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level in the tank is kept above the top of the pipe plate of the steam generator, and because the tank and the heat exchanger function as communicating vessels the pipes are filled with water. Te water in the pipes evaporates due to the heat supplied by the hot medium, so that the weight of the water column in the pipes reduces, causing it to rise. Te mixture of steam and water flows into the separation tank, the water flows downward and the steam flows out through a connection at the top of the tank.
promoters in the pipes somewhat eases this problem – but not enough to make it a viable solution for the factory. Falling film evaporation was the final solution
explored. Tis creates a situation in which no liquid level is maintained over the bundle and no boiling suppression can take place. It also involves a vertical heat exchanger with evaporating water in the pipes. Tis condensate flows downward from the top of the pipes as a thin film. On the way down, part of the water evaporates and is led upward as steam. Due to the thin water film, the heat transfer coefficient to the water is very high. Te non- evaporated water exits the pipes at the bottom and falls into the bottom tank. A fixed water level is maintained in the tank; this level controls the supply of fresh condensate. From this bottom tank, excess water is led to the top of the pipe plate, where a special header ensures that every pipe is supplied with sufficient water and that this water is evenly distributed over the pipe wall.
Te solution is a perfect example of simplicity
Fig. 3. The Thermosyphon reboiler is a vertical heat exchanger that was under consideration for the factory project.
Te problem with this principle in this situation is ‘boiling point suppression’. For example, take an evaporation temperature of 84°C with a corresponding evaporation pressure of 0.556bar. Te vertical pipe length is 6m. Tis 6m of extra water column produces a pressure at the bottom of the pipes of approx. 1.156bar, with a corresponding evaporation temperature of 113°C! As a consequence, the water does not boil in a large part of the pipes, but only though a low heat transfer coefficient. Tis adverse boiling suppression gives rise to a far from optimal heat transfer process. Te second solution assessed was a Kettle type
reboiler. Te advantage of this approach is that no individual separation tank is necessary. Te shell has a much larger diameter than the bundle, creating an area of steam above the bundle with natural separation of steam and water. Te horizontal body includes a water level that is maintained at a few centimetres above the pipe bundle. Here, too, the consequence is boiling point
suppression. Due to the level of liquid, the water pressure in the bottom pipes of the bundle is much higher than the 0.556bar, with the result that the water here does not evaporate. Te highly viscous medium now runs through the pipes and would lead to a very low heat transfer coefficient because of the low Reynolds number. Te use of turbulence
Fig. 4. The falling film solution is a perfect example of simplicity and robustness.
Johan van der Kamp is with Bronswerk Heat Transfer.
www.bronswerk.com
www.engineerlive.com 43
and robustness. Te decision to use steam generators based on the falling film principle results in equipment with the highest possible steam pressure. Tis is the best possible economic balance between CAPEX and OPEX. l
For more information ✔ at
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