Chapter 7 | 407Assuming the argon remaining inside the tank has undergone
a reversible, adiabatic process, determine the final mass in
the tank. Answer:2.46 kg
water and the air in the room, and (c) the entropy generation.
Assume the room is well sealed and heavily insulated.
7–84 Air at 800 kPa and 400°C enters a steady-flow nozzle
with a low velocity and leaves at 100 kPa. If the air under-
goes an adiabatic expansion process through the nozzle, what
is the maximum velocity of the air at the nozzle exit, in m/s?
7–85 An ideal gas at 100 kPa and 27°C enters a steady-flow
compressor. The gas is compressed to 400 kPa, and 10 percent
of the mass that entered the compressor is removed for some
other use. The remaining 90 percent of the inlet gas is com-
pressed to 600 kPa before leaving the compressor. The entire
compression process is assumed to be reversible and adiabatic.
The power supplied to the compressor is measured to be 32
kW. If the ideal gas has constant specific heats such that cv
0.8 kJ/kg K and cp1.1 kJ/kg K, (a) sketch the compres-
sion process on a T-sdiagram, (b) determine the temperature
of the gas at the two compressor exits, in K, and (c) determine
the mass flow rate of the gas into the compressor, in kg/s.
7–86 A constant-volume tank contains 5 kg of air at 100
kPa and 327°C. The air is cooled to the surroundings temper-
ature of 27°C. Assume constant specific heats at 300 K.
(a) Determine the entropy change of the air in the tank dur-
ing the process, in kJ/K, (b) determine the net entropy change
of the universe due to this process, in kJ/K, and (c) sketch the
processes for the air in the tank and the surroundings on a
single T-sdiagram. Be sure to label the initial and final states
for both processes.Reversible Steady-Flow Work
7–87C In large compressors, the gas is frequently cooled
while being compressed to reduce the power consumed by
the compressor. Explain how cooling the gas during a com-
pression process reduces the power consumption.
7–88C The turbines in steam power plants operate essen-
tially under adiabatic conditions. A plant engineer suggests to
end this practice. She proposes to run cooling water through
the outer surface of the casing to cool the steam as it flows
through the turbine. This way, she reasons, the entropy of the
steam will decrease, the performance of the turbine will
improve, and as a result the work output of the turbine will
increase. How would you evaluate this proposal?
7–89C It is well known that the power consumed by a com-
pressor can be reduced by cooling the gas during compres-
sion. Inspired by this, somebody proposes to cool the liquid
as it flows through a pump, in order to reduce the power con-
sumption of the pump. Would you support this proposal?
Explain.
7–90 Water enters the pump of a steam power plant as satu-
rated liquid at 20 kPa at a rate of 45 kg/s and exits at 6 MPa.
Neglecting the changes in kinetic and potential energies and
assuming the process to be reversible, determine the power
input to the pump.ARGON
4 kg
30 °C
450 kPaFIGURE P7–787–79 Reconsider Prob. 7–78. Using EES (or other)
software, investigate the effect of the final pres-
sure on the final mass in the tank as the pressure varies from
450 to 150 kPa, and plot the results.
7–80E Air enters an adiabatic nozzle at 60 psia, 540°F, and
200 ft/s and exits at 12 psia. Assuming air to be an ideal gas
with variable specific heats and disregarding any irreversibili-
ties, determine the exit velocity of the air.
7–81 Air enters a nozzle steadily at 280 kPa and 77°C with
a velocity of 50 m/s and exits at 85 kPa and 320 m/s. The
heat losses from the nozzle to the surrounding medium at
20°C are estimated to be 3.2 kJ/kg. Determine (a) the exit
temperature and (b) the total entropy change for this process.
7–82 Reconsider Prob. 7–81. Using EES (or other)
software, study the effect of varying the sur-
rounding medium temperature from 10 to 40°C on the exit
temperature and the total entropy change for this process, and
plot the results.
7–83 A container filled with 45 kg of liquid water at 95°C
is placed in a 90-m^3 room that is initially at 12°C. Thermal
equilibrium is established after a while as a result of heat
transfer between the water and the air in the room. Using
constant specific heats, determine (a) the final equilibrium
temperature, (b) the amount of heat transfer between the
Room
90 m^3
12 °C
Water
45 kg
95 °CFIGURE P7–83