Dehumidification And Reheat - Honeywell AUTOMATIC CONTROL Engineering Manual

PSYCHROMETRIC CHART FUNDAMENTALS
SUPPLY FAN
90°F DB
52% RH
CWS
CWR
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:
B
C
58°F DB
85% RH
50°F DB
Fig. 27.
1. The entering condition air is 90F dry bulb and 52 percent
rh (Point A). The moisture content is 0.0153 pounds of
moisture per pound of dry air.
2. Air that contacts the spray droplets follows the saturation
curve to the spray temperature, 50F 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 58F dry bulb and 85 percent rh
(Point C). The moisture content is 0.0085 pounds of
moisture per pound of dry air.
4. The moisture removed is:
0.0153 – 0.0085 = 0.068 pounds of moisture per
pound 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
ENGINEERING MANUAL OF AUTOMATIC CONTROL
58°F DB
85% RH
C2597
PUMP
90°F DB
A 52% RH
0.0153 LB/LB
0.0085 LB/LB
75°F WB
55°F WB
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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.
C
B

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.
52
D
A
Fig. 28.
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