NATIONAL BUREAU OF STANDARDS REPORT

5240

PERFORMANCE TESTS OF AN ELECTRO-AIR

"HIGH CAPACITY" ELECTRONIC AIR CLEANER

by

Thomas W. Watson

Henry E. Robinson

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. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-4830 April 18, 1907 5240

PERFORMANCE TESTS OF AN ELECTRO-AIR

"HIGH CAPACITY" ELECTRONIC AIR CLEANER

by

Thomas W. Watson

Henry E. Robinson

Heat Transfer Section

Building Technology Division

to

General Services Administration

Public Buildings Service

Washington 25, D. C.

IMPORTANT NOTICE

NATIONAL BUREAU OF STANT

intended for use within the Gov

to additional evaluation and revit listing of this Report, either in v\ the Office of the Director, Natior however, by the Government age

to reproduce additional copies fi

ess accounting documents Approved for public release by the iy P ut,lished !t is subjected

. . . oduction, or open-literature

Dliector OT the National Institute of js obtained in writing from Standards and Technology (NIST) h permission is not needed, on October 9, 20 1 5.

’ a^'' ,, if ,ha' agency wishes

<^BS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
t'* IS
PERFORMANCE TESTS OF AN ELECTRO-AIR .MODEL 20-C

"HIGH CAPACITY" ELECTRONIC AIR CLEANER

by

Thomas W. Watson and Henry E. Robinson

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 two

aerosols (atmospheric air 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, Pittsburgh 33, Pennsylvania, and was of the electrostatic type. It was identified by the manufacturer as an

"Electro-Air High Capacity Electronic Air Cleaner Model 20-C,"

with one cell rated at 1450 cfm. The power pack nameplate data

furnished by the manufacturer were as follows:

Type B

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

Output: 8,200 volts D. C.

The test unit had a housing with transverse outside dimensions 24 1/8 X 19 5/8 inches and was 20 inches long. The

upstream and downstream faces had special flanges 32 inches

square overlapping those of the duct of the test apparatus by

one inch. The downstream face was adapted to receive a nominal

24 X 19 1/2 X 2-inch afterfilter, which for this unit was a 23 7/8 X 19 7/16 X 1 7/8-inch viscid impingement type air filter

manufactured by the Research Products Corporation of Madison,

Wisconsin. The afterfilter had a net face area of 2.85 square

feet o

-2

The filter cell of the unit was 23 7/8 inches in height,

19 1/2 inches in width, and 17 inches in length, and contained

57 aluminum plates (23 X 10 3/4 inches in dimension) spaced

5/16 inch apart on centers, presenting a total surface area of

approximately 196 square feet. The gross inlet area of the

ionizer assembly was 2.53 square feet and the cell transverse

area was 3.05 square feet.

The manufacturer furnished an adhesive, designated as "Carbide and Carbon Chemicals Company PN-1083 cold water

adhesive," with which the collector plates were oiled by

spraying them from the upstream face. The afterfilter was oiled

in preparation for the test by spraying the media from the

upstream and downstream sides with the same adhesive.

The power pack, connected to a 115-volt 60-cycle supply,

was adjusted by the manufacturers’ 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. TEST METHOD AND PROCEDURE

Efficiency determinations were made by the NBS "Dust-Stop

Method" using the following aerosols: (a) outdoor air drawn

through the laboratory without addition of other dust or contaminant and. (b) Cottrell precipitate. The test method is

described in the paper, "A test Method for Air Filters,"

by R. S. Dill (ASHVE Transactions, Vol. 44, p. 379, 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 1/2

feet 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 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. 41 filter paper. For the atmospheric air 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, 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
’
-3-

same opacity, as ind

missions of the dust

drawn, which were de

transmitted light,

downstream positions

mission readings whe

differed, the effici

icated by the change in

-spot areas before and

termined by means of a The filter papers used

were selected to have

n clean. If the opacit

ency was calculated by

the light transafter the sample was

photometer using

in the upstream and

the same light transies of the dust-spots

means of the formula

Efficiency, percent = 100 I

-

where 0]_ and Op were

and down sire.am, respe

times during which th

tively

,

were drawn

F or the ef f ic ien

aerosol

,

the samples

tl °2

t 2 0l

s of the

tl and

nd d owns respecupstream and downstream were 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 Ag/Al substituted for the ratio ti/t2» where A2 and

A]_ 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 8.2 kv). Two 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

air flows 20 percent greater, and 20 percent less than the

rated air flow.

Foil owing these, two eff

at the ra ted air flow , using

whi ch was dis persed Cottrell

one gram per thousand cubic f

obt ained

,

the process was beg

mix ture of 4 percent cotton 1

c ip itate

,

by weight

,

separate

The lint used for thi s purpos

iciency determinations were made

as an aerosol outdoor air in

precipitate at a concentration of

eet of air. When these had been

un of loading the unit with a int and 96 percent Cottrell prely dispersed into the air stream,

e was No. 7 cotton linters
*
-4-

previously ground in a Wiley mill with a four-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

dirt-loading process was continued until 968 grams of the lint

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

per cfm of unit rating).

