Propane burner - I'm learning/experimenting

[David Collins]

I am trying my first propane burner.  I have read so many different plans, ideas, etc. over the last month or so.  

I am sure the logic of it was lost on me somewhere because . . . it doesn't work.

I am wanting to not just copy someones plan but to really understand it & mess with it.  I enclosed a picture of mine and was hoping someone can give me some advice.  The design is a 2"pipe with a hair dryer on one end to a reducer with the gas line running through it, to a 6" x 1" pipe for the burner end.  The actual gas tube is nothing more than a copper pipe with a small hole drilled in it, pointing toward the end.  I can't light the thing with the hair dryer on, even on low.  If I turn off the dryer I can light it & it is just like a candle burning.

My thoughts on what could be wrong are:

1. the gas line is too close to the end.  Maybe I need to move it back more toward the dryer?

2. there is not enough pipe after the gas inlet to allow the air & gas to mix well.

3. the hair dryer is too much?  Maybe I need more volume & less 'force'?

Any help would be greatly appreciated.

David

[ThomasPowers]

Well if you don't want to follow a known good design; shouldn't you expect to spend 5 to 10 years figuring things out on your own? Isn't it cheating to ask other people to spend their time to troubleshoot your design?

It looks like you are mixing a blown burner design and an aspirated burner design: Do one or the other!

What the heck is the reducer to that small a pipe going into the forge?

Where is the air control on it to allow you to go anywhere from 0 air to 100% air?  Your setup looks like all or nothing?

On my blown burner the pipe is about 2" in diameter, I have about 1 foot of mixer pipe with a 90 deg elbow that goes down to the blower. In the back of the elbow it was drilled and tapped for a small copper pipe that has the end sealed and a hole drilled crosswise (through both "sides") for the gas jet. The elbow is to introduce turbulent flow to mix the propane and air.

I've melted steel accidently in my forge with it.

[Frosty]

Welcome aboard David, glad to have you. If you'll put your general location in the header you might be surprised how many of the Iforge gang live within visiting distance.

If you don't want to learn from a couple hundred years of making gun burners then you need to copy down the specs of that failed attempt, dimensioned drawings are WAY better than photos. Make another one with dimensioned drawings and note the results. Repeat till it works to your satisfaction.

That's the best help I can suggest if you don't want to use an established design, I can't tell you how to build one or even tell you how many obvious mistakes you've made on this one. 

Don't sweat it, lots of the guys on the forum enjoy the same game. Keep us posted on your progress, we LOVE pics.

Frosty The Lucky.

[David Collins]

Thomas Powers & Frosty - Thank you for your help.

Just to clarify, it is not that I don't want to follow other people's designs, etc., it's just that I am trying to learn what the different parts, measurements, etc. are, how, why, and so on. I simply learn better doing.  I wasn't intending anyone to "spend their time troubleshooting my ideas", though in hindsight I can see that is exactly what I did.  

Anyway, I do appreciate the feedback & I'll post again, with photos & drawings, when I get it working.

Thank you,

David

[Frosty]

Okay, I'd think ill but I misspeak all the time, look back and want to go hide.

The working rules of thumb go like this: To make any kind of furnace you need to:

1, Put a flammable mixture of air and fuel in a chamber in enough quantity to make the heat required. AND do it in an appropriate manner.

2, The air fuel mixture has to be in the correct ratio approx 1 pt. propane to 17.5pts. air. There are different approaches to this, build an "on off" burner that has one output and needs no temperature control. A controllable gun type (blown) burner MUST be able to adjust both air and fuel to maintain the correct ratio.

3, Safely and effectively deliver the fuel air mix to the furnace. Most home made burners need a longer mixing tube and a commercial burner and putting a 90* bend in the tube has two positive effects. First it really aids mixing, propane is unfriendly, it doesn't mix easily so you have to stir it up. Second it makes back firing much less likely, I can get into reflected compression waves shoving the flame front away from the available fuel IF you'd like but not now. Take my word for it putting a 90 in the mixing tube is a two fold good thing.

4, The fuel jet. You want it to be relatively large, say 1/8" opening, you want the right quantity of propane to enter the mixing tube at as low a velocity as reasonably possible. You can put the same quantity of gas in a thing through a small hole at higher pressure as a large hole at lower pressure. The practical difference is velocity. You only need enough pressure and velocity as necessary to keep the blower from pushing air back up your gas supply line. It's a world of B-A-D-N-E-S-S to have a flammable ratio in the supply lines and a back burn. Think potential explosive event. Yes?

