Electroheating: clean and energy-efficient

Find it in everything from metals to food

By Morand Fachot

Electroheating, the high-power heating of material using electrical energy, is widespread in many industrial sectors. It is used for processing elements ranging from metals to glass, ceramics, polymers and natural fibres. It is also widely employed in the food industry. In general it is more energy efficient than other industrial processes relying on fossil fuels as well as being significantly cleaner and offering better quality finished products as temperatures can be more precisely controlled.

Aluminum alloy convection conveyor furnace (Photo: Infrared Heating Technologies) Aluminum alloy convection conveyor furnace (Photo: Infrared Heating Technologies)

Not widely known, yet widely used…

Industrial electroheating doesn't ring many bells with most people, yet it is by no means new. It is used in many sectors to produce or process a range of different materials and goods we use and consume, and employs a variety of technologies.

IEC TC (Technical Committee) 27: Industrial electroheating and electromagnetic processing, plays a central role in preparing International Standards for electroheating installations.

Electroheating is used in "big and heavy" industrial applications such as the melting of metals (copper, steel, aluminium and alloys), the forging of steel billets, the hardening of steel parts and the welding, cutting, drying and treatment of various metals and other materials. It is also widely used in lighter processes such as the sterilization of food and the drying of textiles and of ceramic tiles.

While electroheating may be used to bring materials to very high temperatures, in other cases it may be needed to process materials at lower temperatures. These distinct requirements call for different technologies and heating methods such as electric arc, induction and resistance heating, or used with lasers and plasma or in radio-frequency and microwave installations.

Electroheating technologies have a highly significant impact on industrial electricity consumption, accounting for between 20% and 40% of use in the EU, according to UIE (International Union for Electricity Applications) data. As a result, improving the energy efficiency of these technologies is an important objective that figures prominently on TC 27's agenda.

From arc to lasers…

A non-exhaustive overview of some electroheating technologies can give an indication of their importance in today's manufacturing. A number of attempts to employ an electric arc to melt iron were being made as far back as the 19th century. Patents were taken out for EAFs (electric arc furnaces) in the 1880s and the first commercial EAF plant was built in the US in the early 1900s.

The advantages of EAFs in steel production are their relatively low capital cost in comparison with traditional steel mills, and their capacity. This can extend from around one tonne to hundreds of tonnes, allowing the establishment of mini-mills.

EAFs also allow steel to be made from 100% scrap metal, providing considerable savings in energy when compared with primary steelmaking from ores using blast furnaces. Unlike the latter, EAFs can also be started and stopped rapidly, enabling them to cater for variation in demand.

In induction heating, an electrically conductive object (usually metal) is heated by passing an alternating current through an electromagnet. Induction furnaces are used to melt various metals including steel, copper or aluminium, or even precious metals. Their capacities range from less than one kilo to one hundred tonnes. The temperature of the material to be heated can be controlled with complete precision.

Resistance heating is used extensively in electroheating. The process involves current being passed through a set of resistances that act as heating elements and is generally applied in a well-insulated enclosure so as to minimize heat losses. Resistance heating is used to heat treat, form, melt and dry metals; to cook, sterilize and roast in the food industry or to fire and dry ceramic products.

Resistance heating can be indirect: heat from the resistor is transferred to the work piece via conduction (close proximity between resistances and work piece), convection (through the air) or radiation (infrared heating); it can also be direct. Direct resistance heating, also referred to as conductive heating, involves passing current directly through the work piece to be heated.

Other electroheating technologies include use of plasma torches to cut steel plates, microwaves to treat food products, radio-frequency electric fields to dry textiles, and lasers to weld, cut and treat various materials.

Energy-efficient and flexible

Industrial applications of electroheating technologies in many sectors show them to be more energy efficient and cleaner than their "conventional" equivalents that use fossil fuels, especially at higher temperatures. The optimum efficiency of gas furnaces is from 40%-80%, while that of an electric furnace can reach 95%.

However, measuring the emission of CO2 and other noxious gases is complex for electroheating as it depends on the primary energy mix used to generate the electricity that the equipment needs.

From a practical angle, electroheating presents a number of advantages. For coating and curing surfaces of transformers, a Canadian plant found electric IR (infrared) systems to be more energy efficient and cleaner than gas convection ovens. Start-up and shutdown times were shorter, as long preheating and cooling periods were not required; the processing speed was doubled; surfaces alone were treated so the rest of the equipment did not have to sustain high temperatures unnecessarily. Similar findings were obtained when curing polyester coatings on light fixtures using infrared or gas convection ovens.

From surface to deep heating

If infrared or radiation heating is highly efficient in applying heat and curing surfaces, so-called dielectric heating – a term covering RF (radio-frequency) and microwave heating – is more effective for some other applications, since heating occurs inside the material. In both cases the material is heated by an electromagnetic field continuously reversing directions at very high frequency, between 10 and 100 MHz for RF and between 300 and 30 000 MHz for microwave.

As water heats up very fast, RF and microwave heating are used in many industrial applications, such as drying, fixing dye and controlling moisture content in the textile industry. These processes are also used for the sterilization of medical equipment; drying, cooking, heating and sterilization in the food industry and a variety of other applications in industries such as chemical, rubber, paper and wood.

Multiple advantages

Dielectric heating is much faster than conventional techniques: processing times are reduced by as much as 80% and pre-heating and cooling periods are not required. It allows for precise temperature control, resulting in higher product quality than that achieved using conventional heating procedures.

It is also cleaner as no fuels are burned at the production site and the lives of operators are therefore not put at risk. Microwave heating is also safe as electromagnetic waves are kept inside the heating chamber. Dielectric heating systems are smaller than comparable conventional systems, giving space savings of up to 90%.

IEC work essential for the industry

IEC TC 27, created in 1937, prepares International Standards for industrial electroheating and electromagnetic processing.

Electroheating supports a wide range of heating methods. As demands for energy savings, improved product quality and environmental protection grow, the range of applications is expanding. Electroheating also offers interesting prospects in new domains such as nanotechnology and optoelectronics.

TC 27 works extensively where the safety of electroheating installations is concerned, having published 12 International Standards covering all electrical and non-electrical safety aspects. Its objectives for the next 3-5 years include a comprehensive revision of these standards as well as of the large series of test standards, in terms of technological developments and market demands. The TC will also start work on developing safety and test standards for new installations not covered by existing standards.

TC 27 intends to amend existing standards to address EEE (Electrical Energy Efficiency), EMC (Electromagnetic Compatibility) and EMF (Electromagnetic Field) issues in electroheating installations.

The increasing number of technologies being used in electroheating means that the process is constantly evolving and highly flexible, as well as becoming economically more significant and able to be implemented in countless operations. All this points to a very busy agenda for TC 27 in the future.

Aluminum alloy convection conveyor furnace (Photo: Infrared Heating Technologies) Aluminum alloy convection conveyor furnace (Photo: Infrared Heating Technologies)
Heaters in continuous coil coating dryer (Photo: Casso-Solar Technologies) Heaters in continuous coil coating dryer (Photo: Casso-Solar Technologies)
IR / convection oven for automotive dashboards (Photo: Infrared Heating Technologies) IR / convection oven for automotive dashboards (Photo: Infrared Heating Technologies)