Specifications

Page 26
demand is present, the control actuates a motor
which in turn adjusts outdoor dampers to meet cool
ing demand. With outdoor air dampers open, the in
door blower draws in outdoor air for cooling and first
stage compressors are disabled. When heat content
rises above the setpoint, the control de-activates and
the dampers close to the preset minimum position.
First stage compressors are switched to handle all
first stage cooling.
Two types of adjustment may be made at the control.
The first is the control setpoint. The setpoint determines
the temperature and humidity conditions at which the
outdoor air dampers will open and close. The recom
mended setpoint is A." If the economizer is allowing air
which is too warm or too humid into the system, the con
trol may be changed to a lower setpoint (B,C or D). Refer
to enthalpy chart figure 30.
FIGURE 30
ENTHALPY CHART
CHART
CURVE
CONTROL
POINT 50% RH
°F °C
A
B
C
D
73
70
67
63
23
21
19
17
Example:
If the enthalpy control is set at setpoint A" as shown in
figure 30, the following situation could occur. A cooling
demand when the outside air is at 75° and 20 percent
humidity would drive the economizer outdoor air
dampers open to utilize outdoor air for cooling. The
compressor cooling circuit would be disabled. Howev
er, if the outdoor air should change to 70°F (a drop in
temperature) and 70 percent humidity (a dramatic rise
in humidity), the total heat content" of the outdoor air
would rise above the enthalpy control setpoint and de-
activate the damper motor to the preset minimum posi
tion. If cooling demand is still present when the total
heat of the outside air rises above the control setpoint,
cooling demand is routed from the economizer to the
compressor cooling circuit.
b-Minimum Positioner
The second type of adjustment which may be made at
the control is the minimum position of the outdoor
damper blades. Each economizer has a minimum posi
tioner switch (potentiometer) which allows the outdoor
dampers to be adjusted to a preset minimum position.
This allows a preset amount of air exchange at all times
during blower operation. When unit operation stops,
the dampers drive closed. The potentiometer is located
on the enthalpy control face.
c-Enthalpy Sensor
The enthalpy sensor is located on the outside portion of
the outdoor damper blades. The sensor monitors the to
tal heat content of the outdoor air (temperature plus hu
midity) and sends the information to the enthalpy control.
The enthalpy control uses the information to determine if
outdoor air can be used for cooling.
d-Mixed Air Sensor
The mixed air sensor measures the resultant tempera
ture of the mixed air downstream from the evaporator
coil. Temperature is measured in the heating compart
ment (figure 31). The mixed air temperature is used by
the enthalpy control when outdoor dampers are open to
help determine outdoor air damper position. The econo
mizer is factory equipped with a single mixed air sensor
which fits through a factory supplied hole in the panel di
viding unit return and supply air (see figure 31).