Compressor is a complex machine running at high speed. Ensuring full lubrication of crankshaft, bearing, connecting rod, piston and other moving parts of compressor is the basic requirement to maintain the normal operation of the machine. To this end, compressor manufacturers require the use of specified brand lubricants, and require regular inspection of lubricant oil level and color. However, due to the negligence of refrigeration system design, construction and maintenance, the insufficient lubrication of moving parts caused by oil shortage, oil coking deterioration, fluid dilution, refrigerant scouring, and the use of inferior lubricants are quite common. Insufficient lubrication will lead to wear or scratch of bearing surface, and serious accidents of axle holding, piston clamping in cylinder and connecting rod bending and fracture will occur.
I. Insufficient lubrication
The direct cause of wear: insufficient lubrication. Deficiency of oil will certainly lead to insufficient lubrication, but insufficient lubrication is not necessarily caused by lack of oil. The following three reasons can also cause insufficient lubrication: the lubricant can not reach the bearing surface; although the lubricant has reached the bearing surface, but the viscosity is too small to form a sufficient thickness of oil film; although the lubricant has reached the bearing surface, but because of overheating and decomposition, it can not play a lubricating role.
Adverse effects: blockage of oil suction network or oil supply pipeline, oil pump failure and so on will affect the delivery of lubricating oil, lubricating oil can not reach the friction surface far from the oil pump. Oil suction net and oil pump are normal, but bearing wear and clearance are too large to cause oil leakage and oil pressure is too low, which will make the friction surface far away from the oil pump can not get lubricant, resulting in wear and scratch. For various reasons (including the start-up stage of the compressor), the temperature of the friction surface without lubricating oil will rise rapidly, and the lubricating oil will begin to decompose after the temperature exceeds 175 degree C. "Insufficient lubrication - friction - surface high temperature - oil decomposition" is a typical vicious cycle, many malignant accidents including connecting rod holding axle, piston jam are related to this vicious cycle. Insufficient lubrication and oil shortage can be seen in the dismantled compressor. Oil shortage is generally characterized by large area, relatively uniform surface damage and high temperature, while insufficient lubrication is more likely to be caused by wear, scratch and high temperature in some specific parts, such as bearing surface far away from the oil pump.
When the piston moves up and down, the load of the piston pin rotates between the upper and lower parts of the bearing surface, which allows the lubricant to brush the piston pin evenly and provides sufficient lubrication. If the exhaust valve sheet is bent or broken, or the compressor works at high pressure ratio for a long time, it will cause insufficient lubrication and wear on one side of the piston pin, and increase the porosity. When the piston pin has a swing clearance, the piston will be thrown out at the top dead center and hit the valve disc and disc, resulting in an impact sound. Therefore, when replacing the valve disc, the wear of the piston pin should be checked.
2. Oil shortage
Oil shortage is one of the compressor faults that can be easily identified. When the compressor is short of oil, there is little or no oil in the crankcase.
If the oil discharged from the compressor does not return, the compressor will be short of oil. There are two ways to return oil to compressor, one is to return oil to oil separator, the other is to return oil to air pipe. Oil separator installed in the compressor exhaust pipeline, can generally separate 50-95% of the running oil, oil return effect is good, fast, greatly reduce the amount of oil into the system pipeline, thereby effectively prolonging the no-return operation time. It is not uncommon for refrigeration systems with long pipelines, full-liquid ice-making systems and freeze-drying equipment with very low temperature not to return oil or very little oil in ten minutes or even tens of minutes after start-up. The problem that compressor oil pressure is too low and shutdown will occur in poorly designed systems. The installation of high efficiency oil separator in this refrigeration system can greatly prolong the running time of the compressor without oil return, and make the compressor safely pass through the crisis stage of no oil return after start-up.
Unseparated lubricants will enter the system and flow with refrigerant in the tube, forming an oil cycle. When lubricating oil enters the evaporator, on the one hand, because of the low temperature and low solubility, some lubricating oil is separated from the refrigerant; on the other hand, because of the high temperature and low viscosity, the separated lubricating oil is easy to adhere to the inner wall of the tube and the flow is difficult. The lower the evaporation temperature is, the more difficult it is to return oil. This requires that the design and construction of evaporation pipeline and return gas pipeline must be conducive to oil return. The common method is to adopt descending pipeline design and ensure a larger air flow rate. For refrigeration systems with extremely low temperatures, such as - 85 C and - 150 C medical cryogenic chambers, in addition to the selection of high-efficiency oil separators, special solvents are usually added to prevent lubricating oil from blocking capillaries and expansion valves, and to help return oil.
In practical application, the problem of oil return caused by improper design of evaporator and return pipeline is not rare. For R22 and R404A systems, it is very difficult to return oil to full-liquid evaporator, and the design of return pipeline must be very careful. For such a system, the use of high-efficiency oil can greatly reduce the amount of oil entering the system pipeline, effectively prolong the no-return time of the return pipe after the start-up. When the position of the compressor is higher than that of the evaporator, the oil return bend on the vertical return pipe is necessary. The return bend should be as compact as possible to reduce oil storage. The spacing between oil return bends should be appropriate. When the number of oil return bends is large, some lubricants should be added. Care must also be taken in the return pipeline of the variable load system. When the load decreases, the return air speed will decrease, too low is not conducive to return oil. In order to ensure oil return under low load, double risers can be used in vertical suction pipes.
Frequent start-up of compressor is not conducive to oil return. Because the compressor stops running for a very short time, there is no time to form a stable high-speed air flow in the return pipe, so the lubricant can only be left in the pipe.