Analysis and solution of common troubles in refrigeration system of chiller

1 For the refrigeration system with expansion valve, the return liquid is closely related to the improper selection and use of expansion valve Too large type selection of expansion valve, too small superheat setting, incorrect installation method of temperature sensing package, damage of thermal insulation package and failure of expansion valve may cause liquid return 2 For the small refrigeration system with capillary, too much liquid will lead to liquid return 3 When the evaporator frosts seriously or the fan fails, the heat transfer becomes poor, and the non evaporated liquid will cause liquid return 4 Frequent fluctuation of cold storage temperature will also cause failure of expansion valve reaction and liquid return For the refrigeration system which is difficult to avoid the return liquid, the installation of gas-liquid separator and the use of evacuation shutdown (that is, before the shutdown, let the compressor dry the liquid refrigerant in the evaporator) control can effectively prevent or reduce the harm of the return liquid Start with liquid 1 When the return air cooling compressor starts, the lubricating oil in the crankcase blisters violently, which is called start with liquid 2 The bubbling phenomenon during the start-up with liquid can be clearly observed on the oil sight glass 3 The root cause of start-up with liquid is that a large amount of refrigerant dissolved in the lubricating oil and sunk under the lubricating oil suddenly boils when the pressure drops suddenly, and causes the foaming phenomenon of the lubricating oil The duration of foaming is related to the amount of refrigerant, usually several minutes or more A lot of bubbles float on the oil surface, or even full of crankcase Once the intake cylinder is sucked through the intake port, the foam will be reduced to liquid (a mixture of lubricating oil and refrigerant) Obviously, liquid hammer caused by start-up with liquid only occurs in the start-up process 4 Different from the return liquid, the refrigerant that causes the start-up with liquid enters the crankcase in the way of "refrigerant migration" Refrigerant migration refers to the process or phenomenon that when the compressor stops running, the refrigerant in the evaporator enters the compressor in the form of gas through the return line and is absorbed by the lubricating oil, or mixes with the lubricating oil after condensation in the compressor 5 When the compressor is shut down, the temperature will decrease and the pressure will increase Because of the low partial pressure of the refrigerant vapor in the lubricating oil, it will absorb the refrigerant vapor on the oil surface, resulting in the phenomenon that the crankcase air pressure is lower than the evaporator air pressure The lower the oil temperature and the lower the steam pressure, the greater the absorption of refrigerant steam The steam in the evaporator will slowly "migrate" to the crankcase In addition, if the compressor is outdoors, in cold weather or at night, its temperature is often lower than the indoor evaporator, the pressure in the crankcase is also low, and the refrigerant is easy to be condensed into the lubricating oil after being transferred to the compressor 6 Refrigerant migration is a very slow process The longer the compressor is shut down, the more refrigerant is transferred to the oil As long as there is liquid refrigerant in the evaporator, this process will be carried out Because the lubricant that dissolves the refrigerant is heavier, it will sink at the bottom of the crankcase, and the lubricant floating on it can absorb more refrigerant 7, due to structural reasons, when the compressor starts, the crankcase pressure will decrease much more slowly, the foaming phenomenon is not very intense, and the foam will hardly enter the cylinder Therefore, there is no problem of liquid blow up in air-cooled compressor 8 Theoretically, the installation of crankcase heater (electric heater) on the compressor can effectively prevent refrigerant migration After a short-term shutdown (such as at night), keep the crankcase heater energized, which can make the lubricating oil temperature slightly higher than other parts of the system, and refrigerant migration will not occur After a long-term shutdown (such as a winter), heat the lubricating oil for a few or more hours before starting up, which can evaporate most of the refrigerant in the lubricating oil It can not only greatly reduce the possibility of liquid hammer when starting with liquid, but also reduce the danger caused by the flushing of refrigerant But in practical application, it is difficult to maintain the power supply of heater after shutdown or supply power to heater ten hours before startup Therefore, the actual effect of the crankcase heater will be greatly reduced 9 For large