Infrared heater

An infrared heater is a body with a higher temperature which transfers energy to a body with a lower temperature through electromagnetic radiation. Depending on the temperature of the emitting body, the wavelength of the infrared radiation ranges from 780 nm to 1 mm. The relationship between temperature and wavelength is expressed by the Stefan-Boltzmann Law. No contact or medium between the two bodies is needed for the energy transfer.A rough classification of infrared heaters is connected to wavelength bands of major emission of the energy: short wave or near infrared for the range from 780 nm to 1400 nm, these emitters are also named bright because still some visible light with glare is emitted; medium infrared for the range between 1400 nm and 3000 nm; far infrared or dark emitters for everything above 3000 nm.

Elements of infrared heaters

For practical purposes, most infrared heaters are constructed by either using the emission of a flame (usually soot or a heated matrix) or an electrically heated filament as the emitting body. If an electrically operated infrared heater (infrared lamp) is used, the filament is usually protected by a heat-resistant quartz glass tube. Depending on the filament temperature, a filling of the quartz tube with inert gas (e.g. halogen) may be required to prevent filament degradation. These emitters use the same materials and principle as a light bulb.

The most common filament material used for electrical infrared heaters is tungsten wire, which is coiled to provide more surface area. Low temperature alternatives for tungsten are carbon , or alloys of iron, chromium and aluminium (brand name ‘kanthal’). While carbon filaments are more fickle to produce, they heat up much quicker than a comparable medium-wave heater based on a FeCrAl filament.

Industrial infrared heaters sometimes use a gold coating on the quartz tube that reflects the infrared radiation and directs it towards the product to be heated. Consequently the infrared radiation impinging on the product is virtually doubled. Gold is used because of its oxidation resistance and very high IR reflectivity of approximately 95 %

Types of infrared heaters

Infrared heaters are commonly used in infrared modules (or emitter banks) combining several heaters to achieve larger heated areas.

Infrared heaters are usually classified by the wavelength they emit. Near infrared (NIR) or short-wave infrared heaters operate at high filament temperatures above 1800 °C and when arranged in a field reach high power densities of some 100s of kW/m². Their peak wavelength is well below the absorption spectrum for water, making them unsuitable for many drying applications. They are well suited for heating of silica where a deep penetration is needed.

Medium-wave and carbon (CIR) infrared heaters operate at filament temperatures of around 1000 °C. They reach maximum power densities of up to 60 kW/m² (medium-wave) and 150 kW/m² (CIR).

Efficiency of infrared heaters

Theoretically, the efficiency of an infrared heater is 100% as it converts nearly all electrical energy into heat in the filament. The filament then emits its heat by infrared radiation that is directly or via a reflector impinging on the product to be heated. Some energy is lost due to conduction or convection.

For practical applications, the efficiency of the infrared heater depends on matching the emitted wavelength and the absorption spectrum of the material to be heated.

For example, the absorption spectrum for water has its peak at around 3000 nm. This means that emission from medium-wave or carbon infrared heaters are much better absorbed by water and water-based coatings than NIR or short-wave infrared radiation.

The same is true for many plastics like PVC or polyethylene. Their peak absorption is around 3500 nm. On the other hand, some metals absorb only in the short-wave range and show a strong reflectivity in the medium and far infrared. This makes a careful selection of the right infrared heater type important for energy efficiency in the heating process.

Applications of infrared heaters

IR heaters are used in industrial manufacturing processes including curing of coatings; heating of plastic prior to forming; plastic welding; processing glass; cooking and browning food. They are used when high temperatures are required, fast responses or temperature gradients are needed or products need to be heated in certain areas in a targeted way. Their application is difficult for objects with undercuts.

They are also used to provide warmth to suckling animals whose mother cannot or will not provide them with natural warmth as well as to captive animals in zoos or veterinary clinics, especially for lizards and other reptiles, and tropical animals such as birds.

IR heaters are used in low-temperature infrared saunas.

Further reading and references:

Deshmukh, Yeshvant V.: Industrial Heating, Principles, Techniques, Materials, Applications, and Design. Taylor and Francis, Boca Raton, Fl: 2005.

Siegel, Robert and Howell, John R.: Thermal Radiation Heat Transfer. 3rd Ed. Taylor and Francis, Philadelphia, PA:

External links

[http://www.heraeus-noblelight.com/index.php?id=357 Emission spectra of different infrared heater types]

[http://www.pegasusassociates.com/InventionOfLightBulb.html A Short History of the Invention of the Incandescent Light Bulb]