Amal gas injectors for forge use

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  • A couple of us on facebook were mourning the loss of the information on BB about sizing Amal gas injectors for use in forges. Tim, have you got any of the information you posted from there archived that could be reposted here ?

    Yours posts were a goldmine of information on this subject :)

    Im thinking of building a slightly bigger than a 13kg gas bottle forge, wool lined, with cast refractory inner shell, and Josh is planning a similar build. I think Will Catchy built something along the lines of what we are thinking about.

  • Archiving sounds far too organised John. Between BB and Photobucket, it's gone.

    For most of the stuff most normal smiths (oxymoron?) will want to do, the standard Burlen/Amal jetting for Butane seems pretty much ideal.…ors/butane-injectors.html

    It's the Long-Venturi versions that "we" want.

    I've tried the 1/2", 3/4" and 1" Butane-jetted injectors running on Propane and they seem to give an adequate working range: from Austenitizing temperatures with the choke nearly closed (around 800 degC, 1472 degF) to nice hot welding temperatures (well in excess of 1300 degC, 2372 degF) with the choke fully open.

    The highest forge temperature I've measured on the Butane jetting is 1545 degC, 2813 degF and the lowest I've been able to sustain has been 760 degC, 1400 degF.

    I have a vague feeling that Will's welding forge might be running on a slightly smaller jet than the 150 that is normally fitted to the Butane injector, but Owen doesn't seem to find the 150 a problem in his welding forge.

    I'd need to check my jet sizing numbers for going hotter: they are somewhat smaller than the sizes for "general" forging stuff and will get into the properly steel-melty range. Probably unnecessary unless anyone is making Wootz or similar.

    I don't see a need for normal folk to go any bigger than the 1" injector. I had a 2" back at Jack's last hammerin and it was complete overkill. Will had a brief play with a 1 1/2" and decided it was too much, sticking with the 1". That said, if any of the usual suspects feel they need to build something properly big, drop me a PM or phone call.

  • I think Tim is bang on of course. I have had no problem using standard burners but should play with nozzle size just to see if I am not getting the best from my forge. what would be a good renge of jets to get, next time I make an order?

    I use 1" and 3/4" and am unsure which I prefer in a normal forge, (I think the 1" turned down is probably my fave forge at the mo.. ). I use 1" in my welding forge and have done some big 5kg billets and one 75lb sword blade.....

    the only problem I have with them is the jet like roar they make!

    I accidendly bought a 2" and will one day find a use for it Im sure. I do wonder if the next size up above 1" would be better than the one inch noise wise, or 2 x 1" turned down instead of one turned up?

    they are great burners and very turn up and downable. awsome for fluxless welding with a reducing atmosphere. A real game changer.

  • Hello Everyone, Tim, I actually joined the forum yesterday after googling for info on the Amal's and finding your posts here.

    Im building a reasonably large production forge and was wondering if the 1" would be enough, but also now thinking about Owen's suggestion of two burners turned down- that would certainly give me options.

    Ive used forges running them and the atmosphere control is awesome, my hope is that building a big refractory heavy furnace with a decent door would alow me to cut back on gas after its heated up.

    I would PM you more Q's... but i cant find how to do that on this forum...

  • Just a thought Josh, but I wonder if a forge with a door would mess with the back pressure on the burner and cause sub optimal running? The 'close the door' forges I have seen have had blown burners (but generally they have been monstrous things).

  • As have I, so you are covered for availability Josh.

    I don't think the PM facility is available without a paid subscription. If some mutual acquaintance would be good enough to either send me Luddite-friendly contact details for Josh, or can send Josh my mobile number, we can sort something out.

    Best bet is probably for me to send two of the 1" burners for you to have a play with and find what works for you. Once you are happy, you can sort out replacements and/or send them back.

    The difference between the Butane, Propane and Natural Gas burners is "just" the jetting.

    "We" have relatively unusual needs in terms of combustion equipment. Most industrial/commercial combustion processes are trying to get the maximum usable heat from the gas. This generally means a fuel-lean burn with an excess of air.