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

two positions , (1) 12 inches upstream and (2) 15 inches downstream of the test unit. 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

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 afterfilter was determined

separately, by the same means.

4. TEST RESULTS

A summary of the test data giving efficiencies in percent

with the two aerosols and the pressure drop of the complete

unit, including the afterfilter, in inch W. G., at several

rates of air flow, is given in Table 1. A summary of the test

data obtained in the dirt-loading test, conducted at 1450 cfm

is given in Table 2.

Throughout the tests with atmospheric air, electrical

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

on the average, about seven times per hour. However, during

the Cottrell and lint loading test, electrical sparking or

flashing occurred intermittently, the frequency increasing

from about 15 to 20 times per minute at the start to about 30

to 35 times per minute at the end of the loading test.
•

n
-5-

5 . SUMjVIARY

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 air flow at which it was

operated, as shown in Table 1. At the rated air flow (1450

cfm) , the average efficiency on atmospheric air was 91.2 percent. 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

As recorded in Table 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 high level, never falling below 98.4 percent

and ending with a value of 99.3 percent, slightly greater than

at the start of the dirt-loading test. An efficiency determination made with atmospheric air at the end of the dirtloading test indicated that the efficiency of the unit on this

aerosol was higher when it was dirt laden than when it was

clean

The greater part of the pressure drop through the complete

unit was due to the resistance of the afterfilter. It is noted

in Table 2 that in the dirt-loading test, the pressure drop of

the complete unit increased by 0.253-inch W. G., for a total

load of 968 grams. The rise was due chiefly to an increase in

the pressure drop of the afterfilter, 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 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 afterfilter was also satisfactorily cleaned using the same procedure.

C. General.

On completion of the dirt-loading test,

removed from the test duct and examined. The

and bars, and the insulators were generally c

the unit was

ionizer assembly

oated with a
s

*
- 6 -

moderate layer of dust and lint. Dirt deposits were heaviest

on the upstream edges and first three or four inches of the

collector plates, the thickness of the deposits being up to

about 1/32 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. There

was no observable bridging of lint between any of the collector

plates at the conclusion of the tests.

The upstream face of the afterfilter revealed considerable

deposits of lint, somewhat concentrated in two horizontal

strips, each about eight inches wide. The afterfilter was

approximately uniformly darkened by a dust deposit on its

upstream face.

The dirt (dust) deposits on the electrostatic unit and on

the afterfilter 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

0.8 gram.

Cellophane tapes, stretched across the test duct downstream

of the filter unit with the adhesive side facing upstream, indicated upon visual and microscopic examination after exposure to

the air stream throughout the dirt-loading test, that a few particles up to about 200 microns in size had passed through the

unit during the dirt-loading tests. Particles smaller than five

microns were observed in quantity by microscopic examination of

the downstream filter papers and tapes obtained in tests with

both aerosols. A considerable number of lint fibers was also

observed on the downstream tapes that were obtained during the

dirt-loading tests.

Comparison of the numbers of particles on the upstream and

downstream tapes indicated, 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. 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. Nevertheless, the downstream tapes, and the deposits on the afterfilter,

show that a few quite large particles of dust escaped beyond

the electrostatic unit. Whether the large 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.

- 7 -

As the downstream tapes indicate, a few large particles of

dust and lint passed unarrested through the afterfilter.

Assuming that one of the functions of the afterfilter is to

arrest as much as possible of the material escaping the electrostatic unit, the arrestance characteristics of the afterfilter are of major importance in determining the presence or

absence, in the air leaving the complete unit, of such particulate matter.

Table 1

Ionizer

Air Inlet and Plate Pressure Duration

Flow Aerosol* Voltage Drop of Test Eff icienc

cfm kv in . W . G

minpercent

1450 A 0 0.247 20 6 . 6**

1450 A 8 .

.247 180 90.6

1450 A 8.2 .246 180 91.7

1160 A 8.2 .158 180 94.5

1740 A 8.2 .362 180 85.1

1450 C 8.2 .247 10 98.2

1450 C 8.2 .247 10 98.6

*A = Particulate matter in atmospheric air at NBS

C = Cottrell precipitate in atmospheri c air (1 gram/1000 cf)

**Since unit was not energi zed, the eff iciency was chiefly th

of the afterfilter.

Table 2

Air

Flow

cfm

1450

*

(a)

Dirt

Load*

Ionizer

and Plate

Voltage

Primary

Current

Pressure

Drop Ef f ic iency**

grams kv amps in. W.G. percent

29 8.2 0.5 0.249 98 .4( avg .

)

217 8.0 .5 .270 99.0

470 8.1 .5 .317 99.3

742 8.3 .5 .398 98.9

968 8.2 .5 .502 99.3

968 8.2 .5 .502 92.3(a)

Average mixture: 4. 0% lint, 96% Cottrell precipitate by

weight

Efficiency determined with 100% Cottrell precipitate

Efficiency determined with aerosol "A" as in Table 1,

with the unit dirty.

Side elevation

o

. i CM

J.

Filter Frame

VERTICAL SECTION

Air Filler Test Apparatus

Figure I

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