NATIONAL BUREAU OF STANDARDS REPORT

PERFORMANCE TESTS OF A GLASFLOSS

THROWAWAY- TYPE AIR FILTER

Henry E. Robinson

Thomas W. Watson

Report To

General Services Administration

Public Buildings Service

Washington, 25 , D.C,

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

IOOO- 3 O-4 S3 O December 7, 1954 3ftl4

PERFORMANCE TESTS OF A GLASFLOSS

THROWAWAY-TYPE AIR FILTER

Manufactured by

Pittsburgh Plate Glass Company

Pittsburgh, Pennsylvania

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, reprinti

unless permission Is obla

25, D, C. Such permissi

cally prepared If that a

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

irt, is prohibited

rds, Washington

has been specififor its own use,

PERFORMANCE TESTS OF A GLASFLOSS

THROWAWAY- TYPE AIR FILTER 1,

INTRODUCTION

At the request of the Public Buildings Service,

General Services Administration, the performance characteristics of throwaway-type air filters 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 throwaway-type air filter submitted by its

manufacturer at the reauest of the Public Buildings Service and included determinations of dust arresting efficiency with two aerosols (atmospheric air and Cotrell precipitate) , pressure drop and dirt load.

2.

DESCRIPTION OF THE FILTER SPECIMEN

The filter was manufactured by the Pittsburgh Plate

Glass Company, Fiber Glass Division, Pittsburgh, Pennsylvania, and was of the throwaway-type . It was identified

by nameplate as a "Glasfloss Air Filter, Replacement Type”,

nominal 20 x 20 x 2 inches in size, and had media of finefibered glass strewn in a loose pack two inches thick covered

on the faces by retainers of thin brass sheet from which circular discs had been stamped, leaving a grid of circular

openings. The sides of the filter were enclosed in a cardboard

^ edging, leaving a free opening 17-1/2 x 17-9/16 inches, (2.14 ft^).

The glass fibers were lightly coated with a reddish-brown adhesive.

The filter had actual outside dimensions of 19-5/^ x 19-5/^ x 2

inches and weighed 1.09 lbs. when clean. The specimen filter

had an Underwriters Laboratories stamp on the cardboard edging

frame.

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 klr Filters" by

R. S. Dill (ASHVE Transactions, Vol. 44,p379, 193^).

2

For these tests, the filter was installed in the

apparatus and the desired rate of air flow throup-h the

cleaner was established. Samples of air were drawn from

the center of the test duct, at points one foot upstream

and eight feet downstream of the filter and passed through

known areas of hliatman No. 41 filter paper. The areas of

the filter paper used upstream and downstream, or the times

during which the air Wcas sampled upstream and downstream,

were selected experimentally so that the change in transmission of light through the two filter paper spots v/ould

be about the same. The filter efficiency was calculated

by means of the formula

Efficiency, percent 100 tl . Ai 0i

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.

Three efficiency-measuring techniques, or modifications

based on the above formula, were used, depending on the

apparent efficiency of the filter v/ith the different aerosols.

For the tests m.ade, techniques L, K and K were used, as indicated in Table 2.

All light transmission measurements vs're made with the

photometer illumination at a constant intensity as determined by mieasurement on a reference of constant transmission

characteristics. The filter papers used upstream and downstream were selected to have enual .light transmissions when

clean

.

The efficiency of the filter in arresting particulate

matter in atmospheric air was determined by means of tv/o

tests of the L and M types, o.s described above, with the

filter clean, Followinp: these, the efficiency of tlie filter

in arresting Cottrell precipitate was measured by means of
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two N-type tests, after which was begun the process of

loading uhe 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 V7ith a 4-niillimeter screen. At

suitable periods as the loading progressed, the efficiency

of the filter was determined using Cottrell precipitate in

outdoor '^ir. Pressure drops were 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.

4. TEST RESULTS

Table 1 presents data as to the pressure drop, at

several rates of air flow, of the clean filter.

The performance of the filter at ^00 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 l/2 inch. The

downstream surfaces of the media were slightly darkened with

dust at the end of the loading test. No lint was visible

on the downstream face of the media.

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 3/4 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 v;as 2,3 grams, or 0,^6 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.
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downstream of the filter with the adhesive side facing

upstream, indicated upon visual and microscopic examination after exposure to the air stream that some particles of sizes up to approximately 125 microns had passed

through the filter during the dirt-loading tests. Particles much smaller than 5 m.icrons wera observed in

quantity by microscopic examination of the downstream

filter papers obtained in tests with both aerosols. No

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TABLE 1

PRESSURE DROP OF CLEAN OILED FILTER

AIR

FLOW

cfm

FACE

VELOCITY

fpm

PRESSURE

DROP (1)

inch W.G

1200 560 0*2^2

1000 490 .210

SOO 375 .147

600 2^0 .092

( 1 ) Initial values for the clean filter.

TABLE 2

PERFORMANCE OF FILTER AT gOO CFM

Total

Inlet Dirt Pressure Eff. Keas.

Aerosol (1) Load (2) Drop Technique (3) Effici ency

grams inch I'J.G. percent

A 0.147 M 12

- .149 L 12

C 7 ,149 N 74

14 .156 N 76

63 .210 N gl

100 .25g N g2

175 .353 N g3

267 .499 N S 5

A 267 .505 L 24

(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: lint, Cottrell precipitate,

by weight

.

(3) Efficiency measuring technique:

L: Air sampled at eoual rates through equal areas; upstream

sampling time selected to yield approximately equal dustspot opacities both upstream and downstream.

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

times.

N: Air sam.pled at e'^ual rates for eoual times; downstream

areas selected to obtain approximately equal dust-spot

opacities both upstream and downstream.
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THE NATIONAL BUREAU OF STANDARDS

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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

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i1