The reason for the poor oil return of the screw compressor is mainly due to the gas mixture of lubricating oil and refrigerant during operation. Specifically, in the operation of the refrigeration system, the miscible refrigerant and the refrigeration lubricating oil will cause the lubricating oil in the refrigeration compressor to run and the refrigerant will be discharged into the condenser in the form of air mist and microdroplet gaseous. If the oil separator is not effective or the system is not well designed, it will cause poor separation and poor oil return to the system. Of course, the premise is that you are not using fake oil.
What are the effects of poor oil return in screw compressors?
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 continuously replenished with lubricating oil, which will lead to the accumulation of more and more lubricating oil in the system, resulting in a vicious circle, increasing operating costs and reducing operational reliability. In general, it is permissible for the oil-gas mixture to circulate in the system with a refrigerant gas flow rate of less than 1%.
How to solve the problem of poor oil return of screw compressor?
There are two ways to return oil to the compressor, one is the oil return from the oil separator, and the other is the oil return from the return pipe. The oil separator is installed on the compressor exhaust pipeline, which can generally separate 50-95% of the oil, with good oil return effect and fast speed, which greatly reduces the amount of oil entering the system pipeline, thus effectively prolonging the operation time without oil return. It is not uncommon for cold storage refrigeration systems with particularly long pipelines, flooded ice-making systems and freeze-drying equipment with very low temperatures, etc., to not return oil for more than ten minutes or even dozens of minutes after start-up, or the amount of oil return is very small, and the problem of compressor oil pressure is too low and shut down in a poorly designed system. The installation of a high-efficiency oil separator in this refrigeration system can greatly extend the operating time of the compressor without oil return, so that the compressor can safely pass the crisis stage of no oil return after start-up.
The unseparated lubricant enters the system and flows with the refrigerant through the tubes, creating an oil cycle. After the lubricating oil enters the evaporator, on the one hand, due to the low temperature and small 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, and the separated lubricating oil is easy to adhere to the inner wall of the pipe, and the flow is difficult. The lower the evaporation temperature, the more difficult it is to return the oil. This requires that the design and construction of the evaporation line and the return gas line must be conducive to oil return, and the common practice is to adopt a descending line design and ensure a large air velocity. For refrigeration systems with particularly low temperatures, in addition to the use of high-efficiency oil separators, special solvents are often added to prevent the oil from clogging the capillary and expansion valves and to aid in oil return. At the same time, Mr. Wu emphasized that some people use the built-in oil of the air conditioner to replace the external oil, which is ostensibly to save costs, but in terms of the long-term use of the system, it will only greatly increase the operating costs. The efficiency of the system will get worse and worse.
In practice, oil return problems caused by improper design of evaporators and return lines are not uncommon. For the R22 and R404A systems, the return of the flooded evaporator is very difficult, and the system return line design must be very careful. For such a system, the use of high-efficiency oil can greatly reduce the amount of oil entering the system pipeline, and effectively extend the time of no oil return to the air return pipe after start-up.
When the compressor is positioned higher than the evaporator, a return bend on the vertical return line is required. The return bend should be as compact as possible to reduce oil retention. The spacing between the return bends should be appropriate, and when the number of return bends is relatively large, some lubricating oil should be supplemented. Care must also be taken with regard to the return lines of variable load systems. When the load is reduced, the return air speed will be reduced, and the speed is too low to facilitate the oil return. In order to ensure oil return at low loads, the vertical suction pipe can be equipped with double risers.
Moreover, frequent start-up of the compressor is not conducive to oil return. Due to the short continuous operation time, the compressor stopped, and the return air line did not have time to form a stable high-speed air flow, so the lubricating oil had to remain in the pipeline. If the return oil is less than the rush oil, the compressor will be short of oil. The shorter the run time, the longer the pipeline, and the more complex the system, the more prominent the oil return problem becomes. Therefore, in general, do not start the compressor frequently.
Finally, the lack of oil will cause serious lubrication deficiency, and the root cause of oil shortage is not how much oil runs to the screw compressor and how fast it is, but the system oil return is not good. Installing an oil separator allows for fast oil return and extended compressor runtime without oil return. The evaporator and return lines must be designed with oil return in mind. Maintenance measures such as avoiding frequent start-ups, regular defrosting, timely replenishment of refrigerant, and timely replacement of wear parts (e.g., bearings) can also help with oil return.
Contact Person: Mr. Henry Lin
Tel: 86-180 2621 9032