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

It can also be expressed as

(3–19)

where videal
RT/P. Obviously,Z
1 for ideal gases. For real gases Zcan

be greater than or less than unity (Fig. 3–50). The farther away Zis from

unity, the more the gas deviates from ideal-gas behavior.

We have said that gases follow the ideal-gas equation closely at low pres-

sures and high temperatures. But what exactly constitutes low pressure or

high temperature? Is 100°C a low temperature? It definitely is for most

substances but not for air. Air (or nitrogen) can be treated as an ideal gas at

this temperature and atmospheric pressure with an error under 1 percent.

This is because nitrogen is well over its critical temperature (147°C) and

away from the saturation region. At this temperature and pressure, however,

most substances would exist in the solid phase. Therefore, the pressure or

temperature of a substance is high or low relative to its critical temperature

or pressure.

Z

vactual

videal

140 | Thermodynamics

ZZ = 1 Z

> 1

= 1

< 1

IDEAL

GAS

REAL

GASES

FIGURE 3–50

The compressibility factor is unity for

ideal gases.

v, m^3 /kg

T, °C

0

100

200

300

400

500

600

0.001 0.01 0.1 1 10 100

1.6 0.0 0.0

0.5 0.0 0.0

0.2 0.0 0.0

0.1 0.0 0.0

0.1 0.0 0.0

17.3 10.85.0 2.4 0.5 0.0 0.0

37.1 20.88.8 4.1 0.8

7.4 1.3

271.0 56.2 17.6

16.7 2.6

152.7

49.5

25.7

6.0

0.0

20 MPa

10 MPa

5 MPa

1 MPa

100 kPa

10 kPa

0.8 kPa

0.1

7.6

30 MPa

IDEAL GAS

0.0

FIGURE 3–49

Percentage of error
([|vtablevideal|/vtable] 100)
involved in assuming steam to be an
ideal gas, and the region where
steam can be treated as an ideal gas
with less than 1 percent error.