What problems occur when an air compressor lacks oil and has insufficient lubrication?

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An air 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, air compressor manufacturers require the use of lubricants of specified grades, and they demand regular checks on the lubricant level and color. However, due to negligence in the design, installation, and maintenance of refrigeration systems, insufficient lubrication of moving parts is a common issue caused by factors such as oil deficiency in air compressors, oil coking and degradation, liquid backflow leading to dilution, refrigerant erosion, and the use of low-quality lubricants. Insufficient lubrication can cause wear or scratches on the bearing surfaces; in severe cases, it may lead to the shaft seizing, the piston getting stuck inside the cylinder, and subsequent bending or breaking of the connecting rod. The air compressor lacks oil. Lack of oil is one of the compressor faults that is easy to identify; when there is not enough oil in the compressor, there is very little or no lubricant in the crankcase. An air compressor is a special type of pump; as a large amount of refrigerant gas is discharged, it also carries away a small amount of lubricating oil (referred to as oil leakage). Oil leakage from air compressors is inevitable; it’s just that the speed of such leakage varies. Approximately 2-3% of the lubricating oil is present in the exhaust gas of semi-positive displacement piston compressors, whereas 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 air compressor is not returned, the compressor will run out of oil. There are two ways for the air 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 pipe and is capable of separating 50-95% of the oil; 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 well-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 air 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. Once the lubricant enters the evaporator, on one hand, its low solubility due to the low temperature causes some of it to separate from the refrigerant. On the other hand, the high viscosity at low temperatures makes it easy for the separated lubricant to adhere to the inner walls of the tubes, resulting in difficult flow. The lower the evaporation temperature, the more difficult it is to return the oil. This requires that the design, construction of the evaporation pipeline and the return air pipeline must facilitate oil return; a common approach is to use a downward-facing pipeline design and ensure a high airflow velocity. For refrigeration systems operating at extremely low temperatures, such as medical cryostats at -85°C and -150°C, in addition to using efficient oil separators, special solvents are often added to prevent lubricating oil from clogging the capillaries and expansion valves, and to assist in the return of oil. In practical applications, oil return problems caused by improper design of the evaporator and return air circuits are not uncommon. For R22 and R404A systems, it is very difficult to return oil to a full-liquid evaporator; therefore, 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 air 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 oil bends should be appropriate; when there are a large number of such 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 conditions, a vertical suction pipe can use dual standpipes. Frequent starting of the air compressor is not conducive to oil return. Due to the short continuous operation time, the air compressor stops, and no stable high-speed airflow can form in the return pipe, so the lubricating oil remains inside the pipeline. If the return oil volume is less than the oil flowing out, the air compressor will run out of oil. The shorter the operating time, the longer the pipelines, and the more complex the system, the more prominent the oil return issue becomes. For fully enclosed air compressors (including scroll compressors and rotary vane compressors) that lack a hydraulic safety switch, as well as some semi-enclosed compressors, damage caused by frequent startups is relatively common. Compressor maintenance is equally important. During defrosting, the evaporator temperature 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. If the air compressor is lubricated with oil, it also needs to be cleaned regularly, as well as having the oil removed from it. For removing oil from air compressors, it is advisable to use reliable oil removal equipment to make things easier. Oil removal in the compressor systems of the pharmaceutical industry is quite important.