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You are here : Home > Services > Focus Products CARLYNEWS > Components for CO2 (R744)
Components for CO2 (R744)

Refrigeration components for CO2 (R744)

CO2 is a refrigerant that is more and more used because it is considered as an ecological refrigerant : its impact on the ozone layer is null (ODP=0) and its impact on the greenhouse effect is low (GWP=1).
Its thermodynamic properties are excellent and let imagine a good future for this refrigerant, even if its working pressures are much higher than those of HFC’s.

CO2 is a non inflammable refrigerant, non explosive and non dangerous. Its thermodynamic features in low temperatures enable to reduce the volume of refrigerant circuits and to lower the energy consumption. The critical temperature of 30,98°C limits its use.

In a refrigeration system with air condensation, the cycle can work above this critical temperature ; the cycle is called transcritical

In the case when CO2 is used for low temperature refrigeration, it is used either as a heat-conductor refrigerant, or in cascade with another refrigerant (R404A, NH3, R134a, etc.)
The cycle works then below the critical temperature ; it is called subcritical cycle.

The working pressures of CO2 are high, about 140 bar for transcritical cycles and between 40 and 52 bar for subcritical cycles.
The working pressures of the components must enable to cope with the pressure due to ambient temperature in case the refrigeration system stops.
For example, if the ambient temperature is about 32°C (Réf ENV 378-2), the pressure inside the refrigeration system will be more than 74 bar. 
The refrigeration systems working in subcritical conditions with CO2 can be maintained at the right temperature by using an additional refrigeration system or by rejecting the excess pressure towards a storage receiver. 

 

Refrigeration components

1) Transcritical cycle

The refrigeration systems working in transcritical cycle are mostly heat pumps, drink dispensers, and medium temperature refrigeration racks. 
Commercial refrigeration tends to use more and more cascade refrigeration racks working with CO2 in transcritical conditions for the medium temperature stage and in subcritical conditions for the low temperature stage.

The use of CO2 in transcritical cycle requires some products which working pressures are :

Position
in the circuit

Maximum 
working pressure
(bar)

Maximum
working temperature
(°C)

Products
concerned

Compressor discharge

140 bar

160°C

Mufflers, oil separators, valves

Outlet
« gaz cooler »

140 bar

60°C

Filter driers, liquid receivers, sight glasses, valves

Outlet evaporator

40 bar

30°C

Suction accumulators, suction filters, valves

Carbon steels, most currently used, are totally compatible with transcritical applications. Only material thicknesses have to be adapted in order to resist high temperatures and pressures. 
CARLY can offer on request a filter drier and a sight glass for transcritical applications.


2) Subcritical cycle

The installations working with a subcritical cycle are dedicated to low temperature commercial or industrial refrigeration (-25°C to -45°C). 
Contrary to synthetic refrigerants (CFC, HCFC HFC etc.), CO2 works with very high working pressures, especially in low temperature. 
Components located on the liquid line or on the suction line are subject to high pressures and very low temperatures.
Resilience of usual carbon steels does not permit to use them in total safety conditions with low temperatures and high pressures. 
It is then necessary to use either low temperature steels (LTCS), or stainless steel which resilience suits very well.

The use of CO2 in subcritical cycle requires some products which working pressures are :

Position
in the circuit

Maximum 
working pressure
 (bar)

Minimal working
temperature
(°C)

Maximal working
temperature
(°C)

Products
concerned

Compressor discharge

40 (52)*

0

150

Mufflers,
oil separators, vibration eliminators, valves

Liquid line

40 (52)*

-20

0

Filter driers, liquid receivers, sight glasses, dirt filters, valves

Suction line

28

-50

-20

Suction accumulators, suction filters, vibration eliminators

(*) Evolution of working pressures in order to take into account the increase in condensation temperatures.

CARLY offers a range of products compatible with CO2 subcritical applications (see table here under). 
The development of stainless steel products is currently in progress (Filter drier shells, filter driers, suction line accumulators) in order to meet the market demand.

Filter drier shell stainless steel - BCYFilter drier stainless steel - DCYSection line accumulator stainless steel - LCY

 

CARLY products compatible with subcritical CO2 applications :

CO2 products range

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