40_Lessons_on_Refrigeration_and_Air_Conditioning_www.hvac-eng.ir_37

# 40_Lessons_on_Refrigeration_and_Air_Conditioning_www.hvac-eng.ir_37

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0 Thermal efficiency for a heat engine, η HE is defined as: H C H cycle HE Q Q 1 Q W = = η (4.12) where W cycle is the net work output, Q C and Q H and are the heat rejected to the low temperature reservoir and heat added (heat input) from the high temperature reservoir, respectively. It follows from Carnot’s theorems that for a reversible cycle ( H C Q Q ) is a function of temperatures of the two reservoirs only. i.e. H C Q Q ) ( , H C T T φ = . If we choose the absolute ( Kelvin) temperature scale then: H C H C T T Q Q = (4.13) hence, H C H C HE , Carnot T T 1 Q Q 1 = = η (4.14) The efficiency of refrigerator and heat pump is called as Coefficient of Performance (COP). Similarly to heat engines, Carnot coefficient of performance for heat pump and refrigerators COP HP and COP R can be written as C H C C H C cycle C R , Carnot C H H C H H cycle H HP , Carnot T T T Q Q Q W Q COP T T T Q Q Q W Q COP = = = = = = (4.15) where = work input to the reversible heat pump and refrigerator cycle W H Q

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