Posted on March 11 2020
An infrared heater or heat lamp 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 peak of the infrared radiation ranges from 780 nm to 1 mm. No contact or medium between the two bodies 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 780 nm to 1.4 μm, these emitters are also named bright because still some visible light is emitted;
- Medium infrared for the range between 1.4 μm and 3 μm;
- Far infrared or dark emitters for everything above 3 μm.
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
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 (trademark and brand name Kanthal). While carbon filaments are more fickle to produce, they heat up much more quickly than a comparable medium-wave heater based on a FeCrAl filament.
When light is undesirable or not necessary in a heater, ceramic infrared radiant heaters are the preferred choice. Containing 8 meters 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%
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 hundreds 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).
Far infrared emitters (FIR) 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, this also causes the element to glow.
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 is shifted to produce more infrared light. Many heat lamps include a red filter to minimize the amount of visible light emitted. Heat lamps often include an internal reflector.
Heat lamps are commonly used in shower and bathrooms to warm bathers and in food-preparation areas of restaurants to keep food warm before serving. They are also commonly used for animal husbandry. Lights used for poultry are often called brooding 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.
The sockets used for heat lamps are usually ceramic because plastic sockets can melt or burn when exposed to the large amount of waste heat produced by the lamps, especially when operated in the "base up" position. The shroud or hood of the lamp is generally metal. There may be a wire guard over the front of the shroud, to prevent touching the hot surface of the bulb.
Ordinary household white incandescent bulbs can also be used as heat lamps, but red and blue bulbs are sold for use in brood lamps and reptile lamps. 250-watt heat lamps are commonly packaged in the "R40" (5" reflector lamp) form factor with an intermediate screw base.
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 (emitters) are manufactured with three basic emitter faces: trough (concave), flat, and bulb or Edison screw element for normal installation via an E27 ceramic lamp holder.
This heating technology is used in some expensive infrared saunas. It is also found in space heaters. These heaters use low watt density ceramic emitters (usually fairly big panels) which emit long wave infrared radiation. 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