NATIONAL BUREAU OF STANDARDS REPORT

1)807

PERFORMANCE TESTS OF AN ELECTRO-AIR

ELECTRONIC AIR CLEANER

by

Henry E. Robinson

Thomas W. Watson

Report to

General Services Administration

Public Buildings Service

Washington ,25, D. C.

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS

A. V. As! in. Director THE NATIONAL BUREAU OF STANDARDS

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BOULDER, COLORADO

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Circuit Standards.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-I4830 October 10, 1956 I4887

PERFORMANCE TESTS OF AN ELECTRO -AIR

ELECTRONIC AIR CLEANER

by

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.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repr

unless permission Isol 25,D.C. Such permi

cal ly prepared If tha

Approved for public release by the

par1, is prohibi,ed

Director of the National Institute of

dards ’ Washingl0n _ , . rt has been specifi- Standards and Technology (NIST) )rt for its own use . on October 9, 20 1 5.

PERFORMANCE TESTS OF AN ELECTRO-AIR MODEL B-102

ELECTRONIC AIR CLEANER

by

Henry E. Robinson and Thomas W. Watson

1. INTRODUCTION

At the request of the Public Buildings Service, General

Services Adraini strati on, 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 lamp 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 Electro-Air Cleaner

Company, 1285 Reedsdale street, Pittsburgh 33, Pennsylvania,

and was of the electrostatic type. It was identified by the

manufacturer as an '’Electro-Air Electronic Air Cleaner,”

Model B-102, with one cell, rated velocity 333 feet per

minute (1320 cfm total capacity). The power pack nameplate

data furnished by the manufacturer were as follows:

Type B

Input: 60 cycles, 115 volts, 2 amp primary (maximum)

Output: 8,500 volts DC, 20 ma secondary

The test unit had a housing with transverse outside

dimensions 2l\ x 22 7/b inches and was 20 inches long. The

unit was rated at 1320 cfm air delivery, which yields a nominal face velocity of 333 fpm, on the basis of a nominal trans

verse area of about four square feet.

- 2 -

The upstreara and downstream faces had special flanges

30 inches square matching those of the duct of the test apparatus. The face openings were approximately 2 iq x 23 inches

both upstream and downstream. The downstream face was adapted

to receive a nominal 23 x 21 x 2-inch after-filter which for

this unit was a 23 x 20 7/8 x 1 7/8-inch viscid impingement

type air filter, manufactured by the Research Products Corporation of Madison, Wisconsin.

The filter cell of the unit was 23 13/16 inches in

height, 22 9/l6 inches in width, and 17 3/k inches in length,

and contained 57 aluminum plates (23 x 10 3/I4 inches in

dimension) spaced 3/8 inch apart on centers, presenting a total surface area of approximately 196 square feet.

The manufacturer furnished an adhesive (a water-emulsifying oil) designated as "Shell 366-S," with which the

collector plates were oiled by spraying them from both the

upstream and downstream faces. The after-filter was oiled

in preparation for the test by spraying the media from the

upstream and downstream faces with the same adhesive.

The polder pack, connected to a 115-volt, 60-cycle

supply, was adjusted by the manufacturer’s representative

to recommended settings prior to the tests; the ionizer and

plate voltages that resulted were measured by means of an

accurate electrostatic voltmeter.

3. ThST METHOD AND PROCEDURE

Efficiency determinations were made by the NBS "DustSpot Method" using the following aerosols; (a) outdoor air

drawn through the laboratory without addition of otiier dust

or contaminant; (b) kerosene lamp smoke; and (c) Cottrell

precipitate. The test method is described in the paper,

"A Test Method for Air Filters" by R. S. Dill (ASIiVE Transactions, Vol. Iq iq , P. 379, 193®) • The test duct and arrangement are shown in Figure 1. A baffle made of two 3 “inchwide slats was located in the duct about 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 to prevent inleakage of air. The

desired rate of air flow through the air cleaner was established and samples of air were drawn from the center points
*

*
~3

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. I4I 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/lj-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, aggregating one-tenth of the downstream sampling period. Under these conditions an efficiency

of 90 percent 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 sample was drawn, which

were determined by means of a photometer using transmitted

light. The filter papers used in the upstream and downstream

positions were selected to have the same light transmission

readings when clean. If the opacities of the dust-spots

differed, the efficiency was calculated by means of the

formula

Efficiency, percent=10 C

where Op and 0 2 were the opacities of the dust-spots upstream

and downstream, respectively, and tp and tg were the aggregate

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 w ere drawn at

equal rates and for equal times but unequal dust-spot areas

were used to obtain opacities that were approximately equal.

