Analysis and Solutions for Common Faults in Refrigeration Systems

Liquid Return
1. For the refrigeration system using an expansion valve, liquid return is closely related to improper selection and use of the expansion valve. Oversized selection of the expansion valve, too small superheat setting, incorrect installation method of the thermal bulb or damaged thermal insulation wrapping, and malfunction of the expansion valve may all cause liquid return.
2. For a small refrigeration system using a capillary tube, an excessive amount of refrigerant charge will cause liquid return.
3. When the frost on the evaporator is severe or the fan malfunctions, the heat transfer deteriorates, and the unevaporated liquid will cause liquid return.
4. Frequent fluctuations in the temperature of the cold storage will also cause the expansion valve to malfunction and lead to liquid return.
For refrigeration systems where liquid return is difficult to avoid, installing a gas-liquid separator and adopting shutdown after pumping out the liquid refrigerant (i.e., allowing the compressor to pump out the liquid refrigerant in the evaporator before shutdown) control can effectively prevent or reduce the harm of liquid return.
Liquid-carrying Start
1. When a suction-gas-cooled compressor starts, the phenomenon of the lubricating oil in the crankcase bubbling violently is called liquid-carrying start.
2. The bubbling phenomenon during liquid-carrying start can be clearly observed through the oil sight glass.
3. The fundamental cause of liquid-carrying start is that a large amount of refrigerant dissolved in the lubricating oil and settled under the lubricating oil suddenly boils when the pressure drops suddenly, and causes the bubbling phenomenon of the lubricating oil. The duration of the bubbling is related to the amount of refrigerant, usually lasting for a few minutes or more than ten minutes. A large amount of foam floats on the oil surface and even fills the crankcase.

Once the foam is sucked into the cylinder through the intake port, it will turn back into a liquid (a mixture of lubricating oil and refrigerant), which is likely to cause liquid hammer. Obviously, the liquid hammer caused by liquid-carrying start only occurs during the start-up process.
4. Different from liquid return, the refrigerant that causes liquid-carrying start 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 suction pipeline and is absorbed by the lubricating oil, or mixes with the lubricating oil after condensing in the compressor.
5. After the compressor stops, the temperature will decrease while the pressure will increase. Since the partial pressure of the refrigerant vapor in the lubricating oil is low, it will absorb the refrigerant vapor above the oil surface, resulting in the phenomenon that the air pressure in the crankcase is lower than that in the evaporator. The lower the oil temperature is, the lower the vapor pressure is, and the greater the absorption capacity for the refrigerant vapor is. The vapor 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 that of the indoor evaporator, and the pressure in the crankcase is also lower. The refrigerant is likely to be condensed and enter the lubricating oil after migrating to the compressor.
6. Refrigerant migration is a very slow process. The longer the compressor stops, the more refrigerant will migrate into the lubricating oil. As long as there is liquid refrigerant in the evaporator, this process will continue. Since the lubricating oil dissolved with refrigerant is heavier, it will settle at the bottom of the crankcase, and the lubricating oil floating on the top can absorb more refrigerant.
7. Due to structural reasons, the decrease in the pressure of the crankcase when the air-cooled compressor starts is much slower, the bubbling phenomenon is not very violent, and it is difficult for the foam to enter the cylinder. Therefore, there is no problem of liquid hammer caused by liquid-carrying start for the air-cooled compressor.
8. Theoretically speaking, installing a crankcase heater (electric heater) on the compressor can effectively prevent refrigerant migration. After a short-term shutdown (such as at night), keeping the crankcase heater powered on can make the temperature of the lubricating oil slightly higher than that of other parts of the system, and refrigerant migration will not occur. After a long-term shutdown (such as for a whole winter), heating the lubricating oil for several or more than ten hours before starting can evaporate most of the refrigerant in the lubricating oil, which can greatly reduce the possibility of liquid hammer during liquid-carrying start and also reduce the harm caused by refrigerant flushing. However, in practical applications, it is difficult to keep the heater powered after shutdown or to power the heater for more than ten hours before starting. Therefore, the actual effect of the crankcase heater will be greatly reduced.
