Abstract: Refinery process furnaces are disclosed that have horizontal-helical radiant coils and/or improved ladder-type supports for the horizontal tubes. The coils have a tubeside fluid flow path from an inlet pipe section through an alternating series of straight horizontal pipe sections and wide sweep return bends to an outlet pipe section. The horizontal pipe sections are arranged in at least two vertical banks that are parallel and horizontally spaced apart. The bent pipe sweep return bends are arranged in vertical banks at either end of the straight pipe banks. Each bend connects a pair of straight pipe sections in adjacent vertical banks, and each return bend is sloped between horizontal and vertical so that one of the straight pipe sections in the connected pair thereof is elevated with respect to the other. The ladder supports have two opposing columns with a number of vertically spaced support members each having a generally horizontal upper engagement surface.