NATIONAL BUREAU OF STANDARDS REPORT

3385

PERFORMANCE TESTS OF AN AAF CO.

ELECTRONIC PRECIPITATOR

by

Henry E. Robinson

Thomas W. Watson

Report to

General Services Administration

Public Buildings Service

Washington 25, D, C.

<QJBS>

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 /

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section is engaged in specialized research, development, and engineering in the field indicated by its title. A brief description

of the activities, and of the resultant reports and publications, appears on tbe inside of the back

cover of this report.

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Measurements. Electrochemistry.

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^Office of Basic Instrumentation ^Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-4837 June 28, 1954 3385

PERFORMAKCE TESTS OF AN AAF CO.

ELECTRONIC PRECIPITATOR

EIECTRO-CELL Model A

t>y

Henry E. Robinson

Thomas W. Watson

Heating and Air Conditioning Section

Building Technology Division

to

General Services Administration

Public Buildings Service

Washington 25, D. C.

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, re|

unless permission Is

25, D,C, Such pen

cally prepared If tl

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

In part, Is prohibited

andards, Washington

port has been specIHiport for Its own use.
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PEKFORMAUCE TESTS OF AN AAF CO.

MODEL A ELECTRO-CELL ELECTRONIC PRECIPITATOR

1, INTRODUCTION

At the request of the Public Buildings Service^ General Services Administration, the performance characteristics of electrostatic air cleaners were determined to provide information to assist in the preparation of new air

filter specifications.

The test results presented herein were obtained on a specimen electrostatic filter unit submitted by its manufacturer at the request of the

Public Buildings Service and included determinations of dust arresting efficiency with three aerosols (atmospheric air, kerosene laat^) smoke, and

Cottrell precipitate), pressure drop, specific dirt load and cleanability

of the specimen.

2. DESCRIPTION OF THE FILTER SPECIMEN

The cleaner was manufactured by the American Air Filter Compai^, Inc.,

Louisville 8, Kentucky, and was of the electrostatic type. It was identified

by nameplate as an AAF Electro-Cell Model A3, Size 230, 1200 CFM, Serial

553090 . A representative of the mnufacturer, present during the tests, stated

that the nameplate was incorrectly marked, and that in accordance with AAF

Bulletin 252-C it should reads "Model A, Size 230, 15^0 CFM (for a rated air

velocity of 333 feet per minute) Serial 553090." The power pack had the

following nameplate datas

AAF Power Pack

Max. Input I .5 an5)s, 115 V - 60 cy 1 PH 100 watts

Max. Output DC 2.2 MA Ionizer I3 .OKV Collector 6 . 5KV

The test unit had a housing with actual outside dimensions of 24-^ inches

in width, 36 I/8 inches in hei^t and was 24 inches in length, and was rated

as having 4.6 sq. ft. of transverse area, i. e., 1540 CFM air delivery at

333 fpni velocity. Transitions 15 inches in length, with flanges 30 inches

square matching those of the duct of the test apparatus, were used to adapt

the unit for test. The openings formed by these transitions were 24 inches

square both upstream and downstream. The downstream face of the Electrocell housing was adapted to receive a nominal 24 x 31 x 2 inch after-filter,

which for this unit was a 23 3/4 x 31 inch pad of Amer-glas (throwaway type)

sandwiched and compressed to a thickness of approximately 13/4 inch between

two wire grids.

There were seven filter cells in the housing (five full-size and two

half-size) as follows? three similar charged plate cells having outside
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dimensions of 8 3/8 inches in hei^t, 12 inches in lengthy and 20 1/8 inches

in widths and each containing 28 alijminmn plates; two similar grounded plate

cells 8 3/8 inches in height, 12 inches in length, and 21 inches in width;

and each containing 29 al^3lninum plates; two half-size grounded plate cells

k 9/16 inches in height, 12 3/8 inches in length and 21 inches in width,

and each containing 29 aluminum plates o The charged plate and grounded plate

cells were assembled in the housing in position so that the charged plates

meshed with the grounded plates. As assembled, adjacent plates were spaced

approximately ^/l6 inch apart and a total plate surface area of approximately

241 square feet was presented. A stamped hole, about 4^ inches in diameter,

at the top of the ionizer assembly frame, and through which the high potential

bar passes, was closed by the manufacturer's representative with a circular

disc of polystyrene about inch thick.

