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Propane burner problem


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Hello, 

I am new to forges and knife making.  I built a forge and now I am trying to make the burner work.

 

The burner is like a mini Trex style.  Initially I had a propane supply issue that caused the flame to be inside the tube. I got a 60 psi regulator and an unrestricted connector to the tank.  

 

Now I am having trouble keeping it lit at different orifice sizes, and different pressures.  

 

The burner works fine if I close down the choke a little, but if I open it the burner blows itself out. I don't understand why this is happening or what I can do to fix this.  If anyone can help, please clue me in.

 

Specs:  Tube is 1.5 inch diameter, 10 inches long. Flares to 2 inches at the end. I have tried 3 orifices from 0.035" to 0.046" Different sizes do not change the problem. I have also changed the position of the propane jet forward and backward, 1/2 inch each way.  The jet is centered. 

 

Thanks,

Rich

 

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Do you have a pressure regulator?? If it is blowing itself out there is so much press

26 minutes ago, DrRich said:

 

The burner is like a mini Trex style.  Initially I had a propane supply issue that caused the flame to be inside the tube. I got a 60 psi regulator and an unrestricted connector to the tank.  

Sounds like you may be having so much pressure on the gas that it blows itself out! What pressure are you running to the burner? If the pressure is so high that it blows itself out you should smell the unburnt propane it is expelling!! 

 

Here is a link to Trex support. https://hybridburners.com/new-help.html  As with any burner it is the mixture of gas and air that makes the right flame! There is a link to support on that page as well!!

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Did you build that, or buy it?  Looks like there may not be enough air getting in, making it pretty choked at best.  The 0.035 taper MIG tip is the ideal size for this, but you may want to try an 0.028" and see if that helps.  It should be stable at around 7 PSI.

 

Also: Are you trying it in a forge, or in open air?  A properly tuned burner for a forge will not run right in open air.

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60psi is WOW!  First question is, this is a high pressure regulator?  Looking to clarify if you turn down the pressure and it goes back up the tube there's no push power from the regulator.  Most normal regulators for propane are flow regulators. 

 

leads to other questions, how many psi at the regulator? A quarter turn? Full turn of the valve? 

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I made this burner from scrap parts.  

 

The regulator goes up to 60 psi but I was told it did not regulate flow rate.  I started low when the flame would back up the tube and advanced it until it did not, but then it blew itself out without the choke. 

 

Right now I am using it on the bench until I get this resolved. 

 

What if I backed the nozzle out to capture more air before I replace the orifice? I suppose I could replace the nozzle with a thinner pipe. 

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You may have missed this tip, but most venturi burners don't work well in open air.  They need a bit of back pressure to burn properly.  Build a small enclosure for it (some bricks in a stack will do) and then try it and see how it works.  Venturi burners are all about balance, you need the right amount of air and the right flow of gas.  The fact that this is a home build troubles me, not that you can't do it, but if you have something basic wrong, it may never quite work and you could spend a long time trying to tune a burner that is just never going to work.

Having said that, I am not the venturi guru, so take what I've said with an appropriate amount of salt.

 

g

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Ok, Doc, I was going to let it pass but what metal is the tube made from?  It doesn't look like the bronze fitting that is inside it but it's definitely not black pipe.  Are you certain that the alloy will hold up to normal forging temperatures let alone welding temperatures?  It might just be the light on stainless steel but it looks like a cupric alloy.  Not trying to insult your intelligence, just trying to keep everyone safe.

 

Doug

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Thanks for the tips.  I will try the back pressure. Eventually, this will be in my little forge (see picture).  Its a stainless steel trash can lined with insulating brick. 

 

Doug, the tube is 1/16" steel. The color is a trick of the light but it would be a cool color to have.  It was an old steel post or something. I burned off the paint and sanded it. When I was burning off the paint with a propane torch the steel did not even turn red despite the high heat.

The flair is a stainless steel reducer intended for a car exhaust system. 

 

Rich

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If your "flare" isn't a gentle cone inside with the profile of a standard paper cup, it's not doing you any favors either.  If it's just a step into a larger bore, or a bell or hemisphere, it is working against good smooth combustion.  

 

I still think you just aren't getting enough air into the thing.  The tip of the gas injector should be about in the middle of the air inlets.  So yes, if you can back it out that may help, but like Geoff said, running it on the bench isn't telling you anything useful.  It has to be in the forge.  

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Alan,

 

Thanks for the info.  I will try that.  I went with the reducer for a flair because I saw a lot of burner designs that used a reducer used on black pipes and it was bell-shaped.  Do they have a problem with the shape?  

 

Rich

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Even if that burner woks properly, I don't think it will ever be enough for that forge.

1)  I don't know what the dimensions are (cubic inches) but it looks huge.  Unless you plan on forging parts for steam engines, you don't need nearly that much space.

 

2)  Fire brick is a refractory, not an insulator.  Fire brick acts as a heat sink (thermal mass) which has to be heated before you can begin to forge and it has to be kept hot while you are forging.  A forge like this is a gas hog..  A forge like this is the sort of thing you'd see in a production shop running 24/7.  It's not what a hobby smith needs.

 

if you want to keep this I would do the following

1) punch a hole in the back.  A long piece can then fit into your forge.  In addition, your burner (when/if you get it working) will work better.

2)  Line the whole thing in at least an inch of ceramic wool with a hard top coat.  2 inches would be better.

3)  Design a way to close up most of the front.  A table with a stack of brick works, and makes the door easy to reconfigure.  Some sort of hinged affair lined in wool would work.  You only want an opening just a bit bigger than the work piece.  That way you aren't bleeding heat out of the box, and in the hot part of the year you aren't heating your shop.

