By Charles Muhlenforth
A process fired heater has two main sections - radiant and convection. To effectively revamp a process fired heater, both sections must be evaluated.
The radiant section revamp usually centers around modifying the burners. If an increase in capacity is desired, the new burner liberation required can be achieved by changing gas tip drilling or oil gun size depending on the fuel fired. Compare the fuel compositions for significant changes. It is always best to seek the assistance of the original burner manufacturer. The use of preheated air for efficiency gain introduces additional evaluations for the selection. The preheated air temperature will affect burner metallurgy. Also, the burners must be rated both for forced and natural draft to offer some capacity during air preheat system malfunction - typically, fan failure.
Attempting to revamp the radiant section coil also has certain constraints. Rarely can you increase the radiant surface by adding tubes - no available space. Some users have changed tube size, but this can readily lead to heat transfer problems due to spacing.
Coil side pressure drop must be checked for desired flow rates and vapor. In order to minimize pressure drop, the number of parallel passes could be increased. However, the inside heat transfer coefficient will decrease, thus increasing the tube metal temperature; the tube material must be acceptable at the new temperature. External piping changes must also be considered.
A revamp with air preheat alters the amount of heat to the radiant and convection sections. The radiant duty increases resulting in higher radiant flux. Potential metallurgical and/or in-tube coking problems must be considered.
The convection section offers several potential opportunities for revamping for efficiency or capacity. Typically, it is possible to add additional rows to the space provided in the original design (standard practice is to provide space for two future tube rows). The addition of further rows is a function of the design. The general arrangement drawings must be reviewed. The convection coils above the shield section (this section receives direct radiation) which is bare have extended surface either in the form of segmented fins, solid fins, or cylindrical studs. The extended surface must be reevaluated for heat transfer and proper metallurgy. It could be possible to exchange existing extended surface tubes for ones with greater fin or stud density. It is recommended that a physical check of the extended surface tubes be performed, since fins and studs become oxidized with time; fins, if not properly resistance welded, can come loose from the tubes.
Consideration should be given to using a low temperature stream flowing through added tubes as an efficiency source.
Adding an air preheater to the process fired heater usually requires adding internally lined ducting, FD/ID fans, a separate coil in its own enclosure (rarely can you add an air preheater directly to the existing convection section because of structural restraints), new burners with a plenum, and improved instrumentation. Adding air preheat requires duct sizing, burner specification, and fan rating together with actual specification of the air preheater.
The above paragraphs give only a brief overview of revamping a process fired heater. If you wish to undertake a revamp, first prepare a study budget estimate before proceeding to a job. I would recommend that you engage the services of a process fired heater consultant to help you evaluate the study findings, prepare specifications, and assist your engineers with selection and engineering review.