If the opacities of the dust-spots differed, the value of the

efficiency was calculated by means of the formula above, with

the ratio A^/Ap substituted for the ratio t-^/t2, where A2

and Ap were the areas of the dust-spots downstream and upstream respectively.
*
The following procedure was employed in these tests.

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; ionizer and plates 8.14 kv). Three determinations of the efficiency of the clean unit were made at

the rated velocity, using as the aerosol outdoor air drawn

into the test duct through a nearby open window. A determination of efficiency with the unit not energized was also

made. Following these, single determinations were made,

using outdoor air, at velocities 20 percent greater, and 20

percent less, than the rated velocity.

Next, three efficiency determinations 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 duct.

Following these, three efficiency determinations 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 air. When these had

been obtained, the process was begun of loading the unit

with a mixture of L| percent cotton lint and 96 percent

Cottrell precipitate, by weight, separately dispersed into

the air stream. The lint used for this purpose was No. 7

cotton linters previously ground in a Wiley mill with a

L[ -millimeter screen; the lint was dispersed into the air

stream through an aspirator operating at approximately 35> psi inlet air pressure. At suitable 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 899 grams of the lint and

Cottrell precipitate mixture had been fed (i.e., 2/3 gram

per cfm of unit rating).

-5

-

At suitable periods as the dirt-loading process progressed,

strips of transparent cellulose adhesive tape (3/4 inch wide)

were stretched vertically across the test duct near its axis,

with the adhesive side facing upstream. Tapes were located

at three positions (1) 12 inches upstream, (2) 15 inches downstream, and (3) 6 ft downstream, of the test unit; the tapes

at stations (1) 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 sample of the particulate matter in the air flowing past

it, and after suitable times of exposure to 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 (10X) by transmitted

light were made of sections of the tapes corresponding to a position at mid-height’ in the test duct.

The filter cell was 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 of the unit. The cleanability of the after-filter

was determined separately, by the same means.

4 o TEST RESULTS

A summary of the test data, giving efficiencies in percent with the three aerosols and the pressure drop of the

complete unit including the after-filter, in inch W.G., at

rates of air flow corresponding to various face velocities,

is given in Table 1 « A summary of the test data obtained in

the dirt-loading test conducted at the rated face velocity

of 333 fpm I s given in table 2.

Photographs No. 1 , 2 , and 3 included in this report are

10X enlargements of the center 3/4 x 1 -inch sections of cellophane tapes located at the three similarly numbered stations

described under Test Method and Procedure. Tapes No. 2 and . 3 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 unit. This exposure was

during the interval in which the dirt-load increased f rom

- 6 -

I4O to I6I4 grams, as shown in Table 2 . Photograph No. 1

shows a tape exposed at station No. 1 for an equal length of

time. After exposure, the tapes were carefully removed from

the test duct for photographing and microscopic study.

Throughout the tests x^ith atmospheric air, electrical

sparking or flashing in the unit audible to the ear occurred,

on the average, about once per hour. There was no observed

instance of sparking or flashing during the tests with oil

lamp smoke. However, during the Cottrell and lint loading

test electrical sparking or flashing occurred intermittently,

the frequency increasing from about I4O to 50 times per hour

at the start to about 90 to 100 times per hour at the end of

the loading test.

5 . SUMMARY

A, Performance

The efficiency of the air cleaner in arresting the

particulate matter existent in atmospheric air drawn through

the unit varied considerably with the face velocity at which

it was operated, as shown in Table 1 . At the rated velocity

(333 fpm), the average efficiency on atmospheric air (90.2

percent) and that on oil lamp smoke (91.2 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 d etermining efficiencies, although in these results the

differences between comparable efficiency values were consistently of a lesser magnitude.

As recorded in Table 2 , the ionizer and plate voltage

showed a decrease of about 2.5 kv at about 659 grams of load

and a corresponding decrease in efficiency of approximately

ten percent, however, at the beginning of the next test-day

(at a load of 700 grams) the voltages and efficiencies were

found to be at approximately their initial values and remained

at a high level except for a one-minute period during the

ten-minute efficiency determination at 8(36 grams of load.

- 7

-

In these tvjo instances, the voltage was observed to vary

as shown, the first value in each bracket being that at the

start of the efficiency determination, and the last that at

the end.

An efficiency determination made with atmospheric air

at the end of the dirt-loading test indicated that the unit

was approximately as efficient (within about one percent) on

this aerosol when the unit was heavily laden with dirty as

when it was clean.

