![]() Many heaters are specified with a maximum allowable radiant section average heat flux. Peak heat flux represents the maximum heat flux in a specific heater section and can be used to calculate maximum expected oil film and tube metal temperatures (TMT). While average heat flux is a useful concept for high level heater design and evaluation, it does not capture the harshest conditions, which are better represented by ‘peak’ heat flux. For example, the radiant section average heat flux is the total duty absorbed in the radiant section divided by the total radiant tube outside surface area. For fired heater design and evaluation, it is typical to discuss the heat flux for a specific section rather than for the heater as a whole. Heat flux is typically expressed in units of kcal/h-m2 (BTU/h-ft2). ![]() In the context of fired process heaters, heat flux is the amount of heat absorbed through the heater tubes per unit outside surface area. These same design parameters and analytical tools also provide critical insights into practical revamp solutions for existing problem heaters. Peak radiant heat flux is listed on the API heater datasheet, but does the datasheet number really reflect reality? Specific heater layout parameters can heavily influence the answer to this question, and analytical tools are available to explore the question during the design process to avoid surprises at start-up. Excessive radiant tube peak heat flux is a common cause of radiant tube coking. When it comes to heater sizing and layout, radiant section heat flux is one of the most important parameters. Reliable heaters that meet run length expectations are designed by optimising the heater layout and the process side mass velocity. ![]() In crude/vacuum, FCC, delayed coker, and many other units, maximising heater reliability extends run length and increases profitability.įired heaters coke due to high oil temperature and residence time. ![]() Unreliable heaters tend to begin coking rapidly at start-up and require frequent decoking to lower pressure drop and tube metal temperatures. Aside from cases where a heater is simply at maximum firing, many units are limited by poor heater reliability. Fired heaters in refinery heavy oil service are common limits to unit throughput and run length. ![]()
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