Measurement and modeling of pressure drop of HEPA filters clogged with ultrafine particles
Abstract The harmlessness of nanometric aerosols is currently being called into question. Their massive production in industrial production processes or during combustion, whether deliberate or incidental, requires that they be contained. Today, the most widely used means and that providing the best cost benefit ratio is the use of porous, mainly fibrous material, in air filter design. The efficiency of these filtration devices has been the subject of numerous studies and does not require further demonstration, but the prediction of their lifetime, conditioned by the pressure drop which evolves during clogging, still needs to be managed. Predicting pressure drop in the case of very thin, and therefore very clogging, particles remains a major issue in the design of filtration devices. Two phases have been highlighted on a plane fibrous medium: deep filtration and surface filtration preceded by a transition regime where particles are collected both in the depth and on the surface of the medium. Each of these phases is characterized by a different pressure drop evolution. This study focuses on measuring the porosity of the deposits, an important parameter which conditions the resistance of a clogged filter. For this, pseudo spherical alumina particles and aggregates of 10 nm carbon nanoparticles with respectively 350 nm and 73 nm of electrical mobility diameter were used. A device enabling the continuous measurement of the thickness of the deposit during the clogging process was developed. Results are shown for filtration velocities of 0.8 and 0.16 cm s 1. The change in pressure drop of a fibrous medium is then studied for deep filtration, when aerosols penetrate the medium at the beginning of the filtration process. On this basis, a simplified model was developed to predict the pressure drop increase of a filter exposed to ultrafine aerosols.