The Propane Burner

Posted: July 28, 2010 in Burner

As mentioned in the overview, Ron Reil has a lot of good information on his site.  Unfortunately, it’s organized rather badly as far as a “how to” goes, and operates more as a “here are things I’ve done.”  Nuggets of wisdom are scattered around the various pages though, so with a bit of work you can figure some things out.

Distilling the wisdom of the net, this is what I ultimately came up with, but why, and how do they work?

Lets look at a simple Reil burner plan to understand the concepts.



The jet of propane shoots into the tube, and sucks in outside air to mix with and combust.  Ron’s original burner accomplishes the air intake by sucking in air from the tail end of the pipe.  However, since a fair amount of the area is taken up by the jet pipe itself, a “reducer” (actually an “enlarger” in this case) is used to increase the surface area of the intake to  get more air and compress it.  Many people have had success with this simple design… it’s easy and cheap, if not optimal.

Ron created a second burner called the Mongo, which was a 2.5″ diameter monstrosity, and Rupert Wenig scaled it down a bit to be the Mini-Mongo at 1.25″.  This burner is actually quite elegant, and I used this as a basis for my own, simplifying various elements and incorporating improvements mentioned on the site, and used in practice on similar burners from other sources, such as slotted air intakes and a collar choke.

The key differences between the Mongo and Reil burners is that:

1) Mongo burners have a straight-tube injection instead of having a pipe laying perpendicularly to the jet.

2) Air intake comes from holes drilled in the tube wall, not drawn from behind the tube.

Further refinement of the idea is to replace the drilled out pipe-plug jet with a TWECO contact welding tip (14T – 45) and use elongated slots as the air intake rather than simple holes, dramatically increasing the top-end performance since the fuel won’t become oxygen-starved.  In order to eliminate an oxidizing flame at lower settings, a choke sleeve is added to reduce the effective slot length… clever!   As luck would have it, after I figured all of this out, I discovered Michael Porter’s book which includes most of these principles and which probably could have saved me a fair amount of research and investigation.  Still, I was gratified to realize that I was on the right track.  Purchasing the book from Amazon probably wouldn’t be the worst idea if you’re truly interested in the subject!   The T-Rex embodies all of these principles in a very polished and highly accurate commercial package.  If you just want a great burner and money is no object, I understand that they’re excellent.  As for me, I’m rather cheap and I enjoy building things.  The DIY ethic in me won’t allow me to shortcut this, so onto the burner construction…

One thing that initially confused me… different diameter burner pipes had different lengths… why?  Ultimately I discovered that naturally aspirated burners should be following a 9:1 ratio of length to diameter so that full mixing of the fuel can occur.  Shorter lengths are possible, but result in poor operation. 

A word about air velocity, venturi action, step down, etc.  Ultimately the speed of your fuel/air mix is defined by the speed of your jet.   Pulling air into a reducer isn’t going to “speed up” the air any more than it would have without the reducer.  Either way, the maximum speed of the air is the same as the speed of the fuel which caused the vacuum in the first place.   An air reducer inlet like on the reil burner, or some of the modified tube burners (1 1/4″ tube with holes -> reducer -> 1″ burner tube for example) serves solely to increase cross-sectional area to draw air from.  While it is true that the reducer will speed up the air, that simply means that the air is actually moving rather slowly  at the larger intake tube.

If you were unable to increase a lot area, then increasing the suction pressure of the gas by shooting it into a venturi-tube  with a more rapid exit-expansion than inlet-contraction would be one way to try to pull more air in to be burned.  If you have sufficient air intake this shouldn’t gain you anything since the same volume and velocity will be present on the exit from the venturi-tube.

Lastly… exit gas/flame velocity.   The heat content of a given volume of fuel is static.  Assuming a perfect mix of fuel and air, the speed and length of a flame jet is irrelevant because both have combusted their fuel completely and released 100% of its energy into the chamber.  If anything, a slower movement of air allows the air that you just heated to linger longer in your chamber, imparting heat into your metal, walls, etc, instead of shooting out the furnace exhaust for as Dragon’s Breath.

