Stack height best for solid fuel firing

[Mfoxchicago]

Hey guys, is there any specific rule of thumb or computations for determining the optimum height for a stack to be when firing with solid fuel (wood)? In order to prep TULE PRINCESS for cruising on a lake with a fixed bridge, I removed 21" of her original stack. However, our lake has stayed fairly full and clearing the bridge will not be possible for the foreseeable future. Which is ok, but I was wondering if we need that taller stack back.

She was originally designed with solid fuel firing in mind, but has spent most of her life firing with an atomising burner which was added later. For several reasons, we have decided to go with solid wood firing for our application. We've done ok with wood, but do have to assist the draw with her electric stack blower fan at startups and when the temps are really high and the breeze falls dead calm. This past weekend the wind was blowing 10-15mph and when it would really gust, we noticed a spike in the fires ability, on its own, to bring up the pressure. I was wondering if a retro fit of the missing 21" of stack, perhaps even a bit more - in the form of a removable telescoping stack configuration would really help improve the draw and there by the fire's ability to pull more combustible gases up through the boiler tubes.

In pictures of the old boats of the mid to late 1800's, when electric fan stack blowers were not even around - the great river steamers had really, really tall stacks. Was there any calculations for how much feet of stack height was best for X square feet of grating firebox space? Or, just make the darn stack as tall as you can? Your thoughts.

[DetroiTug]

As far as I know, there is no formula for determining this, it's mostly trial and error. Like testing and determining aerodynamics, it's done in a wind tunnel. Of course, using forced draft of some sort like exhausting up the stack, negates any issue with natural draft as it is cancelled out. I know of some steamers where the stack height barely eclipses the canopy roof and they work just fine without induced draft.

I think the really tall stacks were for increased draft and to keep the coal smoke and soot away from the crew and passengers. The same crafts could have probably done equally well with lower stacks. There is such a thing as having too much draft, where the velocity of air moving through the firebox is so fast the hot gasses don't have time to transfer to the heating surface. Where an increased draft may yield a 20% increase in steam production, it may result in a 40% increase in fuel consumption. It's very common in steam car boilers where plate baffles are installed in the boiler exhaust to slow the gases down.

It's all trial and error, there is no exact answer to this question without testing.

It sounds as though when your stack was cut down, it is not drawing as well now, it probably needs to be increased. Most of the inland river steamers ( mine included) have a stack that can be folded back to get under bridges. Some have a clever design with counterbalance weights that make it easy to fold back and then return and lash. That is what I would suggest on yours. There are some YouTube vids of UK canal boats that show this in some detail.

-Ron

[fredrosse]

If you look on youtube there are several old films of steam fishing fleets coming into harbor, under fixed bridges. These steamers all have folding stacks that are turned 90 degrees to get under the bridge, then folded up vertical for cruising. These films should give you an idea of how tall these coal fired steamer stacks should be, approximately. There are some posted on this forum, will try to identify the threads soon.

[barts]

The Dutch coal-fired steam tugs have pretty high stacks:

https://www.youtube.com/watch?v=pP2ciTbhw1U

Basically, taller stacks gives one almost free additional overload steam output
w/o using any steam to force the fire. One thing to consider is that the exhaust
gases should be kept as hot as possible (insulated liner, perhaps) to maximize draft.

- Bart

[Mfoxchicago]

I hear you Bart. I agree. Thanks for the posting and the link.