tailieunhanh - Dust Explosions in the Process Industries Second Edition phần 10

Ống xả mặt cắt ngang của 0,032 m2 và chiều dài m 3 kết thúc trong một hộp 0,73 m3 dập tắt lập phương được trang bị với hai lỗ thông hơi của 0,3 m2 và 0,1 m2 tương ứng. Thông gió của cơn bão đã được thông qua m2 0,032 | Research and development 585 igniter. However in a 20 litre chamber fully developed explosions were generated even with a 5 kJ chemical igniter. The reason for this could be that in the smaller chamber the initial combustion and expansion of the dust cloud was directly supported by the ignition source. The pressure and temperature in the unbumt cloud ahead of the flame would then have increased significantly above ambient when the flame eventually propagated without support from the ignition source. Consequently the self-sustained flame propagation if any would then occur in an adiabatically pre-compressed dust-cloud rather than in a cloud of normal ambient temperature and pressure. These results suggest that great care must be exercised whenever comparatively small chambers in particular closed ones are used for any explosion limit determination. Matsuda and Itagaki 1994 compared dust explosions in a 30-litre explosion bomb with explosion in a 1 m3 vessel. They found that the ranges of explosible concentrations in the 30-litre vessel were considerably wider than those in the 1 m3 vessel for the same dust. A marked increase of the explosible range was found in the 30-litre bomb when increasing the ignition energy from 1 to 10 kJ. This effect was practically absent in the 1 m3 vessel in the ignition energy range 4-20 kJ. Tian Renqu et al. 1994 using a 20-litre explosion bomb found that the minimum explosible concentrations of coal dusts decreased by a factor of two or more when the ignition energy was increased from to 10 kJ. Xu Tianrui et al. 1994 also arrived at the conclusion that the apparent minimum explosible dust concentration determined in a 20-litre bomb depends markedly on the ignition energy. It was found that 10 kJ would be too high to yield realistic results. All this suggests that limiting conditions for flame propagation should be determined in apparatuses of sufficient volumes to prevent significant influence of even quite strong ignition sources

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