The manufacturer furnished an adhesive designated ae "Viscosine BA",

and an applicator for oiling the collecting plates ©f the cells by spraying

them from the upstream face. The ionizer section and pierforated grille were not oiled for the tests. The Amer^glas after-filter had a sli^t viscid

coating on its media as submitted and did not require oiling.

The power pack, connected to a 115“Volt 60-cycle supply, was checked

by the manufacturer's representative as being adjusted to recommended settings

prior to the tests; the ionizer and plate voltages that resulted were measured

by means of a high-resistance voltmeter which was conrpared with an accurate

electrostatic voltmeter.

3o TEST METHOT AMD PBOCEDURE

Efficiency determinations were mde by the MBS "Dust-Spot Method" using

the following aerosols; (a) outdoor air drawn throu^ the laboratory without

addition of other dust or contaminant; (b) kerosene lanip smoke; and (c)

Cottrell precipitate. The test method is described in the paper "A Test

Method for Air Filters" by R. S. Dill (ASSVE Transactions, Vol. 44, P, 379p

1938). The test duct and arrangement are shown in Figure 1. A baffle made

of two 3“inch wide slats was located in the duct about 3^ ft downstream of

the test assembly to intermix the air discharged from it.

For these tests, the unit was installed in the test duct and carefully

sealed t© prevent inleakage of air. The desired rate of air flow throu^

the air cleaner was established and samples of air were drawn from the center

points of the test duct one foot upstream and eight feet downstream of the

air cleaner assembly at equal rates and passed through known areas of Whatman

No. 4l filter paper. For the atmospheric air and oil lamp smoke tests, the

samples were drawn at equal rates through equal areas of filter paper (3/4-inch

diameter spots). The downstream sample was drawn continuously during the test;

the upstream sample was drawn intermittently in a number of one-minute periods

uniformly distributed over the duration of the test, aggre^ting one-tenth of

the downstream sampling period. Under these conditions an efficiency of 90 percent
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would be indicated if the upstream and downstream dust»spots on the filter

papers had the same opacity, as indicated by the change in the light transmissions of the dust“Spot areas before and after the sainple was drawn, which

were determined by means of a photometer using transmitted light o The filter

papers used in the upstream and downstream positions were selected to have

the same li^t transmission readings when clean » If the opacities of the

dust-spots differed, the efficiency was calculated by means of the formula

where 0^^ and O2 were the opacities of the dust°spots upstream and downstream,

respectively, and t^ and t2 were the aggre^te times during which the upstream

and downstream samples, respectively, were drawn

For the efficiency tests with Cottrell precipitate as the aerosol, the

samples upstream and downstream were drawn at equal rates and for equal times

but unequal dust-=spot areas were used to obtain opacities that were approximately ec^lo If the opacities of the dust-spots differed, the value of the

efficiency was calculated by means of the fonmila above, with the ratio

^2/^1 substituted for the radio "^l/t2, where A2 and Ai were the areas of the

dust-spots downstream and upstream respectively,,

The following procedure was employed in these tests o After the clean

and oiled unit had been installed in the test duct, and all discoverable air

leaks into its housing had been sealed, its input and output voltages were

adjusted to recommended values by a representative of the manufacturer;

