PS2Sol - ME 212 PS 2 Solutions 1) A commercial refrigerator...

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1) A commercial refrigerator with refrigerant-134a as the working fluid is considered. The quality of the refrigerant at the evaporator inlet, the refrigeration load, the COP of the refrigerator, and the theoretical maximum refrigeration load for the same power input to the compressor are to be determined. Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible. Analysis ( a ) From refrigerant-134a tables (Tables A-11 through A-13) 0.4795 ! " # ! " # $ ! " # $ ! " # $ % 4 4 4 3 4 3 3 3 2 2 2 1 1 1 kJ/kg 23 . 111 kPa 60 kJ/kg 23 . 111 kJ/kg 23 . 111 C 42 kPa 1200 kJ/kg 16 . 295 C 65 kPa 1200 kJ/kg 03 . 230 C 34 kPa 60 x h P h h h T P h T P h T P 26 $ C Water 18 $ C W in Q L 1.2 MPa 65 $ C Expansion valve Compressor Evaporator Condenser 42 $ C Q H 4 3 2 1 60 kPa -34 $ C Using saturated liquid enthalpy at the given temperature, for water we have (Table A-4) kJ/kg 94 . 108 kJ/kg 47 . 75 C 26 @ 2 C 18 @ 1 $ $ f w f w h h h h ( b ) The mass flow rate of the refrigerant may be determined from an energy balance on the compressor kg/s 0455 . 0 g 75.47)kJ/k 94 kg/s)(108. (0.25 kJ/kg ) 23 . 111 16 . 295 ( ) ( ) ( 1 2 3 2 &' & % % % % R R w w w R m m h h m h h m The waste heat transferred from the refrigerant, the compressor power input, and the refrigeration load are kW 367 . 8 kJ/kg ) 23 . 111 16 kg/s)(295. 0455 . 0 ( ) ( 3 2 % % h h m Q R H kW 513 . 2 kW 0.45 kJ/kg ) 03 . 230 16 kg/s)(295. 0455 . 0 ( ) ( in 1 2 in % % % % Q h h m W R kW 5.85 % % 513 . 2 367 . 8 in W Q Q H L ( c ) The COP of the refrigerator is determined from its definition T Q H Q L W in · 2 · · 4 3 2 1 2.33 513 . 2 85 . 5 COP in L W Q ( d ) The reversible COP of the refrigerator for the same temperature limits is 063 . 5 1 ) 273 30 /( ) 273 18 ( 1 1 / 1
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PS2Sol - ME 212 PS 2 Solutions 1) A commercial refrigerator...

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