Influence of geometric and aeraulic parameters on the clogging of industrial pleated filters by solid particles.
L. Del Fabro, J.C Laborde, J. Lacan, P. Merlin, L. Ricciardi, FILTECH EUROPA 2001, 16-18/10/2001, Dusseldorf.
The communication presents some experimental and numerical results related to a study carried out at the Institut de Protection et de Sûreté Nucléaire (irsn), in collaboration with company FILTRAUTO. This study deals with the modelling of the airflow in pleated filters of nuclear (High Efficiency Particulate Air Filter) and car types, and their clogging by solid particles. Indeed, the dynamic filtration of particles, i.e. the deposit of particles on a pleated filter medium, involves the variations of the filter efficiency and pressure drop. Our study aims at acquiring experimental results and at developing associated modelling on the modification of the pressure drop versus the mass of particles deposited.
The experimental results show that the clogging of the pleated filters by solid particles comprises several stages and in particular a reduction of the filtration surface area is observed at the end of the clogging.
The experimental study is got under way of statistical design of experiments. For each medium, a 24 plan is carried out; the factors of the plan are filtration velocity vf (ranging between 1 and 10 cm/s), height of pleats h (between 27 and 48 mm), distance between two pleats p (between 2 and 3.5 mm) and particles diameters deposited on the pleated filters tested (between 0.1 and 8 µm). The main results will be presented in the paper.
· For identical geometric characteristics of filters and for a given mass of particles deposited, the ratio DP/DP0 (where DP0 and DP are respectively the pressure drop of the clean filter and the pressure drop of the filter for a given mass of particles deposited) increases when the aerodynamic mass median diameter of the particles decreases ; the effect of the particles diameter on the ratio DP/DP0 is the same whatever the medium. This effect results from the influence of the specific area of a particle which is all the higher since the diameter of the particles is low.
· The influence of the geometrical parameters differs according to the type of filter medium. For a car filter, whatever the particles diameter deposited on the filter and the filtration velocity, the increase in the ratio p/h involves an increase in the ratio DP/DP0. It is also significant to note that when this ratio p/h increases, a pleated filter tends towards a flat filter and thus the ratio of the pressure drop tends towards that of a flat filter. For an HEPA filter, there is a predominant effect of p or h according to the particles diameter.
· It can be observed a great influence of the filtration velocity on the evolution of the ratio DP/DP0 versus the mass deposited : whatever the type of filter medium, the ratio of the pressure drop increases when the filtration velocity decreases. This result can be explained by the increase in the density of the dust cake formed on the filter when the filtration velocity decreases and by a better mechanical behaviour of the pleats when the filtration velocity increases.
· For a given mass of particles deposited per unit of surface area, the ratio DP/DP0 is greater for a car medium than for a nuclear medium.
MODELLING OF THE CLOGGING OF PLEATED FILTERS
An adimensional approach, based on the Vashy-Buckingham theorem, is used for modelling the clogging of industrial pleated filters by solid particles. The initial step consists in using the law of pressure drop developed for clean filters – law also deduced from the adimensional approach – and replacing the resistance of the clean filtering medium with that of the clogged medium. The data for the resistances arise from data experiments acquired for flat filtering media. This model seems promising to describe the first stages of clogging and can be improved by incorporating new adimensional numbers to model correctly the reduction of filtering surface area observed.
This work has been done with the collaboration of FILTRAUTO.