Why is the compressor lacking oil and has insufficient lubrication?

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A compressor is a complex machine that operates at high speeds; ensuring adequate lubrication of its moving parts such as the crankshaft, bearings, connecting rods, and pistons is a fundamental requirement for maintaining its proper 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. 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 the compressor, oil coking and deterioration, liquid backflow leading to dilution, coolant 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. 2. Lack of oil – Lack of oil is one of the compressor faults 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 leakage from the compressor 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 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 dozens of 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 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. After the lubricating oil enters the evaporator, due to the low temperature which results in low solubility, a portion of the lubricating oil separates from the refrigerant ； On the other hand, at low temperatures the viscosity is high, and the separated lubricating oil tends to adhere to the inner wall of the pipe, making flow more difficult. 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-flowing pipeline design and ensure a high air flow velocity. For refrigeration systems operating at extremely low temperatures, such as medical cryostats at –85°C and –150°C, in addition to using high-efficiency 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 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 minimize 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 conditions, a vertical suction pipe can employ double standpipes. Frequent startup of the compressor is not conducive to oil return. Due to the short continuous operation time, the compressor stops, and there is not enough time for a stable high-speed airflow to form in the return pipe, so the lubricating oil remains inside the pipeline. If the return oil is less than the oil flowing out, the 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 hermetically sealed compressors (including scroll compressors and rotary vane compressors) that lack a hydraulic pressure safety switch, as well as some semi-hermetically sealed 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. When there is a significant leakage of refrigerant, the return gas velocity decreases. If this velocity is too low, the lubricating oil remains in the return gas line and cannot return to the compressor quickly. The return of lubricating oil to the compressor housing does not mean it returns to the crankcase. In compressors that utilize the principle of negative pressure in the crankcase for oil return, if leakage occurs in the pistons due to wear or other reasons, the pressure in the crankcase rises. The oil return check valve closes automatically as a result of this pressure difference, and the lubricating oil returning through the oil return pipe gets trapped in the motor chamber and cannot enter the crankcase. This is the issue of internal oil return, and it can also lead to a lack of oil. Such accidents occur not only in worn-out old machines; liquid entrapment resulting from refrigerant migration can also cause difficulties in oil return, although this usually lasts for a short period of time, at most a few dozen minutes. When an internal oil return issue occurs, it can be observed that the compressor oil level keeps dropping until the oil pressure safety device activates. After the compressor stops, the oil level in the crankcase returns to normal quickly. The root cause of the internal oil return issue is cylinder leakage; the worn piston assembly should be replaced promptly. The hydraulic safety device shuts down automatically in the event of a lack of oil, protecting the compressor from damage. Hermetically sealed compressors (including rotary and scroll compressors) that lack an oil sight glass and oil pressure safety device, as well as air-cooled compressors, show no obvious symptoms when low on oil and do not shut down; as a result, the compressors gradually wear out and get damaged unnoticed. Compressor noise, vibration, or excessive current may be related to a lack of oil; therefore, it is very important to accurately assess the operating condition of both the compressor and the system. Excessively low ambient temperatures may cause some oil pressure safety devices to fail, leading to compressor wear. The wear caused by a lack of oil in the compressor is generally fairly uniform. If there is little or no lubricating oil, severe friction occurs on the bearing surfaces, and the temperature rises rapidly within a few seconds. If the motor has sufficient power, the crankshaft will continue to rotate; otherwise, the surfaces of the crankshaft and bearings will become worn or scratched. In that case, the crankshaft will be locked by the bearings and stop rotating. The back-and-forth movement of the piston within the cylinder is the same; a lack of oil can lead to wear or scratches, and in severe cases the piston may get stuck inside the cylinder and be unable to move. 3. Insufficient lubrication: The direct cause of wear is insufficient lubrication. A lack of oil will certainly lead to insufficient lubrication, but insufficient lubrication is not necessarily caused by a lack of oil. The following three reasons can also lead to insufficient lubrication: the lubricant fails to reach the bearing surface ； Although the lubricating oil has reached the bearing surface, its viscosity is too low to form an oil film of sufficient thickness ； Although the lubricant has reached the bearing surface, it has decomposed due to overheating and cannot perform its lubricating function. Issues such as clogged oil suction screens or oil supply lines, as well as pump failures, can affect the delivery of lubricant, preventing it from reaching the friction surfaces that are far away from the pump. The oil suction screen and oil pump are in good condition, but issues such as bearing wear and excessive clearance can lead to oil leakage and low oil pressure; as a result, the friction surfaces far from the oil pump do not receive lubrication, causing wear and scratches. Liquid return is a common system issue, and one of the major hazards of liquid return is the dilution of lubricating oil. When the diluted lubricating oil reaches the friction surfaces, its low viscosity prevents the formation of a protective oil film of sufficient thickness, which over time leads to wear. When the amount of returning liquid is high, the lubricating oil becomes very thin; it fails to provide lubrication and instead dissolves and washes away the existing oil film, leading to refrigerant erosion. For various reasons (including during the compressor startup phase), the temperature of the friction surfaces where the lubricating oil is present can rise rapidly; once it exceeds 175°C, the lubricating oil begins to decompose. “\"Insufficient lubrication – friction – high surface temperature – oil breakdown\" is a typical vicious cycle, and many serious accidents, including rod seizure and piston sticking in the cylinder, are related to this cycle. Insufficient lubrication and oil deficiency can be seen in the disassembled compressor. Oil deficiency generally manifests as extensive, relatively uniform surface damage and high temperatures, whereas insufficient lubrication leads more to wear, scratches, and high temperatures in specific areas, such as the bearing surfaces far from the oil pump. As the piston moves up and down, the load on the piston pin alternates between the upper and lower surfaces of the bearing, which allows the lubricating oil to evenly coat the piston pin and provide sufficient lubrication. If the exhaust valve leaf is bent or broken, or if the compressor operates at a high pressure ratio for an extended period, it will result in insufficient lubrication on one side of the piston pin, leading to wear and an increase in the size of the pores. If there is play in the piston pin, the piston will be thrown to the top dead center and strike the valve leaf and valve plate, resulting in a knocking sound. Therefore, when replacing the valve disc, the wear of the piston pin should be checked. 4. Conclusions and Recommendations A lack of oil can lead to severe lubrication deficiencies. The root cause of this issue is not related to the amount or speed at which oil is discharged from the compressor, but rather to poor oil return within the system. Installing an oil separator allows for rapid oil return, extending the time during which the compressor can operate without oil return. The design of the evaporator and return air circuits must take oil return into account. Maintenance measures such as avoiding frequent startups, scheduling defrosting, replenishing refrigerant in a timely manner, and replacing worn piston components promptly also help with oil return. Liquid return and refrigerant migration dilute the lubricating oil, which hinders the formation of an oil film ； Oil pump failures and clogged oil circuits can affect the oil supply volume and oil pressure, resulting in a lack of oil at the friction surfaces ； High temperatures on the friction surface cause the lubricant to decompose, resulting in it losing its lubricating capacity. Lack of lubrication caused by these three issues often also leads to compressor damage. 13926549484: Compressor oil – we highly recommend CPI synthetic lubricants from the United States: www.cpihualai.178b2b.com. The root cause of oil deficiency lies in the system itself. Therefore, simply replacing the compressor or some components cannot fundamentally solve the oil shortage problem.