Carbon dioxide has high density and low viscosity. Its flow loss is small, the heat transfer effect is good. And by strengthening the heat transfer function, it can make up the shortcoming of its low circulation. At the same time, the carbon dioxide has good performance in environment, low cost, easy to obtain, good stability, and good for reducing the volume of the device. Most importantly, it is safe, non-toxic and non-flammable, which has a clear advantage over the R290.
Of course, using carbon dioxide as a refrigerant has its drawbacks. The high critical pressure of carbon dioxide and the low critical temperature also bring many difficulties to it. Whether subcritical cycle or cross-critical circulation, the operating pressure of carbon dioxide refrigeration system will be higher than that of traditional refrigeration and air conditioning system. This will inevitably bring many new requirements to the design of the system and parts. At the same time, there is a relatively low efficiency of carbon dioxide refrigeration system.
Currently, the research and application of carbon dioxide mainly focus on three aspects:
On the one hand is in the field of automobile air conditioning. Due to the large amount of refrigerant emissions, the environmental harm is also great. So the refrigerant with no harm to the environment must be adopted as soon as possible.
The second aspect is the heat pump water heater. Carbon dioxide has a fairly large temperature slip in the supercritical condition. It helps to heat the hot water to a higher temperature.
The third aspect is to consider the good flow performance in low temperature and heat transfer characteristics of carbon dioxide, using it as a cryogenic refrigerant for the cascade refrigeration cycle.
In the complex refrigeration system, the carbon dioxide cycle is operated under subcritical conditions. At this time, carbon dioxide is used as a low-pressure grade refrigerant, and NH3 is used as a high-pressure stage refrigerant. Compared with other low-pressure refrigerants, even at low temperature, the viscosity of carbon dioxide is very small and the heat transfer performance is good. Because of latent heat, its refrigeration capacity is quite large.
At present, the European supermarket has already established several kinds of carbon dioxide cryogenic refrigerants. The operation shows that it is technically feasible. The system is also suitable for low temperature freeze-drying process.
Currently, there are two main technical solutions for the substitution of R22 refrigerants:
One is represented by the Nordic countries and South Korea. It advocated the use of natural engineering as a substitute, such as pure industrial R290, R1270, R744, R600a, R600, R717, etc., and a mixture of HCs classes;
The other is represented by the United States and Japan. It advocates HFCs as a substitute. For example, the Non-azeotropic mixture R410A from United signal corporation, mixture R407C from Dupont and I.C.I., R32 and R152a. The ODP of these refrigerants is 0, which can protect the ozone layer, but it will produce greenhouse effect.