Cascade HC/HFC - CO2 system
Application guide RA8AB102 © Danfoss 10/2010
5
Controls of Cascade system
Control of cascade systems can be divided into:
• Condenser capacity control
• Compressor capacity control
• Cascade injection control
• MT evaporator CO2 flow control
• LT evaporator injection control
Condenser capacity control
Capacity control of the condenser can be ac-
complished via step regulation or speed control
of the fans.
As regulating sensor for the capacity distribu-
tor the condenser pressure should be selected.
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. The reference for the condensing
pressure is set in °C (°F).
Compressor capacity
controls
Pack controller AK-PC 740 (up to 4 compressors)
or AK-PC 780 (up to 8 compressors) controls the
LT suction pressure and is a standard controller
for controlling one suction group in any refrigera-
tion system. The controller is capable of regulat-
ing variable speed of two compressors combined
with one-step compressors of the same or differ-
ent sizes, depending on the choice of coupling
pattern.
A unique feature in the AK-PC 740/780 make it
possible to use the Pc pressure in the LT CO2 side
as control sensor for the HT suction pressure. This
to ensure fast and stable control of the condens-
ing pressure of LT CO2 side.
Low-pressure /
high-pressure coordination
The AK-PC 740/780 is also able to coordinate the
LT and HT start to ensure a smooth operation.
Here the high-pressure compressors can start
either as a result of:
- Load on the high-pressure circuit
- Requirements from the low-pressure circuit
The high-pressure circuit will still ensure that the
low-pressure circuit is only permitted to start
when at least one high-pressure compressor has
started. It will also ensure that security timers and
compressor timers are complied with.
Oil Management /
Oil equalisation
The build in oil management system covers most
systems found on the market to day. Can be used
with CO2 as well as all other conventional refrig-
erants and support input signals from :
• Level switch on compressor
• Level switch on oil separator
• Level switch on oil receiver
• Pressure transmitter on oil receiver
Oil supply to the compressors is managed by
activating solenoid valves with user defined ON/
Off pulse sequences.
HT comp. release
output
LT comp. release.
input
HT comp. request
input
LT comp. request
output
Here both a relay output and an On/off input are used on both control-
lers.
C
ap
. r
eqquest
C
ap
.release