Why does the 30 ton chiller not return oil?

　　Why does the 30 ton chiller not return oil?

　　A screw chiller with poor oil return can cause a large amount of lubricating oil to be trapped in the evaporator pipeline. When the oil film increases by 0.1mm, it directly affects the cooling of the system. On the other hand, the machine is unable to operate due to oil shortage and needs to be continuously replenished with lubricating oil. This can 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 for screw chillers?

　　There are two ways to return oil to the compressor, one is through the oil separator, and the other is through the return pipe. The oil separator is installed on the exhaust pipeline of the compressor and can generally separate 50-95% of the running 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 or have very little oil return after more than ten or even dozens of minutes of startup. Poorly designed systems can cause the compressor to shut down due to low oil pressure. Installing an efficient oil separator in this refrigeration system can greatly prolong the compressor’s oil return free operation time, Make the compressor safely pass the crisis stage of no oil return 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. After lubricating oil enters the evaporator, on the one hand, due to low temperature and low solubility, a portion of the lubricating oil is separated from the refrigerant; On the other hand, with low temperature and high viscosity, the separated lubricating oil is easy 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 that the design and construction of the evaporation and return pipelines must be conducive to the return of oil. 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 the capillary and expansion valves, and to assist in oil return. At the same time, some people currently use the built-in oil separator of air conditioners to replace the external oil separator, which may seem to save costs, but in terms of the long-term operating costs of the system, it will only greatly increase operating costs. The efficiency of the system will become increasingly poor.

　　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 return oil pipeline in the system must be very careful. For such a system, using efficient 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.

　　When the compressor is 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 return bends should be appropriate. When there are many 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. In order to ensure oil return under low load, a vertical suction pipe can be used with double risers.

　　Moreover, frequent start-up of the compressor is not conducive to oil return. Since the compressor stops after a short period of continuous operation, there is no time to form a stable Jet stream in the return pipe, so the lubricating oil can only be left in the pipeline. If the return oil is less than the return 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.