These heaters are characterized by a sub-horizontal coil arrangement. The coil is arranged as a spiral (helical coil) along the cylindrical firebox wall. The burners are floor-mounted and firing upwards. The process flow direction in the radiant coil can be either upwards or downwards (according to the desired process flow regime). These heaters can be designed both with and without a convection section. The main advantages of this kind of heaters are the possibility to drain completely the coil by gravity; the low pressure drop (if compared with a standard vertical cylindrical heater), the reduced plot area requirement and the flexibility in tube spacing allowing the reduction of peak heat fluxes (where required). This kind of heater combines the advantage of vertical-cylindrical heaters with the horizontal tube heater arrangement. The heat duties are in general low (in the range of 1-10 MW) because of the limitation on the number of coil passes that can be easily arranged as helix in the firebox (typically 2-3). This kind of design is typically suitable for process services such as Reboilers, Hot Oil Heaters, Heat Transfer Medium Heaters, Feed preheaters, Regeneration Gas Heaters and Light Crude Heaters.