(input 115 volts j ionizer .13 oij. kv; plates 60 7 kv)o Three determinations of

the efficiency of the cleari unit were mde at the rated velocity, using as

the aerosol outdoor air drawn into the test duct throu^ a nearby open windowo

A determination of efficiency with the unit not energized was also madeo

Following these, single determination® were mde, using outdoor air, at

velocities 20 percent greater, and 20 percent less than the rated velocityo

Next, three efficiency determination® were made at rated velocity, using

as the aerosol outdoor air with the addition of kerosene smoke generated by

an open lamp flame near the inlet to the test due to

Following these, three efficiency detenninations were made at rated

velocity, using as an aerosol outdoor air in which was dispersed Cottrell

precipitate at a concentration of one gram per thousand cubic feet of airo

When these had been obtained, the process was begun of loading the unit with

a mixture of k percent cotton lint and 96 percent Cottrell precipitate, by

wei^t, separately dispersed into the air streamo The lint used for this

purpose was NOo 7 cotton linters previously ground in a Wiley mill with a

4 -millimeter screen; the lint was dispersed into the air stream through an

aspirator operating at approximtely 35 psi inlet air pressure. At suitable

Efficiency, percent s loo
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periods as loading progressed, the efficiency of the unit was determined

using 100 percent Cottrell precipitate in outdoor air<, In these tests, and

during the loading process, the rate of feed of the dispersant was one gram

per thousand cubic feet of air. The pressure drop and the ionizer and plate

voltages of the unit were recorded at intervals during the tests » The dirtloading process was continued until about 1060 grains of the lint and Cottrell

precipitate mixture had been fed (ioCo, 2/3 gram per CFM of unit rating)*

At suitable periods as the dirt-loading process progressed, strips of

transparent cellulose adhesive tape (3/^ inch wide) were stretched vertically

across the test duct near its axis, with the adhesive side facing upstream*

Tapes were located at three positions (l) 27 inches upstream, (2) 30 inches

downstream* and (3) 9 ft downstream, of the test unit; the tapes at stations

(l) and (3) were in the same longitudinal positions in the test duct as the

inlets to the upstream and downstream sampling tubes* The adhesive surface

of such a tape captured a san^le of the particulate matter in the air flowing

past it, and after suitable times of es^osure t© the aerosol, scrutiny of

the tapes by eye and with a microscope afforded considerable information as

to the vertical distribution, the nature, number, and size of the particles

caught at the various stations* Photographic enlargements (lOX) by transmitted light were made ©f sections of the tapes corresponding to a position

at mid-height in the test duct*

The filter cells were removed from the test unit and cleaned by means

of a stream of cold water from a high pressure hose nozzle, directed at and

into the cell plates from both ends ©f the unit*

4* TEST BESULTS

A summary of the test data, giving efficiencies in percent with the

three aerosols, and the pressure drop of the conflate unit including the

after-filter, in inch W*Go, at rates of air flow corresponding to various

face velocities, is given in Table 1* Also, a summary of the test data

obtained in the dirt-loading test conducted at the rated face velocity of

333 fpro given in 'Kble 2*

Photographs No* 1, 2 and 3 included in this report are lOX enlargement®

of the center 3/^xl-inch section ©f each of the cellophane tapes located at

the three similarly-nimibered stations described under Test Method and Procedure* The tapes were exposed at their respective stations simultaneously

during the dirt-loading test for a period in which 124 grams of mixture

were fed to the test imit* This es^osure was during the interval in which

the dirt-load increased from 45 to I69 grams, as shown in Okble 2* After

exposure, the tapes -were carefully removed from the test duct for photographing

and microscopic study*
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“ 5

-

Throughout the tests with atmospheric air electrical sparking or flashing

in the unit audible to the ear occurred intermittently U to 6 times per hour»

There was no instance of sparking or flashing during the kerosene lan^) smoke

tests o However, during the Cottrell precipitate and lint loading test electrical

sparking occurred intermittently, the frequency increasing from 10 to 12 times

per hour at the start to as much as 75 to 90 times per hour at 600 grams of

load, and reducing to about 8 to 10 times per hour at the end of the loading

test.