systems, before shutdown, let the compressor dry the liquid refrigerant in the evaporator (called evacuation shutdown), which can fundamentally avoid refrigerant migration The gas-liquid separator installed on the return pipeline can increase the resistance of refrigerant migration and reduce the migration amount Return oil 1 When the compressor is higher than the evaporator, the return oil 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 return oil bends should be appropriate When the number of return oil bends is large, some lubricating oil should be added 2 The oil return pipeline of the variable load system must also be careful When the load is reduced, the air return speed will be reduced, and too low speed is not conducive to oil return In order to ensure the oil return under low load, the vertical suction pipe can be double riser 3 Frequent start of compressor is not conducive to oil return Since the compressor stops for a short continuous operation time, the lubricating oil can only stay in the pipeline before a stable high-speed air flow can be formed in the return pipe If the return oil is less than the running oil, the compressor will be short of oil The shorter the operation time, the longer the pipeline, the more complex the system, and the more prominent the oil return problem 4 Lack of oil will cause serious lack of lubrication The root cause of lack of oil is not how much and how fast the compressor runs, but the poor oil return of the system Installation of oil separator can quickly return oil and prolong the running time of compressor without return oil 5 The design of evaporator and return line must consider the return oil Maintenance measures such as avoiding frequent start-up, regular defrosting, timely replenishment of refrigerant and timely replacement of worn piston components are also helpful for oil return Evaporation temperature / return gas temperature / return gas pressure 1 When the evaporation temperature is increased by 10 ℃, the motor load can be increased by 30% or even higher, resulting in the phenomenon of small horse pulling big car Therefore, when the low-temperature compressor is used in medium and high temperature system and cold storage for a long time, the compressor will be overloaded for a long time, which will cause great damage to the motor, so that the motor will easily burn down in case of voltage fluctuation and surge 2 The lower the evaporation temperature is, the smaller the refrigerant mass flow is, and the smaller the actual motor power is Therefore, when the air conditioning compressor and medium high temperature refrigeration compressor are used in low temperature, although the actual power consumption of the motor is much smaller than the nominal power, the actual power demand and cooling situation are still too large compared with the low temperature, and the motor cooling is easy to have problems 3 The temperature of return gas is relative to the evaporation temperature In order to prevent liquid return, the general return pipeline requires a return superheat of 20 ℃ If the heat preservation of the return pipeline is not good, the superheat will be far more than 20 ℃ 4 The higher the return air temperature is, the higher the suction temperature and exhaust temperature of the cylinder will be When the return gas temperature increases by 1 ℃, the exhaust gas temperature will increase by 1 ~ 1.3 ℃ 5 For the return air cooling compressor, the refrigerant vapor is heated by the motor when it flows through the motor cavity, and the suction temperature of the cylinder is increased again The heat output of motor is affected by power and efficiency, while the power consumption is closely related to displacement, volumetric efficiency, working condition, friction resistance, etc 6 Some users think that the lower the evaporation temperature is, the faster the cooling speed is In fact, this idea has many problems Although reducing the evaporation temperature can increase the refrigeration temperature difference, the refrigeration capacity of the compressor decreases, so the refrigeration speed is not necessarily fast Moreover, the lower the evaporation temperature is, the lower the refrigeration coefficient is, while the load is increased, the longer the operation time is, the greater the power consumption will be 7 Reducing the resistance of the return pipeline can also improve the return pressure The specific methods include replacing the dirty return filter in time, reducing the length of the evaporation pipe and the return pipeline as much as possible 8 In addition, insufficient refrigerant is also a factor of low return pressure If the suction temperature is too high (1) the refrigerant charge quantity in the system is insufficient, even if the expansion valve is opened to Zui, the liquid supply quantity will not change much, so that the refrigerant vapor overheats