There are two ways to return oil to the compressor, one is oil return from the oil separator, and the other is oil return from the return pipe.
The oil separator is installed on the compressor's exhaust pipeline, and can generally separate 50-95% of the oil that runs off. 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 no oil return. Run time.
Refrigeration systems with extremely long pipelines, full-liquid ice-making systems, and very low-temperature freeze-drying equipment, etc. It is not uncommon that the oil does not return for ten minutes or even tens of minutes after the start-up, or the amount of oil return is very small. A good system will cause the compressor oil pressure to be too low and shut down.
The installation of a high-efficiency oil separator in this refrigeration system can greatly extend the compressor's non-return oil operation time, and make the compressor safe through the crisis stage without oil return after startup. The unseparated lubricating oil will enter the system and flow with the refrigerant in the tube, forming an oil cycle.
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, the lubricating oil with low temperature and high viscosity is easy to adhere to the inner wall of the tube, 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 of the evaporation pipeline and the design and construction of the return gas pipeline must be conducive to oil return. A common practice is to use a descending pipeline design and ensure a large air velocity. For refrigeration systems with extremely low temperatures, in addition to high-efficiency oil separators, special solvents are usually added to prevent lubricants from blocking the capillaries and expansion valves, and to help return oil.
In practice, it is not uncommon for oil return problems caused by improper design of evaporators and return air lines. For the R22 and R404A systems, oil return from the full-liquid evaporator is very difficult, and the system oil return pipe 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, effectively extending the non-oil return time of the return air pipe after starting up.
When the compressor is positioned 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 the oil return bends should be appropriate. When the number of oil return bends is large, some lubricant should be added. The return line of the variable load system must also be careful. When the load is reduced, the return air speed will decrease. Too low speed is not conducive to oil return. In order to ensure oil return under low load, the vertical suction pipe can adopt double riser.
And the frequent start of the compressor is not conducive to oil return. Because the compressor stops for a short period of continuous operation, there is no time to form a stable high-speed air flow in the return pipe, and the lubricating oil can only remain in the pipe. If the oil return is less than the oil return, the compressor will be short of oil. The shorter the running time, the longer the pipeline, the more complicated the system, and the more prominent the oil return problem. Therefore, in general, do not start the compressor frequently.
The lack of oil will cause serious lubrication insufficiency. The root cause of the lack of oil is not how much and how fast the screw compressor runs, but the poor oil return of the system. The oil separator can be installed to return oil quickly and extend the compressor's oil-free running time. The design of the evaporator and return line must take into account return oil.
Cleaning the oil filter in time, avoiding frequent startup, regular defrost, timely replenishment of refrigerant, and timely replacement of wearing parts also help return oil.