16 WGS 130A to 190A OM WGS-5
Sequence of Operation
Compressor Heaters
With the control power on, 120V power is applied through the control circuit Fuse FU7 to
the compressor oil separator heater (HTR-OIL SEP).
Startup/Compressor Staging
During Cool Mode the following must be true to start a circuit operating. The evaporator
and condenser pump (WGS-AW only) outputs must be energized and flow must be
established for a period of time defined by the evaporator recirculate setpoint. Established
flow will be detected by evaporator and condenser water flow switches. The water
temperature leaving the evaporator must be greater than the Active Leaving Water
Temperature setpoint, plus the Startup Delta-T, before a circuit will start. The first circuit to
start is determined by sequence number. The lowest sequence numbered circuit will start
first. If all sequence numbers are the same (default), then the circuit with the fewest number
of starts will start first.
During operation, the slide valves for loading and unloading will be pulsed until the active
leaving water temperature setpoint is maintained. The second circuit start will occur once
the first circuit has loaded to 75% slide capacity or is in Capacity Limit and the water
temperature leaving the evaporator is greater than the active leaving water temperature
Setpoint plus Stage Delta-T. The circuits will load or unload simultaneously through a
continuous capacity control to maintain the evaporator leaving water temperature. If all
sequence numbers are the same, the circuit with the most run hours will be shut down first.
The circuit with the most run hours will stop when the water temperature leaving the
evaporator is less than the Active Leaving Water Temperature Setpoint minus Stage Delta-
T. The last remaining circuit will shut down when the water temperature leaving the
evaporator is lower than the Active Leaving Water Temperature Setpoint minus the Stop
Delta-T.
Automatic Pumpdown
The Model WGS chiller has two separate refrigerant circuits so the refrigerant charge is
stored in the condenser when the circuit is off. Pumpdown to the condenser helps keep
refrigerant from migrating to the compressor. It also helps establish a pressure differential
on start for oil flow. In a normal shutdown, each circuit will close its expansion valve,
causing the evaporator pressure to reach a low-pressure setpoint. Once this setpoint is
reached, or a specified amount of time has elapsed, the running circuit will be shut down.
Chilled Water and Condenser Water Pumps
The chiller’s MicroTech II controller has a total of four pump outputs, two for the
evaporator and two for the condenser (WGS-AW only). There is a manual setting in the
software for the user to select either pump output 1 or 2. It is recommended that the
chiller’s outputs control the water pumps, as this will offer the most protection for the unit.
Cooling Tower Control
The MicroTech II controller can control the cooling tower fans and/or a tower bypass valve.
This provides a simple and direct method to control the unit’s discharge pressure.
Programming directions and the sequence of operation can be found on page 49. Some
m
eans of discharge pressure control is recommended and must be installed if the entering
condenser water temperature to the condenser can fall below 60F.
Condenser Fan Control
The MicroTech II controller can be programmed to cycle air-cooled condenser fans on and
off based on the discharge pressure. Details are on page 66.
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