NATIONAL BUREAU OF STANDARDS REPORT

3779

PERFORMANCE TESTS OF A PLIOTRON

"ELECTRO-STATIC” AIR FILTER

by

Henry E» Robinson

Thomas W. Watson

Report To

Corps of Engineers, U. S. Army

Engineer Research and Development Laboratories

Fort Belvoir, Va*

<NBS3>

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

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

NBS PROJECT NBS REPORT

1000-30-4S30 November 2, 1954 3779

PERFORMANCE TESTS OF A PLIOTRON

"ELECTRO- STATIC” AIR FILTER

by

Henry E. Robinson

Thomas W. Watson

Heating and Air Conditioning Section

Building Technology Division

to

Corps of Engineers, U.S.Army

Engineer Research and Development Laboratories

Fort Belvoir, Va.

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publicatloi

unless permlssl

25, D, C. Sue!

cal ly prepared

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

e or in part, is prohibited

of Standards, Washington

a report has been specifiar report for Its own use,

1. INTRODUCTION

At the request of the Engineer Research and Development

Laboratories, Fort Belvoir, Va. (reference: letters dated

October 22, 1953 and October 27, 1954, File TECRD CB 3-33-01-

001), tests were made to determine the performance characteristics of a Pliotron T, Electro-Static TT air filter manufactured

by the Goodyear Tire and Rubber Company of Akron, Ohio*

The test results presented herein were obtained on a specimen air filter furnished by its manufacturer and included determinations of dust arresting efficiency with two

aerosols (atmospheric air and Cottrell precipitate), pressure

drop, dirt load and cleanability of the specimen.

The manufacturer supplied five specimen filters, three of

the "Screen Vane Type" and two of the "Improved Shredded Design". Each of the filters had the following markings "Pliotron

Electro-Static Air Filter" and "Permanent Type Washable Filter

Type Bl".

The "Screen Vane Type" filter had actual outside dimensions

of 19-5/3 x 19-5/^ x 1 -inch and weighed 5*03 pounds when clean.

It had media consisting of many layers of 12-mesh white plastic

screen piled together and held in place by retaining faces of

l/2-inch wire hardware cloth, enclosed at the edges by a metal

frame having a free opening 13-3/16 inches square (2.30 ft,*

free area)

,

The "Improved Shredded Design" filter had overall dimensions of 19-5/a x 19-5/a x 1-inch and weighed 3.94 pounds.

The filtering media was as follows, starting at the upstream

face: one layer of l/2-inch mesh hardware cloth; one layer

of 14 x 17-mesh plastic screen; a pad of shreds of white plastic,

strewn in a loose pack about 7/a-inch thick; one layer of very

thin glass fiber sheet; one layer of l/2-inch mesh hardware

cloth. The media and screen layers were slightly compressed

2. DESCRIPTION OF THE FILTER SPECIMENS
.
- 2 -

3. TEST METHOD AND PROCEDURE

Efficiency determinations were made by the NBS "Dust-Spot

Method" using the following aerosols: (a) outdoor atmospheric

air drawn through the laboratory without addition of other dust

or contaminant; and (b) Cottrell precipitate, dispersed in the

outdoor atmospheric air. The test method is described in the

paper "A Test Method for Air Filters" by R. S. Dill (ASHVE

Transactions, Vol# 44 F. 379, 1943). In conducting the tests

air was sampled from the test duct at equal rates, from points

one foot upstream and eight feet downstream of the filter, and

drawn through known areas of Whatman No, 41 filter paper. The

areas of filter paper used upstream and downstream, or the

times during which the air was sampled upstream and downstream,

were selected experimentally so that the change in transmission

of light through the two filter paper spots would be about the

same. The filter efficiency was calculated by means of the

formula

Efficiency, percent =100 02 Ti

0l

4 t 2

wher& A represents the dust spot area, 0 the change in light

transmittance of the filter paper as measured before and after

the deposition of dust, and T the time during which the air

sample was drawn. Subscripts 1 and 2 refer to the upstream

or downstream positions, respectively.

