Tuesday, July 3, 2018

Electric Heating Element Design: Nichrome Wire

Nichrome wire heater element
Nichrome wire heating element inside a quartz tube.
(Image courtesy of Wikipedia)
When an electric current passes through a conductive material (a resistor) energy in the form of heat is released. The greater the resistance to electron flow, the greater the heat energy created. The terms resistance and conductance apply to the nature of the conductive material, and it's ability to pass current.

Resistance (measured in ohms and using symbol "R") is defined as the electrical voltage (in volts using symbols "V") divided by the current (in amps, using symbol "I"), or R=V/I. This formula is one variant of Ohm's Law.

The heat, or power, released from the resistor (measured in watts, using the symbol "P") is a function of the supply voltage squared, divided by the conductor resistance. This version of Ohm's Law looks like this: Watts = Voltage squared / Resistance, or P=V2/R.

You can see if the resistance is too high, voltage does not flow, and no heat is produced. For the benefit of simplicity, we'll forego a discussion into superconductors, like those used on MRI machines and mass spectrometers, because their behavior includes complicated magnetic field discussions. Instead, we'll stick to common conductors often used to pass electrical current.

Selecting the right resistive material for a heating element is crucial in order to maximize heat output, heater longevity and energy usage - a conductive material with high resistance that is also easy to work with.

Nichrome alloy, made up of 80% nickel and 20% chromium is by far the most popular resistance heater wire, and is a available in a wide variety of wire gauges and ribbon shapes. It's popularity is performance based - it has high resistance, is easy to apply to many heater configurations, does not oxidize, has a low expansion coefficient and a high melting point. In the design and development of electric heating elements, if you grant that nichrome wire is at the heart of most electric heaters, everything else comes down to packaging and performance.

While there are other materials such as Kanthal (iron / chromium / aluminum) and Cupronickel (copper / nickel), and newer exotic ceramics, the vast majority of electric heaters used in industrial, commercial, OEM, and consumer goods all still rely on the ubiquitous and time proven nichrome alloy.

For more information on electric heating elements, contact BCE by visiting https://bcemfg.com or by calling (510) 274-1990.