NATIONAL BUREAU OF STANDARDS REPORT

5822

PERFORMANCE TESTS OF CAMBRIDGE

"AEROSOLVE" AIR FILTERS

(TYPES 3A-9 5 & 3A-3 5)

by

Thomas W. Watson and Henry E. Robinson

Report to

General Services Administration

Public Buildings Service

Washington 25, D. C.

<NB|>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions anil Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

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Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular projec t or program. Published pajK*rs appear either in the Bureau’s own series of publications or in the

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Inquiries regarding the Bureau’s reports should be addressed to the Office of Technical Information, National Bureau of Standards, Washington 25. D. C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1000-30-4890 March 25, 1958

NBS REPORT

5822

PERFORMANCE TESTS OF CAMBRIDGE

"AEROSOLVE" AIR FILTERS

(TYPES 3A-95 & 3A-3 5)

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 STAf

Intended for use within the G

te additional evaluation and re listing of this Report, either in the Office of the Director, Natl however, by the Government a

to reproduce additional copies

Standards and Technology (NIST) * uch permission Is not needed, on October 9, 201

ogress accounting documents

Kepared if that agency wishes

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
, : ,
PERFORMANCE TESTS OF TYPES 3A-9 5 AND 3A-35

CAMBRIDGE "AEROSOLVE" AIR FILTERS

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

dry-type air filters were determined to provide information

to assist in the preparation of air filter specifications.

The test results presented herein were obtained on specimen dry-type air filters 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 loading performance.

2. DESCRIPTION OF THE FILTER SPECIMENS

The two filters submitted were manufactured by the

Cambridge Filter Corporation of Syracuse, New York, and were

dry-type, replaceable cartridge, air filters, 2^x24x12 inches

in nominal size. They were identified by the manufacturer as

Aerosolve Model 3A~9 5 and Aerosolve Model 3A-35 cartridges.

A permanent steel frame (Cambridge Model 4A-1000) was

furnished for holding the replaceable cartridges during use.

This frame had overall dimensions of 23 15/16 x 23 15/16 x 12

inches

The Model 3A-95 replaceable air filter cartridge had

overall dimensions of 23 3/8 x 23 3/8 x 12 inches. The

filtering media consisted of a mat or blanket about 3/8 inch

thick of apparently un-oiled fine glass fibers, reportedl,

approximately one micron in diameter. In the cartridge, the

blanket was arranged in pleats or vee-folds over wires at

the end of the vees, and its edges were glued to the surrounding frame of fire-resistant two-ply corrugated paperboard.

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

released for use in connection with advertising or sales

promotion.
,

!

.

' * •
The downstream face of the media was supported by a thin glass

fiber sheet. Insertion of the replacement cartridge in the

permanent steel frame additionally reinforces and supports the

filter media by means of wire vees permanently installed in

the steel frame. The inside edge of the replacement cartridge

is pressed against a felt gasket in the steel frame to effect

a dust-tight seal. The net face area of the filter was 3*60

sq. ft., and the area of the blanket effective for filtering

was approximately A3 sq. ft.

The Model 3A-35 air filter cartridge was identical in

size and design, except that the glass fiber blanket was of

coarser texture, the glass fibers having a diameter of approximately 3 microns.

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

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

Transactions, Vol. kk

,

p. 379, 1938)*

For these tests, the filter was installed in the apparatus and the desired rate of air flow through 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 Whatman

No. hi filter paper. The areas of the filter paper used

upstream and downstream and 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

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.

Ap Op Ti

Al

- 01 • Ta
. i i. v

'

•' >

T

, r . i.' J

':

j
3-

For the tests made, techniques L 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 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 in arresting particulate

matter in atmospheric air was determined by means of tests of

the L type, as described 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 which was begun the process of loading the filter

with a mixture of 4 percent 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 pre=

cipitate in outdoor air. Pressure drops were recorded at

intervals during the test. The dirt-loading was continued

until the pressure drop increased to approximately 0.90 inch

W.G. for the 3A-95 filter, and 0.57 inch W.G. for the 3A-35

filter. 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 drops of the

clean filters at several rates of air flow.

The performance of the filters at 1000 cfm is summarized

in Table 2, and is also shown graphically in Figures 1 and 2

for both aerosols A and C.

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

tests showed that the lint fraction of the dirt-load was uniformly deposited on the upstream faces, and had not penetrated

appreciably into the media. In the case of the 3A-95 filter,

the bulk of the dust fraction of the dirt load was found on

the upstream face and had not penetrated into the media beyond

about 1/16-inch. The downstream face of this filter was

observed to be clean and not discolored. In the case of the

3A-3 5 filter, the downstream face of the media was uniformly

darkened at the end of the dirt-loading test. No lint was

visible on the downstream face of either filter.

TABLE 1

PRESSURE DROP OF CLEAN FILTERS

Air Face Pressure

Flow Velocity Drop

cfm fpm inch W.G

3A-95

1200

1000

800

600

28

23

19

l4-

3A- 3 5

0.507

.414

• 322

.234

1200 28 0.172

1000 23 .133

800 19 .09^

600 lb .063
I
TABLE 2

Filter

3A-9 5

3A-35

PERFORMANCE OF FILTERS AT 1000 CFM

Total

osol(l)

Dirt

LoadC2)

Pressure

Drop

Eff. Meas.

Technique ( 3) Eff icienc"

grams inch W.G. percent

A - 0.522 L 92.0

- .423 L 91.5

- .528 L 90.8

- .538 L 2L2

Avg. 91.5

C 10 .538 N 99.7

19 .538 N 99.7

306 .503 N 99.9

5i5 .543 - 533 .603 N 99.9

692 .705 - - 811 .797 - - 909 .865 N 99.8

A 909 .910 L 98.8

909 .918 L 97.7

A - .130 L 25.0

- .130 L 23.5

,130 L 23.8

C 10 .132 N 85.1

19 .133 N 90.5

29 .136 N 93.9

38 .137 N 94.7

A 59 . l5o L 37.7

59 . 150 L 38.5

0 2 57 .160 _ 374 .176 — — 552 .200 N 95.1

790 .252 - — 956 .299 N 95.7

1155 .372 - 1235 .506 N 96.2

1385 .550 - — 1453 .503 N 97.8

1685 . 558 — 1813 .605 N 96.9

1822 .612 N 96.9

A 1822 .580 L 63.1

1822 .570 L 62.8

( Cont 1 d)

TABLE 2 (Cont'd)

Aerosol A: Particulate matter in atmospheric air at NBS

Aerosol C: Cottrell precipitate in atmospheric air (1 gram

per 1000 cf )

Average mixture: 3*9 percent lint, 96.1 percent Cottrell

precipitate, by weight.

Efficiency measuring technique:

L: Air sampled at equal rates through equal areas;

upstream sampling time selected to yield approximately

equal dust-spot opacities of the upstream and downstream filter papers.

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

areas selected to yield approximately equal dust-spot

opacities of the upstream and downstream filter papers.

MODEL 3 A - 9

5

0 200 400 600 800 1000

LOAD REACHING FILTER, GRAMS FIG. I

MODEL

3A-*35

1 N 3 0 d 3d ‘A0N3I0I333

o

O

o

(?> o

OD o

f'- o OID o O

PO o

co

0 'M HONI d 0 d Q 3dnSS3dd

o

200

400

800

1200

1600

2000

LOAD

REACHING

FILTER,

GRAMS

FIG.

2

U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Aslin, Director THE NATIONAL BUREAU OF STANDARDS

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