Inertia collector

Inertial collectors. In inertial collectors, an object is placed in the path of the gas. An example is shown in Fig. 11.1. While the gas passes around the shutters, particles with sufficiently high inertia impinge on them and are removed from the stream. Only particles in excess of 50/um can reasonably be removed. Like gravity settlers, inertial collectors are widely used as prefilters. 

On approaching a collecting body (fiber or liquid droplet), 0 porticle corried along by the gas stream tends to follow the stream but may strike the obstruction because of its inertia. Solid lines represent the fluid streamlines oround a body of diameter Dt, and the dotted lines represent the paths of particles that initially followed the fluid streamlines. X is the distance between the limiting streamlines A and B The fraction of particles initially present in a volume swept by the body that is removed by inertiol interception is represented by the quantity X/Dt, for a cylindrical collector and (X/Dt,) for a sphericol collector... 

Interception A special case of impingement, in which a particle is trapped on a fiber due to the effect of Van der Waals forces rather than inertia. The interception of a particle in a particle collection device occurs when the particle follows a gas streamline round a collector at a distance less than the radius of the particle. 

Interception The inertial impaction model assumed particles had mass, and hence inertia, but no size. An interception mechanism is considered where the particle has size, but no mass, and so they can follow the streamlines of the air around the collector. If a streamline which they are following passes close enough to the surface of the fiber, the particles will contact the fiber and be removed (Figure 8.4). The interception efficiency depends on the ratio of the particle diameter to the cylindrical collector diameter (k= dp/Dc) ... 

Filtration is a physical separation whereby particles are removed from the fluid and retained by the filters. Three basic collection mechanisms involving fibers are inertial impaction, interception, and diffusion. In collection by inertial impaction, the particles with large inertia deviate from the gas streamlines around the fiber collector and collide with the fiber collector. In collection by interception, the particles with small inertia nearly follow the streamline around the fiber collector and are partially or completely immersed in the boundary layer region. Subsequently, the particle velocity decreases and the particles graze the barrier and stop on the surface of the collector. Collection by diffusion is very important for fine particles. In this collection mechanism, particles with a zig-zag Brownian motion in the immediate vicinity of the collector are collected on the surface of the collector. The efficiency of collection by diffusion increases with decreasing size of particles and suspension flow rate. There are also several other collection mechanisms such as gravitational sedimentation, induced electrostatic precipitation, and van der Waals deposition their contributions in filtration may also be important in some processes. 

By performing a radial force balance, Spieiman and Fitzpatrick (1973) determined the radial velocity of a particle attracted to a spherical collector by Loudon forces when particle inertia and Brownian motion are... 

Following Spielman and the aims of this book, our discussion is confined to the capture of particles in liquid suspension from low-speed laminar flows, where the particles are generally small compared with the collector. The two principal transport mechanisms are (a) Brownian diffusion for submicrometer-size particles, and (b) interception of micrometer-size, nondiffusing, inertia free particles with the collector as a consequence of geometrical collision due to particles following fluid streamlines. Inertial impaction, which can be important for gas-borne particles, is usually unimportant for particles in liquids, because the particle—fluid density difference is smaller and the higher viscosity of liquids resists movement relative to the fluid (Spielman 1977). In this section we shall... 

The diffusional flow rate to a cylindrical collector, whose axis is normal to the flow direction, has also been determined by a procedure similar to that outlined for the spherical collector. Although no steady, uniformly valid, inertia free, Stokes solution exists for an unbounded medium, a solution valid near the... 

Let us again first consider collection by a spherical collector assuming the flow to be an inertia free, Stokes flow. The stream function corresponding to the velocity field, defined by Eqs. (8.3.3), is... 

A similar calculation can in principle be carried out for a cylindrical collector. However, a difficulty arises in that there is no solution of the inertia free, Stokes equation for an infinite cylinder in an otherwise unbounded flow. This was already observed in Section 5.1. Nevertheless we can illustrate the low Reynolds number behavior by using the so-called Stokes-Oseen solution for uniform flow of velocity U past an infinite circular cylinder of radius a whose symmetry axis is perpendicular to the flow. Oseen s method accounts in an... 

In the preceding section our analysis for Brownian diffusion assumed the particles were diffusing points, whereas for interception the center of a particle of finite size was assumed to follow the undisturbed streamline near a large collector. In both cases, no other forces were considered to act on the particles, and when they struck the collector it was assumed that they adhered. In reality, however, even in the absence of inertia there may be other external forces acting on the particles, including London forces of attraction, gravitational hydro-dynamic interactions between the particle and collector, and double layer repulsive forces. 

From the inertia free equations of motion and the boundary conditions that the fluid velocity vanishes at large distances from the particle and on the collector surface, it can be shown from dimensional considerations that the force F is expressible in the general form... 

The formulated leading-outs are aetual for intensive operation wet-type collectors in which the basic gear of selection of corpuscles is the gear of the inertia dust separa-tiom... 

Inertial separators involving changes of flow direction of the gas stream, which is thus relieved of dust because the particles, due to their inertia, are not able to follow the gas flow path (Fig. 4). In certain types of separator the dustladen gas impinges on baffles or other bodies and, in being deflected around these, loses its dust particles because of their greater inertia. Such dust collectors are more particularly called impingement separators. 

One of trends of development of wet-type collectors is creation of apparatuses of intensive operation with high carrying capacity on a gas phase that is connected with favorable decrease in gabarits of installations. In these conditions, owing to high relative speed of traffic of liquid and gas phases, work upon effect of a dust separation mechanisms the inertia and direct capture of corpuscles. Such process is realized in impact-sluggish dedusters to which it is possible to refer to the investigated apparatus[l-5]. 

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