Downflowing layers

The thickness of the downflowing layers at the wall of the CFB is typically defined as the distance from the wall to the position of zero vertical solid flux. Measurements of the layer thickness were made on a 12 MW and 165 MW CFB boiler by Zhang, Johnsson and Leckner (1995). They found that the thickness increased for the larger bed. They related data from many different beds (Fig. 19), with the equivalent bed diameter, taken as the hydraulic diameter, using the following form... 

Figure 19. Empirical correlation and experimental data of thickness of downflowing layer at the wall of a CFB as a function of the equivalent bed diameter. (From Zhang etal., 1995.)...

FIG. 16-42 Ion -exchanger regeneration, (a) Conventional. Acid is passed downflow through the cation-exchange resin bed. (b) Counterflow. Regenerant solution is introduced upflow with the resin bed held in place by a dry layer of resin. 

When the heat transfer surface is immersed in the bed at any radial position, the solids particles form a downflowing cluster layer surrounding the surface. The thickness of the cluster layer grows up to reach a Anal constant value. 

The bubble wake released at the free surface, joined with the upflow, constitutes the downflow so that there is no downward return flow through the bubble layer. Under these circumstances, the mean bubble velocity b relative to the column wall is... 

A saturator produces a saturated solution by allowing water to trickle through a bed of coarse granular sodium fluoride solids in excess of the amount required for saturating the water solution. Two types of saturators are available upflow and downflow. In a downflow saturator, the sodium fluoride solids are isolated from the saturated solution by a plastic cone or a pipe manifold and a filtration barrier composed of layers of sand and gravel. The barrier prevents particles of undissolved sodium fluoride solids from infiltrating the solution area under the cone or within the pipe manifold. With the upflow type, no barrier is used, because the water comes up through the bed of sodium fluoride... 

Short-circuiting through the sodium fluoride layer (downflow type). 

Paper chromatography, however, is advantageous in some respects. Chromatoplates are more difficult to prepare and handle in comparison with sheets of paper. Also it is usually easier to elute a compound from a piece of paper cut from a paper chromatogram than it is to scrape out an area of a thin-layer chromatogram for elution. Further, since thin-layer chromatograms cannot readily be run by the downflow technique, the practical length of thin-layer chromatogram is severely limited. 

Intermediate between the mixed layer and the deep sea, at depths between 100 and 1000 m, lies the main thermocline, a region where mixing is imperfect and exchange with the atmosphere is slow but nevertheless faster than in the denser and cooler waters of the deep ocean. At high latitudes, a mixed layer often cannot be discerned and the surface waters mix directly with the deeper strata. Circulation in the main body of the ocean is accomplished by the downward motion of cold surface waters near the poles and a slow updrift at low latitudes. Estimated rates of downflow are 30 x 106 m3/s as given by Munk (1966) and Gordon (1975) for the Antarctic, and (10-30) x 106 m3/s as derived by Broecker (1979) for the North Atlantic. The resulting turnover time of deep ocean waters is 700-1000 yr. 

Downflow columns with a constant height of the layer are convenient for waters with a low content of pollutants. Conversely, for waters with a high... 

Commence simultaneous acid and caustic regeneration 6% HCl solution up flow through the cation bed for 30 min and 4% NaOH solution at 43—49°C downflow through the anion bed for 60 min. The two streams flow out to waste-water neutralisation tank through the interface collector imbedded in the inert resin layer. 

The filter medium is usually graded by size. Three or four layers of different screen cuts, with the finest material on top, are common. Sand is often placed above one or two layers of gravel or pebbles. The top layer is the deepest as well as the finest. An upflow filter then has a solids capacity of20-30kg m , compared with the 5-15 kg obtained in downflow. The brine flow rate must be kept below about 0.3 m min . The fluidization limit, which will depend on the size and density of the filter medium and the properties of the brine, is an absolute hmit which must not be exceeded in an attempt to overcome upsets or to compensate for filter downtime. Some form of assurance that excessive flow will not occur should be part of the design. 

The mechanisms of heat transfer are intimately related to the circulating bed fluid and particle mechanics. Heat is released by reaction throughout the combustor volume. It is carried to the steam-raising surfaces largely by convective flows of solids which have considerably higher thermal capacity than the low density gas phase. Transfer of heat then occurs by contact of the solids with the heat transfer surface in what is generally a downflowing wall layer. 

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