Why is air considered a mixture

Explosive atmosphere

As explosive atmosphere is defined as a mixture of combustible substances with oxygen. The oxygen is mostly present as a component of the ambient air. Gases, liquids or their vapors, but also aerosols from mists or dusts, can be used as flammable substances. In explosion protection, this is considered under normal atmospheric conditions.

The explosiveness depends on the substance used, its flammability and the mixture with air or oxygen. If the mixture is made with an air-oxygen mixture that has a different composition than the air mixture normally present in the environment, the oxygen content must be carefully considered. If the oxygen content falls below a certain substance-dependent value, the so-called limit oxygen concentration, this mixture cannot be ignited. This effect is used in so-called inerting, in which oxygen is replaced by an inert gas - one that is not suitable for oxidation. Furthermore, the concentration of the combustible substance in the air mixture must be between the lower and the upper explosion limit.

An explosive atmosphere can be present intentionally or unintentionally. The former is found in many areas of the production process. An explosive atmosphere occurs wherever the relevant substances are used in the presence of the ambient air. These can be areas in which adhesives or paints dry out or are applied, in the vicinity of fermentation vats in alcohol production, in areas where substances are crushed such as e.g. B. in grain mills or numerous other areas. Unwanted explosive atmosphere is present when an error occurs. This can e.g. B. the bursting of a gas pipeline, the unintentional opening of an exhaust valve or leaks in boilers, which occurs due to lack of maintenance.

Notes on practice

An ignitable mixture can be caused to explode by an ignition source. For example, smoking at a gas station can lead to the following situation. During a refueling process, vapors can escape from the filler neck, which then mix with the air and thus possibly form an explosive atmosphere. When the smoker draws on the cigarette, the ignition temperature can be reached through the increased oxygen supply in the glowing area. If the glow of the cigarette comes into contact with the air / gas mixture, it can ignite. The result is a deflagration, which can result in a fire. In the case of a gasoline bomb, the initially liquid gasoline evaporates and disperses in the air. This explosive atmosphere is ignited, causing a huge explosion. Whether it comes to a deflagration or an explosion therefore depends crucially on the amount of combustible material and the respective framework conditions.

The presence of such an atmosphere can be limited in time and space. So z. For example, when gasoline vapors leak from a crashed tanker, the concentration directly at the leak, together with the ambient air, create an explosive atmosphere. Depending on the wind speed and direction, a cloud spreads, the concentration of which falls due to the dilution effect and falls below the lower explosion limit at a certain distance. In the event of a leak in a pressure vessel, this area can extend up to a distance of 200 m, depending on the wind direction and strength.


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Category: Technical Chemistry