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

128 | Thermodynamics

EXAMPLE 3–1 Pressure of Saturated Liquid in a Tank

A rigid tank contains 50 kg of saturated liquid water at 90°C. Determine the

pressure in the tank and the volume of the tank.

Solution A rigid tank contains saturated liquid water. The pressure and vol-

ume of the tank are to be determined.

Analysis The state of the saturated liquid water is shown on a T-vdiagram

in Fig. 3–31. Since saturation conditions exist in the tank, the pressure

must be the saturation pressure at 90°C:

The specific volume of the saturated liquid at 90°C is

Then the total volume of the tank becomes

V
mv
 1 50 kg 21 0.001036 m^3 >kg 2 
0.0518 m^3

v
vf @ 90°C
0.001036 m^3 >kg¬¬ 1 Table A–4 2

P
Psat @ 90°C
70.183 kPa¬¬ 1 Table A–4 2

EXAMPLE 3–2 Temperature of Saturated Vapor in a Cylinder

A piston–cylinder device contains 2 ft^3 of saturated water vapor at 50-psia

pressure. Determine the temperature and the mass of the vapor inside the

cylinder.

Solution A cylinder contains saturated water vapor. The temperature and

the mass of vapor are to be determined.

Analysis The state of the saturated water vapor is shown on a P-vdiagram

in Fig. 3–32. Since the cylinder contains saturated vapor at 50 psia, the

temperature inside must be the saturation temperature at this pressure:

The specific volume of the saturated vapor at 50 psia is

Then the mass of water vapor inside the cylinder becomes

m

V

v

2 ft^3

8.5175 ft^3 >lbm

0.235 lbm

v
vg @ 50 psia
8.5175 ft^3 >lbm¬ 1 Table A–5E 2

T
Tsat @ 50 psia
280.99°F¬¬ 1 Table A–5E 2

EXAMPLE 3–3 Volume and Energy Change during Evaporation

A mass of 200 g of saturated liquid water is completely vaporized at a con-

stant pressure of 100 kPa. Determine (a) the volume change and (b) the

amount of energy transferred to the water.

Solution Saturated liquid water is vaporized at constant pressure. The vol-

ume change and the energy transferred are to be determined.

Analysis (a) The process described is illustrated on a P-vdiagram in Fig. 3–33.

The volume change per unit mass during a vaporization process is vfg, which

P=^7

0.^1

83

kP

a

T,°C

vf v

90

Sat. liquid

T = 90°C

FIGURE 3–31

Schematic and T-vdiagram for

Example 3–1.

P, psia

vg v

50

Saturated

vapor

P = 50 psia

V = 2 ft^3

T = 280.99F

FIGURE 3–32

Schematic and P-Vdiagram for
Example 3–2.