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THERMODYNAMICS - CASE STUDY

    Introduction



Heat Exchange in the Evaporator
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According to a recent new law, refrigeration systems are no longer allowed to use refrigerant R-12 due to environmental concerns. A cryogenic manufacturing facility has to choose a new refrigerant to replace R-12 used in its refrigeration systems. Refrigerant R-134a is one choice which satisfies the energy requirement. To further evaluate this new refrigerant, the entropy changes need to be determined.

What is known:

  • Refrigerant R-134a enters the evaporator of the refrigeration system as a saturated liquid-vapor mixture at a pressure of 200 kPa.
  • Refrigerant R-134a leaves the evaporator as saturated vapor at a pressure of 200 kPa.
  • Refrigerant R-134a absorbs 150 kJ of heat from the cooled space.
  • The cooled space of the refrigeration system is maintained at -4oC.
     
    Question


 
  • Determine the entropy change of refrigerant R-134a.
  • Determine the entropy change of the cooled space.
  • Determine the total entropy change of the whole process.
     
    Approach



 
  • The evaporator is a heat exchanger which has one inlet and one exit.
  • Refrigerant R-134a enters the evaporator as a saturated liquid-vapor mixture and leaves the evaporator as saturated vapor. Hence the temperature in the evaporator remains constant at the saturation temperature of 200 kPa.