Mad Aubrey

What is the biggest problem you get when throttling back a burner?

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When you try to reduce the heat that a burner is producing, the only way to do it is to throttle back both the air and the fuel being used.
Simply reducing the fuel will create an environment that has too much oxygen in it which means that anything else in the foundry will tend to become oxidized - not a good thing.
Reducing the air will create an environment that has too much fuel in it which means that in the case of using old motor oil a lot of smoke will be created and everything will get a coating of carbon soot or simply filling the foundry with unburned oil which will run out the drain hole - not a good thing either.
So both fuel and air need to be throttled back at the same time so that the air/fuel ratio remains as constant as possible.
However a point will be reached where the flame will not be able to sustain itself and will go out.
When this happens the unburned air/fuel mixture will normally travel into the kiln or foundry where it will meet up with something that is hot enough to ignite it.
This causes an explosion as the mix ignites and this process is repeated rapidly causing a loud continuous popping noise and sending sparks flying all over the place, some times back through the burner and into the blower fan - all of which is dangerous.
In most cases, the only solution is to increase the air/fuel supply which produces more heat than you need which is not what you need but it is something that you simply have to live with.
If you have a situation where you absolutely must have less heat being produced, most of the times I have seen videos where people put a propane burner into the burner pipe to keep the air/oil mixture going which kind of raises the question "why not just use propane from the start?"
That is a very valid point there - so the first thing to do is to figure out a positive way of re-igniting the burner flame should it go out for some or other reason.
The best way is to have some sort of ignition setup like the "hot surface" ignitor used on a lot of domestic boilers/furnaces like this one which go for about US$18 Click image for larger version. 

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ID:	20706 or a "spark" ignitor like this one which goes for about US$15. Click image for larger version. 

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The "hot surface" unit I presume has to be located in the path of the air/fuel mixture to work which means it is being heated by the flame constantly which is a problem as far as the life-span of the unit is concerned but on the positive side, simply needs power to work.
The "spark" ignitor type can be placed just outside of the air/fuel mixture where it exits the nozzle but not near enough so that the spark goes to the nozzle which means that its life-span should be far greater.
On the down side, the "spark" igniter needs some sort of high voltage pulsing supply to make the spark.
However there are many circuits available to make one of these pulsating high voltage gadgets using an old car ignition coil at a very low cost.
Here is one that uses an Arduino to make the pulses - Here is one that uses an Arduino to make the pulses
BUT you still need to purchase the ignitor in either case.
Note to self : Think about using an Arduino to control the entire burner (once you have got it working)

What I have used in the past is the electrodes used in a TIG welder torch that have been set in Pratleys Putty (a 2 part epoxy putty with high heat and insulation qualities) and they worked perfectly - cost almost nothing (old, too short to use TIG electrodes - had putty lying around) and while we were at it, molded a mounting flange into the putty as well.
This setup worked like a charm - we never had to replace any of them because although they do take a bit of heat, it is nowhere near aluminium melting point.

The result was that we had an ignition system that would fire the burner up from cold and would keep the flame going at very low fuel/air mixture rates.

Whichever variant is used we will have the ability to do a cold startup (without lighters or pieces of paper) and the ability to keep the flame going in "idle" mode.