NATIONAL BUREAU OF STANDARDS REPORT

4409

PERFORMANCE TESTS OF TWO "DYCON"

DRY-TYPE AIR FILTERS

by

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

THE NATIONAL BUREAU OF STANDARDS

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Radio Standards. High Frequency Standards. Microwave Standards. • Office of Basic Instrumentation • Office of Weights and Measures
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

IOOO-3 O-I483O November 21, 1955

PERFORMANCE TESTS OF TWO n DYC0Nn DRY-TYPE AIR FILTERS

by

Henry E« Robinson

Thomas W* Watson

Heating and Air Conditioning Section

Building Technology Division

To

General Services Administration

Public Buildings Service

Washington 25, L® C,

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, r Approved for public release by the

unless permission g)j rector Qf the National Institute of

25,D.C. Suchpe _ , , _

Standards and Technology (NIST) on October 9, 20 1 5.

r In part, Is prohibited

standards, Washington

eport has been spe.cifireport for its own use.

UU09

PERFORMANCE TESTS OF TWO "DYCON"

DRY-TYPE AIR FILTERS

by

Henry E. Robinson and Thomas W. Watson

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 new 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 load.

2. DESCRIPTION OF THE FILTER SPECIMENS

The two filters submitted were manufactured by Continental

Air Filters, Inc. of Louisville, Kentucky, and were dry-type

air filters, 2.l\x2l\xQ 3/U inches in nominal size. They were

identified by the manufacturer as Model CA-2I4 with Type A

blanket and Model CA-2lj with Type AX blanket as the filtering

media.

The model CA-2I4 (Type A) air filter cell had overall

dimensions of 23 l/2 x 23 1 /2 x 7 5/8 inches. The filtering

media consisted of an un-oiled blanket of wool-like synthetic fibers, held together by a bonding agent. The blanket

was arranged in vee-folds approximately 7 inches deep, between retainers of 2 -mesh wire on the upstream and downstream

faces. The net face area of the filter was 3.36 sq. ft. and

the area of blanket effective for filtering was approximately

11.75 S Q» ft. The filter cell weighed II4.5 pounds when clean.

The filter model CA-2I4 (Type AX) was identical in size

and design except that two sheets of blanket (Type A and Type

X) were laid in contact in vee-folds with the finer media

(Type X) on the downstream side. The filter cell weighed

15.1 pounds when clean.

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

released for use in connection with advertising or sales

promotion.

2

The manufacturer recommends that for best results and

highest economy, the Dycon CA- 2 I4 cell can be cleaned by

vacuum cleaning with a special nozzle, and occasionally by

washing the media in place with cold water from a hose

nozzle. It is stated that such washing can be repeated up

to about 10 times before the Dycon blanket media needs replacement.

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 a paper "A Test Method for Air Filters” by

R. S. Dill (ASHVE Transactions, Vol. l\b

,

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 knoi^n areas

of Whatman No. l\ 1 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 1

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 with the different aerosols.

For the tests made, techniques L, M and N were used, as indicated in Table 2.

- 3 - 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 two

tests of the L and M types, 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 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 -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 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

the blanket was cleaned in the filter cell by means of a stream of cold water from a high-pressure hose nozzle

directed at and into the filter media beginning with the

downstream face. After drying, the filter was re-installed

in the test apparatus for measurement of its initial pressure drop after the cleaning process, and for subsequent

tests.

i|. TEST RESULTS

Table 1 presents data as to the pressure drops of the

clean filters at several rates of air flow and also the

pressure drops at rated air flow of the same filters after

one cleaning operation with water.

The performance of the filters at 1000 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 Figures 1 and 2, The efficiency of the filters in
•
arresting aerosol A (atmospheric particulate matter), both

initially, and after their resistance had been increased

to 0.5 inch W.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

lint was found on the upstream faces (heavier in the bottoms

of the vee-folds) and had not penetrated the media to any

noticeable degree. The downstream faces of the media were

uniformly darkened by dust. No lint was visible on the

downstream face of either filter.

After the filters had been removed from the test duct,

the section of the duct five feet long downstream of the

unit, and upstream of a 3/14-inch 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 21.0 grams or 1.8 percent

for the Type A media, and eight grams or 0.8 percent for

the Type AX media, of the dust load reaching the filter.

This material consisted for the most part of large dust

particles, and constituted the fall-out in the first five

feet of the duct from the air passed through the filter.

