After being totally confused over what sort of boiler would be most suitable, I decided to make something. It's a small Roberts-type wood-fired boiler with one steam drum and two mud drums. Here's some pictures of it before it got dirty from the fire.
It stands about 15" high, and the two mud drums are 10" apart. The construction.... the mud drums and downcomers are made from electrical conduit (Eeeeek, screeeeem! I hear you say. Well, they're maybe not pressure-rated, but they were thick and at hand so I used them.); 1" for the downcomers and 1 1/2" for the mud drums. The steam/water drum at the top is made from an empty MIG welding gas bottle. The copper heating coils (4 of) are made from 3/8" pipe with flare fittings at the ends (pipe and fittings were bought from BES. The brass fittings are 3/8" flare to 1/4" NPT thread.). The fitting on the top of the steam drum is the steam outlet. The fitting on the end of the drum that looks like a bicycle fitting is (wow) a bicycle fitting; I used this for pressurising the boiler to check for leaks.
The idea is that water is heated in the copper pipes by the fire and rises into the steam/water drum. Cold water flows down the downcomers (which are placed outside the fire, see photos later) and into the mud drums to replace it. Eventually, the water starts to boil.
All the steel parts were welded, which soon taught me that even if a weld looks good, it can still leak like a garden sprinkler! I managed to pack most of the leaks and I put some car radiator sealant in the water to plug up the rest. I know it sounds pretty rough 'n ready but this isn't a "final" boiler so it doesn't matter. Also note the lack of a water level gauge, pressure gauge and safety valve!
I built a little brick hearth to house the boiler, making sure to keep the downcomers outside the fire to ensure convective circulation of the water:
Now we're getting somewhere! It's now steaming (it took about 10 mins to get up to steam):
And here it is running a steam engine (the Stuart Turner 10V described elsewhere)! It ran like the clappers.
And here's some more pics of it generating steam. In the second, I had connected a pressure gauge to see how high it would go; it managed to reach 20psi and would probably have gone a lot further. I thought it best not to put the boiler under too much pressure, just in case!
I've at long last managed to install windows XP on the laptop and I can now capture video! Here's a ~3MB video of the steam boiler running the engine and a whistle. It's about 2 mins long and is in WMV format. Use the video settings controls in Media Player to turn the brightness down a bit!
05/12/10: This is now a YouTube video instead, see below.