Yet another keg furnace

I think a jet engine can get away with dumping a lot of fuel into a small space because it is compressing the air and fuel mixture.
I don't know much about jet engines, but that is my slant on it.

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Pulsejet motors are a good example of using tuned lengths of pipe to achieve compression without moving parts. This particular example shows a motor with gas flow in one direction only, with a reed valved intake and a tuned length exhaust pipe. What I find interesting is that the combustion chamber on the right is noticeably cooler than the tuned pipe. I think it's applicable to my furnace as the furnace has similar pulsations or shockwaves as evidenced by the loud noise it makes. Similar acoustic pulsations in a closed circuit pipe filled with helium makes one of the most efficient stirling cycle motors currently known. I can only guess at what's going on in my furnace, more tests are needed.

 

It is good to remember that the goal of a jet or ramjet engine is to minimize heating of the interior of the engine and to maximize thrust. So, yes, jet engines burn large volumes of fuel and consume prodigious amounts of air as a way of cooling the engine and providing thrust. The metal melters' goals are diametrically opposed to those goals. So, attempting to blast huge amounts of fuel through a small furnace along with corresponding amounts of air turns the furnace into a cool pre-combustion chamber. I'd also be cautious about following potential red herrings like acoustic phenomena associated with furnaces and try to focus on starting the furnace slowly and gradually ramping up low fuel and slow air inputs while observing the response of the furnace. After that is working well, then get fancy and push the boundaries. I would suggest, in other words, first play scales and easy songs, then break new ground with Prokofiev's Piano Concerto No.2.

Denis

 

As far as the sound aspect goes it couldn't be more than a few hundred Watts of sound energy, a drop in the ocean so to speak. Even so it's making sounds like a high speed didgeridoo and I'd like to stop it from happening. It occurred to me that the crucible is roughly halfway up the furnace chamber so there's a cavity below it and a cavity above it with the crucible as a restrictor in the middle, maybe that aids the effect.

 

I'm not sure you could call this a keg furnace anymore, it's 1.5 kegs with an insulating shell..."Keg Plus" maybe?.

When I first started to build this beer keg furnace, the compromise of fitting an A20-A30 crucible in and using dense castable refractory limited the thickness of the wall to about 63mm or 2.5" thickness. To get around this I had hoped to increase the burn rate to compensate for losses. At this point I can melt brass and bronze with no problems but iron is just out of reach, a bit like a MIG15 fighter trying to reach a B36 bomber at cruising altitude and stalling in sight of the target . There's a local Aussie market website called Gumtree where a guy in Perth had listed 1250 deg C high temperature mineral wool insulation for a good price. I bought a couple of boxes which each have three 1 cubic foot blocks of something similar to Kaowool but made by Morgan the foundry supply people. I'd already started to modify the beer keg with a couple of 3mm/ 0.125" thick stainless steel skirt rings to allow an extra 75mm/3" of insulation around the beer keg. Right now the dark straw colour of the stainless indicates the outside of the keg hits about 400 deg C / 750 deg F so heat energy is being lost, hopefully the stainless can withstand the insulated heat without too much damage.

Also I wanted to correct the record on the earlier high fuel consumption, it was actually 30 litres per hour at full throttle not 60 litres as earlier mentioned as I'd had TWO ten minute runs to use up 10 litres of fuel.

 

I was able to fit the insulation today: machined some mild steel spacers for the hinge assembly, a bit of welding, drilling and chassis bashing and only a small amount of claret was spilt from a scratch on my hand. The pyro-bloc has a layered structure and each block easily split into 6 x 50mm thick layers, the split layers can be further torn along one axis . I used a dust mask and gloves when working with the stuff but it was remarkably dust free, I understand it has something like silicone oil added to minimize loose fibres. The furnace used less than 1.5 blocks to fully insulate and by pure luck the steel bars I fitted earlier were more than enough to hold the material in place while the stainless shell was put back in place.

 

Its looking good.
Now I have to go back and read the previous 7 pages to remember what we were doing leading up to this.

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Now that should hold some heat!

Pete

 

Thanks for the comments gentlemen, I may bury a thermocouple next to the keg wall as I'm curious to see how hot it actually gets. Pat: in answer to your question, I've spent a year trying to substitute high burn rates for efficiency and hit a few technical challenges.

 

I ran the furnace yesterday for 2 hours continuous at a wide range of settings: at full throttle, the roughly A20 crucible full of partially melted iron gets to a bright yellow/orange colour. A 1.25Kg round iron weightlifting weight sitting vertically in the lid hole will melt in three minutes and spray hot iron lumps all over the place , while the iron weight sitting in the crucible near the top just gets very soft. At much lower throttle settings and running quite rich with bright incandescent flames coming out the top, the iron weight in the top of the crucible sweats beads of iron and then liquefies. This tends to indicate a flame propagation speed problem as it turns out that a rich flame has a higher propagation speed than a flame with complete combustion so a rich flame will burn fast enough to heat the crucible.

At this point I need to boost the flame propagation speed through one of many methods to release the heat in the chamber rather than just outside the lid if I want faster rates of burn. I'm attaching a photo of my boot soles with some of the shower of iron blobs stuck to them, I had all my protective gear on but a small bit hit my hair too.

 

Awesome! Flying lead! Where do I sign up?

 

Heh heh heh , it's awfully magnetic "lead". I did confirm some of the combustion issues with this last burn, tuning the mix to an incandescent yellow "rich" did allow me to melt 2.5 lbs of iron (a 1.25Kg weight) that sat on top of the crap in the crucible where the flames could play over it. Some fuels have the fastest flame propagation speed when at a rich 0.9 ratio rather than at a 1:1 stoichiometric ratio when in laminar flame mode. A laminar flame example would be a candle flame whereas a blowtorch flame is a turbulent flame and my furnace was melting iron at a relatively low throttle with a sound more of jet engine off in the distance (low rumble) sound rather than the usual oversize blowtorch roar sound.

I'm going to try and increase turbulence (think diesel swirl chambers in the piston crown) as that drastically increases flame propagation speed, see bottom paragraph of : http://eyrie.shef.ac.uk/eee/cpe630/comfun8.html . Other things I can try is starting the flame earlier in the tuyere somehow and increasing temperature of the fuel air mix by conducting heat back towards the fuel nozzles with some copper sheet metal.

Edit: I kept track of fuel consumption accurately this time: 22 litres per hour (5.5 US Gallons) with all 8 nozzles at 55 PSI pump pressure. This was with the throttle wide open and for half an hour run time.