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TEST_2_Sp96 - 2 at the end of the compression process(°C...

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ME 326: THERMODYNAMICS I Test 2, March 29, 1995 1) (33%) Argon gas passes through a well-insulated valve in a supply line at a rate of 0.5 kg/s. Upstream of the valve, the pressure is 10 atm and the temperature is 127°C. Downstream of the valve, the pressure is 1 atm. What is the entropy change of the argon across the valve? Note any assumptions that are required to get your answer. SOLUTION: Because argon is a monatomic gas, c P is constant. First Law for this case, assuming valve is adiabatic and DKE and DPE across the valve are negligable, is so T out =T in . The specific entropy change across the valve for a adiabatic steady-state steady flow is or, if in terms of a rate of entropy change, 2) (33%) A researcher designs a piston cylinder system, and wishes to compress 2 kg of CO 2 gas in the cylinder from an initial condition of P 1 =2 atm, T 1 =50°C to a final pressure of P 2 =20 atm. During the process, there is heat transfer of 200 kJ to the surroundings, which are at 27°C. What is the minimum possible final temperature of the CO
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Unformatted text preview: 2 at the end of the compression process (°C)? DO NOT ASSUME THAT THE SPECIFIC HEAT OF CO 2 IS INDEPENDENT OF TEMPERATURE. SOLUTION: Apply Eq. 5.74, Second Law for a control mass: Minimum T 2 will be if equality is used, so T 2 (using CO2 Tables for s o ) =[5.0292-4.9275)/(5.0560-4.9275)x50+50=89.6°C 3) (34%) A compressor for a fertilizer plant is to take 2 kg/s of ammonia at a pressure of 125 kPa and a temperature of 20°C, and compress it to a final pressure of 500 kPa. The compressor efficiency is 70%. The ammonia leaving the compressor is to be cooled to 40°C in a heat exchanger. ( Note: There are tables of thermodynamic data for ammonia in the text.) a) What power is required to drive the compressor (kW)? b) How much heat transfer must occur in the heat exchanger (kW)? SOLUTION: STATE TABLE: STATE P(kPa) T(°C) h(kJ/kg) s(kJ/kg•K) 1 125 20 (1534.2) (6.4588) 2s 500 (1764.2 6.5488 2a 500 (1862.8) 3 500 40 (1557.9) a) b)...
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