Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual, Page: 62 / 512

ENGINEERING MANUAL OF AUTOMATIC CONTROL
PSYCHROMETRIC CHART FUNDAMENTALS
52
Fig. 26. Air Washer Used for
Cooling and Dehumidification.
Figure 27 is a chart of the air washer process. If a washer is
100 percent efficient, the air leaving the washer is at Point B.
The result as determined by the wet-bulb and dry-bulb
temperatures is Point C and is determined as follows:
Fig. 27.
1. The entering condition air is 32
°
C dry bulb and 52 percent
rh (Point A). The moisture content is 15.5 grams of
moisture per kilogram of dry air.
2. Air that contacts the spray droplets follows the saturation
curve to the spray temperature, 10
°
C dry bulb (Point B),
and then mixes with air that did not come in contact with
the spray droplets resulting in the average condition at
Point C.
3. The leaving air is at 14
°
C dry bulb and 85 percent rh
(Point C). The moisture content is 8.5 grams of moisture
per kilogram of dry air.
4. The moisture removed is:
15.5 g/kg – 8.5 g/kg = 7 grams of moisture per
kilogram of dry air
Figure 28 summarizes the process lines for applications using
washers for humidification or dehumidification. When the water
recirculates, the process is adiabatic and the process line follows
the Constant Enthalpy Line A-C. The water assumes the wet-
bulb temperature of the air as the process line extends. Note that
whenever the washer water temperature is between the dew point
(Point B) and the dry-bulb (Point D) temperature of the air,
moisture is added and the dry-bulb temperature of the air falls. If
the water temperature is above the dry-bulb temperature of the
air (to the right of Point D), both the air moisture and the dry-bulb
temperature increase. Whenever the water temperature is below
the dew point temperature (Point B), dehumidification occurs as
well as dry-bulb cooling. This process always falls on a curved
line between the initial temperature of the air and the point on the
saturation curve representing the water temperature. The exact
leaving air temperature depends upon the construction and
characteristics of the washer.
D
C
B
A
C1849
Fig. 28.
DEHUMIDIFICATION AND REHEAT
Dehumidification lowers the dry-bulb temperature, which
often requires the use of reheat to provide comfortable
conditions. Dehumidification and reheat are actually two
processes on the psychrometric chart. Applications, such as
computer rooms, textile mills, and furniture manufacturing
plants require that a constant relative humidity be maintained
at close tolerances. To accomplish this, the air is cooled below
a comfortable level to remove moisture, and is then reheated
(sensible heat only) to provide comfort. Figure 29 is an air
conditioning system with both a cooling coil and reheat coil.
CWS
CWR
32
°
C DB
52% RH
14
°
C DB
85% RH
C4335
PUMP
SUPPLY FAN
A
C
B
12.5
°
C WB 10
°
C DB
14
°
C DB
85% RH
32
°
C DB
52% RH
15.5 g/kg
8.5 g/kg
C4336
24
°
C WB