NATIONAL BUREAU OF STANDARDS REPORT

5839

PERFORMANCE TESTS OF A SMITH “LIFETIME"

INPINGEMENT TYPE AIR FILTER

by

Thomas W. Watson and Henry E. Robinson

Report to

General Services Administration

Public Buildings Service

Washington 25, D. C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BLKEAIJ OF STANDARDS

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X' ^ -

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-^-890 April 3 , 1958 5839

PERFORMANCE TESTS OF A SMITH "LIFETIME"

IMPINGEMENT TYPE AIR FILTER

by

Thomas W. Watson and Henry E. Robinson

Heat Transfer Section

Building Technology Division

Report to

General Services Administration

Public Buildings Service

Washington 25 , D. C.

IMPORTANT NOTICE

NATIONAL BUREAU OF ST

intended for use within the

to additional evaluation and

listing of this Report, either the Office of the Director, N< however, by the Government

to reproduce additional copii

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

progress accounting documents

)rmally published it is subjected

, reproduction, or open-literature

ision is obtained in writing from Such permission is not needed,

prepared If that agency wishes

<NBS>

U. $. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
*#fe‘;3a SSs;?' T"?3fc*'' ?‘-.-j ;-'

. ».v . . .

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PERFORMANCE TESTS OF A SMITH ‘'LIFETIME”

IMPINGEMENT TYPE AIR FILTER

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

a cleanable impingement type air filter 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 filter 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, dirt load performance characteristics, and cleanability of the specimen.

2. DESCRIPTION OF THE FILTER SPECIMEN

The filter was manufactured by the Smith Filter Corpora

tion, Moline, Illinois, and was of the cleanable viscid type

20 by 20 by 2 inches in nominal size. It was identified by

nameplate as a Smith "Lifetime” air filter. The filtering

media was composed of successive layers of vee-crimped

expanded aluminum sheet arranged, with the vees of adjacent

sheets at right angles, as follows, starting at the upstream

face: one layer expanded metal 3/b by 1/h mesh; eight layer

EM 5/l6 by 1/8 mesh. The layers of media were compressed to

a thickness of about two inches and surrounded at the edges

by an aluminum frame. The filter had actual outside dimen=

sions of 20 by 20 by 2 inches, leaving a free opening l8 1/h

inches square ( 2.31 ft.^ net face area), and weighed b.h lb.

when clean and oiled.

The manufacturer submitted an adhesive designated as

”8-2 Filter Oil" for oiling the filter. This was done in

preparation for the test by spraying the filter faces with

the oil and letting the excess fluid drain off with the

filter standing on edge with one corner low for 19 hours

prior to the test.

*This report is submitted for information only, and is not

released for use in connection with advertising or sales

promotion
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3. TEST METHOD AND PROCEDURE

Efficiency determinations were made by the NBS "DustSpot" 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. A4, p. 379,

1938 ). 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 passed through known

areas of Whatman No. 4l filter paper. The areas of the filter papers 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 1 Efficiency, percent = 100

Ap O2 Tq

1 - lY ^ T2

where 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

and downstream positions, respectively.

Two 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, techniques M and N were used, as

indicated in Table 2.

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.

The efficiency of the filter in arresting particulate

matter in atmospheric air was determined by means of two

tests of the M type, as described above, with the filter

clean. Following these, the efficiency of the filter in

arresting Cottrell precipitate was measured by means of the

N-type tests, after which was begun the process of loading

the filter with a mixture of A percent of cotton lint and

-3-

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 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 dirtloading 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 means of a stream of cold water from a high pressure hose nozzle, directed at and into the filter media.

After being dried, the filter was reoiled by spraying

and, after draining approximately 20 hours, was reinstalled

in the test duct for pressure drop measurements at various

air flows.

4. TEST RESULTS

Table 1 presents data as to the pressure drop, at

several rates of air flow, of the clean and oiled filter,

and also of the same filter after it had been loaded with

dirt, cleaned, and reoiled.

The performance of the filter at 800 cfm is summarized

in Table 2, for both aerosols A and C. The performance of

the filter in regard to aerosol C (Cottrell precipitate in

atmospheric air) is also shown graphically in Figure 1. The

efficiency of the filter in arresting aerosol A (atmospheric

particulate matter), both initially, and after its resistance

had been increased to 0.5 inch W.G., is indicated in Table 2.

Observation of the filter at the end of the dirt-loading

test revealed that the greater part of the arrested dust and

lint was found on the upstream face and had not penetrated the

media beyond a depth of about one inch. The downstream face

of the media was slightly darkened with dust at the end of

the loading test.

The pressure drops recorded in Table 1 indicate that,

after the filter had been subjected to loading with the dustlint mixture and had been cleaned and reoiled, its pressure

drop at 800 cfm was substantially the same. It is believed

that the filter can be considered as satisfactorily cleanable

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

section of the duct 5 feet long downstream of the unit, and

upstream of a 3/^ inch thick wood strip fastened flat across

the bottom of the test duct, was carefully swept out with a fine brush. The amount of material obtained from the duct

by this sweeping was 11.5 grams, or 2.0 percent of the dust

load reaching the filter, constituting the fall-out in the

first 5 feet of the duct from the air passed through the

filter, and consisting for the most part of large dust particles with some visible lint.

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 up to approximately 200 microns, and some lint, had

passed through the filter during the dirt-loading 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.
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TABLE 1

Pressure Drop of Clean Oiled Filter

Air Flow

cfm

Face

Velocity

fpm

Pressure

Drop (1)

in. W.G.

Pressure

Drop (2)

in. W.G.

1200 520 0.170 0.176

1000 ‘<33 .121 .125

800 3‘+6 .077 .080

600 260 .0^7 o oo

•

Initial values for the clean oiled filter

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

cleaning operation, and reoiling.
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TABLE 2

Performance of Filter at 800 OEM

Inlet

Aerosol (1)

Total

Dirt

Load (2)

Pressure

Drop

Eff. Meas.

Techniaue (3) Efficien'

grams in. W.G. percent

A 0.077 M 11

- .077 M 3

0 7 .077 N 51

14 .077 N 51

22 .078 N 52

50 .087 N 52

265 .167 N 59

340 .215 N 61

396 . 266 N 66

489 .380 N 70

571 .511 N 72

A 571

I

—

I

—• M 28

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

Aerosol 0; Cottrell precipitate in atmospheric air

(1 gram per 1000 cf )

(2) Average mixture: ^ percent lint, 96 percent Cottrell

precipitate, by weight.

(3) Efficiency measuring technique:

M: Air sampled at equal rates through equal areas of

filter paper for equal times.

N: Air sampled at equal rates for equal times; downstream area selected to obtain approximately equal

dust-spot opacities upstream and downstream.

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KO in

lN30y3d ‘A3N3l3ldJ3

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9 ’M HONl ‘dOdO 3dnSS3dd

100

200

300

400

500

600

LOAD

REACHING

FILTER,

GRAMS

FIG.

I

U. S. 1)K1*AHTMKNT OF COMMERCE

Sinrliiir Weeks. Srcrclory

NATIONAf- BUHEAU OF STANDABDS

A. V. Asliii. Director THE NATIONAL HUKEAU OF STANDARD.S

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