IOM 1206-1
43
Unit Controller Operation
should use their respective dead bands. Stages five through six
should all use the Stage Up Dead Band 4.
When the saturated condenser temperature is above the target
+ the active deadband, a stage up error is accumulated.
Stage Up Error Step = Saturated Condenser Temperature –
( Stage Up dead band)
The Stage Up Error Step is added to Stage Up Accumulator
once every 5 seconds, only if the Saturated Condenser
Refrigerant Temperature is not falling. When Stage Up Error
Accumulator is greater than 11°C (19.8°F) another stage is
added.
When a stage up occurs or the saturated condenser temperature
falls back within the stage up dead band the Stage Up
Accumulator is reset to zero.
Staging Down
Four stage down dead bands shall be used. Stages one
through four should use their respective dead bands. Stages
five through seven should all use Stage Down Dead Band 4.
When the saturated condenser refrigerant temperature is below
the target – the active deadband, a stage down error is
accumulated.
Stage Down Error Step = (Target - Stage Down dead band) -
Saturated Condenser Temperature
The Stage Down Error Step is added to Stage Down
Accumulator once every 5 seconds. When the Stage Down
Error Accumulator is greater than 2.8°C (5°F) another stage of
condenser fans is removed.
When a stage down occurs or the saturated temperature rises
back within the Stage Down dead band the Stage Down Error
Accumulator is reset to zero.
VFD
Condenser pressure trim control is accomplished using an
optional VFD on the first fan. This VFD control should vary
the fan speed to drive the saturated condenser temperature to a
target value. The target value is normally the same as the
saturated condenser temperature target.
The speed should be controlled between the minimum and
maximum speed setpoints.
VFD State
The VFD speed signal should always be 0 when the fan stage
is 0.
When the fan stage is greater than 0, the VFD speed signal
should be enabled and control the speed as needed.
Stage Up Compensation
In order to create a smoother transition when another fan is
staged on, the VFD compensates by slowing down initially.
This is accomplished by adding the new fan stage up deadband
to the VFD target. The higher target causes the VFD logic to
decrease fan speed. Then, every 2 seconds, 0.1°C (0.18°F) is
subtracted from the VFD target until it is equal to the saturated
condenser temperature target set point.
High IPLV Mode
When the High IPLV Mode setting is ‘On’ and one compressor
is running on the unit, the condenser target setting for the
running circuit may be overridden. In this case, rather than use
the condenser target setting for 33% or 50% (depending on
total number of compressors), the condenser target will be
forced to 21.11°C (70°F).
In addition, when high IPLV mode is active the calculation for
the minimum allowed condenser target (based on LWT) will
be changed. The minimum value will be changed from 23.9°C
(75°F) to 21.11°C (70°F). No other changes to the operation
are made when High IPLV mode is on.
Special Operation For MicroChannel Coil
For units configured with MicroChannel condenser coils, the
fan staging is the same as for a standard coil except there are
two additional conditions which may cause the fan stage to
increase:
•
If the second compressor on a circuit starts, one of the two
condenser fans is running, and the condenser saturated
temperature is higher than the target for 100% capacity, then
the second condenser fan will start immediately.
•
If one of the two condenser fans is running and the saturated
condenser temperature exceeds 56.67°C (134°F) then the
second condenser fan will start immediately.
Both of these additions are in place to deal with temporarily
higher condenser pressure resulting from the lower volume of
the MicroChannel coils. Note that this software version
supports E vintage chillers only up through the AGZ070E at
this time, so all E vintage models supported have four
compressors and four fans.
