Infrared heater
Infrared emitters are components or independently functioning devices that generate infrared radiation that is used for heating or drying purposes. Areas of application are e.g. B. animal husbandry , electric sauna , hall heating, living room heating or in medicine . Flammable gas or electricity is used as the energy source for infrared emitters . The principle of infrared radiation is identical for electrically and gas operated infrared emitters.
In contrast to other heating systems, infrared heaters mainly work by heating the illuminated surface, not by heating the air on the radiator.
In addition to infrared emitters, i.e. devices that emit broadband infrared radiation, there are also devices that only emit infrared radiation in a relatively narrow spectrum; this includes infrared lasers (especially the carbon dioxide laser ) and the infrared light emitting diode .
Infrared heater principle
The principle of the devices is simple and can best be explained with sunbathing on a glacier. Although the ambient temperature is below 0 ° C, it is warm in the sun. This is due to the heat radiation from the sun. Wherever it hits, it is (partially) absorbed and converted into heat, for example on our skin.
Infrared radiation represents only a small part of the electromagnetic spectrum. Radiation with shorter wavelengths than infrared is in the visible or harmful range ( ionizing radiation ) and is therefore undesirable. On the other hand, radiation with a higher wavelength is increasingly poorly absorbed (e.g. radar, radio).
In general, all radiation is absorbed by matter when the respective atoms and molecules can absorb the energy level that corresponds to the respective radiation frequency. The frequency ranges of the molecules in the air differ from those of liquid or solid bodies. For example, infrared radiation only heats air comparatively little, unlike solid bodies. The energy transfer takes place in a targeted and direct manner and differs significantly from convection , i. H. the warming of the ambient air. The aim is to keep the convective loss (rising warm air) as low as possible. By coordinating the emitted frequency and the target (e.g. a specific metal), the transmission can become even more selective.
Infrared heating
Infrared heaters belong to the category of radiant or heat wave heating and are mainly used in halls where conventional convection heaters would be uneconomical, the warm air of which would hang largely useless under the hall roof. This advantage is more pronounced in light emitter systems that work with a higher temperature and a smaller area of the emitter. Of radiant systems a lower risk of fire goes out.
For living areas, heaters are placed behind a protective layer to prevent burns if they touch the hot heater (80–100 ° C for panel heaters, IR heating rods significantly more) for a short time. The heaters are produced in many different versions as a surface, mirror, picture or sphere, fixed or transportable. Due to the uniform radiant heat, the low level of air heating and movement and the associated lower heating requirement, infrared heaters come e.g. B. in large or poorly insulated rooms for use. In contrast to conventional radiators, the room air is not heated in layers, but only the illuminated bodies, which then release the heat again. Infrared heaters are often advertised as very economical heaters with this argument, but when used continuously for space heating, infrared heating is just as inefficient as any other form of electrical heating.
Electrical operation
Basically, light sources are divided according to the type of emission: Infrared lamps, as well as normal incandescent lamps and most light sources, belong to the thermal radiators, that is, they emit radiation based on and according to their temperature. See also black body . In this context, black means that radiation is only emitted due to the body's own temperature and is not "falsified" due to reflections or the like. Conversely, every body with a temperature above zero Kelvin emits thermal radiation.
According to Wien's law of displacement , the “main color” of the thermal radiation of a (black) body only depends on its temperature: the colder it is, the lower the frequency of the radiation it emits. Example: A moderately heated iron wire glows dark red (low frequency); if the wire is heated further, the frequency increases, which is accompanied by a light red, orange, yellow or even bluish-white color. If, on the other hand, the wire is only at room temperature, its radiation is still “below” the darkest red that human eyes can still see, in other words: Such a wire does not seem to glow by itself.
So it depends on the type of lamp and thus the glow temperature how much visible light and how much infrared light it emits. Viewed in this way, relatively cool lamps would be quite efficient infrared lamps, but due to the Stefan-Boltzmann law , the total radiation output of a body decreases with the fourth power of its temperature.
Finally, we briefly refer to Planck's law of radiation . It describes the entire frequency distribution of black body radiation, not just the maximum. A well-known exception to the group of thermal emitters (as a light source) is the light-emitting diode : The radiation emitted is created from an electron transition, which ideally has nothing to do with the temperature inside the diode.
Quartz heater
With a quartz heater, the heating resistor through which the electric current flows is located in a slightly opaque quartz glass tube filled with inert gas . Therefore, the temperature of the wire can be selected higher than with a conventional radiant heater .
