The Steamboating Forum

Hey

I need a chimney - for my new VFT boiler in Froya.

I want it to have the best possible draft (insulated I presume) and stand white paint at the outer barrel/tube.

All design references and ideas are welcome.

Best regards
Jørgen

Jorgen,

I think there were some decent guidelines noted in the "draft" thread: http://www.thesteamboatingforum.net/for ... f=8&t=1264

This has been discussed before, the usual system used in the UK is to have a double skin funnel with an air gap between of about 20 to 30mm depending on funnel size. NO insulation, the reason is to allow a free flow of cooling air through the annular space. This keeps the outer skin cool enough for paint and the flow of air helps keep the top free of soot or at least should do.
Regards
Jack

If practical, pull combustion air down through that gap between the inner and outer tubes.... this will both help prevent burns on the stack and improve boiler efficiency - this gives you a easy air preheater.

=- Bart

I'd have some doubts about how much air you could pull down that annulus. It would be getting warmer as it descends thus the air would rather rise than descend. Also, even though the combustion products are much hotter, and thus more voluminous, than the intake air, the ratio between the cross section of the inner stack and the area of the annulus might not be enough. What I'm trying to say is that I suspect that I wouldn't be able to get enough air down through that gap to feed my fire. If I had forced draft, no problem but I'm trying to keep the boat simple.

Mike

Hi

Yes, pulling air down the gap seemed like a great idea. However, when I was working on my Plan A Blackstaff Wood boiler design I had real difficulty working out how to duct it to the under side of the fire. It all seemed simple at first, but the more detail I developed the more difficult it became. That could, of course, simply be designer failure, but I have never seen the idea used in a launch. The nearest was a rather angular casing for a Blackstaff design that drew air in part way down the casing and ducted it underneath.

Regards

Pete