If the oil return of the Screw Chiller compressor is not smooth, it is a very headache. Generally speaking, the main reason for the poor oil return of the screw compressor is due to the phenomenon of gas mixture between lubricating oil and refrigerant during operation. Specifically, during the operation of the refrigeration system, encountering refrigerant and refrigeration machine lubricating oil immiscibility can cause the lubricating oil in the refrigeration compressor to be discharged into the condenser in the form of mist and droplet gas with the refrigerant during machine operation. If the oil separator is not effective or the system design is not good, it will cause poor separation efficiency and poor system oil return. Of course, the previous question was that you did not use fake oil. What are the impacts of a screw compressor not returning oil?
If the screw compressor does not return oil, it will cause a large amount of lubricating oil to be stuck 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 is out of oil and cannot operate, requiring continuous replenishment of lubricating oil. This will lead to an accumulation of lubricating oil in the system, causing a vicious cycle, increasing operating costs, and reducing operational reliability. In general, it is allowed 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 in screw compressors?
There are two ways for compressor oil return, one is through oil separator oil return, and the other is through return pipe oil return. The oil separator is installed on the exhaust pipeline of the compressor and can generally separate 50-95% of the flowing oil. The oil return effect is good, the speed is fast, and the amount of oil entering the system pipeline is greatly reduced, thereby effectively extending the operation time without oil return. It is not uncommon for cold storage refrigeration systems with particularly long pipelines, full liquid ice making systems, and freeze-drying equipment with very low temperatures to not return oil for more than ten or even tens of minutes after starting up, or for oil to return very little. poorly designed systems can cause compressor oil pressure to be too low and shut down. The installation of an efficient oil separator in this refrigeration system can greatly prolong the non return oil operation time of the screw Chillers compressor, enabling the compressor to safely pass the crisis stage of no return oil after starting up. The lubricating oil that has not been separated will enter the system and flow with the refrigerant in the pipe, forming an oil circulation. In practical applications, oil return problems caused by improper design of evaporators and return pipelines are not uncommon. For the R22 and R404A systems, the return oil of the full liquid evaporator is very difficult, and the design of the system return oil pipeline must be very careful. For such a system, using efficient oil separation can greatly reduce the amount of oil entering the system pipeline, effectively extending the time for no oil return in the return pipe after startup.
After the lubricating oil enters the evaporator, on the one hand, due to the low temperature and low solubility, a portion of the lubricating oil separates from the refrigerant; On the other hand, with low temperature and high viscosity, the separated lubricating oil is prone to adhere to the inner wall of the pipe, making it difficult to flow. The lower the evaporation temperature, the more difficult it is to return oil. This requires the design and construction of evaporation and return pipelines to be conducive to oil return. The common practice is to use a descending pipeline design and ensure a higher airflow speed. For refrigeration systems with particularly low temperatures, in addition to using high-efficiency oil separators, special solvents are usually added to prevent lubricating oil from blocking capillaries and expansion valves, and to assist in oil return. Mr. Wu from Laifukang also emphasized that some people are using the built-in oil separator of air conditioners instead of the external oil separator, which may seem to save costs on the surface, but in terms of long-term system usage costs, it will only greatly increase operating costs. The efficiency of the system will continue to deteriorate.
When the compressor is positioned higher than the evaporator, a 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 the oil return bends should be appropriate. When there are many oil return bends,
Some lubricating oil should be added. The return oil pipeline of the variable load system must also be careful. When the load decreases, the return air speed will decrease, and too low a speed is not conducive to oil return. To ensure oil return under low load, a vertical suction pipe can be equipped with a double riser.
Moreover, frequent start-up of the Chiller Unit compressor is not conducive to oil return. Due to the short continuous operation time, the compressor stopped and there was no time to form a stable high-speed airflow in the return pipe, so the lubricating oil could only remain in the pipeline. If the return oil is less than the running oil, the compressor will run out of oil. The shorter the operating time, the longer the pipeline, the more complex the system, and the more prominent the problem of oil return. Therefore, in general, do not frequently start the compressor. Lack of oil can cause serious lubrication problems, and the root cause of oil shortage is not the amount and speed of oil flowing from the screw compressor, but rather 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 air pipeline must take into account the return oil. Maintenance measures such as avoiding frequent start-up, timed defrosting, timely replenishment of refrigerant, and timely replacement of vulnerable parts (such as bearings) can also help with oil return.