Quartz heater tubes typically have an outer diameter of 8-15 mm and are mostly straight cylindrical. They can be held in place with wire clips made of stainless steel and electrically contacted with 6.3 mm flat connectors. The pipe material consists of quartz-rich glass, pure quartz glass or often the somewhat cloudy quartz material Rotosil. Operating temperatures range from 600 ° C to 950 ° C.
Halogen spotlights
The efficiency of a halogen lamp is higher than that of a quartz lamp. It is also used for cooking under ceramic plates.
Infrared lamps
Infrared lamps (also known as red light lamps or heat lamps) are lamps that emit mostly invisible heat radiation . For this purpose, a mostly red filter is built into the lamp in order to filter out the remaining (non-red) visible light. The light sources used can also contain these filters directly in their glass envelope. In addition to the (still visible) red light component, the emitted radiation mainly only includes so-called near infrared radiation (NIR).
Infrared lamps are used in chick rearing stations and terrariums , for example . They emit infrared radiation in the range that many living things perceive as pleasant. This can be explained by the high proportion of NIR radiation, the most energetic infrared with the highest penetration depth (but still only a few millimeters, see penetration depths of IR radiation ): The resulting heat is generated by the incident radiation just below the skin surface , and not directly on the skin surface, which at least people sometimes find uncomfortable (skin drying out and a feeling of burns). At the same time, despite their power, these infrared lamps do not trigger an escape reflex in animals that otherwise avoid direct sunlight due to their mild and deep red radiation.
The intensity of modern infrared lamps can also be dimmed . The filament then no longer shines bright white to light yellow, but only reddish. The intensity of the infrared light is only slightly lower because of the shift in the radiation maximum (see Wien's law of shift ).
There are also modified infrared lamps that are designed more than infrared emitters . In these, the proportion of visible radiation is further reduced and proportionally more medium infrared (MIR) is emitted. Lamps based on incandescent filaments can still achieve a wavelength range of 5–10 µm. This type of infrared lamp is used when bodies are to be warmed up which are (largely) invisible to the NIR range, i.e. That is, let the corresponding radiation pass unhindered. An example of this is water ice. It is practically transparent in the visible range and in the NIR. It only becomes opaque in the far infrared (FIR), i.e. it absorbs the entire energy of the radiation and is thus heated. Effective “ice thawing infrared heaters” must therefore emit a high proportion of FIR radiation.
Also in industrial heating processes, electric infrared emitters z. B. used in thermoforming .
Gas operation
Radiant heaters in industry and camping, on the other hand, are usually operated with fuel gas , mostly with liquid gas , and less often with natural gas for stationary use . The gas flame heats the incandescent body. Industrial radiant heaters can be used for sole heating of the hall. The installation regulations applicable to gas appliances must be observed for the gas emitters. Because of these properties, they are hardly suitable for living areas. In recent years, more and more patio heaters (also known as “patio heaters”) have been used in outdoor areas such as street cafes. These outdoor heating systems are criticized for their "aesthetics" and harmful effects on the climate, but they are still approved in most German cities. In the case of direct gas-fired devices, there are essentially two types of infrared emitters: light emitters and dark emitters .
Bright emitters
Bright radiators are directly heated by an atmospheric burner and operated with natural gas , petroleum or liquid gas . They are installed on the wall or ceiling. They are called bright radiators because infrared rays are generated by visible combustion of a gas-air mixture on the underside of the device. Ceramic plates glow "brightly". The perforated ceramic plates also form the heart of the light radiators. The gas-air mixture flows through them and burns on their surfaces. The ceramic plate surface is heated up to 950 ° C and emits infrared radiation. Reflectors reflect the radiation downwards into the occupied area.
plates
In the past, the ceramic plates were relatively simple. On average, they had around 1200 holes and were only a quarter the size of today's plates. The surface of the rectangular plates was flat. Developers started improving the ceramic plate back in the 1970s. They recognized that the power output and emissions largely depend on the surface properties and the structure of the plate. Today there are between 3000 and 4000 holes with a diameter of 1–1.3 mm on a board. The surface, the so-called depth effect structure, resembles an evenly arranged honeycomb. They increase the specific surface and thus also the heat transfer area and the radiation yield by around 60%. A small flame burns in every hole. This creates a very hot ceramic surface, although the actual flames remain relatively cool. This reduces the nitrogen oxide (NO x ) values to a barely measurable range. The carbon monoxide (CO) values are in the range of modern condensing boilers, which often use the same ceramic plates and the effect of hot surface and cool flame. A high-quality ceramic plate has a very long service life. Thanks to advanced manufacturing processes, they have an extremely dense and homogeneous structure. This is particularly important with the innumerable interactions between cold and hot, caused by switching on and off over many years of operation.
