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

There are two ways to return oil to the compressor, one is the oil return of the oil separator, and the other is the oil return of the air return pipe. The oil separator is installed on the compressor exhaust pipe. Generally, 50-95% of the oil can be separated. The oil return effect is good, the speed is fast, and the amount of oil entering the system pipeline is greatly reduced, thus effectively extending the operation without oil return. time.

It is not uncommon for cold storage refrigeration systems with extremely long pipelines, full-liquid ice-making systems, and freeze-drying equipment with very low temperatures to return more than ten minutes or even dozens of minutes after starting up, or with very little oil return. Design A bad system will cause the compressor to stop due to low oil pressure. The installation of a high-efficiency oil separator in the refrigeration system can greatly extend the compressor’s non-return operation time, so that the compressor can safely pass the crisis stage of no oil return after the start-up. .

The lubricating oil that has not been separated will enter the system and flow with the refrigerant in the pipe to form an oil circulation. After the lubricating oil enters the evaporator, part of the lubricating oil is separated from the refrigerant due to low temperature and low solubility; on the other hand, low temperature and high viscosity, 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 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 adopt a descending pipeline design and ensure a larger air flow velocity.

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 help return oil. At the same time, some people use the built-in oil of the air conditioner to replace the external oil. On the surface, it saves costs, but in terms of the long-term use cost of the system, it will only greatly increase the operating cost. The efficiency of the system will get worse and worse.

In practical applications, oil return problems caused by improper design of the evaporator and return line are not uncommon. For R22 and R404A systems, the oil return of the flooded evaporator is very difficult, and the oil return pipeline design of the system 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 non-return time of the air return pipe after the start-up.

When the compressor is higher than the evaporator, the oil return bend on the vertical return pipe is necessary. The return bend 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 relatively large, some lubricating oil should be added. The oil return line of the variable load system must also be careful. When the load is reduced, the air return speed will decrease, 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 vertical pipes.

Moreover, frequent compressor startup is not conducive to oil return. Since the compressor stops for a short continuous operation time, there is no time to form a stable high-speed air flow in the return pipe, and the lubricating oil can only stay in the pipe. If the oil return is less than Ben oil, the compressor will be short of oil. The shorter the operating time, the longer the pipeline and the more complex the system, the more prominent the oil return problem. Therefore, in general, do not start the compressor frequently.

In short, lack of oil will cause serious lack of lubrication. The root cause of the lack of oil is not the amount and speed of the screw-type chiller, but the poor oil return of the system. The installation of a high-efficiency oil separator can quickly return oil and extend the compressor’s operation time without oil return. The design of the evaporator and the return gas pipeline must take the oil return into consideration. Maintenance measures such as avoiding frequent starting, timing defrosting, timely replenishment of refrigerant, and timely replacement of wearing parts (such as bearings) are also helpful for oil return