Rob-
The interior dimensions of my furnace are 12.75" bore, and 14" tall.
If I am doing the math right, then my furnace interior surface area is 561 sq.in.
I use about 2.6 gal/hr diesel, and my typical melt time for a #10 crucible of gray iron is about 1 hour (I allow a generous amount of time to make sure I am at pour temperature; I don't have an iron-rated pyrometer).
The 1 gal/hr Delavan nozzle will operate easily up to 4 gal/hr or more (not that you need that sort of fuel flow).
For your furnace, I think the interior surface area is 301 sq.in.
If the heating for various furnace sizes scales approximately linearly, then I would think you could use (301/561) * 2.6 gal/hr = 1.4 gal/hr.
I am not positive the fuel flow rate scales down like that though.
Maybe start with 1.75 gal/hr and see how that works.
To determine the maximum amount of fuel that your sized furnace can completely combust, start with a low fuel flow rate such as 1 gal/hr, and adjust the blower to get about 4" of yellow flames out the lid opening.
Then increase the fuel flow slightly, and the yellow flames should increase in size.
Dial in more combustion air to get the flames back to 4" tall.
Keep increasing fuel flow slightly, and then increasing the combustion air to get back to a 4" flame.
At some point, you will increase combustion air, but you will no longer be able to draw the flames back to 4" tall.
This is the maximum fuel flow that your furnace will completely combust.
Be aware that you may be able to melt iron easily at some level below your maximum fuel burn rate.
I would not use a fuel flow rate above 2.6 gal/hr regardless of anything else.
And if your blower does not produce enough air, then you may not make it through the test described above with an accurate result.
At a glance, I would say you have enough blower, but that is a guess.
Good luck.
Pat J
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