Radiant heating
A radiant heating or radiation heating is a heater whose predominant heat dissipation by infrared radiators in the form of thermal radiation is carried out. It differs from convection heating , which gives off its heat mainly through convection . A distinction is made between two fundamentally different types of radiant heating: the high-temperature radiators and the low-temperature radiators.
General
physics
Every body with a temperature above absolute zero gives off thermal radiation to its surroundings. According to Stefan-Boltzmann's law and Wien's displacement law , the power and wavelength of this radiation depends on the temperature of the radiator:
- With its warming solar radiation, the hot sun radiates (predominantly) short-wave waves , the wavelength is between 0.78 and 1.40 μm, see also the Earth's radiation budget .
- Objects emit long-wave heat radiation at room temperature (20 ° C) , the wavelength is between 1.4 and 3.0 μm, see also the division of the spectral range of infrared radiation )
The active principle of radiant heating, expressed physically correctly - based on the radiation component - is the radiation exchange between the radiator and its surroundings. In the case of high-temperature radiators with a very small area compared to the surroundings, the effect of the environment on the radiator can be neglected. According to the Stefan-Boltzmann law , the emitted power is roughly dependent on the fourth power of the temperature. At higher temperatures (red to yellow glowing heating elements) the proportion of radiation is high and can be directed and bundled using optical reflectors. Thus, the usable energy of the system is higher than with convection or heat conduction.
The thermal radiation (as heating or radiating as cooling) also works in the absence of transmission media (i.e. also in a vacuum , such as, for example, the thermal radiation of the sun through the airless space).
Delimitation to convection heating
The distinction between radiant heating and convection heating is not always easy:
- Radiant heating always has warm components at the same time, which heat the surrounding air and thus generate convection. It is called radiant heating when the majority of its heat is emitted by radiation.
- A convection heater is warm and thus emits heat radiation at the same time. It is called convection heating when the majority of its heat is given off by convection.
- A heating strip produced as convection heat a thin veil against a wall, this thereby heated, which in turn radiates heat.
Principles of application
With radiant heating, the majority of the thermal energy is transferred directly through thermal radiation. All objects and materials that absorb thermal radiation in turn give off heat through thermal radiation and heat conduction to the air passing by. Air heated by thermal radiation gives off heat through thermal radiation and convection . In contrast, with convection heating, efforts are made to heat the room air for the most part and to use it as a heat transfer medium and to distribute the heat in the room by convection (the disadvantage is that air has only a small heat storage capacity ). With both types of heating, heat is also bound in water vapor as heat of evaporation and dissipated by convection and ventilation.
Reach and heat rays of heat
- the top layer of the room walls and the furniture and fittings,
- the occupants of the rooms
- to a small extent the "greenhouse gases" (CO 2 and water vapor) in the room air (see greenhouse effect ).
The main components of the atmosphere, nitrogen and oxygen , as diatomic molecules, have neither a static dipole moment nor a transition dipole moment , they are not excited by radiation of infrared light .
Window glass is practically impermeable to long-wave heat radiation, long-wave heat radiation is either absorbed (according to the heat transfer coefficient or heat transfer coefficient of the type of glass) or reflected back into the room. The reflection depends on the angle of incidence (see also the degree of reflection , reflection factor and total reflection ). In contrast, window glass is almost 100% permeable to incoming short- wave heat radiation from the sun (between 0.2 and 2.5 μm) (see energy transmittance ), this possible heat penetration and the impermeability to room heat is therefore also used in solar thermal collectors and glass houses.
In order to use the effect of re-radiation of heat energy and to avoid heat dissipation into the walls and condensation of the room air humidity on and in the wall, the irradiated room walls, i.e. the " enveloping surfaces " of the living space, should be poorly heat-conducting and good heat-storing. A wallpaper or superior gypsum board is better suited than, for example concrete .
advantages
The advantages of radiant heating in living areas are the lower room air temperature ( with the same subjective feeling of warmth by the residents ) and the reduced air circulation and air stratification, which usually leads to a more pleasant room climate and better thermal comfort ( less dry air and less dust exposure ). In the case of unfavorable air flow conditions ( frequented passageways, high rooms ), radiant heating is the heating of choice for technical reasons alone. Further advantages arise from the heat that is available almost immediately after switching on. This means that rooms that are seldom or only used for a short time can remain heated little or not at all and can still be used with some comfort when entering.
However, all radiant heaters also give off heat by convection. In the case of ceiling spotlights, a cushion of warmth is formed that remains on the ceiling due to the buoyancy of the warm air. The heat cushion (Austrian the heat cushion) gradually takes on the temperature of the radiation surface and because then there is no longer a temperature difference, no heat can be given off by convection into the boundary layer of the air cushion. A "heat curtain" forms in front of walls heated by heat radiation , which is by convection Heat distributes, but can also heat colder wall parts or the ceiling, which then radiate heat or dissipate heat (explained in more detail under heating strips ).
By reducing the energy transport with the help of heated air, the resulting energy losses ( ventilation, leaks ) are also reduced . Leaky windows then help to achieve the necessary air exchange rates in order to remove human-made moisture (sweating, breathing, cooking) from the room air without extra ventilation.
