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Capacity
controller RS8ER302 © Danfoss 2016-02
AK-CH
650
Capacity control of compressors
PI-control and control zones
AK-CH 650 can control up to 6 compressors with up to 3 unloader
valves each. One or two of the compressors can be equipped with
speed regulation.
The calculation of the requested compressor capacity takes place
on the basis of a PI control, but the set up is carried out in the
same way as for a neutral zone controller which is divided into 5
different control zones as shown in below sketch.
The width of some of the zones can be set via the settings “+ Zone
K”, “NZ K” and “- Zone K”.
Furthermore it is possible to adjust zone timers which is equal to
the Tn integration time for the PI controller whenever the suction
pressure is in the zone in question (please see sketch above).
By setting a zone timer to a higher value will make the PI controller
slower in this zone and by setting the zone timer lower will make
the PI controller faster in this zone.
The amplification factor Kp is adjusted as parameter ”Kp S4”
In the neutral zone the controller is only allowed to increase or
decrease the capacity by means of speed control and/or switching
of unloader valves.
In the other zones the controller is also allowed to increase/
decrease capacity by means of starting and stopping compressors.
The last compressor is only allowed to be stopped when the brine
temperature is in the “- Zone” or “- - Zone”
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").
Suction group
Requested capacity
The readout “Requested capacity” is the output from the PI
controller and it shows the actual requested compressor capacity
by the PI controller. The rate of change in the requested capacity
depends upon in which zone the brine temperature is and
whether the brine temperature is stable or whether it is constantly
changing.
The Integrator is looking at the deviation between the set point
and the current temperature only and increases/reduces the
requested capacity correspondingly. The amplification factor
Kp on the other hand only looks at the temporary temperature
changes.
In the “+ Zone” and “++ Zone” the controller will normally increase
the requested capacity as the temperature is above the set point.
But if the temperature is decreasing very fast the requested
capacity might decrease also in these zones.
In the “- Zone” and “-- Zone” the controller will normally decrease
the requested capacity as the temperature is below the set
point. But if the temperature is increasing very fast the requested
capacity might increase also in these zones.
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.
Example:
4 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 at "-Zone" or in
"—Zone"
Brine temperature
