The oil return problem of refrigeration compressors has always been a hot topic in refrigeration systems. Today, I will talk about the oil return problem of screw compressors. Generally speaking, the reason for the poor oil return of the screw compressor is mainly due to the gas mixing phenomenon of lubricating oil and refrigerant during operation. During the operation of the refrigeration system, the refrigerant and the lubricating oil of the refrigerator are mutually soluble, causing the lubricating oil to be discharged into the condenser in the form of aerosol and droplet gas with the operation of the machine and the refrigerant. If the oil separator is not effective or the system design is not good, it will cause poor separation effect and poor system oil return.
1. What problems will occur due to poor oil return:
The poor oil return of the screw compressor will cause a large amount of lubricating oil to stay in the evaporator pipeline. When the oil film increases to a certain extent, it will directly affect the cooling of the system; It will lead to the accumulation of more and more lubricating oil in the system, resulting in a vicious circle, increasing operating costs and reducing operating reliability. Generally, less than 1% of the refrigerant gas flow is allowed to circulate in the system with an oil-air mixture.
2. Solutions for poor oil return:
There are two ways to return oil to the compressor, one is to return oil to the oil separator, and the other is to return the oil to the air return pipe.
The oil separator is installed on the exhaust pipe of the compressor, which can generally separate 50-95% of the running oil. The oil return effect is good and the speed is fast, which greatly reduces the amount of oil entering the system pipeline, thus effectively prolonging the operation without oil return. time.
For cold storage refrigeration systems with particularly long pipelines, flooded ice-making systems, and freeze-drying equipment with very low temperatures, it is not uncommon to see no oil return or very little oil return for ten or even tens of minutes after starting the machine. A bad system will cause the compressor to shut down due to low oil pressure. The installation of a high-efficiency oil separator in this refrigeration system can greatly prolong the operation time of the compressor without oil return, so that the compressor can safely pass through the crisis stage of no oil return after startup. The lubricating oil that is not separated will enter the system and flow with the refrigerant in the tube to form an oil circulation.
After the lubricating oil enters the evaporator, on the one hand, due to the low temperature and low solubility, a part of the lubricating oil is separated from the refrigerant; on the other hand, the temperature is low and the viscosity is large, the separated lubricating oil is easy to adhere to the inner wall of the tube, and it is difficult to flow. The lower the evaporating temperature, the more difficult it is to return the oil. This requires that the design and construction of the evaporation pipeline and the return pipeline must be conducive to oil return. The common practice is to use a descending pipeline design and ensure a large airflow velocity. For refrigeration systems with particularly low temperature, in addition to the selection of high-efficiency oil separators, special solvents are usually added to prevent lubricating oil from blocking capillary tubes and expansion valves, and to help oil return.
In practical applications, oil return problems caused by improper design of evaporator and return gas pipeline are not uncommon. For R22 and R404A systems, the oil return of the flooded evaporator is very difficult, and the system oil return pipeline design must be very careful. For such a system, the use of high-efficiency oil separation can greatly reduce the amount of oil entering the system pipeline, effectively prolonging the time when the gas return pipe does not return oil after starting the machine.
When the compressor is higher than the evaporator, the oil return bend on the vertical return pipe is necessary. The oil return trap should be as compact as possible to reduce oil storage. The spacing between the oil return bends should be appropriate. When the number of oil return bends is large, some lubricating oil should be added. Care must also be taken in the return lines of variable load systems. When the load is reduced, the air return speed will decrease, and the speed is too low, which is not conducive to oil return. In order to ensure the oil return under low load, the vertical suction pipe can use double riser.
Moreover, frequent startup of the compressor is not conducive to oil return. Because the continuous operation time is very short, the compressor stops, and there is no time to form a stable high-speed airflow in the return pipe, so the lubricating oil can only stay in the pipeline. If the return oil is less than the run oil, the compressor will be short of oil. The shorter the running time, the longer the pipeline, the more complex the system, the more prominent the oil return problem. Therefore, under normal circumstances, do not start the compressor frequently.
Lack of oil will cause serious lack of lubrication. The root cause of oil shortage is not how much and how fast the screw compressor runs, but the poor oil return of the system. Installing an oil separator can quickly return oil and prolong the operation time of the compressor without oil return. The design of the evaporator and return line must take oil return into account. Maintenance measures such as avoiding frequent startup, defrosting regularly, replenishing refrigerant in time, and replacing wearing parts (such as bearings) in time also help oil return.
When designing a refrigeration system, the research on the oil return problem is indispensable. Only by considering all aspects, can a safe and reliable refrigeration system be guaranteed.
Post time: Feb-21-2022
