The screw refrigeration compressor belongs to the volume type compressor, which compresses the gas based on the change in volume. Screw refrigeration compressors are further divided into single screw compressors and double screw compressors.
The twin-screw compressor utilizes two screws with spiral grooves placed inside the body to mesh and rotate, and their coordination with the inner wall of the body and the inner wall of the suction and exhaust end seats, causing changes in the volume between the teeth, thereby completing the process of gas suction, compression, and discharge. Let's first take a look at the internal structure of a certain brand of screw compressor. If you are not clear about the diagram or need the information in this article, you can download it for free from our Refrigeration Directory database.
1. Abnormal compression ratio
The compression ratio is not unfamiliar to personnel who know the performance of compressors. The difference between screw machines and piston machines is that piston machines only have under compression, while screw machines have over compression. So what happens when the compression ratio is high or low?
Excessive or in other words, excessive pressure difference indicates that the system is completely deviating from the design value. The main phenomena include high exhaust pressure and temperature, low suction pressure, and high temperature. If the exhaust pressure and temperature are too high, the main adverse consequences are that the lubricating oil in the system is prone to coking, and it is not suitable to form an oil film, which cannot fully lubricate the rotor. If the rotor cannot be effectively cooled and lubricated, it will block and cause damage to the motor. Low suction pressure and high suction pressure temperature mainly affect motor cooling and high exhaust temperature. The consequences are basically equivalent to high exhaust temperature and pressure.
The main impact of being too small is the wet stroke (wet car, frost), and the screw compressor is resistant to wet stroke. In fact, screw machines are more afraid of wet stroke. If a large amount of liquid returns to the compressor, it will cause dilution of lubricating oil, which is equivalent to a high exhaust temperature. Of course, the compression ratio is too small, and some are caused by severe rotor wear and failure during loading and unloading.
2. Compressor overload
The screw machine reported a compressor overload fault, which is usually caused by abnormal current in the compressor. The focus is on troubleshooting the system power supply wiring to see if there are any faults such as phase loss and phase sequence errors. Then, check the compressor wiring, such as the voltage values between the U-V, U-W, and V-W phases of the compressor. Finally, check whether the wiring in the electrical control cabinet is normal. If everything has been tested and everything is normal, it is necessary to troubleshoot the issue with the motherboard.
A certain brand's after-sales service encountered the phenomenon of overload caused by the motherboard when dealing with the screw machine overload, as shown in the figure,
The motherboard was covered in dust, and after replacing the motherboard, the unit ran normally
3. Low condenser efficiency
The low efficiency of the condenser mainly affects the supply temperature and whether liquid can be formed. We know that in an ideal state, the expansion valve is supplied with full liquid, which results in higher system efficiency and maximum refrigeration capacity. Moreover, large units generally have attached storage, mainly used for oil cooling. So maintaining the efficiency of the condenser is particularly important.
The main faults are caused by incorrect selection of cooling methods, insufficient evaporation area, insufficient cooling medium, and insufficient heat exchange. Therefore, during the inspection, key factors such as the fan, water pump, and fins are mainly checked.
If the condensation effect is too good, it is not enough. For example, if the ambient temperature is too low and the condensation effect is too good, it will result in higher efficiency of liquid entering the evaporator. At this time, the suction superheat is very low, and the sensitivity of the expansion valve is low, which will cause start-up liquid hammer. Or if the difference between exhaust pressure and suction pressure is insufficient, it can be fatal for screw machines with differential pressure oil supply.
4. Low or high evaporator efficiency
The low efficiency of the evaporator mainly affects the cooling of the cooled material, while the wet stroke affects the compressor. However, high efficiency can cause excessive suction superheat, which in turn affects the discharge temperature of the compressor.
5. Oil circuit issues
As for the oil circuit, it is mainly reflected in the quality, cleanliness, and return oil temperature of the oil. The main functions of lubricating oil in the screw compressor refrigeration system are lubrication, cooling, and sealing. The temperature of the return oil greatly affects the service life of the screw compressor. Generally, it is recommended to operate at a temperature between 40 and 60 ℃, and some manufacturers also label it as 70 ℃ or 80 ℃. Excessive oil temperature can cause coking of the oil, damage the formation of the oil film, and also affect the exhaust temperature, thereby affecting the compression ratio. Therefore, please pay attention to adjusting the oil temperature when selecting.
The cleanliness of oil is also the cleanliness of the system, and maintaining cleanliness is the main characteristic of screw machines. Screw compressors are not equivalent to piston compressors, and due to structural reasons, the cleanliness requirements for the system are higher than those for piston machines.
6. Other faults
Many types of faults in screw compressors are caused by entanglement in several aspects, such as lubrication failure caused by oil shortage, resulting in bearing jamming, rotor jamming, compressor motor blockage, abnormal compressor rise, and motor burnout. Why is there a shortage of oil or lubrication failure? In fact, it is more caused by high exhaust temperature, liquid hammer and other reasons. So, for maintenance personnel, these all require careful observation.
Oil boiling during startup or operation: This fault belongs to the compressor liquid inlet, or there is too much refrigerant in the lubricating oil. Please adjust the throttle mechanism and check if the refrigerant is overcharged.
Insufficient or high oil level: Insufficient oil level should be considered as an oil separation fault, insufficient oil filling, and difficulty in returning oil to the evaporator. When repairing, pay attention to whether there is no liquid level in the reservoir, and consider the failure of the throttle mechanism or improper installation. If it is too high, it should be considered that the oil filter is blocked and refrigerant is mixed into the oil.
Excessive exhaust temperature: There are many factors that contribute to high exhaust temperature, mainly due to factors such as excessive or insufficient refrigerant, high suction overheating, and unstable working conditions.
Low or fluctuating suction pressure: Low suction pressure is mainly manifested by refrigerant loss, misaligned throttle mechanism, high condensation temperature, liquid hammer, etc.
