26 KOMAC00607-09EN
Outside Air Temperature (OAT) Reset
The Active Leaving Water variable is reset based on the outdoor ambient temperature.
Parameters used:
1. Cool LWT set point
2. Max Reset set point
3. OAT
Reset is 0 if the outdoor ambient temperature is greater than Start Reset OAT set point.
From Start Reset OAT set point down to Max Reset OAT the reset varies linearly from
no reset to the max reset at Max Reset OAT set point. At ambient temperatures less
than Max Reset OAT set point, reset is equal to the Max Reset set point.
75
Cool LWT+Max Reset
(54)
Cool LWT Set-Point
(44)
OAT Reset
60
Max Reset
(10)
OAT (
o
F)
Active
LWT
(
o
F)
Unit Capacity Control
Unit capacity control is performed as described in this section.
Compressor Staging in Cool Mode
The first compressor on the unit is started when evaporator LWT is higher than the
target plus the Startup Delta T set point.
An additional compressor is started when Evaporator LWT is higher than the target plus
the Stage Up Delta T set point.
When multiple compressors are running, one will shut down if evaporator LWT is lower
than the target minus the Stage Down Delta T set point.
The last compressor running will 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 will pass between compressors starting, which is defined by
the Stage Up Delay set point. This delay will only apply when at least one compressor
is running. If the first compressor starts and quickly fails on an alarm, another
compressor will start without this minimum time passing.
Required Load For Stage Up
An additional compressor will 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 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.
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