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Capacity control of compressors
Capacity control
AK-PC 772A can control up to 3 compressors on MT and 2 on LT.
(For parallel compressor operation, however, the system can only
control 2 on MT and 2 on LT.) Each compressor can have up to 3
unloaders. One or two of the compressors can be equipped with
speed regulation.
The cut-in capacity is controlled by signals from the connected
pressure transmitter/temperature sensor and the set reference.
Set a neutral zone around the reference .
In the neutral zone, the regulating compressor controls the
capacity so that pressure can be maintained. When it can no
longer maintain the pressure within the neutral zone, the control-
ler will cut out or cut in the next compressor in the sequence.
When further capacity is either cut out or cut in, the capacity
from the regulating compressor will be modified accordingly
to maintain the pressure within the neutral zone (only where
the compressor has variable capacity).
– When the pressure is higher than the "reference + a half neutral
zone", cut-in of the next compressor (arrow up) is permitted.
– When the pressure is lower than the "reference - a half neutral
zone", cut-out of a compressor (arrow down) is permitted.
– When the pressure is within the neutral zone, the process
will continue with the currently activated compressors. Unload
valves (if present) will activate, depending on whether suction
pressure is above or below the reference value.
Change capacity
The controller will cutin or cutout capacity based on these basic
rules:
Increase capacity:
The capacity distributor will start extra compressor capacity as
soon as the requested capacity has increased to a value, which
allows the next compressor step to start. Referring to below
example - a compressor step is added as soon as there is "Room"
for this compressor step below the requested capacity curve.
Decrease capacity:
The capacity distributor will stop compressor capacity as soon
as the requested capacity has decreased to a value, which allows
the next compressor to stop. Referring to below example - a
compressor step is stopped as soon as there is no more "Room" for
this compressor step above the requested capacity curve.
AK-PC 772A
Suction pressure P0
Capacity controller RS8HF102 © Danfoss 2017-07
Example:
3 compressor of equal size - The capacity curve will look like this
Cut-out of the last compressor stage:
Normally, the last compressor step will only be cut-out when the
required capacity is 0% and the suction pressure is below the
neutral zone.
Operation time first step
At start-up the refrigeration system must have time to be stable
before the PI controller takes over the control. For this purpose at
start-up of a plant a limitation is made of the capacity so that only
the first capacity step will cutin after a set period (to be set via
"runtime first step").
Pump down function:
To avoid too many compressor starts/stops with low load, it is pos-
sible to define a pump down function for the last compressor.
If the pump down function is used, the compressors will be cut-
out when the actual suction pressure is down to the configured
pump down limit.
Note that the configured pump down limit should be set higher
than the configured safety limit for low suction pressure "Min Po".
For the IT compressor, pump down will be controlled by the recei-
ver and MT temperature.
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