9. For a relatively large system, allowing the compressor to pump out the liquid refrigerant in the evaporator before shutdown (referred to as shutdown after pumping out the liquid refrigerant) can fundamentally avoid refrigerant migration. Installing a gas-liquid separator on the suction pipeline can increase the resistance to refrigerant migration and reduce the amount of migration.
Oil Return
1. When the compressor is higher than the evaporator, an oil return bend on the vertical suction pipeline is necessary. The oil return bend should be as compact as possible to reduce the oil storage. The spacing between the oil return bends should be appropriate. When there are a large number of oil return bends, some lubricating oil should be replenished.
2. The oil return pipeline of a variable-load system also needs to be carefully designed. When the load decreases, the return gas speed will decrease, and a too-low speed is not conducive to oil return. To ensure oil return under low load, a double riser can be used for the vertical suction pipeline.
3. Frequent starting of the compressor is not conducive to oil return. Since the compressor stops after running for a very short time, a stable high-speed gas flow cannot be formed in the suction pipeline in time, and the lubricating oil can only remain in the pipeline. If the oil return is less than the oil carryover, the compressor will be short of oil. The shorter the running time, the longer the pipeline, and the more complex the system, the more prominent the oil return problem will be.
4. Lack of oil will cause serious insufficient lubrication. The fundamental cause of lack of oil does not lie in the amount and speed of oil carryover of the compressor, but in the poor oil return of the system. Installing an oil separator can achieve rapid oil return and extend the running time of the compressor without oil return.
5. The design of the evaporator and the suction pipeline must take oil return into account. Maintenance measures such as avoiding frequent starting, regular defrosting, timely replenishing refrigerant, and timely replacing the worn piston components also contribute to oil return.
Evaporation Temperature/Suction Gas Temperature/Suction Gas Pressure
1. For every 10°C increase in the evaporation temperature, the motor load can increase by 30% or even higher, resulting in the phenomenon of a small horse pulling a big cart. Therefore, when a low-temperature compressor is used in a medium-high temperature system or the cooling process of a cold storage lasts too long, the compressor will be in an overloaded state for a long time, which causes great damage to the motor. This makes the motor easily burn out when encountering sudden situations such as voltage fluctuations and electrical surges in the future.
2. The lower the evaporation temperature is, the smaller the mass flow rate of the refrigerant is, and the smaller the actual required motor power is. Therefore, when an air conditioning compressor and a medium-high temperature refrigeration compressor are used in a low-temperature environment, although the actual power consumption of the motor is much less than the nominal power, it is still too large compared with the actual power demand and cooling conditions at low temperatures, and there are likely to be problems with the motor cooling.
3. The level of the suction gas temperature is relative to the evaporation temperature. In order to prevent liquid return, generally, a suction gas superheat of 20°C is required for the suction pipeline. If the insulation of the suction pipeline is not good, the superheat will be far more than 20°C.
4. The higher the suction gas temperature is, the higher the suction temperature and the discharge temperature of the cylinder will be. For every 1°C increase in the suction gas temperature, the discharge temperature will increase by 1 to 1.3°C.
5. For a suction-gas-cooled compressor, the refrigerant vapor is heated by the motor when flowing through the motor cavity, and the suction temperature of the cylinder is increased again. The heat generation of the motor is affected by its power and efficiency, and the power consumption is closely related to the displacement, volumetric efficiency, working conditions, frictional resistance, etc.
6. Some users one-sidedly believe that the lower the evaporation temperature is, the faster the cooling speed will be. In fact, this idea has many problems.
Although reducing the evaporation temperature can increase the refrigeration temperature difference, the cooling capacity of the compressor decreases. Therefore, the refrigeration speed is not necessarily fast. Moreover, the lower the evaporation temperature is, the lower the refrigeration coefficient will be, while the load increases, the running time is prolonged, and the power consumption will increase.
