Have you understood these common debugging and maintenance knowledge?

March 18, 2024

Have you understood these common debugging and maintenance knowledge?


1、 Refluxing

1. For refrigeration systems that use expansion valves, the selection and improper use of return liquid and expansion valves are closely related. Excessive selection of expansion valve, too low setting of superheat, incorrect installation method of temperature sensing package, damaged insulation wrapping, and failure of expansion valve may all cause liquid return.

2. For small refrigeration systems using capillaries, adding too much liquid can cause liquid return.

3. When the evaporator frosts severely or the fan malfunctions, the heat transfer deteriorates, and the unexpired liquid can cause liquid return.

4. Frequent temperature fluctuations in cold storage can also cause the expansion valve to malfunction and cause liquid return.

For refrigeration systems where it is difficult to avoid liquid return, installing a gas-liquid separator and using suction shutdown (i.e. letting the compressor dry the liquid refrigerant in the evaporator before shutdown) control can effectively prevent or reduce the harm of liquid return

2、 Start with liquid

1. The phenomenon of severe foaming of lubricating oil in the crankcase during start-up of a return air cooling compressor is called liquid start.

2. The bubbling phenomenon during liquid start can be clearly observed on the oil level mirror.

3. The fundamental reason for starting with liquid is that a large amount of refrigerant dissolved in the lubricating oil and settled under the lubricating oil suddenly boils and causes foaming of the lubricating oil when the pressure suddenly decreases. The duration of foaming is related to the amount of refrigerant, usually a few minutes or more. A lot of foam floats on the oil surface and even fills the crankcase. Once the cylinder is sucked in through the air inlet, the foam will be reduced to liquid (mixture of lubricating oil and refrigerant), which is easy to cause liquid hammer. Obviously, the liquid hammer caused by liquid starting only occurs during the starting process.

4. Unlike liquid return, the refrigerant that causes liquid start enters the crankcase in a "refrigerant migration" manner. Refrigerant migration refers to the process or phenomenon in which the refrigerant in the evaporator enters the compressor in the form of gas through the return pipeline and is absorbed by the lubricating oil when the compressor stops running, or is mixed with the lubricating oil after condensation in the compressor.

5. After the compressor stops, the temperature will decrease while the pressure will increase. Due to the low partial pressure of refrigerant vapor in lubricating oil, it will absorb the refrigerant vapor on the oil surface, causing the crankcase pressure to be lower than the evaporator pressure. The lower the oil temperature, the lower the vapor pressure, and the greater the absorption of refrigerant vapor. The steam in the evaporator will slowly migrate towards the crankcase. In addition, if the compressor is outdoors, in cold weather or at night, its temperature is often lower than that of the indoor evaporator, and the pressure inside the crankcase is also lower. After the refrigerant migrates to the compressor, it is also easy to condense and enter the lubricating oil.

6. Refrigerant migration is a very slow process. The longer the compressor shutdown time, the more refrigerant will migrate into the lubricating oil. As long as there is liquid refrigerant in the evaporator, this process will proceed. Due to the heavy weight of the lubricating oil that dissolves the refrigerant, it will settle at the bottom of the crankcase, and the lubricating oil floating on top can also absorb more refrigerant.

7. Due to the structure, the crankcase pressure will decrease much more slowly when the air cooling compressor starts, the foaming phenomenon is not very severe, and the foam is difficult to enter the cylinder, so the air cooling compressor does not have the problem of liquid hammer when starting with liquid.

8. In theory, installing a crankcase heater (electric heater) on a compressor can effectively prevent refrigerant migration. After a short shutdown (such as at night), keeping the crankcase heater energized can slightly raise the lubricating oil temperature above other parts of the system, preventing refrigerant migration. After a long period of shutdown (such as a winter), heating the lubricating oil for a few or more hours before starting can evaporate most of the refrigerant in the lubricating oil, greatly reducing the possibility of liquid shock during start-up with liquid, and also reducing the harm caused by refrigerant flushing. However, in practical applications, it is difficult to maintain power supply to the heater after shutdown or to supply power to the heater more than ten hours before startup. Therefore, the actual effect of the crankcase heater will be greatly reduced.

