A coup to avoid the oil return problem of the refrigeration compressor of the piston chiller
A coup to avoid the problem of oil return to the refrigeration compressor of the piston chiller Compressor: It is a complex machine running at high speed. Ensuring adequate lubrication of moving parts such as compressor crankshafts, 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 the bearing surface to wear or scratch, and in serious cases, it will cause the shaft and piston to be stuck in the cylinder and the resulting connecting rod bending and breaking accidents. 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; although the lubricating oil has reached the bearing surface, it decomposes due to overheating If it is lost, it will not lubricate. Adverse effects caused: 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 stage), the friction surface temperature without lubricating oil will rise rapidly, and the lubricating oil will begin to decompose after exceeding 175°C. Insufficient lubrication-friction-high surface temperature-oil decomposition; is a typical vicious circle. Many vicious accidents, including connecting rod shaft holding and piston jamming, are related to this vicious circle. 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 is alternated 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. 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. 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 compressor exhaust pipe. Generally, 50-95% of the oil can be separated. The oil return effect is good, the speed is fast, and the amount of oil entering the system pipeline is greatly reduced, thus effectively extending the operation without oil return. 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 pipe to form an oil circulation. After the lubricating oil enters the evaporator, part of the lubricating oil is separated from the refrigerant due to low temperature and low solubility; on the other hand, low temperature and high viscosity, the separated lubricating oil is easy to adhere to the inner wall of the pipe and flow is 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 particularly low temperatures, such as -85; C and -150; C minimum temperature tanks, in addition to the use of high-efficiency oil separators, special solvents are usually added to prevent lubricating oil from clogging the capillary and expansion valve 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. Frequent compressor startup is not conducive to oil return. Since the compressor stops for a short continuous operation time, there is no time to form a stable high-speed air flow in the return pipe, and the lubricating oil can only stay in the pipe. If the oil return is less than Ben oil, the compressor will be short of oil. The shorter the operating time, the longer the pipeline and the more complex the system, the more prominent the oil return problem. For hermetic compressor without oil pressure safety switch (Including scroll compressors and rotor compressors) and partially semi-hermetic compressors), the damage caused by frequent start-up is more. Compressor maintenance is equally important. During defrosting, the temperature of the evaporator increases, the viscosity of the lubricating oil decreases, and it is easy to flow. After the defrost cycle, the refrigerant flow rate is high, and the retained lubricant will be concentrated back to the compressor. Therefore, the frequency of the defrost cycle and the duration of each time should also be set carefully to avoid large fluctuations in oil level or even oil shock. When the refrigerant leaks too much, the air return speed will decrease. If the speed is too low, the lubricating oil will stay in the air return line and cannot return to the compressor quickly. The lubricating oil returning to the compressor housing does not mean returning to the crankcase. In the compressor using the negative pressure return principle of the crank chamber, if the piston leaks due to wear, etc., the pressure in the crankcase rises, and the oil return check valve automatically closes due to the pressure difference, and the lubricating oil returned from the return pipe stays in In the motor cavity, it is impossible to enter the crankcase. This is the internal oil return problem. The internal oil return problem will also cause oil shortage. In addition to this kind of accidents that occur in worn out old machines, the start-up with liquid caused by refrigerant migration will also cause difficulty in internal oil return, but it usually takes a short time, up to ten minutes. When there is an internal oil return problem, it can be observed that the compressor oil level continues to drop until the oil pressure safety device operates. After the compressor stops, the oil level in the crankcase quickly recovers. The root cause of the internal oil return problem is cylinder leakage, so the worn piston assembly should be replaced in time. The oil pressure safety device will automatically stop when there is no oil to protect the compressor from damage. Hermetic compressors without sight glass and oil pressure safety devices (including rotor and scroll compressors) and air-cooled compressors have no obvious symptoms when oil is lacking, and they will not stop. The compressor will wear out and damage unconsciously. Compressor noise, vibration or excessive current may be related to lack of oil. It is very important to accurately judge the operating conditions of the compressor and the system. Too low ambient temperature may cause some oil pressure safety devices to malfunction, which will cause compressor wear. The wear caused by compressor oil shortage is generally more uniform. If there is little or no lubricating oil, severe friction will appear on the bearing surface, and the temperature will rise rapidly within a few 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 bearing and stop rotating. The reciprocating movement 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. 3. Conclusion The lack of oil will cause a serious lack of lubrication. The fundamental reason for the lack of oil is not how much and how fast the compressor runs, but the poor oil return of the system. The installation of an oil separator can quickly return oil and extend the compressor running time without oil return. The design of the evaporator and the return gas pipeline must take the oil return into consideration. Maintenance measures such as avoiding frequent start-up, defrosting regularly, replenishing refrigerant in time, and replacing worn piston components in time also help to return oil. Liquid return and refrigerant migration will dilute the lubricating oil, which is not conducive to the formation of the oil film; the failure of the oil pump and the blockage of the oil circuit will affect the oil supply and oil pressure, resulting in lack of oil on the friction surface; the high temperature of the friction surface will promote the decomposition of the lubricating oil and make the lubricating oil Loss of lubrication capacity. Inadequate lubrication caused by these three problems often causes compressor damage. The root cause of the lack of oil lies in the system. Therefore, only replacing the compressor or certain accessories cannot fundamentally solve the problem of oil shortage. Therefore, system design and pipeline construction must consider the problem of system oil return, otherwise there are endless troubles! For example, when designing and constructing, the evaporator return pipe is equipped with a return bend and the exhaust pipe is equipped with a non-reverse bend. All pipelines should move along the fluid The direction is downhill all the way, with a slope of 0.3~0.5%.
