I apologise for not posting here for ages but ... well, er, (fit excuses here) - I never got around to actually completing a stove until very recently - even though I began around April.
The thing is, Summer got in the way, when the priorities for building fires eases off. Plus I've had loads of other work to do. And mainly, I've been scavenging and rooting for everything I've needed on the basis that this prototype shouldn't cost me anything (except the obvious labour and consumables).
I've also now got a grandson (first grandchild), now almost 14 months old and I'm astonished how much of my attention he's captivated. So I haven't been fiddling and twiddling as much on personal projects.
But anyway, I've completed what I began in April. I've followed Ppotty's general design and ideas, but very much scaled down. I've used a steel construction throughout, and so all welded (convenient for me), other than the riser shroud (stainless duct pipe), the secondary air pipe (copper) and the insulation (pearlite and high-temp rockwool). It's by no means the ultimate stove even by my standards, let alone other examples built by people here. I haven't gone the extra mile in a few details (firebox insulation, cast riser, routing of primary air) but it's mostly there.
Again, the main reason behind the build was - scaled down (from Ppotty's sizes) for domestic use, and smaller pieces from odds and ends and off-cuts was all that was needed.
Without further rambling, these are the materials I've used for the construction:
1. 13kg Butane cylinder for the heat exchanger. They're only 12.5" diameter and about 17" high (plus the ring base). I cut the top off just after the curve of the shoulder begins. I then welded (all round as a seam) a ring of 40 x 3mm flat bar on the upper rim to give me a vertical ring finish - around which I then welded another ring (formed as a washer - slowly with a propane torch and a light forging hammer) around to give me a bolt-able flange. I think this ring was from 30 x 3mm flat bar. It is drilled equidistantly in 6 places for 8mm bolts and nuts.
On the rear (opposite the burn tube intake), and low down I cut out a slot for the final flue. I made a small flue box from 3mm plate and welded in position all round. The rear of this box has a bolt fixed, rope-sealed access door. The top surface has a 4" hole and short spigot pipe to suit a 4" flue pipe. The box is about 5" wide and 3" deep and protrudes about 6". It's not the greatest access (for hands), being only 3" deep, but being a small cylinder I didn't have too many inches to play with. It's large enough to admit a vacuum-cleaner pipe if and when needed, and for visual inspection.
A problem with gas cylinders is the concave base (internal). The riser pipe doesn't want to sit level (though mine is ultimately welded in position and so cannot move) and there's a problem using shrouds and loose insulation (perlite) - the insulation will dribble out of inevitable gaps from the uneven base. I got round this by inserting a circular disc of 3mm plate inside the cylinder and seating it in a level position and welding it in place. It's not the full diameter of the cylinder, but about 8" - enough to provide a flat internal base to seat the riser and better mating to an eventual shroud pipe. I had a 1" diameter hole already drilled in the centre of this base plate, into which I poured (actually tea-spooned, a laborious experience) loose perlite to fill up the void under this plate. Some nominal insulation under the pipe maybe. The riser pipe base easily covers this 1" hole.
Around the riser I slid an 8" diameter stainless duct pipe, with the required cut away to accommodate to burn tube (and the secondary air-pipe stub - a tricky bit). There is no way the cut-out is accurate enough to retain loose perlite . I wadded a ring of high temp rock-wool around the base of the riser pipe/shroud cavity, then filled the rest of the cavity with loose perlite, finishing with another wad of rock-wool all round the top of the cavity, just to retain the perlite. I also had to wad some rock-wool around the top of the burn tube and where the pipe stub protrudes.
2. A circular disc of 8mm tread-plate. This is the top surface plate of the exchanger and is drilled x 6 to bolt to the exchanger top flange. There's a rope gasket between the two. The plate was an odd piece I found lurking in a steel pile and admittedly a bit rusty to begin, with a few 10mm holes drilled through it from some previous life. I filled those in with weld and cut the circular piece, admittedly a little roughly, freehand with an oxy-propane torch. It's unfortunately about an inch too large in diameter to swing on the lathe, otherwise that edge could have been a little truer. On the underside of this plate I welded a disc (via 1" legs) about 6" diameter, as a sacrificial deflector - to help prevent any possible distortion of the top plate. Once it was welded into position I dished it somewhat in the centre with an oxy-propane torch and a 4lb rounding hammer. This should (may) deflect the riser heat across and down into the exchanger. I think at this point I may have been getting carried away.
3. Schedule 80 steel pipe @ about 107mm ID (about 4.25"). The wall thickness is just short of 10mm. Hence the outside diameter is exactly 127mm (5"). This is the riser pipe. The cut-out in the pipe to match the incoming J tube was cut using a slot drill in a milling machine and was cut to be narrower in width than in height - to accommodate the deflection in the J tube to create the vortex.
