NATIONAL BUREAU OF STANDARDS REPORT

4150

PERFORMANCE TEST OF TWO 1-INCH GLASFLOSS

THROWAWAY-TYPE AIR FILTERS

(IN TANDEM)

by

Henry E* Robinson

Thomas W, Watson

Report to

General Services Administration

Public Buildings Service

Washington 2£, 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

THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

ms project NBS REPORT

1000 - 30-4830 June 13, 1955 4150

PERFORMANCE TESTS OF TWO 1-INCH GLASFLOSS

THROWAWAY -TYPE AIR FILTERS

(IN TANDEM)

Manufactured byPittsburgh Plate Glass Company

Pittsburgh, Pennsylvania

Henry E c .Robinson

Thomas w. Watson

to

General Services Administration

Public Buildings Service

Washington 25, D, C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, r

unless permission

25, D, C. Such pc

cal ly prepared if

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

r In part, is prohibited

itandards, Washington

eport has been speclfireport for its own use,

PERFORMANCE TESTS OF TWO 1-INCH GLASFLOSS

THROWAWAY-TYPE AIR FILTERS

(In Tandem)

lo INTRODUCTION

At the request of the Public Buildings Service,

General Services Administration, the performance characteristics of throwaw ay -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

two one-inch air filters, of the throw aw ay -type , in tandem, submitted by the 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 and dirt

load.

2 0 DESCRIPTION OF THE FILTER SPECIMENS

The filters were manufactured by the Pittsburgh

Plate Glass Company, Fiber Glass Division, Pittsburgh,

Pennsylvania, and were of the throwaway-type e Two similar specimens were submitted,, Each was identified by

nameplate as a

n Glasfloss Air Filter, Replacement Type",

nominal 20x20xl-inch in size, and had media of finefibered glass strewn in a loose pack one inch 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 edges of the filter were

enclosed in a cardboard frame, leaving a free opening

17 5/8 inches square, (2.16 f

t

2

)

„

The glass fibers were

lightly coated with a reddish-brown adhesive. The

filters had actual outside dimensions of 19 5/8 x 19 5/6

x 1 inch and each weighed 0.8? lb. when clean. The filter specimens had an Underwriters Laboratories stamp on

the cardboard frame.

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

- 2 -

atmospheric air. The test method is described in the

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

Transactions, Vol. Ijlj, p 379 , 193&K

For these tests, the two filters were installed in the

apparatus in tandem forming an assembly two inches thick

and the desired rate of air flow through them was establishedo Samples of air were drawn from the center of the

test duct, at points one foot upstream and eight feet

downstream of the filters and passed through known areas

of Whatman No, ill filter paper. The areas of the 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 r A? 0

p

T-. Efficiency, percent = 100 1 - . ~~~ °

' ' Al U 1

i 2 T -Si U . §i fe 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 eff iciency-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 L, M and N

were used, as indicated and described in Table 2,

All light transmission measurements were made with

the photometer illumination at a constant intensity as

determined by measurement 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 combination in arresting particulate matter in atmospheric air was determined

by means of two tests of the Land M types, as shown in

Table 2, with the filter clean. Following these, the

efficiency of the filter in arresting Cottrell precipitate was measured by means of two N-type tests, after

- 3

-

which was begun the process of loading the filter with

a mixture of four 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 the loading

progressed, the efficiency of the filter was determined

using Cottrell precipitate in outdoor air. Pressure drops

were recorded at intervals during the test. The dirtloading was continued until the pressure drop increased to

approximately 0.5° inch W.G. The efficiency was again

determined with Cottrell precipitate and then with atmospheric air as the aerosols.

u o TEST RESULTS

Table 1 presents data as to the pressure drop, at

several rates of air flow, of the clean filter combination.

The performance of the tandem combination at 800 cfm

is summarized in Table 2, for both aerosols A and C. The

performance of the filters in regard to aerosol C (Cottrell

precipitate in atmospheric air) is also shown graphically

in figure 1. The efficiency of the filters in arresting

aerosol A (atmospheric particulate matter), both initially,

and after the resistance had been increased to 0,5 inch

Wo G., is indicated in Table 2.

Observation of the filters at the end of the dirtloading tests revealed that the greater part of the arrested dust and lint had not penetrated the media beyond a depth of about l/2 inch. The downstream surfaces of the

media of the upstream filter of the tandem combination

were discolored; the upstream surfaces of the downstream

filter were slightly discolored; the downstream surfaces

showed little discoloration. Lint deposits were visible

in the upstream l/2 inch of the media of the combination;

but not to a noticeable extent in the remaining media.

