Influence of Pleat Deformation on Pressure Drop for a High-Efficiency Particulate Air Filter: A Small-Scale Experimental Approach

Abstract

For industrial or domestic applications, the wide range of use of pleated filters makes the understanding of their airflow behavior a major issue for designer and users. In all industrial installations dealing with radioactive matter, the containment of pollutants must be ensured. High-efficiency particulate air (HEPA) filters are used as the last purification stage before the air is rejected in the environment. These filters can be used either alone, in the case of nonsensible installation, or coupled with other filtration devices disposed before it where contamination level could be important. The prediction of their pressure drop is very important in nuclear safety to be able to anticipate any dysfunction or rupture of these devices. It has been observed that geometry of the medium has an influence on the pressure drop of a pleated filter. In the case of HEPA filters, no convincing explanation has been brought to explain their airflow behavior. The pressure drop evolution of the filter during the clogging remains difficult to explain by assuming constant pleat geometry. Some studies show that deformation occurs during the filter use, which could induce an increase of the available volume in the pleat and a reduction of the efficient filtration surface. The increase in computation capacity introduces nowadays the possibility to perform complex simulation, taking into account the effect of fluids on sensible devices. This can be the case for simple structural analysis or for more complex analysis such as vibration induced by gas or fluid flow. It is mostly applied to avoid breaking or deformation of safety devices, and this can also be applied to anticipate the fluid behavior of some special devices such as filters. In classical filtration application, properties of the filter are coupled with particle deposition (e.g., changes in geometry and permeability depend on the thickness of the deposit). The studies concerning mechanical properties of filters are mainly performed for liquid filtration and clean filters. For pleated filters, the complexity of this kind of analysis remains the modification of the link between geometry, pressure drop, mechanical strength, and particle transport and accumulation inside the pleat. As a first approach, it has been chosen to combine an experimental and a numerical approach to improve the understanding of filter behavior. In this paper, the pleat deformation will be investigated using a direct nonintrusive laser measurement performed on a single pleat experiment. The rate of filtration surface lost will be estimated using these data and taken into account to evaluate the pressure drop against the filtration velocity. Results obtained show that the pleat deformation is an important parameter, which influences the geometry of the pleat.