(113a) Flux-Selective Coil Design to Improve Process Fired Heater Run Length

Process fired heaters’ coils get coked with time and need to be de-coked to re-start the operation. This downtime, required for decoking operation, reduces the profit margin of the plant. The run length is the biggest limitation on any process fired heater. An increment in fired heater run-length adds up significantly to plant profit margin. Designers and Operators are continuously trying to improve the fired heater design and operation to achieve the maximum possible run length.

The radiant heat flux on the fired heater coils (tubes) is not uniform. Heat flux varies along the circumference of the tubes as well as height of the firebox. The flux variation along the height of the firebox has significant impact on run length. This flux variation is due to the fact that flame hot zone is in the bottom part of the firebox.

Firebox flux variation along the firebox height mostly depends on burner design. There is not much that can be done to make the firebox flux uniform along the height. Therefore, optimizing the coil configuration is the best option to increase the overall process fired heater run length. We know that the fired heater feed is heated from inlet to the outlet of the heater. As the temperature increases along the fluid travel length from inlet to the outlet of the heater, so as the coking tendency of the fluid inside the coil. At the same time, the radiant heat flux is higher in lower section of the firebox compared to that on top zone.

In a typical design, high temperature coils (outlet zone coils) are located in bottom section of the firebox. Therefore, the outlet coil section not only has higher fluid temperature but also experience the highest radiant heat flux. These two factors (higher fluid temperature and higher heat flux) adds up, resulting in generating the highest coking rates. Therefore, the fluid film and tube metal temperature is much higher in the outlet zone coils.

All these flux variation and higher temperatures towards outlet section increases the fluid film temperature. Fluid film temperature is one of the most critical parameter which affects the coking rate. If we can lower the film temperature, especially for tubes towards the outlet (which are more susceptible to coking), this will result in lower coking rates. It is proposed to re-configure the radiant section tube layout so that it reduces the fluid film and tube metal temperature. The work done for a process fired heater using ‘Flux-Selective’ coil design shows a significant reduction in fluid film and tube metal temperature. Paper will provide a case study comparing various coil configuration leading to best ‘Flux-Selective’ coil configuration design which can have the maximum fired heater run length

The design changes will result into a lower fluid film temperature, specially in heater outlet zones where it is most critical. A lower fluid film temperature will result in lower coking rate hence longer run length. Further, as proposed design will have inlet coils in the high flux zone and outlet coils in the low flux zone, this will produce higher vaporization in the inlet side of the radiant coil. This higher vaporization in the inlet section will reduce the residence time. A lower residence time will further reduce the coking rate. Another advantage of this design is a reduction in peak tube metal temperature, this will enhance the tube life and also reduce the tube cost over the life of the furnace. A process fired heater with an increased run length will provide a competitive edge.