static pressure holes of the tube well below this plane at

the end of the duct.

Impact pressure readings obtained by the pitot tube

were observed on a sensitive inclined manometer which had been

calibrated against a Hook gage.

Once the proper transformer tap was selected to obtain

a minimum air delivery rate of 1000 cfm with the fresh air

damper 100 per cent open, the rotational speed of the

blowers was measured. This measurement was made with a stroboscope and also with a tachometer. The tachometer

was applied to the blower shaft with as little pressure as

possible, consistent with no slippage in order not
- 3

-

to affect the motor speed. A deviation in current of 0.03

ampere from that occurring without the use of the tachometer

indicated little change in load during the speed measurement.

Air delivery rates through the discharge grille w^ere

measured under the following angular positions of the

outdoor-air damper:

100 per cent open

'50 per cent closed

75 per cent closed

100 per cent closed

For tests with all fresh-air damper positions, the voltage

impressed on the transformer was maintained at 115 volts

and rotational speed measured and checked in the same

fashion.

With the fresh-air damper 100 per cent open, the

starting current of the unit was measured by means of an

oscilloscope. This instrument was calibrated in terms of

current as a function of vertical displacement of the

electron beam on the screen. During steady operation,

temperatures of the motor casing were measured with a calibrated eopper-constantan thermocouple and Leeds and

Northrup type K-3 potentiometer. Impressed transformer

voltage, motor current, and energy usage readings were made

every 30 minutes of the test. A watthour meter was used for

determining the average power requirement. Wet- and dry-bulb

determinations were made using a sling psychrometer having

a new, clean wick for each test.

The air leakage through the recirculating-air damper

system was measured when the fresh air damper was in the

wide open position. To measure the small quantity of air

leakage occurring in this unit under these conditions, the

air that was leaking was forced to pass through a round 4- inch duct connected to a plenum fitted over the return air

opening. The rate of air flow was measured with the pitot

tube and manometer mentioned above. All openings at other

points in the housing where air leakage into the blowers

could occur , were sealed. Measurement was made using the

arrangement shown in Figure 4 .
-4-

To test the unit for ability to prevent "blow-through”

of winds from outdoors to the schoolroom, smoke-laden air

was forced directly toward the outdoor-air intake grille at

a velocity of 52 mph over about 33 per cent of the grille

area. The test was repeated for an adjacent one-third of

the grille area, and then finally for the remaining third

This test was performed with the outdoor-air damper 100 per

cent open, 50 per cent open, 25 per cent open, and fully

closed. Detection of amount of "blow-through" was made

by looking for an issue of smoke at the recirculating-air

grille.

All tests were performed with the outdoor grille and

medium-mesh screen assembly attached to the unit. A 6 inch

duct connected the grille to the screen assembly replacing

the opening in the masonry in the wall of the school during

normal .operation. Clean filters were used during all the.

tests.

The angle of discharge of the air leaving the discharge

grille was measured with a protractor which was sighted

against 20 threads placed at representative positions in

the grille.

The air delivery of the unit in every case was

calculated using the observed temperature and moisture

content of the air. Since a centrigugal blower is essentially

a constant volume device at a given fan speed regardless

of air density, the volume rate of air delivery would have

been the same if the ambient air had been at standard

conditions. The 70 pitot tube readings measured differences

between total and static pressure in 70 areas of approximately

equal size comprising the total grille area. Velocity for . each area and an average for all areas, was calculated in.

accordance with /the following relation:

where h is the average difference between total pressure

and static pressure, in. of water, as measured with the

pitot tube.

The operating noise level test was performed in the

National Bureau of Standards reverberation room. The fresh

air damper was closed and the unit was placed on a concrete

floor with the fresh air grille next to the wall.

Sound pressure levels were measured at four microphone

stations randomly selected in an area at least 17 feet from

the unit and more than 6 feet from boundary surfaces of the

V = 4(505

Density of Standard Air

Density of Amblent Air
-5-

room. Thus the microphone area was in a reverberant field,

free of direct radiation from the source and interference

from reflecting surfaces. The motor speed selector was in the

’’high” position and the recirculation damper was fully open

when measurements were made. The unit was operated from the

AC power line using a variac to maintain an applied voltage

of 115 volts during operation.

Sound pressure levels were averaged over a 50-second

interval for each octave frequency band, for a given

microphone position. The values thus obtained for' 4

microphone positions were averaged for each frequency band,

giving suitable space-time averages.

The values are rounded off to the nearest 0.5 decibel.

