NATIONAL BUREAU OF STANDARDS REPORT

4973

PERFORMANCE TESTS OF A MICROMAT

DRY-TYPE AIR FILTER

by

Thomas W. Watson

Henry E. Robinson

Report to

General Services Administration

Public Buildings Service

Washington 25, D. C.

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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

The scope of activities of the National Bureau of Standards at its headquarters in Washington,

D. C., and its major field laboratories in Boulder, Colorado, is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section carries out spe- cialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the

inside back cover of this report.

WASHINGTON, D. C.

Electricity and Electronics. Resistance and Reactance. Electron Tubes. Electrical Instruments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation.

Electrochemistry.

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and Lubrication. Engine Fuels. Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. Atomic Physics. Nuclear Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment. AEC Radiation Instruments. Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

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• Office of Basic Instrumentation • Office of Weights and Measures

BOULDER, COLORADO

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Propagation Services. Sun-Earth Relationships.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Systems.

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Systems Application Engineering.

Radio Standards. Radio Frequencies. Microwave Frequencies. High Frequency Electrical

Standards. Radio Broadcast Service. High Frequency Impedance Standards. Calibration

Center. Microwave Physics. Microwave Circuit Standards.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-4830 January 18 , ID 57 4978

PERFORMANCE TESTS OF A MICROMAT

DRY-TYPE AIR FILTER

by

Thomas W. Watson

Henry E. Robinson

Heating and Air Conditioning Section

Building Technology Division

To

General Services Administration

Public Buildings Service

Washington 25, D. C.

IMPORTANT NOTICE

NATIONAL BUREAU OF

intended for use within tl to additional evaluation at listing of this Report, eith the Office of the Director, however, by the Governrm

to reproduce additional c(

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

progress accounting documents

formally published it is subjected

g, reproduction, or open-literature

lission is obtained in writing from

. Such permission is not needed,

ly prepared if that agency wishes

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
'
PERFORMANCE TESTS OF A MICROMAT

DRY -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

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 a

specimen dry-type air 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 and

dirt load.

2. DESCRIPTION OF THE FILTER SPECIMEN

The filter was manufactured by the Microtron Corporation

of Charlotte, North Carolina, and was of the renewable media

type, identified as a standard "Micromat” air filter, 20 x 20

x 2 inches in nominal size. The filter consisted of a flat

sheet of media approximately 20 inches square held in place in

the filter frame by a spring clamp at the edges on the upstream

face and supported on the downstream side by 2 3/4 x 1 1/8-

inch expanded metal mesh. The filter had actual outside dimensions 19 5/8 x 19 11/16 x 2 inches, leaving a free opening

17 1/2 inches square (2.18 ft^) and weighed 4.8 pounds when

clean. Two types of media were submitted and were identified

as three ounce per square yard Micromat media and two ounce

per square yard Micromat media.

This report is submitted for information only, and is not

released for use in connection with advertising or sales

promotion

- 2 -

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 the paper "A Test Method for Air Filters" by R. S. Dill

(ASHVE Transactions, Vol. 44, 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. 41 filter paper. The areas of the filter papers 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

Ti

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.

Three efficiency-measuring techniques or modifications

based on the above formula were used, depending on the .pparent efficiency of the filter with the different aerosols. For

the tests made, techniques L, 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 measurement on a reference of constant transmission characteristics. The filter papers used upstream and downstream were

selected to have equal light transmissions when clean.
.
-3-

The efficiency of the filter in arresting particulate

matter in atmospheric air was determined by means of three

tests of the M and L jrypes , 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 four-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 0.50 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 of the

clean filter at several rates of air flow for both the twoounce and three-ounce media.

The performance of the filter for the two types of media

at 800 cubic feet per minute is summarized in table 2 for two

aerosols. 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 pressure drop had been increased by dirt-loading

to 0.5 inch W. G., is indicated in table 2.

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

tests revealed that the greater part of the arrested lint was

found on the upstream faces and had not penetrated the media

to any noticeable degree. The downstream faces of the media

were uniformly darkened by dust . No lint was observed 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 3/4-inch wood strip fastened flat across

the bottom of the test duct, v/as carefully swept out with a fine brush. The amount of material obtained from the duct by

this sweeping was 1,2 grams or 2.9 percent for the threeounce media, and 8.1 grams or 8.S percent for the two-ounce

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 each media in

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 there was some release or escape of

Cottrell precipitate from the media of the filters (see

table 2) during the efficiency tests with atmospheric air as

the aerosol. For the two-ounce media, this is also shown by

the negative efficiency value and the pressure drop decrease

from 0.515 to 0.505 inches W. G. as noted in table 2,

Table 1

Pressure Drop of Clean Filter

Air Air Velocity

Flow Through Media Pressure Drop , Initial

cfm f pm* inch , W

G.

2-oz. Media 3-oz. Media

1000 419 0.348 0.450

800 367 .250 .320

600 275 .158 .213

*Based on free face area of nominal 20 x 20-inch filter (2,18 ft 2

)
* *
Table 2

• sol (1)

Performance of Filter at 800

Total

Dirt Pressure Eff. Meas

Load (2) Drop Technique

CFM

(3) Efficiency

grams inch W. G. percent

3-ounce Micromat Media

A __ 0.323 M 22

— .360 (4) M 15

- .362 (4) L 20

C 7 .359 N 50

14 .398 N 52

42 . 523 N 53

A 42 . 528 M 15

2-ounce Micromat Media

A - 0.250 M 6

C 7 .278 N 44

14 o 298 N 48

42 .388 N 46

92 .515 N 41

A 92 .505 M -11

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

Aerosol C: Cottrell precipitate in atmospheric air

(1 gram per 1000 cubic feet)

„

(2) Average mixture: 4„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.

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.

(4) A different piece of 3-ounce media was used for these two

measurements after completion of the tests using the first

3-ounce media.

PRESSURE

DROP,

INCH

W.

G.

EFFICIENCY,

PERCENT

IOO

LOAD REACHING FILTER, GRAMS

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of S tandards 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

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

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 agenev of a particular project

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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

complete papers reporting technical investigations; the Technical News Bulletin, which presents

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which provides data for determining the best frequencies to use for radio communications throughout

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Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

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

the National Bureau of Standards (SI.25) and its Supplement '($0.75), available from the Superintendent of Documents, Government Printing Office. Washington 25, D. C.

Inquiries regarding the Bureau’s reports should be addressed to the Offiee of Technical Information, National Bureau of Standards. Washington 25. I). C.