Eventual first build

Hiya chaps

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.

TESTING.

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.

THOUGHTS

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.

Paul.

On a related point to stove building in the UK right now ... have you checked out the crash in steel prices over just this last couple of weeks?

Whilst this seems certain not to be good news long-term, now is the time to buy steel if you can. Prices this week seem to be about 60% of what they were just a month or two ago.

I forgot to include this in the post above.

Hopefully some pics.

well at least you proved that that small one could and will work so mine was just to do with the chimney

nice build there paul and ive still got to read all the info that you wrote up on your system

i think that youve used 10mm steel for the vortex riser just watch it over time for destroying itself

also consider putting in heat bricks at the sides it does make a difference

you may find as i found out is the sides warp out with the heat after a while i used 2.5mm or 3mm stainless steel and that did warp out with the heat with the firebox

have you tried filling it up to top and running it fully as of yet ?

Thanks Gadily

I will keep my eye on the burn tube and riser. I can easily see into the entrance to the burn tube and put a hand right in there if I need to, and of course the top plate of the exchanger is removable.

I think what I can and will do is surround the firebox in combinations of brick and/or firewool. I have put a pad of wool underneath the cylinder, in the cavity formed by the ring base, and also under the firebox which has about one inch clearance off the ground and is on stubby little feet - 16mm round bar.

I expect the inner stainless plates to warp anyway although I have fitted them into guides either side so they can't go far. Stainless steel is a poor conductor and moves all over the place with heat. It's a welder's nightmare sometimes.

I have had a good burn in it today and had the firebox fuller than previously - some old joists about 5" x 3", admittedly full of nails, but old timber. Plus a few very dry boughs and some bits of dry lumber - panelling and such. After a few hours it seems ok. No exterior distortion noted, a definite effect with all 3 air controls - front starting air, side primary air (x 2) and secondary air. The effects of opening or closing these controls is obvious and more or less immediate. The wood consumption has been minor compared to a standard wood-burner for the same heat output. The glass self cleans. There has been no smoke or fume whatsoever back-drafting through the system, not even when the lid is lifted to add more fuel. The flue burns clean after a short while. From the flue-box outlet on the stove to the exit through the roof is a little less than 14 feet, and I have about 2 feet protruding through the roof, and stainless cowl on top of that. The flue pipe is hot - too hot to touch, but as noted above the exchanger is quite small and isn't able to liberate all its heat input before the air is pushed through the system. I'm getting 14 feet length of hot pipe also heating into the air space.

I've got some dry logs here at home and am going to push it to the limit of its capacity tomorrow and note what occurs.

I think I will make a simple grate tomorrow, a removable affair, just to get the fuel up off the solid base a little. I may also put a slight inward bend in those stainless inner side plates to narrow the space slightly at the bottom of the firebox, so as to direct the fuel into the centre as it falls to the bottom. I mill make the grate slightly narrower than full width to accommodate this.

I do want to make a larger one now, avoiding a couple of mistakes and adding some tweaks, such as better routed air and air venting pipes and more aesthetics and less (or no) salvaged scrap metal - well, maybe just a little.

One thing I have noticed is that the 'letter-box' shaped slots in the side walls (primary air) are more than adequately sized for what the firebox needs. They are cut-out slots @ 8" x 2" and still draft the fire when the flaps are virtually closed, leaving only a slight gap because of an non-gasketed opening. Once the flaps are open say 20 degrees the firebox drafts quite fiercely. Open all the way (90 degrees or more) and there is much the same draft as at 20 degrees opening. So the slots could have been cut smaller (say only an inch wide) for this size of firebox and would have been sufficient it would seem. I have already learned that once the fire is going well and all is up to a fierce temperature (which is when I open the secondary gate valve also), the side flaps for primary air can be more or less closed up, leaving just the unavoidable tiny gap around the edges a sufficient draft. If the fire struggles at all, I can just open these 2 side flaps a fraction and it kick-starts the blaze noticeably. As an ultimate control for stubborn starting or burning, I can open the front flap which has a sufficient draft through it to extinguish a match or cigarette lighter held close. It makes the firebox burn fiercely, although quickly if left open.

All in all, the control and the heat output is wonderful, even for this constricted size of exchanger.

Bigger one coming soon.

Paul nice build and thanks for the detailed description.

i choose spinners for my sides id seen them before an i dod like them the added advantage to them better control of air in your system

I found just about 2" or 2 1/2" was adequate enough to allow the heated air to move around system much better and can be really slowed down once at red hot stage then it really gases off inside of the firebox

Thanks for the comment Caotropheus.

