Cascade HC/HFC - CO2 system
Application guide RA8AB102 © Danfoss 10/2010
3
Temperatures and pressures
in cascade systems (contin-
ued)
On systems with low temperatures of the dis-
charge CO2 gas (low superheat), the superheat
of the expansion valve can be the dimensioning
factor for the heat exchanger.
If a CO2 system has high superheat, then desu-
per-heaters need to be used in order to reduce
the load on the high temperature side.
Optimal intermediate pressure in CO2 cascade
systems depends on a number of parameters
(high temperature refrigerant, load pattern etc.).
Generally 2 cases need to be considered:
Operating sequence of cas-
cade systems
1) Systems with load at the medium temperature.
In this case intermediate pressure should be as
high as possible in order to reduce the load at
the high temperature stage. The limitations are
therefore required temperature on the intermedi-
ate level and pressure rating of the system.
2) Systems without load at medium temperature.
In this case the intermediate temperature should
be in the range of -10 - 0°C (due to the high pres-
sure of the CO2 LT) where lower limit is defined
by efficiency and higher by system pressure
rating
In Cascade Systems, it is essential that at least
one compressor in the high temperature side is
running before the first compressor in the low
temperature side can start. Otherwise, the com-
pressor in the low temperature side may be cut
out due to high pressure.
The same sequence is also valid for filling up the
system. First of all, the high temperature circuit
needs to be filled with refrigerant and started up.
When this is done, the CO2 can be filled into the
low temperature system.
The high temperature expansion valve (ETS) to
the cascade heat exchanger should begin simul-
taneously with the high temperature compres-
sors. After this, the valve controls the superheat
of the high temperature gas. LT compressors are
then started up by the CO2 pressure increase on
the suction line.
Danfoss pack controllers such as AK-PC 740 and
AK-PC 780 are specially designed with built in
control functions to coordinate these operations.
Injection into cascade heat
exchanger
Injecting liquid into a plate heat exchanger is
not a trivial matter. The heat exchanger is often
compact and therefore the time constant is very
low. AKV valves are not recommended for this
application.
It is recommended to use motor valves or other
valves that give constant flow. Desuper-heating
of CO2 gas entering the cascade heat exchanger
can also be recommended for three reasons.
One reason is that the gas is often 60°C and
therefore the heat can be rejected to the ambient
or used for heat recovery without problems. The
second reason is to reduce thermal stress in the
heat exchanger. The third reason is that the CO2
gas gives very high heat fluxes which therefore
create unstable conditions on the evaporation
side. Therefore it is recommended to reduce the
superheat on the CO2 side.
Distribution on the CO2 side is also a critical
issue. This is why the heat exchanger has to be
designed for direct expansion to make sure the
mixture of gas and liquid is evenly distributed to
the heat exchanger.
When the heat exchanger is designed for reason-
able pressure drop at part load, the oil transport
and distribution should work under most condi-
tions.