PSYCHROMETRIC CHART FUNDAMENTALS
48
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Figure 18 is the same as the chart shown in Figure 17 except
that it graphically displays the amount of heat added by the
process. Enthalpy (total heat) added is determined by
subtracting the enthalpy of the dry, heated air at Point B from
the enthalpy of the humidified air at Point C as follows:
40.3 – 28.7 = 11.6 Btu per pound of dry air
S
E
N
S
I
B
L
E
H
E
A
T
T
O
T
A
L
H
E
A
T
∆H
S
∆H
T
=
1.0
0.8
0.6
0.5
0.4
0
.3
0.2
0.1
5000
3000
2000
1500
1000
500
-1000
-0.1
-0.3
-0.5
-1.0
-2.0
-4.0
1.0
2.0
E
N
T
H
A
L
P
Y
H
U
M
I
D
I
T
Y
R
A
T
I
O
=
∆ h
∆ W
4.0
0
AB
C
D
55°F DB 90°F DB 92°F DB
0.0164 LB/LB
0.0065 LB/LB
28.7
BTU/LB
29.6
BTU/LB
40.3
BTU/LB
LATENT
SENSIBLE
(0.9 BTU/LB)
TOTAL
ENTHALPY
STEAM
ENTHALPY
1150
REFERENCE POINT
50% RH
C1842
∞
Fig. 18.
The remaining 0.9 Btu is sensible heat. The actual moisture
added per pound of dry air is 0.0099 pounds. The specific
volume of the entering air at Point B is 14 cubic feet per pound.
For a 10,000 cubic feet per minute system, the weight of the
air passing through is:
10,000 ÷ 14 = 714.3 pounds per minute
The weight of the moisture added is:
714.3 x 0.0099 = 7.07 pounds per minute of moisture
Since one gallon of water weighs 8.34 pounds, the moisture to
be added is as follows:
7.07 ÷ 8.34 = 0.848 gallons per minute
The steam raised the temperature of the air from 90F dry bulb
to 92F dry bulb. To find the latent heat added by the steam
humidifier to the air, determine the enthalpy at Point D (the
enthalpy of the heated air without added moisture) and subtract
it from the enthalpy of the humidified air at Point C. This is
as follows:
40.3 – 29.6 = 10.7 Btu per pound of dry air
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