Why is the refrigeration compressor short of oil?

September 5, 2023

Compressor is a complex machine that runs at high speed. It is the basic requirement to ensure the full lubrication of moving parts such as crankshaft, bearing, connecting rod and piston of compressor to maintain the normal operation of the machine. For this reason, compressor manufacturers require the use of designated brand lubricating oil, and require regular inspection of lubricating oil level and color.


Lack of oil is one of the compressor faults that can be easily identified. When the compressor is short of oil, there is little oil or even no lubricating oil in the crankcase.


The compressor is a special air pump, and a large amount of refrigerant gas is discharged, and at the same time, a small part of lubricating oil is carried away (called running oil or running oil). The oil running of compressor is inevitable, but the oil running speed is different.


There is about 2-3% lubricating oil in the exhaust of semi-sealed piston compressor, while 0.5-1% in scroll compressor. For a 6-cylinder compressor with a displacement of 100m3/hr and a crankcase oil storage capacity of 6 liters, 3% oil running means about 0.3-0.8 liters/min, or the compressor runs for more than ten minutes without oil return. If the lubricating oil discharged from the compressor does not come back, the compressor will be short of oil.

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There are two ways for compressor oil return: one is oil separator oil return, and the other is oil return pipe oil return.


The oil separator is installed on the compressor exhaust pipe, which can generally separate 50-95% of the running oil, with good oil return effect and high speed, greatly reducing the amount of oil entering the system pipeline, thus effectively prolonging the operation time without oil return.


It's not uncommon for cold storage refrigeration systems with extremely long pipelines, full-liquid ice-making systems and freeze-drying equipment with very low temperature, etc., to have no oil return or very little oil return for ten minutes or even dozens of minutes after startup. A poorly designed system will cause the compressor to stop due to too low oil pressure. The installation of high-efficiency oil separator in this refrigeration system can greatly prolong the running time of compressor without oil return, so that the compressor can safely survive the crisis stage without oil return after startup.


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The lubricating oil that has not been separated will enter the system and flow with the refrigerant in the pipe to form oil circulation. Lubricating oil enters the evaporator.


On the one hand, due to low temperature and low solubility, part of lubricating oil is separated from refrigerant; On the other hand, the temperature is low and the viscosity is high, so 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 oil. This requires that the design and construction of evaporation pipeline and return pipeline must be conducive to oil return. The common practice is to adopt descending pipeline design and ensure a large airflow speed.


When the compressor is 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 oil return bends should be appropriate. When the number of oil return bends is large, some lubricating oil should be added.


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The oil return pipeline of variable load system must also be careful. When the load decreases, the air return speed will decrease, and 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 risers.


Frequent startup of compressor is not conducive to oil return. Because the compressor stops after a short continuous operation time, there is no time to form a stable high-speed airflow in the return pipe, so the lubricating oil can only stay in the pipeline. If the return oil is less than the running oil, the compressor will be short of oil. The shorter the running time, the longer the pipeline and the more complicated the system, the more prominent the oil return problem is.


When defrosting, the evaporator temperature increases, the viscosity of lubricating oil decreases, and it is easy to flow. After the defrosting cycle, the refrigerant flow rate is large, and the stranded lubricating oil will return to the compressor in a centralized way. Therefore, the frequency and duration of defrosting cycle should be carefully set to avoid oil level fluctuation or even oil shock. When the refrigerant leaks more, the return air speed will decrease, and if the speed is too low, the lubricating oil will stay in the return air pipeline and cannot return to the compressor quickly. Starting with liquid caused by refrigerant migration will also make internal oil return difficult, but it usually takes a short time, up to ten minutes.


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