7. Reducing the resistance of the suction pipeline can also increase the suction gas pressure. Specific methods include timely replacing the dirty and blocked suction gas filter, and minimizing the length of the evaporator pipe and the suction pipeline as much as possible.
8. In addition, insufficient refrigerant is also a factor causing low suction gas pressure.
Too High Suction Temperature
(1) When the refrigerant charge in the system is insufficient, even if the expansion valve is opened to the maximum, the liquid supply amount will not change. In this way, the refrigerant vapor is superheated in the evaporator, causing the suction temperature to rise.
(2) If the opening of the expansion valve is too small, it will result in an insufficient circulation amount of the refrigerant in the system. The amount of refrigerant entering the evaporator is small, the superheat degree is large, and thus the suction temperature is high.
(3) If the filter screen at the opening of the expansion valve is blocked, the liquid supply amount in the evaporator is insufficient, the amount of refrigerant liquid decreases, and a part of the evaporator is occupied by superheated vapor. Therefore, the suction temperature rises.
(4) Other reasons can also cause the suction temperature to be too high. For example, if the insulation of the suction pipeline is not good or the pipeline is too long, it can all lead to a too high suction temperature. Under normal circumstances, the cylinder head of the compressor should be half cold and half hot. If the suction temperature is too high, the entire cylinder head will be hot.
Too Low Suction Temperature
(1) If the refrigerant charge is too much, it will occupy part of the volume in the condenser, causing the condensing pressure to increase, and the liquid entering the evaporator will increase accordingly. The liquid in the evaporator cannot be completely vaporized, making the gas sucked into the compressor carry liquid droplets. In this way, the temperature of the suction pipeline drops, but the evaporation temperature remains unchanged because the pressure has not dropped, and the superheat degree decreases. Even if the expansion valve is closed slightly, there will be no significant improvement.
(2) The opening of the expansion valve is too large. Due to the loose binding of the temperature sensing element, the small contact area with the suction pipeline, or the temperature sensing element not being wrapped with thermal insulation material and the wrong wrapping position, etc., the temperature measured by the temperature sensing element is inaccurate and close to the ambient temperature, causing the opening degree of the expansion valve to increase, resulting in an excessive liquid supply amount.
The Influence of Evaporation Temperature on Refrigeration Efficiency
1. The evaporation temperature has a great influence on the refrigeration efficiency. For every 1-degree decrease in it, the power needs to be increased by 4% to produce the same cooling capacity. Therefore, under the condition of permission, appropriately increasing the evaporation temperature is beneficial to improving the refrigeration efficiency of the air conditioner. The evaporation temperature of a household air conditioner is generally 5 to 10 degrees lower than the air outlet temperature of the air conditioner.

During normal operation, the evaporation temperature is between 5 and 12 degrees, and the air outlet temperature is between 10 and 20 degrees.
Discharge Temperature/Discharge Pressure/Discharge Capacity
1. The main reasons for the too high discharge temperature are as follows: high suction gas temperature, large motor heat generation, high compression ratio, high condensing pressure, the adiabatic index of the refrigerant, and improper selection of the refrigerant.
2. For an R22 compressor, when the evaporation temperature drops from -5°C to -40°C, the general COP will decrease by 4 times, while other parameters change little, and the temperature rise of the gas in the motor cavity will increase by three or four times. Since for every 1°C increase in the suction temperature of the cylinder, the discharge temperature can increase by 1 to 1.3°C. Therefore, when the evaporation temperature drops from -5°C to -40°C, the discharge temperature will rise by about 30 to 40°C. For a suction-gas-cooled semi-hermetic compressor, the temperature rise range of the refrigerant in the motor cavity is roughly between 15 and 45°C.
3. In an air-cooled (air-cooling) type compressor, the refrigerant does not pass through the winding, so there is no problem of motor heating.
