Infrared heater


An infrared heater or heat lamp is a heating appliance containing a high-temperature emitter that transfers energy to a cooler object through electromagnetic radiation. Depending on the temperature of the emitter, the wavelength of the peak of the infrared radiation ranges from to 1 mm. No contact or medium between the emitter and cool object is needed for the energy transfer. Infrared heaters can be operated in vacuum or atmosphere.
One classification of infrared heaters is by the wavelength bands of infrared emission.
  • Short wave or near infrared for the range from to ; these emitters are also named "bright" because still some visible light is emitted;
  • Medium infrared for the range between and ;
  • Far infrared or dark emitters for everything above.

    History

German-British astronomer Sir William Herschel is credited with the discovery of infrared in 1800. He made an instrument called a spectrometer to measure the magnitude of radiant power at different wavelengths. This instrument was made from three pieces. The first was a prism to catch the sunlight and direct and disperse the colors down onto a table, the second was a small panel of cardboard with a slit wide enough for only a single color to pass through it and finally, three mercury-in-glass thermometers. Through his experiment Herschel found that red light had the highest degree of temperature change in the light spectrum, however, infrared heating was not commonly used until World War II. During World War II infrared heating became more widely used and recognized. The main applications were in the metal finishing fields, particularly in the curing and drying of paints and lacquers on military equipment. Banks of lamp bulbs were used very successfully; though by today's standards the power intensities were very low, the technique offered much faster drying times than the fuel convection ovens of the time. After World War II the adoption of infrared heating techniques continued but on a much slower basis. In the mid 1950s the motor vehicle industry began to show interest in the capabilities of infrared for paint curing and a number of production line infrared tunnels came into use.

Elements

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 aluminum. While carbon filaments are more fickle to produce, they heat up much more quickly than a comparable medium-wave heater based on a metal alloy filament.
When light is undesirable or not necessary in a heater, ceramic infrared radiant heaters are the preferred choice. Containing of coiled alloy resistance wire, they emit a uniform heat across the entire surface of the heater and the ceramic is 90% absorbent of the radiation. As absorption and emission are based on the same physical causes in each body, ceramic is ideally suited as a material for infrared heaters.
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 infrared reflectivity of approximately 95%.

Types

Infrared heaters are commonly used in infrared modules combining several heaters to achieve larger heated areas.
Infrared heaters are usually classified by the wavelength they emit:
Near infrared or short-wave infrared heaters operate at high filament temperatures above and when arranged in a field reach high power densities of some hundreds of kW/m. 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 infrared heaters operate at filament temperatures of around. They reach maximum power densities of up to and .
Far infrared emitters are typically used in the so-called low-temperature far infrared saunas. These constitute only the higher and more expensive range of the market of infrared sauna. Instead of using carbon, quartz or high watt ceramic emitters, which emit near and medium infrared radiation, heat and light, far infrared emitters use low watt ceramic plates that remain cold, while still emitting far infrared radiation.
The relationship between temperature and peak wavelength is expressed by Wien's displacement law.

Metal wire element

Metal wire heating elements first appeared in the 1920s. These elements consist of wire made from chromel. Chromel is made from nickel and chrome and it is also known as nichrome. This wire was then coiled into a spiral and wrapped around a ceramic body. When heated to high temperatures it forms a protective layer of chromium oxide which protects the wire from burning and corrosion, and causes the element to glow.

