Summary of features and failures of scroll compressors Deep analysis

The scroll compressor is a positive displacement compressor, and multiple compression chambers are formed by the meshing of the scroll rotor and the scroll stator. With the translation and rotation of the vortex rotator, the volume of each compression chamber continuously changes to achieve gas inhalation and compression. The scroll compressor has a simple structure, few moving parts, high volumetric efficiency, and low operating noise. In recent years, it has been widely used in air-conditioning heat pump units and has the potential to replace piston compressors. However, the application time of scroll compressors in China is not very long, and there are not many analysis of failures during its use. The author of okmarts.com conducted a detailed analysis of some faults that occurred during the actual operation of the scroll compressor and proposed corresponding improvement measures.

1 Main failure of scroll compressor

   The author classifies the compressor failures that have been dealt with in recent years, and there are mainly four types:

  ①The floating sealing ring is damaged, high and low pressure gas blows. According to the structural characteristics of the scroll compressor, in order to provide an appropriate gas pressure on the upper part of the scroll stator, an intermediate pressure channel is opened at the appropriate intermediate compression on the scroll stator to provide an intermediate pressure. A floating sealing device is arranged on the upper part of the intermediate pressure chamber, so the top of the scroll is affected by the exhaust pressure and the intermediate pressure. In addition to balancing the pressure of the compressed gas inside the scroll, it also provides a sealing force between the top and bottom grooves, which is achieved by a floating seal ring. The sealing ring is made of a non-metallic material similar to rubber or plastic. The fault phenomenon is generally manifested as the compressor motor is intact and can be energized, but the discharge pressure of the unit does not increase, the suction pressure does not decrease, there is almost no pressure difference between the suction and discharge, the exhaust pipe is not hot, and the suction The tube is not cold either. The compressor current is very different from the rated value, in fact the compressor is idling.

  ②The scroll plate is damaged. Scroll damage In addition to the above-mentioned damage to the floating seal ring, you can also hear the obvious metal impact sound inside the compressor, which is caused by the crushed metal fragments of the scroll colliding with each other or the compressor shell. sound.

  ③The motor burned out. When the power is turned on, the fuse is blown or the short circuit is tripped, and the compressor cannot be started.

  ④The motor holds the shaft and the bearing is damaged. When the compressor is powered on, a humming sound of the motor in the casing is heard, but it does not run, and the current rises quickly. After a few seconds, the compressor's internal overload protection or external thermal relay protection will act and cut off the power. Sometimes the protector is too late to act, and the locked-rotor current is quickly reached, which may directly cause the motor to burn.

  2 Failure cause analysis and prevention measures

  2.1 After anatomy of several malfunctioning compressors, the author found that the sealing ring was partially melted or broken. The reason is: due to refrigerant leakage and other reasons, the suction pressure decreases (but even if the low-pressure protection device is installed, it may not reach the protection setting value, and the low-pressure protection is not cut off), the suction superheat increases, resulting in The exhaust temperature rises rapidly. At this time, if the exhaust temperature protector is not installed or installed improperly, the system will be seriously overheated. The most effective way to avoid thermal damage to the sealing ring is to install the exhaust temperature protector correctly. The temperature setting of the exhaust temperature protector is generally 125-130℃; the temperature sensor of the exhaust temperature protector is generally installed on the exhaust pipe of the compressor, and the distance between the temperature sensor and the exhaust pipe is not more than 150mm. The fixing must be firm and strict insulation is required; the wiring of the exhaust temperature protector can be connected in series with other protection measures of the compressor (such as high-pressure protection or low-pressure protection) to form a protection for the compressor.

2.2 Scroll damage is generally caused by liquid hammer. There are three main situations: one is that a large amount of refrigerant liquid enters the compressor at the moment of startup; the other is that the water flow of the evaporator is insufficient (the evaporation load is reduced), and the compressor has Liquid back phenomenon; Third, the heat pump of the unit does not work well for defrosting, and a large amount of liquid refrigerant enters the compressor without evaporating, or the liquid in the evaporator (condenser when the heat pump is running) enters and compresses when the four-way valve is reversed. machine.

To solve the problem of liquid hammer or liquid return, we mainly consider the following aspects.

  ① The piping design should avoid liquid refrigerant entering the compressor when it is turned on. This may require an excessive liquid return test on the system, especially the refrigeration system with a relatively large charge. Adding a gas-liquid separator at the compressor suction port is an effective way to solve this problem, especially in heat pump units that use reverse cycle hot gas defrosting.

  ②Before starting up, preheating the compressor oil sump for a long time can effectively prevent a large amount of refrigerant from accumulating in the compressor lubricating oil. It also has a certain effect on preventing liquid shock.

  ③The water system flow protection is indispensable, so that when the water flow is not enough, it can protect the compressor, so as to prevent the unit from liquid back or freezing the evaporator in severe cases. When the flow switch is damaged, repair or replace it in time, and never short-circuit the flow switch.

  2.3 The burning of motor windings is related to the protection of electrical design, or caused by improper operation of the unit.

