How harmful is oil shortage to air compressors

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When the amount of lubricating oil inside the compressor is less than a certain level, the lubrication effect between the internal moving parts will not meet the requirements, resulting in severe friction or even hard damage, and eventually the compressor will be stuck and scrapped.

The root cause of oil shortage is that lubricating oil and refrigerant are mutually soluble, and the solubility is related to temperature and pressure. The lubricating oil will be discharged from the compressor along with the refrigerant and enter the other three major components and pipelines of the refrigeration and air conditioning system. and attachments. At the same time, the oil will circulate back to the compressor shell along with the refrigerant along the pipeline.

Under normal circumstances, the amount of oil running out is equal to the amount of oil running back, reaching a dynamic balance. The dead zone in the system will also retain a part of lubricating oil (the dead zone is the area where the refrigerant flow rate is zero), and the remaining oil in the compressor oil pool should be enough to ensure the lubrication needs of the compressor. However, in some applications, there is a lot of oil that escapes, and there is a lot of lubricating oil remaining in the system and pipelines. There will be situations where oil shortage leads to scrapping. Another situation is that the dryness and purity of the oil in the system are not enough. Moisture, impurities or other incompatible substances or even weak acids affect the quality of the lubricating oil, which will also cause poor lubrication and eventually lead to the quality of the compressor being broken.

To prevent the compressor from lacking oil, two aspects need to be considered

1. Minimize oil escape from the compressor

Professional refrigerant channels can play the role of oil separation, and the OCR is controlled very low, thereby reducing the discharge of lubricating oil from the compressor. At the same time, in terms of system design, it is more effective to install an oil separator to directly transport the oil that escapes back to the intake end of the compressor by relying on the pressure of the refrigerant, so as to increase the amount of lubricating oil in the oil pool of the compressor under the final dynamic equilibrium state. . In addition, the control logic design of the system needs to consider load control, reducing the number of start and stop of the compressor, and correspondingly reducing the discharge of lubricating oil.

2. Increase the amount of oil that runs back to the compressor

The most difficult part of the whole system to return oil is the section from the evaporator to the suction port of the compressor, where the temperature and pressure are low, the viscosity of the lubricating oil is high, and the fluidity is poor, which is relatively difficult for oil return. Due to the high temperature of the hot gas pipeline, if the hot gas pipeline is not very long, there is generally no problem with the refrigerant gas carrying oil, and the solubility of the liquid refrigerant and lubricating oil in the liquid supply pipeline is very good, and generally there is no need to consider the end point. oil problem. The oil can be brought back to the compressor mainly depends on the flow rate of the refrigerant, but if the flow rate is too fast, the pipeline will vibrate too much. Therefore, the proper pipe diameter design is very important for the oil return. Generally, we recommend that the return air pipeline ascends the suction The minimum flow velocity of the air pipeline is 8m/s, the minimum flow velocity of the horizontal or descending pipeline is 5m/s, and the maximum flow velocity is 20m/s. For the pipeline design, please refer to the next issue of Mr. Sha’s classroom.

In fact, in addition to the main factor affecting the oil return, in addition to the main factor of the pipe diameter, it is also related to the control of the system, the way of the pipeline, the layout of the elbow, the level of evaporation temperature, and the dynamic viscosity of the oil. There are also ways to prevent oil shortage. Many, in addition to the most basic pipe diameter design just mentioned, there are many different measures for parallel compressors, variable frequency compressors, etc., such as oil return control logic at part load.