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 Type A media and particles up to 100 microns in size

had passed through the Type AX media during the dirt-loading

tests. Particles smaller than five microns were observed

in quantity by microscopic examination of the downstream

filter papers obtained in tests on the two filters with

each of the aerosols. No lint was observed on the tapes

downstream of either filter during these tests.

At the conclusion of the dirt-loading tests, cellophane tapes also indicated that in the case of the CA 21} Type A filter (see Table 2 ) there was some release or escape

of Cottrell precipitate from the filter media during the

efficiency tests with atmospheric air as the aerosol. This

is also reflected in the low efficiency values (negative in

one case) and the pressure drop decrease from 0.512 to

O.I496 inches W.G. as noted in Table 2 .

- 5 - There was no evidence of release or escape of Cottrell

precipitate from the CA-2ij Type AX filter media during the

corresponding tests with the atmospheric aerosol.

After the loading tests, the filters were removed from

the test duct and subjected to the cleaning process described

under Test Method and Procedure. Complete apparent cleaning

was accomplished with moderate time and care. However, upon

subsequent measurements of pressure drop after the cleaning

and drying operation, it was found, as noted in Table 1,

that the initial pressure drops were slightly higher than

those of the clean as-received filters, in the case of both

the Type A and Type AX filters. The efficiency values for

the two filters after the cleaning procedure were substantially the same as the values obtained with the clean asreceived filters, for both aerosols.

It is believed that a thorough washing with cold water

restores these filters to substantially their original

initial efficiency, with perhaps a slight increase in initial pressure drop for each successive washing. It is felt

that cleaning may be facilitated by using a vacuum cleaner

on the upstream surfaces of the media to remove loose dust

and lint before washing with water is undertaken.
S ’’ \ . .
TABLE 1

PRESSURE DROP OF CLEAN FILTERS

Air Face Pressure Pressure

Flow Velocity Drop (1) Drop (2)

cfm fpm inch W.G. inch W.

G

CA-2l| (Type A Media)

1200 102 0.120

1000 85 .092 0.100

800 68 . O6I4 - 600 51 . OI4O -

CA-2 i| (Type AX Media)

1200 102 0.21*2 c*> 1000 85 .188 0.200

800 68 .136 - 600 51 .092 -

(1) Initial values for the clean filters

(2) Values for the filters after dirt-loading and cleanin

with water.

TABLE 2

PERFORMANCE OF FILTERS AT 1000 CFM

Total

Dirt Pressure Eff. Meas.

Filter Aerosol(l) Load( 2

)

Drop Technique( 3

)

Efficienc

grains inch W. G. percent

CA-24 A 0.092 M 12

(Type A) .092 L 15

C 9 .093 N 73

17 .0914 N 72

28 .096 N 74

203 .123 - -

I4 O 6 .150 N 78

S90 .202 N 85

706 .253 N 85

957 .378 N 83

977 .384 N 84

1189 .512 N 84

u A 1189 .500 M -6 (4)

1189 .496 M 7

After Cleaning and Drying;

w A 0.100 M 14

C 9 101 N 73

TABLE 2 - Cont 'd

Total

Dirt Pressure Eff. Meas.

Filter Aerosol(l) Load( 2

)

Drop Technique (3

)

Efficiency

grams inch W,G. percent

CA-21* A 0.188 M

(Type AX) — .188 L 20

C 9 .190 N 78

i* 17 .192 N 85

278 .2I48 N 87

1413 .286 - 0- 556 .316 N 88

710 .363 N 90

894 .430 - - 10^7 .504 N 91

" A 1057 .504 M 29

After C leaning and Drying

A - 0.200 M 15

C 9 .202 N 79

(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: I}. • 0 percent lint, 96.0 percent Cottrell

precipitate by weight.

(3) 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.

TABLE 2 - Cont'd

Efficiency measuring technique:

M: Air sampled at equal rates through equal areas

for equal times,

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.

Cellophane tapes downstream indicated that Cottrell precipitate was escaping from the filter media during this

measurement.

CD cd h-

±N30d3d < A0N3I0I333 ‘dOdQ

o

3dfTSS3dd

200

400

600

800

1000

1200

LOAD

REACHING

FILTER,

GRAMS

FIG.

I

TYPE

AX

MEDIA

200

400

600

800

1000

1200

LOAD

REACHING

FILTER,

GRAMS

FIG. 2

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

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

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

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

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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 project or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

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Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of

Standards, Washington 25, D. C.