System Operation
Figure 4-14 Set the Master
4.9 Operational Logic
4.9.1 Free Cooling (Economizer) Operation
When Free Cooling (FC) is engaged, the damper opens to introduce cooler air from outside to be delivered to
the shelter. The damper will open when the difference between the indoor and outdoor temperatures is larger
than a predefined minimum (default 3.6°F, configurable under “Free Cooling” in the User Configuration menu)
and the outdoor humidity is below the limit (default 85%).
The supply fan will start at low speed when the damper opening conditions are met and the indoor temperature
is above the Free Cooling Setpoint (configurable under “Room Temperature” in the User Configuration menu).
As the indoor temperature rises, the fan speed will increase to keep the indoor temperature within 2°F of the
main setpoint. If the indoor temperature cannot be maintained below the lead compressor turn on point
(configurable under “Room Temperature” in the User Configuration menu), the compressor will start to assist
FC.
During cold weather, the outdoor damper modulates to keep the supply air temperature above 51.8°F and will
close completely when the supply temperature is below that point (configurable under “Supply Temperature”
in the User Configuration menu). The damper will also close if the humidity is above the limit (default 85%) or
if the AFPD dust sensor triggers a protection event.
If partial free cooling is allowed (default enabled), the damper can remain open when the compressor is
engaged, provided that the outdoor temperature is lower than the indoor temperature. Otherwise, the damper
will close as soon as the compressor starts. For shelters without a secondary pressure relief (e.g. a barometric
louvre), partial free cooling can be disabled (configurable under “Free Cooling” in the User Configuration
menu).
4.9.2 Mechanical Cooling Operation
All AIRSYS Precision Cooling WPUs have variable speed compressors which will adjust their speed to match
the heat load inside the shelter. The compressor logic will vary based on whether free cooling is engaged. If
free cooling is enabled, the compressor will not start until the indoor temperature has reached a predetermined
level above the setpoint (default 2°F). At that point, the compressor will start at full speed and adjust its speed
according to the indoor temperature. These settings can be configured under “Room Temperature” in the Unit
Configuration Menu.
When free cooling is not available, the compressor will follow the logic shown below.
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