This kind of heaters is the most commonly used for very large duties (above 50 MW). In the radiant section, tubes stand vertically along all four walls and across the firebox (dividing the box in different cells). Tubes on the walls are fired from one side (as for vertical-cylindrical and cabin fired heaters) while tubes running across the box are fired on both sides by the floor-mounted burners. The possibility to add “two-side fired” tubes shortens the radiant coil and results in a significant decrease in radiant box volume, compared to horizontal tube cabin heaters and in the possibility to reduce the peak heat flux as required. Each tube pass has an equal number of both walls and center tubes passes in order to keep as uniform as possible the heat distribution between the passes.
The convection section extends over the entire (or almost the entire) length of the radiant section. This kind of design is typically suitable for process services such as Topping and Vacuum services, Reboiler, Heavy Feed preheaters, Hydrocrackers, Visbreakers and Cracking. Process feed are generally 100% liquid, or vaporizing services.