D.Rotblatt

Ribbon Burner Backfire Question

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I've built my first forced air ribbon burner and put it in a new forge.  It runs beautifully...except one thing.  I was hoping I could run it low enough to get an even 1550F or so temp in the forge for heat treating.  When I run it low, maybe 1 lb pressure (the jet is three .052" holes), fan closed up, and gate open only 1/8" and a slightly reducing flame it holds a nice heat treating temp (if a little high), but after a 10 minutes or so it backfires (the flame running back into the burner), I turn it up a little bit and it runs for awhile and then it backfires again, I turn it up....etc.  

In looking for answers, I found one person that said that ribbon burners do that, they backfire after awhile and you have to turn them up.  In all my researches before the build I never found mention of that happening.  Is this a common problem with ribbon burners?  Is there something in the design?

Any thoughts????

The burner is a standard build; 19 holes, 2.5" pipe leading from the fan to the burner.  I'll post a pic:

IMG_6716.JPG

IMG_6710.JPG

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Yes, this is quite common with ribbon burners.  When there is a decent volume of fuel/air moving through the plenum and out through the ports in the burner it takes some heat with it on the way.  When you decrease that flow there isn't as much cooling effect and the heat from the forge starts to creep back into the burner head towards the plenum. 

Your fuel/air mixture will ignite at the first point in your system where the temperature is high enough and the velocity of the flow is slower than the flame front it produces.  So, at low volume of flow you will start to see ignition happen right at the surface of the block with no flame lift at all and then it will ignite inside the plenum.  Ignition inside the plenum produces a burst of higher velocity exhaust which will normally take the flame front back out to the burner block, but only briefly.  Increasing the flow a bit helps with the cooling effect and keeps the fuel/air mixture velocity high enough so the flames don't retreat back into the plenum.

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You may be able to maintain the temperature you're after by changing the size of the openings on the ends of your forge.  This is where a some bricks that can take the heat can help out. You're trying to find an equilibrium point where you have a stable flame on your burner and you're allowing enough heat to escape to maintain your desired temperature. Yes, it's some heat (fuel) wasted, but if you're after a specific temperature inside the forge and you want a stable flame on your burner your options to get there will be limited by the equipment you are using.

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It seems pretty simple to me. If you want to back off your gas the maximum amount, after the forge heats up, then you will need a smaller burner head to keep flow speed adequate to prevent back firing. If you want a fast start up then you'll have to put up with less fuel efficiency after start up. It isn't a problem, but a choice to be made.

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With Mikey's solution in mind another option would be to plug a few of the holes in your burner to have the same effect.  Since you are running a blown burner you do not have the same balancing act to deal with as naturally aspirated burners.  That would keep you from having to cast a new burner and you could work stepwise towards your goal one hole at a time until you get what you want.

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Alternately, has anyone built a forget with two smaller burners, two gas valves, and probably can share the fan. Run them both for welding, and just one for heat treat, etc.?

 

Or, change the fan valve to divert to a cooling jacket around the plenum?

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You have a blown burner, so you can control the temperature by varying the mixture. Leave the air where it is happy and increase the gas.

If you richen up the mixture significantly, the flame temperature will reduce. If you richen it up enough, you can get it down to HT temperatures and this will allow long soak times at optimum temperature. You'll use a lot of gas and you'll need to run it outside, since the rich mixture will produce a huge amount of Carbon Monoxide (and death really doesn't improve your knifemaking). You'll need to reduce the size of the openings too. If you don't, the hot exhaust gases will flow out at the top and draw air in at the bottom. This extra air will cause the temperature to increase. 

As the mixture gets richer, the flame speed will get lower and the likelihood of a backfire due to the flamefront travelling through the burner block will reduce. As the flame temperature comes down, the heat input to the hot face of the block will come down and the likelihood of a flashback due to the back of the block reaching autoignition temperature will reduce. By keeping the airflow relatively high, and increasing the gas flow to get a cool-burning mixture, you will keep the cooling mixture flow through the burner block high and keep the probability of a flashback low.

The extremely rich mixture will help to prevent scaling in the forge and may even help to reduce decarb. If you try this, use a thermocouple and do it outside. The thermocouple is mainly because judging temperature by eye outside is unlikely to give good results and the do it outside part is non-negotiable.

Ribbon burners are very good for certain things, but they are not a magic bullet. Their biggest advantage is that they give short flames. To a large extent, flames are scalable. If you have a particular mixture composition and a particular mixture velocity, your flame will be X times as long as the diameter of your burner port. If we take an X of 4 (purely as an example), we could use a 2" single burner and produce a flame 8" long. Alternatively, we could put the same amount of the same mixture through a multi-port burner (ribbon burner) with 28 holes, each 3/8" in diameter and produce flames 1 1/2" long with the same burner port velocity. It is much easier to build a reasonably small forge that keeps the unburned mixture away from the workpiece if we only need an inch and a half to complete the burn and this is the main reason ribbon burners are used.

Heat-treatment requires much less heat input than welding, or even forging. I have built forges (using commercial Venturi mixers) that can do everything from HT to welding in a single forge, but I really would not recommend it. For the same time, effort and money, I can build a conventional forging/welding forge and a dedicated HT forge, transferring the burner between them, and get much better performance.

