Analysis of insufficient lubrication of air compressor due to lack of oil: Air compressor manufacturers require the use of specified brands of lubricating oil and require air compressors to check the oil level and color of lubricating oil regularly. However, ensuring the adequate lubrication of moving parts such as the crankshaft, bearings, connecting rods, pistons, etc. of the air compressor is a basic requirement for maintaining the normal operation of the machine. For this reason. The air compressor is a complex machine that runs at high speed. Due to negligence in the design, construction and maintenance of the refrigeration system, it is rare for the air compressor to lack oil, oil coking and deterioration, return liquid dilution, refrigerant flushing, and the use of inferior lubricating oil to cause insufficient lubrication of moving parts. Lack of lubrication can cause wear or scratches on the bearing surface. In severe cases, it can cause shaft seizure, piston stuck in the cylinder, and the resulting bending and fracture of the connecting rod.
1. Lack of oil
The air compressor is a special air pump. The vortex compressor is 0.5-1%. For a 6-cylinder air compressor with a displacement of 100m3/hr and a crankcase oil storage capacity of 6 liters, a large amount of refrigerant gas will also carry away a small amount of lubricating oil (called oil running or oil leakage) when being discharged. Oil running in air compressors cannot be prevented, but the oil running speed is different. There is about 2-3% lubricating oil in the exhaust of semi-sealed piston air compressors. 3% oil running means about 0.3-0.8 liters/minute of oil running, and the air compressor accessories run for more than ten minutes without oil return. The lubricating oil discharged from the air compressor does not return. One is oil return in the oil separator, and the air compressor will be short of oil. There are two ways to return oil to the air compressor. The other is oil return in the return air pipe. The oil separator is installed on the exhaust pipe of the air compressor. It can generally separate 50-95% of the oil. The oil return effect is good and the speed is fast. It greatly reduces the amount of oil entering the system pipeline, thereby effectively extending the oil return time. It is not uncommon for cold storage refrigeration systems with particularly long pipelines, full liquid ice making systems, and freeze-drying equipment with very low temperatures to not return oil or return very little oil for more than ten minutes or even dozens of minutes after startup. Poorly designed systems will have the problem of air compressor oil pressure being too low and shutting down. This refrigeration system is equipped with a high-efficiency oil separator, which can greatly extend the oil return time of the air compressor, allowing the air compressor to safely pass the crisis stage of no oil return after startup.
The unseparated lubricating oil will enter the system. An oil cycle is formed. After the lubricating oil enters the evaporator, it flows in the pipe with the refrigerant. On the one hand, due to the low solubility at low temperature, a part of the lubricating oil is separated from the refrigerant; on the other hand, due to the low temperature and high viscosity, the separated lubricating oil is easy to adhere to the inner wall of the pipe, making it difficult to flow. The lower the evaporation temperature, the more difficult it is to return the oil. This requires that the design and construction of the evaporation pipeline and the return air pipeline must be conducive to oil return. A rare practice is to adopt a descending pipeline design and ensure a large air flow rate. For screw air compressors, for refrigeration systems with particularly low temperatures, such as -85 ° C and -150 ° C medical cryogenic boxes, in addition to selecting high-efficiency oil separators, special solvents are usually added to prevent lubricating oil from clogging the capillary and expansion valve and to help oil return. In practical applications. The oil return of a full-liquid evaporator is very difficult. Oil return problems caused by improper design of the evaporator and return air pipeline are not uncommon. For R22 and R404A systems. The system return oil pipeline design must be very careful. For such a system, the use of high-efficiency oil separators can greatly reduce the amount of oil entering the system pipeline and effectively extend the time without oil return in the return air pipe after startup. When the number of return oil bends is relatively large, some lubricating oil should be added. When the air compressor is higher than the evaporator, the oil storage should be reduced. The spacing between the return oil bends should be appropriate. The oil return bend on the vertical return air pipe is necessary. The oil return bend should be as compact as possible. The oil return line of the variable load system must also be careful. When the load decreases. Too low speed is not conducive to oil return. In order to ensure oil return under low load, the return air speed will be reduced. The vertical suction pipe can use double risers. Frequent start-up of the air compressor is conducive to oil return. Since the air compressor stops after a short continuous operation time, there is no time to form a stable high-speed airflow in the return air pipe. The air compressor will be short of oil. The shorter the operation time, the lubricating oil can only remain in the pipeline. The return oil is less than the run oil. The longer the pipeline and the more complex the system, the more prominent the oil return problem. For fully enclosed air compressors (including vortex air compressors and rotor air compressors) and some semi-enclosed air compressors without oil pressure safety switches, the damage caused by frequent start-up is relatively large.
Air compressor maintenance is equally important. The evaporator temperature rises when the air compressor is defrosted. It is easy to flow. After the defrost cycle, the viscosity of the lubricating oil decreases. The refrigerant flow rate is high, and the retained lubricating oil will return to the air compressor in a concentrated manner. Therefore, the frequency of the defrost cycle and the duration of each defrost cycle must also be carefully set to prevent large fluctuations in the oil level or even oil shock. When there is a lot of refrigerant leakage, the return air speed will decrease. It cannot return to the air compressor quickly. Too low a speed will cause the lubricating oil to remain in the return air pipeline. The return of the lubricating oil to the air compressor housing does not mean that it returns to the crankcase. Air compressor using the crankcase negative pressure oil return principle. The pressure in the crankcase rises, and if the piston leaks due to wear, etc. The return oil check valve is automatically closed by the pressure difference, and the lubricating oil returned from the return air pipe is retained in the motor cavity and cannot enter the crankcase. This is the internal oil return problem, which can also cause oil shortage. This accident not only occurs in worn old machines, but also the start-up with liquid caused by refrigerant migration can cause internal oil return difficulties, but usually the time is short, at most ten minutes.
