Refrigeration system, liquid refrigerant flow through the expansion valve, part of the instantaneous vaporization of gas-liquid mixing state, which is the industry referred to as “flash”.
Many people wonder why the liquid refrigerant will occur this change, in fact, the flash is the pressure, temperature and other physical factors play a joint role in the inevitable result of the relationship between the refrigeration system efficiency, stability and equipment life.
This article will explain in detail the definition of flash, causes, hazards, related expansion valve types and countermeasures to help you understand this key principle.
What is Refrigerant Flash?
Refrigerant flash, also known as adiabatic flash evaporation in the industry, in layman’s terms, is the liquid refrigerant flow through the expansion valve such as throttling device, the pressure will suddenly drop, a part of the liquid refrigerant followed by instantaneous vaporization, and ultimately the formation of gas-liquid mixture of the state.
Need to clarify common misconceptions:
flash and vaporization of refrigerant in the evaporator is completely different. Vaporization in the evaporator is the refrigerant to absorb the object being cooled (such as air conditioning, refrigerator air) heat, to achieve the core of the refrigeration link, directly determines the cooling effect.
The refrigerant flash, is the refrigerant itself in almost no exchange of heat with the outside world in the case of their own cooling and the physical state of change, it is not directly related to the realization of the refrigeration function, and more is not the refrigeration process need to deliberately realize the link.
Why does Refrigerant Flash?
The emergence of refrigerant flash, behind a clear physical law, is the result of a variety of physical effects together, specifically can be divided into the following three aspects, the interpretation of easy to understand, does not involve complex professional derivation, to facilitate your understanding.
Expansion Valve Inlet
The refrigerant entering the expansion valve has completed cooling through the condenser and is in a high-pressure, room temperature subcooled liquid state. Its temperature is lower than the boiling point of the current high-pressure environment, the physical properties are stable, and spontaneous vaporization will not occur.
This stable high-pressure liquid state is an important basis for the subsequent flash phenomenon. Because the refrigerant is in a stable liquid state, when the subsequent flow through the valve port pressure drop, there will be “instantaneous vaporization” of the obvious physical changes.
Throttling Moment
Expansion valve valve port is narrow, the refrigerant flow through the circulation space suddenly shrinks, the pressure instantly from the condensing high pressure (such as 3.0MPa) down to the evaporation of low pressure (such as 0.8MPa), this process takes a very short period of time, there is virtually no external heat exchange, belonging to the typical adiabatic throttling process.
Here you need to grasp a key physical properties of refrigerants:
The boiling point of the refrigerant and the pressure strictly corresponds to the lower pressure, the lower the boiling point. Take R32 refrigerant commonly used in domestic air conditioning as an example, in the high-pressure environment of 3.0MPa, its boiling point is about 45℃; while in the low-pressure environment of 0.8MPa, the boiling point is only 7℃.
After the completion of the throttling process, the temperature of the refrigerant has not had time to drop in time, the temperature is much higher than the boiling point of the current low-pressure environment, equivalent to carry excess heat, naturally can not continue to maintain a pure liquid state, which provides a direct condition for the occurrence of flash.
Flashing Occurs
According to the law of conservation of energy, the excess heat will not disappear into thin air, it needs to be consumed in a specific form. At this time, part of the liquid refrigerant will instantly vaporize, and the liquid vaporization process itself needs to absorb a lot of heat.
This part of the absorbed heat is the latent heat of vaporization, and all from the refrigerant itself carries excess heat. Through this vaporization of heat absorption, the temperature of the entire refrigerant system will quickly drop to the current low-pressure environment corresponding to the boiling point, and ultimately the formation of gas-liquid mixing state, which is the flash phenomenon we observed.
What are the Hazards of Refrigerant Flashing
Many people will mistakenly think that the flash can improve the efficiency of refrigeration, but it is not. For conventional refrigeration systems, flash belongs to the harmful ineffective process, and its negative impact is directly related to our experience and equipment life, the specific main points are as follows:
Decrease in Heat transfer Efficiency
Flash formed by the gaseous refrigerant, can not participate in the evaporator heat transfer work, equivalent to “take up space but do not work”. It will occupy the effective heat transfer space of the evaporator, resulting in air conditioning, refrigerators and other refrigeration equipment, the cooling speed becomes slower, the cooling effect is greatly reduced.
For example, the air conditioner does not reach the set temperature for a long time after turning on the air conditioner, and the refrigerator’s cooling capacity decreases, which may be related to the flashing is too serious.
Increase in System Power Consumption
Gaseous refrigerants are far less fluid than liquid refrigerants, increasing the resistance to flow within the piping.
This means that the compressor needs to work harder, which not only leads to a rise in the power consumption of the equipment, but the coefficient of performance (COP) of the unit’s refrigeration also decreases, making the refrigeration equipment even more energy-consuming, and adding a lot of cost to the electricity bill in the long run.
Generate Operation Noise
Flashing process will produce throttle whistling, our daily use of air conditioning when we hear the “ZiZi sound”, mostly from this phenomenon. Although the noise usually does not directly damage the equipment, but will affect the use of experience, especially in the quiet night, its presence will be more obvious.