4. The discharge temperature is greatly affected by the compression ratio (condensing pressure/evaporation pressure, generally 4). Under normal circumstances, the discharge pressure of the compressor is very close to the condensing pressure. When the condensing pressure rises, the discharge temperature of the compressor also rises. The larger the compression ratio is, the higher the discharge temperature will be, and the volumetric efficiency decreases, thus reducing the cooling capacity of the compressor and increasing the power consumption.
5. Reducing the compression ratio can significantly lower the discharge temperature. Specific methods include increasing the suction pressure and reducing the discharge pressure. The suction pressure is determined by the evaporation pressure and the resistance of the suction pipeline. Increasing the evaporation temperature can effectively increase the suction pressure, rapidly reduce the compression ratio, and thus lower the discharge temperature.
6. Practice shows that reducing the discharge temperature by increasing the suction pressure is simpler and more effective than other methods.
7. The main reason for the too high discharge 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 heat dissipation area of the condenser is insufficient, there is scale, the cooling air volume or water volume is insufficient, the temperature of the cooling water or air is too high, etc.). It is very important to select an appropriate condensing area and maintain a sufficient flow rate of the cooling medium.
8. Although the phenomenon of too low discharge pressure is manifested at the high-pressure end, the causes mostly occur at the low-pressure end. The reasons are:
(1) Ice blockage or dirt blockage of the expansion valve, and blockage of the filter, etc., will inevitably cause both the suction and discharge pressures to drop;
(2) Insufficient refrigerant charge;
(3) The hole of the expansion valve is blocked, the liquid supply amount decreases or even stops, and at this time, both the suction and discharge pressures decrease.
9. The insufficient discharge capacity is mainly compared with the designed gas volume of the compressor. If the suction pipeline of the compressor is too long and the pipe diameter is too small, the suction resistance will increase, affecting the suction volume and thus reducing the discharge capacity.
Liquid Hammer
1. In order to ensure the safe operation of the compressor and prevent the occurrence of the liquid hammer phenomenon, it is required that the suction temperature is a little higher than the evaporation temperature, that is, a certain superheat degree should be achieved. The size of the superheat degree can be realized by adjusting the opening of the expansion valve.
2. It is necessary to avoid the suction temperature being too high or too low. If the suction temperature is too high, that is, the superheat degree is too large, it will lead to an increase in the discharge temperature of the compressor. 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 exchange efficiency of the evaporator, but also the suction of wet steam will form a liquid hammer in the compressor. Under normal circumstances, the suction temperature should be 5 to 10°C higher than the evaporation temperature.
Superheat Degree
1. For the commonly used R22 refrigerant, the cooling capacity of the compressor decreases with the increase of the effective superheat. When the superheat degree is 10°C, the cooling capacity is 99.5% of that under saturated evaporation.

When the superheat degree is 20°C, the cooling capacity is 99.3% of that under saturated evaporation. It can be seen that the attenuation of the cooling capacity with the increase of the superheat degree is very small.
2. For the R502 refrigerant, the cooling capacity of the compressor increases with the increase of the effective superheat.
3. Keeping a certain superheat degree of the refrigerant can further prevent the liquid hammer phenomenon from occurring in the cylinder. For a low-temperature refrigeration system, appropriately increasing the effective superheat can make the lubricating oil return to the compressor more smoothly. However, as the suction superheat of the compressor increases, its discharge temperature also rises. Too high a discharge temperature will make the viscosity of the lubricating oil become thin or even carbonize, affecting the normal operation of the compressor. Therefore, the suction superheat should be controlled within a certain range.
Refrigerant Charging
1. When the amount of refrigerant 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 frost; when the amount of refrigerant is too small or there is basically no refrigerant, there is no reaction on the surface of the expansion valve, and only a little hissing sound of the gas flow can be heard.
2. Observe from which end the icing starts, whether it is from the liquid distributor or from the return pipe of the compressor. If it is from the liquid distributor, it means there is a lack of refrigerant, and if it is from the compressor, it means there is too much refrigerant.