COMPARISON OF CAPTURE EFFICIENCIES MEASURED BY TRACER GAS AND AEROSOLTRACER TECHNIQUES

The capture efficiency of an exhaust device was evaluated using a trac er gas (helium diluted in air), and an aerosol tracer of varying parti cle size distributions. Helium concentrations were measured by mass sp ectrometry, and those of the aerosol were evaluated using optical and photometric particle counters in order to follow the temporal evolutio n of the concentrations. This experimental study confirms the theoreti cal results obtained from mathematical simulations (CFD), and from the simple study of the particle relaxation time and sedimentation veloci ty of particles. It demonstrates that the transfer of an aerosol to a square local exhaust system of 20x20 cm(2) is nearly identical to that of a gas when all the following conditions are fulfilled: diameter of particles less than about 30 mu m; low initial particle emission velo city (of a few tens of cm/s); direct capture and short transfer times between the source and the capture system (less than a few seconds). I t is then possible to use the tracer gas technique to measure the capt ure efficiency of an aerosol if the above-mentioned conditions are ful filled.