Polydisperse oil aerosol is used to challenge the “integrity” and measure the leakage of filter media, the filter assembly, framework, and connections, etc. Therefore a common question posed: “How much aerosol is needed to test a typical filter?” The answer: approximately eleven (11) drops for a 24” x 24” x 12” filter operating at 1,000 cfm.
The example below uses 10-ug/l (micrograms per liter) of PAO-4 as an upstream challenge.
Assuming a typical 24″ x 24″ x 12″ HEPA filter has an average of 65 pleats and the pleat size is 22.5″ x 10.5″, this equates to approximately 213 ft2 of media.
To determine the amount of liquid introduced to this filter, the necessary scan time of the filter must be calculated. Using a photometer with industry standard rectangular probe:
Assuming a scan rate at 2in/sec, as specified by most filter leak scan testing standards, 12 seconds is necessary to span one 24” pass. A 24” x 24” cross section requires eight (8) passes, with overlapping strokes, and, thus, a total time of 96 seconds or 1.6 minutes.
The quantity of oil liquid necessary to generate a 10ug/l upstream challenge aerosol for a 1000CFM filter is calculated by:
1,000 cfm x 28.3 (CFM to LPM conversion) x 10 ug/l x 0.00001 (g/ug) = 0.283 grams/min of liquid.
1.6 min x 0.283g / 0.819 g/ml (density of PAO-4) = 0.55ml PAO-4
For low viscosity liquids, it is generally considered appropriate to allow 20 drops per 1 ml of liquid. This would equate to eleven (11) drops of PAO-4 necessary to challenge a 24” x 24” x 12” 1000CFM filter using a 10-ug/l challenge.
For a point of reference, this is comparable to eleven drops of liquid spread over a surface area approximately equal to that of the “inside paint” (i.e. 3 second area) of a basketball court.