New oil burning furnace build

After my existing furnace went underwater during the December 2023 floods, I thought it was time to start work on a truly iron capable furnace. The existing furnace reaches over 1400 degrees C only under special circumstances: with a faster burning fuel or with a smaller crucible that increases the combustion volume.

Both methods do work but for normal fuels like sump oil, diesel and Jet A1, the furnace struggles to get hot enough with an A25 crucible in the furnace. The assumption is the furnace chamber volume is too small to burn fuel fast enough to overcome the heat losses via the chamber walls. Geometry wise the shape that encloses the most volume with the least surface area is a sphere, followed by a cylinder which is what I'll use.

Plugging some numbers into a spreadsheet and using the existing furnace as a starting point, you can see that increasing the bore diameter allows the volume to rapidly increase with modest increases to the surface area. So with an A25 crucible, increasing the bore lets the chamber volume that supports combustion to outpace the surface area (heat loss).

I'm hoping that the larger volume increases the efficiency of harvesting the heat by allowing the burning gasses to "dwell" for longer in the chamber before exiting: the bottom right figure shows 3.37 times the existing volume so the combustion heat should spend longer in the furnace as a result. The surface area increases by 85 percent so heat losses will increase too but not as much as the hoped for gains.

 

I'll cast an inverted "Top hat" liner with the dense 1600 deg C refractory concrete I've bought and make it around 50mm/2" thick. I'm debating whether to cast it into a stainless steel container like a modified keg as that will contain refractory as it inevitably cracks and also prevent oil vapour leaking into the outer layer of fibre refractory which I'll have on the outside of the dense refractory. I've also noticed the fuel spray cools the refractory where it hits and vaporizes as the refractory is black and not glowing yellow so I'm thinking of a a 150mm/6" bore refractory lined tuyere for a short distance of say 20cm/8" or so. That way the fuel spray can be arranged to hit the mouth of the tuyere and vaporize there rather than in the chamber.

 

alot i want to say, but there are fellas here better edumacted, that can probably articulate it better , but just basically, some of your numbers are very suspect Diameter of an A25( Now I know modern Crucibles are all over the place mostly because of the metric system and an A25 and a 25 are different animals so maybe that's the issue),

Also A Keg will not be big enough shell for an A-25 furnace at 16.125 inch OD best you could get would be about 1.25 inch of refractory with nothing behind it for your 35 Cm bore , a 30 gallon metal drum is a better option at 18.5 inch diameter, but step up to a 55 gallon and two full inched of Kaowool easily ???

ill leave the heat loss stuff to the College edumacated fellas, but just from experience it is a larger issue then some think


V/r HT1

 

Hi Henry, I went and measured a Morgan AHM 25 I have in the shed: it's basically a silicon carbide with extra glaze that makes it more flux resistant or something. Regarding the keg size: you're right, I built an 11" bore in a keg and later grafted half a keg on top of that to get my existing furnace which is the first row of measurements in that chart and the 11" bore is too narrow for the 25 crucible with less than an inch gap each side.... I'd been thinking of cutting up two kegs and joining them to get something closer to a 55 gallon drum in diameter. I guess the point I was trying to prove to myself with the chart is that for modest increases in bore size I can fit a lot more fuel and air mix in the chamber at once.

I'm using the Carroll Shelby approach: "There ain't no substitute for cubic inches"

 

I've added a second section with "Freedom units" to match the metric section, the crucible is assumed to be a cylinder for volume calculations rather than a doubly truncated ellipsoid as I'm lazy. The main number to pay attention to is the last column on the right: going from an 11 bore to almost 20" increases the free chamber volume 3.37 times while the surface area only goes up 1.85 times.

 

here rock solid measurements
https://www.morganmms.com/media/0wqbllw2/salamander.pdf,
every knock off i have used was smaller to save material

V/r HT1

 

That agrees with what I have within 10mm for the AHM25. Here's a photo of a Salamander Super A25 from the workshop so 21.2 cm wide by 28.7 cm tall.

 

Hi Kelly, those are all good points: I plan to minimize the refractory from the current 65mm/2.6" down to say 1.5" and back it up with some silica wool based insulation. Time to melt iron was 70 minutes plus and most of that would be due to heating the refractory mass.

