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Blue flames and venturi burner

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by JoshEarl, Oct 2, 2008.

  1. JoshEarl


    Sep 30, 2007
    I'm setting up my new Darren Ellis forge this week, and right now it's running really fuel rich. Blue flames are shooting about six inches out of the front opening. I keep finding sources that say there should be almost no flame coming out of the forge, so I want to get it tuned properly.

    The burner is a venturi burner with a flare on the tip.

    What should I be looking at in terms of tuning it? I think I need more air, but I'm not sure how to get it.

    Adjusting the regulator doesn't change the atmosphere. I get slightly more flames with the gas cranked up, but there's only a 30 percent or so difference between full throttle and the lowest setting.

    Darren has photos of the burner and forge on his site, www.elliscustomknifeworks.com. I got the 6-inch horizontal model.

  2. Ed Caffrey

    Ed Caffrey

    Jul 23, 1999
    In my opinion that is ideal for bladesmithing. The flames burning outside the forge indicates that there is a fuel rich environment, what we often call a "carburizing" flame. It happens when the combustion has consumed all the oxygen inside the forge chamber, and the excess fuel comes outside the forge and gets its oxygen for the surrounding atmosphere. This type of environment minimizes scale, and makes for a much better forging, and forge welding environment.
  3. JTknives

    JTknives KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 11, 2006
    I thought a blue flame meant a lean mix and a yellow flame meant a rich mix, but i guess with enough oxygen you can turn the yellow flame into a blue one.
  4. Dan Zawacki

    Dan Zawacki

    Sep 23, 2006
    blue flame is fuel rich, yellow is fuel lean. Adding oxygen will turn blue to yellow.

    You need to adjust the jetting. Thatis, the amount of gas passing through venturi which is where the air gets picked up. You may need to retreat the gas ject a little from the burner end towards the inlet end of the venturi. Failing that, holes can be drilled along the back side of the venturi. Being a side arm type of burner, the air comes in from only that one big hole near where the gas jet ends. If you back the gas ject off, farther towards the back of that big hole, it should allow the gas to pull a little more air on its way by. If that doesn't work, it's because the one big hole (venturi) is drawing all the air it can, in which case, you need to increase the volume of the venturi, either by drilling a secondary venturi, or grinding this one out to a slightly larger diameter. If you do that, you can drill and tap a hole for a sheet metal flap that you can move to cover the hole to an adjustable degree, which should give you a very easily adjustable atmosphere.

    Also, your burner nozzle may require some adjusting. try backing the burner body out or moving it in in relation to the flare / nozzle position.

    Also, you could try adjusting the volume of gas flow. Volume and pressure are independant, and high pressure with small volume is what makes venturi burners work. You could try a smaller orifice for the gas jet, and then drill that smaller orifice out a little at a time until you get the flame you're after.

    Generally speaking, I agree with Ed's post above, in that a reducing atmosphere is a great place for forging your blades, as it won't allow the steel to pick up oxygen while it is inside the forge. However, a reducing atmosphere will not let the inside of the forge get as hot as it could with a stoichiometric mix of fuel and air (neutral flame) as some of the potential energy is being released outside of the forge body. What that translates into is a lower interior temperature and gas being burned off faster than is necessary. I like to preheat my forge with a neutral atmosphere, then adjust it to be just slightly rich while forging, as a slightly rich flame will maintain the heat inside the forge nicely, but it really has to work to get the forge up to peak temp.

    Of course, unless you're welding, and I mean a lot of welding, or breaking down some pretty big stock, peak temperature may not be at all improtant. I like to run my forge quite hot, as I find it makes the heat more even from front door to back, which makes my job easier.
  5. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    Actually there is a little truth to both arguments. Flame color is in indicator of two things
    1) Flame chemistry - Sodium ions in the flame will cause it to be yellow. Other ions will cause blue, green, and other colors. This is not an indicator of anything that effects forge atmosphere.

