The operating performance parameters such as fuel consumption rate, billet heating time, percentages efficiency and productivity of fuel-fired furnaces under the variable operating parameters such as typed of fuels (solid, liquid, gaseous), % space volume of furnace occupied, shapes of furnaces and air/gas pre-heat temperature were modeled and presented. The rates of fuel consumptions are found to be dependent on fuel types and to increase with increase of gas/air pre-heat temperatures. The billets heating times are found to decrease with increase of gas/air pre-heat temperatures and to be higher in rectangular shaped furnace than for cylindrical shaped furnace of equal internal volume and equal % of volume space occupied by billets. At any given % furnace volume spaces occupied, the % efficiency of operating performance is found to increase with increase of air/gas pre-heat temperatures and higher efficiency is found to occur in cylindrical shaped furnace than that for rectangular shaped furnace under identical operating conditions. Then software can be applied to determine optimum furnaces variable operating parameters to produce optimum design operating parameters for metallurgical furnaces.