NATIONAL BUREAU OF STANDARDS REPORT

9332

PERFORMANCE TEST OF MODEL HP-2A

DRY TYPE REPLACEABLE FILTER

by

Charles M. Hunt

Report to

General Services Administration

Washington, D. C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANIARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

42103-40-4215628 9332

May 27, 1966

PERFORMANCE TEST OF MODEL HP-2A

DRY TYPE REPLACEABLE FILTER

by

Charles M. Hunt

Report to

General Services Administration

Washington, D. C.

IMPORTANT NOTICE NATIONAL BUREAU OF SI

for use within the Government, and review. For this reason, th whole or in part, is not autho Bureau of Standards, Washing'

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

iss accounting documents intended

subjected to additional evaluation

i listing of this Report, either in e Office of the Director, National

i the Government agency for which

:opies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

PERFORMANCE TEST OF MODEL HP-2A

DRY TYPE REPLACEABLE FILTER

by

Charles M. Hunt

1 . Introduction

At the request of the Public Buildings Service of the General

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

were determined. The scope of the investigation included determination

of the arrestance of the particulate matter in laboratory air and of

Cottrell precipitate by the filter, and of pressure drop across the filter

at the rated air flow of 1000 cfm as the dust load was increased from

zero to a final pressure drop in excess of 0.5 inches W.G. The dust-holding

capacity for a final pressure drop of 0.5 in. W.G. was also determined.

2 . Description of Test Specimens

The HP-2A filter consisted of a permanent frame approximately 24 x

24 x 12 inches deep, in which is mounted a replaceable pleated filtering

media formed in twelve vertical pleats or pockets about 11-1/2 inches

deep which fit into and were supported by the frame. The working face

dimensions of the filter proper were 22 x 22 inches giving a face area of

about 3.4 ft^. Due to the pockets the effective media filtering surface

O was 40 ft or more. According to Farr Company Bulletin B-1300-4A, the

rated capacity of this size filter is 1000 cfm.

The filtering media was about 0.1 in. thick and was backed on the

downstream side by a light square-weave net fabric having a thread count

of about twelve threads per inch. 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

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

The upstream face of the filter was treated with a material which imparted

a blue color. The media would burn when held in a flame but combustion

ceased immediately upon removal.

3. Test Method and Procedure

The filters were tested at the rated capacity of 1000 cfm. The

arrestance determinations were made with the NBS Dust Spot Method described

in a paper by R. S. Dill entitled, "A Test Method for Air Filters”

(ASHVE Transactions, 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, or inward flow of air into the test apparatus excerpt through

the measuring orifice. 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 Whatman No. 41 filter paper.

Arrestance determinations were made with the particulate matter in laboratory air as the aerosol and also with Cottrell precipitate injected into

the air stream at an average concentration of about one gram per 1000 cu.

ft. of air.

The relative amount of light passing through the sampling papers

was measured on the same area of each paper 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.

- 2

-
In determining arrestance, the areas of sampling paper upstream and

downstream were selected so as to obtain dust spots of similar density.

The arrestance was then calculated by the equation:

A = ( 1 - 15 X x 100 , V

Su AU y

(1 )

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

change in the amount of light passing through the upstream and downstream

papers due to the dust spots, and Sy and $£> represent the areas of sampling

paper upstream and downstream.

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

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

Cottrell precipitate was dispersed into the air stream at a rate of 1 grata

per 1000 cu . ft. 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

Most of the data was obtained with a single filter, but a second

filter of the same type was tested for initial pressure drop with atmospheric

air

4. Test Results

The amount of dust fed to the filter, the pressure drop across the

filter, and arrestances for atmospheric dust and for Cottrell precipitate

are summarized in Table 1. Most of the arrestance values in the table

are averages of two or more determinations. The same data are plotted in

figure 1 where the two upper curves represent arrestance as a function

- 3-
of total dust fed, and the lower curve shows the pressure drop plotted

against dust fed.

As noted in Table 1, the initial pressure drop across the filter

at the rated capacity of 1000 cfm was 0.060 inches W.G. This value was

duplicated by the second filter.

From the bottom curve in figure 1 the dust fed to the filter when

a pressure drop of 0.5 inches was reached is seen to be 1430 grams. This

corresponds to a nominal dust holding capacity of 1360 grams when corrected

for 4,7% upstream fallout. This figure includes dust passing the filter

as well as dust retained.

Discontinuities are present in the loading curve. The first one

was obtained by raising the flow rate successively to 1500 cfm and 2000

cfm, lowering it to 500 cfm, and then returning it to 1000 cfm as noted

in footnote c in Table 1. The subsequent discontinuities represent

decreases in pressure drop observed when airflow through the filter was

resumed after shutting down overnight or over a weekend. This behavior

has also been observed in sac type filters and has been attributed to

sagging of the filter when the flow is reduced resulting in dust fall-off.

