14–5
THE PSYCHROMETRIC CHART

The state of the atmospheric air at a specified pressure is completely speci-

fied by two independent intensive properties. The rest of the properties can

be calculated easily from the previous relations. The sizing of a typical air-

conditioning system involves numerous such calculations, which may even-

tually get on the nerves of even the most patient engineers. Therefore, there

is clear motivation to computerize calculations or to do these calculations

once and to present the data in the form of easily readable charts. Such

charts are called psychrometric charts,and they are used extensively in

air-conditioning applications. A psychrometric chart for a pressure of 1 atm

(101.325 kPa or 14.696 psia) is given in Fig. A–31 in SI units and in Fig.

A–31E in English units. Psychrometric charts at other pressures (for use at

considerably higher elevations than sea level) are also available.

The basic features of the psychrometric chart are illustrated in Fig. 14–14.

The dry-bulb temperatures are shown on the horizontal axis, and the spe-

cific humidity is shown on the vertical axis. (Some charts also show the

vapor pressure on the vertical axis since at a fixed total pressure Pthere is a

one-to-one correspondence between the specific humidity vand the vapor

pressure Pv, as can be seen from Eq. 14–8.) On the left end of the chart,

there is a curve (called the saturation line) instead of a straight line. All the

saturated air states are located on this curve. Therefore, it is also the curve

of 100 percent relative humidity. Other constant relative-humidity curves

have the same general shape.

Lines of constant wet-bulb temperature have a downhill appearance to the

right. Lines of constant specific volume (in m^3 /kg dry air) look similar, except

they are steeper. Lines of constant enthalpy (in kJ/kg dry air) lie very nearly

parallel to the lines of constant wet-bulb temperature. Therefore, the constant-

wet-bulb-temperature lines are used as constant-enthalpy lines in some charts.

For saturated air, the dry-bulb, wet-bulb, and dew-point temperatures are

identical (Fig. 14–15). Therefore, the dew-point temperature of atmospheric

air at any point on the chart can be determined by drawing a horizontal line (a

line of vconstant or Pvconstant) from the point to the saturated curve.

The temperature value at the intersection point is the dew-point temperature.

The psychrometric chart also serves as a valuable aid in visualizing the air-

conditioning processes. An ordinary heating or cooling process, for example,

appears as a horizontal line on this chart if no humidification or dehumidifica-

tion is involved (that is,vconstant). Any deviation from a horizontal line

indicates that moisture is added or removed from the air during the process.

726 | Thermodynamics

(c) The enthalpy of air per unit mass of dry air is determined from Eq. 14–12:

Discussion The previous property calculations can be performed easily using

EES or other programs with built-in psychrometric functions.

41.8 kJ/kg dry air

 1 1.005 kJ>kg#°C 21 25°C 2  1 0.00653 21 2546.5 kJ>kg 2

h 1 ha 1 v 1 hv 1 
cpT 1 v 1 hg 1

Dry-bulb temperature

Specific humidity,

ω

Saturation line,

φ

= 100%

φ

= const.

Twb

= const.

v h = const.

(^) = const.
FIGURE 14–14
Schematic for a psychrometric chart.
Saturation line
Tdp = 15 °C
Tdb

15
°C
Twb

15
°C
15 °C
15 °C
FIGURE 14–15
For saturated air, the dry-bulb,
wet-bulb, and dew-point
temperatures are identical.