If an explosive atmosphere comes into contact with a heated surface ignition can occur. Not only a hot surface itself can act as an ignition source, but a dust layer or a combustible solid in contact with a hot surface and ignited by the hot surface can also act as an ignition source for an explosive atmosphere .
The capability of a heated surface to cause ignition depends on the type and concentration of the particular substance in the mixture with air. This capability becomes greater with increasing temperature and increasing surface area. Moreover, the temperature that triggers ignition depends on the size and shape of the heated body, on the concentration gradient in the vicinity of the surface, the flow velocity of the explosive atmosphere around the hot surface and, to a certain extent, also on the surface material. Thus, for example, an explosive gas or vapour atmosphere inside fairly large heated spaces (approximately 1 l or more) can be ignited by surface temperatures lower than those measured in accordance with EN ISO/IEC 80079-20-1 or by other equivalent methods. On the other hand, in the case of heated bodies with convex rather than concave surfaces, a higher surface temperature is necessary for ignition; the minimum ignition temperature increases, for example, with spheres or pipes as the diameter decreases. When an explosive atmosphere flows past heated surfaces, a higher surface temperature could be necessary for ignition owing to the brief contact time.
If the explosive atmosphere remains in contact with the hot surface for a relatively long time, preliminary reactions can occur, e.g. cool flames, so that more easily ignitable decomposition products are formed, which promote the ignition of the original atmospheres.
In addition to easily recognizable hot surfaces such as radiators, drying cabinets, heating coils and others, mechanical and machining processes can also lead to hazardous temperatures. These processes also include equipment, protective systems and components which convert mechanical energy into heat, i.e. all kinds of friction clutches and mechanically operating brakes (e.g. on vehicles and centrifuges). Furthermore, all moving parts in bearings, shaft passages, glands, etc. can become ignition sources if they are not sufficiently lubricated. In tight housings of moving parts, the ingress of foreign bodies or shifting of the axis can also lead to friction which, in turn, can lead to high surface temperatures, in some cases quite rapidly.
Hot surfaces can also occur by heating absorbers created by other ignition sources, e.g. electromagnetic waves and ultrasonic waves.
Consideration shall also be given to temperature increases due to chemical reactions (e.g. with lubricants and cleaning solvents).
At all cases an important point for operation of non.electrical Ex devices in possible hazardous environment. Certainly no only for non.electrical Ex.
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