Refrigeration Cycle

• involves a changes of state between vapour and liquid
  • the compressor increases the pressure and temperature of the vapour
  • the vapour has a higher temperature than Th, so heat transfers from the condenser to Th
  • heat leaving the condenser causes the vapour to form a hot liquid (condense)
  • the expansion valve lowers the pressure of the liquid to form a cold mixture of liquid and vapour
  • the liquid vapour mixture has a temperature lower than Tl so heat flows from Tl to the evaporator
  • heat added to the evaporator is absorbed by the liquid (evaporating it) and the vapour (increasing the temperature vapour)
  • repeat

note Qh=Ql + W

Normally we would define efficiency as the useful energy out divided by the work in, however this is not done for the refrigeration cycle since this may have a value greater than 1 (greater than 100%). for this reason we use the term coefficient of performance (CP).

When the refrigeration cycle is used to cool (such as refrigerators and air conditioning systems) the useful energy is Ql. Therefore the coefficient of performance is;

CP= Ql /W - refrigerator or air conditioner

If the useful energy of the system is Qh then we refer to the system as a heat pump and the coefficient of performance is;

CP= Qh/W - heat pump

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