9. For larger systems, allowing the compressor to drain the liquid refrigerant in the evaporator before shutdown (known as vacuum shutdown) can fundamentally prevent refrigerant migration. Installing a gas-liquid separator on the return air pipeline can increase the resistance of refrigerant migration and reduce the amount of migration.

3、 Return oil

1. When the compressor is positioned higher than the evaporator, a return bend on the vertical return pipe is necessary. The oil return bend should be as compact as possible to reduce oil storage. The spacing between the oil return bends should be appropriate. When there are many oil return bends, some lubricating oil should be added.

2. The return oil pipeline of the variable load system must also be careful. When the load decreases, the return air speed will decrease, and too low a speed is not conducive to oil return. To ensure oil return under low load, a vertical suction pipe can be equipped with a double riser.

3. Frequent starting of the compressor is not conducive to oil return. Due to the short continuous operation time, the compressor stopped and there was no time to form a stable high-speed airflow in the return pipe, so the lubricating oil could only remain in the pipeline. If the return oil is less than the running oil, the compressor will run out of oil. The shorter the operating time, the longer the pipeline, the more complex the system, and the more prominent the problem of oil return.

4. Lack of oil can cause serious lubrication problems, and the root cause of oil shortage is not the amount and speed of compressor oil flow, but the poor oil return of the system. Installing an oil separator can quickly return oil and prolong the operation time of the compressor without oil return.
5. The design of the evaporator and return air pipeline must take into account the return oil. Maintenance measures such as avoiding frequent start-up, timed defrosting, timely replenishment of refrigerant, and timely replacement of worn piston components can also help with oil return.

4、 Evaporation temperature/Return air temperature/Return air pressure

1. For every 10 ° C increase in evaporation temperature, the motor load can increase by 30% or even higher, causing the phenomenon of a small horse pulling a big cart. Therefore, if low-temperature compressors are used in medium to high temperature systems and cold storage cooling processes for a long time, the compressor will be in an overloaded state for a long time, causing significant damage to the motor and making it easy to burn out when encountering sudden situations such as voltage fluctuations and surges in the future.

2. The lower the evaporation temperature, the smaller the refrigerant mass flow rate, and the smaller the actual required motor power. Therefore, when using air conditioning compressors and medium to high temperature refrigeration compressors for low temperatures, although the actual power consumption of the motor is much lower than the nominal power, the actual power demand and cooling situation are still too large compared to low temperatures, and motor cooling is prone to problems.

3. The temperature of the return air is relative to the evaporation temperature. In order to prevent liquid return, the general return air pipeline requires a return air superheat of 20 ° C. If the insulation of the return air pipeline is not good, the superheat will far exceed 20 ° C.

4. The higher the return air temperature, the higher the cylinder suction temperature and exhaust temperature. For every 1 ° C increase in return air temperature, the exhaust temperature will increase by 1-1.3 ° C.

5. For a return air cooled compressor, the refrigerant vapor is heated by the motor as it flows through the motor chamber, and the cylinder suction temperature is once again raised. The heat generation of a motor is influenced by power and efficiency, while the power consumption is closely related to displacement, volumetric efficiency, operating conditions, friction resistance, etc.

6. Some users have a biased belief that the lower the evaporation temperature, the faster the cooling rate, but this idea actually has many problems. Although reducing the evaporation temperature can increase the freezing temperature difference, the refrigeration capacity of the compressor decreases, so the freezing speed may not be fast. Moreover, the lower the evaporation temperature, the lower the refrigeration coefficient, while the load increases, the operating time prolongs, and the power consumption increases.

7. Reducing the resistance of the return air pipeline can also increase the return air pressure. Specific methods include timely replacement of dirty and clogged return air filters, and minimizing the length of the evaporator and return air pipelines as much as possible.

8. In addition, insufficient refrigerant is also a factor contributing to low return air pressure.


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5、 Exhaust temperature/exhaust pressure/exhaust volume

1. The main reasons for high exhaust temperature are as follows: high return air temperature, large motor heating capacity, high compression ratio, high condensing pressure, insulation index of refrigerant, and improper refrigerant selection.

