Suction Group; Controlling Sensor Selection - Danfoss AK-PC 781A User Manual

Suction group

Controlling sensor selection

Depending on use, the capacity distributor can regulate according
to the suction pressure P0, a media temperature S4 or separate
control pressure Pctrl in a different refrigeration circuit, e.g. cas-
cade system.
Cap. Ctrl sensor = P0 / S4 /Pctrl
Example 1 – P0
Example 2 – S4 media sensor
When the controlling sensor is selected as S4, P0 is used as a safety
function for low suction pressure and will ensure disconnection of
compressor capacity (frost protection).
Example 3 – Pctrl sensor
When Pctrl is used as controlling sensor, a refrigerant type for this
pressure transmitter must be set, e.g. CO2.
P0 is used as a safety function against insufficient suction pressure
and will ensure disconnection of compressor capacity.
On cascade systems the signal from Pctrl can be used by both the
high-pressure and low-pressure controls either for the controlling
sensor or high-pressure monitoring.
92 Capacity controller RS8HE202 © Danfoss 2017-07
Parallel compression
(Only on CO2 system and control of receiver pressure)
If the controller is to control an IT compressor for parallel compres-
sion, the following regulation settings will be required:
• The system type must be set to LT
• The regulation sensor must be set to Po (the signal received from
the receiver - Prec).
Handling of sensor error
Cap. Ctrl. Sensor = P0
When P0 is used as the regulating sensor, an error in the signal will
mean that regulation continues with 50% cutin in daily operation
and 25% cut-in at night, but for a minimum of one step.
Cap. Ctrl. Sensor = S4
Provided that S4 is used as a regulating sensor, an error in this sen-
sor will mean that regulation continues from the P0 signal, but in
accordance with a reference that lies 5K under the real reference.
If there is an error on both S4 and P0, regulation will continue with
50% cut-in in daily operations and 25% of cut-in in night opera-
tions, but for a minimum of one step.
Cap. Ctrl. Sensor = Pctrl
When Pctrl is used as a controlling sensor, an error in this sensor
will mean that regulation continues after the P0 signal, but in
accordance with a reference that lies 5 K under the real reference.
If there is an error on both Pctrl and P0, regulation will continue
with e.g. 50% cut-in in daily operations and e.g. 25% cut-in in
night operations, but for a minimum of one step.
AK-PC 781A