The accuracy of the sound pressure levels based on a single observation at each of four microphone positions is

given in terms of the standard deviation in db, i.e. the

deviations of the individual sound pressure levels from the

arithmetic mean level. The repeatability of measurements at

any given microphone position throughout the frequency range

was within ±0.2db.

3. TEST SPECIFICATIONS

The specifications supplied by the Government of the

District of Columbia are presented in the following paragraphs.

The tests are to determine that each unit tested

delivers the specified amount of air in cubic feet per minute

(CFM) corrected to standard air conditions. Tests of heating

coil capacity are not required.

The units chosen for testing will be models with room or

recirculated air admitted through louvered lower front

panels. All units are to be tested at free delivery (no

external resistance on unit discharge). Air deflection

vanes, if provided, should be set for no lateral deflection.

Units will be fitted with standard discharge grille normally

furnished.

. Units to be tested will be floor mounted models of a size designed to deliver 1,000 CFM standard air.

Units shall be tested with standard size outdoor air

Intake of standard construction (normally offered by the

manufacturer) in the correct position set at normal distance

from the outdoor air opening to the unit (approximately 6

inches from bird screen side of louver to unit).
- 6-

Units will be fitted with a clean, permanent type

filter. All units tested should be tested with filters

which are identical in construction and resistance

characteristics

During tests, records should be kept of fan motor

voltage, watts, amperes and temperature rise. Peak motor

starting current should be recorded for each unit tested.

Preferably the tests should be conducted in a room of

ambient, temperature of approximately 70°F.

A. Determine fan RPM required to deliver 1,000 CFM standard

air with outdoor air damper 100 per cent open and room . air damper closed. Also, determine amount of air

leakage thru room air damper at this condition.'

B. Set- fan at the RPM obtained in test A above. and determine

CFM at several damper positions with outdoor air damper

100 per cent open to completely closed. Room air damper

, will vary inversely from completely closed to 100 per cent

open.

C. Test "Blow-through” prevention performance, which should

prevent gusts of outdoor air from blowing through the unit

room air intake louver into the room, by setting the

outdoor air damper at various positions and blowing

high velocity air through the outdoor air opening of the

unit. Tests should be made with unit fans operating at

RPM determined in Test A above at free delivery into the

room.

D. At 1,000 CFM standard air delivery, the units shall be

„ tested for operating noise level by an approved method.

Each unit test for noise level shall be identical as to

method.

E. At free delivery into the room through a standard,

normally furnished discharge grille, determine the angle

of discharge leaving the unit and the average leaving

velocity of the air. This test shall be performed with

the unit delivering 1,000 CFM standard air. Deflector

vanes, if provided, shall be set for no lateral deflection.
-7-

4 . TEST RESULTS

The following test results are reported:

TEST A. (l) Fan speed required to deliver 1000 cfm

standard air with outdoor air damper

100 per cent open and room air damper

closed . 680 rpm

( 2 ) Amount of air leakage through room air

damper at this condition 40 cfm

The actual air delivery for a fan speed of 680 rpm was 1065 cfm.

The fan speed for this test was adjusted, as nearly as possible

to an air delivery of 1000 cfm, by the selection of one of

several transformer taps in the electrical supply line to the

fan motor. The transformer tap providing the highest available

output voltage was required to attain this air delivery. The

impressed voltage on the transformer was maintained at 115

volts

.

TEST B. The air delivery rate from the discharge duct, the fan

speed, the motor current, and motor power were observed for

several positions of the outdoor damper and at constant

impressed voltage. The results are summarized in the following

table

Position of

Outdoor-Air Damper

Rotational

Speed of

fan

Air

Quantity

Motor

Current

Impressed

Voltage' Power

Angular Displacement) ~Trpm) (cfm) (amp) (volts

)

(watts

)

100 per cent open 680 1065 . 1.93 115.3 . 180

50 per cent closed 670 1080 2.00 114.6 185

75 per cent close'd 670 1040 1.95 115.0 180

100 per cent closed 710 970 1.80 115.0 170
Ambient

Air Temperatures

Position of

Outdoor-Air Damper

(Angular Displacement)

Dry Bulb

(°F)

Wet Bulb

TFT

Average

Value for “\|K

(in. H20;

100 per cent open 84.1

50 per cent closed 70.0

75 per cent closed 69.0

100 per cent closed 80.0

66 .0.134

67.0 0.137

63.0 O .132

68 .

