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All of my previous forges have been powered by homemade blown burners. I’ve always sized them so that they adhere to the rule of thumb 450 BTU/Cubic Inch of chamber size. I’ve never had a problem welding or forging even large billets.

 

On a whim I decided to give an atmospheric burner a try. I purchased a 1” T-Rex burner and built a new forge. The chamber size is about 450 Cubic inches and fits within the guidelines Rex gives for burner sizing.

 

Here’s the question…even without ITC the forge easily goes to 2,200 F at only 10 psi. According to the BTU chart Rex provides that puts the burner outputting about 65K BTU. That is welding temp at only 144 BTU per cubic Inch and with no ITC. How does that work? Even using the BTU calculator a .045” orifice at 10 PSI is in the 70K BTU range at best.

 

Granted I am using 3 “ K-23 brick for insulation but still that just seems to go against the standard logic of BTU requirements for a forge. Am I just figuring something wrong?

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First thing to say is that I've built a few forges and they seem to work ok. I've spent most of my working life dealing with gas burners and temperature control and I think I understand it fairly well. However, I'm no smith and there are plenty of guys out there with more experience than me.

 

That said, my take on it is this:

 

There are a number of factors that determine how much gas you really need and those rules of thumb are just rules of thumb.

 

Most of the stuff that is out there on BTUs and forges is aimed more at blacksmithing than at bladesmithing. Welding temperatures for mild steel, and particularly for wrought iron, are quite a lot higher than those for typical bladesmithing steels.

 

2200 degF is around 1200 degC and it is certainly possible to weld at that temperature. However, when I've measured the temperature of forges being used for making Damascus, by smiths who are good at making Damascus, they have been around 1300 degC (2372 degF). Even then, it is high-Carbon steel that is being welded.

 

Blades also tend to be able to fit through small openings, so it's quite easy to build a bladesmithing forge with small openings and a large volume. A typical blacksmithing forge will have a much larger front opening and often a large rear opening.

 

In my experience, the area of the openings has much more effect on the heat input needed to maintain temperature than the forge volume does.

 

For the (blacksmithing) rules of thumb, there is probably quite a strong correlation between forge volume and open area when building "normal" forges.

 

The BTU rating of the burner is based on the heat output assuming all the gas is fully burnt. In reality, it seems quite usual to run with a rich (reducing) forge atmosphere to control scaling and for there to be a significant amount of Dragons Breath. The DB represents wasted heat: BTUs that are not actually released in the forge itself.

 

When using MIG tips for gas jets, it should be noted that the tips are sized for a nominal wire size and that I have found the bore of the tip is typically .006-.008" bigger than the nominal tip size (Hybridburners give .007-.009" near the bottom of the page at http://www.hybridburners.com/BTU-charts.html#oneerchart) A .045" nominal mig tip would therefore be .051-.054" in the BTU calculator. The nicely-tapered lead-in probably also raises the discharge coefficient of the jet to at least 0.75 and possibly close to 0.8. This will also affect the BTU calculator results.

 

When I was playing with jet sizes for burners, I built a forge using cheap IFB and with a 6" x 6" x 13.3" chamber. With a single 3" x 3" opening, I got a temperature of 1545 degC (2813 degF) on a 0.6mm (.023") MIG tip measuring between .030" and .031" diameter and 4 bar (59 PSI) gas pressure. BTU calculator gives just under 104,000 BTU/hr (.031", 59 PSI, 0.8 discharge coefficient) which works out to 214 BTU/cu.in (less than half the rule of thumb figure) even at over 2800 degF.

 

DSCF0005_zpsdaf77aa2.jpg

 

DSCF0006_zps51cf6b93.jpg

 

And the effect on the Thermal Ceramics JM23 IFB I'd used to restrict the opening:

 

DSCF0011_zpsa1906ee3.jpg

 

The cheap IFBs held up to the temperature pretty well, despite only being rated to 2300 degF, but are pretty poor insulators compared to the JM23s.

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I am interested as to where you get your 450 BTU per cubic inch from? I would agree with Tim that the size of the openings has a huge effect on forge temp.

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Yeah the 450 BTU/cu in is bandied about all over the web. (ABANA, Iforge, Ron Reil etc) I'm not sure how that number was arrived at though.

 

You make an excellent point. I had not considered the fact that a lot of forges run pretty open in the front and/or back whereas mine has pretty small openings. That plus 3" of insulation probably account for the performance.

 

There may also be some variance in the BTUs actually being generated. I'll have to time the gas consumption and figure out what it is actually putting out.

 

Thanks!

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