流体力学与 及传热课件Settling Separation of the solids from gases.pptVIP

3.3 Settling Separation of the solids from gases; Mechanical separations are applicable to heterogeneous mixtures not to homogeneous solutions. ;The techniques are based on physical differences between the particles such as size, shape, or density. They are applicable to separating solids from gases, liquid drops from gases, and solids from liquids.; Many mechanical separations are based on the sedimentation of solid particles or liquid drops through a fluid, impelled by the force of gravity or by centrifugal force. ; In some situations the subject of the process is to remove particles from a stream in order to eliminate contaminants from the fluid or to recover the particles, ; For coarse solid particles, larger than about 325-mesh（43μm diameter), which may be removed by a gravity settling. ;In the absence of air currents, particles settle toward the floor at a speed equal to their terminal velocities. ;In the following treatment the particles are assumed to be spherical. If the particle is at sufficient distance from the boundaries of the settling chamber and from other particles, so that its fall is not affected by them, the settling process is free settling. ; When the settling of a particle is followed Stokes’ law in a settling chamber, equation 3.2-16 can be applied to the design of gravitational settling chamber:;When the height of the settling chamber is denoted by H, then, the particle will take time tv to approach to the floor ;The particle flows through the settling chamber with the fluid at the velocity u, assuming that the length of the settling chamber is denoted by L, then, the time that the particle moves horizontally from the entrance to the boundary of chamber is ;The particle removed must take time th≥tv, from Eq(3.3-1) and Eq.(3.3-2 ) ;If the capacity of the settling chamber is denoted by Q in m3/s, and the width of the settling container b, the relationship among the capacity of the settling chamber, the velocity of flow, and the di