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Explosion-proof filter to prevent dust explosions
Aug 31, 2016

1, the characteristics of the dust explosion

Dust explosions are suspended in the air of the dust particles come into contact with oxygen in the air full and instantaneous reaction under certain conditions, reaction emits a lot of heat, leads to the phenomenon of high temperature and high pressure. Any dust explosions must have three conditions: the ignition source; fine combustible dust; dust suspended in the air and lower explosive concentration limit.

(1) the dust explosion than complicated and combustible gases, flammable substances, combustible dust suspended in the air, formed dust cloud of the explosive concentration range, in the presence of ignition sources, parts in contact with ignition first ignited and form a small ball of dust. In this small ball of fire burning under the action of the heat released so near the dust around is heated, elevated temperature, ignition and combustion phenomena, so that the ball will rapidly expand to form dust explosion.

Dust explosion in degree of difficulty and intensity and the physical and chemical properties of dust and ambient conditions. In General, the greater the heat of combustion, particles smaller and higher activity of the dust explosion hazard greater; lighter suspended solids flammable substances explosion hazard larger; when the oxygen content in the air, dust prone to ignition, explosion is more intense. Due to water can inhibit explosive effect, so dust and dry gas, then the greater the risk of explosion.

(2) the dust explosion occurs, tends to produce a secondary explosion which is due at the time of the first explosion, a lot of dust in, its concentration exceeds the maximum concentration values of the dust explosion and not explosive. However, when the formation of the first explosion shock or blast the dust back up when in the air mixed with the air, concentrations within the range of dust explosion, followed by secondary explosions. Disasters are often caused by the second explosion is much more serious than the first explosion.

Domestic aluminium production plants of dust explosion accident occurred in 1963 the direct result of exhaust fan impeller and inlet port friction fires. Wind press entrance to the shrimp and underwear three-way airflow is impeded, the dust easily. Especially more likely to stay down dust, once started, the deposition of dust being kicked up, lower explosive limit soon, cause dust explosions.

(3) the dust explosion mechanism of combustible dust in the air will release energy when burned, produces a lot of gas, and release energy fast and slow burning velocity of the powder is exposed to the air in the area. Therefore, the powder with a solid material, grain size smaller than the surface area, the greater burning spread faster. If the solid particle size is fine. And can be suspended, once a little fire to make it burn, you can release large amounts of energy within a very short period of time. This energy is too late to escape into the surrounding environment, resulting in gas space heating and adiabatic expansion, and on the powder produced when burning a lot of gas, will make the system of local pressure, that explode and spread, this is often called a dust explosion.

(4) the dust explosion and the difference between burning large pieces of solid fuel burning is nearly parallel to the advance, such as the burning of coal. The combustion energy released slowly. The heat and gas can escape quickly. Burning of combustible dust, in a well-ventilated case to form an open flame burning, and in poorly ventilated conditions. Can form the smokeless flame or smouldering.

There are several stages to the burning combustible dust: the first stage, the powder is heated the surface; the second phase of surface gasification, overflow volatile the third stage, the essential distributed angry combustion.

Ultrafine powder explosion is a more complex process, due to the scale of dust clouds are usually small, and flames spread fast, hundreds of metres per second, in the Center fire ignition of dust, flammable in less than 0.1s time throughout the dust cloud. During this process, if dust is burnt, the highest pressure is generated if the unburned, generates lower pressure. Combustible particles can burn out, depending on the particle size and the depth of burn.

(5) according to their flammability classification of combustible dusts powder can be divided into two categories: one for fuel; one is non-flammable. Classification of flammable powders and standards vary in different countries.

United States to be dangerous goods, flammable powders classified as grade, while one of the metal powder, carbon dust, grain dust into different groups. United States developed the classification is made according to the measured powder dust explosion occurred at standard test device when the boost speed to classify, and divided into three levels. Our country has not seen on the classification of combustible dusts ready-made standards.

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