in the evaporator to increase the suction temperature (2) The opening of the expansion valve is too small, resulting in insufficient circulation of system refrigerant, less refrigerant entering the evaporator, high superheat, and high suction temperature (3) the filter screen at the expansion valve port is blocked, the liquid supply in the evaporator is insufficient, the liquid quantity of refrigerant is reduced, and a part of the evaporator is occupied by superheated steam, so the suction temperature is increased (4) if the suction temperature is too high due to other reasons, such as the poor insulation of the return pipe or the pipe is too long, the suction temperature can be too high Under normal circumstances, the compressor cylinder head should be half cold and half hot If the suction temperature is too high, the cylinder head will be all heated Suction temperature is too low (1) too much refrigerant is charged, which occupies part of the volume of the condenser and increases the condensing pressure, thus increasing the liquid entering the evaporator The liquid in the evaporator can not be completely gasified, so that there are liquid micro drops in the gas sucked by the compressor In this way, the temperature of the return pipe decreases, but the evaporation temperature does not change because the pressure does not decrease, and the superheat decreases Even if the expansion valve is closed, there is no significant improvement (2) the opening of expansion valve is too large Due to the loose binding of the temperature sensing element, the small contact area with the air return pipe, or the temperature sensing element is not wrapped with thermal insulation material and the wrong binding position, the temperature measured by the temperature sensing element is not accurate and close to the ambient temperature, which increases the opening degree of the expansion valve action, resulting in excessive liquid supply Evaporation temperature has a great influence on refrigeration efficiency 1 Evaporation temperature has a great influence on refrigeration efficiency When it is reduced by 1 degree, the power needed to produce the same cooling capacity needs to increase by 4% Therefore, if conditions permit, it is beneficial to increase the evaporation temperature properly to improve the refrigeration efficiency of air conditioner The evaporation temperature of the household air conditioner is generally 5-10 degrees lower than the air outlet temperature of the air conditioner In normal operation, the evaporation temperature is 5-12 degrees and the air outlet temperature is 10-20 degrees Exhaust temperature / exhaust pressure / exhaust volume 1 The reasons for high exhaust temperature are as follows: high return gas temperature, large heating capacity of motor, high compression ratio, high condensation pressure, insulation index of refrigerant and improper selection of refrigerant 2 For R22 compressor, when the evaporation temperature is reduced from - 5 ℃ to - 40 ℃, cop will be reduced by 4 times in general, while other parameters will not change much, and the temperature rise of gas in the motor cavity will be increased by 3 or 4 times As the cylinder suction temperature increases by 1 ℃, the exhaust temperature can increase by 1 ~ 1.3 ℃ Therefore, the evaporation temperature will decrease from - 5 ℃ to - 40 ℃, and the exhaust temperature will rise about 30 ~ 40 ℃ The temperature rise range of the refrigerant in the motor cavity of the return air cooled half sealed compressor is about 15-45 ℃ 3 In air-cooled (air-cooled) compressors, the cooling system does not go through the winding, so there is no heating problem of the motor 4 The exhaust temperature is greatly affected by the compression ratio (condensation pressure / evaporation pressure, generally 4) Under normal conditions, the discharge pressure of the compressor is close to the condensation pressure When the condensation pressure increases, the discharge temperature of the compressor also increases The larger the compression ratio is, the higher the exhaust temperature is, and the smaller the gas transmission coefficient, so that the refrigeration capacity of the compressor is reduced and the power consumption is increased 5 Reducing the compression ratio can significantly reduce the exhaust temperature, including increasing the suction pressure and reducing the exhaust pressure Suction pressure is determined by evaporation pressure and suction line resistance Increasing the evaporation temperature can effectively increase the suction pressure, rapidly reduce the compression ratio, and thus reduce the exhaust temperature 6 Practice shows that it is more simple and effective than other methods to reduce exhaust temperature by increasing suction pressure 7 The main reason for the high exhaust pressure is the high condensation pressure (air in the system; refrigerant charge