OMM 1130
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2
15
When staging down, one compressor on each circuit will be left on until each circuit has only one
compressor running. In other words, the compressor, on either circuit, with the most run-hours will stop
first. The compressor with the most run-hours on the
other
circuit will stop next. One compressor on
each circuit will be running. The third compressor off will be the one, on
either
circuit, with the most
run-hours. The remaining compressor will be the last off. See the following description of pumpdown.
Table 2, Staging in Cool and Glycol Mode
Description
Occurs When:
Action Taken
Stage #1 ON
(See Notes
Below)
Lvg Evap T > Evap LWT SP + (DB/2) +
Startup Delta T
Available compressor with least starts,
ON
Stage #2 ON
After Stage Up Delay times out then, LVG
Evap T > Evap LWT SP + (DB/2)
Available compressor on the other
circuit with least starts, ON
Stage #3 ON
After Stage Up Delay times out, then LVG
Evap T > Evap LWT SP + (DB/2)
Available compressor on either circuit
with least starts, ON
Stage #4 ON
After Stage Up Delay times out then, LVG
Evap T > Evap LWT SP + (DB/2)
Remaining compressor, ON
Stage #4 OFF
After Stage Down Delay times out then, LVG
Evap T < Evap LWT SP – (CB/2)
Compressor with most run hours, OFF
Stage #3 OFF
After Stage Down Delay times out then, LVG
Evap T < Evap LWT SP – (DB/2)
Compressor on the other circuit with
most run hours, OFF
Stage #2 OFF
After Stage Down Delay times out then, LVG
Evap T < Evap LWT SP – (DB/2)
Compressor on either circuit with most
run hours, OFF
Stage #1 OFF
After Stage Down Delay times out then, LVG
Evap T < Evap LWT SP – (DB/2)-StopDelta T
Remaining compressor, OFF
Note 1: DB (Dead Band) = Evap Water Delta T x .3
Manual Compressor Disable Logic
Logic is available that allows the operator to manually enable and disable compressors. When a
compressor is disabled, it is considered unavailable to start in the staging logic. This allows a damaged
compressor to be taken offline while the remaining compressor can still provide some cooling
•
The Compressor Disable set points are found on Compressor Set Points screens three and four.
•
A running compressor cannot be disabled until it has been shutdown.
•
If all of the compressors on a circuit are disabled, then the circuit will be disabled.
•
If both circuits have all of their compressors disabled, then the Unit State will remain Off
Automatic Pumpdown
WGZ units are equipped with single pumpdown control. When the last compressor running on either
circuit is ready to shut off, the liquid line solenoid valve (LLSV) is closed first and the compressor
continues to run until the pumpdown pressure is reached, at which time the compressor shuts off. The
shut off pressure is set at 15 psi below the Low Evaporator pressure Unload setpoint.
When the first compressor on a circuit starts, the LLSV opens simultaneously.
Manual Pumpdown
When the Pumpdown Switch is in the pumpdown position, all compressors except #1 and #2 will shut off.
Then the Liquid Line and Hot Gas Bypass Valves will close. The operating compressor will pump out the
refrigerant. When the Suction Pressure is at 40 psig, the compressors will stop.
Chilled Water and Condenser Water Pumps
The chiller MicroTech II controller can be programmed to start and stop the system chilled water and
condenser water pumps. They may also be controlled by the BAS or manually. Programming directions
and the sequence of operation can be found beginning on page 38.
Cooling Tower Control
The cooling tower fans and/or the tower bypass valve can be controlled by the MicroTech II controller.
This provides a simple and direct method to control the unit’s discharge pressure. Programming
Summary of Contents for WGZ030DW
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