Filter casing has a very important role in filtration systems. Its shape must assure mainly an equal air flow distribution through the filtering cartridge. Designing priority is to regulate the flow without spin which sometimes may be the cause of noise. Besides, well designed casing is often a filter itself where particles of contaminants are separated in the field of centrifugal force. Although it traps only larger contaminants fractions of high specific gravity (i.e quarc dust), but the mass of trapped contaminants is significant and sometimes comparable with those trapped on the filter medium. Another important factor is an easy access to the filtering cartridge during replacement. Realising all these functions with a small volume in the bonnet is quite a difficult task requiring hard work of an experienced team of designers.