In the HP-2A filter, however, the frame appears to give good support to

the filter. Nevertheless, irregularities in the pressure-drop curve

were obtained. The form of this curve suggests that if loading were

carried on continuously day and night a slightly lower dust holding

capacity might be obtained as suggested by the dashed line. However,

the dust-holding requirement for a type-C filter, according to General

Services Administration Standard Air Conditioning Specification of

December 1964, is 275 grams per 1000 cfm of specified capacity, and the

capability of the filter exceeded this requirement substantially.

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The initial arrestance for Cottrell precipitate was 78.7 percent,

and an arrestance of 93.0 percent was obtained after about 1490 grams of

dust, net, had reached the filter. The average arrestance was 87.3

percent, based on the area under the curve in figure 1 up to a dust load

corresponding to a final pressure drop of 0.5 in. W.G.

A repeatable initial arrestance with atmospheric dust was not

obtained, as noted in footnote d of Table 1. However, arrestance improved

as air was drawn through the filter. One suspected source of difficulty

is that small amounts of the fire-proofing or binding agent may have dusted

out of the filter during the first few hours. This was suggested by

spurious negative arrestances which were obtained initially with the

filter, after the duct had been checked out for arrestance with no filter

in place. Also, small but visible amounts of blue discoloration could be

observed when the hand was passed lightly over the upstream surface of

the second filter.

Arrestance of atmospheric dust became repeatable after loading with

test dust was begun and reached a final value of 58.0 percent after 1490

grams (net) of dust had reached the filter. The average arrestance

calculated from the middle curve of figure 1 was 43.3 percent when integrated

up to the amount of dust fed when a pressure drop of 0.5 in. W.G. was

reached

In Table 2 the performance of the Farr HP-2A filter operating at

its rated capacity of 1000 cfm is compared with the requirements for type-C

filters according to General Services Administration "Standard Air Conditioning Specifications" of December 1964.

- 5-
FOOTNOTES FOR TABLE 1

a. At the conclusion of the measurements, 78 grams of dust were collected

upstream from the filter. This corresponds to 4.7% of the total dust

fed, or conversely only 95.3% of the dust reached the filter. If it

is assumed that the percentage of fallout was constant throughout the

dust feeding period, each value in this column may be multiplied by

0.953 to obtain a corrected estimate of the dust reaching the filter.

b. The apparatus was shut down overnight and over weekends. These pressure

drop readings were obtained when measurements were resumed.

c. At this point the flow rate through the filter was measured at 1500,

2000, 500, and 1000 cfm. The pressure drop did not return to its

original value. The flow rates and corresponding pressure drops were

obtained in the following sequence,

cfm

In.

W.G.

1 . 1000 0.115

2. 1500 .209

3. 2000 .334

4. 500 .031

5. 1000 .096

d. No satisfactory initial arrestance with atmospheric dust was obtained.

After a sequence of negative values, arrestances of 9.0, 18.0, and

41.8 percent were obtained. On the second filter an initial arrestance

of 10.4% was obtained after air had been drawn through the filter for

two to three hours.
Table 1

Performance of Farr HP-2A Filter at 1000 cfm

t Fed

;rams)

Pressure Drop

a Across Filter

(in. W.G.)

Arrestance

Atmospheric Cottrell

Dust Ppt.

% %

First Filter

0 0.060 d 78.7

139 .088

171 .099 34.3 81.2

289 .122

. 115 b

.096 c 393 .120

518 .149

622 .176

.171 b

726 .201 48.6 87.4

810 .230

893 .265

.264 b

976 .301 46.8 92.1

1018 .328

.320 b

1143 .397 50.0 92.6

.296 b

1268 .389

1393 .494 51.4 93.7

.470 b

1518 .582

1560 .612 58.0 93.0

.576 b

1643 .722

Second Filter

0 0.060 10.4 —

Footnotes on preceding page.
Table 2

Performance of Farr KP-2A filter compared with GSA requirements for typeC filters.

Farr

HP-2A

Nominal dust holding capacity when 0.5

in. W.G. is reached, grams per 1000 cfm 1360

of capacity.

Average arrestance for Cottrell ppt.,

percent. 87.3

Average arrestance for atmospheric

dust, percent 44.5

Requirement

Type-C filter

275

80

not specified
PRESSURE

DROPINCHES

W.G.

ARRESTANCE

-

PERCENT

FARR HP - 2 A FILTER IOOO CFM

90

80

70

60

50

40

30

20

10

0

FIG. I TOTAL DUST FED - GRAMS


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