Almost all properties of the â parallelâ filter modelâ a system of parallel, cylindrical, staggered and regularly arranged fibres, can be treated theoretically, but these properties differ considerably from those of real filters. On the contrary, the theoretical treatment of the â fanâ model, obtained by turning in the parallel model all fibre rows in their planes by an arbitrary angle, is very difficult, but its properties are the same as those of real filters with a perfectly homogeneous structure. The â degree of inhomogeneityâ of a real filter is the reverse ratio of its resistance and of the particle capture coefficient on its fibres (in absence of inertial deposition) to those in an equivalent fan model. This makes it possible to calculate the filter efficiency from its resistance.