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

Chapter 8 | 483

entire process, including the conversion of electric energy to
heat energy. Answers:(a) 1.507 kg, (b) 689 kJ

8–128 What would your answer to Prob. 8–127 be if heat
were supplied to the pressure cooker from a heat source at
180°C instead of the electrical heating unit?

8–129 A constant-volume tank contains 20 kg of nitrogen
at 1000 K, and a constant-pressure device contains 10 kg of
argon at 300 K. A heat engine placed between the tank and
device extracts heat from the high-temperature tank, produces
work, and rejects heat to the low-temperature device. Deter-
mine the maximum work that can be produced by the heat

heated by passing them through an oven at 1300°F at a rate of

300 per minute. If the plates remain in the oven until their

average temperature rises to 1000°F, determine the rate of heat

transfer to the plates in the furnace and the rate of exergy

destruction associated with this heat transfer process.

8–134 Long cylindrical steel rods (r
7833 kg/m^3 and

cp
0.465 kJ/kg · °C) of 10-cm diameter are heat-treated by

drawing them at a velocity of 3 m/min through a 6-m-long

oven maintained at 900°C. If the rods enter the oven at 30°C

and leave at 700°C, determine (a) the rate of heat transfer to

the rods in the oven and (b) the rate of exergy destruction

associated with this heat transfer process. Take T 0 
25°C.

8–135 Steam is to be condensed in the condenser of a steam

power plant at a temperature of 60°C with cooling water from

a nearby lake that enters the tubes of the condenser at 15°C at

a rate of 140 kg/s and leaves at 25°C. Assuming the condenser

to be perfectly insulated, determine (a) the rate of condensa-

tion of the steam and (b) the rate of exergy destruction in the

condenser. Answers:(a) 2.48 kg, (b) 694 kW

8–136 A well-insulated heat exchanger is to heat water

(cp
4.18 kJ/kg · °C) from 25°C to 60°C at a rate of 0.4 kg/s.

The heating is to be accomplished by geothermal water (cp


4.31 kJ/kg · °C) available at 140°C at a mass flow rate of

0.3 kg/s. The inner tube is thin-walled and has a diameter of

0.6 cm. Determine (a) the rate of heat transfer and (b) the

rate of exergy destruction in the heat exchanger.

Ar

10 kg

300 K

HE W

QH

QL

N 2

20 kg

1000 K

FIGURE P8–129

engine and the final temperatures of the nitrogen and argon.
Assume constant specific heats at room temperature.

8–130 A constant-volume tank has a temperature of 800 K
and a constant-pressure device has a temperature of 290 K.
Both the tank and device are filled with 20 kg of air. A heat
engine placed between the tank and device receives heat from
the high-temperature tank, produces work, and rejects heat to
the low-temperature device. Determine the maximum work
that can be produced by the heat engine and the final temper-
atures of the tank and device. Assume constant specific heats
at room temperature.

8–131 Can closed-system exergy be negative? How about
flow exergy? Explain using an incompressible substance as
an example.

8–132 Obtain a relation for the second-law efficiency of a
heat engine that receives heat QHfrom a source at tempera-
ture THand rejects heat QLto a sink at TL, which is higher
than T 0 (the temperature of the surroundings), while produc-
ing work in the amount of W.

8–133E In a production facility, 1.2-in-thick, 2-ft  2-ft
square brass plates (r
532.5 lbm/ft^3 and cp
0.091 Btu/lbm
· °F) that are initially at a uniform temperature of 75°F are

Oven, 1300°F

Brass

plate, 75°F

1.2 in.

FIGURE P8–133E

Wa t e r

25 °C

60 °C

Brine

140 °C

FIGURE P8–136