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

Chapter 8 | 479

Review Problems

8–95 Refrigerant-134a is expanded adiabatically in an
expansion valve from 1.2 MPa and 40°C to 180 kPa. For
environment conditions of 100 kPa and 20°C, determine
(a) the work potential of R-134a at the inlet, (b) the exergy
destruction during the process, and (c) the second-law effi-
ciency.

8–96 Steam enters an adiabatic nozzle at 3.5 MPa and
300°C with a low velocity and leaves at 1.6 MPa and 250°C
at a rate of 0.4 kg/s. If the ambient state is 100 kPa and 18°C,
determine (a) the exit velocity, (b) the rate of exergy destruc-
tion, and (c) the second-law efficiency.

8–97 A 30-L electrical radiator containing heating oil is
placed in a well-sealed 50-m^3 room. Both the air in the room
and the oil in the radiator are initially at the environment
temperature of 10°C. Electricity with a rating of 1.8 kW is
now turned on. Heat is also lost from the room at an average
rate of 0.35 kW. The heater is turned off after some time
when the temperatures of the room air and oil are measured
to be 20°C and 50°C, respectively. Taking the density and the
specific heat of oil to be 950 kg/m^3 and 2.2 kJ/kg
°C, deter-
mine (a) how long the heater is kept on, (b) the exergy
destruction, and (c) the second-law efficiency for this
process. Answers:(a) 2038 s, (b) 3500 kJ, (c) 0.046

exchanger at 350°C. Determine (a) the rate of steam produc-

tion, (b) the rate of exergy destruction in the heat exchanger,

and (c) the second-law efficiency of the heat exchanger.

8–99 The inner and outer surfaces of a 5-m 
6-m brick

wall of thickness 30 cm are maintained at temperatures of

20°C and 5°C, respectively, and the rate of heat transfer

through the wall is 900 W. Determine the rate of exergy

destruction associated with this process. Take T 0 0°C.

8–100 A 1000-W iron is left on the ironing board with its

base exposed to the air at 20°C. If the temperature of the base

of the iron is 150°C, determine the rate of exergy destruction

for this process due to heat transfer, in steady operation.

8–101 One method of passive solar heating is to stack gal-

lons of liquid water inside the buildings and expose them to

the sun. The solar energy stored in the water during the day is

released at night to the room air, providing some heating. Con-

sider a house that is maintained at 22°C and whose heating is

assisted by a 350-L water storage system. If the water is heated

to 45°C during the day, determine the amount of heating this

water will provide to the house at night. Assuming an outside

temperature of 5°C, determine the exergy destruction associ-

ated with this process. Answers:33,548 kJ, 1172 kJ

8–102 The inner and outer surfaces of a 0.5-cm-thick, 2-m


2-m window glass in winter are 10°C and 3°C, respec-

tively. If the rate of heat loss through the window is 3.2 kJ/s,

determine the amount of heat loss, in kJ, through the glass

over a period of 5 h. Also, determine the exergy destruction

associated with this process. Take T 0 5°C.

8–103 An aluminum pan has a flat bottom whose diameter

is 20 cm. Heat is transferred steadily to boiling water in the

pan through its bottom at a rate of 800 W. If the temperatures

8–98 Hot exhaust gases leaving an internal combustion
engine at 400°C and 150 kPa at a rate of 0.8 kg/s is to be
used to produce saturated steam at 200°C in an insulated heat
exchanger. Water enters the heat exchanger at the ambient
temperature of 20°C, and the exhaust gases leave the heat

Water

15 °C

4.6 kg/s

Mixture

45 °C

Mixing

chamber

Sat.

vapor

0.23 kg/s

FIGURE P8–94

Room

10 °C

Radiator Q

FIGURE P8–97

HEAT

EXCHANGER

350 °C

Water

20 °C

Sat. vap.

200 °C

Exh. gas

400 °C

150 kPa

FIGURE P8–98

30 cm

Q

BRICK

WALL

20 °C 5 °C

FIGURE P8–99