Waste Heat Recovery – Heater Modification
If we decide to recover waste heat, we can add more coils to the convection section or add a convection section to increase convection section duty. Doing so may have two negative effects on draft. Additional tube coils increase pressure drop in the convection section, and cooler flue gas decreases the draft effect of the stack.
Lengthening the stack to increase draft is often feasible. Some heaters were constructed with unused height in the convection section. Putting additional coils into these heaters is relatively inexpensive. (The design of any new heater can specify that space is to be left for two additional rows of finned tubes in the convection section. The cost is low.) A small increment of convection area can be gotten by increasing the fin density. Retrofitting finned tubes or replacing bare tubes with finned ones means a new tube coil and new tube sheets. It is unlikely that the downtime needed to modify an existing convection section can be justified, although sometimes one is removed and replaced by a larger one.
If additional space is needed, a chamber must be built nearby. Stack gas must be ducted to and from that location, at significant cost. An induced draft fan may be needed. A possible cost-saving measure is that flue gas from two or more heaters may be handled in a central location. Interaction effects on heater draft must then be reviewed.