A furnace is a chamber, most often insulated, in which heat is transferred from a high-temperature source to a sink. The types of furnaces used are numerous and depend on the requirements of the heating process. Section 319, Section 320, Section 321, Section 322, Section 323, Section 324 and Section 325 are concerned with fuel-fired furnaces, in which the heat is supplied by combustion of a fuel within the furnace chamber and transferred to the sink by a combination of radiative and convective heat transfer processes. The most important applications of fuel-fired furnaces in the refinery and petrochemical industry are process heaters and steam generating boilers. In the wider chemical industry one finds many other examples including high-temperature kilns such as cement kilns, metal melting and reheating furnaces, glass production furnaces, thermal oxidizers as well as hot-water boilers.

The design of furnaces and the prediction of their performance have considerably improved in recent years, albeit it can still rely heavily on empirical data and engineering experience. This situation is a consequence of two related problems: first, the many factors in addition to the heat transfer calculation that must enter into the basic design of a furnace, such as, for example, the permissible heat fluxes, peak-to-mean flux ratios, heat sink arrangement, combustion volume, and burner size and placement; and second, the formidable difficulties of developing a general unified theory for predicting the performance of all furnaces. Nevertheless, a number of theoretical models for furnace calculations have been developed, and it is the purpose of these sections to present some of these and to indicate their application to combustion chamber design.

The material is arranged as follows. A brief description of the types of process heaters, boilers and industrial furnaces currently in use is given in Section 320. General aspects of heat transfer in furnaces are discussed in Section 321. This is followed by four sections in which various furnace models are described, starting with the simple stirred-reactor model and progressing through to advanced models based on numerical solution of the equations governing a turbulent, chemically reacting flow in a furnace chamber.