  ①Due to the evaporator freezing and cracking, after the fluorine system and the water system collude, the compressor is burnt due to the water entering the compressor. The reasons are: a. Due to various reasons (dirty water filter, unreasonable water pump matching, etc.), the water system flow is small, and the water flow protection fails or is short-circuited, and the evaporator freezes and cracks the refrigerant copper pipe; b. In winter, the water system of the unit stores water and no antifreeze measures are taken, and the evaporator freezes and cracks.

  ②The load is abnormal. Frequent actions of the built-in thermal protector of the compressor may cause the contacts to stick and cause the protector to lose its function and burn the motor. Lubrication failure and increased frictional resistance are the main causes of abnormal load. To solve the problem of lubrication failure, we mainly start from the following aspects: prevent liquid back, prevent lubricating oil from overheating (high exhaust temperature), and solve the problem of oil return or lack of oil in the system.

③If the electrical design does not have overload protection or overcurrent protection, the following dangers may occur: If the compressor current is large, the motor windings continue to work at a higher temperature, and this temperature is not enough to activate the built-in thermal protector. In this case, the insulation layer of the coil may be damaged and the motor may be short-circuited and burned. Increasing external overload protection is an effective means to prevent the motor from burning out, such as thermal relay protection. The setting value of the thermal relay should be able to disconnect when it does not exceed 140% of the rated current of the compressor. In addition, it is very important that the power cord of the unit passes through the air switch.

  ④ There are impurities in the system, which corrode and wear the compressor motor coil, causing the motor to burn out.

⑤Frequent start and stop of the motor will make the contacts of poor quality contactors prone to adhesion. In this way, relying on the contactor to disconnect all protection controls (high and low pressure protection, exhaust temperature protection, water flow protection, etc.) All failures, the compressor is in an unprotected state, causing the motor to burn out. Therefore, the contactor should be selected correctly.

  ⑥ Insufficient cooling is also one of the reasons for the burning of the compressor motor. Hermetic scroll compressors are generally cooled by refrigerant, and a large amount of refrigerant leakage will cause insufficient cooling of the compressor. Of course, a large amount of refrigerant leakage will generally cause low pressure protection or thermal protection, but if the low pressure protection and thermal protection fail, it will cause the compressor to burn.

  2.4 The motor holds the shaft and the root cause of the bearing damage is the lubrication problem. It is divided into the following situations:

  ①The lubricating oil is insufficient, or there is a problem with the oil return of the system. In this case, the bearing cannot be fully lubricated, and the bearing wear is aggravated, which eventually causes the motor to hold the shaft. Therefore, the mass ratio of refrigerating oil to refrigerant is generally required to be greater than 0.35, otherwise refrigerating oil must be added. In addition, the on-site replacement of the compressor will also cause the loss or deterioration of the refrigerating oil. At this time, it is necessary to supplement part of the refrigerating oil.

  ② Impurities in the system enter the bearing, the bearing is worn or stuck, causing the motor to hold the shaft.

According to the analysis of the relevant researchers, the main components of impurities are copper, iron, carbides, etc., which are mainly produced during the production process, such as welding nodules or welding slag entering the copper pipe during brazing; copper scraps when cutting the copper pipe Into the system; poor lubrication during compressor operation will increase the high temperature caused by friction, and high temperature will cause the carbonization of lubricating oil or other organic matter; the system is not fully evacuated, there is moisture, or the moisture control in the refrigerant is not pure enough. It will cause "copper plating" in the system, which will eventually produce powdery copper and iron particles in the system; the work site is not clean, and dust and impurities enter the system. In response to these problems, we should improve the production process, strictly enforce process discipline, and improve the technical level of workers.

③There is no preheating or insufficient preheating time before starting up, causing a large amount of refrigerant in the system to migrate to the compressor. Liquid refrigerant accumulates more in the compressor, and the density of liquid refrigerant is higher than that of refrigerated oil, so it is compressed The refrigerant accumulated at the bottom of the machine is divided into two layers, the upper part is refrigerating oil (rich oil phase), and the lower part is refrigerant (rich refrigerant phase). The refrigerant in the lower part prevents the refrigerating oil from entering the bearing lubrication (equivalent to raising the refrigerating oil level). Almost all refrigerant enters the bearing lubrication surface when the machine is started, which leads to poor lubrication and foamy refrigerating oil Liquid refrigerant entering the compression chamber will also cause liquid compression.

An effective measure to prevent refrigerant migration and dissolution in lubricating oil is to use a heater. By heating the oil, the temperature of the oil is higher than that of other parts of the system, and the refrigerant in the lubricating oil evaporates [51, but the oil is heated The power of the device should not be too high, otherwise it will overheat the oil, reduce the viscosity, or even carbonize, and cause improper bearing lubrication. Therefore, the oil heater should be selected correctly.

Another problem is that even if the unit is designed with a heater, some users often ignore this in the actual application process, causing the problem of no preheating or insufficient preheating time before starting up. Therefore, if possible, it is recommended Set up on the electrical control system to ensure sufficient warm-up time. When the preheating time is generally 12-24h, the oil temperature is 10-12℃ higher than the ambient temperature.

  3 Conclusion

Once the hermetic scroll compressor is damaged, it is almost impossible to repair it. The compressor can only be replaced. The loss caused by this is relatively large. Therefore, how to deal with the common failures of the hermetic scroll compressor in design The preventive measures are very important.