The use of electricity for heating purposes in the process industries is not widespread but it is increasing, particularly for special applications. The power may be purchased via the local electricity grid, or more often than not in a large chemical complex it will be generated by the recovery and transformation of waste heat via steam turbines and alternators.
The following are some of the advantages of electricity (a secondary form of energy) that need to be set against its relatively high cost compared with primary sources of energy such as coal and oil.
It is clean in operation
The energy is of constant quality
It is convenient and versatile
Control is relatively simple.
There are a number of different ways that the energy can be utilized, providing an opportunity to optimize cost and convenience. Common forms of electrical heating include:
Resistance heating, which involves passing an electric current through a resistance to generate heat. It is probably the most common method of using electrical power for process heating. The electric current may pass through an external resistance (indirect heating) or through the material to be heated (direct heating). It has been used for food processing [Skudder and Biss (1987)].
Induction heating utilizes the transfer of energy from a coil to the workpiece via an alternating magnetic field. Traditionally, the technique has been used for metal heating, but in more recent times it has been applied to chemical reactors [Hobson and Day (1985)].
Dielectric heating involves subjecting the material to be heated—provided it contains suitable molecules capable of excitation—to the effects of an electric field alternating at radio or microwave frequencies. It has been used in polymerization and curing processes.
Infrared heating depends on radiation effects. The principle is that an element (resistance-heated) radiates heat energy that may be focused or reflected in the same way as light energy, and can therefore be directed as determined by the process requirements. The principles of Radiative Heat Transfer discussed elsewhere in this Encyclopaedia apply to this technique. Uses of infrared heating include drying of sheet material and spray-painted articles.
Hobson, L. and Day, J. (1985) Induction heating of vessels. Int. J. Electr. Eng. Educ. 2: 129.
Skudder, P. and Biss, C. (1987) Aseptic processing of food products using Ohmic heating. Chem. Engr. Feb: 26.