NATIONAL BUREAU OF STANDARDS REPORT

9474

Performance Test at 2000 cfm

of Model HP-2A Dry Type Replaceable Filter

Manufactured by

Farr Company

Los Angeles, California

by

Charles M. Hunt

Report to

General Services Administration

Public Buildings Service

Washington, D. C.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 provides measurement and technical information services

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

42103-40-4215630

January 16, 1967

NBS REPORT

9474

Performance Test at 2000 cfm

of Model HP-2A Dry Type Replaceable Filter

Manufactured by

Farr Company

Los Angeles, California

by

Charles M. Hunt

Report to

General Services Administration

Public Buildings Service

Washington, D. C.

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government. E and review. For this reason, the whole or in part, is not authors Bureau of Standards, Washington

the Report has been specifically p

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

accounting documents intended

jbjected to additional evaluation

isting of this Report, either in Office of the Director, National

the Government agency for which

)ies tor its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Performance Test at 2000 cfm

of Model HP-2A Dry Type Replaceable Filter

Manufactured by

Farr Company

Los Angeles, California

by

Charles M. Hunt

Performance Test at 2000 cfm

of Model HP-2A Dry Type Replaceable Filter

Manufactured by

Farr Company

Los Angeles, California

by

Charles M. Hunt

1 . Introduction

At the request of the Public Buildings Service of the General

Services Administration the performance characteristics of a Farr HP-2A

filter were determined at an airflow rate of 2,000 cfm. A previous HP-2A

filter was tested at 1,000 cfm and the results were reported in NBS Report

No. 9332 of May 27, 1966. In the present tests arrestances were determined with the dust in laboratory air as the test aerosol as well as with

Cottrell precipitate dispersed in laboratory air. The pressure drop

across the filter was determined as the dust load was increased from

zero to a final value corresponding to a pressure drop greater than 0.5 in.

W.G*

2. Description of Test Specimen

The HP-2A filter was similar to the previous specimen described in

Report 9332. It consisted of a permanent rubberized wire frame approximately 24 x 24 in. in face dimensions and 12 inches deep in which was

mounted a replaceable filtering media formed into twelve vertical pleats
2

about 11-1/2 in. deep. The pleats fit into and were supported by the

frame. The same frame was used in both the earlier tests described in

Report No. 9332 and the present tests. The working face dimensions of

the filter proper were 22 x 22 inches, giving a face area of about 3.4

2 2

ft . Due to the pleats the effective media filter surface was 40 ft

or more. According to Farr Company Bulletin B-1300-4E the rated capacity

of the 24 x 24 x 12 inch HP-2A filter is 2000 cfm. In NBS Report 9332

1000 cfm was erroneously quoted as the rated capacity of this filter.

The filtering media was about 0.1 inch thick, and a light squareweave net fabric having a thread count of about twelve threads per inch

was attached to the downstream side. Seen under the microscope, the fibers

of the media appeared to be cotton. When they were mounted in water,

patches of insoluble non-fibrous material could be seen, which may be

some kind of chemical treatment, possibly a binder or fire retarder.

The upstream face of the filter was treated with a material which imparted a blue color. The present media appeared to have less non-fibrous

material than the previously tested media, and there was no easily removable excess of blue material on the upstream face. The media would

burn when held in a flame but combustion ceased immediately upon withdrawal.

3 . Test Method and Procedure

The filters were tested at 2,000 cfm. The arrestance determinations were made by means of the NBS Dust Spot Method described in a paper

by R. S. Dill entitled, "A Test Method for Air Filters" (ASHVE Transactions,
- 3 -

Vol. 44, page 379, 1938). The filter under test was installed in the

test apparatus and carefully sealed to prevent any by-pass of air

around the media. After establishing the correct air flow rate through

the filter, samples of air were drawn from the center points of the test

duct 2 feet upstream and 8 feet downstream of the test specimen. Each

sample of air was passed through known areas of Whatman No. 41 filter

paper. With one filter arrestances were measured with the particulate

matter in laboratory air as the test aerosol and also with Cottrell

precipitate. With a second filter only Cottrell precipitate was used.

The amount of light passing through the sampling papers was measured

on the same area of each paper in a photometer before and after test, and

the two sampling papers used for any one arrestance determination were

selected to have the same light transmission when clean.

In determining arrestance, the areas of sampling paper upstream

and downstream were selected so as to obtain dust spots of similar optical

density. The arrestance was then calculated by the equation:

where A represents the percent arrestance, AU and AD represent the change

in the amount of light transmitted through the upstream and downstream

papers due to the dust spots, as indicated by the photometer readings,

x 100 ( 1 ) U

and S and represent the areas of sampling paper upstream and downstream.
- 4 -

The filter was loaded with test dust consisting of 96 parts by

weight of Cottrell precipitate, and 4 parts of No. 7 cotton linters which

had previously been ground in a large Wiley mill having a 4-mm. exit screen.

The Cottrell precipitate was dispersed into the air stream at a rate of

about 1 gram per 1,000 cubic feet of air, and the cotton linters were

dispersed separately after each 20-gram increment of Cottrell precipitate.

Arrestance determinations were made initially and at selected intervals

in the dust loading process.

