The Best burner ever

The most important, and probably least understood component of the home foundry is the burner, it's sole role in life, is to get the inside of the furnace to working temperature in as short a time as possible. There are two main categories of a burner,

• Self aspirated Burner

• Forced air burner

The first of these, (an example of one of my designs shown above) draw's air into the burner tube by inducing a low pressure area behind the jet of gas, this produces turbulence, forcing the gas and air to mix inside the burner tube, with the gas in a proportion close to what is required for combustion. The fitting of a flair on the end of the burner tube, as shown only serves to slow the velocity of the gas/air mixture emitting from the tube so that the flame front, the point where the gas ignites, is not blown away by the force of the gas/air mixture coming out of the burner. But this is only important when the burner is outside of the furnace.

When the burner is in the furnace, the flame is forced into a vortex, and as more gas/air mixture is blown in it is ignited by the confined flame generally under these conditions the higher the velocity of the gasses the better.

These burners work very well and with a little bit of tweaking will produce good working temperatures, if you look on the web you will find many examples of this type of burner, some of which are very impressive, unfortunately they all suffer from the same defect in that they will not tolerate any undue back pressure.

This will mean that the exhaust port must be tuned so that the combustion product can flow through the exhaust port without excessive restriction and of course the bigger the exhaust port the greater the quantity of heat that will escape.

A simple test here is that having lit the burner and heated the furnace view the exhaust gasses after dark, if you have a flame extending out of the furnace then you have too much fuel going in, any flame extending out of the furnace is not heating the metal but contributing to global warming with no payback.

So there comes a point where you cannot put any more in with out causing the back pressure to exceed beyond the point where this type of burner will not draw in any more air, flash back and ignite internally, What do you do now?? well  put it out of course and build a forced air burner!!

Forced air burner

My first forced air burner as shown in the link above originally use a shaded pole AC motor and had a maximum rotational speed of about 1500 RPM when driving a fan, I then changed this for a 12V permanent magnet electric motor which runs at about 3500 RPM driving the same fan, this had dramatic effects on the efficiency on the performance of the furnace and I could with a small piece of card over the air inlet run the furnace at different temperatures, this was fine until one day it stopped mid melt..............almost total disaster. I noticed just in time that the supply leads had shorted, a small piece of card saved the day by inserting it between the contacts.

That afternoon an electric grass mower then came into possession which was swiftly dismantled where I found a universal motor.

Well it certainly blew air! It cleaned out the furnace from dust on first blow it was almost as powerful as a leaf blower just smaller.

So now I had to design a new burner,

New burner

Up till now all of my burners were built out of mild steel which have all suffered from melting and burning away under prolonged exposure to flames, especially since I do push up the temperature to melt cast iron from time to time, so I used a piece of heavy gauge stainless steel tube with an outside diameter of 42mm (1.650 Inch) and a bore of 35mm (1.375 Inch) 

Stainless steel will melt at the same or slightly higher temperature to steel but it has a much higher resistance to the effects of oxidization at elevated temperatures and thus will provide a longer service period.

The basic concept is so simple it is hardly worth describing but here goes, The fan assembly and Aluminium mounting tube remain unchanged, the burner tube has been shortened, the diameter increased to 35mm (1.375 Inch) with the stainless steel tube fitting inside the mild steel tube and welded together, The mounting flange is welded to the mild steel tube so that it can be bolted to the Aluminium mounting tube, the total length of the assembly is 230mm  (9.055 Inch) a  change however is made to the method of feeding the gas into the burner, here I turned a 15mm (5/8 Inch) slab of brass so that it would fit snugly inside the mild steel tube and held in place with two 1/8 by 1/12 Inch BSP nipples only one will feed the gas, a hole was drilled centre tapped 1/8 BSP and a brass bung fitted with a 1.5mm hole drilled on each face, much bigger than before. In actual fact it is the remains of my oil burner spray nozzle, which is why it is odd in shape.

This method of feeding the gas has the advantage that it is easy to fit and remove.

Burner Tube

Here are the components of the burner assembly, the brass boss in the centre has be turned so that it has a radius suitable to fit inside the larger steel tube

Boss 

This shows the boss fitted inside the larger steel tube the compressed air fitting is used to couple the gas supply and is extremely useful at time when the gas freezes or unexpectedly runs out giving the ability to very quickly change gas supply.

I have added a drawing of the burner construction here   

The New motor on the burner is as been said a universal motor, and is controllable via altering the phase angle of the supply.
This is very useful as there must be more than a trillion phase angle Speed controllers, err, light dimmers out there on the market, I for one did not believe that it would work so having an old unit lying around tried it and to my astonishment it worked!!! so by coupling up the motor and dimmer you have the ability for total versatility in both air flow and gas flow into your furnace.

The best burner ever