OMM 1130 15
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 36F.
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 39F and the Shutdown Temp above 36F, 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.
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
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