GAS CYCLE REFRIGERATION
DRAW BACK IN CARNOT CYCLE WHEN GAS IS USED AS REFRIGERANT
- Due to isentropic compression, very high pressure and due to isothermal heat rejection very high volume of gas is developed.
- Isothermal heat transfer with a gas is not possible practically due to very small specific heat of the gas.
- Small irreversibilities in a gas cycle causes much increase in work due to narrowness of the cycle.
REVERSED BRAYTON OR JOULE OR BELL COLEMAN CYCLE
- In this cycle, the two isothermal process of carnot cycle is replaced by is two isobaric process.
- These changes decrease the refrigerating effect and increase the work done.
- This cycle is used for aircraft refrigeration.
- Its COP is less than that or vapour compression cycle.
q0 =Cp (T1 - T4 )
Isentropic compressor work
Isentropic expander work
Net work =
r = Pressure Ratio =
- If Pk increases or P0 decreases pressure ratio (r) increases and so COP of the cycle decreases
- Minimum pressure is generally fixed as being atmospheric pressure.
- It the maximum pressure increases, refrigerating effect and work done both increases but COP decreases, as increase in work done is more than that of refrigerating effect.
Application to Aircraft Refrigeration
- The gas cycle is exclusively used in aircraft refrigeration systems of military and commercial aircraft.
- Only air is used as working substance in this application. Hence it's called Air
- Power per tonner of refrigeration is more compared to vapor compression system.
TYPE OF AIR REFRIGERATION CYCLE
- It can work at suction pressure higher than atmospheric pressure.
- It has more COP as pressure ratio is reduced.
- It needs heat exchanger for refrigeration process.
- Its weight & cost are more.
- It can work at atmospheric pressure as suction pressure.
- Its COP is less
- It does not require heat exchanger for refrigeration process as fresh air is directly led to conditional space.
- Its weight & cost is less.
ADVANTAGE OF AIR REFRIGERATION CYCLE OVER VAPOUR COMPRESSION CYCLE
- Leakage of refrigerant is tolerable.
- In open type system no heat exchanger is required.
- Refrigerant being air is available in mid atmosphere.
- Cabin pressurization and air conditioning can be done simultaneously.
- Initial compression of the air is obtained by ram effect.
- Conversion of high kinetic energy of the ambient air into enthalpy and pressue rise in an air craft is called ram effect.
Ram Efficiency =