How many blown burners do I need?

J

James Mul.

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Dec 29, 2014
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Im not sure if anyone has posted anything about this, I haven't been able to find anything online about this, but i know with venturi burners the general rule is one for every 300-350 cubic inches. I havent found anything as specific for blown burners though. I am currently making plans to build a new gas forge out of an 11 gal air compressor tank and i am wondering how many i will need. The tank is roughly 23" by 13" and I will be using 2 or 3" of kaowool and at a minimum plistix to coat it with a few firebricks for the floor. Any help or advice is appreciated.
 
The answer is that it depends on how big the burners are.

The 350 cu in for Venturi burners seems to be for the 3/4" ones. I have used from 1/2" up to 2" burners with commercial Venturi mixers.

http://amalcarb.co.uk/downloadfiles/amal/amal_gas_injectors.pdf

The 1/2", 3/4" and 1" seem to cover most bladesmithing needs. I built a 1 1/2" for a friend to use in his vertical forge, but he tried it and went back to the 1".

For blown burners, the Physics allows much more freedom over sizing.

It is best to have only one mixing point from the point of view of control; The mixture composition is what adjusts the flame temperature (Degrees), while the volume flow of mixture is what adjusts how much flame there is, effectively the heat input (BTU/hr or kW).

The downside of multiple burners on a single mixer is that everything downstream of the mixer is full of a potentially explosive gas/air mixture, so if the burner burns back, there is a bigger bang.

Unless the forge needs to be a really strange shape for some specific process, I'd use a single burner every time.

Depending on whether you use 2" or 3" of Kaowool, you'll come in somewhere between 750 and 1500 cu in. It will use a lot of gas and 3" of Kaowool will help to keep the consumption down. In my, admittedly limited, experience, the size of the openings is probably a more important factor in burner sizing than the chamber volume.
 
For a blown vertical forge of normal knife maker size, you typically need one burner.
 
I have found one burner does fine in most general use situations. Two can be used once the length gets to 24", but as Tim was pointing out, burner and blower size is probably far more important than the number of burners in a larger volume forge. For your setup, use lots of insulation, a thick refractory coating over the wool, ITC-100, and a 1.5" blown burner with a 150CFM blower.

The only real use for multiple burners is evenness of heating in HT forges. These use multiple smaller burners, and are built a bit differently as well.

Tim,
The way to eliminate the gas mixture filled manifold on a multiple burner forge is to inject the gas separately near each burner port. Just use a manifold and the one blower for the air feed. This also allows you to tune each burner separately for more even heat distribution.
 
Thanks for all the advice. I was thinking of going with two burners for a HT forge as I don't really have a lot of cash or time to make a bunch of different forges and I would prefer to have one that is more versatile. I was planning on basing the design off of Fred Rowe's "Vulcan's Chariot" With the gas being fed through a needle valve into the T. a few inches off of the chamber. I planned on incorporating a shutoff valve into each individual gas line so I could run the burners individually when Im not doing any heat treating. Im not sure if there was any other significant difference in a two burner vs. a one burner forge. Is it possible to heat treat with a single burner and still get even heat throughout?
 
Last edited:
Stacy E. Apelt - Bladesmith said:
Tim,
The way to eliminate the gas mixture filled manifold on a multiple burner forge is to inject the gas separately near each burner port. Just use a manifold and the one blower for the air feed. This also allows you to tune each burner separately for more even heat distribution.
Click to expand...

I completely agree; a big air plenum feeding identical burners fitted with identical gas jets will dramatically reduce the risk and will even allow single-point adjustment. I think that tuning each burner individually loses the operational simplicity of one global adjustment point though.

Perhaps the main reason I like to have a single adjustment point is that I am not skilled enough to identify and control a large number of variables: I like things to be as simple as possible. I appreciate it's not a particularly laudable reason, but I think it's a valid one and I'm fairly sure I'm not alone in having it.

Building multiple burners to be identical to the point where they can all be adjusted with a single gas control and a single air control and all respond identically is not particularly difficult, but it is starting to peck away at one of the big advantages of blown burners, which is that they can be built "more-or-less right" and still be tuned afterwards to perfectly match the requirements of the application.

I'd probably go with fairly large MIG tips as the identical gas jets and I'm pretty sure the burners would still be much less finicky to build than a similar number of identical Venturi burners.

I don't think a multiple-blown burner setup will ever be a problem for anyone who thinks about the design and understands it. It's just that the understanding usually seems to come from experience (often of getting it wrong and having to put it right) and there does not really seem to be very much out there on the web that explains things well enough for a beginner to avoid getting it wrong in the first place.
 
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