by colin saengdara » Mon Dec 30, 2013 9:52 pm
Hi everybody. I still can't find my build photos so I took some new ones of the externals of the stove and the hookup to the chimney. As I said before, this one is a mock up to figure out answers to lots of questions I had about how small a system size was practical for a batchbox. It's ugly because it still has all the clay mortar all over it, but it's gong to be taken apart soon enough to reuse the bricks upstairs. This is the view from the front:
It's a two bell stove. The heat riser exits right below the clay "burner". I added the burner in an attempt to dissipate the highly localized hot spot above the heat riser exit and as an attempt to give more surface contact with the skillet. In the real stove, I plan to have the skillet in direct contact with the flame. Then I will be able to close the stove by means of 1 the skillet, 2 a cast refractory insert the same size and shape as the skillet , or 3 a heat riser extension and small barrel.
Pictured is the cob "burner"
So, one of the things I learned was that there was a tremendous amount of heat coming out of this little guy. I temporarily fire bricked the top so that I could test fire it with plenty of mass to absorb the heat output. This current setup absorbs all the heat readily and because the fire brick is exposed, it radiates pretty well too. But the final design will be a cast refractory top with the round skillet and a rectangular skillet as the primary absorptive loads. I plan to use a low density mix for the flue gas path and high density for the exposed top. Then my wife wants a rack, like oven rack, to cover the non-cooking surface of the 2nd bell so she can dry wood, and keep things warm there.
Pictured is the combustion chamber opening. It consists of my original casting lengthened 4.5" by dense fire brick to make it more convenient sized for fuel. Because of the draw from the chimney, there is always negative pressure even when the stove is cold. So lighting is as simple as loading it and burning 1 small piece of newspaper over the face of the fuel. Then putting the burning bricks in front of it to give it the correct amount of primary air. From there it takes off on it's own.
Because it's a test of the bell, the stove's secondary air is not hooked up. I regret this lazy decision, because it's not burning as cleanly as it had on it's own in the test bed. If I had hooked up the P channel, I wouldn't be stuck wondering how much of that is because of the still unburned fuel condensing on the cold fire brick, and how much is because of the lack of P channel.
Unfortunately, I think that the heat riser needs to be extended. Even with the P channel in the test setup flames were regularly reaching 2.5 to 3 ft into the chimney flue. Ideally I would like to have the flames crown just below the cooking surface. That is why I plan to have a gas path that is still lightweight refractory, to give the gases more time to burn. I'm hoping that a red hot skillet bottom might assist in the final combustion stage, but if not, the cast refractory insert will effectively extend the heat riser a great deal more. With that in place, the gases will not hit an absorptive surface until they reach the front cooking surface. This is about double the length of the heat riser suggested by the formula Peterberg developed. (.72 x 3.125 x 10= 22.5" But this is not the application it was developed for. That's why I need to have a good plan for providing high enough temperatures even in the cooking zones to achieve complete combustion. This is where I kick myself for not just hooking up my P-channel, it might make a huge difference here. The last thing I really need right now is an extra variable!
This last photo is the bricked top. the gas exits below the burner and proceeds under the top layer counterclockwise to waterfall down into the bell at the lower right brick.
Happy Holidays! - Colin