Common problems of after-sales maintenance of refrigeration units

1. Liquid back
1. For refrigeration systems using expansion valves, liquid return is closely related to the improper selection and use of expansion valves. Excessive selection of the expansion valve, too small superheat setting, incorrect installation of the temperature sensing bulb, damaged adiabatic dressing, and failure of the expansion valve may cause liquid flood back.
2. For small refrigeration systems that use capillary tubes, excessive liquid addition will cause liquid backflow.
3. When the evaporator is severely frosted or the fan fails, the heat transfer becomes poor, and the unevaporated liquid will cause liquid back.
4. Frequent temperature fluctuations in the cold storage will also cause the expansion valve to malfunction and cause liquid backflow.
For refrigeration systems where liquid return is difficult to avoid, the installation of a gas-liquid separator and the use of pump-down shutdown (that is, let the compressor drain the liquid refrigerant in the evaporator before shutdown) control can effectively prevent or reduce the harm of liquid return.
2.Start with liquid
1. When the return air cooling compressor is started, the phenomenon that the lubricating oil in the crankcase foams violently is called start with liquid.
2. The foaming phenomenon when starting with liquid can be clearly observed on the oil sight glass.
3. The root cause of starting with liquid is that a large amount of refrigerant dissolved in the lubricating oil and sinking under the lubricating oil suddenly boils when the pressure drops suddenly and causes the lubricating oil to bubble. The duration of bubbling is related to the amount of refrigerant, usually several minutes or ten minutes. A lot of foam floated on the oil surface and even filled the crankcase. Once sucked into the cylinder through the intake duct, the foam will be reduced to liquid (a mixture of lubricating oil and refrigerant), which can easily cause liquid hammer. Obviously, the hydraulic shock caused by starting with liquid only occurs in the starting process.
4. Unlike liquid flood back, the refrigerant that caused the start with liquid enters the crankcase by means of "refrigerant migration". Refrigerant migration refers to the process or phenomenon in which the refrigerant in the evaporator enters the compressor in gas form through the return line and is absorbed by the lubricating oil when the compressor stops running, or is mixed with the lubricating oil after being condensed in the compressor.
5. After the compressor is stopped, the temperature will decrease and the pressure will increase. Because the partial pressure of refrigerant vapor in the lubricating oil is low, it will absorb refrigerant vapor on the oil surface, causing the phenomenon that the crankcase air pressure is lower than the evaporator air pressure. The lower the oil temperature and the lower the vapor pressure, the greater the absorption power of refrigerant vapor. The steam in the evaporator will slowly "migrate" to the crankcase. In addition, if the compressor is outdoors, when the weather is cold or at night, its temperature is often lower than that of the indoor evaporator, and the pressure in the crankcase is also lower. After the refrigerant migrates to the compressor, it is easy to be condensed and enter the lubricating oil.
6. Refrigerant migration is a very slow process. The longer the compressor is down, the more refrigerant will migrate into the lubricant. This process will proceed as long as there is liquid refrigerant in the evaporator. Because the lubricant with dissolved refrigerant is heavier, it will sink to the bottom of the crankcase, and the floating lubricant can absorb more refrigerant. To
7. Due to structural reasons, the pressure of the crankcase will decrease much more slowly when the air-cooled compressor is started, the foaming phenomenon is not very severe, and the foam is difficult to enter the cylinder, so the air-cooled compressor does not have the problem of liquid hammering when starting with liquid. To
8. In theory, the installation of a crankcase heater (electric heater) on the compressor can effectively prevent the migration of refrigerant. After a short period of shutdown (such as at night), keeping the crankcase heater energized can make the lubricating oil temperature slightly higher than other parts of the system, and refrigerant migration will not occur. After a long period of shutdown (such as a winter), heating the lubricating oil for several or ten hours before starting up can evaporate most of the refrigerant in the lubricating oil, which can greatly reduce the possibility of liquid shock during startup with liquid It can also reduce the harm caused by refrigerant erosion. However, in practical applications, it is difficult to maintain the heater's power supply after shutting down or to supply power to the heater ten hours before starting up. Therefore, the actual effect of the crankcase heater will be greatly reduced. To
9. For larger systems, let the compressor drain the liquid refrigerant in the evaporator before shutting down (called pump-down shutdown), which can fundamentally avoid refrigerant migration. The installation of a gas-liquid separator on the return gas pipeline can increase the resistance of refrigerant migration and reduce the amount of migration. To
 3. oil return
1. When the compressor is higher than the evaporator, the oil return bend on the vertical return pipe is necessary. The return bend should be as compact as possible to reduce oil storage. The spacing between the oil return bends should be appropriate. When the number of return bends is large, some lubricant should be added. To
2. The oil return pipeline of the variable load system must also be careful. When the load is reduced, the air return speed will decrease, too low speed is not conducive to oil return. In order to ensure the oil return under low load, the vertical suction pipe can adopt double vertical pipes. To
3. Frequent starting of the compressor is not conducive to oil return. Since the compressor stops for a short continuous operation time, there is no time to form a stable high-speed air flow in the return pipe, and the lubricating oil can only stay in the pipe. If the oil return is less than Ben oil, the compressor will be short of oil. The shorter the operating time, the longer the pipeline and the more complex the system, the more prominent the oil return problem.
