Petrie Ltd

What is Infrared Heating?
Infrared (IR) heating is a process within a family of heating technologies known as electro-heat techniques designed to heat materials. Not dissimilar to dielectric heating in that the principle is that portions of the electromagnetic spectrum are utilised as the primary energy source to heat a material, it differs in that the target materials are not necessarily electrically-insulating (dielectric), but can be both insulators and conductors.

The application of electromagnetic radiation may be direct, where the electromagnetic energy is irradiated directly onto a material in order to heat it; or indirect, where the electromagnetic energy is used to heat an appliance which is directly applied to the material. This is fundamentally different from conventional heating techniques, based on conduction and convection, where the primary energy source to heat a material is through migration of heat flux through the material. For IR heating, the application is almost ubiquitously direct.

Fundamentals of IR Heating
The basis of IR heating is that a higher temperature body transfers energy to a lower temperature body through electromagnetic radiation in the infrared region of the electromagnetic spectrum. The wavelength of the IR radiation depends on the temperature of the emitting body, and is typically in the range of 780 nm to 1 mm. Since the energy transfer is electromagnetic no contact or medium between the two bodies is needed in order to heat the material.

Practicalities of IR Heating
Industrial IR heaters are typically constructed from a bank of electrically heated tungsten, carbon or iron alloy filaments, which acts as the emitting body. This filament is protected by a heat-resistant quartz glass tube and is usually filled with an inert gas to prevent degradation of the filament. This is the same principle as the conventional light-bulb. The quartz tube often has a gold coating around it which deflects and directs the IR radiation towards the product. The use of gold serves to maximise the IR reflectivity, reflects approximately 95% of incident IR radiation, and is highly resistant to oxidisation which is an issue in moist industrial environments.

IR heaters are classified by the wavelength they emit. Short-wave IR heaters, also known as near-infrared (NIR) heaters, operate at high-filament temperatures of around 1800°C and can be arranged to give power densities of hundreds of kilowatts per square metre (kW/m²). Medium-wave IR and carbon-infrared (CIR) heaters, owing to the fact they have carbon filaments, operate at temperatures of around 1000°C and can be arranged to give power densities of up to 60kW/m² (medium-wave) and 150kW/m² (CIR) respectively.

Short-wave IR heaters are typically not used for high-moisture applications, such as drying processes, because the peek wavelength is below absorption level of water. They are mainly used for deep-penetration applications. Medium-wave and CIR heaters are better suited to moisture dependant applications and small-scale products. Large-wave, also known as far-infrared (FIR) heaters are typically used in low-temperature processes and thus have restricted usage in industrial applications.

Typical applications of IR heating are plastic forming, and welding, glass processing, cooking and browning of food. In food applications IR heating is often used with dielectric heating to obtain the dual benefits of rapid volumetric heating along with browning capabilities in order to obtain the desired product textures.

Advantages of IR Heating
The advantages of IR heating for industrial processes are:

• Efficiency – The filament end of the IR heater has near 100% efficiency since it converts nearly all electrical energy to heat. With careful heater design this can translate into very high efficiencies throughout the heater by matching the emitted wavelength from the filament to the absorption spectrum of the product and ensuring the deflectors maximise the energy impinging on the product. There are small losses due to conduction and convection between the filament and product.

• High-temperature – The high-temperatures of the filament translate into high heating temperatures for the product. This makes IR heating ideally suited to applications where temperature gradients or high-tolerance surface is necessary.

• Rapid Heating – IR heating can dissipate extremely high power densities within a product resulting in very fast heating times. This is an advantage for direct heating applications where the need to minimise the effects of heat conduction are essential to avoid damaging the product. It is also an advantage in indirect heating applications where heating times are an important parameter.

• Controllability – In IR heating, the power supplied can be regulated very accurately. This allows safe and precise control of an applicator even when applying large power or rapid heating rates.

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