108 McQuay OM 920-1
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.
Table 73: R-407C Condenser Fan Setpoints
Unit Size
CondFanOutputA
(MCB-D07)
CondFanOutputB
(MCB-D08)
CondFanOutputC
(MCB-D06)
Setpoint Differntial Setpoint Differential Setpoint Differential
015 60 5 - - - -
018 0 5 - - - -
020 0 5 - - - -
025 65 5 - - - -
030 65 5 - - - -
036 65 5 - - - -
040 60 5 - - - -
045 55 5 - - - -
050 50 5 - - - -
060 15 5 - - 70 5
070 30 5 - - 70 5
075 65 5 0 5 75 5
080 65 5 0 5 75 5
090 50 5 0 5 75 5
105 0 5 70 5 35 5
To Next Page
Loading...
Need help?
General
