The zone network method was employed for calculating heat transfer not only in furnaces similar to well-sti(?)ed boxes but in long furnaces with temperature gradients of flue gases. Equations for heat transfer between surfaces(reflecting surfaces and sink) in a radiant section with grey flue gases were established by simulating corresponding electric networks. The long radiant section was subdivided into several sections by imaginary planes along its axis and equations for each section were linked by the co...