Fire Space for small solid fuel boilers

[SL Ethel]

I am looking for guidance for how much firespace is required for very small solid fueled boilers. I am putting together a power plant for a canoe, where weight and center of gravity are obviously critical. I am thinking of very small roberts, express, or 2-drum type boiler, with a 4" steam drum. Construction will be all copper with a 100 psi working pressure. I'm contemplating about 5 square feet of heating surface for a 1/4 hp engine.

I'd like to burn wood, but I'm not sure if this is pragmatic. If not, I'd go with coal or charcoal. I know there are locomotives with tiny fireboxes burning coal - are there good design rules out there for very mall solid fuel boilers? Everything I've found seems to be geared toward full sized practice. Has anyone had good experience burning wood in a very small boiler? And if so, what were the design features?

Frankly, feedback on any aspect of this boiler concept would be welcome. I've built model boilers and a full size boiler, but never anything straddling the line between the two like this. The goal is to have something robust and fairly relaxed to operate for relatively long river cruises.

Thanks!
Scott

[barts]

Some random thoughts:

The smaller the wood pieces you burn, the less grate area you'll need (see pellet stoves for examples). Rainbow has about 1.5 sq feet of grate area, and makes 5-6 hp. Power is proportional to grate area, so you'll likely be limited by the size of the pieces of wood you want to split. For power & energy density, it's hard to beat mesquite charcoal broken into small pieces. You'll get more power out if you exhaust out up the stack as in locomotive practice. With a copper boiler and solid fuel, remember you'll want to be able to dump the fire if you lose the ability to pump water into the boiler. Any silver soldered joint exposed to the radiant heat of the fire must ALWAYS be covered with water, otherwise the joint will fail (I've seen two copper 3 drum express boilers lose a tube this way). A 5 sq ft boiler will make plenty of steam; you can go somewhat smaller. A Bolsover design similar to the Blackstaff boiler is a good choice as well.

- Bart

[Kelly Anderson]

On the Blackstaffe boiler I built for my launch Reciproca, the grate area is 6.5" x 15.5". The distance from the bottom of the steam dome to the grate is 6.1". She would always smoke a little, so combustion wasn't entirely complete, but was always a good steamer.

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[SL Ethel]

Hi Kelly and Bart,

Thank you both for the feedback. It's that 6.1" number I was wondering about - that gives me a great reference to work with. Bart, my intention was try firing on hardwood wood chips or homemade charcoal. My goal is to have the boiler be large enough that it can supply adequate steam for normal cruising on natural draft without too much fuss. My first boiler on my larger boat would only maintain pressure crammed absolutely full of wood with the stack blower working, and that was not a very satisfying way to operate.

Thanks!
Scott