Ok, so I've had the firebox filled up with seasoned logs and more dry joist timber and had it burning very fiercely.  There have been no problems.  There is no noticeable distortion of the firebox and all the flaps and the door still hinge nicely.  The exchanger top-plate is still nice and flat, which is no doubt due to the sacrificial under-plate that is fitted.  The exchanger body becomes very hot indeed (although I haven't had it going dull red or anything like that).

The primary purpose of making it smaller, for the sake of more compact and aesthetic looks in a smaller space (than a workshop) is all well and good in one way but as noted, the exchanger (cylinder) is too small to liberate it's heat before it goes through the system and into the final flue.  

I'm now very much aware of this and where I was leaning towards smaller sizes for more compactness, I am now leaning towards much larger for output and efficiency.  I suppose this is obvious.  I must have had a brain-storm when first conceiving the design size.  Plus, I've also fitted this model at work, where aesthetics don't matter as much.  I'm not going to make one for home from a gas cylinder and off-cuts, but a much nicer one from new parts - sometime soon.  For a work model I can use a much larger cylinder (47kg propane tank) and it doesn't have to have maximum aesthetics.

I might yet partially destroy this smaller prototype model, as in cut off the firebox as a whole - it could be used on another model, even the bigger one I'm planning.  I could also use the top-plate elsewhere too.  This would leave the cylinder alone with welded-in riser pipe and its insulated shroud.  If I lengthen the burn tube a few inches (pieces of 10mm plate or flat) I could preserve this model as an outdoor cooking stove, feeding fuel just into the burn tube.  I will have to remove the flue box and block up the cut-out, leaving the body of the cylinder intact.  It can still have the secondary air tube in position, though on a shorter stub of copper pipe.

So, I will be chopping and changing already, at an early stage but this is what happens until you iron out all the pros and cons at the beginning.  I've proved the design myself now, not that I didn't already believe the excellent videos and commentary provided by people here and elsewhere on YT.

Gadily - I agree that spinners are a good air-control and convenient in how fine they can be opened, one thread at a time.  It's now obvious that the primary air control slots were more than adequate and smaller opening could have been used.  Having said that, I now aim to route primary air beneath the exchanger, as Ppotty has done, which will result in one common intake.  I may use a larger spinner plate on that intake.

More to follow.

This is the deflector plate I fitted under the exchanger top-plate. I overlooked uploading this picture in the original post.

all very nice and nice explanation as well on your build

you do realise that you can add a heat exchanger at back of that system just by creating another up and down before exit to the chimney

im just going for a more compact system however im going to be pushing at the high heat like ppottys new build and add heat exchangers at the side

im going for a 14" width and 21 1/2" length on system, height i havent decided on that as of yet

Just a quick post here about the performance and observations about this stove.

I've been running it for over a month now, almost every working day - which includes weekends often too.

The stove is seriously hot, despite a lot of heat entering the flue pipe.  The wood consumption is satisfyingly light.  There has been no blow-back or any other fume problems.  The only real downside to it is that the exchanger is just too small to liberate all its heat.

I won't destroy this stove and at some future point may well add fins, hot-air tubes, or even increase the height of the cylinder (exchanger) and riser pipe.

I've been inspecting the J tube and riser pipe regularly for degradation.  So far there is no sign of anything amiss.  The interior of these rocket tube parts is perfect, even scoured clean you may say.  There is no pitting, burn-out, de-lamination or anything else of concern.

In this last week, I've become a little better at running the stove, having had time to learn its quirks.  I've now managed to find the sweet-spot as regards air intakes and had some serious firebox temperatures (un-insulated), causing what I suspect is initial gasification and thus a rise in exchanger heat output.

I'm not sure exactly of the performance of the secondary air, but am opening up the valve nonetheless, once the system seems up to a fierce temperature.  If there is any problem with this, it will be down to a near horizontal secondary air pipe, rather than a gentle up slope.  Being a first build with no plans, I overlooked the exact positioning  of all the parts and had a clash with the path of the secondary air in relation to the primary air intake slots.  There is no down slope on any part of the pipe, but a pronounced up slope would have suited me better.  Next time...

Thanks Gadily about suggesting a 2nd heat-exchanger.  I could do that, but there isn't room where it is sited.  Also, the work and materials involved in making such a thing is better employed elsewhere at this point in time.  It's worth some thought though, so thanks again.


