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Capacity control of compressors
PI-control and control zones
AK-PC 783 can control up to 8 compressors. 5 on MT and 3 on LT
or 4 on MT and 4 on LT. Each compressor can have up to 3 unload-
ers. 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 which is divided into 5 different
control zones as shown in below sketch.
Suction pressure P0
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 Po"
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.
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").
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 pressure is and whether the
pressure is stable or whether it is constantly changing.
The Integrator is looking at the deviation between the set point
and the current pressure only and increases/reduces the requested
capacity correspondingly. The amplification factor Kp on the other
hand only looks at the temporary pressure changes.
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Capacity controller RS8GN202 © Danfoss 11-2013
In the "+ Zone" and "++ Zone" the controller will normally increase
the requested capacity as the suction pressure is above the
set point. But if the suction pressure 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 suction pressure is below the
set point. But if the suction pressure 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"
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".
AK-PC 783
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