Prototyping a spinning cup oil burner

A two foot deep 1/4” hole ina two foot shaft?! That would be a challenge—-but not that tough. Think gun barrel. But I am not talking anything like that deep a bore. Say the spindle in the motor is 6 at most inches or so. That is all you need. And that is easy.

 

That first concept is pretty much what I'm planning for a Mk2 prototype, just a longer fuel tube that extends into the cup. It should be possible to have a sheet metal motor enclosure over the motor, belt drive and pulley so that no forced air can escape. The commercial units seem to have a larger cup/lower RPM (6000 revs) combination, so a larger cup should directly translate to lower RPM.

The 1930's apartment steam plant spinning cup burners were made this way with a hollow shaft AC induction motor running at lower speeds according to one eyewitness description from an engineer who saw them as a boy in his New York neighbourhood. For a direct drive unit, it might be possible to press out the motor shaft and then press in a long tube in it's place and rebalance the rotor. There's no doubt this design is more elegant.

For now, I'll push the Mk1 unit to it's logical conclusion with a steel tube running beside the spinning shaft before making a U turn up to the inside of the cup. If I use a thin capillary for the U turn and keep it an inch or so away from the cup it may work perfectly fine with the odd drip from collected spray hitting it. I'll use steel brake line as it handles higher temps than copper.

If I went the direct drive-hollow shaft route some 4130 thick wall seamless tubing sold for homebuilt planes and drag cars would have a reasonable level of finish, straightness, concentricity to use for a hollow shaft

 

I'm still a little amazed at the rpm these cups spin at... I have no clue of the physics involved but it seems really excessive.

 

“If I went the direct drive-hollow shaft route some 4130 thick wall seamless tubing sold for homebuilt planes and drag cars would have a reasonable level of finish, straightness, concentricity to use for a hollow shaft”


DOM tubing might be another choice for your high speed shaft. It is manufactured to closer tolerances for ID, OD, straightness, and roundness

“DOM Tubing (Drawn over Mandrel) is a cold drawn 1020 to 1026 or ST52.3 grade electric resistance welded tube with all the flash removed prior to drawing to size. Cold drawing DOM to size allows for higher yield and tensile strengths. In comparison to other tubing, DOM is produced to more exact OD and ID tolerances. This allows for minimal machine time and has the highest weld strength possible.

• DOM tubing is made from 1020 / 1026 or ST52.3 Steel
• Meets ASTM A513 Type 5
• Stocked Size Range: 3/16” – 14” OD; .028” - .625” wall thickness
• DOM tubing is stocked in 17'-24’ random lengths
• Cut to length service available
• Suitable to hone sizes
• Custom sizes available upon request
• All the common off road sizes available

Benefits of DOM Tubing (Drawn over Mandrel):

• Uniform wall thickness with close OD and ID tolerances
• Uniform grain structure and controlled hardness
• High Yield and Tensile Strength
• Smooth and Clean OD and ID surfaces
• OD and ID concentric
• Excellent machining characteristics

Common applications:

• Hydraulic Cylinders
• Auto and Truck Axles
• Suspension parts and shock absorbers
• High Speed Shafts and rollers
• Bearings and Spacers
• Roll cages
• Agricultural components and many other machined tubular parts.”

FWIW

 

It's just like flicking water droplets off the end of something wet, the faster you do it, the smaller the droplets get. Once you get past a few tens of thousands of RPM, the droplets get very tiny as the centrifugal force overcomes the surface tension of the liquid: at those speeds the drops just can't hang on!.

So this is a jury rigged 1/4" O.D. copper fuel line for the spinning cup, the tube has some gentle bends to avoid kinks and also to pass through the plane of the spray droplets away from the cup to try and minimise the amount of droplets intercepted by the tube. By positioning the tube bend vertically, it may act a bit like a plumbing trap by going uphill relative to the control valve and possibly minimising fuel dripping out the 3 feet or so of fuel laden tube past the cut off/throttle valve. The brass gas valve body is drilled for the 1/4" O.D. copper tube and will be soft soldered into the body. There is no nozzle at the end of the tube, it just ends inside the cup periphery.

 

Not sure if it's the 'end game' solution, but what kind of duty cycle do you think you can get out of a dremel...?

What I'm also seemed to be doing, is losing the sense of scale with this project. What you have pictured above with the fuel line would go into a massive burner tube.

 

Dump that stupid 3ft long flex shaft or CUT IT. The duty cycle issue with the dremel isn't about the motor. It's the stupid hose for a connector they use inside it. But this is just a test rig so who cares if your 6dollar dremel dies a fiery death.

A much smaller motor can be worked into the solution later. For now, shove this thing in a 4inch pipe and light the wick! I wanna see some flames.

 

4 inches???

He's going to need a 8" pipe for that setup.

Me have an idea for ya but want to draw it up later on instead of attempting to explain it.
Picture is worth a thousand words.................

Not busting your balls Marks, I applaud your enthusiasm.

 

I think that Dremel-like object in the photo is branded "Firebrand" because the makers have a sense of humour. It did come with spare brushes so there's some expectation of life, come to think of it I did route the 50 and 60Hz strobe notches around the rim of a record player turntable pattern with a 3mm carbide bit in that unit, so it already has four continuous hours use.

