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Unformatted text preview: Prob 11.5 PROBLEM 11.2 PROBLEM STATEMENT: A refrigeration system using refrigerant R134a is to have a capacity of 2 tons. The cycle is the ideal vapor compression cycle. The evaporator exit pressure is 0.15 MPa, and the condenser inlet temperature is 60C. Determine the compressor power requirement in kilowatts. DIAGRAM DEFINING SYSTEM AND PROCESS: GIVEN: R134a, ideal vapor compression cycle, & IN Q = 2 tons T 2s = 60C, P 1 = 150 kPa FIND: & IN,COMPRESSOR W (kW). ASSUMPTIONS: SFSS, NKEPE GOVERNING RELATIONS: 1. Refrigeration capacity, IN,EVAP IN,EVAP 1 4 1 4 Q Q = m(h -h ) m = (h -h ) & & & & 2. Ideal compressor power requirement, & & IN,COMPRESSOR 2s 1 W =m(h -h ) QUANTITATIVE SOLUTION: From the property tables for R134a. 1 V 1 v 2s 2s 2 2s 3 4 L h = h (150 kPa) = 237.01 kJ/kg (interpolated from Table 15s) s = s (150 kPa) = 0.931 kJ/kg = s (interpolated from Table 15s) h = h(T = 60 C, s = 0.931 kJ/kg ) = 283.1 kJ/kg (Table 16s @ P 1400 kPa) h = h = h (1400 kPa) =125.26 kJ/kg (Table 15s @ P 1400 kPa) Hence, from the two governing relations, the compressor power requirement is 2s 1 IN,COMP IN,EVAP 1 4 h -h kW 283.10- 237.01 W = Q = 2 (tons) 3.52 = 2.9 kW h -h ton 237.01-125.26 & & DISCUSSION OF RESULTS: The unit tons defines the total rate of cooling, IN,EVAP Q & . This term had its origin in the days when ice was manufactured in large plants and distributed to homes for preserving food in ice-boxes. A ten-ton refrigeration plant could produce approximately 20,000 lbm of ice per day. P 1 = P 4 T 1 3 2s 4 P 2s = P 3 s P 3 4 1 2s Prob 11.5 PROBLEM 11.5 PROBLEM STATEMENT: A Carnot engine operating between two temperature reservoirs of 817C and 25C rejects 20 kJ/s. The engine drives the compressor of an ideal vapor compression refrigerator whose inlet and exit pressures are 0.17 and 1.1 MPa, respectively. Calculate the coefficient of performance and the refrigerator's capacity in tons if the refrigerant is R-134a. DIAGRAM DEFINING SYSTEM AND PROCESS: GIVEN: Refrigerant 134a in an ideal vapor compression refrigerator P 1 =0.17 MPa, P 2 =1.1 MPa Carnot heat engine, T C =25C, T H =817C, C Q 20 kJ/s = & FIND: Coefficient of performance and refrigeration capacity, ( 29 IN,EVAP Q tons & ASSUMPTIONS: 1. Ideal refrigeration cycle (no P in evap. or condens., reversible compressor) 2. SFSS, NKEPE GOVERNING RELATIONS: 1. HE,OUT C C HE,CARN H H H W T Q 1 1 T Q Q = =- =- & & & & 2. Coefficient of performance, IN.EVAP 1 4 IN,COMP 2 1 q h -h COP = = w h -h 3. Refrig capacity: IN,EVAP IN,EVAP 1 4 1 4 Q Q = m(h -h ) m = (h -h ) & & & & QUANTITATIVE SOLUTION: First, determine the power output of the Carnot engine....
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- Spring '08