by aflatoxin » Fri 18 Aug 2006, 22:58:08
There are some design features that can prevent smoking in something like this. I work around industrial furnaces, so I've learned a few things. A lot of these design elements are applicable to solid-fuel combustion only. Gas fuels are a lot different. I've built several wood-burning stoves over the years, and making one of these things work properly is actually a challenge.
These pointers should give you something to work with:
Sizing of the stove is important so the fire can be maintained at a relatively constant rate. If it's too big, then it has to start and shut down often, and that could cause poor performance and smoke. Far better to make it too small, and run the heck out of it. If it's not quite big enough for the coldest days, get a blanket.
Stacks must be tall enough and be insulated. Whats more, they need to be the correct diameter. This ensures that the gas travels at a sufficent velocity, and stays hot enough to prevent the creosote from condensing inside the stovepipe.
A cold surface can quench the fire, preventing it from staying hot enough to burn efficiently. Good designs use firebrick barriers and arches to reflect enough of the radiant heat of the fire back into the firebox to keep it burning correctly. The boiler tubes/plates do not directly "see" the fire, instead they rely on radiant energy reflected onto the heat exchanger. The boiler tubes also take the heat from the hot combustion gasses, but not so much as to cause condensation of the creosote.
Finally, the grate design is important. Good designs preheat the combustion air and impinge this heated air onto the fire with enough force to dislodge, to a small extent, the ash that accumulates on the wood. A shaker grate can periodically knock the ash into the ashbin.
The boiler at my friends place is about 20 years old, and it has pretty simple controls (thermostats). It sometimes smokes pretty bad, especially if it is really jammed full of wood.
I'm sure that a more modern design with computerized air dampers, stoker, and propane-assist ignition would be pretty clean. All new woodstoves sold in the US have to meet certain EPA emissions requirements, so they shouldn't smoke much.
If I was going to build a wood boiler, I would put the boiler tubes between the smoke shelf and the flue, about 8 inches above the physical location of the top of the wood. I would size the tubes so they had a surface area of about 30-40% of the total area of the smoke shelf to prevent quenching. I would use stainless steel pipe, probably 1" NPT. I would use a convection type heat transfer system between the water tubes and the steam drum, and a pump off the steam drum (water reservoir.) to the heating system. The boiler tube design must allow for expansion when heated so they don't get stressed and break I would pressure test it for about 2x or more the working pressure of the system, and make sure that the pressure relief valve was big enough.
If I was going to use this on a radiant system, there would have to be a provision to protect the tubing if the water temperature got too hot.
Hope this helps.
