Chapter 7

7-11 Chapter 7 Example Refrigerant 134a is the working fluid in an ideal vapor-compression refrigeration cycle that communicates thermally with a cold region at 0 C and a warm region at 26 C. Saturated vapor enters the compressor at 0 C and saturated liquid leaves the condenser at 26 C. The mass flow rate of the refrigerant is kg/s. Determine (a) the compressor power, in kW, (b) the refrigeration capacity, in tons, (c) the coefficient of performance, and (d) the coefficient of performance of a Carnot refrigeration cycle operating between warm and cold regions at 26 and 0 C, respectively. Solution ---------------------------------------- ---------------------------------------- ---------- CondenserEvaporatorExpansionvalveCompres sor4123 QLQHPLPHTsLiquidVaporIsenthalp1234 Wabc26 Co0 Co Specific Internal Specific Specific Type Temp Pressure Volume Energy Enthalpy Entropy Quality Phase C MPa m3/kg kJ/kg kJ/kg kJ/kg/K 1 R-134a 0 1 Saturated Vapor 3 R-134a 26 236 0 Saturated Liquid 2 R-134a Superheated Vapor (a) The compressor work is give by cW = m (h2 - h1) = ( kg/s)( - ) kJ/kg = kW (b) The refrigeration capacity, in tons, is LQ = m (h1 - h4) = ( kg/s)( - 236) kJ/kg = kW LQ = ( kJ/s)(60 s/min)1 ton211 kJ/min = ton 6 Moran, M.

Design a refrigeration system, which supplies two levels of ammonia refrigerant at 10oC and at - 10oC to two exchangers requiring duties of 850 and 2,500 kW, respectively. Draw a process flow diagram (PFD) showing major equipment, flow rates in kg/s of refrigerant, duty of each heat exchanger, duty, and horsepower of each compressor.

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  System, Refrigeration, Ammonia, Refrigeration systems

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