The two documents of the Montreal Protocol and the Kyoto Protocol on the Ozone Depleting Substances Convention clearly indicate the timeliness requirements for the replacement and phase-out of CFCs and CFC-containing HCFCs. In June 1991, China joined the London Amendment to the Montreal Protocol and gradually carried out specific work on the phase-out of CFCs refrigerants. Since the establishment of the Montreal Protocol, the international refrigeration industry has actively carried out research on CFCs and alternative substances for HCFCs, but most of them are limited to the air-conditioning and commercial refrigeration fields, and the CFCs low-temperature refrigerants with the evaporating temperature of -10 0 - 60 superimposed low-temperature watersheds. There are fewer reports of alternative substance application studies.
In recent years, with the rapid development of China's economy, cascaded cryogenic units with an evaporation temperature of -100 - 60 have been widely used, but most of the domestic cascaded cryogenic units still use CFCs such as R13. Therefore, the cascade of cryogenic refrigeration units of new HFCs refrigerants that replace CFCs such as R13 is becoming well known and understood. This article will discuss the performance of various aspects of cryogenic refrigerants used in two-stage cascade cryogenic refrigeration systems.
1Multi-layer low-temperature refrigeration system is composed of two single-stage refrigeration compression cycles. The high-temperature refrigeration cycle uses medium-temperature refrigerants such as R22, R404A and R507A, while the low-temperature stage uses low-temperature refrigerants such as R13 and R23. The condensation of the evaporative and cryogenic refrigerants is connected by an evaporative condenser. In order to prevent the refrigerant vaporization pressure of the low-temperature refrigeration system from being too high due to an increase in ambient temperature after shutdown, an expansion vessel is usually connected in parallel in the low-temperature suction line.
2 HFCs low temperature refrigerant performance analysis 2.1 The physical and chemical properties of low temperature refrigerants are generally expected to be suitable for a certain type of vapor compression refrigeration cycle refrigerant has good thermodynamic properties, good heat transfer and flow properties, good physical Chemical properties, good compatibility with lubricating oil, environmental friendliness, non-toxicity, low flammability and good electrical insulation properties, etc., in addition, people also require refrigerants with good economics and easy to obtain. Based on the above factors, the low temperature refrigerants and their physical and chemical properties of the cascaded cryogenic refrigeration system with evaporation temperature of -100 - 60 are listed.
R13 is a refrigerant for CFCs, R503 is an azeotropic refrigerant containing CFCs, R23 is a refrigerant for HFCs, R508B is an azeotropic refrigerant mixture of HFCs consisting of R46 and R116 with a mass percentage of 46,54, while R170 is HCs refrigerant.
Therefore, from an environmentally friendly point of view, R13 and R503 are refrigerants that are eliminated and banned, while R23, R508B and R170 are refrigerants that can be used for a long time. Several of the listed refrigerants are non-toxic, and all refrigerants other than R170 are non-flammable, so the application of R170 is limited in terms of flammability. From the perspective of convenience and economy of refrigerant acquisition, the R170 has advantages over several other refrigerations. R170, although highly flammable, has low economics, excellent thermodynamic properties at low temperatures, and good interfacial properties with low-temperature lubricants, making it widely used in large industrial refrigeration systems. For small and medium-sized cascade cryogenic refrigeration systems, R23 and R508B are usually used as low-temperature refrigerants, taking into account the overall economics of the refrigeration system. This article focuses on R23 and R508B cryogenic refrigerants for small to medium sized cascade cryogenic refrigeration systems.
2. 2 Impact of R23 and R508B on system performance
For several low temperature refrigerants at evaporating temperature - 84 4, condensing temperature - 35, undercooling 5 6 , suction temperature - 17 8 , compressor clearance volume 4%, isentropic compression index 0 7 theoretical performance The calculations were performed and the results are as shown. Considering the economic aspects of refrigeration system cooling capacity and energy efficiency ratio, R503 is the best performance low temperature refrigerant, but R503 is a banned refrigerant with poor environmental friendliness. For R23 and R508B, the performance of R508B is better than that of R23. It is especially important that the exhaust temperature of R508B is much lower than that of R23 under the same working conditions and is allowed in the compressor. Safe operation of the exhaust gas temperature range is very beneficial in terms of ensuring the lubricating properties of the lubricant and prolonging the compression life.
Therefore, considering the effects of different refrigerants on the refrigeration capacity and energy efficiency ratio of the refrigeration system, combined with its influence on the operating life of the compressor and its impact on the environment, R508B is most suitable as an azeotropic refrigerant of HFCs type. Low-temperature refrigerant for small and medium-sized cascade cryogenic refrigeration systems.
2. 3 R23 and R508B heat transfer performance is well known, the thermal conductivity of the refrigerant, viscosity, latent heat of vaporization, specific pressure specific heat capacity and liquid density directly affect its boiling evaporation and condensation heat transfer performance. The characteristics of low-temperature refrigerants and their heat transfer performance at low temperatures are rarely mentioned in public reports. The trend of the influence of low temperature refrigerant on its heat transfer performance with the decrease of temperature is analyzed qualitatively. The heat transfer coefficient of R23 under forced convection boiling and membrane condensation was analyzed. It can be seen that as the temperature decreases, the heat transfer coefficient of the low-temperature refrigerant also decreases.
