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What is Gas Cycle Refrigeration ? and Determination of Its COP

Refrigeration can also be accomplished by means of a gas cycle. In the Gas cycle, also called Bell-Coleman Cycle, an expander replaces the throttle valve of a vapor compression system, because the drop in temperature by throttling a real gas is very small. For an ideal gas, enthalpy is a function of temperature only, and since in throttling enthalpy remains unchanged, there would not be any change in temperature also. Work output obtained from the expander is used as an aid in compression, thus decreasing the network input. The ideal gas refrigeration cycle is the same as the reverse Brayton cycle. Since there is no phase change , the condenser and evaporator in the vapor compression system are here called the cooler and refrigerator respectively. At 1, the air is isentropically  compressed from p1 to p2(1-2), after which is cooled at a constant pressure p2 to state 3 and then expanded isentropically to cool to state 4. The COP of the refrigeration cycle, assuming the gas to be ideal, is given by , 

COP = Q2 / Wnet = (h1 - h4) / ((h2-h1)- (h3-h4))

        = (T1 - T4) /  ((T2-T1)-(T3-T4)) 

       For isentropic compression and expansion 

T2 / T1 = (p2 /p1) ^((y-1)/y)  = T3 / T4 


COP = T4 / (T3-T4) 

Also 

COP  = 1 /  ((rp^((y-1)/y)) -1)

where p2 is the pressure after compression and p2 is the pressure before compression.



The COP of a Gas cycle refrigeration is low. The power requirement per unit capacity is high. Its prominent application is in aircrafts and missiles, where the vapor compression refrigeration system becomes heavy and bulky. 

The Compressed air is available and is a small percentage of the amount handled by the compressor of a turbojet or a supercharged aircraft engine. Large amounts of cool ambient air are available for cooling the compressed air. In addition to cooling, the replacement of stale air in the cabin is possible. At high altitudes, the pressurization of cabin air is also possible. Because of these considerations, air cycle refrigeration is favored in aircrafts. 

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