AWP suction filters are combined with a stop valve and are primarily connected to the suction side of screw-type compressor units. They protect the compressor medium circulation from soiling. When the compressor unit is switched off, the integrated check valve prevents the compressor from rotating in the reverse direction. The suction filter is spring loaded and improves the closing of the check valve. Types 572/551 also use an additional hot gas connection. They are fitted with a check valve and with a replaceable strainer insert made of rustproof metal mesh. The check valve is attached to the bottom of the suction filter casing with hexagonal nuts and is maintenance-free. The strainer is self-centring and is held in the bottom of the casing. The cold gas refrigerant flows through the strainer from the inside to the outside. As soon as the flow of the medium slows down and comes to a standstill, the check valve closes. This prevents the screw-type compressor from reversing and thus no oil is lost.