There are a variety of designs for fired tubular heaters. Some of the more commonly used designs are the box, cylindrical, and cabin designs. The tubes in the radiant section of the heater are called radiant tubes. The heat pickup in these tubes is mainly through radiation from the firebox flame, radiating flue gas components, and the incandescent refractory. The shock or shield tubes are located at the entrance to the convection section. Because these tubes absorb both radiant and convective heat, they usually receive the highest heat flux. These bare rows protect or "shield" any extended surface tubes above them from absorbing too much heat and coking or damaging them.
Beyond the shock bank is the convection section where heat pickup comes from the combustion gases, primarily through convection. Convection tubes are commonly finned or studded to increase the surface area for heat transfer. Sometimes, the lowest rows of these extended surface tubes can absorb more heat per unit bare tube surface area than the radiant tubes.