Chemical Engineering Journal 166 (2011) 616–623 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Modelling the pressure drop across HEPA filters during cake filtration in the presence of humidity A. Joubert a,b,∗ , J.C. Laborde a , L. Bouilloux a , S. Chazelet b , D. Thomas b a IRSN - DSU - SERAC – Laboratory for the Experimental Study of Containment, Air cleaning and Ventilation, BP 68, 91192 Gif-sur-Yvette Cedex, France b Nancy Université – Laboratoire Réactions et Génie des Procédés – CNRS, 1 rue Grandville, BP 20 451, 54001 Nancy Cedex, France article info Article history: Received 1 September 2010 Received in revised form 7 November 2010 Accepted 8 November 2010 Keywords: HEPA filter Pressure drop Modelling Relative humidity Cake filtration Hygroscopicity abstract The modelling of the variation in the pressure drop across a HEPA filter during cake filtration in the presence of humidity was studied. A semi-empirical model was developed that considers the pressure drop across the clogged filter as the sum of the pressure drops across the clean filter and across the cake, which is itself decomposed into several successive layers. The pressure drop across each layer of particles depends on its state of equilibrium with the air humidity. The model includes the specific resistance of each layer of particles and its variation when the cake is exposed to a flow of moist air. This model applies for relative humidity below the deliquescent point of the aerosol if it is hygroscopic. The results show good agreement between the calculated values and experimental values for flat filters clogged with aluminium oxide and sodium chloride test aerosols. The model also revealed the influence of filtration time on the evolution of the pressure drop across a clogged filter in the presence of humidity, especially with a hygroscopic aerosol. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Air cleaning is a major concern in many sectors of industry. In the nuclear industry, aerosols are the main vectors of contamina- tion and they are captured by pleated High Efficiency Particulate Air (HEPA) filters. These filters are used to maintain containment of radioactive substances in nuclear facilities and provide the final barrier before discharge to the external environment both under normal operating conditions and under accident conditions. Some accident scenarios can lead to a significant increase in the humid- ity of the gaseous effluent. A line break with steam release or the use of water spray for fire extinction, leading to the emission of aerosols, are two examples. It is therefore essential to be able to predict the behaviour of the filters in terms of performances and particularly the evolution of their pressure drop in humidity con- ditions, as this could be detrimental to maintaining containment in nuclear facilities. The presence of humidity during the clogging of flat HEPA filters, constituted of glass fibres, was studied experimentally by Gupta et al. [1]. The conclusions of this study, confirmed by Miguel [2] using polyester-fibre filters, have revealed the influence of the hygroscopicity of the aerosols on the evolution of the pressure ∗ Corresponding author at: Ecole des Mines de Nantes, 4 rue Alfred Kastler, BP 20722, 44307 NANTES Cedex 3, France. E-mail addresses: aurelie.joubert@mines-nantes.fr (A. Joubert), Dominique.Thomas@ensic.inpl-nancy.fr (D. Thomas). drop across the filters. First, for a non-hygroscopic aerosol or a hygroscopic aerosol when the air humidity is below its deliques- cent point, the linear increase in filter pressure drop during the build-up of a cake at the surface of the filtering medium is much lower than when the air humidity is high. According to Gupta et al. [1], the effect of humidity on the evolution of the pressure drop across the filter can be attributed to an increase in the particle-to- particle adhesive forces as the air humidity increases. Thus, during cake formation, the particles tend to form a more open structure giving a lower pressure drop for the same mass of collected parti- cles. Experimental study realized prior to this modelling work [3] has confirmed the results of Gupta et al. [1] and highlighted that influence of humidity is more significant with the decreasing of par- ticle diameter. The hypothesis proposed to explain this behaviour is associated with the adsorption of water vapour on the surface of the particles deposited. Indeed, liquid water is formed by capillary con- densation of the vapour into capillaries or fine pores of the particles. According to Butt and Kapple [4], liquid water can be formed even far above the dew point of the surrounding atmosphere. Thus, water modifies the adhesive force between particles leading to a restruc- turing of the deposit (to a structure with a smaller specific surface area or to a non-homogenous structure due to the local creation of preferential passageways by the formation of aggregates) which causes a reduction in the specific resistance. This effect is more sig- nificant when the diameter of the particles is smaller because the structure of the deposit with submicron-sized particles presents a larger specific surface area due to the dendritic structure of the deposit [5]. Thus, this structure allows the adsorption of a greater 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.11.033