Deck 8: Exergy: A Measure of Work Potential

Question 1

Heat is transferred through a plane wall steadily at a rate of 750 W. If the inner and outer surface temperatures of the wall are 30°C and 15°C, respectively, and the environment temperature is 15°C, the rate of exergy destruction within the wall is

A)37 W
B)731 W
C)7200 W
D)9600 W
E)1.9 W

Answer 1

37 W

Question 2

Kerosene enters an adiabatic piping system at 20°C at a rate of 12 kg/s. It is observed that the temperature rises by 0.2°C in the pipe due to friction. If the environment temperature is also 20°C, the rate of exergy destruction in the pipe is

A)70 kW
B)4.8 kW
C)0.40 kW
D)5.8 kW
E)24 kW

Answer 2

4.8 kW

Question 3

A heat engine receives heat from a source at 1200 K and rejects the waste heat to a sink at 400 K. If the thermal efficiency of the engine is 0.40, the second-law efficiency of this heat engine is

A)40%
B)27%
C)60%
D)67%
E)83%

Answer 3

60%

Question 4

A pool contains 800 tons of water at an average elevation of 75 m. The maximum amount of electric power that can be generated from this water is

A)16,700 kWh
B)164 kWh
C)163,500 kWh
D)16,300 kWh
E)588,600 kWh

Question 5

An office is maintained at 25°C in winter by electric resistance heaters. If the outdoor temperature is 10°C, the second-law efficiency of the resistance heaters is

A)5.3%
B)5.0%
C)60%
D)67%
E)100%

Question 6

A 15-kg iron ball whose specific heat is 0.45 kJ/kg.°C is at a uniform temperature of -10°C. For an environment temperature of 25°C, the exergy content of this ball is

A)Less than zero
B)0 kJ
C)1.0 kJ
D)15 kJ
E)236 kJ

Question 7

Choose the wrong statement below:

A)The thermal efficiency of all reversible heat engines is 100%.
B)For all reversible processes, the second-law efficiency is 100%.
C)The thermal efficiency of a heat engine cannot be greater than its second-law efficiency.
D)The second-law efficiency of a process is 100% if no entropy is generated during that process.
E)The coefficient of performance of a heat pump can be greater than 1.

Question 8

A thermal-energy reservoir supplies heat steadily to a heat engine at 1800 K at a rate of 600 kJ/s. The maximum amount of power that can be produced by this heat engine in an environment at 300 K is

A)100 kW
B)250 kW
C)720 kW
D)500 kW
E)600 kW

Question 9

Air is throttled from 100°C and 900 kPa to a pressure of 200 kPa at a rate of 1.2 kg/s in an environment at 25°C. The change in kinetic energy is negligible, and no heat transfer occurs during the process. The power potential wasted during this process is

A)0
B)129 kW
C)13 kW
D)154 kW
E)840 kW

Question 10

Steam enters a turbine steadily at 4 MPa and 400°C and exits at 0.2 MPa and 150°C in an environment at 25°C. The decrease in the exergy of the steam as it flows through the turbine is

A)0 kJ/kg
B)445 kJ/kg
C)458 kJ/kg
D)1921 kJ/kg
E)597 kJ/kg