Information dynamics
Carbon dioxide has the advantages of high density and low viscosity, low flow loss and good heat transfer effect. By strengthening heat transfer, it can make up for the shortcomings of low circulation. At the same time, carbon dioxide environment performance is good, low cost and easy to obtain, good stability, conducive to reducing device volume. Most importantly, it is safe, non-toxic and non-flammable, which has obvious advantages over R290.
Of course, the use of carbon dioxide as refrigerant also has shortcomings. The high critical pressure and low critical temperature of carbon dioxide also bring many difficulties for it to be used as refrigerant. Whether subcritical or transcritical, the operating pressure of carbon dioxide refrigeration system will be higher than that of traditional refrigeration and air conditioning system, which will inevitably bring many new requirements to the design of system and components. At the same time, the efficiency of carbon dioxide refrigeration system is relatively low.
At present, the research and application of carbon dioxide mainly focus on three aspects:
On the one hand, in the field of automotive air conditioning, because of the large amount of refrigerant emissions and the great harm to the environment, it is necessary to adopt refrigerants that are not harmful to the environment as soon as possible.
The second aspect is heat pump water heater. There is a considerable temperature slip in the supercritical heat release of carbon dioxide, which is conducive to heating hot water to a higher temperature.
The third aspect is to use carbon dioxide as low-temperature refrigerant in cascade refrigeration cycle considering its good low-temperature fluidity and heat transfer characteristics.
In cascade refrigeration system, carbon dioxide cycle operates under subcritical conditions. Carbon dioxide is used as low pressure refrigerant and NH3 as high pressure refrigerant. Compared with other low-pressure refrigerants, even at low temperatures, the viscosity of carbon dioxide is very small and the heat transfer performance is good, because its refrigeration capacity is quite large by using latent heat.
At present, several cascade refrigeration systems using carbon dioxide as cryogenic refrigerant have been established in supermarkets in Europe. The operation shows that the system is technically feasible and suitable for cryogenic freeze-drying process.
At present, there are two main technical schemes for the replacement of R22 refrigerant in the world:
One is represented by Nordic countries and South Korea, which advocates the use of natural refrigerants as substitutes, such as pure refrigerants R290, R1270, R744, R600a, R600, R717, and mixtures of HCs.
The other is the use of HFCs as substitutes represented by the United States and Japan, such as the non-azeotropic mixture R410A of United Signal Company, the mixture R407C of DuPont Company and I.C.I Company, and the ODP of R32 and R152a. These refrigerants are all 0, which can protect the ozone layer, but will have a greenhouse effect.
At present, the prospect of carbon dioxide in the domestic market is somewhat like "looking at flowers in the fog", as Wang Liqun said, they all know its good, but really use less. Domestic air-conditioning industry can not see the development of carbon dioxide for the time being, and it has just taken a step in the domestic refrigeration market. However, in the field of heat pump water heater, the core fittings standard of carbon dioxide heat pump water heater, GB/T 26181-2010, will be promulgated in China. Le Hongsheng, the Minister of the heat pump propulsion Office of Shanghai Hitachi Electrical Appliances Co., Ltd., who participated in the formulation of the standards, said that although the heat pump water heater using carbon dioxide refrigerant has not yet come out in China and the compressor of the core component is still in the research and development stage, "the formulation of this advanced standard will play a good role in promoting the market of the products."
Properties of CO2 Refrigerant
(1) Main advantages of CO2 refrigerants
1. CO2 is a natural substance, ODP = 0, GWP = 1. The use of CO2 as refrigerant has no destructive effect on the atmospheric ozone layer. It can reduce the global greenhouse effect. It has a wide range of sources and does not need to be recycled. It can greatly reduce the cost of refrigerant substitution, save energy, fundamentally solve the pollution problem of compounds to the environment, and has a good economy.
2. CO2 is safe, non-toxic, non-flammable, and has good thermal stability. Even at high temperature, it will not decompose harmful gases. In case the leak is harmless to human body, food and ecology.
3. CO2 has thermophysical properties suitable for refrigeration cycle and equipment. Molecular weight is small, refrigeration capacity is large, refrigeration capacity per unit at 0 C is 5-8 times higher than that of conventional refrigerants, so for refrigeration systems with the same cooling load, the size of the compressor can be significantly reduced, the weight of the compressor can be reduced, and the whole system is very compact; lubrication conditions are easy to meet, and there is no corrosion to the common materials of the refrigeration system, which can improve the sealing performance of the open compressor and reduce leakage.