According to the architect and specialist book author Konrad Fischer , with radiant heating (and skirting board heating) the "building envelope surfaces" would always be warmer than the air by absorbing heat radiation, the air would never be cooled below the dew point , the interior plaster could not get wet and mold would not grow; with convection heating, on the other hand, the air would always be warmer than a wall, which means that the dew point could be undershot on one wall. Fischer essentially took over the theses of the architect Claus Meier , "with a more radiation-intensive heating system, the room air temperature can be reduced significantly compared to the convection-optimized heating system - the energy savings are therefore enormous" . Because the humidity of the indoor air (source: breathing, evaporation, drying, plants, etc.) should be removed from living spaces by air exchange , more energy would be lost with the air exchange with convection heating (in which the room air is heated) than with body or traveling warming Radiant heater. Thus (according to Fischer) heating systems based on thermal radiation, even with leaky windows, would be more efficient than those with convection heating and thermal insulation. Failure to change the air in the pleasantly warm air would often lead to condensation of moisture in the interior and as a result, in connection with organic nutrients (from binders, paint, wallpaper adhesives, paper wallpaper) to serious black mold pollution (moisture in external thermal insulation systems, on the other hand, is caused by capillary action or penetration into joints Condensation or other forms of precipitation ; see more detailed information on moisture # Moisture in building components ).
Low temperature heater
Low-temperature radiators are usually radiators that are used to emit heat energy generated or stored in other ways. Active media are either warm water (via heating coils, radiators or heating strips ) or flat electrical heating mats (made of graphite or soot ) embedded in foils . Common designs are underfloor heating , wall heating or ceiling heating systems , but there are also special designs ( e.g. component heating ) for special requirements.
Due to their functional principle, radiant heaters in the low temperature range require a much larger room-side radiation surface than a convector heater. They can be integrated into room-delimiting components ( advantage: no space requirement of their own - disadvantage: poor maintainability ) or they can be applied as flat construction elements on the wall or ceiling ( advantage: easy maintenance - disadvantage: for walls: possible lack of space ).
High temperature heater
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With the high-temperature radiators, the heating energy is generated on or in the radiating component and radiated at a high temperature. This form of heating is used to emit heating energy over a greater distance or to a greater extent. Examples are
- electric radiant heaters, e.g. B. Bath additional heating and changing table warmer , as well
- Gas heater, e.g. B. the catalytic construction heater , patio heater (in everyday language also known as a patio heater , which is also registered as a trademark, and in Austria called Heizschwammerl ) and industrial indoor workplace heating .
Due to the high temperature, there is at least a risk of burns, usually even a risk of fire, which must be countered by taking appropriate precautionary measures ( shielding, mounting outside the work area, distances to other objects ).
After rapidly increasing popularity, there are bans on the use of gas-powered patio heaters in public establishments such as restaurants due to the high carbon dioxide emissions in many larger cities and German federal states. There is no ban on occasional private use.
Electrical operation
In the case of radiant heating, a heating coil or a heating rod through which an electrical current flows is heated and thereby emits heat in the form of infrared rays . Behind the incandescent body there is a mirror that directs the infrared rays in one direction. Since the incandescent body heats up to several hundred degrees Celsius, contact protection is always attached. The structure of the incandescent body is a heating coil wound around a ceramic core for insulation against short circuits. The heating coil of the incandescent body is also called the incandescent filament.
Today, electric infrared heaters are used in many areas, e.g. B. in catering and in the private sector as outdoor heating, additional heating in bathrooms and wherever short-term heat is required. Special changing table heaters are designed for the changing area of babies, mostly these are designed with 600 W heating power and with splinter protection (because of the quartz heating rod). Newer devices have an automatic switch-off (usually after 10 or 20 minutes of heating operation).
A special form of the electric radiant heater is the red light lamp, in which the thermal energy generated by the filament is high enough to be able to act medically from a smaller distance. But here, too, a risk of fire cannot be ruled out ( unnoticed contact with the substance during irradiation ).
An important special form of the electric radiant heater is the incandescent lamp . With this design, most of the energy used is converted into thermal radiation. Compared to other types of construction, a high but still low part of the radiation (approx. 5% of the energy used) is emitted in the visible range of the spectrum. This is achieved by enclosing the incandescent body in a protective gas , which enables the incandescent body to reach higher temperatures.
literature
- A. Kollmar and W. Liese: The radiation heating. 4th edition. R. Oldenbourg, Munich 1957.
- Bernd Glück: Radiant heating - theory and practice. Verlag für Bauwesen, Berlin / CF Müller-Verlag, Karlsruhe 1982, ISBN 3-7880-7157-5 . Excerpts online
Web links
Individual evidence
- ↑ Claus Meier: Building physics of the historical window , information documents of the German Castle Association eV, Advisory Board for Restoration, PDF file
- ↑ F.Frieß: Interaction of radiation with glass and glass coatings PDF file
- ↑ Konrad Fischer: The temperature control of the building envelope surfaces
- ^ Hermann Rietschel: H. Rietschels textbook of heating and ventilation technology. Springer-Verlag, 2013, ISBN 978-3-662-25438-7 , p. 75 ( limited preview in the Google book search).
- ↑ Prof. Meier's controversial contributions to energy saving 5
- ↑ Konrad Fischer: The temperature control of the building envelope surfaces 21
- ↑ a b Meier, C .: Practical Guide to Monument Preservation No. 7, Old Buildings and Thermal Insulation - 13 questions and answers. Information publications of the Deutsche Burgenvereinigung eV, Marksburg, Braubach, 1999; cited in The temperature control of the building envelope surfaces 21 .
- ↑ Prof. Dr.-Ing. habil. Claus Meier: Are we insulating ourselves in the dead end? Thermal insulation and energy saving ordinance. Contradictory and absurd , lecture on the occasion of the Backsteintage 2001, January 30/31, 2001 in Hildesheim / Westerstede, (PDF file)
- ↑ That's how dangerous the patio heater really is. In: welt.de. January 30, 2008, accessed April 19, 2016 .
- ↑ Heating mushrooms must stay off in winter. In: Süddeutsche Zeitung . December 13, 2017, accessed December 21, 2018 .
- ↑ Cities forbid patio heaters - No "killer mushrooms" in the café . In: taz . November 11, 2007, accessed December 21, 2018 .