Reflectors
In order to meet the requirements for high power output, there are also fully insulated devices in addition to uninsulated devices. The insulation ensures that the heat transfer to the outside of the reflector is very low. This creates a hot air cushion in the heater, the reflectors get hot and in turn radiate heat. This effect is called combined radiation. Another “reflector”, but in the form of a grid, the so-called radiation grid, is located directly under the ceramic plates. It causes some of the radiation from the ceramic plates to be reflected back towards them. The radiation is converted into heat on the surface and the temperature of the ceramic plate begins to rise, a "ping-pong" of radiation.
Infrared power
The performance of the devices has increased rapidly in recent years. Whereas devices used to achieve an average of only 40–50%, the emitted power (radiation factor) is now between 65% and 77%. The radiation factor is therefore a direct indicator of the energy yield.
Areas of application
Bright radiators are particularly suitable for higher halls with ceiling heights of over 6 m, for heating poorly insulated halls or for outdoor heating. They are used in industry, workshops, exhibition halls, museums, warehouses, aircraft hangars, churches and many other areas of application. The heating of bearings to keep condensate free and the heating of football stadiums can be named as special applications.
Regulations for exhaust gas routing
The exhaust gases from bright radiators can be discharged indirectly via the room air due to the almost pollutant-free combustion. A fresh air supply of 10 m³ / (h · kW) must be guaranteed.
regulation
The spotlights can be regulated either in stages or modulating. Depending on the planning and temperature profile of the hall, different temperatures can be realized in a room. In this way, individual temperature requirements for individual zones or workplaces can be flexibly addressed. The devices are operated by simple timers or complex controls that regulate the on and off processes with radiation sensors. Modern controls automatically determine the optimum switch-on time. PC connection or connection to the building management system is also possible.
Dark radiator
Dark radiators also generate heat by burning an oxygen-gas mixture, but in closed burners with radiant tubes. The combustion is not visible, hence the name dark radiator. The generated hot gases heat the surface of the radiant tubes, which give off the heat mainly as radiation. Natural and liquid gas as well as heating oil are used as fuel, although only a few manufacturers offer the latter.
construction
Dark radiators are relatively simple devices consisting of a burner, a fan, a radiation tube and reflectors arranged above. A linear or U-shaped tube serves as a jet surface. The burner, which is mounted at one end of the radiant tube, generates a flame that extends relatively far into the tube. Modern devices work with an oppressive system. This means that the fan sits in the same place on the burner and “pushes” the flame far into the radiant tube. This creates a long and laminar flame that heats the heater evenly over its entire length. In addition, the fan is not exposed to the hot exhaust gases. Older designs still use the suction fan at the other end of the pipe. The fans generate a negative pressure that transports the gases through the jet pipe. However, the fans are always exposed to the hot gases, which results in a shorter service life for the fans. The radiant tube is covered by a reflector that directs the heat radiation into the desired area. To increase the radiation factor, the reflector sheet can be backed with thermal insulation made from mineral fibers.
Infrared power
The pipe surface temperature is between 300 and 650 ° C, depending on the performance and design. Depending on the design and burner technology, dark radiators work with radiation factors between 45% and 55% due to the relatively low temperatures. The radiation level of modern, isolated devices (radiation factor up to 77%) can hardly be increased and will never reach the high level of bright radiators. In practice, this is put into perspective by the fact that with bright radiators in closed halls, ventilation must be installed that removes much of the heat that is introduced. The developers are reaching their limits by complying with the legally prescribed exhaust gas values.
Exhaust system
Dark emitters have to discharge their exhaust gases directly from the hall. These are discharged from the hall via appropriate exhaust pipes either individually for each device or collected from several devices directly via a chimney. The exhaust system requires an annual inspection by the chimney sweep.
Areas of application
Dark radiators emit less intensity than bright radiators, but due to their length they supply a larger radiation field per device. Due to the lower surface temperature, they can be used in rooms with a ceiling height of approx. 4 m.
regulation
The regulation corresponds to that of bright radiators. In the case of collective exhaust systems, the exhaust fans only have to be controlled.
