Honeywell AUTOMATIC CONTROL Engineering Manual page 51

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 70F dry-bulb and 60F wet-bulb lines cross (Fig. 3,
Point A), the following additional values can be determined:
26.3 D
BTU/LB
56% RH
13.505 CF/LB
A
B
54°F DP
70°F DB
Fig. 3.
— Relative humidity is 56 percent (Point A)
— Volume is 13.505 cubic feet per pound of dry air
(Point A)
— Dew point is 54F (Point B)
— Moisture content is 0.0088 pounds of moisture per pound
of dry air (Point C)
— Enthalpy (total heat) is 26.3 Btu per pound of dry air
(Point D)
— Density is 0.074 pounds per cubic foot (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 77F (Fig. 4, Point A), the following
values can be read:
— Wet-bulb temperature is 67.5F (Point A)
— Volume is 13.8 cubic feet per pound of dry air (Point A)
— Dew point is 62.5F (Point B)
— Moisture content is 0.012 pounds of moisture per pound
of dry air (Point C)
— Enthalpy is 31.6 Btu per pound of dry air (Point D)
— Density is 0.0725 pounds per cubic foot (reciprocal of
volume)
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
C
enthalpy (total heat) of the air is 31.6 Btu per pound of dry air
0.0088 LB/LB
(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
60°F WB
C1829
content. Project a constant enthalpy line to the enthalpy scale
(from Point C to Point E). Point E enthalpy represents sensible
heat of 18.7 Btu per pound of dry air. The difference between
this enthalpy reading and the original enthalpy reading is latent
heat. In this example 31.6 minus 18.7 equals 12.9 Btu per pound
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.
18.7 BTU/LB
SENSIBLE
HEAT
A
41
PSYCHROMETRIC CHART FUNDAMENTALS
31.6 BTU/LB
D
62.5°F DP
B
A
67.5°F WB
13.8 CF/LB
E 77°F DB
Fig. 4.
31.6 BTU/LB
D
LATENT HEAT
B
E
C 77°F DB
Fig. 5.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
60% RH
C
0.012 LB/LB
C1830
60% R.H.
C1831