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Hi,
My plant is struggling with a thermosyphon reboiler start-up. The heat input to the reboiler is by condensate level control. The heating medium is a 21 bar steam which has been let down from 45 bar saturated steam without desuperheating. The process side is water at 112°C. The problem with these reboiler is that it can't be started up easily without losing thermosyphon. During start-up, the steam is slowly opened up using a globe valve on the steam inlet line to the reboiler. The reboiler will seem to be doing fine until when the chest pressure reaches 4.5 bar and it will then stop thermosyphoning.
Is it possible that there is film boiling involved here? For water systems, what is the maximum allowable temperature difference before one can get film boiling?
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Reboilers
Started by Guest_Guest_*, Feb 13 2004 09:41 AM
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siretb
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Posted 16 February 2004 - 05:20 AM
I will assume a vertical thersiphon.
There are several reasons that may cause a "shut down" or instability.
Please cheack the following
- vaporization per pass should not be too high. do not exceed 30%
- the inlet line could be too large. In such case, as the effective heat flux increases some vapor can back flush and block the siphon. You may try a distribution plate with orifices that will restrict the flow
- there is a critical flux (critical heat flux, CHF). (Kcal/hr/m2 or BTu/hr/ft2 of effective area, usual (organics) is aroud 15000Btu/hr/ft2, can be as low as 4000, can also be higher) above it you may get a film or just you have too many bubbles that hinder the transfer and flux)
- you mention you have 112°C water. Do you have salts (that may cristallize due to too high a loacl vapor fraction or too high a skin temperature)?
- check non condensables in the steam
It may be a good idea to add a circulation pump. Before doing that I would check the following: (design) expected heat transfer coef, vaporization per passs, specific heat flux (Btu/hr/ft2).
Of course the fact that the steam is not de-superheated is not a favorable factor, I would not use superheated steam if given the choice.
There are several reasons that may cause a "shut down" or instability.
Please cheack the following
- vaporization per pass should not be too high. do not exceed 30%
- the inlet line could be too large. In such case, as the effective heat flux increases some vapor can back flush and block the siphon. You may try a distribution plate with orifices that will restrict the flow
- there is a critical flux (critical heat flux, CHF). (Kcal/hr/m2 or BTu/hr/ft2 of effective area, usual (organics) is aroud 15000Btu/hr/ft2, can be as low as 4000, can also be higher) above it you may get a film or just you have too many bubbles that hinder the transfer and flux)
- you mention you have 112°C water. Do you have salts (that may cristallize due to too high a loacl vapor fraction or too high a skin temperature)?
- check non condensables in the steam
It may be a good idea to add a circulation pump. Before doing that I would check the following: (design) expected heat transfer coef, vaporization per passs, specific heat flux (Btu/hr/ft2).
Of course the fact that the steam is not de-superheated is not a favorable factor, I would not use superheated steam if given the choice.
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