Damage to Core Components
This is the most serious danger of flickering. If the flash is too violent, it will lead to the expansion valve adjustment failure, which in turn affects the normal operation of the entire refrigeration system; in extreme cases, the incompletely vaporized liquid refrigerant will enter the compressor, triggering the “liquid shock” failure.
The failure will directly damage the compressor cylinder, valve and other core components, significantly shorten the service life of the equipment, and may even lead to the direct scrapping of the compressor, resulting in extremely high maintenance costs.
Expansion Valve: Core Components Related to the Flash Hair
Expansion valve is the core component of the refrigeration system, common types of thermal expansion valve and electronic expansion valve. Need to clarify the misunderstanding: the core function of the two is not to control the flash, flash is only a throttling “by-product”, its core mission is to ensure stable and safe operation of the system, only the principle of work and adapt to the scene there are differences.
The core logic of the work of the two expansion valve is the same, are by adjusting the valve opening to control the refrigerant flow, the principle is not complex, the specific work process and the difference is as follows:
Thermal Expansion Valve
Thermal expansion valve relies on the temperature-sensitive package (filled with temperature-sensitive media) to sense the evaporator outlet refrigerant superheat, through the thermal expansion and contraction of the temperature-sensitive media, drive the valve needle core to move, and then adjust the valve opening, control refrigerant flow, the specific work process is as follows:
When the air conditioning load becomes large, the evaporator outlet temperature rises, the temperature-sensitive medium will be thermal expansion, pushing the needle core to open the valve, increase the refrigerant flow rate, to ensure that the cooling effect is not affected, to meet the use of demand;
When the air-conditioning load becomes small, the evaporator outlet temperature decreases, the temperature-sensitive medium will be cold contraction, the needle core reset and close the valve, reduce the refrigerant flow to prevent the liquid refrigerant into the compressor triggered by the liquid shock failure, to protect the core component safety.
Its advantages are simple structure, low cost, high reliability, widely used in household air conditioners, small refrigerators and other small and medium-sized refrigeration equipment; the disadvantage is that the regulation of precision is slightly lower, the response speed is relatively slow, the inhibition of the flash effect is limited.
Electronic Expansion Valve
Electronic expansion valve consists of controller, stepping motor and valve body, no need for temperature-sensitive package, through the sensor to collect the key parameters of the system, the controller drives the motor to accurately adjust the valve opening, to achieve precise control of refrigerant flow, the specific work process is as follows:
When the system load changes (such as air conditioning regulation temperature, ambient temperature fluctuations), the sensor will quickly feedback the relevant parameters, the controller quickly calculate the amount of regulation, drive the motor to rotate, accurately open or close the valve, perfectly matching the current refrigeration demand;
When the air-conditioning load becomes large, the evaporator outlet temperature rises, the valve will automatically open, increase refrigerant flow, to ensure the cooling effect; when the air-conditioning load becomes small, the evaporator outlet temperature decreases, the valve will automatically close the small, reduce the refrigerant flow, to prevent the occurrence of liquid shock failure.
Both expansion valves will not actively control the flash, but will indirectly affect the flash intensity by adjusting the refrigerant flow, of which the electronic expansion valve to inhibit the effect of better; the core purpose of the two are to protect the compressor, to protect the stability of the system, only in the structure, accuracy and adaptability of the scene on the difference.
How to Reduce the Hazards of Refrigerant Flashing
For the various types of hazards brought about by the flash, the industry has formed a mature response measures, the following three points are highly practical, can effectively reduce the negative impact of the flash, whether it is the equipment design stage or the daily maintenance process, all have reference value:
Improve Condensate Side Subcooling
The key to reduce the flash is to reduce the heat carried by the liquid refrigerant entering the expansion valve, which is the most effective way to improve the condensing side of the degree of subcooling.
In practice, domestic air conditioners are usually retrofitted with return heaters, and large refrigeration units are specially set up with subcoolers, which can significantly reduce the incidence of flashbacks, and the effect is intuitive, and is also the most widely used means of coping in the industry at present.
Optimization of Expansion Valve Runner Design
Through the optimization of the expansion valve valve runner structure, can reduce the pressure loss in the process of refrigerant throttling, so that the pressure drop more gently, not only can effectively inhibit the intensity of the flash, but also reduce the throttle whistle generated by the operation of the noise, taking into account the stability of the system operation and user experience.
This measure is mainly used in the design and manufacturing stage of the equipment, which can reduce the adverse effects of flashing from the source.
Reasonable Matching System Conditions
In the design stage of the refrigeration system, set the condensing temperature and evaporating temperature reasonably, minimize the difference between the high and low pressure of the system, reduce the enthalpy difference in the process of refrigerant throttling from the root, thus reducing the probability of flashing, and avoiding the hazards of flashing from the design level.
Such measures need to be combined with specific types of refrigeration equipment and use scenarios, targeted design adjustments.
Conclusion
Refrigerant flow through the expansion valve after the flash is a normal phenomenon under the physical law, no need to deliberately avoid. As long as you master its laws, good system design planning and routine maintenance, you can avoid its harm, to protect the stability and durability of refrigeration equipment.
This professional knowledge is closely related to the daily use of air conditioners, refrigerators, read the principle of flash, not only can understand the core logic of refrigeration, but also more scientific use of equipment, judgment of abnormalities, reduce maintenance costs.
