There are many different names that are used for the combustion chamber containing the heating system and the material being heated, often referred to as the load. They are broadly categorized here as low temperature (< 2,000 °F or 1,100 °C) and high temperature (> 2,000 °F or 1,100 °C). Low-temperature combustion chambers — or combustors — include, for example, dryers and heaters. Dryers are combustors that are typically used to remove moisture from materials (Keey, 1997) while heaters are typically refractory-lined chambers are used to heat solids and liquids.

High-temperature combustors include, for example, furnaces and kilns (Baukal, 2000). Furnaces are refractory-lined chambers used for heating and melting solids. Kilns (pronounced “kills”) are combustors used to bring about physical and chemical changes in materials and are a type of furnace that is usually associated with thermal processing of nonmetallic solids such as in the ceramic, cement, and lime industries (Shires, 1997). The chemical changes are often through calcination that may involve water evaporation, volatile evolution, and even partial combustion of the load material.

There are many good resources in the literature on industrial combustors such as heaters and furnaces. Griswold’s book (Griswold, 1946) discusses fuels, burners, refractories, and industrial combustors. The two-volume set of books by Trinks et al. (2004) is considered by many to be a good reference that deals with all aspects of furnaces including heat transfer and heat recovery. Reed’s book (Reed, 1981) focuses on process heater operations including a discussion of heat transfer. Blokh (1988) discusses many aspects of heat transfer in steam boiler furnaces. Hewitt et al. (1994) have written a chapter on furnaces that includes heat transfer calculations and mathematical models. Schlesinger et al. (1996) have written a chapter on fuels and furnaces that includes information on different types of fuels, carbonizing coal, gasification, incineration, and electric furnaces and ovens. Khavkin (1996) has written a book focusing on the design of combustion systems. Deshmukh’s book (Deshmukh, 2005) on industrial heating includes chapters on steady state heat transfer, transient conduction, and refractories among others. Cone (2006) has written a chapter on furnaces with an extensive section on heat transfer. Mullinger and Jenkins (2008) have written a book on process furnaces that includes chapters on heat transfer, flames and burners for furnaces, and combustion and heat transfer modeling.

The following sections present a classification of furnaces by the firing method, load processing method, heat transfer medium, and geometry. As it will be seen in Section 320, each of these aspects has a unique influence on the heat transfer performance of the system. In Section 321, the different types of industrial furnaces are described.

Gas recirculation systems, typically used to increase the overall process thermal efficiency and reduce NOx emissions are described in Section 322. Section 323 presents the basic heat transfer relationships used in furnaces.