STULZ E
2
SERIES CONTROLLER FOR PERIMETER SYSTEMS OPERATION MANUAL
49
The controller monitors the outdoor air and ceiling return air
conditions and adjusts the damper setting to get the mixed
air conditions as close as possible to the desired temperature
setpoint. If the outdoor air conditions are outside of the user
adjustable setpoints, the damper remains closed at 0%.
In the simple case where both the outdoor air and ceiling
return air temperatures are above the temperature setpoint
and outdoor conditions are within the economizer operating
envelope, the colder of the two temperatures is selected which
then drives the damper to 0% (full ceiling return air) or 100%
(full outdoor air). Temperatures on either side of the setpoint
cause the damper to open to a "New Damper" position such as
20% as shown in the above screen.
The damper position is determined by the air temperatures,
but the resultant mixed air dewpoint is also taken into account.
In this example, the ceiling return (room conditions) started at
52.1 degrees dewpoint and the outdoor air is at 50 degrees
dewpoint. The incoming mix is at 51.6 degrees dewpoint. If the
A/C unit or the overall conditioned space has no mechanism for
humidifi cation, the ceiling return dewpoint will drop which then
causes the mixed dewpoint to drop even lower. Eventually, the
room dewpoint will match the dewpoint of the outside air. The
speed at which the dewpoint drops depends on the airfl ow rate
and the size of the room.
6.3 Enabling Economizer Operation
6.3.1 Economizer Option Screen
Economizer Option
Enable Economizer: No
Run economizer at
all times No
Monitor mix temp: No
From the Service>Economizer Option screen the economizer
mode is enabled by entering Yes in the fi eld of the fi rst line. If
Yes is entered in the second line it allows economizer damper
control to continue operating even when the A/C unit is turned
off. In the third line the economizer mixed air monitoring function
may be selected by the user. With the Monitor mix temp
economizer selection set to Yes, the controller modulates
the damper according to the external sensors and adjusts it
accordingly if the mixed air sensor is not at the setpoint. This
is intended for use in closed duct systems and allows for any
inaccuracies in the damper system to be accounted for. If
Monitor mix temp is set to No, the controller will simply
move the damper based on the external sensors and the desired
setpoint. This is used for an open duct system where the external
air comes into the room, but not solely through the A/C unit.
The economizer normally operates within the outdoor
temperature boundaries of Econ High Temp and Econ Low
Temp and between the dewpoint boundaries of Dehumidify
Figure 11 depicts the use of the expanded dewpoint where
the lower dewpoint offsets of C and D have been replaced by
C
1
and D
1
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Test Economizer
Test OA air : 50 °F
Test OA DP: 50 °F
Test Mix Temp 72°F
Econ OK? Yes
After entering the test values of the outdoor air temperature,
humidity and desired mix the controller determines if those
values lie within the economizer zone. You then enter the ceiling
return air temperature and dewpoint.
Test Economizer
Test Ret air 77.5°F
Test Ret DP 52.1°F
OA Damper: 20
DP set 51.6
Test Mix <-> Damper
Adjust Mix Temp: 72°F
New Damper 20
New Mix DP 51.6°F
Adjust Damper: 000
New Mix Temp 77.5°F
New Mix DP 52.1°F
The temperature setpoint and dewpoint may be changed in
the Control menu if desired. The controller displays the damper
setting for the input conditions (20%) and the resulting dewpoint
(51.6). The operator can vary the damper setting to see how it
effects the overall mixed air temperature and dewpoint coming
into the A/C unit.
Figure 11. Example Economizer Zone
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