The air compressor is a complex machine that operates at high speed. Ensuring adequate lubrication of the crankshaft, bearings, connecting rods, pistons and other moving parts of the air compressor is a basic requirement to maintain the normal operation of the machine. For this reason, air compressor manufacturers require the use of specified brands of lubricating oil and require regular inspection of the lubricating oil level and color. However, due to negligence in the design, construction and maintenance of the refrigeration system, it is common for air compressors to suffer from insufficient lubrication of moving parts due to lack of oil, oil coking and deterioration, liquid back dilution, refrigerant flushing, and the use of inferior lubricants. Insufficient lubrication will cause the bearing surface to be worn or scratched. In severe cases, it may cause the shaft to be held, the piston to be stuck in the cylinder, and the resulting connecting rod to bend or break.
Lack of oil
Lack of oil is one of the easily identifiable compressor faults. When the air compressor is short of oil, there is little or even no lubricating oil in the crankcase. The air compressor is a special air pump. When a large amount of refrigerant gas is discharged, a small amount of lubricating oil is also entrained (called running oil or running oil). It is unavoidable for air compressors to run out of oil, but the speed of oil running is different. The operation time of the air compressor without oil return is more than ten minutes. If the lubricating oil discharged from the air compressor does not come back, the compressor will be short of oil. Generally speaking, there are two ways to return oil to an air compressor, one is oil return from the oil separator, and the other is oil return from the air return pipe.
When an internal oil return problem occurs, you can observe that the compressor oil level continues to drop until the oil pressure safety device operates. After the compressor is shut down, the oil level in the crankcase recovers quickly. The root cause of the internal oil return problem is cylinder leakage, and worn piston components should be replaced in time.
The oil pressure safety protection device will automatically shut down when there is lack of oil to protect the compressor from damage. Fully enclosed compressors (including rotor and scroll compressors) and air-cooled compressors without oil sight glasses and oil pressure safety devices will have no obvious symptoms and will not shut down when they are short of oil. The air compressor will unknowingly Wear damage. Air compressor noise, vibration or excessive current may be related to lack of oil. It is very important to accurately judge the operating status of the air compressor and system. Too low ambient temperature may cause some oil pressure safety devices to malfunction and cause compressor wear.
The wear caused by the lack of oil in the air compressor is generally relatively uniform. If there is little or no lubricating oil, severe friction will occur on the bearing surface and the temperature will rise rapidly within seconds. If the power of the motor is large enough, 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 bearings and stop rotating. The same goes for the reciprocating motion of the piston in the cylinder. Lack of oil will cause wear or scratches. In severe cases, the piston will get stuck in the cylinder and cannot move.
Insufficient lubrication
The direct cause of wear is insufficient lubrication. Lack of oil will definitely cause insufficient lubrication, but insufficient 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, but the viscosity is too low to form a sufficiently thick oil film; the lubricating oil has reached the bearing surface, but it decomposes due to overheating If it falls off, it will no longer provide lubrication.
Blockage of the oil suction net or oil supply pipeline, oil pump failure, etc. will affect the delivery of lubricating oil, and the lubricating oil cannot reach the friction surface away from the oil pump. The oil suction screen and oil pump are normal, but bearing wear, excessive clearance, etc. cause oil leakage and low oil pressure. The friction surface away from the oil pump will not get lubricant, causing wear and scratches.
Liquid backflow is a common system problem, and one of the major hazards of liquid backflow is diluting lubricating oil. After the diluted lubricating oil reaches the friction surface, the viscosity is low and it cannot form a protective oil film of sufficient thickness, which will cause wear over time. When the amount of liquid return is relatively large, the lubricating oil will be very thin, which will not only fail to provide lubrication, but will also dissolve and wash away the original oil film, causing refrigerant washout.
Due to various reasons (including the startup stage of the compressor), the temperature of the friction surface that does not receive lubricating oil will rise rapidly. After exceeding 175°C, the lubricating oil will begin to decompose. "Insufficient lubrication - friction - high surface temperature - oil decomposition" is a typical vicious cycle. Many vicious accidents, including connecting rods holding shafts and piston cylinders stuck, are related to this vicious cycle.
Insufficient lubrication and lack of oil can be seen in a disassembled air compressor. Lack of oil generally manifests itself as large-area, relatively uniform surface damage and high temperatures, while insufficient lubrication is more likely to result from wear, scratches and high temperatures in some specific parts, such as the bearing surface far away from the oil pump.
As the piston moves up and down, the load on the piston pin is alternated between the upper and lower parts of the bearing surface, which allows the lubricating oil to brush across the piston pin evenly and provide adequate lubrication. If the exhaust valve plate is bent or broken, or the compressor is exposed to high pressure for a long time, it will cause insufficient lubrication and wear on one side of the piston pin, resulting in large pores. If there is a rocking clearance in the piston pin, the piston will be thrown out at the top dead center and hit the valve piece and valve plate, causing a knocking sound. Therefore, when replacing the valve plate, the piston pin wear should be checked.
conclusion and suggestion
Lack of oil will cause severe insufficient lubrication. The root cause of the lack of oil is not the amount of oil in the air compressor and its slowness, but poor oil return to the system. Installing an oil separator can quickly return oil and extend the operation time of the air compressor without oil return. The design of the evaporator and return air piping should take oil return into consideration. Maintenance measures such as avoiding frequent starts, timing defrost, timely replenishment of refrigerant, and timely replacement of worn piston components can also help oil return.
Liquid return and refrigerant migration will dilute the lubricating oil, which is not conducive to the formation of oil film; oil pump failure and oil circuit blockage will affect the oil supply and oil pressure, resulting in a lack of oil on the friction surface; high temperature on the friction surface will promote the decomposition of the lubricating oil, causing the lubricating oil to decompose. Loss of lubrication ability. Insufficient lubrication caused by these three problems often causes damage to the air compressor.
The root cause of oil starvation lies in the system. Therefore, just replacing the air compressor or some accessories cannot fundamentally solve the problem of oil shortage.