Heat Exchange in the Evaporator

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 -4^oC.

• 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.

• 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.