Fired Heater Classification According to Firing of the Radiant Coil
There are three ways the radiant coil can be fired:
• single fired with process fluid in tubes (Figures 100-2, 100-3, 100-4 and 100-8)
• double fired with process fluid in tubes (Figures 100-5, 100-7, and 100-9)
• with the fire in a submerged “fire tube” (Figure 100-10)
Single Fired, Process Fluid in Tubes. Tubes are placed about one tube diameter away from a refractory wall, roof, or floor and are heated from one side. This is an economical arrangement and is used in most heaters. Its disadvantage is that the ratio of peak to average flux (discussed in Section 200) is high; this makes tubes subject to local overheating, which causes bowing and hot spots.
Double Fired, Process Fluid in Tubes. Burners are on both sides of the tubes. Heating is more even. Firebox volume and the cost of supporting tubes are increased. This design is common in critical hydroprocessing heaters (Figure 100-9) and steam reforming heaters (Figures 100-5 and 100-7).