NATIONAL BUREAU OF STANDARDS REPORT

6214

PERFORMANCE TESTS OF AN EXPANSIBLE FIBROUS MEDIA AIR FILTER,

FLANDERS EXPANSIBLE, NO. 15

MANUFACTURED BY

FLANDERS FILTERS, INC.

RIVERHEAD, NEW YORK

by

Carl W. Coblentz and Paul R. Achenbach

Report to

General Services Administration

Public Buildings Service

Washington 25* D. C.

<5J5§>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

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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

complete papers reporting technical investigations; the Technical News Bulletin, 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 (SI.25) and its Supplement (10.75), available from the Superintendent of Documents, Government Printing Office, Washington 25. D. C.

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

NBS PROJECT ms REPORT

1000-30-4830 6214

November 18, 1956

PERFORMANCE TESTS OF AN EXPANSIBLE FIBROUS MEDIA AIR FILTER,

FLANDERS EXPANSIBLE, NO, 15

MANUFACTURED BY

FLANDERS FILTERS, INC.

by

Carl W. Coblentz and Paul R. Achenbach

Air Conditioning, Heating, and Refrigeration Section

Building Technology Division

to

General Services Administration

Public Buildings Service

Washington 25j D. C.

IMPORTANT NOTICE

NATIONAL BUREAU C

Intended for use within

to additional evaluation

listing of this Report, e the Office of the Direct) however, by the Govern

to reproduce additional

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

•y or progress accounting documents

s Is formally published it Is subjected

Intlng, reproduction, or open-ilteratwr*

permission is obtained in writing front D< C. Such permission 3s not needed,

iflcaily prepared If that agency wishes

<NBJp>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Performance Tests of an Expansible Fibrous Media Air Filter

Flanders Expansible No. 15

by

Carl W. Coblentz and Paul R. Achenbach

1. Introduction

At the request of the Public Buildings Service., General

Services Administration, the performance characteristics of a

Flanders expansible dust filter were determined using the NBS

Dust Spot Method of testing. The scope of this investigation

included determination of the arrestance of atmospheric

dust and Cottrell precipitate, pressure drop, and dust holding capacity of the test specimen and how these characteristics

were affected by the expansible feature of the filter.

2 . Description of Test Specimen

The filter was manufactured and supplied for test purposes by Flanders Filters, Inc., of Riverhead, New York; it

was identified as "Expansible Filter No. 15".

The filter pack consisted of a stack of 1/2 inch thick

fiberglas blankets perforated in a checkerboard pattern. It

was approximately 6 in. thick initially and could be compressed

to about one half this thickness with the mechanism provided.

In order to increase the effective filtering area of the media,

each layer of the glass fiber, -except the first and last, was

perforated by 121 holes 1-1/2 inches square in a regular

checkerboard pattern. A wall of glass fiber 1/2 inch thick

separated adjacent holes in each direction. The upstream layer

of glass fiber was perforated with 6l holes and the downstream

layer was perforated with 60 holes of the same size as those

in the other layers, but so spaced that alternate holes in

the checkerboard pattern were blocked at the upstream and downstream sides of the filter pack by these surface layers. Thus,

a glass fiber filter media 1/2" thick was provided when the

filter was fully expanded with filtration occurring as the air

passed through the walls between adjacent holes or through the

upstream and downstream layers where they covered the square

holes. The face of the filter pack measured 23" x 23" corresponding to a face area of 3-67 sq ft. The effective filter

area provided by the channels was approximately 7 sq ft when

the filter pack was fully compressed and 14.5 sq ft with the

media fully expanded.

- 2 -

By expanding the filter suddenly when a certain pressure

drop had been produced by the accumulated dust load, the designer

of this filter claimed that the surface layer of dust was broken

up and carried deeper into the filter medium, thus reducing the

pressure drop caused by the dense surface load.

The expansion of the filter pack was controlled by a pressure switch that sensed the pressure drop across the filter and

operated a small 1RPM electric motor which in turn released,

by one notch at a time, the springs that compressed the filter

pack. A signal light was provided to indicate the need for

replacing the filter peck when the maximum pressure drop had

been reached after the medium had been fully expanded. The

filter pack was installed in a steel frame, which also supported

the expansion mechanism, and could be easily replaced.

For this test, a signal light was installed, parallel with

the pressure switch, to indicate the operation of the electric

motor that performed the expansion of the filter pack.

3 . Test Method and Procedure

Arrestance determinations were made with the NBS "Dust Spot

Method” using the following aerosols: (a) air drawn from the

laboratory without addition of other dust or contaminant and

(b) Cottrell precipitate. The test method is described in the

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

Transactions, Vol. 44, p. 379 j 1938).

For this test, the unit was installed in the test duct

and carefully sealed to prevent inleakage of air except through

the measuring orifice. The desired rate of air flow through

the filter was established and samples of air were drawn from

the center points of the test duct one foot upstream and eight

feet downstream of the test specimen at equal rates and passed

through known areas of Whatman No. 4l filter paper. The ratios

of the upstream and downstream filter paper areas were selected

to obtain a similar increase of opacity, and with air drawn

through both samplers for the full period, the arrestance was

computed by the following formula:

A 100 x

AD

AU

x 100

-3-

where and are the downstream and upstream filter paper areas

and AD and AU the respective changes in opacity of the samplers.

Each test with atmospheric dust required from one to two

hours and, in order to make the increase of opacity of the two

samplers more nearly equal, the upstream sampling air was drawn

through the sampler for only part of the time. If M T" is the

percentage of the time during which air was drawn through the

upstream sampler while air through the downstream sampler was

drawn continuously, the formula for computing arrestance would

be

x 100

The two filter papers used for each test were selected to

have the same opacity readings when clean.

