Hammerstein operators preserving disjointness

A filter-drier for a heat pump system is installed in flow lines in which the flow of refrigerant is reversed when the system changes from the heating to the cooling mode. Two side-by-side filter-drier units, joined to the flow line by a pair of Y-connectors, are so installed with check valves at the inlet and outlet of each, to filter the refrigerant flowing in one direction through a low pressure filter-drier unit for the heating mode and to filter the high pressure refrigerant flowing in the other direction through the other unit, a more porous filter-drier, for the cooling mode. The outlet check valves permit refrigerant flow through only one filter, and the inlet check valves prevent entrainment, into the flow out of that filter, of contaminants previously trapped in the other filter.