4. Lack of oil will cause serious lack of lubrication. The root cause of the lack of oil is not the amount and speed of the compressor, but the poor oil return of the system. The installation of an oil separator can quickly return oil and extend the compressor running time without oil return.
5. The design of evaporator and return gas pipeline must take oil return into consideration. Maintenance measures such as avoiding frequent starting, timing defrosting, timely replenishment of refrigerant, and timely replacement of worn piston components also help to return oil.
4. Evaporation temperature/return air temperature/return air pressure
1. When the evaporation temperature increases by 10℃, the motor load can increase by 30% or even higher, causing the phenomenon of small horse-drawn carts. Therefore, if the low-temperature compressor is used in the medium-to-high temperature system and the cold storage cooling process lasts too long, the compressor will be overloaded for a long time, which will cause great damage to the motor, causing the motor to encounter unexpected situations such as voltage fluctuations and surges in the future. It is easy to burn at times.
2. The lower the evaporating temperature, the smaller the refrigerant mass flow, and the smaller the motor power actually required. Therefore, when air conditioning compressors and medium-high temperature refrigeration compressors are used at low temperatures, although the actual power consumption of the motor is much smaller than the nominal power, it is still too large compared to the actual power demand and cooling at low temperatures, and the motor cooling is easy problem appear. To
3. The return air temperature is relative to the evaporation temperature. In order to prevent liquid return, the return gas pipeline generally requires a return gas superheat of 20°C. If the return air pipe is not well insulated, the superheat will far exceed 20°C. To
4. The higher the return air temperature, the higher the cylinder suction temperature and exhaust temperature. Every time the return air temperature increases by 1°C, the exhaust temperature will increase by 1 to 1.3°C.
5. For the return air cooling compressor, the refrigerant vapor is heated by the motor when it flows through the motor cavity, and the cylinder suction temperature is once again increased. The amount of heat generated by the motor is affected by power and efficiency, while the power consumption is closely related to displacement, volumetric efficiency, working conditions, friction resistance, etc.
6. Although reducing the evaporation temperature can increase the freezing temperature difference, the refrigeration capacity of the compressor is reduced, so the freezing speed is not necessarily fast. What's more, the lower the evaporation temperature, the lower the refrigeration coefficient, but the load increases, the operating time is prolonged, and the power consumption will increase. To
7. Reducing the resistance of the return air pipeline can also increase the return air pressure. The specific methods include timely replacement of the dirty return air filter, and minimize the length of the evaporation tube and the return air pipeline. To
8. In addition, insufficient refrigerant is also a factor of low return pressure.
5. the temperature is too high
1. The refrigerant charge in the system is insufficient, even if the expansion valve is opened to the maximum, the liquid supply will not change, so that the refrigerant vapor in the evaporator will overheat and the suction temperature will increase.
2. The opening of the expansion valve is too small, resulting in insufficient refrigerant circulation in the system, less refrigerant entering the evaporator, high overheating, and high suction temperature. To
3. The expansion valve port filter is blocked, the liquid supply in the evaporator is insufficient, the amount of refrigerant liquid is reduced, and a part of the evaporator is occupied by superheated steam, so the suction temperature rises. To
4. The suction temperature is too high due to other reasons, such as poor heat insulation of the return air pipe or too long pipe, which can cause the suction temperature to be too high. Under normal circumstances, the compressor cylinder head should be half cold and half hot.　
6. he temperature is too low
1. Too much refrigerant charge, occupying part of the condenser volume and increasing the condensing pressure, and the liquid entering the evaporator increases accordingly. The liquid in the evaporator cannot be completely vaporized, so that the gas sucked by the compressor contains liquid droplets. In this way, the temperature of the return air duct drops, but the evaporation temperature does not change because the pressure does not drop, and the degree of superheat decreases. Even if the expansion valve is closed, there is no significant improvement. To
2. The opening of the expansion valve is too large. The temperature sensing element is too loosely bound, the contact area with the air return pipe is small, or the temperature sensing element is not wrapped with insulation material and its wrapping position is wrong, etc., resulting in inaccurate temperature measurement of the temperature sensing element, close to the ambient temperature, causing the expansion valve to operate The opening degree increases, resulting in excessive liquid supply. To
7. The influence of evaporating temperature on refrigeration efficiency
1. The heating temperature has a great influence on the refrigeration efficiency. When it decreases by 1 degree, the same cooling capacity needs to increase the power by 4%. Therefore, if conditions permit, appropriately increasing the evaporating temperature is beneficial to improving the cooling efficiency of the air conditioner. of. The evaporating temperature of household air conditioners is generally 5 to 10 degrees lower than the air outlet temperature of the air conditioner. During normal operation, the evaporating temperature is 5 to 12 degrees, and the outlet temperature is 10 to 20 degrees.