Looks like I'll be putting the 1:4 turndown ratio to the test, I will be wearing my BROWN overalls for the first test when it happens. The test stand is an old perlite-fireclay-concrete lined furnace from some Internet casting website and I understand it melts well before cast iron temps are reached. The spinning cup is going into a 4" stainless steel thin wall pipe...once I get some more of the stuff.

Anyway I have to fabricate some 4" PVC pipe air plumbing from the blower and also fit some kind of air control valve like a butterfly valve or shutter unit. I might even fit a solenoid fuel cut off valve so I can pull the plug as I'm running away.

So the ultimate goal of this burner is to have a fast rate of fuel burn like my friend's propane furnace: flat out, it can melt an A20 full of bronze (60lbs) in 20 minutes from a cold start and 15 minutes once it's warmed up.

 

My idea which I was pondering upon is to drill some small holes at an angel within the inner - outer section of the spinner around the drive shaft. If that makes sense to ya.
Possibly creating a vacuum which would hopefully draw oil from a sump filled with oil and housed inside a tube within the burner tube itself.
Of course the spinner would half to be backed up against the sump wall to draw the oil out.
There would be an inner tube and outer tube to the design.
The outer tube would allow introduction of air to the flame.

Yes?
No?

Faulty idea?

 

You're missing that pic or another 900 words or so. Either way, I'm not totally grasping what you're trying to explain...

 

The Cats Meow would be a submersible motor with a combined pump mixed into the configuration.

That way the motor would not only drive the spinner/shaft but force feed it as well.
The entire setup would have to be encapsulated into it's only housing box & burner tubing all in one.
A sealed box with pipe attached and all the fixing incorporated within.

 

Pondering at the moment.

Once I "think" I have it figured out I'll draw a doodle.

 

Was reading the comments and came up with this sketch, a bit tricky to machine in one piece but do-able, would be easier to machine as two pieces with a separate cup and tapered inner hub.

I've obtained some 4" diameter stainless tube and some PVC plastic pipe and fittings to start work on the blower plumbing and throttle slide valve tomorrow. I was reading the Sunflame brochure for their "Third Generation" rotary cup burner technology: apparently it's an integral motor on the shaft with variable speed and a separate air blower where the earlier technology was a centrifugal blower and cup sharing a common motor and shaft. The new tech has a wider turn down ratio as they can vary cup and blower independently.
https://www.sunflame.net/pdf/burner_eng.pdf

 

Now your getting the idea.

I was thinking the drive shaft would be flush with the inner bucket of the cup and would have a hollow core which would serve as the fuel/oil delivery channel.

Of course the fuel/oil would have to be force feed via some sort of pump or pressure feed from a oil bath reservoir.

 

What about his?



Design flaws, fixing, stand by Houston.

 

Revised

 

Mark’s has come up with a very clever idea! The centrifugal force induced by the spinning cup slings the air and oil to create a vacuum in the cup drawing oil into the cup and yet no seals are needed. Very neat! Machining that cone except the cold-end cap as a single piece and then tig welding the cap on the cold end and finishing the machining would be no problem.

Minor tweak suggestion: In order to induce less turbulence and possible damming of the air, it might be good to have the fuel enter the cold end of the cup through a loose-fitting disc incorporating the fuel tube(s) rather than discrete stand-alonge tubes as drawn.tubes.

One other way to get the fuel into the cup, obviously, would be to simply run it down the drive shaft if the shaft were in fact a tube. The high rpms may be causing concern as to whether a coupling could easily be made to withstand those rpms. I would point put that if a 1/4” o-ring were fitted to the cold end, at 20000 rpm the outer surface of the ring would only be traveling at 1250 feet per minute or about 14 miles per hour. At that low surface speed the diesel fuel would provide more than adequate lubrication to prevent the ring from burning up. After all, the seals in the bearing of a common 20000 rpm drive motor are much larger and hold up well. So, just using a basic o-ring on the cold end of the hollow drive shaft could also get the fuel where it would need to be. For my money, the simplest approach would be direct drive dc motor with a hollow drive shaft loctited into the bored spindle of the drive motor spinning the cup and simple o-ring coupling for fuel on the cold end of the shaft outboard the motor would get my vote.

That said, I think Mark’s idea is really cool.

 

Here's an image of a cross section of a common spinning cup burner unit with an integral blower. It uses a relatively short hollow shaft and a belt drive unit and apart from the integral blower on the same shaft it is what I'd eventually go for. There's one spinning tube/drive shaft and one fuel tube running up the middle into the cup. I don't know that I'd have primary, secondary and tertiary air circuits unless I had fuel spray hitting the sides of the air tube. So everything in yellow is probably the easiest setup for a home user to build.

 

Have started sorting out the PVC blower plumbing using 4" PVC pipe which is really about 4&1/4" for some archaic reason. The blower has an adapter attached, the unit is a 3/4Hp induction motor driven blower intended for a kids jumping castle. The slide valve to vary the airflow is now complete. I have about one metre of 100mm (4") stainless tube for the hot stage.


Trepanning the acrylic plastic sheet for a snug fit of the female PVC joiners:



Slide valve complete with 1/8" aluminium slide, the side spacers are made of the same thickness sheets but with a tape shim, it has a good snug friction fit with no tendency to fall out.