7–62C Starting with Eq. 7–34, obtain Eq. 7–43.
7–63C What are Prand vrcalled? Is their use limited to
isentropic processes? Explain.
7–64C Can the entropy of an ideal gas change during an
isothermal process?
7–65C An ideal gas undergoes a process between two spec-
ified temperatures, first at constant pressure and then at con-
stant volume. For which case will the ideal gas experience a
larger entropy change? Explain.
7–66 Oxygen gas is compressed in a piston–cylinder device
from an initial state of 0.8 m^3 /kg and 25°C to a final state of
0.1 m^3 /kg and 287°C. Determine the entropy change of the
oxygen during this process. Assume constant specific heats.
7–67 A 1.5-m^3 insulated rigid tank contains 2.7 kg of car-
bon dioxide at 100 kPa. Now paddle-wheel work is done on
the system until the pressure in the tank rises to 150 kPa.
Determine the entropy change of carbon dioxide during this
process. Assume constant specific heats. Answer:0.719 kJ/K
406 | Thermodynamics
helium during this process, assuming (a) the process is
reversible and (b) the process is irreversible.
7–72 Air is compressed in a piston–cylinder device from
90 kPa and 20°C to 400 kPa in a reversible isothermal process.
Determine (a) the entropy change of air and (b) the work done.
7–73 Air is compressed steadily by a 5-kW compressor
from 100 kPa and 17°C to 600 kPa and 167°C at a rate of 1.6
kg/min. During this process, some heat transfer takes place
between the compressor and the surrounding medium at
17°C. Determine the rate of entropy change of air during this
process. Answer:0.0025 kW/KCO 2
1.5 m^3
100 kPa
2.7 kgFIGURE P7–677–68 An insulated piston–cylinder device initially contains
300 L of air at 120 kPa and 17°C. Air is now heated for
15 min by a 200-W resistance heater placed inside the cylin-
der. The pressure of air is maintained constant during this
process. Determine the entropy change of air, assuming
(a) constant specific heats and (b) variable specific heats.
7–69 A piston–cylinder device contains 1.2 kg of nitrogen
gas at 120 kPa and 27°C. The gas is now compressed slowly
in a polytropic process during which PV1.3constant. The
process ends when the volume is reduced by one-half. Deter-
mine the entropy change of nitrogen during this process.
Answer:0.0617 kJ/K
7–70 Reconsider Prob. 7–69. Using EES (or other)
software, investigate the effect of varying the
polytropic exponent from 1 to 1.4 on the entropy change of
the nitrogen. Show the processes on a common P-vdiagram.
7–71E A mass of 15 lbm of helium undergoes a process
from an initial state of 50 ft^3 /lbm and 80°F to a final state of
10 ft^3 /lbm and 200°F. Determine the entropy change of
100 kPa
17 °CAIR
COMPRESSOR
5 kW600 kPa
17 °C 167 °CFIGURE P7–737–74 An insulated rigid tank is divided into two equal parts
by a partition. Initially, one part contains 5 kmol of an ideal
gas at 250 kPa and 40°C, and the other side is evacuated. The
partition is now removed, and the gas fills the entire tank.
Determine the total entropy change during this process.
Answer:28.81 kJ/K
7–75 Air is compressed in a piston–cylinder device from
100 kPa and 17°C to 800 kPa in a reversible, adiabatic
process. Determine the final temperature and the work done
during this process, assuming (a) constant specific heats and
(b) variable specific heats for air. Answers: (a) 525.3 K,
171.1 kJ/kg, (b) 522.4 K, 169.3 kJ/kg
7–76 Reconsider Prob. 7–75. Using EES (or other)
software, evaluate and plot the work done and
final temperature during the compression process as func-
tions of the final pressure for the two cases as the final pres-
sure varies from 100 to 800 kPa.
7–77 Helium gas is compressed from 90 kPa and 30°C to
450 kPa in a reversible, adiabatic process. Determine the final
temperature and the work done, assuming the process takes
place (a) in a piston–cylinder device and (b) in a steady-flow
compressor.
7–78 An insulated rigid tank contains 4 kg of argon gas at
450 kPa and 30°C. A valve is now opened, and argon is
allowed to escape until the pressure inside drops to 200 kPa.