Prototyping a spinning cup oil burner

This is a thread on yet another oil burner type that I think has some significant advantages worth investigating: the "Spinning Cup" oil burner. The burner atomises the liquid fuel by centrifuging it off the sharp rim of a spinning cup sideways into the forced airflow which then hopefully carries the fuel mist into the furnace to be combusted.

Advantages are:

• Low maintenance with no nozzles to clog or erode from dirty oil particles
• A high turn down ratio of 1:4
• Wide tolerance of fuel types from kerosene viscosity to bunker oil viscosity.
• Reasonably simple to make, although the cup would need a metal lathe ideally.
• No need for compressed air, just electricity for the motor.

Spinning cup burners have been in common use from the 1930's to the current day and were used in apartment steam plants as well as ship steam plants and ship's incinerators today. A dedicated unit has a centrifugal blower with a spinning cup at the centre both driven off the same shaft.

I first went down this path after getting some oil on a tiny wire wheel mounted in a motorised tool similar to the Dremel tools. An experiment showed it worked quite well and generated a fine mist and once power was cut to the tool it was noticed that all the wires had been flung off the cheapo wire wheel and only the tiny disc periphery was atomising the liquid. After that I built a larger, more rugged test bed, with the idea of fitting it into a furnace.

It's probably about half scale from what I really want and the cup design is shallow compared to commercial units, but it worked well enough to show the principle and a more sophisticated unit with a hollow rotating shaft with cup at one end would allow a stationary tube to feed oil through the shaft to the cup without shadowing the mist or interfering with airflow. It would also allow a quieter induction motor with belt drive to be used.






Here's a quick demo of the spinning cup in operation, first liquid is water, second liquid in the brown bottle is kerosene and the third is ISO46 viscosity (thin) hydraulic oil.

 

That is a pretty interesting concept.
It seems to atomize well.

 

For something built using the "TLAR" (That Looks About Right) principle it works better than I have any right to expect. Later versions will have a larger, much deeper cup to increase the centrifugal forces and even the fuel flow up the cup. I should try and jury rig a forced air burner using that test bed rig.

 

A couple of thoughts, fully knowing that this is just a TLAR build thus far...

1) I would think that a less aggressive taper on your cup would provide a more consistent mist.
2) The big complication with such a burner is the rotary union needed to introduce oil into the tube that is used to spin the cup. Obviously such unions do exist but the diy'ing something that won't wear itself out in short order will be an interesting aspect.

 

Pure speculation to follow: For the kinds of RPMs involved, it would be interesting to consider an air bearing union arrangement. High pressure in the oil stream should not be encountered, in fact the slinging of the oil away from the center of the spinner should induce suction. So an "air bearing"-style seal should provide a non-wearing means of providing a durable functional union.

Why might such a seal not work? Introduction of bearing air into the fuel stream might be an issue. That problem might be mitigated if a hydrodynamic design (as is used in hard drives for example) were used instead of a hydrostatic design.

Musings of an idle (waiting for grandkids) mind.

 

The cup geometry is going to be a major design issue, both the inside and outside for the airflow. I'll crib some design cues from a commercial unit: the 6000 RPM commercial units are a deep cup with a shallow taper to a larger diameter that would give thick oils time to flow evenly. I expect a home furnace job will need some playing around with RPM to find settings for low and high throttle settings for the cup and also vary the airflow.

There is some air introduced into the cup on commercial units, the diagram below is for a ship's incinerator unit burning heavy fuel oil and shows the air route in blue. My plan is to have a hollow shaft with a decent diameter bore hole, it should be possible to have a fuel tube made from straight hard drawn tube either steel or copper (aircon or hydraulic tubes) that goes up the bore centre with a few metal shield ball bearings to prevent contact with the bore when it's spinning. The metal shield bearings would give some seal against air into the cup although the commercial units introduce some air into the cup for some unknown reason.

So a Mk2 unit would probably have:

• A longer, wider, shallow taper cup cloned from a commercial unit.
• Hollow drive shaft with at least a 1/2" bore.
• Stationary fuel tube with ball bearing co-axial spacers to the spinning bore.
• Belt drive variable speed motor.

 

I wonder if a free-spinning pinwheel turbulator would work.
It would be powered from the combustion airstream, and would sort of sling the oil and break it up.

 

I think it would work but would be beyond my meagre research program. If I have variable cup rpm, variable fuel flow and variable airflow, I should be able to tune the unit for best performance/atomisation at different throttle settings. A turbulator would require a careful design and experimentation program to get good performance at all settings and may reduce the turn down ratio if reduced flow doesn't spin it as fast. I see that sample diagram shows some fixed swirl turbulators right up at the cup rim.

 

It should be scalable to whatever size you need, one of it's advantages is the wide range of throttling 1:4 which I understand is pretty good. Anyway more experiments are in order....

