Where should upstream challenge aerosol be introduced into a filter system that is being tested?
Adequate aerosol mixing upstream can usually be obtained by introduction at least ten duct diameters upstream of the filters, or by introducing it upstream of baffles or turning vanes in the duct. When neither of these methods is practical, a Stairmand disk located four to six duct diameters upstream will provide satisfactory mixing. A Stairmand disk is a plate with the same geometric shape as the duct section that blocks the central half of the duct area. Air flowing past the disk creates vortices on the leeward side that compel turbulent and thorough mixing of the introduced aerosol and the dilution air stream. The required diameter of the stairmand disk may be calculated using the following information. The Stairmand disk diameter equals the pipe (or duct) diameter divided by the square root of two (1.414213).
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