Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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PSYCHROMETRIC CHART FUNDAMENTALS
ENGINEERING MANUAL OF AUTOMATIC CONTROL
41
heat) line. The protractor nomograph, in the upper left corner,
is used to establish the slope of a process line. The mechanics
of constructing this line are discussed in more detail in the
STEAM JET HUMIDIFIERS section.
The various properties of air can be determined from the chart
whenever the lines of any two values cross even though all
properties may not be of interest. For example, from the point
where the 21
°
C dry-bulb and 15.5
°
C wet-bulb lines cross (Fig. 3,
Point A), the following additional values can be determined:
Fig. 3.
— Relative humidity is 56 percent (Point A)
— Volume is 0.845 cubic meters per kilogram of dry air
(Point A)
— Dew point is 12
°
C (Point B)
— Moisture content is 8.75 grams of moisture per kilogram
of dry air (Point C)
— Enthalpy (total heat) is 43.5 kilojoules per kilogram of
dry air (Point D)
— Density is 1.163 kilograms per cubic meter (reciprocal
of volume)
Figure 4 is another plotting example. This time the dry-bulb
temperature line and relative humidity line are used to establish
the point. With the relative humidity equal to 60 percent and
the dry-bulb temperature at 25
°
C (Fig. 4, Point A), the following
values can be read:
— Wet-bulb temperature is 19.5
°
C (Point A)
— Volume is 0.86 cubic meters per kilogram of dry air
(Point A)
— Dew point is 17
°
C (Point B)
— Moisture content is 12.0 grams of moisture per kilogram
of dry air (Point C)
Fig. 4.
Figure 5 is the same as Figure 4 but is used to obtain latent
heat and sensible heat values. Figures 4 and 5 indicate that the
enthalpy (total heat) of the air is 56.0 kilojoules per kilogram
of dry air (Point D). Enthalpy is the sum of sensible and latent
heat (Line A to E + Line E to D, Fig. 5). The following process
determines how much is sensible heat and how much is latent
heat. The bottom horizontal line of the chart represents zero
moisture content. Project a constant enthalpy line to the enthalpy
scale (from Point C to Point E). Point E enthalpy represents
sensible heat of 25.5 kilojoules per kilogram of dry air. The
difference between this enthalpy reading and the original
enthalpy reading is latent heat. In this example 56.0 minus 25.5
equals 30.5 kilojoules per kilogram of dry air of latent heat.
When the moisture content of the air changes but the dry-bulb
temperature remains constant, latent heat is added or subtracted.
Fig. 5.
D
B
A
C
8.75 g/kg
43.5 kJ/kg
12
°
C DP
56% RH
0.845 m /kg
21
°
C DB
15.5
°
C WB
C4320
3
— Enthalpy is 56.0 kilojoules per kilogram of dry air
(Point D)
— Density is 1.163 kilograms per cubic meter (reciprocal
of volume)
C
D
A
E
B
LATENT HEAT
25.5 kJ/kg
SENSIBLE
HEAT
60% R.H.
C4322
25
°
C DB
56 kJ/kg
25
°
C DB
C
A
D
E
56 kJ/kg
12 g/kg
17
°
C DP
19.5
°
C WB
0.86 m
3
/kg
C4321
60% RH
B