vaporization
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Why is it necessary to drain the liquid first and then release pressure when emptying a device? 1. For ordinary liquids, if pressure is released first and then the liquid is drained, there will be no pressure in the drainage device, so the liquid cannot be discharged; even if there is a pressure difference, the liquid cannot be emptied completely. 2. For substances with low boiling points, releasing pressure first causes the liquid to vaporize rapidly, leading to a sharp drop in temperature, which may result in supercooling. Materials are prone to cold shock at low temperatures, which severely affects the device’s lifespan. 3. Additionally, if the substance contains water, a hydrocarbon-water mixture can form, blocking the pipes. These are the reasons why it is necessary to drain the liquid first and then release pressure when emptying a device; the main reason is the second one: releasing pressure first causes rapid vaporization of the liquid and a sharp drop in temperature, potentially leading to supercooling. Regarding this point, I wonder what the difference is between this phenomenon and flashing – does flashing also result in supercooling due to a sharp drop in temperature? I hope experts can give some advice. Thank you.
Regarding flash evaporation, why is it necessary to first drain the liquid and then relieve the pressure when emptying the equipment? In my opinion, the principle behind this is similar to that of normal flash evaporation: flash evaporation occurs when the pressure drops suddenly below the saturated vapor pressure at the current temperature. What differs from normal flash evaporation is that the driving force for this type of flash evaporation is not sustained; it is only partial and localized. If the gases produced by flash evaporation are not removed, the pressure will increase again, causing the system to reach a two-phase equilibrium state and stopping further flash evaporation. The term \"supercooling\" is relative; strictly speaking, this spontaneous flash evaporation is caused by a pressure difference, and it does not involve heat absorption that could lead to deformation – it’s merely an attempt at elastic deformation. I welcome any criticism from experts.
It can certainly be understood as flashing: it was originally part of a high-pressure system, and when the pressure dropped suddenly, the liquid in that high-pressure system turned into saturated vapor as a result of flashing. Simply put, when a liquid evaporates into a gas, heat is absorbed in this process, causing the temperature of the system to drop; it is possible for extremely low temperatures to occur. Additionally, it can also be understood from the perspective of energy conversion: flashing, expansion, and the performance of work result in a decrease in the system’s internal energy and a drop in temperature. Contrary to the view expressed above, as the liquid vaporizes into gas upon pressure release, there is some compensation for the pressure; however, since the pressure release process continues, the vaporization also continues, and thus the temperature may keep dropping until all the pressure has been released or the liquid has completely vaporized. Ultra-low temperatures are still possible. Therefore, it is not possible to release pressure first; the liquid must be drained first before pressure can be released.
It seems to be related to the relationship between the throttling effect and flashing; with flashing, the temperature doesn’t seem to change significantly, but the temperature effect of the throttling effect is apparent. The specific differences should be explained in terms of the chemistry of the material – I can’t remember right now