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

Chapter 7 | 417

7–182 The explosion of a hot water tank in a school in
Spencer, Oklahoma, in 1982 killed 7 people while injuring 33
others. Although the number of such explosions has
decreased dramatically since the development of the ASME
Pressure Vessel Code, which requires the tanks to be designed
to withstand four times the normal operating pressures, they
still occur as a result of the failure of the pressure relief
valves and thermostats. When a tank filled with a high-
pressure and high-temperature liquid ruptures, the sudden
drop of the pressure of the liquid to the atmospheric level
causes part of the liquid to flash into vapor, and thus to expe-
rience a huge rise in its volume. The resulting pressure wave
that propagates rapidly can cause considerable damage.
Considering that the pressurized liquid in the tank eventu-
ally reaches equilibrium with its surroundings shortly after
the explosion, the work that a pressurized liquid would do if
allowed to expand reversibly and adiabatically to the pressure
of the surroundings can be viewed as the explosive energyof
the pressurized liquid. Because of the very short time period
of the explosion and the apparent calm afterward, the explo-
sion process can be considered to be adiabatic with no
changes in kinetic and potential energies and no mixing with
the air.
Consider a 80-L hot-water tank that has a working pressure
of 0.5 MPa. As a result of some malfunction, the pressure in
the tank rises to 2 MPa, at which point the tank explodes.
Taking the atmospheric pressure to be 100 kPa and assuming
the liquid in the tank to be saturated at the time of explosion,
determine the total explosion energy of the tank in terms of
the TNT equivalence. (The explosion energy of TNT is about
3250 kJ/kg, and 5 kg of TNT can cause total destruction of
unreinforced structures within about a 7-m radius.) Answer:
1.972 kg TNT

7–185 The inner and outer surfaces of a 2-m 2-m win-

dow glass in winter are 10°C and 3°C, respectively. If the

rate of heat loss through the window is 3.2 kJ/s, determine

the amount of heat loss, in kilojoules, through the glass over

a period of 5 h. Also, determine the rate of entropy genera-

tion during this process within the glass.

7–186 Two rigid tanks are connected by a valve. Tank A is

insulated and contains 0.2 m^3 of steam at 400 kPa and 80

percent quality. Tank B is uninsulated and contains 3 kg of

steam at 200 kPa and 250°C. The valve is now opened, and

steam flows from tank A to tank B until the pressure in tank

A drops to 300 kPa. During this process 600 kJ of heat is

transferred from tank B to the surroundings at 0°C. Assuming

the steam remaining inside tank A to have undergone a

reversible adiabatic process, determine (a) the final tempera-

ture in each tank and (b) the entropy generated during this

process. Answers:(a) 133.5°C, 113.2°C; (b) 0.916 kJ/K

Hot water

tank

80 L

2 MPa

FIGURE P7–182

7–183 Using the arguments in the Prob. 7–182, determine
the total explosion energy of a 0.35-L canned drink that
explodes at a pressure of 1.2 MPa. To how many kg of TNT
is this explosion energy equivalent?

7–184 Demonstrate the validity of the Clausius inequality
using a reversible and an irreversible heat engine operating
between the same two thermal energy reservoirs at constant
temperatures of TLand TH.

Low-temperature reservoir at TL

REV.

HE

QH

Wnet,rev

QL

IRREV.

HE

QH

Wnet,irrev

QL, irrev

High-temperature reservoir at TH

FIGURE P7–184

600 kJ

A

0.2 m^3

steam

400 kPa

x = 0.8

B

3 kg

steam

200 kPa

250 °C

FIGURE P7–186

7–187 Heat is transferred steadily to boiling water in the

pan through its flat bottom at a rate of 500 W. If the tempera-

tures of the inner and outer surfaces of the bottom of the tank