Page 269 - Dust Explosions in the Process Industries
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Generation of Explosible Dust Clouds  24 7


               across which an induction spark was passed. The dust was placed in a piece of a narrower
              tube attached to one end of the explosiontube, and the dust cloud was formed in the region
               of the spark by means of a short blast of compressed air. Engler (1907) used glass flasks
               of 250 to 500 cm3capacity as explosionvessels (see Figure 1.57).In the spherical explo-
               sion bomb of 1.4 liter capacity used by Trostel and Frevert (1924), the dust was placed
               in a small cup near the bottom of the bomb, and the dispersing blast of air was introduced
               through a glass tube, entering the bomb through the bottom and, having a bend of 180°,
               facing the opening downward toward the dust heap in the dispersion cup. In their explo-
               sion vessels of  1 liter capacity, Boyle and Llewellyn (1950) and Eckhoff (1970) used
               arrangements practically identical to that introduced by Trostel and Frevert.
                 The well-known Hartmann apparatus, which was first described by Hartmann et al.
               (1943), consists of a vertical cylinder having a volume of about 1.2 liter, supported by
               a metal bottom part shaped like a cup, in which the powder is placed (see Chapter 7).
               The dispersing airblast is introduced axially from below and deflected downwardtoward
               the dust heap by means of a small conical “hat” or “mushroom.”As discussed by Dorsett
               et al. (1960),this apparatus, in the form of either an open tube or a closed bomb, has been
               used to determine the numerous values of minimum ignition energy, minimum explosi-
               ble dust concentration,rates of pressure rise, maximum explosionpressure, and so forth
               that have, through the years, been published by the U.S. Bureau of Mines.
                 Carpenter (1957) used a slightly modified form of the Hartmann apparatus, the main
               features, however, being identical. In a subsequent work, Carpenter and Davies (1958)
               used a smaller, detached dust dispersion cup of 2 cm diameter fitted in the lower part of
               a cylindrical275 em3combustion chamber.Meek and Dallavalle(1954) employed a rather
               large explosion chamber of  about 60 liters. The dust was dispersed from a polished
               funnel-shaped cup, fitted with a special dispersing cone.
                 Vanous versions of the transient air blast method have been used in a number of other
               investigations.Nagy et al. (1971) adapted this technique over a wide range of explosion
               vessel volumes, ranging from 1 liter to 14 m3.Moore (1979) employed the method in
               three different vessels of volumes from 1to 43 liters; and Enright (1984) used it in three
               vessels of volumes from 1 to 20 liters.
                 The simplest version of the transient air blast method, based on just directing a blast of
               air towards a dust heap, was found to give a rather poor dispersion of very fine, cohesive
               dusts.To improve dust dispersion, more refined versions of the airblast method were devel-
               oped, based on forcing the dust/air suspension through narrow nozzles (see Section 3.7).
                 This was done, for example, by Helwig (1965), who generated his dust clouds from
               the  100 cm3 cylindrical “whir1ing”chamber shown in Figure 3.30. The chamber was
               placed inside the 43 liter explosionbomb. By means of a blast of compressed air admit-
               ted through the bottom of the whirling chamber, the dust was fluidized and the fluidized
               suspension forced through a number of holes in the chamber lid at the top. There is little
               doubt that the nozzle dispersion mechanism discussed in Section 3.7 played an essen-
               tial role in this process.
                 In his 1m3explosion vessel, Bartknecht (1971)used a dust dispersion systemby which
               the dust was forced at high velocity by high pressure airthrough a number of 4-6  mm diam-
               eter holes in a U-shaped tube of  19 mm internal diameter. Barthecht’s 1 m3vessel and
               dust dispersion system has later been adopted as an IS0 standard (InternationalStandards
               Organization, 1985).From what has been said in Section 3.7, it is quite clear that this stan-
               dard test method produces a high degree of dispersion, even for very cohesive dusts.
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