14 WGZ030D through WGZ200D OMM 1130
41.5°F
40.0°F
38.5°F
½DB
½DB
EWT
50.0°F
10.0°F
40.0°F
Evap Delta-T Set
LWT Set
Start Delta T
46.5°F
36.5°F
Stop Delta T
If the outside air temperature higher than 75.0°F the condenser fan staging logic changes to bring on
the first fan on when the condenser pressure is greater than 140 psi.
The standard condenser fan staging logic would start the first condenser fan when the condenser
pressure is higher than 200.0 psi.
The last condenser fan on each circuit will not shut down until the condenser pressure drops below
140.0 psi regardless of the outside air temperature
Sequence of Operation
The following sequence of operation is typical for WGZ water chiller models. The sequence can vary
slightly depending upon options.
Compressor Heaters
With the control circuit power on and the control stop switch S1 off, 115V power is applied through the
control circuit fuse Fl to the compressor crankcase heaters HTR1, HTR2, HTR3, and HTR4.
Start-up/Compressor Staging
When compressors start and stop.
Stage Up Temp is the LWT temperature at which the next compressor to start will stage up (start) after at
least one compressor on the unit has started and is running.
Start Up Temp is the LWT at which the first compressor starts. The start up temperature equals the stage
up temperature plus the Start Delta temperature. A high Start Delta will keep the unit off longer and
reduce unit cycling at low loads. However, this high Start Delta will cause a larger excursion from the
LWT setpoint before the unit starts.
Stated another way, the Start Delta is the number of degrees above the Evap LWT setpoint, plus ½ the
Dead Band, that determines when the first compressor starts. The Start Delta is in effect for only the first
start after all compressors have been off. Additional compressor starts and stops are determined by the
LWT in respect to the dead band only. The dead band is automatically set at 30% of the EvapDeltaT
selected in menu 3. The following sequence would occur for the settings shown below:
EvapDelta T=10.0F Dead Band=3.0F StartDelta=5.0F StopDelta=2.0F LWT=40.0F
Figure 7, Staging/Starting Temperatures
For a warm start-up (no compressors
running), the first compressor will start at any
temperature above 46.5F. Each subsequent
compressor will start after the Stage Up Timer
has timed out and if the temperature is above
the dead band, 41.5F in this case.
If the LWT stays above 41.5F, all of three (or
5) remaining compressors will eventually
stage on after the Stage Up Timer times out
between each stage.
At some point, the chilled water temperature will be dropping and begin to approach the point when
compressors should begin staging off, which is the LWT setpoint minus ½ of the Dead Band, 38.5F in
this case. If the LWT remains below LWT setpoint minus ½ Dead Band and the Stage Down Timer times
out, additional compressors will stage off. The last compressor will stage off when the LWT falls below
the LWT Setpoint minus ½ the Dead Band minus the Stop Delta T. The stop Delta T is in effect for only
the last compressor running.
If the temperature climbs above 38.5F all running compressors will remain on. No compressor staging
occurs within the Dead Band. The next-on compressor will start when the chilled water temperature
reaches 41.5F and the Stage Up Timer times out.
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