Fault analysis and treatment of excessive oil consumption of the refrigeration compressor of piston chiller Analysis and Treatment of Excessive Oil Consumption of Refrigerating Compressor of Piston Chiller
Compressor is a complex machine running at high speed. Ensuring adequate lubrication of moving parts such as compressor crankshaft, bearings, connecting rods, pistons, etc. is the basic requirement for maintaining the normal operation of the machine. For this reason, compressor manufacturers require the use of specified grades 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, the lack of oil in the compressor, the deterioration of the oil coking, the dilution of the liquid back, the flushing of the refrigerant, the use of inferior lubricating oil, etc. cause insufficient lubrication of the moving parts. Insufficient lubrication will cause wear or scratches on the bearing surface. In severe cases, it will cause shaft holding, piston jamming in the cylinder and the resulting connecting rod bending and fracture accidents. Phenomenon 1: Insufficient lubrication The direct cause of wear: insufficient lubrication. The lack of oil will definitely cause insufficient lubrication, but the lack of lubrication is not necessarily caused by the lack of oil. Insufficient lubrication can also be caused by the following three reasons: the lubricating oil cannot reach the bearing surface; the lubricating oil has reached the bearing surface, but the viscosity is too small to form an oil film of sufficient thickness; the lubricating oil has reached the bearing surface, but it decomposes due to overheating If it is lost, it will not lubricate. The resulting adverse effects: blockage of the oil suction network 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 net and the oil pump are normal, but the bearing wear, excessive clearance, etc. cause oil leakage and low oil pressure, which will prevent the friction surface away from the oil pump from getting lubricating oil, causing wear and scratches. Due to various reasons (including the compressor start-up phase), the friction surface temperature that does not get lubricating oil will rise rapidly, and the lubricating oil will begin to decompose after exceeding 175°C. Insufficient lubrication-friction-high temperature on the surface-oil decomposition; is a typical vicious cycle, many vicious accidents including connecting rod holding shaft, piston clamping cylinder are related to this vicious cycle. Insufficient lubrication and oil shortage can be seen in the disassembled compressor. Oil shortage generally manifests as large area, relatively uniform surface damage and high temperature, while insufficient lubrication is more abrasion, scratches and high temperature in some specific parts, such as the bearing surface away from the oil pump. When the piston moves up and down, the load of the piston pin alternates between the upper and lower parts of the bearing surface, which allows the lubricant to brush the piston pin evenly and provide sufficient lubrication. If the exhaust valve piece is bent or broken, or the compressor is operated at a high pressure ratio for a long time, it will cause insufficient lubrication and wear on one side of the piston pin, and the pores will increase. If the piston pin has sloshing clearance, the piston will be thrown out at the top dead center and strike the valve disc and valve plate, producing a knocking sound. Therefore, when replacing the valve, check the wear of the piston pin. Phenomena 2: lack of oil 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 lubricant in the crankcase. The compressor is a special air pump, a large amount of refrigerant gas is discharged while also entraining a small part of lubricating oil (called running oil or running oil). Compressor running oil is inevitable, but the speed of running oil is different. There is about 2-3% lubricating oil in the exhaust of the semi-sealed piston compressor, and 0.5-1% for scroll compressors. For a 6-cylinder compressor with a displacement of 100m3/hr and a crankcase oil storage of 6 liters, 3% of oil flow means about 0.3-0.8 liters/minute of oil flow, or the compressor runs without oil return The time is ten minutes. If the lubricating oil discharged from the compressor is not returned, the compressor will run out of oil. There are two ways to return oil to the compressor, one is oil return from the oil separator, and the other is oil return from the air return pipe. The oil separator is installed on the exhaust pipe of the compressor, and generally can separate 50-95% of the oil. The oil return effect is good, the speed is fast, and the amount of oil entering the system pipe is greatly reduced, thereby effectively extending No oil return operation time. It is not uncommon for cold storage refrigeration systems with extremely long pipelines, full-liquid ice-making systems, and freeze-drying equipment with very low temperatures to return more than ten minutes or even dozens of minutes after starting up, or with very little oil return. Design Bad system will cause the problem of compressor oil pressure is too low and shut down. The installation of a high-efficiency oil separator in this refrigeration system can greatly extend the operating time of the compressor without oil return, so that the compressor can safely pass the crisis stage of no oil return after starting up. The lubricating oil that has not been separated will enter the system and flow with the refrigerant in the tube to form an oil circulation. After the lubricating oil enters the evaporator, on the one hand, a part of the lubricating oil is separated from the refrigerant due to the low solubility and low temperature; on the other hand, the low temperature and the high viscosity make the separated lubricating oil easily adhere to the inner wall of the pipe, and the flow is more difficult. 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 pipeline must be conducive to oil return. The common practice is to adopt a descending pipeline design and ensure a larger air flow velocity. For refrigeration systems with extremely low temperatures, such as the lowest temperature tanks at -85°C and -150°C, in addition to 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 applications, oil return problems caused by improper design of the evaporator and return line are not uncommon. For R22 and R404A systems, the oil return of the flooded evaporator is very difficult, and the system's oil return pipeline design must be very careful. For such a system, the use of high-efficiency oil separator can greatly reduce the amount of oil entering the system pipeline, and effectively extend the non-oil return time of the air return pipe after starting up. When the compressor is higher than 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 the oil return bends should be appropriate. When the number of return bends is large, some lubricant should be added. The return line of the variable load system must also be careful. When the load is reduced, the air return speed will decrease, too low speed is not conducive to oil return. In order to ensure the return of oil under low load, the vertical suction pipe can use double riser.
