An eight-stage cascade impactor, with cut-sizes ranging from 0.056 to 18 pm at a flow rate of 30 L/min, has been developed, calibrated, and tested in field programs. This impactor, called a microorifice uniform deposit impactor, has several features not normally found in cascade impactors. Ultrafine particles as small as 0.056 pm are collected while keeping the pressure drop moderate by utilizing multiple nozzles (up to 2000) as small as 52 pm in diameter. At each stage the collected particles are deposited uniformly over the entire impaction plate by rotating the impaction plate relative to the nozzles and by the correct radial placement of the nozzles. The particle cutoff characteristics (collection efficiency curves) of each stage and interstage losses were determined by means of monodisperse aerosols. The cutoff characteristics were found to he sharp and similar for all stages.
A coating technique for applying uniform, thin layers of grease onto impactor substrates is described. The technique involves dissolving or diluting the grease in a solvent, applying the grease solution to the substrate, the spinning the substrate at a high speed. The coating thickness is controlled by the concentration of grease in the solution and by the spin speed. Apiezon-L vacuum grease and silicone oil were tested with this technique. Coating thicknesses between 0.3 and 9 y m were successfully generated on the polycarbonate film. The coated substrates and uncoated polycarbonate were tested for particle collection efficiency with polystyrene latex in two stages (0.56-and 1.0-ym cut-size) of a microorifice uniform deposit impactor. In the range of coating thicknesses tested, the collection efficiency of Apiezon-L was strongly related to the coating thickness; large coating thicknesses greatly improved particle collection. For the silicone oil, collection efficiency was high for all the coating thicknesses. The thinnest coating of silicone oil performed as well as the thickest coating of Apiezon-L grease.
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