Generally the rule of thumb for venturi burners is 1 burner per 350 cui.  My welding forge is approximately 900cui and I heat that to welding temps with a single fan (150 cfm), 2 output common rail burner running at about 3 psi.  But blowers and venturi are different animals

 

g

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2 hours ago, DrRich said:

Do they have a problem with the shape?  

 

Yes.  An actual burner flare is a smooth taper between 1:12 and 1:14, with a tiny step at the small end.  You can buy one or machine one yourself, or, and this is the cheapest and best way to do it, just make it in the refractory lining of the forge.  

 

The bell reducer just creates turbulence and a spot for pre-ignition.  If you see one on a burner design, the maker had no idea what they were doing.  This is fluid dynamics.  The tiny step on a true burner flare introduces a tiny bit of turbulence that acts as a flame holder while the smooth expanding taper guides the developing flame front smoothly into the forge, not unlike a rocket nozzle.  Unlike rocket nozzles, which are parabolic to concentrate thrust, the linear taper creates a nice spread of the flame front.

 

Bell reducers and simple step-ups do none of this, they just introduce a big ol' ball of turbulence that screws with proper burn.  Venturi burners are finicky enough to tune properly without introducing crap to the system.  People who don't know any better hear about flares and think maybe a bell reducer will work.  You can force them to work, of course, or the idea wouldn't keep spreading, but they are a hindrance rather than a help.  I think they got the idea from an older burner design that used a bell reducer as the air intake.  That does work, but not as well as newer designs.  Same with trying to tune a burner to look like a torch flame in the open air rather than in a forge.  You can make it work, but it won't run right in the forge.  

 

All of this is why a lot of folks like a blown burner.  Simply dump gas into a tube of fan-blown air.  No MIG tips, no tiny orifices, no careful balance of static choke and PSI.  Just a needle valve on the gas line and a sliding choke on the air supply and you can tune it however you like.

 

Final tip: 99% of forges and venturi burner designs you see on eBay, and at least 75% on YouTube, are utter crap designed solely to separate you from your money.  They kinda sorta work, a bit.  They can get things hot, sure.  But they are not efficient and in some cases are patently unsafe. 

 

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Edited to add: The smooth tapered flare is not necessary, it just spreads the flame a bit and tends to create a more stable flame front.  A straight pipe will work just fine in most cases.

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The inside of my forge is 14" wide at the floor, 17" long which would give it a volume of about 425 cubic inches.  I have a brick wall to cover the front.  I used bricks that were intended to be insulating (the first picture), not the fireplace brick (the second picture).  Since I have not used it yet so I don't know just how insulating it is or not. 

 

Thanks for the info about the flair too. 

 

Rich

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The pictures went in backwards, but I think you know what I mean. 

 

Rich

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Sorry, I used pre-completion measurements and had a typo on the volume.  The inside is 10.5 inches wide and 14 inches deep, giving a volume of about 610 cubic inches. 

 

Rich

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The insulating bricks are okay, but you may want to give them a thin wash of refractory mortar like Satanite and/or a coat of IR reflectant like ITC 100 to get the best results.  They are far better than the hard firebricks, heat-transfer wise, but are still heat sinks rather than true insulation.  I have a little forge made from two of them hollowed out that I use for a lot of blade work.  The shell gets plenty hot.

 

In the event you ever want to forge-weld in there, you definitely need a topcoat of something.  Flux eats soft brick like gasoline on cotton candy.

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40 minutes ago, Alan Longmire said:

like gasoline on cotton candy.

This makes me want to hang out in your shop watching you go at it more than anything else up to this point (and that is saying something).  :lol:

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I assume that this is  the proper shaped flare.  Next question, if you guys don't mind, is:  How big is the gap supposed to be between the start of the flare and the main tube?  I heard it was in the neighborhood of 1/8 inch.

 

Thanks, Rich

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I'm sure that will be fine.  I do have two observations (or two more, I guess)

 

That looks like more than 1/12.  It will probably work just fine.

 

You may be overthinking this.

 

I would do 2 tests on your setup.  I would build a little test enclosure, bricks will be fine, open at both ends.  First, take the flare off and see if your burner will work inside the box.  Then try it with the flare.  If it won't run in either condition, it's time for a redesign.  If it works better one way or the other, then you've gotten close.

A thing to remember is that burners like this have been around for at least 200 years (though not at this scale) and they work pretty well, so just get it built and stop fussing over it.

 

In my collection of things I have a number of designs for burners.  Sandia Labs built a burner with 3 inlets, each one passed through an exhaust stack so that the waste heat could preheat the gas, giving you more bang for your buck.

 

I also have an article about a guy who built his burner nozzle out of heavy gauge stainless and then put a water jacket around it.  This was to prevent heat erosion of the burner tip, and it cost a fortune.  Sure was techie, though.

 

The very first gas forge I ever saw was in the 1980's.  The body was a tube of expanded mesh steel.  The back end had an old Ford hubcap wired over it.  1 wrap of uncoated Kaowool.  The burner was a piece of black pipe.  About half way down the pipe a hole had been drilled and that was direct connected to the gas tank.  The blower was a second hand hair drier.  Worked fine and it glowed like a pumpkin.

 

Do you want to build the perfect forge burner?  Or do you want to beat some steel?  Don't get me wrong, I understand the lure of building tools, and if that's what you want to do, that's great.

 

Geoff

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What Geoff said.  Looks a little too flared from the outside, but the inside may be fine.  Like I said, it's not really necessary anyway.

 

Follow Geoff's advice to see how it works.  Might be fine!

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I was planning to cut the length of the flare but I need to decide first if there needs to be a gap between the main tube and the start of the flare.  Many pictures  have seen don't have one, but some do. 

 

Rich

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