This kind of heater 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 into 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.
Compared to horizontal tube cabin-heaters, the possibility to add “two-side fired” tubes shortens the radiant coil and significantly decreases the radiant box volume. It also can reduce the peak heat flux as needed. Each tube pass has the same number of walls and center tubes’ passes to keep the heat distribution among passes as uniform as possible.
The convection section typically extends over the entire length of the radiant section.
This kind of design is generally suitable for process services such as topping and vacuum, reboiler, heavy-feed preheaters, hydrocrackers, visbreakers and cracking. Process feeds are generally 100% liquid or vaporizing services.