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 Wien's displacement law. No contact or medium between the two bodies is needed for the energy transfer.

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 1400 nm, these emitters are also named bright because still some visible light is emitted;
• Medium infrared for the range between 1400 nm and 3000 nm;
• Far infrared or dark emitters for everything above 3000 nm.

A household infrared electric heater

Elements

Infrared heaters use either a fuel to heat an emitter, or an electrically heated filament as the emitting body. An infrared lamp will protect the filament with a heat-resistant quartz glass tube. Some heat lamps used halogen lamps. These emitters use the same materials and principle as an incandescent 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 aluminum (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.

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

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.

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 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 microns. 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.

Quartz heat lamps

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 microns. The enclosed filament operates at around 2500 K, producing more shorter-wavelength radiation than open wire-coil sources. Developed in the 1950s at General Electric, these lamps produce about 100 watts/inch (4 w/mm) and can be combined to radiate 500 watts per square foot (54000 watts/square m). 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 28 kW/ square foot (300 kW/square meter).

Clear Quartz Element Example

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.

Much of the infrared and visible energy released is caused by the direct heating of the quartz material, 97% of the near infrared is absorbed by the silica quartz glass tube causing the temperature of the tube wall to increases, this causes the silicon-oxygen bond to radiate far infrared rays.^{[citation needed]} 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.^{[citation needed]}

Quartz heater

Quartz tungsten

Quartz tungsten infrared heaters emit medium wave energy reaching operating temperatures of up to 1500°C (medium wave) and 2600°C (short wave). It reaches top temperatures within seconds. Peak wavelength emissions of approximately 1.6 microns (medium wave infrared) and 1 micron (short wave infrared).

Carbon

Carbon heaters are relatively recent and produce high quality, long wave far infrared heat, but the problem is they do not produce a big amount of it. For this reason it is not a popular solution for space heating, but it is widely use in many infrared saunas.

Gas-fired

There are two basic types of infrared radiant heaters.

• Luminous or high intensity
• Radiant tube heaters

Radiant tube gas-fired heaters used for industrial and commercial building space heating burn natural gas or propane to heat a steel emitter tube. Gas passing through a control valve flows through a cup burner or a venturi. The combustion product gases heat the emitter tube. As the tube heats, radiant energy from the tube strikes floors and other objects in the area, warming them. This form of heating maintains warmth even when a large volume of cold air is suddenly introduced, such as in maintenance garages.

The efficiency of an infrared heater is a rating of the total energy consumed by the heater compared to the amount of infrared energy generated. While there will always be some amount of convective heat generated through the process, any introduction of air motion across the heater will reduce its infrared conversion efficiency

Far-infrared

This heating technology is used in some expensive infrared saunas. It is also found in space heaters. These heaters use 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, and infrared rays are directional, only the person who is receiving directly the far infrared rays will feel warm in the room. Therefore, in the case of far infrared space heaters, it is extremely important to make a study of the room and place the far infrared panels directed to the places where people stay most of the time (sofa, dining table, bed, etc.).

Efficiency

Electrically-heated infrared heaters radiate up to 86% of their input as radiant energy. Nearly all the electrical energy input is converted into infrared radiant heat in the filament and directed onto the product by reflectors. 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.

Ceramic elements operate in the temperature of 300°C to 700°C (572°F - 1292°F) producing infrared wavelengths in the 2000 to 10000 nm range. Most plastics and many other materials absorb infrared best in this range, which makes the ceramic heater most suited for this task.^{[citation needed]}

Applications

Infrared heater cooking döner kebab

IR heaters are used in industrial manufacturing processes including curing of coatings, shrink tunnels, heating of plastic prior to forming, plastic welding, processing glass,and 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 as well as to captive animals in zoos or veterinary clinics, especially for lizards and other reptiles, and tropical animals such as birds.

Health effects

In addition to the dangers of touching the hot bulb or element, infrared heaters may cause indirect thermal burns when the skin is exposed for too long or the heater is positioned too close to the subject. Individuals exposed to large amounts of infrared radiation over an extended period of time may develop depigmentation of the iris and opacity of the aqueous humor, so exposure should be moderated.

Further reading

• 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.