The choice of an appropriate furnace type depends on the specific application, as there are many possibilities. One important consideration is whether materials will be processed in a batch or continuous mode (discussed in Section 320.B). Another important consideration is the kind of fuel that will be used (Johnson and Auth, 1951). Other considerations include the furnace geometry and required temperature range. The processing method and furnace geometry are briefly considered next. Trinks et al. (2004) have specifically limited their definition of industrial furnaces to only those processes that raise the temperature of the material (mainly metals), but do not cause any phase changes (melting or vaporization) or any chemical changes (e.g., calcining). Even with that narrow definition of furnace, they list 61 applicable processes ranging in material temperatures from 300 °F to 3,250 °F (150 °C to 1,790 °C). Here, industrial furnace is more broadly defined to include processes that not only raise the material temperature, but that may also cause phase changes and chemical reactions. However, only processes firing some type of fuel are considered here. Other methods, such as electric heating, are not included in the following discussion.

Khavkin (1996) devotes a significant portion of his book on combustion system design to the use of unlined combustors. These unlined chambers are commonly used, for example, in gas turbine engines and internal combustion engines, but they are not commonly used in industrial combustion because of the high operating temperatures. Khavkin presented a number of designs where no refractory lining is needed, which can save weight, space, and costs. These designs were developed in Russia, but at present they have not become popular elsewhere because of the technical challenges and potential problems associated with the high heat flux rates from flames to unprotected metal walls. Therefore, nearly all of the furnaces considered here are refractory lined to enhance thermal efficiency and reduce the chances for injury to personnel from high temperatures.

A. Firing Method

There are two main methods used to heat materials, often referred to as the load, namely, direct and indirect firing. The method is normally chosen based on the process requirements. For example, if the material being processed should not come into contact with the combustion products, then indirect firing is used. If there are no such requirements, direct firing is usually preferred as it is normally more thermally efficient and straightforward.

(a) Direct Firing