Prefilters applied in some types of round filters are the additional filter media, generally made of unwoven fabric and installed for exploitation in dustiness or when longer replacement periods are required. Prefilters are meant for coarse filtering the air and unclogging the specific filter medium. Using prefilters extend the filter exploitation period, as the absorptivity is improved in terms of improving the ability of gathering contaminants in the filter which do not cause exceeding the limit of air flow resistance.