    2) Flame temperature - As the flame gets hotter, the spectrum radiated goes from yellow to blue. A pure propane fuel flame (no pre-mixed oxygen) will burn at about 1000F, and be a flickering yellow flame. Add in oxygen and it will go from pale yellow, to dark blue to bright blue, to very light (almost invisible) blue, at around 560nm. This indicates a properly mixed flame. The more the oxygen, the more the blue (Sorry Dan).

    BUT, In a forge chamber, pre mixed air/fuel burns.

    With a balanced mixture and volume the fuel all burns in the chamber with no visible flame. There will be no "Dragon's Breath" ,since all the fuel is consumed before leaving the chamber - this is a balanced, or neutral atmosphere, and is good for general forge work.

    Add a little less fuel and the there will still be no flame coming out the forge port, but there will be extra oxygen in the forge chamber. This is called an oxidizing atmosphere, and is usually not a good thing in a forge .All this will give you is a hot forge.Damascus makers sometimes don't care about anything but how hot it is, and use this type of atmosphere (but would be better off with a neutral chamber).

    Add a little extra fuel, and the flame rolls out the forge mouth (Dragon's Breath). The chamber has an excess of fuel and a dearth of oxygen, so the extra fuel burns outside of the forge. Since this fuel is mixing with air at the time of burning, it may look somewhat yellowish in color, due to a lower burning temperature. However, if the fuel is coming out of the forge in a superheated state (from a 2500F forge chamber) it will burn quite efficiently, and look bluish.The flame color only indicates whether the forge is up to heat of not.
    This situation is called a reducing atmosphere, or carburizing atmosphere, and is just about perfect for most forge work.

    The balance you want is to have a neutral forge (hottest) and just go slightly past that with a little extra fuel.You should see some light flames from the forge port.

    HOWEVER ,those forges with roaring flames shooting out two feet, are not indicating anything but the fact that there is too much fuel pressure,mixed with too much air. Turn down the air and the fuel until there is just enough fuel/air in the chamber and you will have a BALANCED and EFFICIENT forge. This is where a blown forge is much superior to a venturi forge. The air/fuel mix can be completely controlled down to below 1PSI, if desired.

    So -Blue is good, but it doesn't exactly tell you the fuel mix.
  6. Sweany


    Dec 3, 2002
    What Stacy said,
    You cain't carry 10 lbs of potatoe's in a 5 lb sack.

    Flames coming out the door only heat the smith.
  7. JoshEarl


    Sep 30, 2007
    Thanks, guys. This is helpful so far.

    Stacy, the blue flames start immediately, not when the forge heats up. Basically it looks the same all the time, with 6" to 9" of blue flame shooting out. So I don't think the issue is superheated gasses, but extra fuel like you mentioned. I'm looking to get a slight reducing atmosphere while minimizing CO output.

    I think I'll start by trying to adjust the flare positioning and the position of the nozzle in the tube. I'd like to avoid modifying the burner itself if possible.

    It's also sputtering a bit when I drop the pressure, so I am pretty sure that it needs more air.

  8. cdent


    Aug 28, 2005
    Hi Josh,
    At low pressure, it might need less air. You might try blocking half the intake with maybe a piece of tape temporarily and run it at maybe 4 psi. Put the mig tip right where Darren suggests in the body, and the burner will be choked down a bit for your smaller chamber size. If it works for you, a swinging plate choke over the air intake may be a simple worthwhile modification.

    Best of luck with the project, Craig
  9. Dan Zawacki

    Dan Zawacki

    Sep 23, 2006
    Wow, neat!

    Great info, thanks Stacy!
  10. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    A venturi burner is tuned to get the proper mix. The simplest way to set it up is to clamp it in a vise or such to run it out of the forge. Obviously this has to be done in such a way as to not ignite the place. Turn on the gas and ignite the burner. Adjust gas pressure to get a steady flame.Open the choke plate about 3/4 the way. Move the gas jet in and out (if possible with the burner you have), to get the smoothest flame. Reduce the pressure until it starts to sputter. Adjust the choke and jet position again. Keep this up until you have the burner running at the lowest pressure that it will run smoothly at. Then adjust the air choke to get the best looking flame. Now the burner should run good at higher pressures, as well as at its lowest pressure. The next step is to re-install the burner and adjust the flare position for the best chamber flame.It may need a little adjustment of the choke once re-installed to compensate for any back pressure.

