Why does a refrigeration compressor run out of oil?

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A compressor is a complex machine that operates at high speeds; ensuring proper lubrication of its moving parts such as the crankshaft, bearings, connecting rods, and pistons is a fundamental requirement for maintaining its normal operation. To this end, compressor manufacturers require the use of lubricants of specified grades, and they demand regular checks on the lubricant level and color. Oil deficiency is one of the compressor failures that is easy to identify; when a compressor lacks oil, there is very little or no lubricating oil in the crankcase. A compressor is a special type of air pump; as a large amount of refrigerant gas is discharged, it also carries away a small amount of lubricating oil (known as oil leakage). Oil carryover in compressors is inevitable; it’s just that the rate of oil carryover varies. Approximately 2-3% of the lubricating oil is present in the exhaust gas of semi-positive displacement piston compressors, while it is 0.5-1% in scroll compressors. For a 6-cylinder compressor with a capacity of 100 m3/hr and a crankcase oil volume of 6 liters, 3% oil leakage means an oil loss rate of approximately 0.3–0.8 liters per minute, or the compressor can operate without oil return for around ten minutes. If the lubricating oil from the compressor is not returned, the compressor will run out of oil. There are two ways for the compressor to return oil: one is through the oil separator, and the other is through the return air pipe. The oil separator is installed on the compressor exhaust line and is generally capable of separating 50-95% of the oil mist. It provides an effective oil return with fast speed, **reducing the amount of oil that enters the system pipes, thereby effectively extending the operating time without oil return. In cold storage refrigeration systems with particularly long pipelines, flooded ice-making systems, and freeze-drying equipment operating at very low temperatures, it is not uncommon for no oil to return to the compressor or only a very small amount of oil to return after the system is started, over ten minutes or even several dozen minutes. Systems that are not properly designed may experience problems such as excessively low compressor oil pressure, which leads to shutdown of the compressor. Installing an efficient oil separator in such a refrigeration system can **extend the time during which the compressor can operate without oil return, allowing it to get through the critical phase right after startup when there is no oil return. The lubricating oil that has not been separated will enter the system and flow along with the refrigerant within the pipes, thus forming an oil circulation. Lubricating oil enters the evaporator. On the one hand, due to the low temperature and resulting low solubility, some of the lubricating oil separates from the refrigerant ； On the other hand, at low temperatures the viscosity is high; the separated lubricating oil tends to adhere to the inner walls of the pipe, making it difficult for it to flow. The lower the evaporation temperature, the more difficult it is to return the oil. This requires that the design and construction of both the evaporation piping and the return piping must facilitate oil return. A common practice is to adopt a descending piping design while ensuring a relatively high air flow velocity. In practical applications, oil return problems caused by improper design of evaporators and return piping are quite common. For R22 and R404A systems, it is relatively difficult to return oil to a full-liquid evaporator, and great care must be taken in the design of the system’s oil return piping. For such systems, using high-efficiency oil can **reduce the amount of oil entering the system pipes, thereby effectively extending the period during which no oil returns in the return pipe after the system is started up. When the compressor is located higher than the evaporator, an oil return bend on the vertical return pipe is necessary. The return oil bend should be as compact as possible to reduce oil accumulation. The spacing between the return bends should be appropriate; when there are a large number of return bends, some lubricating oil should be added. Care must also be taken with the return oil lines in variable load systems. When the load decreases, the return air velocity drops, and a too low velocity is not conducive to oil return. To ensure oil return under light load, a vertical suction pipe can use double standpipes. Frequent compressor startups are detrimental to oil return. Since the compressor shuts off after only a very short period of operation, a stable high-speed airflow fails to develop in the return pipe; consequently, the lubricating oil remains trapped in the piping. If the oil return is less than the oil pumped, the compressor will be short on oil. The shorter the operating time, the longer the pipelines, and the more complex the system, the more prominent the oil return issue becomes. During defrosting, the temperature of the evaporator rises, the viscosity of the lubricating oil decreases, allowing it to flow more easily. After the defrost cycle, the refrigerant flow rate is high, and the retained lubricating oil returns to the compressor in large quantities. Therefore, the frequency of the defrost cycle and its duration per cycle also need to be carefully set to avoid significant fluctuations in oil level or even oil slugging. When there is a significant refrigerant leak, the return gas velocity decreases; if this velocity becomes too low, lubricating oil will accumulate in the return gas piping and fail to return to the compressor quickly. Liquid carryover caused by refrigerant migration can also lead to difficulties in internal oil return, but this usually lasts for a short period of time, at most a few dozen minutes.