Single-Fired/Double-Fired Heaters

Single or double firing refers to heaters where the radiating planes or burner flames are oriented to the radiant tubes.The single-fired heater has one radiating plane on the right with the tubes against the refractory wall while the double-fired version has radiating planes on both sides of the tubes. The dashed line represents the average heat flux on the tubes while the solid line represents the actual heat flux on the tubes.The front (toward the radiating plane) 60° of the tube represents the peak heat flux to the tubes. As noted for a single-fired tube the peak to average ratio is about 1.8 for a standard two-diameter tube spacing. The ratio is only 1.2 on the double-fired tube.This means that a double-fired radiant section only requires two-thirds of the surface area of a single-fired design, assuming the same peak heat flux or tube metal temperature (TMT); since peak heat flux determines TMT. This is useful to increase velocities in critical fouling services, lower the pressure drop or make a more compact radiant section.