    When all this is done and running right, make a few knives. After selling a couple, build a blown burner and keep the venturi burner for some other project ( or when there is a total blackout for days). You will never regret going to a blown forge, especially if you are going to do Ht with it.
  11. WadeH


    Apr 24, 2006
    Why is a blown forge better for HT? The one I am building now I plan to try to HT in it.
  12. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    With a venturi, you have to have a high enough gas flow to draw the air in. With a blown forge, the air and fuel are both under pressure (low pressure). You can turn down the gas and air until the flame is barely there, if you so wish. Most venturi forges are great for forging, but are hard to get down to below 1500F and hold there, as is needed for good HT control. Sputtering ,back-flame (the flame burning in the burner tube), and flame-out are a real problem as the gas pressure drops. Also, the fuel consumption is lower on a blown forge. They typically run at 2-5 PSI to do what a venturi does at 5-15 PSI. A blown forge can be run at a higher rate than a venturi,too. This is a plus for casting/melting operations, and for damascus welding. Finally, a blown forge can be adjusted to attain a desired forge atmosphere by simply turning a knob or two.

    Don't get me wrong, a properly adjusted venturi is fine. I do most of my general forging on a well set up NC Whisper Lowboy. But for a controlled forge (PID controller and solenoid valve), or one that you want to try and hold at a certain temperature ( adjust the air and gas flow rate), a blown forge wins hands down.
  13. JoshEarl


    Sep 30, 2007
    Stacy, great info. I may build a blown burner for this one at some point, because I will be heat-treating with it.

    Last night I put the ITC-100 coating on the inside, and also moved the burner further back in the burner tube. The flames were still coming out at about the same level, but now they had orange mixed in with the blue. So I think I was getting a little more air.

    I'm not sure how to move the gas jet back in the tube. It looks like it should unscrew a bit from the pipe, but the parts don't move easily. I think it's time for me to take it apart and learn how it's put together...

    The flare on the end is secured with set screws, so should I pull the collar back or push it forward to get a leaner mix?

    Thanks again for all the help.

  14. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    Don't worry about the flame color telling you much. A blue flame is fine. It is the way it is burning and at what pressure that you want to set. Don't mess with the flare much. The gas jet pipe can often be moved in and out to adjust the venturi effect. You want it to pull as much air in as possible. The air can then be adjusted with the choke plate (assuming here is one). If you have questions about adjusting the burner, call the maker.

    BTW- I can't see your burner, but the orange flames probably mean there is not enough air.
  15. JoshEarl


    Sep 30, 2007
    Well, in my time-honored tradition of soliciting advice, picking out a few things and ignoring the rest... I pulled my burner out this evening, clamped it to a piece of I-beam, and tried to figure out how to adjust it. The first time I lit it, the flame had a green tinge to it, and it was pretty big. Ron Reil's site has some photos on it, and the flame looked a lot like his image of a burner that's running rich.

    Nothing seems to move on this burner except for the flare, so I decided to try messing with it. The flare was set right at the end of the burner pipe, so I slid it back down the pipe maybe 3/4" of an inch. The flame looked more blue and smaller, and it ran more evenly through a range of pressures.

    I turned it off and tried sliding it back another 3/4" or so. This time it wouldn't even light, so I moved it forward 1/2" or so, and I was getting a smaller blue flame. Nice.

    Reinstalled it in the forge, and the flames coming out of the front seem manageable. There is still more flame than I would like, but it's a lot less intense. I think I now have a nice reducing atmosphere. It doesn't start "huffing," as Ron puts it, when I drop the pressure. I think I'll be able to do a nice job with my heat-treats, as it seems to run well in the low-orange range.

    Thanks for all the help--especially Stacy.


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