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Publisher Summary This chapter explores the different processes through which heat is transferred in a furnace, namely, conduction, convection, radiation, and electrical heating. The primary objective of a furnace is to transfer thermal energy to the product. In a solid, the flow of heat by conduction is the result of the transfer of vibrational energy from one molecule to the next, and in fluids it occurs in addition as a result of the transfer of kinetic energy. Under transient conduction conditions, the temperature gradient through the system varies with time. Heat transfer by convection is attributable to macroscopic motion of a fluid and is thus confined to liquids and gases. Convective heat transfer is in reality the conduction of heat through a flowing fluid to a fixed surface, whereby the conductivity is defined by a convective heat transfer coefficient. All materials radiate thermal energy in the form of electromagnetic waves. When radiation falls on a surface it may be reflected, transmitted, or absorbed. The fraction of energy that is absorbed is manifested as heat. The physical laws governing thermal radiation are well established but are insufficient to describe technical processes quantitatively. Heat can be transferred by other parts of the electromagnetic spectrum, particularly in the long wave region, in and beyond the infrared. The primary source of energy for heating in this region is electricity. If there is a temperature difference between two parts of a system, then heat will be transferred by one or more of three methods. |
