Questions about falling-film reboilers
Thread Content
Regarding falling-film reboilers, the information available on Baidu seems rather vague; there are a few issues that I’m not sure about. I hope experts in heat exchangers can help me out. 1. It is said that in a falling film reboiler, the liquid is spread into a film by gravity and then flows down along the pipes; so shouldn’t the static pressure at the upper and lower parts of such a heat exchanger be similar? Is that tower under vacuum? Is the heat exchanger also under vacuum, or is there some other pressure there? If the heat exchanger is not in a vacuum state, is there still a throttle valve at the outlet to create a vacuum? If the upper and lower parts of the heat exchanger are under vacuum, does that mean a throttle valve also needs to be installed at the pump outlet to create a vacuum? 2. Both are falling-film reboilers suitable for use under vacuum conditions. However, I fail to see how they’re applicable in this context. There’s already a pump at the bottom of the tower that pumps the material; the pump’s outlet pressure is sufficient to overcome the resistance of the heat exchanger. An ordinary heat exchanger should be adequate. Since the components used are the same, why bother using a falling-film reboiler?
In towers operating under vacuum pressure conditions, the materials in the upper and lower sections of the falling-film reboiler evaporate under almost the same vacuum pressure (as the pressure drop due to gas flow in the tube side is virtually negligible). When a forced-circulation vertical reboiler is used, due to the effect of hydrostatic pressure on the lower part of the tube side, the vaporization pressure of the material becomes 20–30 kPa higher than the vacuum pressure inside the tower. Consequently, the material’s vaporization temperature rises significantly; there is also a long and inefficient subcooling section, with a high-temperature zone occurring at a certain point upward from the bottom of the tube side. This is not acceptable for materials with high boiling points, sensitivity to heat, and a tendency to coking; it can even result in the temperature of the heat source in the reboiler not being sufficient to meet the requirements of the material being vaporized.
Thank you for your reply. Can it be understood in this way: 1. Since the flow inside the tubes of a falling-film reboiler is discontinuous (in the form of film flow), there is no static pressure, and the pressures on the upper and lower sides are essentially the same. 2. Is there positive pressure at the outlet of the circulation pump? It was mentioned above that the upper side of the heat exchange tube is under vacuum. So, which component absorbs most of the pressure at the pump outlet? Is it a liquid distributor? 3. If it is changed to a horizontal forced circulation system, will the effect of static pressure be eliminated?
This post was last edited by wanlirn on 2022-8-6 at 13:23. The key to a falling film reboiler lies in film evaporation; once the liquid is turned into a film, there is naturally no static head as in traditional thermosyphon reboilers； The pressure inside the heat exchange tubes, both at the top and bottom, is essentially equal to the pressure inside the tower ； The circulation pump here can be considered part of an open system; it is necessary to pump the liquid at the bottom of the tower to the inlet located at the upper part of the heat exchanger. The pressure loss in the pipes can be ignored. Of course, there must be positive pressure at the pump outlet, otherwise it would be impossible to pump the liquid from the bottom of the tower upward. . . . It seems you’re a bit confused about the four concepts of pump head pressure and static head pressure, as well as the kinetic and potential energy of liquids; find a book and study it carefully
Thank you for your reply, sir. I basically understand it now. However, I still have one question: The outlet of the pump is under positive pressure, while the upper inlet of the heat exchange tube is under vacuum (roughly equivalent to the tower pressure). Wouldn’t the pressure drop along the piping from the pump outlet to the upper inlet of the heat exchange tube (including the distributor, etc.) be at least 1 atmosphere (the difference between positive pressure and vacuum)? In other words, does the distributor account for most of this pressure drop?
At least one throttle valve should be installed in the pipeline from the outlet of the circulation pump to the upper inlet of the falling film evaporator, in order to prevent the circulation pump from being overloaded and damaging its motor. It is certainly difficult to ensure that the excess head at the outlet of the circulation pump is consumed by the liquid distribution distributor under different loads, and moreover, the pressure drop across the distributor follows an inverse relationship to the \"flow rate~head\" curve of the centrifugal pump.
This post was last edited by wanlirn on 2022-8-8 at 12:53 regarding the pressure difference in your vacuum. Why don’t you think about it? Isn’t there also a negative pressure at the pump inlet? For example, consider a pump that draws water from a tank: the inlet of the pump is at atmospheric pressure, and so is the outlet. So do you still need to take atmospheric pressure into account when calculating the pump’s head? It’s just offset. . Take your system as an example: if the elevation of the feed inlet to the heat exchanger is 6 meters and the center of the pump inlet is at 0 meters, then a pump with a head of 6 meters will suffice. The pressure loss in the pipelines is very small and can be ignored. Of course, in practical applications it’s necessary to have a certain margin of safety; or if no pump with exactly that parameter is available, a valve can be installed at the pump outlet to prevent excessive flow and high pump current, and that will do
I’ve learned a lot; there are things I myself would never have thought of