Hello rocket people.
So I've managed to get a 2nd stove built over this summer. It is an improvement in design and increase in size from my first attempt. I have fitted it at work in the same position as the initial build, swapping it to fit the same final flue. It is modelled even more closely on Ppottys large gas cylinder design, being more or less the same general size.
I have used some more of the same riser pipe as I did in my first build - that is, a Schedule 80 steel pipe @ 5" outside diameter (4.25" bore). I have however internally sleeved this pipe with stainless steel - just sheet about 1mm thick. The riser has an outer shroud of stainless steel ducting pipe at 8" diameter. The space created by this shroud is insulated with ceramic wool (around the base and up and over the burn tube) and then filled up with Perlite.
The heat exchanger body is an old 47kg propane cylinder. The top plate is 10mm steel plate. On the underside is a steel disc welded onto little legs (an inch) to deflect the main up-draught heat off the underside of the top plate - to avoid distortion. I have cut out an inspection hole on one side of the exchanger body and covered this with a removable plate, rope-sealed. This inspection plate is on the same side as the secondary air pipe, allowing me to inspect this and attend if anything goes amiss. The flue box is at the back and has a generous (rather deep) spigot pipe that accepts the final flue. The gap between the top of the riser pipe and the underside of the heat-deflector plate is 2 inches. The heat exchanger has a hollow base that I have filled with ceramic wool to provide insulation beneath the exchanger body. The total height of the exchanger is exactly 40", plus a 10mm top-plate.
The horizontal burn tube is fabricated from 10mm steel plate and fully welded, inside and out, to the cut-out riser pipe. There is a 'kick' to one side again to help with a vortex flame as it enters the riser pipe. It is about 8" long and has internal dimensions of about 4" top to bottom x 6" wide, angling down to about 3.5" wide at the entrance to the riser.
The firebox is again 3mm tread plate, although the lid (door) is 6mm plate. The glass viewing window in the lid is one of those pieces that come with a typical (inexpensive) security light - the type that take up to 500W halogen bulbs and usually come with a motion sensor. These are a good resource just for that piece of glass, which is nice, thick stuff - about 5 or 6 mm thick. I wasn't sure if this glass would take the heat, but so far (just 3 burns up to now) it has held up fine. There is a front air flap, low on the front panel, rope-sealed when closed and provided with a screw clamp to tighten it shut when not required. This flap is used to kick-start the fire upon starting or will be useful if ever a difficult burn is experienced (which should be never), or when any re-fuelling is done and it is desired to get the fuel going quickly. The air supply when this flap is open is immediate and promotes rapid burning. The firebox has a removable grate set 2" high off the base. The front air flap is situated on the lower 2" of the front panel so the air intake at this point is just under the grate.
The firebox is a generous size, mainly in depth, so I can fit long pieces vertically. The outside dimensions are about 14" square x 20" deep at the rear x 18" deep at the front. Discounting the 2" of grate space, one can easily feed a 16" log into the box. The front to rear internal dimension remains about 14", while the side to side dimension is now about 9"-10", given the hollow air chambers to the sides.
I have routed the primary air via two square tubes (50x50x3mm SHS) that pass through
the heat exchanger body. There are spinner plate controls on the input end of these two tubes. The tubes enter the exchanger at the rear, pass through the body and exit and are then routed around the sides of the firebox. There are slots cut into the firebox side walls and the primary air tubes (now reduced to 40 x 40 x 3mm SHS, with one of their four sides now cut away) are welded directly over these slots. Because of a gentle up-slope to these tubes, rear to front, the side entry slots to the firebox are about one third the way up the box. There are side chambers formed from stainless plates inside the box and the air, once entering the slots, is forced to rise up these side chambers and out and across the top of the firebox - all the same as my last build and how Ppotty has designed his air delivery to the fire.
