Discussion in 'Burners and their construction' started by Mark's castings, Feb 4, 2018.
ditto to what Myford said...
I was finally able to do a test burn on the new design, initially I did a couple of water tests and discovered the blower air would try to blow the cup shaft off the fuel tube causing the drive belt to come off. I did reverse the belt earlier for a few runs on the bench but it kept coming off the pulleys, probably due to the shallow rim on the shaft and not enough crowning on the driven pulley. Water testing also revealed the cup is a bit short for the job as water can enter the cup in a stream and do a lazy spiral off the edge with poor atomisation. Various half arsed attempts to diffuse the flow so it hit the cup didn't work and I bit the bullet and machined up a stainless steel disc with a tubular section on one side that is a neat fit for the bore of the fuel tube. There are four holes to allow fuel to enter the cup and the gap between the rim of the disc and the bore of the cup is close. The water now tends to get evenly spread by the cup rotation and has a chance to get centrifuged. I ran it for about 45 minutes with a break around the 30 minute mark of 10-15 minutes when I ran out of fuel and then ran it for 15 minutes after that. I barely got about 10 lbs of scrap brass melted, definitely not fluid enough to pour a casting. So far the unit runs really well and can be turned up quite high but the flame flow in the furnace is an issue as the flame only does one turn around the inside before exiting, I'll raise the crucible height with a second half brick under it and see if that helps things.
The only other issue was oil inside the stainless air tube, the ball valve appears to leak even when turned fully off so when the cup unit is withdrawn on shutdown, there's oil leaking out the cup in the air tube. Other improvements would be a sheet metal hatch in the cover in case the belt came off, right now the entire cover has to be removed to do this.
Hmmmm So what's the plan?
I'll open up the cover....yet again ... and see what I can find out regarding the oil leaking, it was dribbling water too during testing, too much volume for what would be in the line after the ball valve.
The flames also seem to be above the crucible mostly so I'm planning on raising the height of the crucible on it's firebrick plinth to see if that helps. This unit doesn't have the Mk2's surging when hot so it solved the surging of the U tube when hot. I think it would be usable once there's some fine tuning done on the combustion in the furnace chamber. J Vibert's concept of a design with a spinning shaft and external bearings is looking to be a better bet: it's entire length spins the fuel before it hits the cup, the bearings can be away from the heat and it can be solidly built. If I were to build another unit it'll be a rear fed, belt driven shaft with external bearings.
This one (Mk3) does everything it's supposed to do (easy ignition, no compressor, no pump, high rate of burn) but it feels a bit complex and finicky.
It's taken a while to get organised enough to do another furnace run: I finished the heat shield for the crucible lifter by fitting a 3/8" layer of mineral wool insulation between two stainless discs above the crucible opening. I bought some thick leather boots, even the tongue is leather, but synthetic laces. My overalls have legs long enough to go over the top of the boots and I borrowed a chrome leather apron to add some protection. Fuel is no longer a problem: a local fish and chip shop has 80 litres of used cooking oil a week for free, also a local tyre shop has scrap wheels for AUD $10 each for 8 Kg minimum weight wheels.
So this time there have been some slight changes: the jumping castle blowers turn out to have a motor cooling vent that bleeds air off the blower to cool the motor. I blocked 80% of it with silicone RTV on the secondary blower as lot of air was coming out of it and hopefully the air pressure is increased a bit. In the furnace I noticed most of the flames were above the crucible on a previous run so I added a second fire brick chunk to double the height of the plinth that the crucible rests on. This gives a larger combustion chamber/swirl chamber under the crucible now and hopefully a bit more time for the fuel in the chamber to burn fully. I filtered the oil through some large scouring pad material which got a decent amount of large solids and left some particles like a fine sand size in the oil which should traverse the system fine. The furnace started easy, there was some oil vapour smoke leaking out the cracks which disappeared once warmed up, so the oil must be burning faster and the cracks in the wall could be closing when hot too. The mystery leak of oil turned out to be caused by the static spreader disc inside the spinning cup: it creates a chamber full of oil that allows oil flow backwards through the cages of the roller bearings and between the 16mm spinning tube and the static 12mm tube before exiting as a spray below the angle grinder motor and filling the chamber with an oil mist..... .
Anyway the furnace was run with the fuel valve at about 45 degrees open or 20% of maximum flow rate on the throttle and tuned for a semi transparent orange, leaner flame rather than a bright yellow flame. The furnace was run for about ten minutes to preheat and the crucible was full of dull red molten aluminium in 20 minutes or 30 minutes total burn time. The furnace used 12 litres over 45 minutes or 16 litres per hour consumption at the 20% of max flow setting. I used my crucible gripper to get the pot out of the furnace, this time the heat shield was above the rim of the furnace and there was no hurry or heat at all when lifting the crucible out of the furnace although I expect bronze will be a different story altogether.
So I'd made a styrofoam safety guard model for a 10" grinding wheel and packed it in sand, being a large surface area and forgetting to place weights on top of the sand it failed badly. But that's ok I was able to get the furnace running hot enough to melt the perlite/silica sand/concrete/fireclay refractory. I'm pleased with how well the burner runs and will use it a bit before going to a Mk4 design.
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