I wish I had 3 phase, just a 15 hp rotary converter. I looked at an big old induction heater at auction back before the small ones were popular, it was as big as a truck!
Calculation for forge capacity
5" x 5" x 30" = 2250 cu. in. divided by 1728 cu. in./ cu. ft. = 1.3 cu. ft. of steel
Raise 1.3 cu.ft. to 2250 deg. F. from 50 deg. F.
Specific of steel= .165 Btu per lb. per F deg.
1 cu ft of steel = 490 lbs., times 1.3 cu ft = 637 lbs
Raise 2150 deg. F deg net
.165 Btu/lb/F deg times 637 lbs times 2150 deg.F = 226,000 Btu's with no heat loss
Furnace size to be approx 2 ft x 3 ft. x 1.5 ft. = 27 sq. ft. of interior furnace area
A estimate of heat loss and refractory soak for walls are 4 1/2" or thicker. Gross heat input required to balance heat loss to fire-brick walls, the heat stored in the furnace wall as well as the heat which flows through the wall. An estimate of a 430,000 Btu's, with a cold start to temp of approximately 60 minutes.
226,000 Btu plus 430,000 Btu's = 657,000 Btu required as an estimate. Knowing that I may heat more or less, take more or less time to heat up, but knowing that I have the capacity to get a load up to forging temp in a reasonable time and that once the forge soaks out I would have some turn down capability to reduce heat input. Granted it is all an estimate but just trying to get as close as possible so that when the unit is built it will do the job that iI need to do.
Looking into the 750 burner http://hauckburner.thomasnet.com/viewitems/gas-burners/pbg-packaged-gas-burners? not cheap but as a complete unit the efficiency and the hugh turndown would be an advantage.