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7.30 A rigid, insulated tank contains 0.6 kg of air, initially at 200 kPa, 20°C. The air is stirred by a paddle wheel until its pressure is 250 kPa. Using the ideal gas model with cy 5 0.72 kJ/kg ? K, determine, in kJ, (a) the work, (b) the change in exergy of the air, and (c) the amount of exergy destroyed. Ignore the effects of motion and gravity, and let T0 5 20°C, p0 5 100 kPa.
7.31 As shown in Fig. P7.31, two lb of ammonia is contained in a well-insulated piston–cylinder assembly fitted with an

electrical resistor of negligible mass. The ammonia is initially at 20 lbf/in.2 and a quality of 80%. The resistor is activated until the volume of the ammonia increases by 25%, while its pressure varies negligibly. Determine, in Btu,
(a) the amount of energy transfer by electrical work and the accompanying exergy transfer.
(b) the amount of energy transfer by work to the piston and the accompanying exergy transfer.
(c) the change in exergy of the ammonia.
(d) the amount of exergy destruction.
Ignore the effects of motion and gravity and let T0 5 60°F,
p0 5 1 atm.

(a) determine the final temperature, in °F, and final pressure, in lbf/in.2
(b) evaluate the exergy destruction, in Btu.
(c) What is the cause of exergy destruction in this case? Let T0 5 70°F, p0 5 1 atm.

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