The heat transfer mechanism for these inline furnaces are not always fully understood by many in the field. When heating with say a walking beam furnace at lower temperatures, the method of heat transfer is convection. As the gas circulates around the pipe the heat is transferred to the surface. This process is relatively slow and a rough rule of thumb is one hour per inch of thickness to get to temperature. In our high temperature tunnel furnace, the operating temperature is 1950 F. At this temperature the transfer is almost all radiant. The heat moves from the walls, roof and floor directly to the pipe. The pipe is being bombarded from every direction. As the pipe rotates within the furnace the uniformity of heating is excellent.
Heat transfer is strictly dependant on the radiant surface temperature. Radiant heat transfer rate goes up by the fourth power of the temperature. As you increase the temperature even 100 F significantly increases the rate of transfer to the pipe. In comparison to a box furnace with good convection the pipe may take only 3 or 4 minutes to come to temperature compared with 20 to 30 minutes in the box. The rate is also determined by the distance to the wall, the burner placement, the flue location, and the materials of construction. ESG has optimized these variables.