Blown burner size/forge body?

Kentucky

Kentucky

Joined
Dec 13, 2008
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Building g a new blown forge. Need a fairly big working space. Looking at final inside dimension of roughly 8”-9” wide, 6”-7” high and appx 10”-11” deep.. insulation will be 1”x2 kaowool layer with kast o lite 30 shell, then itc100 or plistex.. will have a cast floor likely. Haven’t decided that yet.
What size burner tube would you recommend for this size forge? 1 1/4”-1 1/2”?
 
The usual advice for NA burners is around 300-350 cu in per 3/4" burner.

Your dimensions give a range of 480-693 cu in.

A 1" burner has around 1.8 times the area of a 3/4" burner, so should handle 540-630 cu in and will probably work pretty well.

To some extent, your blower will affect the sizing, but pretty much any blown burner should be able to flow more mixture than a NA burner of the same area, so I don't think capacity will be a problem.

The turndown limit on many burners is when the mixture speed in the burner tube drops below the flamefront speed and the flame runs back up the burner tube. Keeping the tube diameter small and the mixture speed high should help with the turndown.

I think 1" is pretty safe and going up to 1 1/4" just-in-case may lose you efficiency.

I can say from personal experience that it is deeply annoying when you've built a well-insulated, efficient forge but cannot actually run it efficiently because the burner will not turn down enough.
 
timgunn1962 said:
The usual advice for NA burners is around 300-350 cu in per 3/4" burner.

Your dimensions give a range of 480-693 cu in.

A 1" burner has around 1.8 times the area of a 3/4" burner, so should handle 540-630 cu in and will probably work pretty well.

To some extent, your blower will affect the sizing, but pretty much any blown burner should be able to flow more mixture than a NA burner of the same area, so I don't think capacity will be a problem.

The turndown limit on many burners is when the mixture speed in the burner tube drops below the flamefront speed and the flame runs back up the burner tube. Keeping the tube diameter small and the mixture speed high should help with the turndown.

I think 1" is pretty safe and going up to 1 1/4" just-in-case may lose you efficiency.

I can say from personal experience that it is deeply annoying when you've built a well-insulated, efficient forge but cannot actually run it efficiently because the burner will not turn down enough.
Click to expand...

What size exhaust port or door would be optimal?

Hoss
 
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If we say the maximum temperature needed will be around 2500 degF, that's 1644Kelvin. If we assume an ambient temperature of freezing, 32 degF, 273K, we would need 1644/273 = 6.02 times the inlet area to get the exhaust gases away at the same velocity as the incoming mixture if the number of reactant molecules equalled the number of product molecules.

For a Propane forge, completely burning the fuel.
C3H8 + 5O2 → 3CO2 + 4 H2O, so the reaction increases the gaseous molecule count from 6 reactant molecules to 7 product molecules.

Burning rich, to Carbon Monoxide, we get
2(C3H8) + 7(O2) --> 6CO + 8H2O, an increase from 9 reactant molecules to 14 product molecules.

If we take the worst case and assume that air is 20% O2 and the other 80% does not react, we'll need to increase the exhaust area by a factor of:
80% + (14/9 * 20%) = 111%

6.02 x 1.11 = 6.69.

The "internet rule of thumb" is to allow a minimum of 7 times the inlet area for the exhaust and that seems to tally pretty well with the above.

For a 1" ID burner, a minimum exhaust area of 5.5 square inches would be appropriate.

The optimum open area for the intended process may be something completely different of course, with the size of port required for work access being the main consideration for most folk. If there is a door that can be opened to get the work in and out and closed during normal operation, the exhaust area with it closed should be 7 times the inlet area.
 
That agrees with what I have found. I usually go 10:1 to be sure and say for a 1" blown burner you need a 2X3" port front and 2X2" port rear . For damascus forges and ones that will run wide open for welding, the port size is rarely an issue unless you try to block it up with too many firebricks. Just stick with 10:1 and it will be fine.
 
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