PSYCHROMETRIC CHART FUNDAMENTALS
ENGINEERING MANUAL OF AUTOMATIC CONTROL
49
Recalling that the steam added 25.5 kilojoules per kilogram
of dry air, the total heat added is:
5.4 kg/s x 25.5 kJ/kg = 137.7 kilojoules per
second
Summarized, a steam humidifier always adds a little sensible
heat to the air, and the Process Line B–C angles to the right of
the 32°C starting dry-bulb line because of the added sensible
heat. When the process line crosses the moisture content lines
along a constant dry-bulb line, only latent heat is added. When
it parallels a constant, horizontal moisture line, only sensible
heat is added.
AIR WASHERS
Air washers are also used as humidifiers particularly for
applications requiring added moisture and not much heat as in
warm southwestern climates. A washer can be recirculating as
shown in Figure 19 or heated as shown in Figure 20. In
recirculating washers, the heat necessary to vaporize the water
is sensible heat changed to latent heat which causes the dry-
bulb temperature to drop. The process line tracks the constant
enthalpy line because no total heat is added or subtracted. This
process is called “adiabatic” and is illustrated by Figure 21.
Point A is the entering condition of the air, Point B is the final
condition, and Point C is the temperature of the water. Since
the water is recirculating, the water temperature becomes the
same as the wet-bulb temperature of the air.
Fig. 21.
The next two psychrometric charts (Fig. 22 and 23) illustrate
the humidifying process using a heated air washer. The
temperature to which the water is heated is determined by the
amount of moisture required for the process. Figure 22 shows
what happens when the washer water is heated above the air
dry-bulb temperature shown at Point A. The temperature of the
water located at Point B on the saturation curve causes the
system air temperature to settle out at Point D. The actual
location of Point D depends upon the construction and
characteristics of the washer.
As the humidity demand reduces, the water temperature moves
down the saturation curve as it surrenders heat to the air. This
causes the water temperature to settle out at a point such as Point
C. The final air temperature is at Point E. Note that the final air
temperature is above the initial dry-bulb temperature so both
sensible and latent heat have been added to the air.
C2598
PUMP
SUPPLY FAN
Fig. 19. Recirculating Air Washer.
C2599
PUMP
HEAT EXCHANGER
HWS
HWR
SUPPLY FAN
Fig. 20. Heated Air Washer.
A
B
C
D
SATURATION
CURVE
C1844
E
Fig. 22.
C1843
C
A
CONSTANT
ENTHALPY
LINE
B
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