isentropic efficiency of 80 percent, the power produced by
the turbine is
(a) 194 kW (b) 291 kW (c) 484 kW
(d) 363 kW (e) 605 kW
7–228 Water enters a pump steadily at 100 kPa at a rate of
35 L/s and leaves at 800 kPa. The flow velocities at the inlet
and the exit are the same, but the pump exit where the dis-
charge pressure is measured is 6.1 m above the inlet section.
The minimum power input to the pump is
(a) 34 kW (b) 22 kW (c) 27 kW
(d) 52 kW (e) 44 kW
7–229 Air at 15°C is compressed steadily and isothermally
from 100 kPa to 700 kPa at a rate of 0.12 kg/s. The minimum
power input to the compressor is
(a) 1.0 kW (b) 11.2 kW (c) 25.8 kW
(d) 19.3 kW (e) 161 kW
7–230 Air is to be compressed steadily and isentropically
from 1 atm to 25 atm by a two-stage compressor. To mini-
mize the total compression work, the intermediate pressure
between the two stages must be
(a)3 atm (b)5 atm (c) 8 atm
(d) 10 atm (e) 13 atm
7–231 Helium gas enters an adiabatic nozzle steadily at
500°C and 600 kPa with a low velocity, and exits at a pres-
sure of 90 kPa. The highest possible velocity of helium gas at
the nozzle exit is
(a) 1475 m/s (b) 1662 m/s (c) 1839 m/s
(d) 2066 m/s (e) 3040 m/s
7–232 Combustion gases with a specific heat ratio of 1.3
enter an adiabatic nozzle steadily at 800°C and 800 kPa with
a low velocity, and exit at a pressure of 85 kPa. The lowest
possible temperature of combustion gases at the nozzle exit is
(a) 43°C (b) 237°C (c) 367°C
(d) 477°C (e) 640°C
7–233 Steam enters an adiabatic turbine steadily at 400°C
and 3 MPa, and leaves at 50 kPa. The highest possible per-
centage of mass of steam that condenses at the turbine exit
and leaves the turbine as a liquid is
(a)5% (b) 10% (c) 15%
(d) 20% (e)0%
7–234 Liquid water enters an adiabatic piping system at
15°C at a rate of 8 kg/s. If the water temperature rises by
0.2°C during flow due to friction, the rate of entropy genera-
tion in the pipe is
(a) 23 W/K (b) 55 W/K (c) 68 W/K
(d) 220 W/K (e) 443 W/K
7–235 Liquid water is to be compressed by a pump whose
isentropic efficiency is 75 percent from 0.2 MPa to 5 MPa at a
rate of 0.15 m^3 /min. The required power input to this pump is
422 | Thermodynamics
(a) 4.8 kW (b) 6.4 kW (c) 9.0 kW
(d) 16.0 kW (e) 12 kW
7–236 Steam enters an adiabatic turbine at 8 MPa and
500°C at a rate of 18 kg/s, and exits at 0.2 MPa and 300°C.
The rate of entropy generation in the turbine is
(a) 0 kW/K (b) 7.2 kW/K (c) 21 kW/K
(d) 15 kW/K (e) 17 kW/K
7–237 Helium gas is compressed steadily from 90 kPa and
25°C to 600 kPa at a rate of 2 kg/min by an adiabatic com-
pressor. If the compressor consumes 70 kW of power while
operating, the isentropic efficiency of this compressor is
(a) 56.7% (b) 83.7% (c) 75.4%
(d) 92.1% (e) 100.0%Design and Essay Problems
7–238 It is well-known that the temperature of a gas rises
while it is compressed as a result of the energy input in the
form of compression work. At high compression ratios, the
air temperature may rise above the autoignition temperature
of some hydrocarbons, including some lubricating oil. There-
fore, the presence of some lubricating oil vapor in high-
pressure air raises the possibility of an explosion, creating a
fire hazard. The concentration of the oil within the compres-
sor is usually too low to create a real danger. However, the oil
that collects on the inner walls of exhaust piping of the com-
pressor may cause an explosion. Such explosions have
largely been eliminated by using the proper lubricating oils,
carefully designing the equipment, intercooling between
compressor stages, and keeping the system clean.
A compressor is to be designed for an industrial applica-
tion in Los Angeles. If the compressor exit temperature is not
to exceed 250°C for safety consideration, determine the max-
imum allowable compression ratio that is safe for all possible
weather conditions for that area.
7–239 Identify the major sources of entropy generation in
your house and propose ways of reducing them.
7–240 Obtain the following information about a power
plant that is closest to your town: the net power output; the
type and amount of fuel; the power consumed by the pumps,
fans, and other auxiliary equipment; stack gas losses; temper-
atures at several locations; and the rate of heat rejection at the
condenser. Using these and other relevant data, determine the
rate of entropy generation in that power plant.
7–241 Compressors powered by natural gas engines are
increasing in popularity. Several major manufacturing facilities
have already replaced the electric motors that drive their com-
pressors by gas driven engines in order to reduce their energy
bills since the cost of natural gas is much lower than the cost
of electricity. Consider a facility that has a 130-kW compressor
that runs 4400 h/yr at an average load factor of 0.6. Making
reasonable assumptions and using unit costs for natural gas
and electricity at your location, determine the potential cost
savings per year by switching to gas driven engines.