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Unformatted text preview: Assignment 8
Availability or Energy D55 A freezer is maintained at 20°F by removing heat from it at a rate of 75 Btufmin.
The power input to the freezer is 0.70 hp, and the surrounding air is at 75°F. Determine
(a) the reversible power, (3)) the irreversibility, and (c) the secondlaw efficiency of this freezer.
Answers: (a) 0.20 hp, (3:) 0.50 hp, (c) 28.9 % D56 The radiator of a steam heating system has a volume of 20 L and is ﬁlled with
superheated water vapor at 200 kPa and 200°C. At this moment both the inlet and the
exit valves to the radiator are closed. After a while it is observed that the temperature of
the steam drops to 80°C as a result of heat transfer to the room air, which is at 21°C.
Assuming the surroundings to be at 0°C, determine (a) the amount of heat transfer to the
room and (b) the maximum amount of heat that can be supplied to the room if this heat
from the radiator is supplied to a heat engine that is driving a heat pump. Assume the heat engine operates between the radiator and the surroundings.
Answers.“ (a) 30.3 k], (b) 114.8 kJ D57 A 1.2m3 insulated rigid tank contains 2.13 kg of carbon dioxide at 100 kPa. Now
paddlewheel work is done on the system until the pressure in the tank rises to 120 kPa.
Determine (a) the actual paddlewheel work done during this process and (b) the
minimum paddlewheel work with which this process (between the same end states) could be accomplished.
Answers: (a) 87.0 kl, (b) 7.66 k] D58 Steam enters an adiabatic turbine at 6 MPa, 600°C, and 80 mfs and leaves at 50 kPa,
100°C, and 140 mr's. If the power output of the turbine is 5 MW, determine (a) the
reversible power output and (b) the secondlaw efﬁciency of the turbine. Assume the surroundings to be at 25°C.
Answers: (a) 5.81 MW, (13) 86.1 % D59 Steam expands in a turbine steadily at a rate of 15,000 kg/h, entering at 8 MPa and
450°C and leaving at 50 kPa as saturated vapor. Assuming the surroundings to be at 100
kPa and 25°C, determine (a) the power potential of the steam at the inlet conditions and (b) the power output of the turbine if there were no irreversibilities present.
Answers: (a) 5513 kW, (5) 3899 kW g: COHBge of Engineering #5 i I// m
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 Spring '09
 Davis

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