Carbon dioxide (CO₂)

Eco-technologies for the refrigeration business

Physical and chemical properties

Chemical formula: CO₂
Relative molecular mass: 44
Gas constant: 189 J/kg·K
Boiling/sublimation point at 101.3 kPa: -78.4°C (this temperature corresponds to change in phase between solid and gaseous state)
Critical temperature: 31.1°C
Critical pressure (abs.): 73.6 bar
Solubility in water: up to 88 ml in 100 g H₂0 at 20°C

Impact on human beings and environment

Carbon dioxide is A1 safety group: nontoxic (low-toxic), inflammable. In high concentration, CO₂ is an asphyxiating gas. Carbon dioxide is heavier than air, so when leaked in closed premises it accumulates at the floor level.

According to GOST EN 378-1-2014, the oxygen threshold value (oxygen deficit limit) is 0.07 kg/m³, the practical limit for occupied spaces is 0.1 kg/m³.

The practical limit refers to the concentration of a refrigerant that does not harm a human being or require immediate evacuation in case of casual leakage and release of all the volume into the room. The practical limit is used to specify the maximum refrigerant charge of a specific refrigeration plant.

Another risk consideration for CO₂ is high working pressure, so to prevent accidents, safety valves and pressure switches are used.

Carbon dioxide’s ODP is zero, and its impact on climate is taken as a unit of global-warming potential (GWP). GWP of many traditional refrigerants—CFC, HCFC, HFC—is hundred and thousand times higher than that of CO₂.

Production of carbon dioxide

In industry, carbon dioxide is generated from stack gases or as a by-product of various chemical processes, e.g. decomposition of natural carbonates or production of alcoholic drinks.

CO₂ can be a by-product of generating pure oxygen, nitrogen, and argon at air separation units.

Use of carbon dioxide

The refrigeration industry has been using carbon dioxide since the XIX century. According to the international nomenclature (R-numbering), CO₂ is referred to as R-744.

Carbon dioxide has high thermal conductivity, relatively low viscosity, low critical point, and high triple point. The gas density ensures high heat transfer with air. The pressure loss in the pipeline almost does not influence the efficiency of cooling due to high working pressure. Due to the high volumetric capacity, CO₂ systems are more compact.

Transcritical and subcritical refrigeration cycles

At atmospheric pressure, carbon dioxide exists only in a solid or gaseous phase: it is called dry ice at temperatures below -78.4°C, and at higher temperatures evaporates.

At the triple point—at 5.2 bar and -56.6°C—carbon dioxide has three phases at equilibrium.

The critical point is at 31.1°C at 73.8 bar, and with temperature rise, carbon dioxide turns into a gas (supercriticality).

CO₂ refrigeration cycle with a range of working temperatures and pressures below the critical point and above the triple point is a subcritical cycle. It is a standard refrigeration cycle of a vapor compression refrigerating machine where heat is transferred from a cooler body (medium) to a warmer one through the change in the refrigerant aggregate state accompanied by heat absorption or release.

CO₂ refrigeration cycle with heat abstraction at temperatures above the critical one (31.1°C) and without condensing is a transcritical cycle. Since no condensation takes place, the heat exchanger where the refrigerant releases heat in the environment is not a condenser but a gas cooler.

Transcritical refrigerating plants

Basic transcritical refrigerating plant

The basic transcritical plant consists of a compressor, gas cooler, evaporator, and expansion device.

The basic plants are not fitted with pressure controls; they operate at optimum high pressure and maximum output under constant conditions.

In more complex systems, a thermal valve regulates the cooling temperature, and a low-pressure receiver compensates the load swing at the high-pressure side. Heat exchange in such systems is arranged between the compressor suction line and the gas cooler discharge line.

Recently, many different CO₂ transcritical refrigeration units have been developed, including compact packaged screw compressors for fishing vessels.

Transcritical cycle can also be used in air-water heat pumps developed in Japan and consuming 68% less electrical energy than electrical heaters to heat water to 90°C. Air-water CO₂ heat pumps are environmentally friendly because they use low-GWP refrigerants, consume less energy, and heat water without burning fossil fuels.

Transcritical booster (two-stage) systems

Transcritical booster system for commercial use

Low-temperature systems used for, e.g. freezing, operate at the high discharge temperature that is reached by a two-stage (booster) compression where the refrigerant (CO₂) is discharged by the low-temperature compressor to the suction port of the medium-temperature compressor. To cool the gas between two compressors, throttling expansion of the liquid refrigerant from the high-temperature stage, and the gas released from the receiver through the pressure-control valve is applied.

Cascade refrigerating plants and secondary refrigerant systems

As separate plants, CO₂ refrigerating plants operating only in the subcritical cycle are not widespread; they are used as low-temperature stages of cascade systems. The heat released during condensation of carbon dioxide is absorbed by the evaporating refrigerant of the high-temperature circuit (usually ammonia). Examples of NH₃/CO₂ systems are described in a separate section.

There are systems with CO₂ used at two stages: the high-temperature stage operating in the subcritical cycle, and the low-temperature stage, can operate in the transcritical cycle if the ambient temperature is high.

Carbon dioxide can serve as a secondary refrigerant. It is cooled by a basic refrigeration machine using synthetic or natural (hydrocarbons, ammonia) refrigerants, and is pumped by a centrifugal pump. An example of a system with the ammonia chiller and carbon dioxide as a secondary refrigerant is described in a separate section.

As a secondary refrigerant, carbon dioxide has the advantage of requiring a pump of smaller capacity, and high volatility, so absorption of latent heat in the consumer’s heat exchanger results in partial evaporation of CO₂ already there.

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Carbon dioxide (CO₂)

Physical and chemical properties

Impact on human beings and environment

Production of carbon dioxide

Use of carbon dioxide

Transcritical and subcritical refrigeration cycles

Transcritical refrigerating plants

Transcritical booster (two-stage) systems

Cascade refrigerating plants and secondary refrigerant systems

Additional materials

Carbon dioxide (CO2)

Physical and chemical properties

Impact on human beings and environment

Production of carbon dioxide

Use of carbon dioxide

Transcritical and subcritical refrigeration cycles

Transcritical refrigerating plants

Transcritical booster (two-stage) systems

Cascade refrigerating plants and secondary refrigerant systems

Additional materials

Carbon dioxide (CO₂)