Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 7 | 415

entropy generation during this process. Answers:(a) 442 kPa,
69.5°C, (b) 0.258 kJ/kg K

7–166 Refrigerant-134a enters a compressor as a saturated
vapor at 200 kPa at a rate of 0.03 m^3 /s and leaves at 700 kPa.
The power input to the compressor is 10 kW. If the surround-
ings at 20°C experience an entropy increase of 0.008 kW/K,
determine (a) the rate of heat loss from the compressor,
(b) the exit temperature of the refrigerant, and (c) the rate of
entropy generation.

7–167 Air at 500 kPa and 400 K enters an adiabatic nozzle
at a velocity of 30 m/s and leaves at 300 kPa and 350 K.
Using variable specific heats, determine (a) the isentropic effi-
ciency, (b) the exit velocity, and (c) the entropy generation.

7–171E A piston–cylinder device initially contains 15 ft^3 of

helium gas at 25 psia and 70°F. Helium is now compressed in

a polytropic process (PVnconstant) to 70 psia and 300°F.

Determine (a) the entropy change of helium, (b) the entropy

change of the surroundings, and (c) whether this process is

reversible, irreversible, or impossible. Assume the surround-

ings are at 70°F. Answers:(a) 0.016 Btu/R, (b) 0.019 Btu/R,

(c) irreversible

7–172 Air is compressed steadily by a compressor from 100

kPa and 17°C to 700 kPa at a rate of 5 kg/min. Determine the

minimum power input required if the process is (a) adiabatic

and (b) isothermal. Assume air to be an ideal gas with variable

specific heats, and neglect the changes in kinetic and potential

energies. Answers:(a) 18.0 kW, (b) 13.5 kW

7–173 Air enters a two-stage compressor at 100 kPa and

27°C and is compressed to 900 kPa. The pressure ratio across

each stage is the same, and the air is cooled to the initial tem-

perature between the two stages. Assuming the compression

process to be isentropic, determine the power input to the

compressor for a mass flow rate of 0.02 kg/s. What would

your answer be if only one stage of compression were used?

Answers:4.44 kW, 5.26 kW

Air

500 kPa

400 K

30 m/s

300 kPa

350 K

FIGURE P7–167

7–168 Show that the difference between the reversible
steady-flow work and reversible moving boundary work is
equal to the flow energy.

7–169 An insulated tank containing 0.4 m^3 of saturated
water vapor at 500 kPa is connected to an initially evacuated,
insulated piston–cylinder device. The mass of the piston is
such that a pressure of 150 kPa is required to raise it. Now
the valve is opened slightly, and part of the steam flows to
the cylinder, raising the piston. This process continues until
the pressure in the tank drops to 150 kPa. Assuming the
steam that remains in the tank to have undergone a reversible
adiabatic process, determine the final temperature (a) in the
rigid tank and (b) in the cylinder.

150 kPa

0.4 m^3

sat. vapor

500 kPa

FIGURE P7–169

7–170 One ton of liquid water at 80°C is brought into a
well-insulated and well-sealed 4-m 5-m 7-m room ini-
tially at 22°C and 100 kPa. Assuming constant specific heats
for both air and water at room temperature, determine (a) the
final equilibrium temperature in the room and (b) the total
entropy change during this process, in kJ/K.

Water

80 °C

4 m × 5 m × 7 m

ROOM

22 °C

100 kPa

Heat

FIGURE P7–170

100 kPa

27 °C

AIR

COMPRESSOR

(1st stage)

900 kPa

(2nd stage)

27 °C

Px Px

W

Heat

FIGURE P7–173