Three efficiency-measuring techniques, or modifications

based on the above formula, were used, depending on the apparent efficiency of the filter with the different aerosols. For

the tests made with atmospheric air, techniques L, M and N were

used, as indicated in Table 2. For equal changes in light

transmission of the upstream and downstream papers, the corresponding efficiencies, as determined from the formula, are 15,

zero, and 50 percent, respectively. For the tests made with

Cottrell precipitate as the aerosol, the usual procedure, designated as N in Table 2, was followed. The filter efficiency

corresponding to equal changes of light transmission of the upstream and downstream papers in this case is 50 percent.

All light transmission measurements were made with the

photometer .illumination at a constant intensity as determined

by measurements on a reference of constant transmission characteristics. The filter papers used upstream and downstream

were selected to have equal light transmissions when clean.

- 3 -

In accordance with a request of the Engineer Research

and Development Laboratories, the pressure drops at various

rates of air flow were measured for both types of filter

submitted, with the object of selecting for the performance

tests the type of filter having the lower resistance. This

was found to be the "Improved Shredded Type", a specimen

of which was then tested as indicated below, at an air flow rate

of $00 cfm. In all tests, except as noted in Table 2, the frame

of the filter was electrically grounded*

The efficiency of the selected filter in arresting the particulate matter in atmospheric air was determined by means of

four tests of the L and M types, as described above, with the

filter clean. Following these, the efficiency of the filter

in arresting Cottrell precipitate was measured by means of

three N-type tests, after which was begun the process of loading the filter with a mixture of 4 percent of cotton lint and

96 percent of Cottrell precipitate, by weight, separately dispersed in the air stream. The lint used for this purpose was

No. 7 cotton linters previously ground in a Wiley mill with a

4-millimeter screen. At suitable periods as loading progressed,

the efficiency of the filter was determined using Cottrell precipitate in outdoor air. The pressure drop was recorded at intervals during the test. The dirt-loading was continued until

the pressure drop increased to approximately 0.50 inch W.G, The

efficiency was again determined with Cottrell precipitate and

then with atmospheric air as the aerosols.

The filter was then removed from the test duct and cleaned

by immersing it in a bath of luke warm water with detergent

added. Following this, a fine spray of cold water from a hose

nozzle was used to dislodge lint particles that were not easily

removed by the above method.

After being dried over-night, the filter was reinstalled

in the test duct and the pressure drop at various air flows

was measured. Next, the efficiency of the filter was determined

with atmospheric air and with Cottrell precipitate as aerosols.

These measurements were made with the filter frame electrically

grounded, and electrically ungrounded.

4. TEST RESULTS

Table 1 presents data as to the pressure drops, at

several rates of air flow, of a specimen of each of the

two types of filter submitted. Values of the dust

arrestance or efficiency of each of the types, using

atmospheric air as the aerosol, are also given. Because

of its lower initial resistance, further tests were conducted on the shredded design filter. Table 1 also gives

data on the pressure drop of this filter after it had been

loaded with dirt and cleaned.

The performance of the "Improved Shredded Design"

filter at 300 cfm is summarized in Table 2. The table

gives data on the efficiency of the filter in arresting

Cottrell precipitate, and on its pressure drop, as the

filter was loaded with a mixture of 4 percent lint and

96 percent Cottrell precipitate by weight. The efficiency

of the filter in filtering atmospheric air is given for the

clean specimen, and for the specimen when it was loaded

with a dirt deposit to a pressure drop greater than 0.50

inch W.G. Efficiency determinations with atmospheric air

and with Cottrell precipitate are also recorded for the

filter after it had been cleaned following the dirt-loading

test. Two measurements were made with the filter frame not

grounded as noted in the table; the filter frame was grounded

during all other measurements.

Throughout the tests, the relative humidity of the air

entering the test duct was within the range from 27 to 56$,

at dry bulb temperatures ranging from 73 to 32 °F.

