In a heat exchanger unit for conditioning a first fluid, particularly a refrigerant in an air conditioning system, including a heat exchanger, the heat exchanger comprises first and second tube element units for heat exchange between a first fluid flowing through the tube elements and a second fluid flowing over the tube elements, wherein the first fluid is supplied first to the first tube element unit so as to flow upwardly therethrough and via an intermediate outlet connector to a compressor stage from where it is transferred to the top of the second tube element unit which is disposed above the first tube element unit and flows downwardly through the second tube element unit to an expansion stage wherein the energy released during expansion in the expansion stage is utilized for the compression of the first fluid in the compressor stage.