radiator
A radiator is a body that emits heat mainly through thermal radiation . It usually consists of a metal that conducts heat well and its surface usually has a high emissivity . Colloquially, heating or cooling elements (see there) are sometimes referred to as radiators - these emit the heat primarily via heat conduction to the adjacent air layer, where it is then transported further in the air by convection .
function
Radiators are heat exchangers . A medium can radiate heat (mostly in the infrared range ) through the radiator . The hotter, the greater the amount of heat that can be radiated per time span and area. Double the absolute temperature (in Kelvin ) causes 16-fold energy radiation ( Stefan-Boltzmann law ). When calculating the net amount of heat transferred, the radiation on the radiator must also be taken into account ( radiation exchange ).
An emissivity that is as high as possible in the range of the maximum emission improves the heat radiation. The range of the maximum emission shifts depending on the absolute temperature from the mid-infrared at room temperature to the near-infrared at temperatures of several 100 to 1000 ° C. The emissivity is determined by the surface material and condition. A rough or rugged surface increases the emissivity. Oxide layers on metal and ceramics have a very high emissivity in the mid-infrared. In the near infrared it is more difficult to produce surfaces with high emissivity.
Combination with other heat transfer mechanisms
The share of heat radiation in the total heat output is, for example, insignificant in hot water heating systems due to the low absolute temperature, so that the terms convector or heating or cooling element would be more appropriate. The term radiator for radiators is used in different regions.
The emissivity can still be improved even at low temperatures by using a high emissivity. The decisive factor is the emissivity in the mid-infrared; it is almost 100% with a paint coating of any color.
Infrared radiation is hardly absorbed by the surrounding air, so the radiation only slightly heats the air. Instead, the radiation heats up surfaces that it hits; it is largely reflected on metallic surfaces. This is why radiant heaters often have a parabolic reflector to direct the radiation into the room.
Examples of radiators
- Radiant heaters quickly heat the floor and surfaces, for example in a bathroom or an infrared cabin .
- Very powerful radiant heaters, for example for heating a building shell, are called infrared heating cannons .
- Large, high halls sometimes have ceiling heating , especially if radiators on the floor would be a hindrance.
- In electron tubes , the grid and anode have to be cooled; they often have blackened cooling fins or ribs. In the case of radiation-cooled tubes with glass bulbs, there is no convection or heat conduction, as the space between the electrodes and the bulb is evacuated .
- Spacecraft can only be cooled by radiation. For this purpose, they have blinds that can be closed or opened on the outer skin, depending on the amount of sunlight. See also temperature control in space travel .