3. 150 x 10mm flat plates for the 'J' tube. These were formed into a square(ish) 'box' tube roughly 6" height x 5.25" wide for several inches of length (can't remember, haven't got my drawings here) and then narrowed on the left side to give a final aperture width of 3.5". This fabricated J tube matches the cut-out in the riser pipe and is oriented so the pipe is twisted slightly off-centre (to the right), rather than square-on to the J tube. This is for encouraging the vortex pattern in the draft.
I didn't make this J tube quite long enough in hindsight. There may have been some slight miscalculation at the time. As a result the J tube only protrudes through the heat exchanger (gas cylinder) by about an inch - which made for a very close firebox later on. I've realised this mistake since but it's not really been a big problem and I will live with it for this model. I would have preferred about 3" protrusion had I not miscalculated. I didn't have any more lengths of 10mm flat or 10mm plate at the time, and didn't want to be piecing little bits together.
I MIG welded this 10mm section J tube to the riser pipe with some very solid welds. With some careful edge prep grinding I was able to lay down welds on the inside of the joint (Vee joints, filled) as well as the outside of the piece. The riser pipe was open ended at the bottom at this point, allowing access. When all was welded I then closed off the bottom of the pipe with a final 10mm plate (which also butt welds in line with the base of the J tube). This base plate was oversize (for the pipe diameter) and forms a larger flat base that helps seat the riser truly.
On the right side of the J tube, just before it meets the riser pipe is a 1" hole. Over this hole I welded a short piece (about 2") of iron pipe (still within the body of the cylinder when all is fitted up). I had this pipe in the lathe first and reamed the inside bore for an inch or more depth so it's a tight fit for a piece of 22mm copper pipe to go into.
Where the J tube emerges from the cut out in the cylinder, it is fully seam welded around all the joint. This then means the cylinder and the riser pipe/J tube are now all one piece and cannot be dismantled without destruction. It also begins to make it quite heavy. But in addition it makes the whole thing very solid and at this point no rope or other gasket materials are needed. There is no chance of air or fume intake or exhaust.
4. A firebox all put together from scraps of 3mm tread-plate, with however a 6mm plate lid (feed door). I made 'letter-box' style slots in both sides for primary air feed with hinged flaps as controls. I put in a front flap as well at the base of the front plate. This is mainly to assist starting and is rope-sealed so that it closes off completely once the fire is going well enough. The effect of opening this front flap is instant as it draws air directly onto and across/through the burning fuel.
There are false sides as per Ppotty's design, so the primary air slots on the sides feed air into these side chambers, travel up and across the top gap (about 20mm from the lid) and down onto the fire. I bent short 90 degree deflectors to the top of the inner plates to channel the air across the top a little, as per recommended. The inner plates are stainless steel sheet, about 1mm thick. They seem to be holding up so far.
I didn't put a grate or shelf in this firebox and am burning on the solid base. The J tube inlet begins only about 1" up from the base so there isn't really room to be raising the fire too much. I could easily retro-fit one, probably removable. I've cleaned out the firebox so far quite easily with a vacuum-cleaner.
The firebox is about 12" wide at the rear, 10" at the front. The side air chambers are about 1.5" wide x the whole depth and height. I think it's about 10" deep, front to rear. It's 12" high at the rear, sloping down to 10" high at the front. It's basically a 10" x 10" x 10" box or a little more with the tapering dimensions (minus the air chambers internally).
I wondered long about the fit of the firebox to the burn tube. Therein came the problem of little protrusion through the cylinder. It would be a bit of a struggle to make a flange and to gain access to it properly when required. I could have lengthened it from that point, or made an extension off the back off the firebox - but again, I didn't want to be adding little pieces ... and tbh I just wanted to get on with it at that point. I've made this stove in fits and starts, in between everything else.
So, what I did was take the cylinder, with welded-in, insulated burn tube/riser, to the outside world and built a small stick fire on two bricks in front of the burn tube. I had no top on the cylinder at this point and no final flue fitted (I hadn't cut the exhaust slot in the cylinder either). Sure enough the fire pulled into the burn tube and up the riser very well, particularly once blazing brightly. It was basically just a ground fire built right in front of the construction - though I did push some sticks just into the opening of the burn tube. The burn was also behaving as a vortex. Indications so far were encouraging.
That small fire was only burning for 20 minutes, as a brief test and with no top plate on the exchanger liberated all its heat out of the open exchanger and certainly the open riser. I had to let the thing cool for just 10 minutes before I could grab the device with fingers under the top ring flange. Nonetheless I could still feel a good heat coming out of the open riser, being positioned somewhat above it. When I got it back indoors I carefully put my hand and arm down into the riser and felt a very nice heat there - enough still to burn on contact. Three hours later the inside of that riser pipe was still warm.
I then fabricated the flue box and had ready a 9' 6" length of flue pipe. I fitted the top plate to the cylinder and the flue pipe and then repeated the ground fire in front of the device. Again, good signs and pulling through the system very well.