4. The viscosity of CO2 saturated liquids is small. The kinetic viscosity of CO2 saturated liquids is only 5.2% of NH3 and 23.8% of R1 2 at 0 C. The flow resistance of fluids is small and the heat transfer performance is better than that of CFC refrigerants, which can improve the heat dissipation of fully enclosed refrigeration compressors.
Of course, the use of carbon dioxide as refrigerant also has shortcomings. The high critical pressure and low critical temperature of carbon dioxide also bring many difficulties for it to be used as refrigerant. Whether subcritical or transcritical, the operating pressure of carbon dioxide refrigeration system will be higher than that of traditional refrigeration and air conditioning system, which will inevitably bring many new requirements to the design of system and components. At the same time, the efficiency of carbon dioxide refrigeration system is relatively low.
At present, the research and application of carbon dioxide mainly focus on three aspects:
On the one hand, in the field of automotive air conditioning, because of the large amount of refrigerant emissions and the great harm to the environment, it is necessary to adopt refrigerants that are not harmful to the environment as soon as possible.
The second aspect is heat pump water heater. There is a considerable temperature slip in the supercritical heat release of carbon dioxide, which is conducive to heating hot water to a higher temperature.
The third aspect is to use carbon dioxide as low-temperature refrigerant in cascade refrigeration cycle considering its good low-temperature fluidity and heat transfer characteristics.
In cascade refrigeration system, carbon dioxide cycle operates under subcritical conditions. Carbon dioxide is used as low pressure refrigerant and NH3 as high pressure refrigerant. Compared with other low-pressure refrigerants, even at low temperatures, the viscosity of carbon dioxide is very small and the heat transfer performance is good, because its refrigeration capacity is quite large by using latent heat.
At present, several cascade refrigeration systems using carbon dioxide as cryogenic refrigerant have been established in supermarkets in Europe. The operation shows that the system is technically feasible and suitable for cryogenic freeze-drying process.
At present, there are two main technical schemes for the replacement of R22 refrigerant in the world:
One is represented by Nordic countries and South Korea, which advocates the use of natural refrigerants as substitutes, such as pure refrigerants R290, R1270, R744, R600a, R600, R717, and mixtures of HCs.
The other is the use of HFCs as substitutes represented by the United States and Japan, such as the non-azeotropic mixture R410A of United Signal Company, the mixture R407C of DuPont Company and I.C.I Company, and the ODP of R32 and R152a. These refrigerants are all 0, which can protect the ozone layer, but will have a greenhouse effect.
At present, the prospect of carbon dioxide in the domestic market is somewhat like "looking at flowers in the fog", as Wang Liqun said, they all know its good, but really use less. Domestic air-conditioning industry can not see the development of carbon dioxide for the time being, and it has just taken a step in the domestic refrigeration market. However, in the field of heat pump water heater, the core fittings standard of carbon dioxide heat pump water heater, GB/T 26181-2010, will be promulgated in China. Le Hongsheng, the Minister of the heat pump propulsion Office of Shanghai Hitachi Electrical Appliances Co., Ltd., who participated in the formulation of the standards, said that although the heat pump water heater using carbon dioxide refrigerant has not yet come out in China and the compressor of the core component is still in the research and development stage, "the formulation of this advanced standard will play a good role in promoting the market of the products."
Properties of CO2 Refrigerant
(1) Main advantages of CO2 refrigerants
1. CO2 is a natural substance, ODP = 0, GWP = 1. The use of CO2 as refrigerant has no destructive effect on the atmospheric ozone layer. It can reduce the global greenhouse effect. It has a wide range of sources and does not need to be recycled. It can greatly reduce the cost of refrigerant substitution, save energy, fundamentally solve the pollution problem of compounds to the environment, and has a good economy.
2. CO2 is safe, non-toxic, non-flammable, and has good thermal stability. Even at high temperature, it will not decompose harmful gases. In case the leak is harmless to human body, food and ecology.
3. CO2 has thermophysical properties suitable for refrigeration cycle and equipment. Molecular weight is small, refrigeration capacity is large, refrigeration capacity per unit at 0 C is 5-8 times higher than that of conventional refrigerants, so for refrigeration systems with the same cooling load, the size of the compressor can be significantly reduced, the weight of the compressor can be reduced, and the whole system is very compact; lubrication conditions are easy to meet, and there is no corrosion to the common materials of the refrigeration system, which can improve the sealing performance of the open compressor and reduce leakage.
4. The viscosity of CO2 saturated liquids is small. The kinetic viscosity of CO2 saturated liquids is only 5.2% of NH3 and 23.8% of R1 2 at 0 C. The flow resistance of fluids is small and the heat transfer performance is better than that of CFC refrigerants, which can improve the heat dissipation of fully enclosed refrigeration compressors.