The greater part of the pressure drop through the complete

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 complete unit increased by Child inch W,G. for

a total dirt-load of 899 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,

B, Cleanability

The filter was subjected to the cleaning process desdescribed under Test i'iethod and Procedure, No difficulty

was experienced in thoroughly cleaning the ionizer and

collector sections of the unit, using moderate care. The

after-filter was also satisfactorily cleaned using the same

procedure

„

C * General

On completion of the dirt^loading test, the unit was

removed from the test duct and examined. The ionizer assembly and bars, and the insulators were generally coated i^ith

a moderate layer of dust and lint. Dirt deposits were

heaviest on the upstream edges and first two or three 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 negative plates. A continuous,

but thinner, layer of dirt was deposited over the remaining

area of the collector plates, extending to the aft edges.

Considerable bridging of lint fibers from one collector plate

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

bridging appeared to be wholly from the center of the unit

downstream to the aft edges of the plates.

0 -

The upstream face of the after-filter revealed considerable deposits of lint, somewhat concentrated in two

horizontal strips each about five inches wide. The afterfilter media was approximately uniformly darkened by a dust

deposit on its upstream face.

The dirt (dust) deposits on the electrostatic unit and

on the after-filter appeared to be well saturated with oil.

After the unit had been removed from the test duct, the duct

section downstream of the unit was carefully swept out with

a fine brush. The amount of material obtained from this

sweeping was 0.3 grams.

The cellophane tape samples obtained at stations (1),

(2), sind ( 3 ), 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 five microns in actual size, and a distribution of

larger particles up to a few as large as hOO microns, as well

as many fibers of lint. (In these photographs l/l 6 inch

corresponds to an actual dimension of about 160 microns.

)

Photograph No. 2 for the tape 1^ inches downstream of the

unit, shows a few large particles up to about 150 microns

in size, but very few fine dust particles considering their,

number upstream, of the unit. Photograph No. 3 , for the tape

6 ft downstream of the unit shows fewer of the larger particles of Photograph No. 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

-, 9

-

particles were passed through the unit 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 complete unit, of such particulate

matter as is shown in the downstream tape photographs.

Table 1

Ioni zer

Face Inlet ^ and Plate Pressure Duration

loci ty Aerosol''** Voltage Drop of Test Efficiency

f pm kv inch W. G. minutes percent

333 A 0 0.178 30 4.8**

333 A 8.i| .178 180 90.0

333 A 8.1] .178 180 90.4

333 A 8.4 .178 180 90.2

266 A 8.4 .118 180 94-2

[[00 A 8.4 .255 180 86.1

333 S 8,4 .177 180 90.6

333 S 8,4 .177 180 91 o 2

333 S 8.4 .178 180 91.7

333 C 8, It .178 9 98.6

333 C 6,4 .179 9 98.3

333 C 8.4 .179 10 97.7

-''A = Particulate matter- in atmospheric air at NBS,

S = Kerosene lamp smoke in atmospheric air,

w C = Cottrell precipitate in atmospheric air (1 gram/1000 cf )

,

Since unit was not energized, the efficiency was chiefly

that of the after-filter.

Table 2

Face Dirt Ionizer and Output Pressure

Velocity Load'* Plate Voltage Current Drop Eff i ci ency'*'*

f pm grams kv ma inch W.G. percent

333 ho 8oh 0.5 0.178 98.2 (Avg)

54 8.4 .5 .180 98.7

I6I4 boh *5 .181 98.9

333 boh .5 .182 98.5

502 8.5 *5 .189 99.0

659 (5.9) . 6 .212 88.8

(6.2) • 6

700 8.5 • 5 .220 96.1|

(8.5) * 5

886 C5.8)(a) . 6 .293 96.1

(8.5) *5

899 8.5 ®5 .294 98.8

899 8.5 .5 .293 89 . 2 ( Av g )

( b

)

Aver age mixture; 3*9% lint. 96.1% Cottrell priecipi tate

by weight.

Efficiency determined with 100% Cottrell precipitate.

(a) Voltage decreased to 5*8 kv for one minute during 10“

minute efficiency determination.

(b) Efficiency determined with aerosol ,T A

n as in table 1,

with the unit dirty..

LINT

> DUST

" -ryiy////7:77^ // V r^///7/^/V////////////}/}////////))/////////////A

Bl

To

ow3r

6'ljl 2'~o\'

Up Stream Duct Down Stream Duct

Side elevation

Filter Frame

VERTICAL SECTION

Air Fi If er Test Apparatus

Figure I

Photograph No 1

Photograph No. 2

• /

Photograph No. 3

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

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Reports and Publications

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

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

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

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publications should be addressed to the Office of Scientific Publications, National Bureau of Standards, Washington 25, D. C.
NBS