Experiments with the thermocouples showed the refractory disc didn't improve the maximum temperature and slightly slowed down heating. It does give a beautiful evenly dispersed flame up the furnace walls so maybe a washer shaped disc would give even flames up the crucible sides. The disc was present for the successful iron runs but I've chalked it up to the mystery fuel that softened the plastic buckets. I'll have enough chamber height for a refractory disc on top of the plinth in case I need that, it's only a few extra inches and extra height eliminates the annoying cyclone spin on top of the molten metal.

I like your drawing of the furnace sections, I'll probably cast a cylinder tube that fits a socketed base like you've drawn. The ring round the top would be recessed/socketed to fit the cylinder tube. I'll whip up a drawing and add it to this post.

 

I think dwell time helps with heat transfer and compensates for slow burning oil, certainly the extra volume can accommodate more fuel burnt without blowing it out the top unburnt. I'll keep the option of the firebox/toroidal/donut racetrack that can be added later as it improves the vaporization of fuel spray and greatly evens out the flame distribution. There's a good chance a larger chamber will make the flame distribution uneven and the toroidal path cures that: I'll just make a disc with a hole in the middle so the flame go up the crucible sides this time and not the furnace bore sides.

I think my existing furnace with less than an inch of gap between the bore and the crucible was an edge case of not enough combustion happening near the crucible to overcome losses. Turning up the fuel and airflow just blew more flames out the top and supercharging was not an option with leaks. With forced air cooling and even water cooling of equipment there comes a point where the flow speed is too high and things get actually worse due to boundary layers preventing conduction transfer.

Before the extra half a keg extension, the current furnace had only 2.5cm/1" clearance between the crucible and the lid which gave rise to the slag and molten aluminium tornadoes, at worst I aim to have a lid gap equal to the bore air gap and a generous lid opening.

So in short the existing furnace chamber is too narrow....adding extra length doesn't fix things and having a shape that gives the most volume for the least surface area should help with conduction/radiation losses.

 

I'm digging out the Morgan furnace brochure for rough dimensions to use....of course their smallest furnace weighs 500 kilos so I aim to be below 1/4 of that weight without trying hard. I think they give a rough idea of furnace sizes to aim for. There's no mention of the chamber sizes in their brochure unfortunately. Looking at the furnace drawing there's about 2/3rd insulation and 1/3rd air gap....so dimension A of 640mm - 197 for an A20 crucible gives 443mm or 17.44 inches.
Divide that by two for each side and you have 221mm, 2/3rd for insulation is 147mm and 74mm for air gap, That's close enough to 5.8" of insulation and 2.9" airgap around the mouth of an A20 crucible.

 

I just plan to have to refractory concrete as thin as practical: when it cracks I'd like it to hold together say 1.5" or 40mm thick and the rest 10cm or 4" will be insulating wool. I'd like a 4" air gap all round the crucible mouth. I can always neck it down but can't widen it later.

 

alot more solid Numbers and measurements on the Mifco B 301

https://mifco.com/foundry-furnaces/high-speed-melters/

Scroll down for the Hot links for exact measurements and diagrams


V/r HT1

 

I see the B series will melt some cast irons with gas.....gas seems to make life much easier with furnace design and would be the fuel of choice if it was cheap over here. The B301 would be what I'm aiming for with maybe an extra inch or two on the bore diameter for oil use. It's 13" bore would give a minimum 2.5" air gap at the top rim of an A20 crucible vs the estimated 2.9" of the Morgan oil burner.

 

Gas is easier to clean up a spill, oil is just messy . That B702 would have an air gap of 4" for a 40 size crucible down to 2.75" for a 70 crucible.


I'll cast the refractory bore cylinder first, I have some 12" diameter PVC pipe I can slit and expand to say 16" bore and fit some inner wood circles and massage with a heat gun to the expanded diameter. If I use a keg slit lengthwise, I can weld in some extra from another keg and bring it 19" diameter and that would cast the refractory to 1.5" wall thickness. The same wooden form can cast the socketed furnace base and the socketed ring for the top to cap off the fibre insulation.