help In a piston cylinder device, 5kg of Air at 200kPa and 227 degree C is compr
ID: 1863246 • Letter: H
Question
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In a piston cylinder device, 5kg of Air at 200kPa and 227 degree C is compressed to 500kPa at 100 degree C. Determine the entropy change of the air. If during this process, there is only 200kJ transferred to the surrounding of 20 degree C, determine the entropy change of the surrounding. Will this process occur or not according to the second law of thermodynamics? (Answer: -2.79kJ/K; 0.68kJ/K; not) Nitrogen at 900kPa and 300 degree C is expended adiabatically and reversibly in a closed system to 100kPa. Determine the minimum nitrogen temperature after the expansion. (Answer: 305.9K)Explanation / Answer
3.
At 200 kPa and 227 deg C, from air properties we get s1 = 7.19 kJ/kg-K
At 500 kPa and 100 deg C from air properties we get s2 = 6.63 kJ/kg-K
Entropy change = m*(s - s1)
= 5*(6.63 - 7.19)
= -2.8 kJ/K
T_surr = 20 deg C = 20+273 K = 293 K
Surrounding entropy change = Q / T_surr
= 200 / 293
= 0.68 kJ/K
Net entropy change of universe = -2.8 + 0.68 = negative.
Since net entropy change of universe is negative, process is not possible.
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