It is precisely because people know less about the heat transfer performance of low-temperature refrigerants, so it has certain influence on the design and selection of low-temperature heat exchangers. It is recommended that system designers should compare and analyze the low-temperature heat exchangers and correct them with experimental data. In order to achieve low cost heat exchangers to achieve economic and reasonable.
2. 4 Selection of R23 and R508B Lubricants The two-stage cascade cryogenic refrigeration system has an evaporation temperature of -100 - 60. Therefore, in addition to the characteristics of general refrigerant lubricants, low-temperature lubricants must have a sufficiently low pour point. The temperature is such that the lubricating oil can return to the compression from the suction side of the evaporator or the like, so that the refrigeration system can operate normally.
R13 and R503 refrigerants are usually formulated with mineral oil, such as domestic N46 refrigeration oil, but the mineral lubricating oil is easily separated from the refrigerant at low temperature and condensed and dewaxed, blocking the throttle mechanism of the refrigerator and accumulating in the evaporator to affect heat transfer. And the system is in normal operation. It is recommended to add substances such as dehydrated pentane to the refrigeration system when necessary to prevent dewaxing of the lubricating oil. It should be noted that if a substance such as dehydrated pentane is added, it is necessary to control the proportion so as not to change the flammability of the refrigerant.
R23 forced convection boiling heat transfer coefficient and membrane condensation heat transfer coefficient with temperature change trend R23 and R508B are HFCs refrigerant, can be used with POE ester lubricants, lubricants suitable for R23 are also applicable to R508B, and low temperature The lubricating oil has a better solubility in R508B than R23. According to the recommendations of each compressor manufacturer and the unit of the author, POE ester lubricants with ISO viscosity grades of 22 and 32 can be used as lubricants for R23 and R508B refrigerants, and some lubricants recommended as R23 and R508B are listed. Related performance. It should be noted that when the listed lubricants are used in different compressors, it should also be consulted whether the compression manufacturer needs to add appropriate amounts of additives.
POE ester lubricating oils do not have the problem of coagulation dewaxing due to their inherent characteristics. However, the presence of lubricating oil in the evaporator also affects heat exchange. Therefore, a high-efficiency oil separator is required in a low-temperature refrigeration system. Only if the exhaust gas does not exceed 5 ppm to ensure that the lubricant is not concentrated in the evaporator, resulting in reduced system performance or even compressor failure.
3 R23 and R508B in system design When R23 and R508B are used to cascade cryogenic refrigeration systems, in addition to some of the problems mentioned above, the following issues should be noted in the system design.
3. 1 R23 refrigeration system charge The same temperature and pressure R23 gas and liquid specific volume ratio R13 and R503 are larger, so the R23 cascade cryogenic refrigeration system needs to re-check the volume of the expansion vessel. literature
It is pointed out that the refrigerant charging capacity of R23 refrigeration system is about 10 20% lower than that of R13 or R503 system. The measured data of the refrigerant shows that the refrigerant charge can be reduced by about 30% compared with R13. Therefore, the author suggests to optimize the refrigerant charge of the system through experimental tests, so that the low temperature system is easier to start and has good performance.
3. 2 Compressors for R23 and R508B Although cascaded cryogenic refrigeration systems have been widely used, few compressor manufacturers have announced the performance parameters of their compressors for R23, R508B and other refrigerants, and there is no special R23. Compressors developed by refrigerants such as R508B. Most of the compressors used in the market for cascading cryogenic refrigeration systems are ordinary compressors. Therefore, when selecting compressors, the following principles can be followed: 3 2 1 Select low-temperature piston and screw compressors designed for industrial and commercial refrigeration. 3 2 2 In order to improve the system energy efficiency ratio, it is recommended to use a low-power motor compressor under the same exhaust volume; the operating pressure range of the 3 2 3 compressor design should be in the design operating pressure range of the low-temperature stage of the cascaded cryogenic refrigeration system. Adapt to avoid safety accidents; 3 2 4 The material of the selected compressor should be compatible with the selected refrigerant and lubricating oil to avoid problems such as material corrosion. If necessary, obtain relevant information from the compressor manufacturer; 2 5 Pay attention to the application range of the compressor, paying particular attention to the minimum suction temperature and the maximum exhaust temperature limit of the compressor. In the design of the low temperature system, it must be taken into consideration and satisfied to avoid compressor failure; 3 2 6 When a screw compressor is selected, a compressor with a suitable internal volume ratio can be selected according to the designed operating range to fully improve the system efficiency.
4 Conclusions This paper discusses the effects of HFCs low temperature refrigerants R23 and R508B replacing R13 and R503 on the performance of refrigeration systems, and points out that R508B is a very good alternative refrigerant for HFCs.
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