    Most combustion equipment uses both primary air (drawn in before the nozzle: in this case at the Venturi) and secondary air (air that comes in after the nozzle). Most forges only use primary air. Any secondary combustion usually takes place outside the forge as the Dragons Breath and does not contribute to the conditions in the forge itself.

    We don't want excess (primary) air, because excess air means that there is free Oxygen in the forge and this will react with our steel causing scaling. We therefore want a fuel-rich burn, giving a reducing forge atmosphere and much less scaling. The extra fuel used is an acceptable tradeoff for the reduced scaling.

    Combustion chemistry is not quite as simple as the stuff most of us did in school, where most of the reactions were effectively irreversible. At high temperatures, the reactions are reversible and reach an equilibrium point that depends on the reactant concentrations and the temperature. This means that there is not a definite point at which the mixture changes from scaling to not-scaling.

    There is more of a very wide grey band, going from very-scaling-indeed to hardly-scaling-at -all.

    Very-scaling-indeed tends to be quite close to the stoichiometric mixture: the mixture at which all the fuel gas combines with all the Oxygen in the air, leaving neither fuel nor Oxygen after the flame. This is also the mixture at which the flame temperature is hottest.

    As the mixture becomes more fuel-rich, the forge atmosphere becomes more reducing (less tendency to scaling) and the flame temperature reduces.

    At the rich extreme of the range, the flame temperature is down to about 800 degC and the burn is so rich that unburned Carbon soot is deposited on surfaces. It seems that this "should" give minimal scaling and reduced decarburization, but I have not done any scientific testing to confirm/deny this.

    Going a little richer is just about possible, but not far below 800 degC, things are not hot enough to sustain combustion and the forge will stop burning.

    The jet size determines the mixture with the choke fully open: smaller jet, leaner mixture and, as long as it does not go beyond the stoichiometric point, higher flame temperature.

    The choke allows the user to reduce the proportion of air, so it is no problem to run the burner richer than the choke-fully-open mixture.

    Adjusting the choke gives control of both the flame temperature and the "reducing-ness" of the flame (together). Adjusting the gas pressure gives control of the amount of flame.

    The physics of the Venturi means that, once the choke position is set, the mixture ratio (air:fuel) will remain pretty much constant over quite a wide pressure range.

    For our purposes, the optimum jetting is that which will give us a high enough flame temperature for our hottest intended use with the choke fully open.

    I did quite a bit of fiddling with jet sizes, starting from scratch with a lashed-up forge and some expensive temperature-measuring equipment, to try to establish my idea of optimum jetting. I came up with 30, 60 and 120 jets for the 1/2", 3/4" and 1" LV injectors respectively. I was trying for rich-but-near-stoichiometric with the choke fully open.

    When I was fiddling about with Heat-Treat forges a little later on, I was having a little difficulty reaching temperature on a longer-than-previously-tried forge and tried the 60 jet from "my" 3/4" burner in the 1/2" burner. It gave the higher temperature because it flowed twice as much gas as the 30 jet, but also gave "better" control at low temperatures: less temperature change for a given rotation of the choke.

    When I tried it in a more conventional (but small) forge, it would still exceed the 1300 degC I think of as welding temperature, so I consider it good.

    A bit more experimenting convinced me that the 60, 90, 150 progression for the 1/2", 3/4" and 1" would do anything most smiths will realistically need.

  • well thank goodness for that! I just ordered a couple of no. 60 jets for my 3/4" burner.

    It's nice to see you again Tim. As it were. Seems like years ago you were dropping off that little grinder at my place.

    John, I'd be interested to see the construction of your new forge. I've just built a smallish general forging forge but have been thinking about building an upright style forge for welding in for a little while now. What refractory are you using for the cast liner? The dense stuff i have is incredibly course and "porous" for lack of a better description after casting.

    Perhaps I should have used a vibrator on it during the casting process. (ahem.)

    All the best


  • Looks great!

    I really do fancy trying out an upright forge.

    I decided to reline the first forge I built today - picked it up and the whole lot slid out in one go. :rolleyes: Then I remembered the ceramic blanket is in our storage unit along with the rest of our stuff. Sigh.

    All the best