How Does A Heating Coil System Work?

A heating coil consists of a coil of resistance wire, typically made from a nickel-chromium (nichrome) or iron-chrome-aluminum alloy with high resistivity and a high melting point, connected to an electrical power supply. When an electric current flows through the wire, heat is generated, converting electrical energy into heat through a process known as "joule heating". This heat can then be used to heat the air in your home or in appliances like toasters, hair dryers, or coffee machines.

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The coil's materials are chosen because they resist corrosion and have a high melting point, allowing them to operate properly at high temperatures.

A "heating coil" is just one form of electric heating element. Resistance wires, strips, or ribbons can be configured in other shapes, all of which produce heat in the same way via the electrical current from a flow of electricity.

Heating Elements

by Chris Woodford. Last updated: March 25.

Fire was one of humankind's earliest and greatest discoveries, made over one million years ago. In our modern age of jet engines, space rockets, steel skyscrapers, and synthetic plastics, smoke and flames might seem positively prehistoric. But all of these inventions rely on fire in one crucial way or another.

Fire is brilliant, but it's not very convenient. Sometimes it takes ages to get a fire going; for instance, coal-powered steam locomotives need to be fired up several hours before they can pull trains. Other times, fire breaks out unexpectedly, threatening lives, buildings, and possessions. Wouldn't it be great if fire were as easy to control as electricity, so you could turn it on and off at a moment's notice? That's the basic idea behind heating elements. They are the "fire" inside electric heaters, showers, toasters, stoves, hair dryers, clothes dryers, soldering irons, and other handy household appliances. Heating elements give us the power of fire with the convenience of electricity. Let's take a closer look at what they are and how they work!

A typical electric fire standing on a stone hearth features three electric heating elements ("bars") mounted in white ceramic behind a silver-colored safety grid. These elements glow red hot when in use, emulating a primitive yet effective source of heat.

Making Heat from Electricity

In school, we learn that some materials conduct electricity well, while others do so poorly. Good conductors are called conductors, and poor ones are called insulators. Conductors and insulators are often described by their resistance to electric current flow. Conductors have low resistance, while insulators have high resistance. In an electrical circuit, resistors can control current flow. For example, using a dial to increase resistance can lower the current in a loudspeaker circuit, thus turning down the volume.

Resistors work by converting electrical energy to heat energy, getting hot when electricity flows through them. But it's not just resistors that do this; even a thin piece of wire will get hot if enough electricity flows through it. This is the basic idea behind incandescent lamps, which use a thin wire filament that glows white hot when enough electricity flows through it. However, these lamps produce more heat than light, making them inefficient for illumination purposes but quite effective as heating elements.

If we focus on the heat generated instead of light, we find that traditional incandescent lamps are quite efficient as heating elements. These lamps convert about 95% of the electricity they consume into heat. However, a lamp's heat output is localized and insufficient for broader applications like space heating. For such purposes, we need a more robust material that can endure repeated heating and cooling, maintain a reasonable temperature, and provide more widespread heat. This is the essence of a heating element: a sturdy electrical component designed to emit heat when a significant electric current flows through it.

What is a Heating Element?

A typical heating element is a coil, ribbon, or strip of wire that emits heat similar to a lamp filament. When an electric current flows through it, the wire glows red hot, converting electrical energy into heat and radiating it in all directions.

Materials

Heating elements are usually either nickel-based or iron-based. The nickel-based ones are often made of nichrome, an alloy consisting of about 80% nickel and 20% chromium. Nichrome has a high melting point, doesn't oxidize easily, doesn't expand too much when heated, and has a relatively stable resistance.

In water heaters, the nichrome element is covered with an outer sheath made of stainless steel, tin-coated copper, or INCOLOY® (an iron-nickel-chromium superalloy). This sheath is insulated from the heating element by magnesium oxide, a material that conducts heat well but not electricity.

Types of Heating Elements

There are various types of heating elements. Sometimes nichrome is used bare; other times it's embedded in a ceramic material to make it more robust and durable. The size and shape of a heating element depend on the appliance it fits into and the area over which it needs to produce heat. For example, hair curling tongs have short, coiled elements, while electric radiators have long bar elements. Electric stoves have coiled heating elements just the right size to heat cooking pots and pans.

Different appliances have varying visibility of their heating elements. In electric toasters, nichrome ribbons are visible and glow red hot. Electric radiators use glowing red bars, while electric convector heaters contain heating elements positioned in front of or behind electric fans for faster heat distribution. Electric kettles have concealed elements, operating just below the boiling point of water to ensure safety.

For other applications, heating elements are completely concealed for safety reasons, such as in electric showers and hair curling tongs. Electronic cigarettes also use battery-powered heating elements to generate aerosols.

Designing Heating Elements

Designing heating elements involves multiple factors, including determining how the element will be used. For instance, a heated rear window in a vehicle requires an element that does not obstruct the driver's view, adheres permanently to the glass, and is powerful enough to melt frost and snow quickly.

Thor Hegbom's book on the subject lists approximately 20-30 factors that affect the performance of a heating element. These include the voltage and current, element length and diameter, material type, and operating temperature. Specific types of elements have additional considerations, such as wire diameter and coil form for coiled elements or ribbon thickness and width for ribbon elements.

Heating elements do not work in isolation; they must fit into a larger appliance and function correctly during use. Engineers must consider how the element will be supported, how it will behave under different conditions, and how to ensure it does not sag or overheat. The goal is to design a product that is effective, economical, durable, and safe.

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