Screw refrigeration compressors are volumetric compressors. Since they have been used since 1934, due to their excellent performance, no wear and tear, and large unit cooling capacity, they have dominated small to large and medium-sized refrigeration systems. So what types of failures are prone to occur in the screw compressors for fluorine refrigeration during use, let’s take a closer look below!
1. Abnormal compression ratio
2. Low efficiency and failure of the condenser
3. Low efficiency and failure of evaporator
4. Oil circuit system failure
5. Electrical failure
1. Abnormal compression ratio
Compression ratio is familiar to anyone who knows about compressor performance. But what is the use of compression ratio? Is it just a computing tool designed, in fact, it is not.
The difference between a screw machine and a piston machine is that the piston machine will only undercompress, while the screw machine will overcompress.
Affected by the structure, the screw machine has an important data, that is, the internal volume ratio, the English abbreviation Vi, for most screw compressors, the Vi is fixed. From the perspective of maintenance and operation, the value of the internal volume ratio is very similar to the value of the external compression ratio (the absolute pressure ratio of the condensing pressure and the evaporating pressure), and the efficiency of this compressor is the highest.
So what happens when the compression ratio is large or small?
If it is too large, or the pressure difference is too large, it proves that the system completely deviates from the design value. The main phenomena are that the discharge temperature and pressure temperature are too high, the suction pressure is low, and the temperature is high.
If the exhaust pressure and temperature are too high, the adverse consequences are mainly that the lubricating oil in the system is easy to coke, it is not suitable to form an oil film, and the rotor cannot be fully lubricated.
Low suction pressure, high suction pressure temperature mainly affects motor cooling, and high exhaust temperature. The consequences are basically equivalent to high exhaust temperature and pressure.
If it is too small, it mainly affects the wet stroke (damp car, inverted frost). In some materials, the screw compressor is resistant to wet stroke, including some of our designs, and salesmen like to promote it like this. In fact, screw machines are more afraid of wet strokes. If a large amount of liquid returns to the compressor, it will cause dilution of the lubricating oil, and the consequence is equivalent to high exhaust temperature.
Of course, the compression ratio is too small, and it is also caused by serious wear of the rotor and failure of loading and unloading.
2. The efficiency of the condenser is low
The low efficiency of the condenser mainly affects the temperature of the liquid supply and whether it can form liquid. We know that the expansion valve is ideally supplied with full liquid. In this way, the efficiency of the system is higher and the cooling capacity is the largest. Moreover, large units basically have attached storage, which is mainly used for oil cooling. Therefore, it is particularly important to maintain the high efficiency of the condenser. The failure is mainly caused by wrong selection of cooling method, insufficient evaporation area, insufficient cooling medium, and insufficient heat exchange. Therefore, the key points such as fans, water pumps, and fins are mainly checked during inspection.
Speaking of this, the condensation effect is too good. For example, if the ambient temperature is too low, the condensation effect is too good, resulting in higher efficiency of liquid entering the evaporator. At this time, the suction superheat is very low and the expansion valve sensitivity is low, which will cause Start hydraulic shock. Or the difference between the exhaust pressure and the suction pressure is insufficient, which is fatal to the screw machine with differential pressure oil supply.
3. Evaporator efficiency is low or high
The low efficiency of the evaporator mainly affects the cooling of the object to be cooled, while the wet stroke affects the compressor. And high efficiency will cause the suction superheat to be too high, which will affect the compressor discharge temperature.
Judgment of wet stroke
Wet stroke, under the condition of low temperature, the judgment is actually relatively simple, mainly judged by the suction frost line of the compressor, but what about the condition of the air conditioner? By dew? Especially for chillers, if there is a problem in judgment, it will cause problems such as breakage and water ingress. Therefore, it can be judged according to the pressure-enthalpy diagram, or the value of the exhaust temperature minus the temperature after condensation. If the value is less than 30K, it can be judged as a wet stroke.
Let me say one more thing here, expansion valve, I don’t have a separate list (see my book maintenance of expansion valve). The expansion valve is not a universal regulating valve, and not all working conditions meet the adjustment requirements of the expansion valve. Especially big horse-drawn carts.
