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

7–174 Consider a three-stage isentropic compressor with
two intercoolers that cool the gas to the initial temperature
between the stages. Determine the two intermediate pressures
(Pxand Py) in terms of inlet and exit pressures (P 1 and P 2 )
that will minimize the work input to the compressor.
Answers: Px(P^21 P 2 )1/3, Py(P 1 P 22 )1/3

7–175 Steam at 6 MPa and 500°C enters a two-stage adia-
batic turbine at a rate of 15 kg/s. Ten percent of the steam is
extracted at the end of the first stage at a pressure of 1.2 MPa
for other use. The remainder of the steam is further expanded
in the second stage and leaves the turbine at 20 kPa. Deter-
mine the power output of the turbine, assuming (a) the
process is reversible and (b) the turbine has an isentropic effi-
ciency of 88 percent. Answers:(a) 16,291 kW, (b) 14,336 kW

416 | Thermodynamics

12.5 MPa and 500°C at a rate of 25 kg/s and exits at 10 kPa

and a quality of 0.92. Air enters the compressor at 98 kPa and

295 K at a rate of 10 kg/s and exits at 1 MPa and 620 K.

Determine (a) the net power delivered to the generator by the

turbine and (b) the rate of entropy generation within the tur-

bine and the compressor during this process.

20 kPa

STEAM

TURBINE

(1st stage)

6 MPa

500 °C

(2nd stage)

1.2 MPa

10%

90%

FIGURE P7–175

7–176 Steam enters a two-stage adiabatic turbine at 8 MPa
and 550°C. It expands in the first stage to a pressure of 2
MPa. Then steam is reheated at constant pressure to 550°C
before it is expanded in a second stage to a pressure of 200
kPa. The power output of the turbine is 80 MW. Assuming an
isentropic efficiency of 84 percent for each stage of the tur-
bine, determine the required mass flow rate of steam. Also,
show the process on a T-sdiagram with respect to saturation
lines. Answer:85.8 kg/s

7–177 Refrigerant-134a at 140 kPa and 10°C is com-
pressed by an adiabatic 0.7-kW compressor to an exit state of
700 kPa and 50°C. Neglecting the changes in kinetic and
potential energies, determine (a) the isentropic efficiency of
the compressor, (b) the volume flow rate of the refrigerant at
the compressor inlet, in L/min, and (c) the maximum volume
flow rate at the inlet conditions that this adiabatic 0.7-kW
compressor can handle without violating the second law.

7–178E Helium gas enters a nozzle whose isentropic effi-
ciency is 94 percent with a low velocity, and it exits at 14
psia, 180°F, and 1000 ft/s. Determine the pressure and tem-
perature at the nozzle inlet.

7–179 An adiabatic air compressor is to be powered
by a direct-coupled adiabatic steam turbine
that is also driving a generator. Steam enters the turbine at

Air

comp.

Steam

turbine

1 MPa

620 K

98 kPa

295 K

10 kPa

12.5 MPa

500 °C

FIGURE P7–179

7–180 Reconsider Prob. 7–179. Using EES (or other)

software, determine the isentropic efficiencies for

the compressor and turbine. Then use EES to study how vary-

ing the compressor efficiency over the range 0.6 to 0.8 and the

turbine efficiency over the range 0.7 to 0.95 affect the net work

for the cycle and the entropy generated for the process. Plot the

net work as a function of the compressor efficiency for turbine

efficiencies of 0.7, 0.8, and 0.9, and discuss your results.

7–181 Consider two bodies of identical mass mand specific

heat cused as thermal reservoirs (source and sink) for a heat

engine. The first body is initially at an absolute temperature

T 1 while the second one is at a lower absolute temperature T 2.

Heat is transferred from the first body to the heat engine,

which rejects the waste heat to the second body. The process

continues until the final temperatures of the two bodies Tf

become equal. Show that when the heat engine

produces the maximum possible work.

Tf 1 T 1 T 2

HE W

QH

QL

m, c

T 1

m, c

T 2

FIGURE P7–181