The most significant difference between air refrigeration cycles and conventional vapour compression cycles is that the refrigerant (dry air) normally stays in the gas phase throughout the cycle. Air will only begin to behave like a normal refrigerant if its temperature falls to below -140oC (its critical temperature). 

The ideal reverse-Joule cycle is shown schematically and on a pressure-enthalphy diagram, click on icon below to down load image: 

Click on icon to down load air cycle schematic. 
Click on icon to down pressure-enthalpy diagram.
Because air cycles involve sensible heat transfer (the refrigerant stays in the gas-phase) much greater refrigerant (air) flow rates are required than in comparable vapour compression systems which include latent heat transfer.  For this reason continuous dynamic machinery are normally used for the compressor and expander.  Most current air cycle plant use turbines because these can be made compact and can achieve high efficiencies. Modern expansion turbines can achieve efficiencies around 90%.


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