Question about finned tubes in a flue gas reheater 1

[Yobbo]

L.S.,

We apply a few flue gas reheaters with finned tubes in one of our power plants. The fin density is 196 fins per meter. The flue gas in question may still contain some fly ash after passing the E-filter. The reheater is situated after the wet desulphuriser. Therefore the fluwe gas contains quite some condensate droplets or aerosols. The acidity is quite high ( pH = 0 .. 2 ). The finned tubes tend to corrodate heavily and clogg. In a german book about heat exchangers I read that the spacing between two fins in normal industrial applications should not be less then 4 mm. For dirty industrial gases they even advice to increase the spacing to about 15 mm.

I would like to hear from fellow collegues whether they have dealt with the same kind of problem, whether the fin spacing really is a determining factor and what fin and pipe should be selected to prevent extensive corrosion.

Karel Postulart, The Netherlands
Nuon Power Generation

 

[ione]

Don't know whether you've already come across the attached paper, but if not it'a worth the effort to give a read.

 

[speco]

Yobbo,

I think this is a fairly rare application, so there may not be much response to your question. I have worked on a few of these types of heaters, but not many.

It appears that your heater is a direct flue gas heater from your description, and probably uses steam as a heat source.

First, I think the design you have is very likely to become fouled from carry-over from the scrubber. It's also very likely to corrode as well.

The fin spacing you have now is very close. Fewer fins would be better. However, if you have the space for it, a bare tube design would be even better than that. I am hesitant to suggest materials, but some type of stainless would probably be a good choice for this.

The alternate heating system that I have seen heats air, rather than flue gas. The air is heated to a much higher temperature, then blended with the cool flue gas to get the bulk temperature well above the dew point. This type of heater can use ordinary aluminum fins with a very close fin spacing (typically about 10 fins/inch or 394 fins/meter).

Regards,

Speco

 

[Yobbo]

Hello Ione and Speco,

Thank you for your usefull responses to my thread. As it is rather impossible to prevent any carry over from the scrubber I think I will try to size a heat exchanger with a transfer area as large as feasibly possible with straight pipes.

Karel Postulart, The Netherlands
Nuon Power Generation

 

[EdStainless]

Where there is a chance of carry over from the scrubber you should run smooth tubes.
Do you have a washdown method (ala soot blowers)?
I have seen this service eat up super-austenitic and super-ferritic tubing. The first bundles (wettest) were replaced with C276 and the super-ferritic was used on the others.

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Plymouth Tube

 

[Yobbo]

Hello Edstainless,

Also thank you for your usefull comment. I will take it to heart and consult our corrosion specialist on your suggestion.

With best regards.

Karel Postulart, The Netherlands
Nuon Power Generation

 

[chicopee]

I don't know about the mass rate and temperature of the flue gases, however, may be you should consider passing the flue gases thru another boiler.

 

[rmw]

If your heat transfer equipment was sized such that it required the extended surface (fins), then it will be woefully short on performance as a bare tube. However, once your finned surface becomes clogged, then it is virtually useless, so bare tubes may give better performance than clogged fin tubes even though the performance is reduced. What is inside the tubes as the heating medium?

rmw