 

Maybe to cancel out a possible low pressure zone which may prevent the oil from misting optimally..? I'm thinking the low pressure zone created by the spinning cup may inadvertently cause the collapse of the spray pattern once the atmosphere gains the advantage rushes to the center, taking the misted oil with it.

Not sure if I'm explaining things clearly, but trust me, it makes sense in my mind...lol

 

It may also have something to do with keeping the flame front away from the cup, or even keeping the flame lit??. That diagram shows a flame pretty close to the cup. I wish this monsoon rainy weather would ease up so I can put the assembly in a stainless tube and apply some forced air.

 

Not that don't have enough imaginary projects on my list, but I have been contemplating how to build a variant of spinning cup burner.

I think I'd probably do a "tube within a tube" approach to get the oil feed into the cup. Then use a belt to drive the spinning mech. I think that might be the easiest approach for the home brew guy.

Then again..., It's not like you need a tight seal to prevent fuel seepage between a potential union and spinning shaft. That would allow you to directly drive the spinning cup. Downfall there is you would loose the mechanical advantage of gearing between motor/cup if you need it for sake of rpm. Which raises another point of interest for me. Exactly how fast does this cup need to spin for sake of efficiency...?

 

ok... two concepts:

The belt drive with no union:

Direct drive with a union:


Obviously there's some details missing in both pics but the important conceptual stuff is all there..lol

I think if you have a motor capable of the necessary rpm, (whatever that may be) then I prefer the direct drive. It would be even nicer if you could get the combustion air stream to blow over for cooling, but if you size it right I don't think overheating will be much of an issue. The union would be better placed after the introduction of the combustion air that way any seepage would just get blow down the burner tube and into the furnace. The union "seals" could be fairly rudimentary, as leaks aren't really much of a concern I think.

 

Rather than incorporating a belt drive and the complexities of very high rpm belt drives, I would investigate using a 20000 rpm universal motor with a spindle of adequate size like this one

https://www.zoro.com/ametek-lamb-un...MIlK-26NyR2QIVSXJ-Ch3H1AMMEAQYCSABEgLD7vD_BwE

Then I would, using a lathe, accurately center bore the spindle 1/4" or so for its full length. Then loctite in a 1/4" od tube to both feed oil and carry the spinner. It would not be terribly difficult to do that. Being a universal motor, RPMs are easily adjusted on the fly as desired. The motor would sit outside the straight tube portion of the burner tube and the 1/4" tube would be carried on spaced bearings down to the burner/spinner end.

 

Great motor... I spent some time trying to find something similar (120v) but was apparently looking in the wrong places...lol.

I'm struggling to envision your approach... What's the point in boring the a spindle to .250" just to glue in a .250"od tube...? Regardless of that, I would think simply buying a .250" id tube would be far easier then attempting to drill/bore a small od shaft ~12" long. (https://www.mcmaster.com/#standard-metal-tubing/=1bgcjc3)

I agree the direct drive approach being the better course of action. That said, a flat belt pulley arrangement would be very easy to implement if someone was inclined to do so.

 

My thought was that you take say a 2 foot length of 1/4 tube and insert it into the motor shaft exactly concentric to the motor shaft. The extra length heads down the burner tube carrying oil and supporting the spinner at the furnace end.f On a lathe boring accurately concentrically is fairly easy. Then there are no pulleys and belts to deal with pulling the shaft off center, needing balancing etc. It just seems to me like a good design approach. Running a pulley drive at 20000 rpm does not seem like a trivial problem.

 

Im envisioning a 3/8 tube acting as both a drive shaft for the cup as well as an oil supply tube. An extralong drill bit of the appropriate diameter with the shank cut off inserted and brazed inside the tube. Does it seem that the oil would get pulled through the tube?

Pete

 

Ah.... makes a tad more sense now. Originally it was a .250" id spindle with a .250"od tube loctited into it.... At least that's the way it read.

Not nearly as bad as you think. Small light weight flat belts are a vacuum cleaner section away at your local hardware store, and the pulleys are just disks of equal size. You may have to go so far as crowning the pulleys or turning a valley in them to guide the belt. Extremely easy if you have the fore mentioned metal lathe.

As far as boring a .250" hole 2ft into a small diameter shaft... Although I'm not a machinist I've done more than my fair share of metal lathe work, and what you're suggesting would be difficult in terms of tooling (who owns something that bores a .250" hole and has a reach of 2ft?), and even worse in terms of multiple set up changes to complete the task with any amount of accuracy. Spend the $10 and buy a tube ready made.

What you're thinking mimics my second pic that illustrates the direct drive approach. The only thought provoking aspect is getting the oil into the spindle. A 'tee' shaped union with maybe loosely fitting o-rings or barely oversized bushings would do the trick I think. At the rpms everyone seems to be suggesting, the friction at the "seals" of this union will break down very quickly if actually in contact with the spindle. The fuel feed would need to be under some level of pressure I think as well. Not sure mere head pressure from a raise bucket would be able to force oil through the rather smallish holes in the spindle tube while it's ripping away at 20,000rpm.