What are the reasons for poor oil return of screw machine?

August 1, 2023

The poor oil return of screw compressor is mainly due to the phenomenon of gas mixture of lubricating oil and refrigerant during operation. Specifically, in the operation of the refrigeration system, when the refrigerant and the refrigerator lubricating oil are mutually dissolved, the lubricating oil in the refrigeration compressor will be discharged into the condenser in the form of gas mist and droplets with the operation of the machine. If the effect of oil separator is not good or the system design is not good, it will cause the problems of poor separation effect and poor oil return of the system. Of course, the premise is that you didn't use fake oil.


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What are the effects of poor oil return of screw compressor?


Poor oil return of the screw compressor will lead to a large amount of lubricating oil staying in the evaporator pipeline. When the oil film increases by 0.1mm, it will directly affect the cooling of the system. On the other hand, the machine cannot run due to lack of oil, and it needs to be replenished constantly, which will lead to more and more lubricating oil in the system, resulting in a vicious circle, increasing operating costs and reducing operating reliability. In general, it is allowed to circulate the oil-gas mixture in the system when the refrigerant gas flow rate is below 1%.


How to solve the problem of poor oil return of screw compressor?


There are two ways for compressor oil return, one is oil separator oil return, and the other is oil return pipe oil return. The oil separator is installed on the compressor exhaust pipe, which can generally separate 50-95% of the running oil, with good oil return effect and high speed, greatly reducing the amount of oil entering the system pipeline, thus effectively prolonging the operation time without oil return. It's not uncommon for cold storage refrigeration systems with extremely long pipelines, full-liquid ice-making systems and freeze-drying equipment with very low temperature, etc., to have no oil return or very little oil return for ten minutes or even dozens of minutes after startup. A poorly designed system will cause the compressor to stop due to too low oil pressure. The installation of high-efficiency oil separator in this refrigeration system can greatly prolong the running time of compressor without oil return, and make the compressor safely pass through the crisis stage without oil return after startup.


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The lubricating oil that has not been separated will enter the system and flow with the refrigerant in the pipe to form oil circulation. After the lubricating oil enters the evaporator, on the one hand, part of the lubricating oil is separated from the refrigerant because of its low temperature and low solubility; On the other hand, the temperature is low and the viscosity is high, so the separated lubricating oil is easy to adhere to the inner wall of the pipe and it is difficult to flow. The lower the evaporation temperature, the more difficult it is to return oil. This requires that the design and construction of evaporation pipeline and return pipeline must be conducive to oil return. The common practice is to adopt descending pipeline design and ensure a large airflow speed. For the refrigeration system with extremely low temperature, in addition to the high-efficiency oil separator, special solvent is usually added to prevent the lubricating oil from blocking the capillary and expansion valve, and to help return oil. At the same time, General Manager Lai Fu Kang-wu stressed that at present, some people use the built-in oil of air conditioners to replace the external oil, which on the surface saves the cost, but in terms of the long-term use cost of the system, it will only greatly increase the operating expenses. The efficiency of the system will get worse and worse.


In practical application, the problem of oil return caused by improper design of evaporator and air return pipeline is not uncommon. For R22 and R404A systems, it is very difficult to return oil from the flooded evaporator, so the design of the oil return pipeline of the system 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 and effectively extend the time without oil return in the return pipe after startup.


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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 the number of oil return bends is large, some lubricating oil should be added. The oil return pipeline of variable load system must also be careful. When the load decreases, the air return speed will decrease, and too low speed is not conducive to oil return. In order to ensure the oil return under low load, the vertical suction pipe can adopt double risers.


Moreover, the frequent start of compressor is not conducive to oil return. Because the compressor stops after a short continuous operation time, 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 running oil, the compressor will be short of oil. The shorter the running time, the longer the pipeline and the more complicated the system, the more prominent the oil return problem is. Therefore, in general, do not start the compressor frequently.


Finally, the lack of oil will cause serious lubrication shortage. The fundamental reason for the lack of oil is not how much and how fast the screw compressor runs, but the poor oil return of the system. The installation of oil separator can quickly return oil and prolong the running time of compressor without oil return. The design of evaporator and air return pipeline must take oil return into account. Maintenance measures such as avoiding frequent start-up, defrosting regularly, replenishing refrigerant in time and replacing wearing parts (such as bearings) in time are also helpful for oil return.


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