oriented openings the manifold means defines in the range of from 6 to 36 upwardly oriented openings laid out to produce a cylindrically shaped curtain of flue gases along an inwardly facing surface of the set of radiant tubes the upwardly oriented openings are preferably defined by nozzles to assist in creating a curtain of relatively cool gases the manifold means and the conduit means are preferably sized to inject in the range of 15 to 60 of the flue gases into the radiant heating furnace section depending on process needs a preferred embodiment of the invention is a method for increasing capacity and reducing coking in fired heaters using flue gas injection technique as described above part of the flue gas leaving stack is withdrawn and sent to a flue gas fan flue gas injection is carried out in the radiant section of the fired heaters it reduces the fire box temperatures and cools the flame and flue gases this in turn reduces the coking rates and heat absorption in the radiant section typical prior art heaters are of two major types vertical cylindrical and cabin the drawing illustrates only the vertical cylindrical heater but the invention can also be applied to cabin heaters as well in the vertical cylindrical heater the radiant tubes are laid out in a circle in a horizontal manner the flow of fluid.