5c SUMMARY

A. Performance

The efficiency of the air cleaner in arresting the particulate matter

existent in atmospheric air drawn throng the unit varied considerably with

the face velocity at which it was operated, as shown in Table 1. At the

rated velocity (333 the average efficiency on atmospheric air (90.0

percent), and that on ©il lamp smoke (91c 3 percent), were very nearly the

same. The efficiencies are reported to three significant figures obtained

from the test data. In reporting thus, however, it is considered desirable

to point out that an uncertainty on the order of one or two percent is

possible in determining efficiencies, althou^ in these results the differences

between comparable efficiency values were consistently of a lesser magnitude.

The greater part of the pressure drop throu^ the conrplete unit was due

to the resistance of the after-filter. It is noted in Table 2 that in the

dirt-loading test, the pressure drop of the conrplete unit increased by 0.0^0

inch W.G. for a total dirt-load of IO63 grams. This rise was due chiefly

to an increase in the pressure drop of the after-filter, as a result of an

accumulation of cotton lint and of comparatively large particles of Cottrell

precipitate on its media.

As recorded in ^ble 2, the ionizer and plate voltages remained substantially constant throughout the dirt-loading test. The efficiency of the

unit on Cottrell precipitate as the aerosol remained at a hi^ level, never

falling below 9^.9 percent and ending with a value of 98°? percent, approximately equal to that at the start of the loading test. An efficiency determination made with atmospheric air at the end of the dirt-loading test

indicated that the unit was as efficient on this aerosol when dirt laden,

as it was when clean,

B, Cleanability

The filter cells were subjected to the cleaning process described under

Test Method and Procedure. No difficulty was experienced in thoroughly cleaning

the ionizer and collector sections of the unit, using moderate care. The

after-filter was not cleaned since it was of the throwaway type.
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- 6 -

C o General

On completion of the dirt-loading test, the xmit was removed from the

test duct and examined. Dirt deposits were heaviest on the upstream edges

and first 3 or 4 inches of the collector plates, the thickness of the deposits

being up to about l/32 inch. The heaviest deposits were observed on the

leading edges of the grounded plates. A continuous, but thinner, layer of

dirt was deposited over the remaining area of the collector plates, extending

to the after edge. Considerable bridging of lint fibers from one collector

plate to another, spanning the gap between them, was observed; such bridging

appeared to be most extensive in alternate plate spacings and in the narrowest

plate spacings. The perforated upstream grille and ionizer assembly were fairly clean since they were not oiled prior to the tests.

The upstream face of the after-filter revealed considerable deposits

of lint, somewhat concentrated in five vertical strips each about 3 inches

wide. The after-filter media was approximately unifonnly darkened by a

dust deposit on its upstream face, and slightly darkened by dust visible on

its downstream face. However, there were eight very dark patches located

at the vertical edges of the upstream face of the after-filter approximately

1x2 inches in size four on each side. On close examination, it was observed

that these patches conformed somewhat to air-paths (or leaks) between the

outer ground plates and the unit housing. It seemed evident that a portion

of the air flow by-passed the collecting plates and that some of the entrained

unarrested aerosol was caught by the after-filter. It is difficult to say

how much effect these by-pass leaJss had on the over-all performance of the

unit. However, a reduction of these leaks would result in a higher efficiency.

The dirt (dust) deposits on the electrostatic unit, and on the afterfilter, appeared to be we11-saturated with oil. Microscopic examination

of the cellophane tape exposed at station (2) showed a few droplets of what

appeared to be oil, but the number and amount of such droplets were so small

that oil entrainment in the effluent air is considered of no practical importance, After the unit had been removed from the test duct, the section

downstream of the unit was carefully swept out with a fine brush. The amount

of material obtained from the duct by this sweeping was 0,6 gram.

