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

Compared to saturated vapor, superheated vapor is characterized by

Lower pressures (PPsatat a given T)

Higher tempreatures (TTsatat a given P)

Higher specific volumes (vvgat a given P or T)

Higher internal energies (uugat a given P or T)

Higher enthalpies (hhgat a given P or T)

Chapter 3 | 133

EXAMPLE 3–6 Internal Energy of Superheated Vapor

Determine the internal energy of water at 20 psia and 400°F.

Solution The internal energy of water at a specified state is to be deter-

mined.

Analysis At 20 psia, the saturation temperature is 227.92°F. Since T

Tsat, the water is in the superheated vapor region. Then the internal energy at

the given temperature and pressure is determined from the superheated

vapor table (Table A–6E) to be

u
1145.1 Btu>lbm

EXAMPLE 3–7 Temperature of Superheated Vapor

Determine the temperature of water at a state of P
0.5 MPa and h


2890 kJ/kg.

Solution The temperature of water at a specified state is to be determined.

Analysis At 0.5 MPa, the enthalpy of saturated water vapor is hg
2748.1

kJ/kg. Since hhg, as shown in Fig. 3–41, we again have superheated

vapor. Under 0.5 MPa in Table A–6 we read

T, °C h,kJ/kg

200 2855.8

250 2961.0

Obviously, the temperature is between 200 and 250°C. By linear interpola-

tion it is determined to be

T
216.3°C

3 Compressed Liquid

Compressed liquid tables are not as commonly available, and Table A–7 is
the only compressed liquid table in this text. The format of Table A–7 is very
much like the format of the superheated vapor tables. One reason for the lack
of compressed liquid data is the relative independence of compressed liquid
properties from pressure. Variation of properties of compressed liquid with
pressure is very mild. Increasing the pressure 100 times often causes proper-
ties to change less than 1 percent.

h

T

0.5 MPa

hg h > hg

FIGURE 3–41

At a specified P, superheated vapor

exists at a higher hthan the saturated

vapor (Example 3–7).