5 Attachment(s)
New masonry gasification arch build.
Hi All, I am new to this forum, I have been observing for a few years now, but am just now posting. I am about to start a new arch, and thought that I would share my design ideas, and hopefully post progress shots of construction, as well as observations on its performance.
A quick bit about myself, I am a masonry contractor living in northern Vermont. I recently relocated back to the property that I grew up on, which has a formidable sugarbush behind the house. There are an estimated 325 taps back there, the farmer neighbors have been tapping them for as long as I remember (30+ years), but they are amenable to me "taking back" 50 or so taps to work myself. Over the years I have been paying close attention to masonry heaters and such, and have been paying close attention to the nature of combustion, and firebox designs. I have been mulling this particular design over in my head for a while, and just recently committed it to paper (Sketchup, actually).
The general philosophy of this design is to create a firebox that effectively gets hot enough for pyrolisis to occur, then administer preheated secondary combustion air directly below the pan, and as far forward as possible. My illustration shows a baffle above the firebox that kicks the gases forward, just above which is a cast refractory hump that has a bunch of small orifices in it. I figure that if I can mix the gases at this point, complete combustion takes place where it matters most.
I will do a little walk through of the design from the bottom up;
The ash pit, illustrated without a clean out door but will likely include one, is roughly 9" tall, and has two 2" schedule 40 steel pipes piped in from the rear. These pipes will have holes drilled at a 45* angle facing the cast iron grate (i.e. looking at it from the front, the right pipe will have holes drilled at 10:30, and the left pipe will have holes at 1:30). These pipes will obviously deliver air underneath the fire. They will run through the back wall into the illustrated rear chamber.
The firebox floor will have two cast iron grates that together make an opening that is roughly 16" long by 9" wide. The left and right side of the floor will be ramped towards the grates to move ash and coals toward the grates and ash pit. The firebox is deep enough to hold a good amount of coals below the level of the feeding door. The door will be a cast iron bake oven door that I took off of a Tulikivi masonry heater recently. The baffle at the top of the firebox is above the door, because I want to try to protect the door from the heat of secondary combustion to the best of my ability. It may be moot, the door may be doomed to begin with, but there is no harm in trying.
This is drawn for a 2x4 flat pan, I figure that if the firebox design proves effective, I can lengthen the heat exchange area without having to modify the firebox. I would be interested to see how well this design scales up for larger rigs.
The key feature of my design is the cast refractory heat exchanger/secondary air delivery piece. You can see in my illustration the air is piped into the floor below the rear of the arch, it is then drawn forward through channels in this cast refractory floor, and is injected just above the firebox. My perspective, as an armchair pyroengineer with a 12th grade education, is that the hump shown at the front of this cast piece is pretty critical. There are three sets of holes facing 9:00, 10:30, and 12:00 on this hump, they are all offset from one another. I am counting on the hump constricting and mixing the gases in that one concentrated area. The cross sectional area above that hump matches that of a 10" round flue pipe. I feel that the venturi effect (as best as I understand it) will further decrease the pressure right there, passively drawing in secondary air. Obviously, without a blower, the entire firebox would be a negative pressure zone, but I suspect that the throat area will exacerbate the negative pressure.
The gases flow underneath the pan to a standard base stack for a 24" wide rig, this is unillustrated. The base stacks seem to be available for 7" pipe, as well as 10", I am opting for the 10" as I believe that it will increase the draft. In the chimney sweep world they say that you need the three Ts for proper combustion, that is Time, Temperature, and Turbulence. I am hoping that this design is optimized for this. Time is addressed by the available length underneath the pan between the hump (or throat) and the base stack. Temperature is addressed by having the firebox inuslated with 1" of ceramic wool blanket between the firebrick and the exterior masonry. And Turbulence is addressed by the venturi effect created by the hump/throat, rapidly mixing gases seems to be key to good combustion.
I have some brick on hand leftover from a job that I will build the exterior out of, I think they will be more stable then cement blocks. The top of the arch will be covered by a stainless steel chase cover that a local chimney supply company will fabricate for me. Hopefully it is easily visible in the attached drawings. It basically just spans the two layers of masonry, and runs down each side an inch or so, effectively acting as a "clip" of sorts that will mechanically bind the two layers together. I did not illustrate a small flange that will center the pan. I acknowledge that I may encounter clearance issues with draw off valves and thermometers, but I will cross that bridge when I get there.
I will build this in my garage, and have the rig lifted out by my farmer neighbor. The first year of sugaring will take place outside of my garage, I will see how this design performs, and make sure that it is appropriately sized for my needs, then eventually build a sugar house for a permanent installation. Additionally, every year I attend a trade show locally to peddle my wares, and often bring down impractically heavy displays of my work. I will plan on bringing this rig down this year for display. I have not calculated weight, but my gut tells me in the neighborhood of 3,000 pounds. Heavy for sure, but manageable with available equipment. It will be built on a reinforced concrete slab, and experience has shown me that structures of this nature are surprisingly moveable.
The pipes for combustion air are stubbed into the void below what might be the flue pan. I am hoping that this rig will function effectively passively, but I wanted to leave the option open of being able to easily add a blower if I see fit. This should be easily plumbed to do such a thing. Further, I think that it will be good to be able to experiment with primary/secondary air ratios. For example, I suspect that at start up, it will be good to have a lot of under-fire air, and as things heat up shifting over to more over-fire air will be better. Perhaps some type of butterfly valves in these pipes will be good for these adjustments. This is another variable that I will leave for experimentation, as there are limits to my constructive speculation.
I think that is all for now, hopefully the details of my design have been more intriguing than boring (you wouldn't be reading this sentence if you found the previous content boring). I would appreciate any feedback, and concerns that may not have crossed my mind. I am hoping to get this thing constructed in the next three weeks or so, so hopefully this will be a fairly active thread, with construction photos and such. Now let's see if I can attach some drawings....
Attachment 15116
A view of the outside of the arch.
Attachment 15117
A 3/4 cutaway view of the arch, hopefully clearly illustrating the firebox, throat, and heat exchanger.
Attachment 15118
A cross section of the arch, illustrating where and how combustion air is delivered.
Attachment 15120
A cross section of the firebox.
For some reason I was unable to load a cross section of the cast refractory heat exchanger floor thing. You will have to take my word that it is there, just a hollow floor that leads to the perforated throat.
Thank you for your time, -Alfred N. Dedam, Old Goat Masonry (I am only 33 though, I don't know why I named my business this).
