New oil burning furnace build

I found this gravel filter tank for free in my travels today: I think it'll make a decent pressure feed oil tank for my furnace. The thing looks new to my eyes.

 

Today I got back to work after a long break and opened up a hole for the tuyere pipe, summer is here with a vengeance and it's 33 degrees C in the shade. First I had to weld some steel tabs to the steel base around the furnace liner to keep it in place when tipped on it's side, the top has the steel ring but there's nothing on the base to stop it from moving sideways. I transferred the floor measurement to the outside of the liner and marked out an oval where the tuyere should be. With the furnace chocked in position to stop it rolling, I used a 6mm masonry drill on the liner and drilled holes at the right angle and made as many holes as I could while trying to avoid punching refractory chunks off the inside wall as much as possible.

Once the hole was opened up a bit, the stainless tube was fitted so a felt pen could transfer the liner curve to the stainless tuyere pipe so it could be cut out and deburred. The stainless tube is in place now but there'll need to be a strategy to seal up the 10mm gaps and keep the grout in place: I think I'll mount a sheet metal ring around the tuyere on the outside and fill the area with refractory. I have some clay I use for cracks on the inside that hardens up nicely after one firing session, it could fill the gaps on the inside.

 

The tuyere pipe is now set in a block of refractory. I reused the bore PVC form and butted the 10cm bore stainless tuyere up against it. I set up a rectangular sheet metal box to contain the refractory and filled with the same refractory as the liner is cast from. It was mixed extra wet to allow for the dry liner to soak up some of the water and then vibrated by hand until the refractory had made it trhough to the inside and was visible down the tuyere pipe....hopefully it's flowed into all the gaps.

 

I was able to cast the furnace lid today: it has a slight cone on the inside and if I hadn't run out of refractory the top would have been conical to give a constant thickness. The theory behind this idea is that the steel band round the outside constrains the refractory which will inevitably crack, but being cast in a cone it will wedge itself tight. The steel band is two coils of 1mm galv steel tack welded together ever couple of inches and with several spots welded to give bumps to key in the refractory so there are no steel tabs on the bottom like my other furnace. As I hand vibrated the refractory, there was a fair bit of water on the surface so I added handfuls of dry refractory mix to soak it all up and it vibrated in fine, a little bit more effort but worth it.

I cast the lid on top of the top ring of the furnace with a sheet of polythene as the release agent, the steel band stays in place and the PVC tube in the middle gets heated until soft to allow easy removal.
So far the entire furnace has used 6 and 1/2 twenty kilogram bags of refractory for 130Kg total.

 

It's really coming together nicely Mark. What sort of mechanism are you planning for the lid?

 

I think I'll opt for the basic vertical shaft hinge so you stick a handle in a couple of holes and lift the lid so it can pivot and swing sideways off the furnace. The parallelogram hinge can be finicky to set up and get a good seal and uses a lot of steel.

 

The lid has the inner cone and pvc exhaust pipe removed and given a wipe down before assembling. The exhaust refractory is only 3cm thick at the centre hole which isn't much, I'll probably add wool on top as well to boost heat retention. I'm going to opt for a more conventional lid mechanism by having a lifting cam operated by a lever that doubles as the handle to swing the lid sideways.

The hot air gun is set on low giving an air temperature of 70 degrees C internally.
Edit: After an hour or so the air temperature internally has reached 109 degrees C.
Edit 2: After two hours the internal air temp is 125 degrees C and the floor is 85 degrees C. The outside bore is quite hot to touch maybe 60 degrees C.

 

The heat gun for an initial 'warm-up' is brilliant, I may borrow that idea for my project.

 

That's a cheapo hot air gun: I had to support the metal nozzle with a small steel block hidden inside the tuyere to take the weight, otherwise it will melt into the plastic housing. Normally you can't hold it for very long as the case gets too hot to hold....sort of limiting the duty cycle in a way. So it's one thing to be aware of when running a hot air gun unattended for long periods.

 

So the lid hinge mechanism is under construction: it's a pipe in pipe with a fairly sloppy fit that I hope allows the lid to seal against the bore. If it was a super stiff precise hinge then alignment could be an issue. With the rear gap closest to the hinge being 35mm open, the front is 20mm open, this is all due to the inner vertical pipe being 5mm less in diameter than the outer pipe bore. I'm starting on the lid cams which should give 60mm or so height lift when operated and also lock in the over center position. I'll add primitive steel rollers to the cam lobes.

 

60mm seems like a lot of travel and friction with those pipes being in contact. The lid just needs enough space to clear with some extra for wear-in droop. I’m envisioning putting a slightly beveled ring on top of the flange of the lower pipe with a complementary ring with matching interior bevel on the upper pipe so they mate, such that the pipes are held apart while raising and lowering. That way the friction is limited to the contact between the ring surfaces and your alignment is assured. It’s frustrating when you lower the lid and it doesn’t lower.
Just my 2 cents.

