Chapter 7 | 421350 kW power is produced by the turbine, determine the effi-
ciency of the turbine. Take the density of LNG to be 423.8
kg/m^3. Answer:72.9 percent
7–220 Argon gas expands in an adiabatic turbine from 3
MPa and 750°C to 0.2 MPa at a rate of 5 kg/s. The maximum
power output of the turbine is
(a) 1.06 MW (b) 1.29 MW (c) 1.43 MW
(d) 1.76 MW (e) 2.08 MW
7–221 A unit mass of a substance undergoes an irreversible
process from state 1 to state 2 while gaining heat from the
surroundings at temperature T in the amount of q. If
the entropy of the substance is s 1 at state 1, and s 2 at state 2,
the entropy change of the substance sduring this process is
(a)ss 2 s 1 (b)ss 2 s 1
(c)ss 2 s 1 (d)ss 2 s 1 q/T
(e)ss 2 s 1 q/T
7–222 A unit mass of an ideal gas at temperature Tunder-
goes a reversible isothermal process from pressure P 1 to pres-
sure P 2 while losing heat to the surroundings at temperature
Tin the amount of q. If the gas constant of the gas is R, the
entropy change of the gas sduring this process is
(a) sRln(P 2 /P 1 )(b) sRln(P 2 /P 1 ) q/T
(c) sRln(P 1 /P 2 )(d) sRln(P 1 /P 2 ) q/T
(e) s 0
7–223 Air is compressed from room conditions to a
specified pressure in a reversible manner by two compres-
sors: one isothermal and the other adiabatic. If the entropy
change of air sisotduring the reversible isothermal compres-
sion, and sadiaduring the reversible adiabatic compression,
the correct statement regarding entropy change of air per unit
mass is
(a) sisotsadia0(b) sisotsadia0(c) sadia 0
(d) sisot0(e) sisot 0
7–224 Helium gas is compressed from 15°C and 5.40
m^3 /kg to 0.775 m^3 /kg in a reversible and adiabatic manner.
The temperature of helium after compression is
(a) 105°C (b) 55°C (c) 1734°C
(d) 1051°C (e) 778°C
7–225 Heat is lost through a plane wall steadily at a rate of
600 W. If the inner and outer surface temperatures of the wall
are 20°C and 5°C, respectively, the rate of entropy generation
within the wall is
(a) 0.11 W/K (b) 4.21 W/K (c) 2.10 W/K
(d) 42.1 W/K (e) 90.0 W/K
7–226 Air is compressed steadily and adiabatically from
17°C and 90 kPa to 200°C and 400 kPa. Assuming constant
specific heats for air at room temperature, the isentropic effi-
ciency of the compressor is
(a) 0.76 (b) 0.94 (c) 0.86
(d) 0.84 (e) 1.00
7–227 Argon gas expands in an adiabatic turbine steadily
from 500°C and 800 kPa to 80 kPa at a rate of 2.5 kg/s. For3 barLNG, 40 bar- 160 °C, 55 kg/s
Cryogenic
turbineFIGURE P7–213Fundamentals of Engineering (FE) Exam Problems
7–214 Steam is condensed at a constant temperature of
30°C as it flows through the condensor of a power plant by
rejecting heat at a rate of 55 MW. The rate of entropy change
of steam as it flows through the condenser is
(a)1.83 MW/K (b)0.18 MW/K (c) 0 MW/K
(d) 0.56 MW/K (e) 1.22 MW/K
7–215 Steam is compressed from 6 MPa and 300°C to 10
MPa isentropically. The final temperature of the steam is
(a) 290°C (b) 300°C (c) 311°C
(d) 371°C (e) 422°C
7–216 An apple with an average mass of 0.15 kg and aver-
age specific heat of 3.65 kJ/kg · °C is cooled from 20°C to
5°C. The entropy change of the apple is
(a)0.0288 kJ/K (b)0.192 kJ/K (c)0.526 kJ/K
(d) 0 kJ/K (e) 0.657 kJ/K
7–217 A piston–cylinder device contains 5 kg of saturated
water vapor at 3 MPa. Now heat is rejected from the cylinder
at constant pressure until the water vapor completely con-
denses so that the cylinder contains saturated liquid at 3 MPa
at the end of the process. The entropy change of the system
during this process is
(a) 0 kJ/K (b)3.5 kJ/K (c) 12.5 kJ/K
(d) 17.7 kJ/K (e)19.5 kJ/K
7–218 Helium gas is compressed from 1 atm and 25°C to a
pressure of 10 atm adiabatically. The lowest temperature of
helium after compression is
(a) 25°C (b) 63°C (c) 250°C
(d) 384°C (e) 476°C
7–219 Steam expands in an adiabatic turbine from 8 MPa
and 500°C to 0.1 MPa at a rate of 3 kg/s. If steam leaves
the turbine as saturated vapor, the power output of the tur-
bine is
(a) 2174 kW (b) 698 kW (c) 2881 kW
(d) 1674 kW (e) 3240 kW