Sequence Of Operation - Daikin WGZ030DW Installation, Operation And Maintenance Manual

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 set
point before the unit starts.
Stated another way, the Start Delta is the number of degrees
above the Evap LWT set point, 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=10.0°F
Dead Band=3.0°F
StartDelta=5.0°F
StopDelta=2.0°F
LWT=40.0°F
Figure 35: Staging/Starting Temperatures
For a warm start-up (no compressors running), the first
compressor will start at any temperature above 46.5°F. Each
subsequent compressor will start after the Stage Up Timer
has timed out and if the temperature is above the dead band,
41.5°F in this case.
If the LWT stays above 41.5°F, all of three (or 5) remaining
IOM 1322-1 • WATER-COOLED SCROLL COMPRESSOR
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 set point minus ½ of the
Dead Band, 38.5°F in this case. If the LWT remains below LWT
set point 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 Set point
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 38.5°F 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 41.5°F and the Stage
Up Timer times out.
However, in some circumstances this methodology can cause
the LWT to drop to dangerously low levels, with the evaporating
temperature below the freeze point, before stopping. In
the example shown in Figure 7, the Shutdown Temp (last
compressor off) would be 36°F.
This would result in a refrigerant evaporating temperature
approaching freezing, so the rule is amended to read:
If the Cool Leaving Water Temperature (LWT) set point is
less than half the Control Band
above 39.0° F the Stage Down temperature is calculated as:
Stage Down Temperature = Cool LWT – (Cool LWT - 39.0°
F), and the
Shutdown Temperature = Cool LWT – (Cool LWT - 39.0° F)
– Stop Delta T
This keeps the Stage Down Temp above 39°F and the
Shutdown Temp above 36°F, as the maximum Stop Delta T
allowed is 3-degrees.
Which compressor starts and stops . One compressor per
circuit will start before starting the second compressor (or
third) on any circuit. In other words, the compressor with the
lowest number of starts will start first. The compressor with the
lowest number of starts on the other circuit will start next, so
that one compressor on each circuit will be running. The third
compressor on will be the compressor on either circuit with the
fewest starts. The remaining compressor will be the last on. If
a circuit is unavailable for any reason, the second compressor
on the operating circuit will stage on. Only two (or three)
compressors (on the one circuit) will be operating.
There is a 150 second delay after power-up before any
compressor is allowed to start. 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.
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Sequence of Operation

Sequence of Operation
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