Patent ID: 8852440

Claim:
A filter unit comprising: an inlet ( 12 ) and an outlet ( 13 ) for a flow medium to be filtered, a first filter cartridge ( 14 ) having a first filter element, a second filter cartridge ( 15 ) having a second filter element, and a switching device ( 16 ) for facilitating an alternating filter operation of the first or the second filter cartridge ( 14 , 15 ), the first filter cartridge being implemented as a main cartridge ( 14 ) and the first filter element being implemented as a fine filter which provides a fine filtering operation, the second filter cartridge being implemented as an auxiliary cartridge ( 15 ) and the second filter element being implemented as a coarse filter which provides a coarse filtering operation, a differential pressure switch ( 9 ) having a first and second pressure sensor ( 8 a , 8 b ) being assigned to a main filter ( 2 ), implemented as the main cartridge ( 14 ), and, upon exceeding a predetermined pressure loss, the differential pressure switch triggering an alarm, wherein the pressure loss in the auxiliary cartridge is set to a same threshold value as the pressure loss in the main cartridge with a contaminated fine filter, at which the differential pressure switch reacts and triggers an alarm such that, during changing of the main filter, the pressure loss of the flow medium flowing through the coarse filter of the second filter element, when the course filter is in a clean state, causes the differential pressure switch to continue triggering the alarm, and the differential pressure switch only discontinues triggering of the alarm once the first filter cartridge is replaced and the flow medium again flows through the first filter cartridge at a reduced pressure loss following changing the filtering back to the fine filtering operation.