Enough theory… onto the building!

The Mini-Mongo is a little larger than necessary for my relatively small forge, and I’d just as soon cut down on the pipe length.  As a result, the KCrucible burner is 9″ long, 1″ in diameter made out of:

1) One 10″ black iron pipe nipple (threaded on both ends),

2) One 1″ end cap

3) Two 1″ pipe couplings

4) One 2″ x 1/8″ brass pipe nipple

5) One Tweco 14T -45 welding tip

6) a little Sculpy or other clay

7) silver bearing lead-free solder

 

Creating the Air Intake

Take the 10″ pipe nipple and draw four lines, about 3.5″ long, evenly spaced around the outside of the pipe.  Use a dremel grinding bit to create four divots in the metal along each line.  The  end divots shouldn’t be quite AT the end.  The point is to create a depression for the tip of a drill bit so that we can drill out four holes, at which point we can dress up the rest by connecting them.  The spacing from the ends of the line should be half of your drill bit width.  I used the biggest bit I could find, though I’m not sure of the actual size off the top of my head.

Once the divots are complete, clamp the pipe to your workbench (I used spare lumber to create a trench for the pipe to lie in) and drill out the holes.

Now, use a dremel to grind/cut out the excess metal between the holes and create a nicely finished slot.  Round off the edges along the outside and inside to reduce drag which can disrupt airflow. 

When complete, you can wipe the pipe with oil and put a propane torch on it for a while to carbonize the oil and create a protective coating for the area you just cut.  This will help prevent rusting.

Creating the Tip Assembly

The tip assembly is much simpler than the Wenig version.  Take the end cap and screw it onto the pipe. Grind a divot at the top outside, dead center of the pipe (not the cap).   This matters because these are cast-iron parts, and the screw  is going to be somewhat lopsided.

Drill through the cap with a bit that is the same size as the thickness of the 2″ brass nipple or slightly smaller.  Use a Dremel grinding tip to enlarge the hole until the nipple can just be pushed through.  Look down the end of the pipe and check for centering.  Adjust the hole dimensions as necessary to get good alignment.

Take out the brass pipe nipple and insert the TWECO tip into the end of it until the TWECO threads don’t show.  It should be a close fit.. snug but not binding.  Heat the tip and the nipple with your torch, then apply the solder to the joint.   The solder will flow into the gap.  Work all the way around the junction.  When done, allow it to cool and wash it off, then blow into the brass end of the nipple while holding your finger over the TWECO outlet.  You shouldn’t feel or hear any air escaping.  You could aid this test by spraying the joint with soapy water and checking for bubbles.   If there’s still a leak, reheat it and remedy.

Insert the soldered part back into the cap and push the clay into the inside of the cap, surrounding the brass insert and holding it snugly within the cap. 

If you’ve got it, take a circle of foam or cardboard the size of the inner diameter of the pipe and put a small hole right in the center of it.  Put the tip through the hole and screw the cap onto the pipe.

Check to make sure that the tip is centered by looking down the end of the pipe, and making sure that the tip stays in the same position through the air intake windows as you rotate the pipe.  Adjust the tip, cap, etc, as necessary until you get good alignment.  When done, remove the foam tip, pack the clay tightly and double-check the alignment.

With the cap in place, solder the brass nipple to the outside of the iron cap.  The solder will flow into the joint between them to solidify the connection, start to harden the clay, and I built up the solder a bit on the outside surface (it grips the lacquer) to provide additional stress relief.   A little tidying up with a Dremel grinding bit, and we’re done with this!

Creating the Choke

The choke is simply a sliding collar of metal that can obstruct the air intake slots.  This allows you to adjust the amount of air to match the volume of propane being used at any given time so that you can maintain a slightly reducing flame.

