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Operation Manual
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Light Load Stage Down in Cool Mode
When multiple compressors are running, one will shut down if all running compressors are at a capacity lower than the
Load Stage Down set point and the evaporator LWT is less than the target plus the Stage Up Delta T set point. A minimum
amount of time will pass between compressors stopping as a result of this logic, which is defined by the Stage Down Delay
set point.
Light Load Stage Down in Heat Mode
When multiple compressors are running, one will shut down if all running compressors are at a capacity lower than the
Load Stage Down set point and the condenser LWT is greater than the target minus the Stage Up Delta T set point. A
minimum amount of time will pass between compressors stopping as a result of this logic, which is defined by the Stage
Down Delay set point.
Maximum Circuits Running
If the number of compressors running is equal to the Max Circuits Running set point, no additional compressors will be
started.
When multiple compressors are running, one will shut down if the number of compressors running is more than the Max
Circuits Running set point.
7.12.3
Compressor Staging in Ice Mode
The first compressor will start when evaporator LWT is higher than the target plus the Startup Delta T set point.
When at least one compressor is running, the other compressors will start only when evaporator LWT is higher than the
target plus the Stage Up Delta T set point.
All compressors will be staged off when evaporator LWT is less than the target.
Stage Up Delay
A fixed stage up delay of one minute between compressor starts is used in this mode. When at least one compressor is
running, the other compressors will start as quickly as possible with respect to the stage up delay.
7.12.4
Staging Sequence
This section defines which compressor is the next one to start or stop. In general, compressors with fewer starts will
normally start first, and compressors with more run hours will normally stop first. Compressor staging sequence can also
be determined by an operator defined sequence via setpoints.
Next To Start
The next compressor to start must meet the following requirements:
Lowest sequence number of those compressors available to start
•
if sequence numbers are equal, it must have the least starts
•
if starts are equal, it must have least run hours
•
if run hours are equal, it must be the lowest numbered compressor
Next To Stop
The next compressor to shut down must meet the following requirements:
Lowest sequence number of the compressors that are running
•
if sequence numbers are equal, it must have the most run hours
•
if run hours are equal, it must be the lowest numbered compressor
7.12.5
Compressor Capacity Control In Cool Mode
In Cool mode, evaporator LWT is controlled to within
0.2 °C
of the target under constant flow conditions by controlling
capacity of the individual compressors.
Compressors are loaded with a fixed step scheme. The rate of capacity adjustment is determined by the time between
capacity changes. The farther away from the target, the faster compressors will be loaded or unloaded.
The logic projects ahead to avoid overshoot, such that the overshoot does not cause the unit to shut off due to evaporator
LWT dropping below the target minus the Shutdown Delta T set point while there is still a load on the loop at least equal
to the minimum unit capacity.
Capacity of the compressors is controlled so that when possible their capacities are balanced.
Circuits that are running in manual capacity control or running with active capacity limiting events are not considered in the
capacity control logic.
The compressor capacities are adjusted one at a time while maintaining a capacity imbalance that does not exceed 12.5%.