A high velocity jet is safe but tends to force the propane against one wall of the mixing tube and requires more length to mix the air fuel. What you really want is the fuel to enter the general center of the air flow at about the same velocity. Turbulence will do the rest.

Those are the gun burner basics to the best of my knowledge but I'm a naturally aspirated burner guy more than a gun burner guy and the rules are different.

Then there's the last of the cycle. Introducing the mix into the furnace chamber, forget where it's pointed, vortex vs, localized, etc. Just getting the right kind of fire in the chamber. By necking the diameter of the mixing tube down you're increasing the velocity of the mix so it MUST spend less time in the chamber and shed less heat to the furnace walls for you to use. Ideally you want the velocity of the mix to be JUST above it's rate of propagation or flame front velocity. Either term is accurate. This prevents the flame from firing back up the mixing tube, (backfiring or burning back.) While at the same time keeping the fire IN the furnace chamber as long as possible.

The type furnace we use as gas forge is called a "reverberatory wall furnace". I don't really understand the definitions of the words and what reverberate has to do with furnace walls but this is what it does. The flame heats the forge wall, the wall radiates intense IR and the IR is what heats our steel. The flame isn't very effective as a direct heat source on the irons in the fire. It's not zero but the bulk of the heating is done by IR from the forge walls.

And those are the basics. Details as discovered. 

Frosty The Lucky.

[David Collins]

This is exactly what I was looking for.  Thank you very much!  Now I got myself some thinkin' to do.  

What do you want to bet that when I figure it all out it will be exactly like all the others?  I know it will, because they are tried & true, but this is still fun.

Thank you again!

David

[Frosty]

Wrong again David. If you make ONE like all the others you'll have to make hundreds maybe thousands.

You're welcome, my pleasure.

Frosty The Lucky.

[Latticino]

A couple more minor issues regarding safety:

1. NEVER light a blown burner without the air source operating.  This can result in having the gas both at the burner outlet and at the blower (especially if you have a flow restriction at the outlet, like a pipe reducer or multi-port/ribbon burner block).  If you have a blower motor that is not "sparkproof" or even worse has heating elements, you can get combustion in the blower when you finally turn it on.
2. Disable the heating elements in your hair drier for this application and develop some way of modulating its output more than High/Low (waste gate/inlet shroud...)
3. Don't use plumbing putty to seal up the connection between the gas inlet and mixing tube.  If you don't have the ability to weld or braze that connection, find a way to do it with gas rated swaged fittings.  Leaks at that location are bad.
4. Another method for getting the turbulence that Frosty and Thomas mentioned is to have your gas port pointed back towards the blower (which will always be on when the burner is operating, right?)
5. You may want to rethink your choice of materials, depending on how you eventually use the burner in the forge.  Currently they are fine, if you keep the blower on all the time the burner is in use and afterwards until the entire assembly (burner and forge) cools down.  Otherwise there is a chance that the radiant heat from the forge will backfeed into the burner after it is shut down.

[JHCC]

You could always go with solid fuel (coal, charcoal, wood, etc) and use the hair dryer as a blower. I'm just sayin'....

[Mikey98118]

There is some method in your madness. The two reducers would probably create too much restriction to airflow, without the positive pressure of the hair blower (for safety's sake do not turn on that heater). I would normally consider the mixing tube to be too short, but that setup will cause more than normal spin for air/fuel mixing. However, that much spin may also lead to trouble with the flame, forcing you to use a longer mixing tube to calm it down, or to install internal fins for that purpose.

So, why not use the heating elements to gain a hotter flame. In the first place you will only gain about 140 F in flame temperature; a minor deal. What you will lose is a cold incoming fuel/air mixture, which is a BIG deal. that burner is going to be placed in heating equipment, where its end will become yellow hot. It only takes about half that heat to start combustion in that very flammable incoming mixture. The nice cold incoming gas and air is the only thing preventing combustion level heating in the mixing tube, where it can run merrily back down the length of your burner!!!

You show imagination, and some feel for design; that is good. In the Burners 101 thread you can find a lot of background information that you need to design sensibly.

[Frosty]

Another point for not using hot combustion air is it's less dense so you need more air to achieve a proper air fuel ratio. Using the heat elements is more dangerous, less efficient and tends to burn out blow driers sooner, they're not really meant to blow hot air for many ours at a time like a good forge session.

Frosty The Lucky.