The influence of lubricating oil on the evaporation process of mixed refrigerant

Comparison of evaporative heat transfer performance between R502 and its substitutes R502 is mainly used for quick freezing of refrigerators and food. The currently developed substitutes are all mixed refrigerants, such as R507, R404a, etc. SMSami et al. conducted a series of experimental studies on R502 and some of its substitutes R507, R404a, R408a, and the quaternary mixture R32/R125/R143a/R134a (40/25/25/10wt%), using grooved tubes. The refrigerant contains about 1% oil, and the mass flow rate is lower than 110kg/m2s. They found that the evaporative heat transfer coefficients of the refrigerants from high to low are: R408a, R502, quaternary mixture, R404a, R507, and the pressure drop from high to low are: R507, R404a, R408a, R502, the order of the quaternary mixture The pressure drop is between R502 and R507. z*Near SMSami, etc. also compared the evaporation process of R502 and its substitutes R404a, R408a, R507, R407b in a four-row finned tube (with micro-fin strengthening) type heat exchanger that exchanges heat with air 18 It is found that when the mass flow rate is the same, the evaporation heat transfer coefficient from high to low is: R502, R408a, R507 and R407b, R404a is between R502 and R407b, in terms of pressure drop, R407b>R502>R404a>R408a. When the mass flow rate is lower than 130kg/m2s, it is higher than the above four refrigerants, when the mass flow rate is higher than 130kg/m2s, it is lower than R407b, but still higher than the other three refrigerants. NKattan et al. studied the two near-azeotropic refrigerants of R502, R404a and R402a, and found that the evaporation heat transfer coefficient of R404a is slightly higher than that of R502, and that of R402a is slightly lower than that of R502.　　　　The influence of lubricating oil on the evaporation process In actual refrigeration systems, unlike most of the studies mentioned above, the circulating refrigerant always contains a certain proportion of lubricating oil, and the presence of lubricating oil will change the evaporation process. Lubricating oil can be regarded as a high boiling point component. After being added to the refrigerant, the boiling point of the mixture will increase. According to calculations by OZrcher of JRhome, if 5% lubricating oil is added to R134a and evaporating at a pressure of 0.293MPa (corresponding to the evaporation pressure of pure R134a at 0°C), the evaporation temperature will increase significantly after the dryness is greater than 0.8. When the dryness is 0.93, the evaporation temperature will increase by 8.015°C. Lubricating oil will also affect the pressure drop in the evaporation process, because the viscosity of the refrigerant/lubricating oil mixture is different from that of pure refrigerant.　　　　 In order to find out the influence of lubricating oil on the heat transfer of R134a, SJEckels et al. conducted a series of experimental studies. When using lubricating oil of model 169-SUS, in the smooth pipes with diameters of 9.52mm and 12.7mm, as the lubricating oil content increases (0～5%), the heat transfer coefficient first slightly increases (0～12%). ), and then decrease (15% to 30%); if it is a micro-finned tube, low concentration has no effect on heat transfer, and high concentration reduces the heat transfer coefficient. When using 150-SUS lubricating oil, the heat transfer coefficient in the smooth tube does not change at low concentrations, but it decreases at high concentrations, and the heat transfer coefficient in the micro-finned tube always decreases. When using lubricating oil of model 369-SUS, as the content of lubricating oil increases, the evaporation heat transfer coefficient in the two types of tubes will decrease. In terms of pressure drop, no matter what kind of lubricating oil is added, the pressure drop will increase as the content increases. For example, after adding 4.9% of lubricating oil of type 169-SUS, the pressure drop in the smooth tube will increase by 30%, slightly The pressure drop in the finned tube will increase by 10%.
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