Heat lamps

A heat lamp is an incandescent light bulb that is used for the principal purpose of creating heat. The spectrum of black-body radiation emitted by the lamp can be shifted to produce longer wave-length infrared light by reducing the filament temperature.
However, many popular heat lamps for near infrared are constructed as flood lamps from glass and an internal reflector and use filament temperatures around 2000 K and a red filter to minimize the amount of visible light emitted. Such often 250 watt heat lamps are commonly packaged in the "R40" form factor with an intermediate screw base.
Heat lamps are commonly used in food-preparation areas of restaurants to keep food warm before serving and for animal husbandry. Lights used for poultry are called brooder lamps. Aside from young birds, other types of animals which can benefit from heat lamps include reptiles, amphibians, insects, arachnids, and the young of some mammals. Ordinary household white incandescent bulbs can be used as heat lamps, as was exploited by Heatball in order to circumvent the phase-out of incandescent light bulbs, but these are very bright unless voltage-reduced or filtered and specific lamps with color filters are sold for animal use.
Heat lamps can be used as a medical treatment to provide dry heat when other treatments are ineffective or impractical.

Ceramic infrared heat systems

Ceramic infrared heating elements are used in a diverse range of industrial processes where long wave infrared radiation is required. Their useful wavelength range is 2–10 μm. They are often used in the area of animal/pet healthcare too. The ceramic infrared heaters are manufactured with three basic emitter faces: trough, flat, and bulb or Edison screw element for normal installation via an E27 ceramic lamp holder.

Far-infrared

This heating technology is used in some expensive infrared saunas. It is also found in energy efficient space heaters. They are usually fairly big flat panels that are placed on walls, ceilings or integrated in floors. These heaters emit long wave infrared radiation using low watt density ceramic emitters based on carbon fibre technology. More efficient designs use carbon crystals, a combination of carbon fibre, integrated with nanotechnology, transforming carbon into nanometer form. Because the heating elements are at a relatively low temperature, far-infrared heaters do not give emissions and smell from dust, dirt, formaldehyde, toxic fumes from paint-coating, etc. This has made this type of space heating very popular among people with severe allergies and multiple chemical sensitivity in Europe. Because far infrared technology does not heat the air of the room directly, it is important to maximize the exposure of available surfaces which then re-emit the warmth to provide an even all round ambient warmth. This is known as radiant heating.

Quartz heat lamps

Halogen lamps are incandescent lamps filled with highly pressurized inert gas combined with a small amount of halogen gas ; this lengthens the life of the filament. This leads to a much longer life of halogen lamps than other incandescent lamps. Due to the high pressure and temperature halogen lamps produce, they are relatively small and made out of quartz glass because it has a higher melting point than standard glass. Common uses for halogen lamps are table top heaters.
Quartz infrared heating elements emit medium wave infrared energy and are particularly effective in systems where rapid heater response is required. Tubular infrared lamps in quartz bulbs produce infrared radiation in wavelengths of 1.5–8 μm. The enclosed filament operates at around, producing more shorter-wavelength radiation than open wire-coil sources. Developed in the 1950s at General Electric, these lamps produce about and can be combined to radiate. To achieve even higher power densities, halogen lamps were used. Quartz infrared lamps are used in highly polished reflectors to direct radiation in a uniform and concentrated pattern.
Quartz heat lamps are used in food processing, chemical processing, paint drying, and thawing of frozen materials. They can also be used for comfort heating in cold areas, in incubators, and in other applications for heating, drying, and baking. During development of space re-entry vehicles, banks of quartz infrared lamps were used to test heat shield materials at power densities as high as. In 2000, General Electric launched the first quartz waterproof lamp alongside British infrared heating manufacturer Tansun.
Most common designs consist of either a satin milky-white quartz glass tube or clear quartz with an electrically resistant element, usually a tungsten wire, or a thin coil of iron-chromium-aluminum alloy. The atmospheric air is removed and filled with inert gases such as nitrogen and argon then sealed. In quartz halogen lamps, a small amount of halogen gas is added to prolong the heater's operational life.
The majority of the radiant energy released at operational temperatures is transmitted through the thin quartz tube but some of that energy is absorbed by the silica quartz glass tube causing the temperature of the tube wall to increase, this causes the silicon-oxygen bond to radiate far infrared rays. Quartz glass heating elements were originally designed for lighting applications, but when a lamp is at full power less than 5% of the emitted energy is in the visible spectrum.