ESIE10-01 Functional Control
Part 2 – Functional Description 2–35
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2.2.9 Unit Capacity Control
Unit capacity control will be performed as described in this section. All unit capacity limits described in
following sections must be applied as described.
Compressor
staging in cool
mode
The first compressor on the unit should be started when evaporator LWT is higher than the target plus
the Startup Delta T set point.
An additional compressor should be started when Evaporator LWT is higher than the target plus the
Stage Up Delta T set point.
When multiple compressors are running, one should shut down if evaporator LWT is lower than the
target minus the Stage down Delta T set point.
The last compressor running should shut down when the evaporator LWT is lower than the target
minus the Shut Down Delta T set point.
■ Stage Up Delay
A minimum amount of time should pass between compressors starting, which is defined by the
Stage Up Delay set point. This delay should only apply when at least one compressor is running.
If the first compressor starts and quickly fails on an alarm, another compressor may start without
this minimum time passing.
For units in Ice mode, a fixed stage up delay of one minute between compressor starts should be
used in this mode. When at least one compressor is running, the other compressors should start
as quickly as possible with respect to the stage up delay.
■ Required Load for Stage Up
An additional compressor should not be started until all running compressors are at a capacity
higher than the Load Stage Up set point, or running in a limited state.
■ Light Load Stage Down
When multiple compressors are running, one should 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 should pass between compres-
sors 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 should be started.
When multiple compressors are running, one should shut down if the number of compressors run-
ning is more than the Max Circuits Running set point.
Load
Stop Last Unload
Unload
+ Stop
Load + Start Start first
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