MIL-PRF-46736E
4.4.4 Dust capacity. Unless otherwise specified by the procuring activity (see 6.2), after
pretest preparations, an absolute filter of proper size shall be installed between the air cleaner and
the flow meter. The air cleaner setup shall remain as specified in 4.4.3. Airflow shall be started
and adjusted to rated flow of the element. The initial restriction at the start of the test and at the
measurement points shown at figure 1 shall be measured. Coarse dust shall be fed at the rate as
specified in 4.3.2f. A fiberglass batting shall be installed in the absolute filter frame at all times. It
shall be replaced with a clean fiberglass batting when the net static pressure drop across the
absolute filter reaches 2.48 kPa (10 in. of water) above the original restriction. All batting shall be
weighted before and after testing as specified in A.3.2.1. The dust capacity test shall be
terminated when the air flow restriction reaches 4.98 kPa (20 in. of water) at the rated air flow or
when the specified dust capacity in hours is achieved. The absolute filter shall be removed taking
care to remove all entrapped dust. The accumulated efficiency shall be determined using the dust
efficiency equation in 4.4.3 including the specified accuracy of all measurements, and dust
efficiency shall not be less than 99.9% in reference to the dust capacity test. This requirement
shall be met when the assembly is operated at variable airflows and fed coarse grade dust. During
dust capacity test, an accumulative efficiency shall be measured. The accumulated efficiency after
completion of dust capacity test shall not be less than 99.9%. This requirement shall be met when
the assembly is operated at variable airflows and fed coarse grade dust.
4.4.5 Post service airflow restriction. Following the dust capacity test, the filter element
shall be removed from the air cleaner. The filter element shall be cleaned by using compressed air
of not more than 690 kPa (100 psig) to remove the dust. The air hose nozzle shall be inserted
into each pocket or serration of the filter element to blow out the deposited dust, using a back and
forth motion along the length of each serration and a direct flow into each pocket from the inside
out. After cleaning, the element shall be reassembled into the air cleaner. Airflow shall be started
and adjusted to the rated flow of the element. The restriction shall be noted.
4.4.6 Backfire. The filter element used in the efficiency test shall be cleaned, soaked for
5 hours in water, then removed from the water and allowed to drip for 5 minutes. The element
shall be installed in the air cleaner and the assembly shall be set up as shown in figure 2. The
capacity of the compressed air chamber shall equal 579 cubic centimeter (cm3) for each m3/min
[1 cubic in. (in3) for each ft3/min] of rated airflow of the element. Starting at 69 kPa (10 psig) in
the chamber, the compressed air shall be released instantly to the assembly. The air pressure in
the chamber shall be increased in increments of 69 kPa (10 psig) until 690 kPa (100 psig) is
reached, or until structural damage occurs. The test shall be repeated 5 times at each pressure.
4.4.7 Differential pressure test. Following post service airflow restriction, the filter
element assembly need not be installed in its intended air cleaner housing for the test. The filter
element shall be connected to a flow source capable of a minimum 12.44 kPa (50 in. of water)
negative pressure (vacuum) at the rated air flow of the filter element as specified in figure 3. Dust
shall be fed to the filter element at a rate not to exceed 88.3 g/m3 (2.5 g/ft3) until the differential
pressure (pressure drop) reaches 12.44 kPa (50 in. of water). At this condition, the filter element
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