Instruction manual

McQuay OM 920 107
Operator’s Guide
In addition to the fans controlled by the VFD there are up to three staged condenser fan output
that are controlled based on sump temperature. The condenser fan outputs controlled by these
outputs are mechanically prevented from operating when no compressors on that circuit are
operating. When compressors are operating on both circuits, the number of fans operating on
the two circuits is the same since these output are wired to turn on the same fans on each
circuit.
The number of staged condenser fan outputs operating will increase if the time since the last
change in the number of operating fans exceeds the Evap Condensing Stage Time and the
Sump Temperature exceeds the Max Sump T. The number of fans operating per circuit will
decrease if the time since the last change in the number of operating fans exceeds the Evap
Condensing Stage Time, and the Sump Temperature drops below the Min Sump T.
After the two lead condenser fans are turned off via the VFD, they are turned back on as soon
as the Sump Temperature rises above the Maximum Sump Temperature. This transition from
Stage zero to stage one is the only transition that does not require the Stage Timer to expire.
All of the condenser fans need to be run periodically when evaporative condensing is used to
prevent damage to the motors due to their high humidity environment. As indicated above, the
condensing fans controlled by the VFD always run when evaporative operation is in effect so
no special control of these fans is required. The fan motors controlled directly through the
staged condenser fan outputs are turned on and off based on time on and time off. When a
condenser fan output must be turned on for normal operation, the output that has been off for
the longest period of time is started. When condenser fan must be turned off for normal
operation, the output that has been on for the longest period of time is turned off.
A unit equipped with evaporative condensers can not operate in the cooling state below 40°F.
If the OAT Compressor Lockout Temperature on units with evaporative condensing is lowered
below 40°F, it will immediately be set back up to 40°F.
Freeze Protection Valve Control
When the Sump Temperature for Evaporative Condensing units gets too cold, the sump needs
to be emptied to prevent freezing. The Sump Dump output is used to control a Freeze
Protection Valve. This output is turned on if the sump temperature drops below the Freeze
Protection Setpoint, default = 35°F.
This output is turned back off if the sump temperature rises above this setpoint by more than
the Sump; Dump Differential, (default 3°F) and the unit is in the Mechanical Cooling State.
The requirement for Mechanical Cooling prevents the unit from continuously be emptied and
filled as the ambient temperature as measured by the sump temperature sensor varies below
35°F and above 38°F even when cooling was is not required.
If the unit is equipped with a Dolphin system and the pump does not run for five days, the
sump needs to be emptied. This would occur when cooling is not required and the OAT
remains below 35°F. Sump heaters are used to keep the sump temperature above 35°F when
the OAT drops below that value so an additional means of turning on the Freeze Protection
Valve is required. In addition to the requirement that the Sump Pump output be turned on to
open the valve when the sump temperature drops below 35°F, the Sump Pump output is to be
turned on if both of the following are true:
• Dolphin System= Yes
• The Sump Pump has been off for 120 hours or more