4. Test Results

Two filters were tested for arrestance and pressure drop as the

dust load was increased from zero to a terminal value. The results

obtained with the first filter are shown in Table 1. In this case arrestance was determined with both atmospheric dust and Cottrell precipitate

as the test aerosols. Most of the arrestance determinations were done in

duplicate, and the individual values are presented from left to right in

the table in the order which they were determined. The data indicates

that initial arrestance improved rapidly with repetition even when the

amount of dust added to the filter was quite small. This was true with

both atmospheric dust and Cottrell precipitate, although the first two

values with the latter were imprecise due to poor match between the upstream and downstream dust spots as mentioned in footnote d of Table 1.
5

Table 1. Performance of First Farr HP-2A

Filter at 2000 cfm

Pressure

Atmospheric3 Cottrell 3 Dust a Drop 8 Across

Fed Filter Dust Precipitate

(grams) ( inches W.G.

)

(percent) (percent)

0 0.170 0, 5.

4

C

, 14.8 54.

0

d

, 66.

9

d

72.4, 74.0

16 .176

141

266

391

.224

.284

•327

32.4, 25.0 75.6, 76.6

-15.1°, -39.3 80.0

a. The dust fed and pressure drop values were obtained before determining

corresponding arrestance values shown at right. The small amounts of

dust added in determining Cottrell arrestance are included in the subsequent dust load.

b. The duct and filter frame without media were tested for atmospheric

dust arrestance. A value of zero was obtained before installing the

filter

.

c. After making these arrestance determinations the upstream section of

the duct was removed and the filter mounting inspected for leaks. None

were found. The upstream section of the duct was replaced and the next

arrestance determination was made.

d. The precision of these arrestance values is less than that of the other

Cottrell arrestances in the table because of poor match between the upstream and downstream dust spots. Nevertheless they demonstrate an

increase in arrestance with repetition.
- 6 -

An atmospheric dust arrestance of zero was obtained initially, but this

increased with time as air was drawn through the filter. Somewhat similar

behavior was observed in the earlier test of an HP-2A filter at 1,000 cfm

(NBS Report 9332, May 27, 1966). Increased arrestance was obtained after

141 grams of dust had been fed to the filter, but negative values were

obtained after 391 grams of dust were added. This suggests that some unloading may have occurred at the higher dust load. At this point an

arrestance of 80% was obtained with Cottrell precipitate, and the test was

discontinued. A little dust was found on the floor of the duct downstream

from the filter, enough to show a streak when the finger was drawn across

the surface. However, the amount was small compared with that found after

testing some typical prefilters. Since the concentration of dust in normal

air is low during an atmospheric dust spot determination, release of a very

small amount of dust would be enough to produce a negative arrestance value.

A second HP-2A filter was then tested, determining arrestance with

Cottrell precipitate only. The results are shown in table 2. Data from

table 1 and table 2 are also plotted in figure 1. The upper curve in the

figure represents arrestance of Cottrell precipitate and the lower curve

pressure drop across the filter, both plotted as functions of dust fed.

According to table 2 the initial pressure drop of the second filter was

0.166 in. W.G. Dust was fed to the filter until a pressure drop of 0.565

in. W.G. was obtained and 760 grams of dust had been fed to the filter.
7

Table 2. Performance of Second Farr HP-2A

Filter at 2,000 cfm

Dust 3 ,b

(grams)

Pressure Drop3

Across Filter

(in. W 0 G 0

Arrestance3

of Cottrell Ppt.

(Percent)

0 0.166 70.

7

d

, 77.0, 78.8

12 . 174c

95 .212

157 .229 80.0, 77.3

270 .284

333 .515

395 •337 80.0, 80.0

528 .389 — . 410° 77.1, 80.0

607 .459

669 .495 82-3, 83.9

677 .517

760 .565 82.5, 84.3

The dust fed and pressure drop values were obtained before determining

corresponding arrestance 'values shown at right. The small amounts of

dust added in determining Cottrell arrestance are included in the subsequent dust load.

b. 0.2 gram of dust were recovered from the duct upstream from the filter.

From this it is concluded that essentially all of the dust fed reached

the filter.

c. These values were obtained when measurements were resumed after shutting

down overnight.

d. The precision of this arrestance value is less than that of the other

values in the table because of poor match between the upstream and

downstream dust spots.
8

From figure 1 a dust holding capacity of 670 grams up to a pressure drop

of 0.5 inches W. G. is obtained. This corresponds to grams per 1,000

cfm of air flow at the rated capacity of 2,000 cfm. The dust holding

requirement for a type C filter is 275 grams per 1,000 cfm of air flow when

loaded up to a pressure drop of 0.5 in. W. G. at the rated capacity of

2,000 cfm.

The initial arrestance of the second filter was 75.5% based on the

average of the first three values in table 2. The average arrestance up

to a pressure drop of 0.5 in. W.G. was 79.7 ± 1.3%. The measure of repeatability used here is the pooled standard deviation (1) of all of the duplicate Cottrell arrestance determinations in both tables, omitting those

values where poorly matching dust spots were obtained. A rounded estimate

of the average arrestance is 80 ± 1%>. The GSA requirement for a type C

filter is 80% average arrestance with Cottrell precipitate.

5. References

1. "Statis tical Methods for Chemists", W. S. Youden, Wiley & Sons, Inc.,

pages 12, 16.

USCOMM-4CS -X
PRESSURE

DROP

(IN.

W.G.)

ARRESTANCE

(PERCENT)

100

DUST FED (GRAMS)

FIS. I. PERFORMANCE OF FARR HP-2A FILTER AT 2000 CFM.