Instruction manual

106 McQuay OM 920
Operator’s Guide
Evaporative Condensing Control (RTU)
The evaporative condensing option for Rooftop units uses the heat absorbed by evaporating
water as well as air drawn across a bank of tubes with refrigerant flowing through them to
condense hot refrigerant to a liquid. Water is pumped from a sump beneath the condenser
tubes to nozzles above the coil that spray water onto the bank of tubes. The refrigerant in the
tubes is cooled and condensed as some of this water is evaporated when it strikes the hot tubes
and is carried away by condenser fans. Using this method, the refrigerant can be cooled to a
lower temperature than is the case with a normal finned condenser that transfers heat directly
to the air.
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.
Condenser fan # 1 on each circuit is turned on and off via ModBus communications with a
single VFD. The speed of these two fans is controlled via ModBus communications with the
VFD. These two fans will always operate at the same speed.
Condenser fans #2 and # 3 on each compressor circuit is turned on and off using the two
outputs on the Main Board that are used for standard condenser fans. Relays are driven by
each of these outputs so that fans associated with a circuit that is not operating will not be
turned on.
Sump Pump Control
The pump output is turned on whenever the unit is in the Cooling state and the Sump Pump
Fail condition described below is false. The Sump Pump is turned on before any compressor
or condenser fan is turned on.
A Sump Pump Fail condition exists if the Sump Pump status is open after the Sump Pump
Output has been turned on for more than the Sump Pump Delay time (Default equals 30
seconds). The Sump Pump Fail condition remains for one cooling stage time. The Sump Pump
is turned off and the maximum Cooling Stage is set to zero when the Sump Pump Fail
condition exists. A Sump Pump Fail Problem that disables cooling becomes active if Cooling
has been limited to Stage Zero due to Sump Pump Fail conditions three times between 2:00
AM of one day and 2:00 AM of the next day.
If the unit is equipped with a Dolphin system, the sump pump is run every three days to reduce
scaling. The Pump output is turned on for one hour if all of the following are true:
• Dolphin System= Yes
• Sump Pump has been off for more than 72 hours but less than 120 hours
• The OAT is greater than 35°F
• The Freeze Protection Valve is Closed
Sump Temperature Control
All condenser fans are off when all compressors are off. Whenever any compressor is turned
on, Condenser Fan # 1 (the lead condenser fan) on both circuits is turned by sending a signal
to the VFD via the ModBus network internal to the unit. Condenser fan # 1 for both
compressor circuits are on even if no compressors are operating on one of the circuits.
Condenser Fan # 1 on both circuits are turned off when all compressors are off, and they are
turned off as described in paragraphs that follow when the sump temperature gets low.
The VFD is controlled and monitored by the main controller via ModBus network internal to
the unit. Whenever any condenser fan is on, signals are sent to the VFD to control connected
fans to a calculated setpoint. The speed varies from a minimum value (25% default) to 100%
as the sump temperature varies from the Min Sump Temp to the Max Sump Temp setting.