Jason Fry Posted July 8, 2022 Share Posted July 8, 2022 I've got a forge shell, vertical, 2" thick cast liner. 250 cubic inch interior. I have a blown burner on 1" pipe. I know how to basically tweak the gas and air to get a bit of dragon's breath and a neutral to reducing atmosphere. I've changed from a 3/4" venturi that will weld clean stacks but struggles on long soaks. I'm changing burners to get closer to yellow/white than orange/yellow. My question is this: What happens if my blower is just too much? Choke down the blower and the gas a bit more? Leave the air alone but mess with the PSI at the regulator and the flow at the needle valve? Sorry I haven't been around in quite a while. Work internet was blocking the site. Now I work from home Quote Link to comment Share on other sites More sharing options...
Latticino Posted July 8, 2022 Share Posted July 8, 2022 There are a couple of different ways to reduce the airflow from a blower. If the motor is capable of speed control you can reduce the RPM. On most blowers, but not all, you can "ride the fan curve" by increasing the resistance on the blower outlet with a valve or slide gate of some sort (some can go unstable at elevated external pressure). On some blowers you can reduce the airflow by restricting the inlet (but this can be a problem for others, overheating the motor). You can also add a waste gate that directs some of the air elsewhere (possibly to a slot diffuser at the opening of you forge to direct the dragons breath upwards). Lots of options, but first you have to ask yourself: How do I know I have too much air? How much fuel/air mixture is needed to burn to maintain a stable flame at the forge temperature I need for the forge I built (changes at different interior temperatures)? Should I reconfigure the burner outlet before reducing airflow? Quote Link to comment Share on other sites More sharing options...
Jason Fry Posted July 8, 2022 Author Share Posted July 8, 2022 I kinda get that. I kind of figure I can set either the gas PSI and adjust the needle to get it right, or choke down the air and adjust the needle to get it right at a lower temp. What I don't get... what are the limitations? If I'm pushing too much gas and air into too small a space, will it just not tune right until I turn it down? I assume some trial and error to find a setting that holds a consistent high welding heat, but also some trial and error to set a regular forging heat? Quote Link to comment Share on other sites More sharing options...
Buzzkill Posted July 8, 2022 Share Posted July 8, 2022 If you have too much gas and air being pushed into the forge anywhere near the correct ratios you will get a large amount of dragon's breath out of the openings. Too little will cause a flameout or just low temps. Too much air (not enough fuel) will cause oxidation (forge scale) to form while the piece is still in the forge. Too much gas (not enough air) causes excess CO to be produced, and in extreme cases will limit the temps you can reach. I haven't run a blown burner in a while, but I always tuned by eye and ear. Try to find the spot with the loudest flames, which should be a medium blue color, and then I'd back the air off just a hair so I'd stay on the rich side of neutral to avoid scale forming in the forge. If you have a pressure gauge and your gas input line is restricted enough to show pressure, you may want note the pressure where you got the temps you want for general forging and welding. After that you can set the gas pressure and use the air controls to tune the flame and you should end up at the same temperature you had before. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted July 8, 2022 Share Posted July 8, 2022 An important part of heating the forge is the dwell time of the hot gasses. They don't transfer their BTUs *immediately* so too much gas/air into the forge is just wasting fuel as you get the hot gasses exiting before they can dump their heat. Quote Link to comment Share on other sites More sharing options...
Jason Fry Posted July 8, 2022 Author Share Posted July 8, 2022 (edited) That also makes sense. No point in all the hot stuff burning out the doors. Thanks for that thought. Trial and error Edited July 9, 2022 by Mod30 Excessive quoting Quote Link to comment Share on other sites More sharing options...
