A connection between cake thickness h_c, filtrate volume V and filter area A can be derived out of a mass balance of the filter cake and the filtrate, aside from this being dependent on the porosity ε and the densities of the solids and the liquid:

As can be recognized out of the formula, the kappa-factor is to be interpreted also as a concentration measure in dependence of the solids volume concentration c_V of the suspension.

Old design of leaf filters (1905), employed for fine filtration (fine filter) and usually works with overpressure. The Kelly filter possesses vertical, rectangular filter blades, which are installed in sequence in a lying pressure tank. For the filtration it will be filled entirely with the suspension to be separated. The filtrate discharge is performed for each filter element through the lid. After the ending of the filter process the filter container is pulled away from the plate package, which is to be cleaned. The overpressure in Kelly filters lies usually at several bars. At difficult to filter suspensions one can work with filter aids.

The kinematic viscosity ν is expressed in Stokes (1 St = 1 cm2/sec). It is related to the dynamic viscosity η via the fluid density ρ:

Device for removal of filter cake after the demoisturing step. The knife can actually have a cutting function, such as the peeler knife in peeler centrifuges, or only function as a deflecting plate, as in drum filters and disc filters with scraper discharge.