Observation of the filter at the end of the dirt-loading test revealed that there was little dust or lint on the

plastic fly screen layer of the filter. Some dust and lint

was observed to have been arrested in the pad of plastic

shreds. The greater part of the arrested dust and lint was

found on the upstream face of the glass fiber sheet at the

downstream side of the filter. This sheet, white when the

filter was clean, was much darkened by dust at the end of

the loading test.
'

:
- 5 -

Cellophane tapes, stretched across the test duct

downstream of the filter with the adhesive side facing

upstream, indicated upon visual and microscopic examination after exposure to the air stream that large numbers

of particles of sizes from 5 to 200 microns, and considerable lint, had passed through the filter during the Cottrell

precipitate tests. Particles much smaller than 5 microns

were observed in quantity by microscopic examination of

the downstream filter papers obtained in tests with both

aerosols; it was not possible to be certain as to the smallest size limit of particles caught upon the cellophane tapes,

but particles at least as small as 5 microns were observed

with both aerosols.

In cleaning the filter, it was found that immersing

and moving the filter in a lukewarm detergent solution readily removed the dust from the media, but did not satisfactorily

remove lint fibers from its interior. Most of the lint was

removed later by careful flushing with a fine cold water spray

from a hose nozzle, but the latter operation required much

care, and was not entirely satisfactory, as indicated by the

greater pressure drop of the filter after it was cleaned than

when it was new and clean.
.

1;

' •
- 6 -

TABLE 1

Air Face Pressure Pressure Efficiency

Specimen Flow Velocity Drop ( 1

)

Drop (2) Aerosol A

cfm fpm inch W.G. inch W.G. percent

Screen Vane 400 174 0.123 „ o. c=» Type 600 261 .250 -- - - 300 348 .417 — 15 (3)

1000 435 .635 -- --

Shredded 400 174 0.071 «=> «o c oo Design 600 261 .130 .143 — — 300 348 .212 .227 15 (4)

1000 435 .315 .337

(1) Initial values for the clean filter.

(2) Values for the test filter after the dirt-loading test

and cleaning operation.

(3) Value for one measurement.

(4) Average value for four tests shown in Table 2.

- 7 -

TABLE 2

Performance of "Improved Shredded Design” Filter at S00 CFM

Total

Inlet Dirt Pressure Eff. Meas.

Aerosol (1) Load (2) Drop Technique (3) Efficiencv

grams inch W.G. percent

A •• — 0.214 L 13

.216 L 16

.217 L 19

«3 — .217 M 12

15 Avg.

C 7 .217 N 41

15 .220 N 43

25 *222 N 43

109 340 N 42

147 .421 N 41

1$5 .498 N 40

193 .533 N 42

42 Avg.

A 193 .543 L 15.0

After Cleaning

A ex»c .230 L 9

.232 L 9 (4) -- .232 N 16

C .236 N 41

-- .239 N 40 ( 4 )

(1) Aerosol A: Particulate matter in atmospheric air at NBS.

Aerosol c: Cottrell precipitate in atmospheric air

(1 gram per 1000 cf)

(2) Average mixture: l+%> lint, 96% Cottrell precipitate, by weight.

(3) Efficiency-measuring technique:

L: Air sampled at e^ual rates through equal areas; upstream

sampling time $5% of that downstream.

M: Air sampled at equal rates through equal areas for equal

times*

N: Air sampled at eoual rates for equal times; downstream

area 50% of that upstream*.

(4) Frame of filter electrically ungrounded; in all other

tests the frame was electrically grounded,,

5. DISCUSSION OF EFFICIENCY RESULTS

The efficiency of the tested specimen in arresting the

particulate matter in the atmosphere at the time of the tests

is indicated by the results given in Table 2 to be about 15

percent. Agreement of the tests of the L, M and N types is

not perfect, but in view of the fact that the efficiencies

"shot for" in conducting these tests (15, zero and 50 percent,

respectively) differed so widely, the consensus is good that

the atmospheric air efficiency is in the neighborhood of

15 percent.

For reasons having to do with the problem of obtaining

not-too-dark filter paper spots, tests on the filter with

Cottrell precipitate as the aerosol were made only ]Dy the

method of test (N-type) ordinarily used here in conducting

such tests. The results obtained in the Cottrell precipitate

tests are therefore directly comparable with those obtained

here in tests of other filters with Cottrell precipitate.

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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

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