Danfoss ADAP-KOOL AK-PC 783, Design Manual, Page: 110 / 128

110 Capacity controller RS8GN202 © Danfoss 11-2013 AK-PC 783
Capacity control of the condenser can be accomplished via step
regulation or speed control of the fans.
• Step regulation
The controller can control up to 6 condenser steps that are cut in
and out sequentially.
• Speed control
The analog output voltage is connected to a speed control. All
fans will now be controlled from 0 to max. capacity. If an ON/
OFF signal is required it can be obtained from a relay output.
Regulation can be carried out based on one of the following
principles:
- all fans operate at the same speed
- Only the necessary number of fans is cut in.
- Combination with one fan speed regulated and the rest step
regulated.
Capacity control of condenser
The cut-in condenser capacity is controlled by the condenser pres-
sure’s actual value and depends on whether the pressure is rising
or falling. Regulation is performed by a PI controller which may
however be changed into a P controller if the design of the plant
necessitates this.
PI regulation
The controller cuts in capacity in such a way that the deviation
between the actual condensing pressure and the reference value
becomes as small as possible.
P regulation
The controller cuts in capacity that depends on the deviation
between the actual condensing pressure and the reference value.
The proportional band Xp indicates the deviation at 100%
condenser capacity.
The adjustments are carried out using amplification factor Kp,
where Kp = 100/Xp.
Regulating sensor selection
The capacity distributor can either regulate from the condenser
pressure PC or from the average temperature S7.
Cap. Ctrl sensor = Pc / S7
If the control sensor is selected for media temperature S7, then Pc
is still used as the safety function for high condenser pressure and
will therefore ensure cut-out of the compressor capacity when
condenser pressure is too high.
Handling sensor errors:
Cap. Ctrl. Sensor = Pc
If Pc is used as the control sensor, an error in the signal will result
in condensator capacity being controlled as a function of the
connected compressor capacity. The compressor regulation will
remain normal.
Cap. Ctrl. Sensor = S7
If S7 is used as the control sensor, an error in this sensor will re-
sult in further regulation that follows the Pc signal, but in accord-
ance with a reference that is 5K over the actual reference. If there
is an error on both S7 and Pc, 100% condenser capacity cuts-in,
but the compressor regulation remains normal.
Reference for condensing pressure
The reference for the regulation can be defined in two ways. Either
as a fixed reference or as a reference that varies according to the
outdoor temperature.
Fixed reference
The reference for the condensing pressure is set in °C.
Floating reference
This function allows the condensing pressure’s reference value to
vary according to the outdoor temperature within a defined area.
By combining floating condensing pressure with electronic
expansion valves a lot of energy saving can be achieved. The
electronic expansion valves enables the controller to decrease
the condensing pressure according to outdoor temperature
and thereby reduce energy consumption by around 2% for each
degree the temperature can be decreased.
PI regulation
The reference is based on:
- the outdoor temperature measured with Sc3 sensor
- The minimum temperature difference between the air
temperature and the condensing temperature at 0% compressor
capacity.
- the condenser’s dimensioned temperature difference between
the air temperature and the condensing temperature at 100%
compressor capacity (Dim tmK)
- how large a part of the compressor capacity has been cut in.
The lowest possible ΔT at low load is typically set to 6 K to ensure
subcooling and that the fans stop when the compressors stop.
Set the dimensioned difference (dim tm) at max. load (e.g. 15 K).
The controller will now contribute with a value to the reference
which depends on how large a part of the compressor capacity
has been cut in.
Condenser