Flow vertical, coarse solids

To accommodate efficient heating of coarse solids, it would be desirable to break the fall of these particles during their descent by means of baffles in order to prolong their residence time, as shown schematically in Fig. 13 (Kwauk, 1979b). In this respect, a good baffle needs to cover up to 100% of the cross-sectional area traversed by vertical flow, and yet permit oblique passage as near to 100% as possible. Also, baffles should distribute solids laterally in order to give uniform solids population in the heat transfer apparatus. Thus, conceptually, an ideal baffle plate should consist of a cellular array, structurally robust, of deflectors made of infinitesimally thin sheet materials. 

The flow behaviour of suspensions of coarse particles is completely different in horizontal and vertical pipes. In horizontal flow, the concentration of particles increases towards the bottom of the pipe, the degree of non-uniformity increasing as the velocity of flow is decreased. In vertical transport, however, axial symmetry is maintained with the solids evenly distributed over the cross-section. The two cases are therefore considered separately. 

Rushton (35) was the first to draw attention to the errors associated with wall sampling. Sharma and Das (37) mentioned that the mechanism of particle collection using an opening flush with the wall is different from the concept of isokinetic sampling. Moujaes (38) used wall sampling to measure solids concentration in upward vertical slurry flows. He found the sample concentration to be consistently lower than the true values in the pipe, especially with the coarse sand particles. 

It is customary to divide suspensions into two broad categories - fine particle suspensions in which the particles are reasonably uniformly distributed in the liquid with little separation and coarse suspensions in which particles, if denser than the liquid, tend to separate out and to travel predominantly in the lower part of a horizontal pipe (at a lower velocity than the liquid) in a vertical pipe the solids may have an appreciably lower velocity than the liquid. Although, this is obviously not a very clear cut classification and is influenced by the flow rate and concentration of solids, it does nonetheless provide a convenient initial basis for classifying the flow behaviour of liquid-solid mixtures. 

---