Danfoss Cascade HFC, Cascade HC Application Manual
Application guide
AKS 32 R
AKS 32R
GD/DGS
AKS 11
AKS 2050
AKS 11
AK-PC 740/780
GD/DGS
AKS 11
AKS 32R
AKD 102
AKS 11
DCR
Pump
SGN
AKS 2050
AKS 11
EKC 313
EVR
ETS
DCR
AKD 102
AKD 102
AK-CC 450
AKS 11
AKVH
AK-CC 550 A
AKS 11
AK-PC 740/780
Danfoss
R64-2050.11
AKS 32R
AK-SC 255/
AK-SM 350/
AK-SM 720/
AKA 245
AKS 32R
AKS 11
EVRH
Cascade HC/HFC - CO2 system
How to control the system
www.danfoss.com/co2
2 Application guide RA8AB102 © Danfoss 10/2010 Cascade HC/HFC - CO2 system
General description
EKC 313
AK-CC 550
AK-CC 450
Cascade systems are typical not used in FR applications with traditional refrigerants. There are a
few reasons for this such as the need to maintain
two different refrigerants in one system; system
control strategy (especially that of a cascade heat
exchanger) is more complex. At the same time
using CO2 in cascade systems gives a number of
advantages:
• Eciency of the system is high even in the hot
climates
• Only a small amount of refrigerant is needed for
high temperature stage
• Temperature dierence for cascade heat exchanger is relatively low
• On the high side various refrigerants can be
used ex HC/HFC or NH3.
Ammonia/CO2 cascade systems have the highest
efficiency of all. If HFC is to be used at a high tem-
perature stage, R134a is a preferable option due
to its thermo dynamical properties and lower
(compared to R404A) GWP potential.
Temperatures and pressures
in cascade systems
Intermediate temperature in a cascade system is
selected based on the required temperature for
high temperature cases in a store which means
they can be cooled by CO2 directly. Intermediate
temperature can also be optimised for the highest energy efficiency if the system is used for low
temperature only.
Since a cascade system actually consists of two
different refrigeration systems which are interfaced but isolated at the cascade heat exchanger,
the design working pressure for each can be
different. CO2 design pressure is normally based
on the availability of components and is equal to
40-45 bar (corresponding to +5 - +10°C).
In order to prevent pressure from increasing
above the previously mentioned measurements,
standstill systems are recommended. Safety
valves should have the highest setting. Stand still
pressure can be achieved by raising the desing
pressure to 80-90 bar.
For example:
CO2 side
• System design working pressure (saturated suction temperature): 40 bar (+5°C)
• Safety valve settings: 36 bar (-10% MWP)
• System emergency relief setting: 34 bar (-1°C)
• CO2 discharge pressure setting: 30 bar (-5°C)
The higher the efficiency of the cascade heat
exchanger, the lower the difference between
the condensation temperature of CO2 and the
evaporating temperature of the refrigerant on
the high temperature side. As the temperature
difference on the cascade condenser increases,
the overall efficiency of the refrigeration system
decreases!
AK-PC 740/780
AK-PC 740/780
Cascade HC/HFC - CO2 system Application guide RA8AB102 © Danfoss 10/2010 3
Temperatures and pressures
in cascade systems (continued)
On systems with low temperatures of the discharge 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 desuper-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:
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 intermediate 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 pressure of the CO2 LT) where lower limit is defined
by efficiency and higher by system pressure
rating
Operating sequence of cascade systems
Injection into cascade heat
exchanger
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 compressor 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.
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.
The high temperature expansion valve (ETS) to
the cascade heat exchanger should begin simultaneously with the high temperature compressors. 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.
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 reasonable pressure drop at part load, the oil transport
and distribution should work under most conditions.
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.
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