Danfoss AK-PC 781A

108 Capacity controller RS8HE202 © Danfoss 2017-07 AK-PC 781A
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 8 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. If the refrigerant
is CO2 and transcritically regulated, use a temperature sensor Sgc,
located at the outlet for the gas cooler.
Cap. Ctrl sensor = Pc / S7 / Sgc
If the regulation 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 regulation 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 regulation sensor, an error in this sensor will
result in further regulation that follows the Pc signal, but in ac-
cordance 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.
Cap. Ctrl. Sensor = Sgc
In the event of failure this sensor switches over to Shp if installed.
If Shp cannot provide a signal, the controller switches over to an
"emergency cooler sequence" that attempts to maintain regula-
tion.
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 within a defined range. The reference varies according to the
outdoor temperature and the connected compressor capacity.
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.
The measured outdoor temperature is also used by the controller
to optimise the regulation algorithm. The function can be
compared to a variable Kp value, which is higher during warm
periods and lower during cold periods. There is no setting.
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.
Condenser