Impactors

Slow-speed hammer mills or impactors are more difficult to maintain, and jaw crushers have not been required. 

Impactors are size reduction devices that strike (impact) the coal which is then thrown against a hard snrface or against other coal particles. The coal is typically contained within a cage, with openings on the bottom, end, or side of the desired size to allow pulverized material to escape. 

FIGURE 6.11 A reversible rotor-type impact mill. (From Meyers, R.A., Ed., Coal Handbook, Marcel Dekker, New York, 1981, p. 192.) 

---

The fast-flowing narrow liquid stream has a high relative linear velocity with respect to the slower flow of the argon gas stream. This leads to breaking up the liquid stream into fast-moving droplets, which strike the impactor bead and form much smaller droplets. 

A sample solution is drawn or pumped into a V-shaped groove cut into the end of a capillary tube. The crossed gas and liquid streams form an aerosol. An impactor bead can be used to provide an even smaller droplet size. 

B. W. Loo, J. M. JaMevic, and F. S. Goulding, "Dichotomous Virtual Impactors for Large Scale Monitoring of Airborne Particulate Matter," in B. Y. H. Liu, ed., Eine Particles, Aerosol Generation, Measurement, Sampling and Analysis, Academic Press, Inc., New York, 1976, pp. 311—350. 

For acid mists, the Brink impactor is often used (Fig. 10) (17). The mist is first drawn through a cyclone to remove particles larger than 3 fim. A five-stage impactor is used to classify mist particles of diameter 0.3—3.0 fim. 

FIG. 14-115 Experimental collection efficiencies of rectangular impactors. C is the Stokes-Ciinningbam correction factor Pp, particle density, g/ond U, superficial gas velocity, approaching the impactor openings, cm/s and ig, gas viscosity, P. Calveri, Yung, and Leung, NTIS Puhl. PB-24S050 based on Mercer and Chow, J. Coll. Interface Sci., 27, 75 (1.96S).]... 

For determination of the aerodynamic diameters of particles, the most commonly apphcable methods for particle-size analysis are those based on inertia aerosol centrifuges, cyclones, and inertial impactors (Lundgren et al.. Aerosol Measurement, University of Florida, Gainesville, 1979 and Liu, Fine Paiiicles—Aerosol Generation, Measurement, Sampling, and Analysis, Academic, New York, 1976). Impactors are the most commonly used. Nevertheless, impactor measurements are subject to numerous errors [Rao and Whitby, Am. Ind. Hyg. A.s.soc.]., 38, 174 (1977) Marple and WiUeke, "Inertial Impactors, in Lundgren et al.. Aerosol Measurement and Fuchs, "Aerosol Impactors, in Shaw, Fundamentals of Aerosol Sci-... 

Rotor Impactors The rotor of these machines is a cylinder to which is affixed a tough steel bar. Breakage can occur against this bar or on rebound from the walls of the device. Free impact breaking is the principle of the rotor breaker, and it does not rely on pinch crushing or attrition grinding between rotor hammers and breaker plates. 

Penn.sylvania Cru.sher Corp. offers a variety of impactor types granulators, Bradford breakers, roll crushers, and jaw crushers. (See Table 20-15.)... 

Consider the impact of a semi-infinite space on a plate of thickness dp, separated from an identical plate by a gap of width d. If the impactor and plates are all composed of the same materials, what is the subsequent behavior Plot in both Lagrangian and Eulerian coordinates. 

Tilt. The flat impactor must be delivered onto the specimen with a minimum of tilt between the impacting surfaces. Long projectiles help to minimize tilt. 

Asay, J.R., L.C. Chhabildas, and L.M. Barker (1985). Projectile and Impactor Designs for Plate-Impact Experiments, Sandia National Laboratories Report SAND85-2009. 

Because a filter sample includes particles both larger and smaller than those retained in the human respiratory system (see Chapter 7, Section III), other types of samplers are used which allow measurement of the size ranges of particles retained in the respiratory system. Some of these are called dichotomous samplers because they allow separate measurement of the respirable and nonrespirable fractions of the total. Size-selective samplers rely on impactors, miniature cyclones, and other means. The United States has selected the size fraction below an aerodynamic diameter of 10 /xm (PMiq) for compliance with the air quality standard for airborne particulate matter. 

The collection technique involves the removal of particles from the air stream. The two principal methods are filtration and impaction. Filtrahon consists of collecting particles on a filter surface by three processes—direct interception, inertial impaction, and diffusion (5). Filtration attempts to remove a very high percentage of the mass and number of particles by these three processes. Any size classification is done by a preclassifier, such as an impactor, before the particle stream reaches the surface of the filter. 

Fig. 14-6. Schematic diagram of a four-stage cascade impactor. Source Giever, P. M., Particulate matter sampling and sizing, in "Air Pollution," 3rd ed., Vol. lil (A. C. Stem, ed.). Academic Press, New York, 1976, p. 41,...

Impactor 1. Koni meter Impaction on gel-coated disc 60-100... 

Impact damage Damage caused by the impact of a 25 mm diameter hemispherical impactor with 135 J of kinetic energy, or with that kinetic energy required to cause a dent 2.5 mm deep, whichever is least. 

Aerodynamic Diameter The aerodynamic diameter of a particle is defined as that of a sphere, whose density is 1 g cm " (cf. density of water), which settles in still air at the same velocity as the particle in question. This diameter is obtained from aerodynamic classifiers such as cascade impactors. 

Impactors utilize aerodynamic forces to separate the various particle size classes. This is consistent with the particles aerodynamic diameter. The operation... 

---