Latticino Posted July 8, 2022 Share Posted July 8, 2022 2 hours ago, Jason Fry said: what are the limitations The limitations on your burner operation are also related to a number of factors, so it isn't a simple answer. The forge interior temperature directly influences the flame front speed (the velocity of the flame burning back towards the fuel/air source at the outlet of the burner). You want this flame to be stable, and located right at the outlet of the burner. As I've noted on numerous occasions, it is a balance between the velocity of the air/gas mix at the outlet (set by the volumetric flowrate) and the flame front speed. If it is too far off, it can easily "flame out" and, hopefully reignite (forge will appear to be "huffing". If too far into the burner outlet you can get preiginition in the mixing chamber (anything from "popping" to small explosions). Burner outlet area may need to be adjusted to achieve a "sweet spot" (though there are also burner outlet designs that work at a wider range - flame retention outlets and multiport outlets). As is pretty logical, a larger outlet will support more fuel/gas mixture at a lower flow speed, typically leading to a more "powerful" burner, but one that may not be able to be turned down as far without burning back into the mixer. You also have to be careful, again for flame front speed reasons, when turning down your forge from high to low fire. My recommendation is always to turn off the gas and just run on the blower until the forge drops to the desired temperature to avoid having the flame burning back faster than the lower air/fuel mixture can support. Overall length of flame can also be a factor (as Thomas stated), with how long the dwell time is inside the forge. Longer dwell leads to more energy transfer. Choice between oxidizing and reducing flames have been well described by Buzzkill. There are also characteristic forge interior sizes that work better with different flame configurations (both forge interior volume - for proper combustion, and forge width opposite the burner outlet - for properly developed flame length). I expect the 1" burner may be too large for your 250 cubic inch forge, but that is hard to tell without experimentation. You can always choke it down to a 3/4" outlet with a reducer to see. Best and most efficient path to increasing forge temperature IMHO is to ensure you have good operable doors. This is easier with a blown burner, so you are already on the road to success. Good luck. Quote Link to comment Share on other sites More sharing options...
Jason Fry Posted July 9, 2022 Author Share Posted July 9, 2022 Well, it worked. Smashed some ball bearings, forged some mild, and did three billets of damascus this morning before I ran out of propane. It is hard to judge temp by color, and you can't see the dragons breath, when it's SUPER bright sunlight, but that's what I had. Even so, by color it was getting a couple of shades hotter than it was before, with not malfunctions. Scale wasn't out of line compared to before, and I did manage to catch a nice piece of scale in my shoe Quote Link to comment Share on other sites More sharing options...
Frosty Posted July 9, 2022 Share Posted July 9, 2022 So long as it's not forming scale IN the forge it's not too lean. Hot steel will always scale up in ambient air when you pull it out. Having to work in direct sunlight stinks, you'll learn to judge how the metal is working by sound and feel so you'll know when to return it to the fire. When it's ready to come out is tougher, try running your forge at night and judge how hot it CAN get. Pay attention to how the burner sounds and what the gauge says so you can repeat that temperature when you can't see the color accurately. Once you have a good idea of how hot the forge is your project will be THAT temp when it's the same color as the forge. You can tell that much in bright sunlight. If you repeat the above and make note of each temp you use you'll be able to repeat them in bright sunlight or pitch black. You don't turn scrolls as hot as works well for general forging ad forging at welding temps can damage the steel while melting off thin sections. It may take you a while to develop a reference guide for how you work but once you have one you'll never have to wonder what the temp in your forge is. Be aware your reference sheet only applies to YOUR forge until you change it and you're still tinkering it to work well so get a couple reference settings and sounds for now. Hmmm? Frosty The Lucky. Quote Link to comment Share on other sites More sharing options...
Irondragon Forge ClayWorks Posted July 9, 2022 Share Posted July 9, 2022 Another trick is to have a "shadow box" one large enough to stick the stock in as soon as it comes out of the forge. If the color looks good in the shade of the box go to hammering, if not back in the forge. Quote Link to comment Share on other sites More sharing options...
Jason Fry Posted July 10, 2022 Author Share Posted July 10, 2022 Yeah I stand in front of the forge and shadow it and can judge decently. With the power hammer, I can feel when it stops moving. I'm sure it would still show dark red in shadows. I've used this forge for 4 years now, welded a few dozen damascus billets. Today was just day 1 for the blown burner, not for me or the forge I have at least three more solid sessions of forge welding before this one's anywhere close. Here's the WIP for the knife I'm doing. Quote Link to comment Share on other sites More sharing options...
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