0.122

Cross sectional area of discharge duct 1.95 sq.ft.

TEST C. There was no evidence of "blow-through” when smoke-laden

air was forced directly into the outdoor-air intake grille at

a velocity of 52 mph over about 33 per cent of the grille area,

for outdoor damper positions of 100 per cent open, 50 per cent

closed, 75 pe r cent closed, and 100 per cent closed (angular

displacement )

TEST D.

Sound Pressure Levels in db, re 0.0002 dyne/cm2

Mid-frequency

Octave Bands Sound Pressure Levels Standard Deviation

cycles/sec db db

50 51.5 1.0

100 56.5 0.5

200 56.5 0.5

400 57.0 0.5

' 800 56.5 . 0.5

1600 53.0 0.5

3150 46.0 0.5

6300 • 39.5 1.0

Computed Overall Sound Pressure Level 63.5

The sound pressure levels given above were normalized to a reference room absorption of 10 square meters. The air temperature was 71°F ±2 and the relative humidity was 72 % ± 2 in

the reverberation chamber.

TEST E. The angle of discharge of the air through the discharge

grille was 70° ±5° referenced to the horizontal. The average

leaving velocity with the outdoor damper 100 per cent open

was 545 fpm.
-9-

While the unit was operating with the outdoor-air damper

100 per cent open, peak starting current was 4 ± .2 amperes.

The maximum temperature rise of the motor casing was 40°F above

an ambient temperature of 85 °F. The maximum temperature was

reached after 2.5 hours of operation.

5. APPENDIX

The following information on components of the unit

ventilator was supplied by the manufacturer.

Motors: Permanent - split, capacitor type motor, rated

1/8 HP, 700 rpm, 115 volt, single phase, 60 cycle, sleeve

bearing, 48 frame, with thermal protection.

We have three suppliers of these motors. Century rated

1.5 amps. General Electric rated 2.0 amps, Westinghouse

rated 1.75 amps.

This motor has two windings, main winding and a starting

or auxiliary winding, the capacitor being in series with

the auxiliary winding. By means of an autotransformer

with various voltage taps ranging from 115 to 45 volts,

we vary the voltage to the main winding only, thus

varying the motor speeds.

The autotransformer is supplied by either the Apex or

Dormeyer Transformer Companies.

Fans : We manufacture the fan wheels used in our unit

ventilators . Specifications of materials used are:

Fan wheel rims

Fan wheel blades

Fan wheel center

Fan wheel hub

- aluminum,

- aluminum,

disk - steel,

- steel.

16 ga. (.051")

20' ga , ( .032" ) 16 ga. (.060”)

12 ga. ( .105")

Fabrication of this wheel is all by mechanical means

except that the hub is spot welded to tne center disk. A

socket head type set screw is provided for attaching the wheel

to the fan shaft.

Filter : We manufacture the filters used In our Unit

VentTlators . The filter in question is our permanent type

with overall dimensions of 12” x 43-13/16” x 1”.

The frame is constructed of 22 ga. (.030”), zinc coated

steel.
- 10-

The media is constructed of three layers of #14 mesh

galvanized wire screen. The cross-section of each layer has

a specific configuration.

After fabrication the. filter is sprayed with a light

coat of "SH" or "TEH Viscosine.

This particular size filter offers a resistance of

0.030 Inches of water at 1000 cfm.

USC0MM-NJ3S-DC
.ure
1






U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

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

'The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the 'following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering! * n Reid indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover. WASHINGTON, IJ. C.

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Dielectrics. High Voltage.

•Metrology. Photometry and Colorimetry. Refractometry. Photographic Research. Length. Engineering Metrology.

Mass and Scale. Volumetry and Densimetry.

Heat. Temperature Physics. Heat Measurements. Cryogenic Physics. Equation of State. Statistical Physics.

Radiation Physics. X-ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipment.

Nucleonic instrumentation. Neutron Physics.

Analytical and Inorganic Chemistry. Pure Substances. Spectrochemistry. Solution Chemistry. Standard Refer- ence Materials. Applied Analytical Research. Crystal Chemistry.

Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Rheology. Combustion

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Crystallography.

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Instrumentation. Engineering Electronics. Electron Devices. Electronic Instrumentation. Mechanical Instru- ments. Basic Instrumentation. Physical Chemistry. Thermochemistry. Surface Chemistry. Organic Chemistry. Molecular Spectroscopy. Ele- mentary Processes. Mass Spectrometry. Photochemistry and Radiation Chemistry.

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