2. For the R22 compressor, when the evaporation temperature decreases from -5 ° C to -40 ° C, the COP generally decreases by four times, while other parameters do not change much. The temperature rise of the gas in the motor chamber will increase by three to four times. Due to the fact that for every 1 ° C increase in cylinder suction temperature, the exhaust temperature can increase by 1-1.3 ° C. Therefore, when the evaporation temperature decreases from -5 ° C to -40 ° C, the exhaust temperature will increase by about 30-40 ° C. The temperature rise range of the refrigerant in the motor chamber of the return air cooling semi sealed compressor is approximately between 15 and 45 ° C.

3. In air-cooled (air-cooled) compressors, the refrigeration system does not go through the winding, so there is no problem with motor heating.

4. The exhaust temperature is greatly affected by the compression ratio (condensation pressure/evaporation pressure, usually 4). Under normal circumstances, the discharge pressure of the compressor is very close to the condensation pressure. When the condensing pressure increases, the exhaust temperature of the compressor also increases. The higher the compression ratio, the higher the exhaust temperature, the lower the gas transmission coefficient, and thus the refrigeration capacity of the compressor decreases and the power consumption increases.

5. Reducing the compression ratio can significantly reduce the exhaust temperature, including increasing the suction pressure and reducing the exhaust pressure. The suction pressure is determined by the evaporation pressure and the resistance of the suction pipeline. Raising the evaporation temperature can effectively increase the suction pressure, quickly reduce the compression ratio, and thus lower the exhaust temperature.

6. Practice has shown that reducing exhaust temperature by increasing suction pressure is simpler and more effective than other methods.

7. The main reason for high exhaust pressure is that the condensation pressure is too high (there is air in the system; there is too much refrigerant filling, and the liquid occupies the effective condensation area; the condenser has insufficient heat dissipation area, scaling, insufficient cooling air volume or water volume, and high cooling water or air temperature). Choosing the appropriate condensing area and maintaining sufficient cooling medium flow rate is very important.

6、 Liquid hammer

1. In order to ensure the safe operation of the compressor and prevent liquid hammer phenomenon, it is required that the suction temperature be slightly higher than the evaporation temperature, that is, there should be a certain degree of superheat. The magnitude of superheat can be achieved by adjusting the opening degree of the expansion valve.

2. Avoid high or low suction temperature. Excessive suction temperature, i.e. excessive superheat, will cause an increase in compressor exhaust temperature. If the suction temperature is too low, it indicates that the refrigerant is not completely evaporated in the evaporator, which not only reduces the heat transfer efficiency of the evaporator, but also causes the suction of wet steam and the formation of compressor liquid hammer. Under normal circumstances, the suction temperature should be 5-10 ℃ higher than the evaporation temperature.

7、 Superheat

1. For the commonly used R22 refrigerant, the cooling capacity of the compression mechanism decreases with the increase of effective superheat. When the superheat is 10 ℃, the cooling capacity is 99.5% of the cooling capacity under saturated evaporation. When the superheat is 20 ℃, the cooling capacity is 99.3% of the cooling capacity under saturated evaporation. It can be seen that the cooling capacity decays very little with the increase of superheat.

2. For R502 refrigerant, the cooling capacity of the compression mechanism increases with the increase of effective superheat.
3. Maintaining a certain degree of superheat in the refrigerant can further prevent liquid hammer phenomenon in the cylinder, and for low-temperature refrigeration systems, increasing the effective superheat appropriately can make the lubricating oil return to the compressor more smoothly. But as the suction superheat of the compressor increases, its exhaust temperature also rises. Excessive exhaust temperature can cause the viscosity of the lubricating oil to become thinner or even carbonized, affecting the normal operation of the compressor. Therefore, the suction superheat should be controlled within a certain range.

8、 Fluorination

1. When the fluorine content is low or the regulating pressure is low (or partially blocked), the valve cover (corrugated pipe) of the expansion valve and even the inlet will frost; When there is too little or almost no fluorine, the surface of the expansion valve has no reaction and can only hear a faint sound of airflow.

2. From which end does ice start, from the separating head or from the compressor return pipe? If there is a lack of fluorine from the separation head, then there is an excess of fluorine from the compressor.







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