So I'm on with another one, this time using a 47kg propane cylinder as the heat-exchanger.  This will be the last one (it's only the 2nd ) using a gas cylinder.  I aim to swap it, like for like (but it will be bigger) with the existing one, using exactly the same location and flue pipe - at work.  It will become my true working prototype, and available for viewing to any interested parties hereabouts.

Again, I am basing it on Ppotty's design and general sizes (firebox, piping, etc) but with a couple of extra tweaks that he suggested - that I thought about anyway.

I'm going to run the primary air feed pipes INSIDE the heat exchanger.  They will exit the exchanger front and rear and will be fully welded up all round at those points.  The rear will be the intakes and have controls - probably spinner plates.  I will probably route the two (left and right) into one larger pipe and thus have the one control.  

(One negative thought about this is possible burn-out of these tubes over time.  I'm looking to use 50 x 50 x 3mm steel box-section for each tube.  It's not very thick, to withstand the heat inside the exchanger, over time.  It will be a nightmare to replace them once the whole stove is built up and welded firmly.  Stainless steel box-section tubes might be one way to overcome this, if only in that length inside the exchanger.)

The exits of the tubes on the front of the exchanger cylinder then need routing to input into the firebox.  I'm tempted to run these INSIDE the firebox, discharging at the top of the box.  This will then do away with the need for false side-chambers and the traditional routing, up and over.  However, one thought about this is degradation of these pipes also, over time.  Again, stainless box-section throughout might overcome this.  Alternatively, replaceable firebox piping is an option.  They could slide over (or flange bolt to) short intake spigots at the point of entry into the firebox.

However I do like the side openings (up and over) and routing the piping this way to cut-out slots in the sides of the firebox, as per Ppotty's design.  I suspect these is a nice increase of air-speed as the air rushes over the inner side-plates, and as the air can only go initially upwards upon entry to the firebox, this is also an advantage.  I'm dithering at the moment as to which route to take.

Any thoughts here?

One thing I've thought about is the provision of slopes, in the right direction, for the various air tubes.  The Primary Air will input at the rear of the cylinder (I will side-branch to one side at the end, for convenience of access to the control.)  The air goes into the pipe/pipes at the rear, the pipes enter the heat-exchanger, then exit the exchanger and then enter (or route closely - welded contact even) the firebox.  At all times I want this piping to be gently rising, rear to front - exchanger to firebox.  

Thus it will enter the heat exchanger at the lowest point possible.  I aim to incorporate the intake as part of the box that is the unit providing the flue outlet spigot (it will be separated of course, fume-wise), so scavenging some heat by conduction.  The pipe(s) then has to enter the exchanger, rising slightly through the exchanger, missing the secondary air pipe (one side), exit the exchanger, keep gently rising a few more inches of length and enter the firebox - either direct entry or side-slot feeding.  

If I go for side-feeding, it's likely the slots won't be at the base of the firebox sides.  The gentle rise throughout will prevent this.  It may be the slots in the firebox sides will be up to half-way up - maybe a little less.  This isn't necessarily a problem.  The air will already have been heated fiercely and so the loss of a few inches of passage inside the firebox shouldn't be an issue.  Alternatively if I do enter the primary air pipes to run inside the firebox, and around the top, I've already gained the height necessary to encourage a nice up-slope angle.

Conversely, the secondary air pipe slopes in the opposite direction - front to rear upwards - firebox to exchanger.  I aim to use copper pipe for this again.  In this case it may well be possible to run this pipe through the firebox false side-chamber (on one side), and crucially, it will be underneath its twin, the primary air input pipe, in the free space mentioned above.  It can then rise steadily through the depth of the firebox, out the back (all insulated her btw) and into the front (or side) of the heat-exchanger cylinder and then into the rocket tube.

It will all be a tricky manoeuvre of piping and angles and accurate cut-outs and fit-up and some nice welding to finish.

Once I get this one all sorted out and the fabrication logistics firmed up, I'm looking to make more and thereon to fabricate my own heat exchanger units, probably using 3mm plate steel.  Various shapes and styles could be made, mainly for aesthetic appearance and can be adorned or ornamented with extras for a pleasing effect.  All pictures to follow, in time.

Thanks for reading - a not very quick post after all.

paul take a look at this right at the start



and all it would involve is highering your chimney exit

Nice build! Thanks for the detailed description and photos.

Thanks CV104.

I'm on with a bigger one now. Pictures and explanation in due course.