Oil fired furnace update: 16 months maintenance

In September 2019 I added some mineral wool insulation round my dense refractory lined beer keg furnace as it was hitting 360 deg C/680 deg F on the outside skin and losing significant heat. The fibre was wrapped around the outside of the keg and while it was an afterthought mod, it turned out to be pretty good in that the dense refractory can crack while the stainless steel beer keg mechanically holds it together and acts as a confinement for any oil vapours and smoke that can escape through cracks that inevitably form in the dense castable refractory.

Over time it became apparent that a few pinholes in the keg caused by my crappy stick welding can let smoke into the fibre insulation where it condenses and soaks the wool. Yesterday after a fairly successful test run that melted half a brake rotor, the furnace leaked enough fuel to catch fire on the outside. So today, I finally bit the bullet and stripped the outer shell off and took a look for vapour leaks. I used some Dow Corning 702 silicone that happily stands up to molten aluminium temps and smeared a thick layer over my welds with their holes. I'm hoping that will withstand the heat and keep the insulation from soaking up further oil vapour.


The fuel soaked fibre insulation, it's noticeably heavier.



Some silicone over the lid hinge brackets, about three holes in the stainless keg.



Blower cart and fuel pump, the pink pulley has been resleeved with steel and two grub screws to improve reliability after it fell off once in use due to a sloppy fit.

 

How does the tapered tip work? Have thought about trying it myself.

 

It's an adaptation to using a 4" bore tuyere which gave slower airflow, the flame only went halfway round the chamber and did a right angle turn upwards. The crucible was unevenly heated as a result. With the air nozzle the bore went from 104mm /4" down to 65mm/2.6" bore and the brass fuel nozzle restricted the size even more. The blower is relatively high pressure unit with a narrow chord/width and larger diameter so it can cope with the air cone.

The sheet stainless cone also gives some shielding of the brass nozzle from the flames swirling round the chamber.

 

Insulating on the outside of the shell (your beer keg) is generally not a good idea. It is better to insulate it on the inside. But then you should probably rebuild your furnace.

To prevent this, is getting a larger shell (another larger beer keg or a barrel) and line the outer shell with the fiber which can be standard insulation rock wool which you already use, but then completely and put the existing beer keg inside the outer shell. Obviously, you have to make the entry point for the burner on the same place in the outer shell and be sure that no flames leak into the layer between the two shells.

That is what I did when I put my old Mizzou concrete lined furnace (which lost too much heat as Mizzou is a poor insulator) inside a Kaowool lined barrel and that works flawlessly.

 

While having a metal skin between the lightweight fibre insulation and the dense calcium aluminate based concrete was an afterthought it has a surprising advantage of tightly retaining the concrete as it develops cracks from thermal cycling and it also provide a physical barrier to oil vapours (or it would if I could do airtight stick welds). Performance wise it, seems to run fine with 15Kg of iron melted in 30 minutes from a cold start with an A25 crucible and in 20 minutes when hot, so I see no need to rebuild it at this stage.

 

Ya, Mark that’s not bad performance for anybody! Are you at pouring temp in 30 minutes? That’s very fast. What external temp do you hit with the added external wool?

I tried to look up 702 Silicone and could not find it. Is that number correct.

Denis

 

Correction 2: I went and reread my account of the furnace run here: https://forums.thehomefoundry.org/index.php?threads/oil-furnace-swirl-experiments.855/

I had 9 Kgs of iron in a furnace that had been running for 12 minutes already: so 32 minutes to melt 9Kgs in an already preheated furnace. Apologies for the faulty recall, the second run was 15Kgs in 42 minutes and 62minutes from cold to poured and shut down.


Correction 1: it's Dowsil 732 silicone RTV acetic cure, I found out about it at the foundry to stick the sand mold halves together and attach the pouring cups. It outperforms all other brands so there must be fillers in the other brands or something similar. As far as the furnace speed goes, the first time it happened, I was running around getting sand moulds assembled and other prep work done and took a look at the half hour mark. When I poked the solid crust on top it was that roiling yellow white iron underneath with that weird optical effect where you're not sure if the iron is white or black or silver (if that makes sense). I was so taken by surprise I had to spend another 15 minutes to put on my work trousers, long sleeved shirt, safety boots, spats, leather apron etc. so it was 45 minutes plus before I could pour. That was where I made a small iron bearing spindle but didn't place the core right.

 

On my ceramic fiber lined furnace it can develop a crack and lets unburnt oil vapour escape between the steel shell and ceramic fiber lining. I made sure that the steel shell was air tight when I welded it so the only place the oil vapour can escape is in between the furnace and lid. To stop that happening I use silicone gutter sealant. If it is not done properly hot air can escape and cooks the sealant (turns into a white powder).

 

So, Ironsides, it sounds like I should try the gutter sealant as a gasket between the body and lid of my nearly complete heat treat oven. The two will butt together flat to flat of 1x1x1/8 angle iron. I’ve been dithering as to what to use. Easy enough to try. The oven will be electric and max temp of 1750 on occasion, 1150 more commonly

Denis

 

Any favored sources of supply? I had been planning and may still begin with a simple strip of 1" wool rigidized. Easy to scrape off if it does not work out. It sounds and looks like the paper might be nicer.

There is some 1/8 thick paper in a 300" roll on Amazon at a reasonable price.

Denis

 

Kelly, thanks for the info about the ceramic paper. I was previously unaware of this option for use as a seal. Looks like good stuff. I will do a trial run or two with simple wool as a gasket. Provided that shows no need for modification of the layout of the furnace, I will get some paper to use as a gasket. I expect to run some electrons throught the Kanthal today---lots of them!

Denis

 

Do you have any pictures of your setup?