Why is the compressor short of oil, and why does the evaporator have an oil film

Why is the compressor short of oil, and why does the evaporator have an oil film

The compressor is a complex machine that runs at high speed. Ensuring adequate lubrication of the compressor crankshaft, bearings, connecting rods, pistons and other moving parts is the basic requirement for maintaining the normal operation of the machine. For this reason, compressor manufacturers require the use of designated brands of lubricating oil and require regular inspections of the lubricating oil level and color.

Lack of oil is one of the compressor faults that can be easily identified. When the compressor is out of oil, there is little or no oil in the crankcase.

　　 Compressor is a special air pump. When a large amount of refrigerant gas is discharged, it also entrains a small part of lubricating oil (called running oil or running oil). Compressor rushing is inevitable, but the running speed is different.

　　 semi-hermetic piston compressor has about 2-3% of the lubricating oil in the exhaust gas, while the scroll compressor has 0.5-1%. For a 6-cylinder compressor with a displacement of 100m3/hr and a crankcase oil storage capacity of 6 liters, 3% oil rushing means about 0.3-0.8 liters/min of oil rushing, or the compressor runs without oil return The time is ten minutes. If the lubricating oil discharged from the compressor does not return, the compressor will be short of oil.

　　 There are two ways to return oil to the compressor: one is to return the oil from the oil separator, and the other is to return the oil from the return pipe.

The oil separator is installed on the compressor exhaust pipe, 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 pipeline is greatly reduced, thereby effectively extending the operation without oil return. time.

It’s 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 A bad system will cause the compressor to stop due to low oil pressure. 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. The lubricating oil enters the evaporator.

　　 On the one hand, due to low temperature and low solubility, part of the lubricating oil is separated from the refrigerant; on the other hand, with low temperature and high viscosity, the separated lubricating oil is easy to adhere to the inner wall of the pipe and it is 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 pipeline must be conducive to oil return. The common practice is to adopt a descending pipeline design and ensure a larger air flow velocity.

　　In practical applications, oil return problems caused by improper evaporator and return line design are not uncommon. For the r22 and r404a systems, the oil return of the flooded evaporator is relatively difficult, and the oil return pipeline design of the system 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 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 oil 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 oil return under low load, the vertical suction pipe can adopt double vertical pipes.

　　 The frequent start of the compressor 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.

　　 The temperature of the evaporator increases during defrosting, and the viscosity of the lubricating oil decreases, making it easy to flow. After the defrost cycle, the refrigerant flow rate is large, and the retained lubricating oil will return to the compressor in a concentrated manner. Therefore, the frequency of the defrost cycle and the duration of each time must also be carefully set to avoid large fluctuations in the 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 start-up with liquid caused by refrigerant migration can also cause difficulty in internal oil return, but it usually takes a short time, up to ten minutes.