Draft

[smokestackmag]

I have a Mosquito Enterprises 55 sq ft boiler with their fore and aft compound in their hull. It's a nice set up, although very heavy. I am beginning to burn Bituminous Coal. I took a friend out on a test run on the Connecticut River last fall and he recommended that I use a taller stack to get more draft as we were using the draft inducer all the time. I found in a junkyard a piece of stainless the right size to increase the stack height by about 4'. Does anybody have a feeling for this? Will 4' be enough? If I fit a manometer how much should it read? Oh and where should the manometer tube be plugged in to? I was thinking that it should be just above the upper tube sheet of the VFT boiler. Any help here would be very much appreciated.

Tim Lynch
SL Flying Cloud
Oxford, CT
Editor Smokestack

[fredrosse]

What is the height change from the furnace to the top of the boiler? Also from the top of the boiler to the top of the existing stack? Also, if available, do you have a stack temperature when burning wood or coal? How many tubes, and tube inside diameter? This data will allow a rough calculation of draft changes. Draft measurements can be made at the outlet of the boiler (base of stack), but also measuring draft in the furnace is relevant.

Burning coal generally requires more draft to get the combustion air up thru the coals (under-fire air), so a thin bed of coals is needed if draft is light. However if there are holes in the fire bed, then the combustion air can escape thru the grates and miss the fire bed, so with mild draft you need to keep a close watch, adding coal often and lightly. Wood fires generally do not have much draft required thru the fire bed compared to coal.

[barts]

Take a look at pictures of Dutch steam tugs, which were powered by coal, to see what the stacks looked like.
http://www.debinnenvaart.nl/binnenvaart ... boten.html

I rode on this one in 2000 from Rotterdam to Dordretcht:



- Bart

[smokestackmag]

Fredrosse,

Thank you for answering my query. I'll get the info today and post. And I'll see you at Waterford! And maybe GISF?

Bart, as well, thank you.

Yes, the tall stack on the tug is suggestive of a need I have.

Tim Lynch
SL Flying Cloud

[smokestackmag]

Fred and Bart,

Here's the dimensions:

The boiler tubes are 22 1/2" long, 1" ID and there are 83 tubes. There is about 12" from the top tube sheet to the base of the stack. Adding things up, there is 62" from the fire to the top of the existing stack and I have an extension piece that adds 42" to all this making the total 130". Stack temp varies with steam draft induction or not from 400°F to Oh My Goodness over 1000°F if I really open up the steam. That's with wood and without the stack extension. I don't recall the temps with coal but they weren't outrageous and I had to run the steam draft induction all the time.

Thank you very much for your help

[87gn@tahoe]

With fear of not being as scientific about it as Bart or Fred, I would say that 400* is near ideal as it seems there would be little wasted heat up the funnel. I would think that anything over that would warrant a stack damper to slow the flue gasses and allow the heat more time to transfer into the water within. Like a flue damper on a home wood stove allowing the heat to linger a while longer and more effectively heat one's house rather than heating the neighborhood,

[Steamboat Mike]

Hi Tim, before you go to a lot of expense and effort on extensions you might want to check the integrity of the air seal in you existing stack and boiler. It is astonishing how small a leak can kill draft through the fire. I thought my system was tight until I "tested" it. I had a smokey fire, and for some reason now forgotten, I covered the stack. The amount of smoke that poured from the supposedly tight firedoor was amazing. If the door is not truly airtight most of the air through the firebox will be overfire air and not of much help with intense combustion. I made a glass rope gasket and positive clamping latch for it and now have much better draft and combustion. Another place to check is the boiler shell to smoke hood connection. If it is not welded or a bolted flanged connection chances are it will leak. Smoke pipe joints are also notorious. Red high temp silicone sealant will work up to 500 degrees. It is flexible and easy to apply. It does require some design innovation to keep the joints from overheating but it can be done. An air leak effectively makes the remainder of the stack ineffective above that point. As to measuring the draft, it is measured in small fractions of an inch of water column for natural draft. With this little to start with it is easy to lose a lot of draft from a very few leaks. I think the best place to measure is through a 1/4" hole in the fire door that is plugged with a screw when not being used.
You may find that you do not need extra height in your stack, then again you may find that you do.
Happy steaming, best regards, Steamboat Mike

[smokestackmag]

Steamboat Mike,

Thanks for your answer! I remember when you went with me and we burned wood and charcoal for the first time in my boiler on Lake Winnie. And you tried to seal the breeching with some 'Goo Pucky' stuff in a tube. I haven't bothered with that since because I thought that the air/gases going through the boiler are at such a rate and volume that this would be of little value to the whole. Perhaps I need to rethink the whole thing.

[S. Weaver]

Tim, diameter is also a factor. I didn't think I saw that. Also with bituminous, some over-fire is required for efficient combustion of the gases. But as Mike was alluding to, any over-fire can have a negative effect on the overall draft.

I was just reading a B&W book during the holiday and there was a great discussion on stack sizing and draft calculations. One factoid was that except for induced vacuum via the venturi effect of a stack blower, draft is never a matter of draw, but a matter of "the push" of the colder, denser air displacing the warmer, less-dense air. The column of warmer air in the stack becomes a measurable event over the ambient air in the pan. I was enthralled at least ...

[smokestackmag]

Gentlemen,

My stack diameter is 9" if that helps.