It is a matter of maximizing the annular space around the collar of the outer Refractory Well and the Inner Diameter of the nozzle and for ease of installation of the nozzle insulating components. A 6" Sch 80 nozzle provides 1.5 inches of annular space between the Refractory Well collar and the I.D. of the nozzle. This allows plenty of space for installation of the packing material around the collar of the Refractory Well. A 4" Sch 80 nozzle leaves a 0.5 inch space between the Refractory Well collar and the I.D. of the nozzle, adequate space but not optimum. The 6" nozzle size is preferred. 4" nozzles are acceptable.