Air cycle air conditioning: analysis of different configurations

Air conditioning for passenger transportation needs a global approach. For a railway operator, maintenance cost can overcome energy cost and air conditioning failure can be a serious burden to customer satisfaction. Sealed refrigeration circuits introduce a failure mode. Air cycle refrigeration systems, although less efficient, can be conceived as open cycles, avoiding the need for a closed circuit. From the experience accumulated in existing air cycles for terrestrial use, low pressure cycles seem best suited for this application. These systems expand ambient air in a turbine, lowering its temperature. This cold flow draws heat from the passenger compartment air in a heat exchanger and then is brought back to ambient pressure by a compressor. Part of the compression work comes from the turbine. Different configurations are possible. The compression/expansion sequence can be inverted and the heat exchanger can be used to cool the compressed air before expansion. This reduces the exchanger size, but makes dehumidification more complex. In this paper, some basic alternative configurations are analyzed, accounting for air humidity. Sensible and latent heat recovery is pursued. An attempt to reverse the cycle in order to have a heat pump is included.