Honeywell AUTOMATIC CONTROL, Engineering Manual

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PSYCHROMETRIC CHART FUNDAMENTALS
50
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Fig. 23.
Figure 23 illustrates a heated washer where the water
temperature is between the dry-bulb and wet-bulb temperatures
of the air. The air is humidified but also cooled a little. Point B
represents the initial and Point C the final temperature of the
water with reduced humidity demand. Point A represents the
initial and Point E the final temperature of the air. The location
of Points D and E depends on the construction and
characteristics of the washer. The temperature of the water in a
C
B
D
E
A
SATURATION CURVE
C1845
washer is always located on the saturation curve. Note that the
dry-bulb temperature of the air is reduced as it passes through
the washer. This happens because some of its heat is used to
evaporate the water; however, the humidity of the air rises
considerably. In this case, some of the sensible heat of the air
becomes latent heat in the water vapor, but the enthalpy of the
air is increased because of the heat in the water.
VAPORIZING HUMIDIFIER
Vaporizing and water spray humidifiers operate on the
principal of breaking water up into small particulates so they
are evaporated directly into the air. This process is essentially
adiabatic since the enthalpy lines of the water vapor for 32 and
212F are so close. The enthalpy of water at 32F is zero and at
212F it is 180 Btu per pound. If air at Point A (Fig. 24) is
humidified by 212F water, the process follows a line parallel
to line C-D and the 80F WB line and ends at a point such as
Point B. The actual water temperature of a vaporizing or water
spray humidifier will be between 32 and 212F and will usually
be around room temperature so using the zero enthalpy line
(C-E) as reference will not introduce a significant error into
the process.
Fig. 24. Psychrometric Chart Showing Line A–B Parallel to Line C–D.
C
32 ° F WATER = 0 BTU/LB
212 ° F WATER = 180 BTU/LB
E
D
80 ° F WB LINE
B
A
CONSTRUCTION LINE, FOR LINE A -B,
PERPENDICULAR TO LINES C-D AND A-B
C1846
S
E
N
S
IB
LE
HEAT
T
O
T
A
L
HEAT
∆
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
ENTHALPY
HUM
IDITY RATIO
=
∆
h
∆
W
4.0
0
∞