8. exhaust temperature / exhaust pressure / exhaust volume
1. The main reasons for the high exhaust temperature are as follows: high return air temperature, large heating capacity of the motor, high compression ratio, high condensation pressure, adiabatic index of the refrigerant, and improper refrigerant selection.
2. For R22 compressor, when the evaporating temperature decreases from -5°C to -40°C, the COP will generally decrease by 4 times, and other parameters will not change much, and the temperature rise of the gas in the motor cavity will increase by three or four times. As the cylinder suction temperature increases by 1°C, the exhaust temperature can increase by 1 to 1.3°C. Therefore, when the evaporation temperature is reduced from -5°C to -40°C, the exhaust steam temperature will rise by about 30-40°C. In the return air cooling type semi-hermetic compressor, the temperature rise of the refrigerant in the motor cavity is roughly between 15 and 45°C.
3. In the air-cooled (air-cooled) compressor, the refrigeration system does not pass through the windings, so there is no motor heating problem.
4. The exhaust temperature is greatly affected by the compression ratio (condensing pressure/evaporating pressure, generally 4). Under normal circumstances, the discharge pressure of the compressor is very close to the condensing pressure. When the condensing pressure increases, the compressor discharge temperature also increases. The larger the compression ratio, the higher the exhaust temperature and the reduction of the air delivery coefficient, which reduces the cooling capacity of the compressor and increases the power consumption.
5. Reducing the compression ratio can significantly reduce the exhaust temperature. Specific methods include increasing the suction pressure and reducing the exhaust pressure. The suction pressure is determined by the evaporation pressure and the resistance of the suction line. Increasing the evaporation temperature can effectively increase the suction pressure and rapidly reduce the compression ratio, thereby reducing the exhaust temperature. To
6. Practice shows that reducing the exhaust temperature by increasing the suction pressure is simpler and more effective than other methods.
7. The main reason for excessive exhaust pressure is that the condensing pressure is too high (there is air in the system; the refrigerant charge is too much, and the liquid occupies the effective condensing area; the condenser has insufficient heat dissipation area, fouling, cooling air volume or insufficient water volume , Cooling water or air temperature is too high, etc.). It is very important to choose a suitable condensing area and maintain sufficient cooling medium flow.
8. Exhaust pressure is too low. Although the phenomenon is manifested in the high-pressure side, the reason is mostly at the low-pressure side.
9. Insufficient air displacement is mainly due to the fact that the compressor suction pipe is too long and the pipe diameter is too small compared with the compressor’s design air volume, which increases the suction resistance, affects the suction volume and reduces the discharge volume. .
9. liquid strike
1. In order to ensure the safe operation of the compressor and prevent liquid hammer, it is required that the suction temperature be higher than the evaporation temperature, that is, it should have a certain degree of superheat. The degree of superheat can be achieved by adjusting the opening degree of the expansion valve. To
2. Avoid excessively high or low temperature. If the suction temperature is too high, that is, the overheating is too high, which will cause the compressor discharge temperature to rise. If the suction temperature is too low, it means that the refrigerant is not completely evaporated in the evaporator, which not only reduces the heat exchange efficiency of the evaporator, but the suction of wet steam will also cause compressor liquid hammer. The suction temperature should be 5-10℃ higher than the evaporation temperature under normal circumstances. To
10. overheating
1. For the commonly used R22 refrigerant, the cooling capacity of the compressor decreases with the increase of the effective superheat. When the superheat is 10℃, the cooling capacity is 99.5% of the cooling capacity under saturated evaporation, and when the superheat is 20 At ℃, the cooling capacity is 99.3% of the cooling capacity under saturated evaporation. It can be seen that the attenuation of the cooling capacity with the increase of superheat is very small.
2. The refrigerant maintains a certain degree of superheat, which can further prevent the liquid hammer phenomenon in the cylinder. For low-temperature refrigeration systems, appropriately increasing the effective superheat can make the lubricating oil return to the compressor smoothly. But as the compressor suction superheat increases, its discharge temperature also rises. Excessive discharge temperature will make the lubricating oil viscosity thinner and even carbonize, affecting the normal operation of the compressor, so the suction superheat should be controlled Within a certain range.
11. Add fluoride
1. When the amount of fluorine is small or its regulating pressure is low (or partially blocked), the valve cover (bellows) of the expansion valve and even the liquid inlet will be frosted; when the amount of fluorine is too small or there is basically no fluorine, the appearance of the expansion valve No response, only a faint sound of airflow can be heard.
2. See from which end the icing starts, whether it is from the dispensing head or the return pipe from the compressor. If the dispensing head is short of fluorine, the compressor is more fluorine.