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Compressor is a complex machine running at high speed. To ensure sufficient lubrication of compressor crankshaft, bearing, connecting rod, piston and other moving parts is the basic requirement to maintain the normal operation of the machine. For this reason, compressor manufacturers require the use of a designated grade of lubricating oil, and require regular checks of the oil level and color.
Lack of oil is one of the compressor faults that is easy to identify. When the compressor is short of oil, there is little or no oil in the crankcase.
Compressor is a special air pump, a large number of refrigerant gas is discharged at the same time also entrain a small part of the lubricating oil (called running oil or running oil). Compressor half-sealed piston compressors have about 2-3% lubricating oil in the exhaust, while scroll compressors have 0.5-1% lubricating oil. For a 6-cylinder compressor with a displacement of 100m3/hr and a crankcase oil storage of 6 liters, 3% oil rush means about 0.3 to 0.8 liters of oil run per minute, or a compressor running time of more than 10 minutes without oil return. If the lubricating oil from the compressor does not come back, the compressor will be short of oil.
There are two ways to return oil from the shrinking machine: one is to return oil from the oil separator, and the other is to return oil from the return pipe.
Oil separator installed on the compressor exhaust pipe, generally can separate 50-95% of the oil, oil return effect is good, fast, greatly reduce the amount of oil into the system pipeline, so as to effectively extend the operation time without oil return.
The pipeline is particularly long cold storage refrigeration system, full liquid ice system and very low temperature freeze-drying equipment, etc., more than ten minutes or even dozens of minutes after the boot of the oil or oil return is very small situation is not rare, poor design of the system will appear compressor oil pressure is too low and stop the problem. The high efficiency oil separator installed 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 of no oil return after starting.
The lubricating oil that has not been separated will enter the system and flow with the refrigerant in the tube, forming an oil circulation. The lubricating oil enters the evaporator.
On the one hand, due to low temperature and small solubility, a part 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 tube wall, and the flow is more difficult. 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 pipe must be conducive to the return of oil. The common practice is to use the design of the down type pipeline and ensure the greater airflow speed.
In practical application, the oil return problem caused by improper design of evaporator and return pipe is not rare. For r22 and r404a systems, full evaporator oil return is relatively difficult, the system return line 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 prolong the return pipe after startup without oil return time.
When the compressor is higher than the evaporator, an 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. The number of oil return bends is more, and some lubricating oil should be added.
The return line of the variable load system must also be careful. When the load is reduced, the return speed will be reduced, too low speed is not conducive to oil return. In order to ensure oil return under low load, the vertical suction pipe can be double riser.
Frequent starting of the compressor is not conducive to oil return. Because the continuous operation time is very short, the compressor stops, and there is no time to form a stable high-speed airflow in the return pipe, and the lubricating oil can only be left in the pipeline. If the return oil is less than the rush oil, the compressor will run out of oil. The shorter the run time, the longer the line, and the more complex the system, the greater the return problem
Defrosting evaporator temperature increases, lubricating oil viscosity decreases, easy to flow. After the defrosting cycle, the refrigerant flow rate is high, and the residual lubricating oil will be concentrated back to the compressor. Therefore, the frequency of defrosting cycles and the duration of each cycle should be carefully set to avoid large fluctuations in the oil level and even oil shocks. If the refrigerant leaks too much, the air return speed will decrease. If the speed is too low, the lubricating oil will stay in the air return pipe and cannot quickly return to the compressor. Starting with liquid caused by refrigerant migration will also cause difficulties in internal oil return, but usually the time is short, more than ten minutes.
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