The return oil of screw compressor is not smooth, which will cause a lot of lubricating oil to stay in evaporator pipeline. When the oil film increases to a certain extent, it will directly affect the system cooling. Machine oil shortage can't run on the other hand, need to constantly add lubricating oil, which can lead to the oil in the system, create a vicious cycle, increase the operation cost, lower reliability. In general, the circulation of a mixture of oil and gas under 1% of the refrigerant gas flow is allowed in the system.
Ii. Unobstructed oil recovery solution:
There are two ways to return the compressor oil. One is to return the oil from the oil separator and the other is to return the oil from the gas pipe.
Oil separator is installed on compressor exhaust pipe on the road, generally can isolate 50-95% of the oil, oil effect is good, quick speed, greatly reduce the amount of oil pipeline into the system, thus effectively prolong operation time without oil return.
Line special long cold storage refrigeration system, full liquid type ice-making system and low temperature freeze-drying equipment, boot after more than ten minutes or a few minutes without oil or oil return quantity very little case is not rare, bad design system will appear compressor oil pressure is too low and downtime. The installation of high efficiency oil separator in this refrigeration system can greatly extend the operation time of the compressor without oil return, so that the compressor can safely pass the crisis stage without oil return after starting up. Unseparated lubricating oil will enter the system and flow in the tube with the refrigerant, forming an oil circulation.
When the lubricating oil enters the evaporator, on the one hand, due to the low temperature and low solubility, part of the lubricating oil is separated from the refrigerant. On the other hand, the temperature is low and the viscosity is high, the separated lubricating oil is easy to attach 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 evaporative pipeline and the return pipeline must be conducive to the return of oil. The common practice is to adopt the design of the descending pipeline and ensure a high airflow speed. Special solvents are usually added to a particularly low temperature refrigeration system in addition to a high efficiency oil separator to prevent lubricating oil from blocking capillaries and expansion valves, and to help return oil.
In practical application, it is not uncommon to find oil recovery problems caused by the improper design of evaporator and return pipe. For R22 and R404A systems, it is very difficult to return the oil of the liquid-filled evaporator, and the design of the system return pipeline 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 time of no oil return to the return pipe after starting up.
When the compressor is higher than the evaporator position, the vertical oil return bend on the return pipe is required. Return oil bend as tight as possible to reduce oil storage. The distance between the oil return bend should be appropriate, the amount of oil return bend is large, some lubricating oil should be added. The return pipeline of the variable load system must also be careful. When the load is reduced, the return gas speed will be reduced, and the low speed is not conducive to the return oil. In order to ensure the oil recovery under low load, the vertical suction pipe can be used with double vertical pipe.
Moreover, frequent starting of the compressor is not conducive to oil recovery. Because the compressor stopped running for a very short period of time, there was no time to form a stable high speed air flow in the return pipe, so the lubricating oil could only be left in the pipeline. The compressor will run out of oil if the return oil is less than the running oil. The shorter the operation time, the longer the pipeline, the more complex the system, the more serious the problem of oil return. So in general, do not start the compressor frequently.
Lack of oil will cause serious lubrication deficiency, the root cause of the lack of oil is not the amount and speed of screw compressor running oil, but the system return oil is not good. Installation of oil separator can quickly return oil, to extend the compressor running time without oil return. The design of evaporator and return pipe must consider the return oil. Maintenance measures such as avoiding frequent start-up, regular defrosting, timely replacement of refrigerants, and timely replacement of wear and tear parts, such as bearings, are also helpful.
When designing the refrigeration system, it is essential to study the problem of oil recovery. Only by taking all aspects into consideration can a safe and reliable refrigeration system be ensured.
The last one：Key points of oil replacement in screw refrigeration compressor