There is copper pipe secondary air delivery routed into the heel of the burn tube/riser pipe. However on this model I have managed to run the pipe inside the firebox, laying along the side wall, underneath the chamber for primary air on that side (the primary air chamber only begins about one third the way up as explained above). I have covered the copper pipe inside the firebox with a loose stainless steel channel section, folded out of sheet, just to protect it from crushing from any logs dropped in there.
The following areas are rope-sealed with new fire-rope: Heat exchanger top-plate; firebox lid; exchanger inspection plate; front air-flap; intake and exit points for two primary air tubes and a secondary air copper pipe.
Thoughts so far:
As per usual, these can be tricky to light if one does not follow the principle of starting a good stick fire inside the burn tube. Allowing a fire inside the firebox too early is fatal for preventing smoke production and leakage out of the firebox lid and even the primary air intakes, right at the back.
It is a little more difficult to get going than the smaller previous model. However there is more mass of steel and pipe to get hot before the burn begins properly.
I did fail to get things going adequately before allowing the fire to burn back into the firebox on the first burn - and even worse on the 2nd burn. Impatience really and inattention to detail. I had quite a good bit of smoke leaking into the workshop space until I could coax the fire into the burn tube again and get the stove heated properly. It wasn't a problem as it is a large shop and I can open a wide front door and have a large extractor fan on the rear wall and can thus replace a large volume of air quickly.
Yesterday, on a 3rd burn, I lit the stove properly (at the mouth and inside the burn tube) and was more patient. The riser is a thick steel pipe (10mm wall thickness) and takes time to get properly warm. The 10mm top-plate is stone cold (or steel cold) initially and it is a good indicator that the riser is warming adequately. When this top-plate is noticeably warm to the touch seems about the right time to allow the fire to burn back into the firebox.
What I actually did was stack crumpled paper and sticks to the front two-thirds of the firebox while creating an definite firebreak area between the burn tube intake and this main fuel pile. I allowed the burn-tube fire to crackle away for a few minutes (until the top-plate is good and warm) and then I lit the main fuel pile by pushing a long spill of newspaper into the front air-slot, under the grate and lighting it at the front. The draw through this front flap is very positive and the flame is sucked inside, along the length of the paper spill and ignites all the main pile of paper and fuel. At this point the system is up and running.
On the 3rd burn yesterday, following the above procedure, it was very successful. I had no smoke exiting the firebox lid (although I did improve the rope seal a little before lighting - I used slightly fatter rope) or the air tubes. The glass has sooted up quite a bit and so it was difficult to see, but this was caused with poor starting on burns 1 and 2. I addition I haven't had the firebox anywhere near full or the stove going full tilt as yet.
My 3rd burn yesterday used at most about half a 2-gallon bucket of wood, plus a few initial sticks for the burn-tube fire.. A good part of the main fuel was just dry sticks and small dry boughs needed as initial kindling. There were at most about 5 pieces of 3"x2" lumber, no more than about 6" long. This fuel caused the heat exchanger and top plate to become sizzling hot. The firebox too gives off a lot of heat. I can directly feel the heat from the stove from about 6 feet away. This small fuel pile was still burning 1 hour after ignition and had a fair degree of life in it still. At this point I added 2 more pieces of 3"x2" at about 12" long. This fuel and the remainder of the first small load was still burning 2 hours later.
I am totally made up with the stove. I think it will only get better as I learn to tweak it properly and find the sweet-spots. I am mildly apprehensive about the result of a full firebox and some decent dry tree logs. it could go nuclear.
I realise there is one more process to accomplish - that is to line the firebox interior with fireboard and to make a sheet metal sleeve for around the exterior, into which will be added outside firebox insulation - probably ceramic wool.
Isn't fireboard expensive to buy? I aim to make some of my own in the next week or so. I can't decide whether to make a fondue cement and Perlite mix, or one of sodium silicate (water glass) and sand. Any thoughts appreciated.
I hope these pictures upload ok. In fact I will put all the pics in a 2nd post.
Thanks for reading.