Chapter 7 | 419the entire system cools to the surrounding air temperature of
70°F. Determine the total entropy generated during this
process.
7–198 Air enters the evaporator section of a window air
conditioner at 100 kPa and 27°C with a volume flow rate of 6
m^3 /min. The refrigerant-134a at 120 kPa with a quality of 0.3
enters the evaporator at a rate of 2 kg/min and leaves as satu-
rated vapor at the same pressure. Determine the exit tempera-
ture of the air and the rate of entropy generation for this
process, assuming (a) the outer surfaces of the air conditioner
are insulated and (b) heat is transferred to the evaporator of
the air conditioner from the surrounding medium at 32°C at a
rate of 30 kJ/min. Answers: (a)15.9°C, 0.00193 kW/K,
(b)11.6°C, 0.00223 kW/K
7–202 Repeat Prob. 7–200 by assuming the piston is made
of 5 kg of copper initially at the average temperature of the
two gases on both sides.
7–203 An insulated 5-m^3 rigid tank contains air at 500 kPa
and 57°C. A valve connected to the tank is now opened, and
air is allowed to escape until the pressure inside drops to 200
kPa. The air temperature during this process is maintained
constant by an electric resistance heater placed in the tank.
Determine (a) the electrical energy supplied during this
process and (b) the total entropy change. Answers:(a) 1501
kJ, (b) 4.40 kJ/K
7–204 In order to cool 1-ton of water at 20°C in an insu-
lated tank, a person pours 80 kg of ice at 5°C into the
water. Determine (a) the final equilibrium temperature in the
tank and (b) the entropy generation during this process. The
melting temperature and the heat of fusion of ice at atmo-
spheric pressure are 0°C and 333.7 kJ/kg.
7–205 An insulated piston–cylinder device initially contains
0.02 m^3 of saturated liquid–vapor mixture of water with a
quality of 0.1 at 100°C. Now some ice at 18°C is dropped
into the cylinder. If the cylinder contains saturated liquid at
100°C when thermal equilibrium is established, determine
(a) the amount of ice added and (b) the entropy generation
during this process. The melting temperature and the heat of
fusion of ice at atmospheric pressure are 0°C and 333.7 kJ/kg.Sat.
vaporR-134a120 kPa
x = 0.3AIR
100 kPa
27 °CFIGURE P7–1987–199 A 4-m 5-m 7-m well-sealed room is to be heated
by 1500 kg of liquid water contained in a tank that is placed in
the room. The room is losing heat to the outside air at 5°C at
an average rate of 10,000 kJ/h. The room is initially at 20°C
and 100 kPa and is maintained at a temperature of 20°C at all
times. If the hot water is to meet the heating requirements of
this room for a 24-h period, determine (a) the minimum tem-
perature of the water when it is first brought into the room and
(b) the entropy generated during a 24-h period. Assume con-
stant specific heats for both air and water at room temperature.
7–200 Consider a well-insulated horizontal rigid cylinder
that is divided into two compartments by a piston that is free
to move but does not allow either gas to leak into the other
side. Initially, one side of the piston contains 1 m^3 of N 2 gas
at 500 kPa and 80°C while the other side contains 1 m^3 of He
gas at 500 kPa and 25°C. Now thermal equilibrium is estab-
lished in the cylinder as a result of heat transfer through the
piston. Using constant specific heats at room temperature,
determine (a) the final equilibrium temperature in the cylin-
der and (b) the entropy generation during this process. What
would your answer be if the piston were not free to move?
7–201 Reconsider Prob. 7–200. Using EES (or other)
software, compare the results for constant spe-
cific heats to those obtained using built-in variable specific
heats built into EES functions.
0.02 m^3
100 °CIce
18 °CFIGURE P7–2057–206 Consider a 5-L evacuated rigid bottle that is sur-
rounded by the atmosphere at 100 kPa and 17°C. A valve at
the neck of the bottle is now opened and the atmospheric air
is allowed to flow into the bottle. The air trapped in the bottle
eventually reaches thermal equilibrium with the atmosphere
as a result of heat transfer through the wall of the bottle. The
valve remains open during the process so that the trapped air
also reaches mechanical equilibrium with the atmosphere.
Determine the net heat transfer through the wall of the bottle
and the entropy generation during this filling process.
Answers:0.5 kJ, 0.0017 kJ/K
7–207 (a) Water flows through a shower head steadily at a
rate of 10 L/min. An electric resistance heater placed in the