The compressor will be short of oil if the lubricating oil discharged during operation cannot return. There are two oil return modes for compressor, one is oil return from oil separator, and the other is oil return from air return pipe. The oil separator is installed on the exhaust pipeline of the compressor, which can generally separate 50-95% of the running oil. This oil return effect is very good and fast, greatly reducing the amount of oil entering the system pipeline, thus effectively extending the operation time without oil return. It is not uncommon that the refrigeration system of the cold storage with extremely long pipeline, the full liquid ice making system, and the freeze-drying equipment with very low temperature will not return oil for ten or even ten minutes after starting up, or the amount of oil returned will be very small. The system with unreasonable design will have the problem of compressor shutdown due to low oil pressure. The installation of high efficiency oil separator in this refrigeration system can greatly prolong the compressor's non oil return operation time, so that the compressor can safely pass the critical period of non oil return after startup.
The lubricating oil that has not been separated will enter the system and flow with the refrigerant coagulant in the pipe to form an oil circulation. After the lubricating oil enters the evaporator, on the one hand, due to low temperature and low solubility, part of the lubricating oil is separated from the refrigerant; On the other hand, due to low temperature and high viscosity, the separated lubricating oil is easy to attach to the inner wall of the pipe, and it is difficult to flow. The lower the evaporation temperature, the more difficult the oil return. This requires that the design and construction of evaporation pipeline and gas return pipeline must be conducive to oil return. The common practice is to adopt a descending pipeline design and ensure a large air flow speed. For refrigeration systems with extremely low temperatures, such as - 85 ° C and - 150 ° C medical low temperature boxes, in addition to using efficient oil separators, special solvents are usually added to prevent the capillary tube and expansion valve from being blocked by lubricating oil, and to help oil return.
In practical application, oil return problem caused by improper design of evaporator and gas return pipeline is not uncommon. For R22 and R404A systems, the oil return of the full liquid evaporator is very difficult, and the design of the oil return pipeline of the system must be very careful. For such a system, the use of efficient oil separator can greatly reduce the amount of oil entering the system pipeline and effectively extend the time of no oil return in the return pipe after startup. When the compressor is higher than the evaporator, the oil return bend on the vertical return pipe is necessary. The oil return bend should be as compact as possible to reduce oil storage. The spacing between oil return bends should be appropriate. When there are many oil return bends, some lubricating oil should be added. The oil return pipeline of the variable load system must also be careful. When the load is reduced, the gas return speed will be reduced, and too low speed is not conducive to oil return. In order to ensure oil return under low load, the vertical suction pipe can be double risers, which can prolong the service life of the compressor for a period.