The easy way to do this would have been to use another appropriately size pipe that would get a good fit.  I was having trouble finding one, so I decided to use a 1″ pipe coupling and grind out the threads.   It works, and looks pretty cool,  but it took more time than I’d have liked.

Creating the Flare

For proper flame behavior outside of the furnace (and certainly doesn’t hurt inside a furnace) you need a flared end to your burner.  I’ve been told that it’s important, but don’t understand it well.  As near as I can tell, the purpose of the flare is to reduce the speed of the flame jet by causing the same quantity of air/fuel to exit the pipe via a larger hole.  This keeps the flame from blowing itself out and behaving more like a candle, leaving the flame stably situated at the end of the pipe.. the flame can continually ignite the gasses that are exiting.  A nice smooth transition will allow this slowing, without creating back-pressure which could reduce fuel and airflow.

Research by those who understand these elements suggests that a 1:12 ratio is the best taper for a flare, though I suspect others would probably work fine.  This means that if you start with a 1″ diameter, it will double (increase by 1″) over the course of 12″.

BackyardMetalCasting has a tutorial on creating flares from rolled sheet metal.   I didn’t like it.  Zoeller sells stainless steel flares, but that didn’t appeal to me as a first-attempt sort of thing… maybe my fallback.  Ron Reil had suggested that it was possible to cast a flare into the furnace body itself.  That’s interesting.   But what if I want to use the burner outside of the furnace?

That’s when I had an idea to create a hybrid flare… a metal collar with refractory material within it, in the flare shape.  The metal will hold and protect the somewhat more fragile refractory and the insulating refractory will help keep the iron burner from oxidizing from the excessive heat of the furnace as well.. sounds like a win-win!

My flare is a 1″ pipe coupling as above with the choke sleeve, but with the threads only removed from one end.  The pipe is filled with refractory around a tapered wooden (and greased) cone and allowed to dry.   A little more work to smooth and sand, and we’ve got ourselves a flare.

To create the wooden cone I found a 2″ x 2″ x 12″ wooden stick.   Draw a line down the center on 4 edges.  At one end, put marks .5″ from both sides of the center lines.  .5″ + .5″ = 1″ which is our starting size based on the inner diameter of the burner tube.  The end size needs to conveniently be 2″, which it is already.   Draw lines from the 2″ corners to the ends of the 1″ corners.  That’s our taper.

We don’t need 12″ of flare though.  Measure out  4-5″ from the 1″ side and cut.   Using a bench grinder, dremel, etc, remove the wood that is outside the taper line to form a cut off cone.

Grind out the threads on one end of the iron flare jacket, and go a little  more on the outlet end to support a thicker refractory wall.   Make sure that the burner pipe’s inner diameter doesn’t narrow at the end.. grind it down to be flat.  Screw on the flare jacket then insert the wooden cone into the pipe for fit.  Grind down the pipe or wood as appropriate to get it snug so that you seen little-to-no light leaking around the flare when you look down the tube at a light source on the inlet end.  Build a collar out of aluminum tape (or other rigid body) that extends another .25″ past the end of the iron flare jacket.

Lubricate the cone and mix a small amount of refractory.   Spoon in the refractory around the inside of the jacket and base towards the bottom, then insert the cone.  Continue filling the jacket from the end until it fills the extension collar.  I let it sit for a half-hour to harden a little then removed it, but if it’s sufficiently lubricated AND smooth then you can pull it out later.   I gave a little twist and it dislodged.  I had to go back after to the fact and patch in a bit that broke off and re-smooth/shape.

So, we let it dry and ultimately will fire it up.   We’re done!   The interface between the 2″ brass nipple and a propane hose we’ll deal with later after I get my regulator and hose and can figure out the adapter I’ll need.

Update! 

Make sure to read Burner Modifications prior to construction of this burner to save yourself time.

Comments
  1. [...] and exit gas velocity and flame stability. Here is the end result as it deviates from my initial Propane Burner [...]

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