The following procedure was employed in testing the filter:

After the unit had been installed in the test duct and sealed

against air leakage, the pressure switch was adjusted by the

manufacturer’s representative to operate the expander at a

pressure difference of 0.8 in. W.G. The pressure drop across

the compressed filter was then determined for the rated air

flow rate and for airflows 20 percent lower and higher than

the rated flow rate.

The initial arrestance value at the rated air flow rate,

using the laboratory air as the aerosol, showed a value which

was considered too low by the manufacturer’s representative.

An examination of the filter pack showed that the discoloration

of the portion of the first and last layers, which covered the

square holes- was excessive. According to the manufacturer’s

representative, these end layers were cut from the ends of a

roll of glass fiber and were considerably thinner than the

nominal 0.5 in.

The end layers of a second filter pack were then installed

in addition to the original ones. When the first filter pack

was again installed in the test apparatus, it was found that

the pressure drop at the rated air flow rate of 1000 cfm had

increased from 0.448 in. W.G. to 0.665 in. W.G. By expanding

the filter pack one step, the pressure drop decreased to 0.568

In. W.G. and when it was expanded a second step, it decreased

to 0.480 in. W.G. The manufqcturer ’ s representative decided

to use this step for the beginning df the test, pointing out

A = 100

A AD

T x 7— x —

Att AU

-4-

that the additional end layers had produced a too-tightly packed

filter and would reduce the dust holding capacity.

The pressure drop of the modified filter was then determined

for the same three air flow rates and two determinations of the

arrestance were made at the rated air flow rate with atmospheric

air. Thereafter,, the filter was loaded with a mixture of 96

percent Cottrell precipitate and 4 percent cotton lint, by weight,

in a concentration of one gram per 1000 cu ft of air. The Cottrell precipitate and the lint were dispersed separately into

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. It was dispersed into the air stbeam through an aspirator operating at about 35 psi Inlet air pressure. At intervals,

the arrestance of the filter was determined using 100 percent

Cottrell precipitate in laboratory air. Also, several more

arrestance determinations with atmospheric dust were made.

The pressure drop was recorded as it increased with the

dust load, and also the decrease of the pressure drop when the

filter pack was expanded by one step.

The filter was loaded beyond its regular pressure drop after

it had been fully expanded to determine the rate of rise of the*

pressure drop when overloaded.

4. Test Results

A summary of the test results is presented in Table I which

shows the cumulative dust loads, the pressure drop, and the

arrestance values determined with atmospheric dust and Cottrell

precipitate . at an air flow rate of 1000 cfm. The pressure drop

immediately before and after each of the three expansions of

the filter pack is also given. Figure 1 shows the values of

this table in a graph.

-5-

Table I

Performance of Flanders Expansible Dust Filter, No. 15

Total

Load Pressure Drop Arrestance Test Dust b

grams in. W.G. %

0 0.480 51.6° A

10 0.482 82.7 C

238 0.657 95.6 C

415 0.8l2 a/0.640b 95-8° C

602 0 .840a/0. 680b 56 .

A

633 0.724 95-

C C

727 0.8l5a/0.650b 62.2 A

892 0.810 93-0° C

944 1.10 - 965 1.23 62.2 A

996 - 95.

C C

a Pressure drop before expansion

b Pressure drop after expansion

c Average of two tests

d Test Dust

A Particulate matter in atmospheric air in the

laboratory

C Cottrell precipitate

The pressure drop of the clean filter as modified prior

to the tests at different air flow rates was as follows:

Air Flow Rate, cfm Pressure Drop, in. W.G.

800 O. 36 O

1000 0.480

1200 0.608

Two values are given in Table I for the pressure drop at

total loads of 415* 602, and 727 grams respectively. These

two values correspond to the pressure drop of the filter

immediately before and after it was expanded by one step.

The pressure drop of the clean filter increased from 0.480

in. W.G. to 0.812 in. W. G. as 415 grams of dust were introduced

into the system. At that time it expanded and the pressure drop

was reduced to 0.640 in. W.G. As the loading was continued,

the next two expansions took place after 187 grams and 125 grams

additional dust were introduced with similar effect on the pressure drop.

- 6 -

5 . Discussion and Conclusions

Figure 1 shows that the total dirt holding capacity of the

test specimen under these conditions of test was about 890 grams

at a final pressure drop of 0.84 in. W.G. with the media fully

expanded. It seems probable that the dirt holding capacity

would have been appreciably higher if the first two expansion

st&ps could have been used during the test. The average arrestance during the loading period, determined from the curves in

Figure 1, was approximately 94$ for Cottrell precipitate and

58$ for the atmospheric dust in the vicinity of the laboratory.

The fact that the end layers of both filter packs submitted for test were considered faulty by the manufacturer,

indicates that the production control was probably inadequate

and that the results reported are not necessarily indicative

of the performance of the commercial product.

At the end of the loading test, the filter medium was again

compressed and the pressure switch of the expander was reset.

After introducing a small amount of Cottrell precipitate, the

expander functioned to expand the media. It was observed that

a considerable amount of dust was carried through the filter as

a result of the mechanical shock of the expander action and the

movement of the media, appearing as a dust cloud at the outlet

of the exhaust blower. This observation indicates that some

of the dust load penetrated the entire media during the expanding

process. The filter was not under observation for dust penetration at the time of expansion during the major part of the loading tests summarized in Table I. It seems probable that the

same thing would have occurred to a greater or lesser degree at

each expansion. An expansion mec^nism that provided less

mechanical sh-O.ck when it functional would probably alleviate

this dust penetration of the media.

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FLANDERS EXPANSIBLE DUST FILTER

300 400 500 600 700 800 900 1000

DUST LOAD GRAMS 1

U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A, Y. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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