For the extra cost of a second, smaller, burner , the HT forge can be made to work very well indeed. I have built several electric HT ovens and am convinced the HT forges are better for a hobby knifemaker working in Carbon steels. The electric ovens are undoubtedly better for stainless steels, particularly those that benefit from ramp/soak programs. They need less attention in use, so are better for the professional as well. For the hobby maker though, the reduction in scaling (and maybe decarb) with the forge is a definite advantage. 

For my purpose-built HT forges, I normally use a 1/2" burner in an 8" diameter, 20"-22" long chamber and it is ample. The chamber is a 2-foot long 10" thinwall pipe lined with 1" of kaowool and has 1" kaowool board disks pushed in for the ends.  

 

 

 

Edited by timgunn1962
Miscalculation

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Thanks for all the replies!

Buzzkill and Mikey; Excellent thoughts!  Smaller burner/holes or less holes. The easiest is to plug up some holes - a little ceramic wool in a few of the holes.  I'll give that a try and see what happens.  As it is I have much to much pressure from the fan, so back pressure will not be a problem. 

Timgunn; nice discussion of the problem.  I already tried a richer mixture this morning (I always use a pyrometer) and got a steady 1550F +/- 10 with a much more even heat inside the forge. I have yet to see if this will work on a long burn.  I've used the rich mixture technique on my previous forge many times.  My studio has a 6.5' tall 4' wide window next to the forge and an 8' wide sliding door across from it - plus a ceiling fan.  Plenty of cross draft = no CO poisoning.  I really wanted a dual purpose forge, it's not the money, it's the space, I just have too much stuff.   Want to do fluxless forge welding and HT in the same forge (been playing with it).  If not I'm modifying the forge now to take a removable kiln shelf.  The shell of this forge is a small helium tank I added about 6" length to, 1" of Kaowool, lined with Zircon heat "reflector" about 1/16" thick.  So the inside is 7.5" diameter and 12" long with a flat bottom.  I should be able to HT a knife up to 16" long or so.  For swords I have a different kiln.  I also have an electric kiln with DIY PID controller, It's also about 12" deep and wide (old ceramic kiln I got for a song).  I use it for HT on stainless knives, but there is a depth issue and I can't get a reducing atmosphere unless I modify it further to take an inert gas intake.

In regards to a second forge just for welding; if I really get into doing a lot more forge welding, I'll build a vertical forge - just cause I haven't ever used one and I love to experiment.  I have a whole roll of Ceramic wool, some old helium tanks for shells, and several Mikey burners I built years ago (I think one of them is a 1.25" burner....that would be more then enough).

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Shrike - good thoughts too.  Would love a simpler fix then building two burners, though I'll keep that in mind for future builds.  Perhaps the same plenum with a divider lengthwise and two intakes - so the top and bottom row(s) can be turned on independently.  Fairly easy build.

The plenum is not getting hot, it's the actual refractory where the holes are that's in the forge.  The face gets hot, and because the gas is set to such a low volume, it is burning back into the holes - it's not a single backfire, but a phlegmy whistle (don't know how else to describe it). 

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Good Morning,

Like what Buzz said above. If you have to turn down the gas pressure to decrease the heat, make a metal slide to block off some of the nozzle ports. To turn down the gas pressure, you also must turn down the air volume. If the flow is too low, the flame will start to burn back up into the Ribbon burner head, causing your backfires. If you restrict the quantity of available burner ports, you can turn down the heat. You must still have sufficient flow to keep the flame face working. The whistle is the flame burning back up your fuel line.

To do controlled Heat Treating, this application of a Forge may not be ideal. Electric Heat Treating Oven/Furnace will still be the choice of stability. Stable temperature for Tempering is most desirable.

Neil

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Thanks Swedefiddle, I'll play with it in the next few days.  I like the idea of plugging some holes and seeing what that does.  It's really a no-brainer, but since the idea didn't even drift by in the back of my mind, what does that make me????  :huh: Was focusing on making a wheeled cart for the forge today (and cleaning....endless cleaning).  

Ideally a molten salt bath would be the way to go for HT...but I don't do enough to warrant that.  

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I agree that plugging up some holes is the easiest way to go. You'd be surprised at how few holes you need to have a nice hot forge. I wound up plugging about half the holes on my burner and still have to throttle it back or I have welding heat. Fewer holes save fuel too. 

With your blower I would plug the bottom row. That way you maintain a better swirl.

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OK.  First plugged the lower row of holes.  Burner was not as stable as it was before.  Unplugged the outer two holes on the lower row, and either I got better at judging the air/gas mix or it was more stable.  Either way, worked well with 11 holes.  Got an even 1550F with a reducing flame for 10 minutes or so - no backfire, but I'll have to do a longer test later. Like an hour or so.  I then turned it up.  Damper opened 1/4, fan wide open and it will get to welding temps easy I think.  I'm now cleaning and rearranging the studio to make more room, so I'll try some welds in a week or two.

Problem Solved!!!  Either plug up some holes or run a very reducing atmosphere (with doors open).

Thanks all!

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