The increase in weight of the upstream and downstream filters due to dirt load was 0.35 and 0.05 pounds, respectively, for a total of 0.L[3 lb. reaching the filter.

After the filters had been removed from the test dust,

the section of the duct five feet long downstream of the

unit, and upstream of a 3 /k 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 0.5 gram, or

0.26 percent of the dust load reaching the filter, constituting the fall-out in the first five feet of the duct

- 4 - from the air passing through the filters, and consisting

for the most part of large dust particles.

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 some particles of

sizes up to approximately 125 microns had passed through

the filters during the dirt-loading tests. Particles much

smaller than five microns were observed in quantity by

microscopic examination of the downstream filter papers

obtained in tests with aerosols A and C, No lint was

observed on the tapes during these tests.

5. DISCUSSION OF RESULTS

In using throwaway air filters, the possibility

exists of using, for example, two one-inch units in tandem in place of one two-inch unit. Since most of the

dirt is arrested on the upstream media, renewal in the

case of the tandem combination would consist of disposal of the loaded upstream one-inch filter, moving

the downstream filter to the upstream position, and

setting a new one-inch unit downstream of the other.

Depending on performance, and the difference in cost of

new two-inch and one-inch units, the tandem arrangement

might have advantages.

The data in this report on a tandem arrangement for

one-inch Glasfloss filters, and that in National Bureau

of Standards Report No, 3814 on a two-inch Glasfloss

filter, obtained under the same test conditions, allow

a comparison of the two installations to be made. The

pertinent performance data are summarized in the table

below. Cost data are not included.

One Two-Inch Two One-Inch

Initial P.D. at 800 cfra 0.15 0,21

Dirt load at 0*5M P.D., grams

Avg, dust-spot efficiency,

from initial P,D to 0 o 5"

260 180

P.D. , % 81,5 83

As indicated in this report, the downstream unit of

the tandem arrangement gained only 0.05 lb in weight during

thef .testis As .'compared to- 0^35^1b^f&r the upstream filter.

- 5 -

It is concluded that practically speaking the downstream

unit was, at the end of the test, as good as a new one,

for use in the upstream position

.

The average dust-spot efficiency was almost the same

for both arrangements, but the tandem combination had

only about 70%> of the dirt-holding capacity, at a final

pressure drop of 0„5 inch W.G., of the two-inch unit„

This is believed to be due to the greater initial pressure

drop of the tandem pair, resulting possibly from the

doubling of the metal face retainers,, An economic comparison of the merit of one arrangement over the other

should take into account the more frequent changes of

filters in the tandem arrangement, as well as the unit

cost of the filters. It would appear that the one-inch

unit would have to cost not more than l80/260, or 70%, as

much as a two-inch unit to make the tandem arrangement as

satisfactory as one two-inch unit, not counting the expense of making the more frequent changes.

TABLE 1

Pressure Drop of Clear Oiled Filter

Air Face Pressure

Flow Velocity Drop ( 1

)

cfm fpm inch W. Go

1200

1000

800

600

0 ® 395

. 302

o 211

olljO

(1) Initial values for the clean filter

TABLE 2

Performance of Filter at 800 CFM

Total

Inlet Dirt Pressure Eff. Meas.

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

grams inch W 0 G„ percent

A 0.211 M 9

- .211 L 12

C 7 .222 N 76

14 .228 N 79

37 .259 N 82

100 .349 N 84

194 .523 N 87

A 194 • 523 L 34

Cl) Aerosol A:

Aerosol Cs

Particulate matter in atmospheric air at

Cottrell precipitate in atmospheric air

(1 gram per 1000 cf).

NBS.

(2) Average mixtures l[% lint, 96% Cottrell precipitate, by

weight

.

C3) Efficiency measuring techniques

Ls Air sampled at equal rates through equal areas;

upstream sampling time selected to yield approximately equal dust-spot opacities both upstream

and downstream.

Ms Air sampled at equal rates through equal areas

for equal times.

Ns Air sampled at equal rates for equal times;

downstream areas selected to obtain approximately

equal dust-spot opacities both upstream and downstream,,

PRESSURE

DROP,

INCH

W.

G.

EFFICIENCY,

PERCENT

j~f Ft 1 -|— : T

: 1

I

;

1 ’ ~

j

' 0 100 ' 200 ‘ 300

LOAD REACHING FILTER, GRAMS FIG.

I

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

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maintenance of the national standards of measurement and the provision of means and methods

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