Advantages and disadvantages
Infrared rays do not need a “carrier medium” to transport their energy . This means that they get from the device to the illuminated surfaces with almost no loss, instead of through the convection of the air. Of course, all bodies that have been heated by infrared radiation inside a hall, for example, also heat the air by conduction . However, this effect may be significantly less than it would be with conventional systems. Since the air is not heated directly, there may be fewer warm air cushions under the roof. Depending on the device type and manufacturer, this should, under certain conditions, save energy compared to conventional electric heaters. Infrared heating can therefore be advantageous if a large room is only to be heated locally without heating the entire room air. The use can also be useful if z. B. basement rooms should only be used a few hours per week, because surfaces heat up very quickly under infrared radiation.
However, in order to permanently heat an entire room, including the room air, to a certain temperature, infrared heating uses just as much energy as any other electric heating. Therefore, consumer advice centers usually advise against infrared heaters in the home due to the high operating costs.
Efficiency
Infrared heaters operated with electricity emit up to 86% of the energy supplied as radiation. The filaments emit the heat by means of infrared radiation. The loss of energy is due to the lines and convection, but overall (as with any other electrical heating system) almost 100% of the energy is converted into heat. However, a high energy efficiency of an individual component does not necessarily mean high energy efficiency. If you include the energy output in electricity generation and distribution, direct electrical heating is usually inefficient in providing low-temperature room heating. If primary energy with a carbon dioxide content is used to generate electricity (crude oil, natural gas), the result is a high level of climate pollution due to carbon dioxide emissions. Since electrical energy is usually three to four times more expensive than, for example, thermal energy from gas heating, the operating costs are relatively high.
absorption
Basically, the absorption of the radiation of an infrared radiator depends on its emitted wavelength in relation to the absorption spectrum of the material to be heated (see infrared radiation ). Careful selection of the appropriate infrared emitter is necessary in order to increase absorption.
Middle or classic (normal) infrared (MIR; wavelength: 3–50 µm): Water, for example, has an absorption spectrum with a peak of around 3.0 µm. This means that the emission radiation from medium-wave emitters or carbon infrared emitters is better absorbed by water and water-based layers (human body) than the short-wave radiation. The same applies to numerous plastics such as polyvinyl chloride (PVC) or polyethylene . Their absorption peak is around 3.5 µm.
Near infrared (NIR; wavelength: 0.78–3.0 µm): Conversely, a large number of metals absorb infrared radiation only in the short wave range and show a high level of reflection for long and medium waves. Ceramic heating elements work with a temperature between 300 and 700 ° C and generate infrared radiation in the wavelength range from 2–10 µm. Most plastics and numerous other materials absorb the infrared radiation best in this area, which is why ceramic emitters are preferred for these materials.
Depth effect
The infrared spectral range is divided into the following sections (DIN 5031). The depth effect under the skin layers, which can be felt as warmth despite air movements, is decisive for use in outdoor areas.
| Infrared range | Wavelength in nm | Area | Penetration depth in mm |
|---|---|---|---|
| IR-A (near infrared) | 780 to 1,400 | short-wave | to 5.0 |
| IR-B (near infrared) | 1,400 to 3,000 | medium wave | to 2.0 |
| IR-C (mid infrared) | 3,000 to 50,000 | long wave | up to 0.3 |
| IR-C (far infrared) | 50,000 to 1,000,000 | long wave | up to 0.3 |
See also
Web links
Individual evidence
- ↑ Heating element in the form of a tubular component, patent EP1119220A2, accessed December 25, 2019.
- ↑ Almut F. Kaspar: Klimakiller: patio heaters heat up the mind. In: Stern. November 12, 2007.
- ↑ Tobias Kniebe: Patio heater - climate change on the terrace. In: Süddeutsche Magazin. (“The Principle”) January 10, 2008.
- ↑ Electric heating mostly uneconomical. ( Memento of the original from May 15, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In consumer advice center. German Consumer Association , January 28, 2016, accessed on May 15, 2016.
- ↑ 2008 ASHRAE Handbook - Heating, Ventilating, and Air-Conditioning Systems and Equipment. (IP Edition) American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2008, ISBN 978-1-60119-795-5 , Table 2, p. 15.3.
- ↑ Günther Frey et al .: Study on the energy efficiency potentials by replacing electrical power in space heating. (PDF) izes gGmbH, Bremer Energie Institut, February 28, 2007, accessed on May 24, 2016 .
- ↑ Matthias Morfeld: cross section rehabilitation, physical medicine and naturopathy: a case-oriented textbook . Urban & FischerVerlag, 2007, ISBN 978-3-437-41178-6 ( limited preview in Google book search).
- ↑ Christian Raulin, Bärbel Greve: Laser and IPL technology in dermatology and aesthetic medicine . Schattauer Verlag, 2003, ISBN 3-7945-2236-2 ( limited preview in Google book search).