Pete

 

My latest does about 25mm of lift but it takes some 250 degrees to go from closed to locked open. It would be nicer if it were less.

 

It's all subject to change, right now I'm going for the maximum possible lift all other things equal. I'd also been warned to give clearance in case of crud hanging off the inside of the lid and interfering with the opening as a friend of mine has had that experience while melting bronze ingots through the lid hole.

I'll be lucky to get 90 degrees of handle movement and part of that will have to be over centre to give locking in the open position.

 

The lid lever handle and cam is finished, I have a couple of 430 stainless steel rollers with 8mm stainless bolts to hold them. The bolts have un-threaded shoulders so they can be done up tight and still allow the rollers to spin. I used 5mm mild steel sheet welded to the galvanized steel pipe to form a fork that pivots off a bolt though the vertical pipe part of the lid. There's 20mm lift at the back and 6mm lift at the front so way less than I predicted and it does go "Over centre" to lock the lid in the upright position when the handle is fully down.

 

Today I made the sheet metal skin from 0.5mm 304 stainless sheet, I had to cut a 120cm wide sheet into two 60cm pieces to have enough to go round the circumference. The sheets go to within 3cm of the top of the furnace so I may extend it to protect the edge of the refractory ring. The furnace is resting on a couple of concrete blocks as the sheet skin goes past the round base by 10mm or so, I'll carefully fold the sheet over and shrink it a bit with a hammer so it folds around the round mild steel base. I did that on the last furnace without too much trouble but this time I'll be working upside down. The skin is in two sheets of metal with the larger having a hole cut for the tuyere pipe and using a full length of it 170cm or so. The second sheet is just 50cm long and will make a great inspection hatch.

I'll run some 10mm diameter 304 tubes through the walls for thermocouples in a few places about halfway up the bore. I'll run a few loops of thin wire round the diameter to help retain the mineral wool insulation and fully insulate the furnace before putting the sheet skin back on.



There's a gap in places that will have to be filled with refractory cement.

 

I was able to peen the stainless sheet over the edge of the base, I had to heat a lot of it red hot to get it to lay flat. As far as panel beating quality, I'd describe it as "Rough but honest". I wire brushed the loose rust off the base and gave it a coat of silver zinc galvanize paint and commenced work on the fibre insulation. I'm using some Morgan Pyro Blocs which are 1 foot cubes of silica fibres with some added silicone oil to keep the fibres attached. By cutting tapered pie slice wedges I was able to insulate the entire base with one and a half blocks.


Stainless furnace skin:


Zinc paint to hide the stick welds:



Morgan Pyro Bloc fibre insulation:



Base insulation in place:



Edit: The furnace has the fibre insulation stuffed in tightly and the skin refitted, in addition I added a 12.5mm stainless tube to the bore for thermocouples to be fitted. There's a lot of gaps to be filled with mullite clay but the furnace is more or less complete. Just have to overhaul the fuel pump and test all that stuff again.

 

I found my grey clay I use for patching the furnace, it's some mystery clay that I dug from a roadside cutting because it was a brilliant white colour when dry. It holds up well to furnace temperatures and vitrifies to a glossy cream colour that is pretty durable. The top ring had about a 6mm gap as I made sure the weight rests on the steel frame and not the top of the refractory cylinder. I did about four-five laps with the clay using a steel strip to pack it tight into the gap. The thermocouple port was grouted, it's a bit lower than I anticipated, about plinth height and I checked the tuyere clay grouting after it's been heated twice and it's quite hard with no signs of cracks or shrink. The clay is relatively dry compared to potter's clay so I just check it after it's dried a bit for shrinkage just in case. The heat gun has identified a few hot spots in the skin where the fibre refractory may have gaps so I'll have to take note of the spots and remove the skin and pack a bit more fibre into any gaps. For now the furnace is more or less complete. I'll pinch my plinth from the other furnace for the first few runs.

 

Fantastic furnace build Mark. Thanks for sharing that.
Mark

 

It remains to be seen if it'll work as advertised, testing is delayed after a 12m tall tree fell over a week ago and damaged the rear of the shed. I'd almost moved the furnace to a spot where it would have been hit by the tree trunk. It's covered in thorny vines that makes things difficult to remove. You can see it snapped off in the storm about 1/3rd of the way up the trunk. This photo is taken after lots of chainsawing of branches and 12 wheelbarrow loads of branches, trunk, vines etc. have been removed. Even fallen over, the tree was as high as the shed before trimming and you can see where it hit and bent three purlins and roofing iron.

 

That is annoying when nature gets in the way of hobbies like that