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I Forge Iron

Reducing scale in a gas forge

Ted Ewert

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I've done some research on scale: what it is composed of, how it forms, etc. It's mostly a few different varieties of iron oxide (mainly FE3O4 which is magnetite), combined with the oxides of whatever other alloys are in the steel. No real news here for the veterans.

I've tested my forge for external air infiltration and it has none. I have tight barn doors on the front, and a high enough internal pressure differential so that all cracks and orifices are blowing out hot gas. I am also running a blower which is probably delivering a considerably higher flow of gas compared to a NA burner.

Therefore, I assume scale can only form in this atmosphere from oxygen which has not been "burned" and consequently combined into a non reactive gas. 

I'm bringing this topic up because I have noticed a significant reduction in scale forming on my steel since I introduced a gas mixing tube into the airflow. This corresponds with having to reduce the airflow about 80% to get a clean burn. 


The thorough mixing of propane and air seems to be more difficult than normally assumed. This subject has been discussed numerous times here and I can attest to its importance.  When mixed well, everything works much better in the forge: efficiency increases, heat increases, scale decreases and the flame remains quiet and steady. 

The advantage of a blown forge is that better mixing can be accomplished in a variety of ways without changing the geometry of the input plumbing. I've replaced the original mixing tube with one which has a bunch of little holes (#50 bit), and added a screen across the inside of the pipe to increase turbulence and mixing. 

Unfortunately, I broke my ribbon burner by putting a little too much torque on it, but poured and drilled a new one which needs a day or two to cure. I'm trying out 1/4" tubes with 3/8" expansion ports this time since I get a little more turbulence in the smaller tubes.

I just found this interesting and worth mentioning to the guys who are still new to this whole business like me. I'm sure the more experienced guys here have bright , shiny steel emerging from their highly tuned forges everytime. ;)



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You don't say where the scale is forming, in or outside the forge.

Getting propane and air to mix thoroughly can be problematical. It mixes more readily in low psi conditions. With or without turbulence NA burners are inherently low psi devices. It's not much but it's a factor.

Frosty The Lucky.

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 using a venturi burner, I've noticed that when my intake choke is wide open, I get more scale than I do when I choke it down a bit till I start to see reducing dragons breath.  I've thought about the blown designs and wondered if forcing more air into the forge might increase scaling.  

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I'm talking about scale forming inside the forge. Not much you can do about it once it's outside. 

The pressure in a 2 inch pipe is pretty low, even with a blower. 

16 minutes ago, MotoMike said:

I've thought about the blown designs and wondered if forcing more air into the forge might increase scaling.  

With the right mixture it won't. Thats the whole point here. If the gas and air are properly mixed, and in the right ratio, all the oxygen gets used in the flame and there is little if any to react with the steel. 

When I had a poor mix, I could see the scale forming in the forge. 

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Moto, one benefit would be that you can include feature in a blown burner that will force the gas to mix in ways not easily or practically done in a naturally aspirated burner. Additionally, if the blower is a high pressure type, you can potentially have forge with smaller exhaust openings without effecting the air to fuel ratio as much: same amount of backpressure could cause issues to an NA. 

I think it just depends what you need to do, or what work arounds fit your st up, since they both have pros and cons.

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Two points: First, how sure are you the fuel air ratio is stoichiometric? That's just a matter of adjustment so I don't have suggestions.

Second, if it is, then where do you think your mixing strategy is falling short?

Propane and air do not mix easily. It's common for a burner to be running rich and still have turbulence with oxygen heavy zones. Ron Reil did some pretty sophisticated testing back when. He metered both the fuel and air so he knew within a couple 1/10s % of what they were and he had to adjust things pretty rich to prevent any scaling in the forge. 

Commercial small scale propane furnace gun burners often inject the propane into the blower vanes. This isn't terribly practical nor safe on a home build, there is an increased fire danger. It's still attractive, good thing I'm not into gun burner. :ph34r:

If the blower generates a high enough velocity flow then you can use cavitation initiators to shatter the propane droplets and mix them thoroughly in the collapsing vacuum of the cavitation. 

This is part of the advantage of a NA burner with a tapered mixing tube. As soon as the flows pass the throat they are under decreasing pressure and flowing in a vortex so mixing time is longer. It's not much advantage but it's there. A gun burner is under a positive pressure again pretty low but it is less advantageous for complete mixing. 

One thought is to use perforated metal for your swirl strip. I don't know how many or what size perforations are better though they do need to be round. You want to induce a whistling action in the stream, it indicates a degree of cavitation and the whistling sound itself causes agitation in the flow to enhance mixing. Stringing tight wires across the mixing tube will aid mixing but not in a vortex.

Frosty The Lucky.

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