I made the firebox, at this point without a lid, but with all primary side air slots and flaps and the front starting flap. I drilled the cylinder at the correct point (tricky) and inserted a shortish stub of 22mm copper pipe into the iron pipe intake. I didn't seal around this copper pipe at this point.
I took a few breaths and said to hell with it and welded the firebox to the protruding burn tube. However I only tack welded it into position in all four corners, merely to fix it in place temporarily (maybe). Outside we went again and another test burn began. This time all the parts were in place although not completed.
Obviously the visible gap all around the burn tube to firebox joint would draw some air into the system. I have a piece of oven door glass that I merely placed over the top of the firebox. It's oversize by a distance but adequately covers the opening. I have got a good flat finish to the firebox edges (no wobble of the glass plate) but inevitably it wouldn't be a total seal against any air draw. The small gap all around the copper pipe would draw some air (or leak out?) into the cylinder.
However - it performed wonderfully. It got steaming hot (cylinder and flue - and firebox) and burnt very little timber in proportion, over about 3 hours. I think I used a handful of sticks and 3 cubes of wood - the type used in pallet construction - the smaller ones about 3" cubed. The flue was clean through 90% of the burn - just starting and the odd puff of something. Obviously I played around with the air flaps and could see definite effects.
So - I came back indoors with it and welded up that firebox to the burn tube fully, inside and out. The burn tube is of course 10mm section thickness so I was able to get some very nice fillet welds all around that joint. So, the whole thing is now as one. It's heavy - but it's not to heavy to carry if you put your mind to it. I've welded some carrying handles to it in strategic places. Two people can easily carry it. Obviously the flue pipe detaches, as does the heavy top plate and the firebox lid. I fitted a glass strip into the firebox lid, which is only 10 x 1.5" (out of a coal-fired room-heater door) but sufficient to get a view of what's occurring in the firebox. I fitted a longer piece of copper pipe and shaped it to route forwards and hug the firebox. Ive fitted a gate valve to the end as a secondary air control.
I did worry about the firebox being too large - too HIGH, in relation to the height of the riser tube. There isn't many inches in difference. I think the cylinder was about 17" effective internal height onto which I then raised it with a 40mm ring. I then sacrificed this gain with the top plate deflector fitted underneath the plate. There is about 2" gap (or slightly less) between the top of the riser and the deflector. So the riser may only be 15" effective height. I think it's closer to 16" somehow. I'll check my numerous drawings next opportunity. However the height of the firebox is 12" at the rear - it's not a lot of difference.
I did wonder whether I might have to shave off 2 inches or so of height off the firebox to widen this difference, hence why I didn't make the firebox lid until the end or fully seal up (weld) the joints initially. However, it's turned out fine it seems, at least from the point of view of drafting and pulling through the system. Having said that I've not yet had the firebox full or loaded near to the top.
It is economical with fuel. It does burn clean. For a crude and prototype version, I'm very impressed.
The main observation so far is the cylinder (heat exchanger) is too small. I knew it was small of course because this was partly the idea but I reckon I've overdone it and gone too small. What is happening is that the system cannot liberate enough of the heat generated through such a small exchanger before that heat is pushed through the system - hence a very hot flue pipe. This doesn't bother too much me with this model - I've fitted it at work and the flue pipe is central and within the building for about 14 feet until it exits the roof. I'm getting some heat off it into the air space.
I could burn less fuel, slow the air down to minimal and could have had a smaller firebox and just burn sticks. I could fin the heat exchanger or fit vent pipes and so on. With this model, I'm not going to bother with any more mods or additions.
So the next plan is to make a much better example. More aesthetic as well as even more efficient. Better air routing. All new steel and parts. Larger and more interesting shapes - hexagon or octagon heat exchangers for eg.
I'm drawn now to cast riser pipes of course and will have some questions there soon. I've been listening to videos about water-glass and perlite mixtures alone which sounds interesting. Is it feasible?
However there may well be a debate about my use of a plain steel burn tube and riser. It's not stainless and obviously not refractory material. It's 10mm wall thickness though and I've seen more than a few rocket stoves on You tube now that utilise steel burn tubes of lesser thickness. I also know what it takes to burn through 10mm plate. In addition, I have a self-made coal forge wherein the firepot is fabricated from 10mm plate. Recently, I had a periodic clean-out of the forge bed and pot (ash and tiny clinker and general inspection) and the sides and base of the pot are flawless and as new. Even the drilled pipe cap which is the tuyere is undamaged. These parts have experienced several hundred hours of a forge fire with air blast. It takes a lot to burn through 10mm plate, without an oxy torch or plasma.
The main advantage I gained in using steel throughout all the jointed parts was the ability to weld them and strongly. It made the build more convenient. I am a welder by trade and so by inclination.
Anyway there you go for now. I will try to put some pics up next, if I can remember how.
Sorry for the immensely long post.