When the internal oil return problem occurs. Until the oil pressure safety device is activated. After the air compressor is shut down, it can be observed that the oil level of the air compressor is constantly decreasing. The oil level of the crankcase recovers quickly. The root cause of the internal oil return problem is cylinder leakage, and the worn piston assembly should be replaced in time. The oil pressure safety device will automatically shut down when there is no oil. There are no obvious symptoms when there is no oil, and the air compressor is not damaged. There is no oil sight glass and oil pressure safety device for fully enclosed air compressors (including rotor and scroll air compressors) and air-cooled air compressors. It will not shut down, and the air compressor will be worn and damaged unknowingly. The noise, vibration or current of the air compressor is too large, which may be related to the lack of oil. It is very important to accurately judge the operating status of the air compressor and the system. Low ambient temperature may cause some oil pressure safety devices to fail, which will cause wear of the air compressor. The wear caused by lack of oil in Hefei air compressors is generally uniform. If there is little or no lubricating oil. The temperature will rise rapidly in a few seconds. If the power of the motor is large enough, there will be severe friction on the surface of the bearing. The crankshaft will continue to rotate, and the crankshaft and bearing surfaces will be worn or scratched, otherwise the crankshaft will be locked by the bearing and stop rotating. The reciprocating motion of the piston in the cylinder is the same. Lack of oil will cause wear or scratches. In severe cases, the piston will be stuck in the cylinder and cannot move.
2. Insufficient lubrication
The direct cause of wear is lack of lubrication. Lack of oil will definitely cause insufficient lubrication. However, the viscosity is too small, but insufficient oil lubrication is not necessarily caused by lack of oil. The following three reasons can also cause insufficient lubrication: the lubricating oil cannot reach the bearing surface; the lubricating oil has reached the bearing surface. It cannot form an oil film of sufficient thickness; the lubricating oil has reached the bearing surface, but it has decomposed due to overheating and cannot play a lubricating role. Blockage of the oil suction net or oil supply pipeline, oil pump failure, etc. will affect the delivery of lubricating oil. However, bearing wear, excessive clearance, etc. cause oil leakage and low oil pressure, and the lubricating oil cannot reach the friction surface far away from the oil pump. The oil suction net and oil pump are normal. The air compressor accessories will prevent the friction surface far away from the oil pump from getting lubricating oil, causing wear and scratches.
Liquid backflow is a rare system problem. One of the hazards of low viscosity and return liquid is that it dilutes the lubricating oil. After the diluted lubricating oil reaches the friction surface, it cannot form a protective oil film of sufficient thickness, which will cause wear over time. When the amount of return liquid is relatively large, the lubricating oil will be very thin, not only can it not play a lubricating role, but it will also dissolve and wash away the original oil film, causing refrigerant scouring. Due to various reasons (including the start-up phase of the air compressor), the temperature of the friction surface that does not receive lubricating oil will rise rapidly. Many serious accidents, including connecting rod seizure and piston cylinder seizure, are related to this vicious cycle. The lubricating oil begins to decompose after exceeding 175 ° C. Lack of lubrication - friction - high temperature on the surface - oil decomposition " A typical vicious cycle. Lack of lubrication and oil shortage can be seen in the disassembled air compressor. Lack of oil is generally manifested as large-area, relatively uniform surface damage and high temperature. Such as the bearing surface away from the oil pump. Insufficient lubrication causes more wear, scratches and high temperature in some specific parts. When the piston moves up and down. And provide sufficient lubrication. If the exhaust valve plate is bent or broken, the load on the piston pin is alternated between the upper and lower parts of the bearing surface. This allows the lubricating oil to brush the piston pin evenly. Or the air compressor temporarily operates at a high pressure ratio, which will cause insufficient lubrication and wear on one side of the piston pin, and the pores will increase. If the piston pin has a shaking gap, the piston will be thrown out at the top dead center and hit the valve plate and valve plate, causing a knocking sound. Therefore, when replacing the valve plate, the wear of the piston pin should be checked. .
3. Conclusion and Suggestions
Lack of oil will cause serious lack of lubrication. Rather, the system has poor oil return. The air compressor is equipped with an oil separator to quickly return oil. The fundamental reason for lack of oil is not how much oil the air compressor runs and how fast it runs. Extend the air compressor's oil return-free operation time. The design of the evaporator and the return air pipeline must take oil return into consideration. Maintenance measures such as preventing frequent starts, timed defrosting, timely replenishing refrigerant, and timely replacing worn piston components are also helpful for oil return. Liquid return and refrigerant migration will dilute the lubricating oil. This will lead to lack of oil on the friction surface; high temperature on the friction surface will cause the lubricating oil to decompose, which is conducive to the formation of oil film; oil pump failure and oil line blockage will affect the oil supply and oil pressure. This will cause the lubricating oil to lose its lubricating ability. Lack of lubrication caused by these three problems often causes damage to the air compressor.