Carbon dioxide (also called CO2 or R744) is a substance in nature and also a very good refrigerant. In fact, it has been used as a refrigerant for a hundred years.
1. History of CO2 as a Refrigerant
From the end of the 19th century to the 1930s, CO2 (R744), ammonia (R717), SO2 (R764), methyl chloride (R40), etc. were widely used as refrigerants.
In 1931, once CFCs refrigerants represented by R12 were developed, they quickly replaced CO2 in terms of safe refrigerants with their non-toxic, non-flammable, non-explosive, non-irritating characteristics, moderate pressure and high refrigeration efficiency. CO2 was gradually no longer used as a refrigerant.
CO2 Refrigerant Is Valued Again
Due to the adverse effects of CFCs on the ozone layer and global warming, protecting the environment and finding substitutes for them have become issues of common concern to the world. New alternative working fluids must be harmless to the ozone layer. HFC-type working fluids have become important substitutes because they have no depletion on the ozone layer. In particular, HFC134a has been mass-produced and applied as a substitute for CFC12.
Although HFCs do not destroy the ozone layer, they are chemically stable and can accumulate after release, which ultimately leads to a significant greenhouse effect. Although people can work hard to synthesize better working fluids, due to the large amount of refrigerants used, a considerable part will inevitably leak into the atmosphere. Any large amount of artificial synthetic substances discharged into nature will have an impact on the environment, so now a common view is to use natural working fluids.
Former President of the International Institute of Refrigeration, G Lorentzen, from Norway vigorously advocated the use of natural working fluids from 1989 to 1994. In particular, he had played a good leading role in the research, promotion and application of CO2. Since then, the research and application of CO2 refrigeration devices has once again become a hot topic that is valued worldwide.
2. Advantages and Disadvantages of Carbon Dioxide Refrigerant
2.1 Advantages of Carbon Dioxide Refrigerant
● Carbon dioxide is a natural substance (ODP=0, GWP=1). Using it as a refrigerant has no destructive effect on the atmospheric ozone layer, and can reduce the global greenhouse effect. It has a wide source, low price and good economic efficiency, can greatly reduce the cost of refrigerant replacement, save energy, and solve the problem of environmental pollution caused by chemical compounds.
● Carbon dioxide is safe, non-toxic, non-flammable, non-explosive, and has good thermal stability. It will not decompose harmful gases even at high temperatures, and its leakage will not harm the human body, food, and ecology.
● Carbon dioxide has thermal properties that are compatible with refrigeration cycles and equipment. It has a small molecular weight and a large refrigeration capacity. The unit refrigeration capacity at 0°C is 5 to 8 times higher than that of conventional refrigerants. Therefore, for refrigeration systems with the same cold load, the size of the compressor can be significantly reduced, and the entire system is very compact for the reduced weight. Lubrication conditions are easy to meet, and there is no corrosion to common materials in the refrigeration system. It can improve the sealing performance of open compressors and reduce leakage.
● Carbon dioxide has a low viscosity. The kinematic viscosity of saturated liquid carbon dioxide at 0°C is only 5.2% of NH3 and 23.8% of R12. The flow resistance of the fluid is small, and the heat transfer performance is better than that of CFC refrigerants, which can improve the heat dissipation of hermetic refrigeration compressors.
2.2 Disadvantages of Carbon Dioxide Refrigerant
● It cannot sustain life. If the concentration is too high, it will cause damage to human respiratory organs and even suffocation and death.
● It has a high critical pressure and a low critical temperature. The critical temperature of CO2 is Tc=31.1℃ and the critical pressure is Pc=7.3MPa while the critical temperature of water is 374℃ and the critical pressure is 22MPa.
● Whether it is a subcritical cycle or a transcritical cycle, the operating pressure of the CO2 refrigeration system will be higher than that of the traditional refrigeration and air-conditioning system, which brings many difficulties to the design of the system and components, and the manufacturing cost is relatively high.
3. Application of Carbon Dioxide in Refrigeration Industry
The research and application of carbon dioxide are mainly concentrated in three aspects:
● One of them is the field of automotive air conditioning, in which the amount of refrigerant emissions is large and the harm to the environment is also great.
● The second is the field of heat pumps. There is a considerable temperature slip when carbon dioxide releases heat under supercritical conditions, which is conducive to heating water to a higher temperature (above 90℃) at extremely low ambient temperatures.
● The third is the field of cascade refrigeration cycle. Carbon dioxide has good low-temperature flow performance and heat transfer characteristics, and therefore it is used as a low-temperature refrigerant in the CO2/NH3 cascade refrigeration cycle.
As mentioned earlier, excessive carbon dioxide concentration can cause damage to human respiratory organs and even suffocation and death, so CO2 leakage monitoring is required. The relevant requirements are as follows:
If the carbon dioxide concentration in somewhere exceeds the limit, only trained personnel are allowed to enter;
The alarm apparatus must emit sound and light alarms at the same time, and cover both indoors and outdoors;
The ventilation system and the alarm apparatus must not use the same power supply;
The alarm apparatus must be able to start ventilation system, and preferably also can start the shut-off valve;
The vent and detector must be installed at the possibly lower position;
The detector must monitor the CO2 concentration, not the hypoxic conditions.
3.1 Application of Carbon Dioxide in Cascade Refrigeration System
The CO2/NH3 cascade refrigeration cycle is composed of the NH3 high-temperature refrigeration cycle and the CO2 low-temperature refrigeration cycle. The two independent refrigeration systems are coupled through the evaporator-condenser (intermediate heat exchanger). The new system can not only meet the appropriate evaporation temperature when evaporating at a lower evaporation temperature, but also meet the moderate condensation pressure when condensing at ambient temperature.
In the figure: 1-2-3-4-1 is the low-temperature CO2 refrigeration cycle with throttle valve; 5-6-7-8-5 is the high-temperature refrigeration cycle with throttle valve.
Because the heat release process of the low-temperature stage (process 2~3 in the figure) and the heat absorption process of the high-temperature stage (process 5~8 in the figure) occur in the evaporator-condenser, and there is no heat exchange with the environment. Therefore, it can be considered that the evaporation load of the high-temperature stage cycle is equal to the condensation load of the low-temperature stage cycle, and the heat transfer temperature difference in the evaporator-condenser is generally 5~8 ℃.