The cellophane tape san^sles obtained at stations (l), (2) and (3)5. as shown in Photographs No. 1, 2 and 3^ respectively, indicate in a general

way the performance of the complete unit. Photograph No. 1 shows many

particles under 5 microns in actual size, and a distribution of larger

particles up to a few as large as 400 microns, as well as mny fibers of lint,

(in these photographs, I/16 inch corresponds to an actual dimension of about

160 microns,) Photograph No, 2, for the tape 30 inches downstream of the

unit, shows a few large particles up to about I50 microns in size, but very

few fine dust particles considering their number upstream of the unit. Photogrph No, 3? for the tape 9 ft downstream of the unit, shows an absence of the
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- 7 -

larger particles of Photograph 2, which apparently settled out of the air

stream. No significant amount of lint fibers is visible in either downstream

tape photograph, or was observed by microscopic study of the tapes.

Comparison of the numbers of particles on the upstream and downstream

tapes indicates, in an obvious way, a high order of efficiency for the unit

in arresting Cottrell precipitate, as is also indicated by the discoloration

test results presented in Table 2. The latter results show a considerably

higher efficiency for the unit when Cottrell precipitate was being received

in the air stream than when the aerosol was outdoor air or kerosene lamp

smoke. The overall efficiency of the unit on particles of the sizes found

in Cottrell precipitate appears therefore to be better than on the finer

particles in outdoor air or kerosene smoke. Nevertheless, the downstream

tapes, and the deposits on the after-filter, show that a few quite large

particles of dust escaped beyond the electrostatic unit. Whether the large

particles were passed through the iinit because they were not arrested at all, or were caught and later dislodged from the collector plates by electrical

sparking, is not known from these tests.

As the photographs show, a few large particles passed unarrested through

the after-filter. Assuming that one of the functions of the after-filter is

to arrest as much as possible of the material escaping the electrostatic unit,

the arrestance characteristics of the after-filter are of major importance in

determining the presence or absence, in the air leaving the con5>lete unit,

of such particulate matter as is shown in the downstream tape photographs.
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Table 1

Face

Velocity

Inlet

Aerosol*

Ionizer

Voltage

Plate

Voltage

Pressure

Drop

Duration

of Test

Efficiency

percent

fpm kv kv inch WG minutes

333 A 0 0 0.273 18 15.8**

333 A 13 A 6.7 .272 180 90.3

333 A 13.6 6.8 .272 180 89.7

333 A 13.6 6,8 .272 180 90.0

266 A 13.6 6.8 .180 180 93.9

to A 13.6 6.8 .370 180 Ql.k

333 S 13.6 6.8 .272 20 91.2

333 S 13.6 6.8 .272 20 91.7

333 S 13.6 6.8 .272 20 91.0

333 C 13.6 6.8 .272 10 98.5

333 C 13.6 6.8 .272 9 98.3

333 C 13.6 6.8 .272 10 98.6

* A = Particulate matter In atmospheric air at NBS.

S = Kerosene lao^ smoke in atmospheric air.

C = Cottrell precipitate in atmospheric air (l gram/lOOO CF).

** Since unit was not energized, the efficiency was chiefly that of the

after-filter.
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Table 2

Face

Velocity

Dirt

Load*

Ionizer

Voltage

Plate

Voltage

Pressure

Drop Efficiency**

fpm grams kv kv inch W.G. percent

333 13.6 6.8 0.272 98.5 (avg.)

169 13.6 6.8 .273 —

183 13.6 6.8 .273 97.9

327 13.8 6.9 .279 98.0

487 13.8 6.9 .283 98.1

615 13.8 6.9 .293 97.9

742 13.8 6.9 .297 97.8

870 13.6 6.8 .301 96.9

998 13.8 6.9 .312 98.2

1047 13.6 6.8 .312 98.8

1063 13.8 6.9 .312 98.7

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\init dirty.
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Filter Frame

Down Stream Duct

VERTICAL SECTION

Arr Pilfer Test Apparotus

Figure 1

Photograph No. 1
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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 Ofl&ce: 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 ($1.00). Information on calibration services and fees can be

found in NBS Circular 483, Testing by the National Bureau of Standards (25 cents). Both are

available from the 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