4. Oil circuit problem
For the oil circuit, it is mainly reflected in the quality of the oil, cleanliness, oil return temperature, etc. The main function of the lubricating oil in the refrigeration system of the screw compressor is to lubricate, cool down and seal.
In addition, it also has the function of noise reduction and shock absorption, but there is a lot of controversy in the industry, mainly because the oil will form air bubbles in the motor part, and the air bubbles will eliminate noise, but some manufacturers think it is useless, and the gas-liquid is difficult Control, so instead add foam suppressant.
Shock absorption is mainly for the lubrication of rolling bearings, and this effect is not obvious, so the above two functions cannot be regarded as the main functions.
The temperature of the oil return greatly affects the service life of the screw compressor. Generally, the recommended operating temperature is between 40 and 60°C, and some manufacturers also mark 70°C or 80°C. Excessively high oil temperature will cause coking of the oil and damage the formation of the oil film. The oil temperature also affects the exhaust temperature, which in turn affects the compression ratio. Therefore, please pay attention to the adjustment when selecting the oil temperature.
oil cleanliness
The cleanliness of the oil is also the cleanliness of the system. Maintaining cleanliness is the main feature of the screw compressor. The screw compressor is not equal to the piston compressor. Due to structural reasons, the cleanliness of the system is higher than that of the piston compressor. Due to the high speed of the meshing rotor, some foreign objects are quickly sucked into the compressor, causing damage to the meshing rotor, especially some small particles of metal or foreign objects, which will break through the interception of the suction filter (including some relatively large foreign objects, The damage to the filter screen due to suction is not uncommon), or even the assembly problem of the machine itself, causing the parts to fall off and get stuck between the rotors. It is bound to cause direct damage to the motor. Although small metal particles do not act directly, they impact the oil film of the rotor, resulting in poor lubrication of the rotor bearing, cylinder sticking, and biting of the bearing box. The most terrible thing is that small particles will form a short-circuit chain and directly cause damage to the motor.
Acidic lubricating oil compressors often smell the burnt smell of lubricating oil when they are turned on for analysis. The temperature is very high when the metal surface is severely worn, and the lubricating oil starts to coke when it is above 175oC. If there is a lot of water in the system (vacuum pumping is not ideal, lubricating oil and refrigerant have a large water content, air enters after the negative pressure return air pipe is broken, etc.), the lubricating oil may become acidic. Acidic lubricating oil will corrode copper tubes and winding insulation. On the one hand, it will cause copper plating; on the other hand, the acidic lubricating oil containing copper atoms has poor insulation performance, which provides conditions for winding short circuit.
For screw compressor units, many fault types are caused by several aspects. For example, the lubrication failure caused by lack of oil causes the bearing to be stuck, the rotor is stuck, and then the compressor motor is blocked, the compressor meets abnormal rise, and the motor burn. And why lack of oil or lubrication failure? In fact, it is more caused by high exhaust temperature, liquid shock and other reasons. Therefore, for maintenance personnel, these are all things that need careful observation and hard thinking before they can be repaired and perfected.
1. Oil boils during startup or operation
This fault is due to the liquid entering the compressor, or there is too much refrigerant in the lubricating oil. Please adjust the throttling mechanism to check whether the refrigerant is overcharged.
2. The oil level is insufficient or too high
If it is insufficient, it should be considered whether it is an oil fault, the amount of refueling is insufficient, and it is difficult to return oil to the evaporator. When maintaining, pay attention to whether there is no liquid level in the liquid reservoir. It should be considered that the throttling mechanism is faulty or caused by unreasonable installation.
If it is too high, it should be considered that the oil filter is blocked and the refrigerant is mixed into the oil.
3. The exhaust temperature is too high
There are many factors for high exhaust temperature, mainly due to too much or too little refrigerant, too high suction superheat, and unstable working conditions.
4. Low or fluctuating suction pressure
The main manifestations of low suction pressure are lack of refrigerant, imbalance of throttling mechanism, high condensing temperature, liquid shock, etc.
Post time: Dec-05-2022
