Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev

Mechanical Engineering SSC JE (Technical)

Mechanical Engineering : Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev

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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.

Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev
Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev
Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev
Refrigeration effect 
q0 =Cp (T1 - T4 )
Heat rejected
Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev
Isentropic compressor work
Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev
Isentropic expander work
Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev
Net work =
Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev
r = Pressure Ratio =
Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev

  •  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

Refrigeratin Cycle.

  •  Power per tonner of refrigeration is more compared to vapor compression system.

TYPE OF AIR REFRIGERATION CYCLE
Closed 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.

Open Cycle

  •   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.

Ram Effect

  •  Conversion of high kinetic energy of the ambient air into enthalpy and pressue rise in an air craft is called ram effect.

Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev

Ram Efficiency = 
Chapter 5 Gas Cycle Refrigeration - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev

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