Results of various authors on the average porosity of different shapes of grains (spheres, cylinders, lumps, Rashing rings) are codified, and relationships are derived between the porosity of fixed granular bed and the ratios of vessel diameter to grain diameter and the shape of the grains. A formula is proposed for engineering evaluation of bed porosity, the accuracy of the calculation from which is improved as a result of the statistical representativeness of an array of experimental data.A fixed granular bed (FGB) of sorbent, catalyst, or inert packing, which is charged into an industrial vessel, is a complex system. The free-volume fraction of the FGB is known as the porosity of the granular bed. Here, the internal pores of individual grains are disregarded in the calculation. The average porosity ε of an FGB is a statistical characteristic of the bed, which is required for thermal and hydraulic analyses of contact and adsorption vessels.The porosity of FGB will depend on the method of charging, the shape of the grains, the ratio of the vessel diameter D to the grain diameter d, and a number of other parameters [1].The exact porosity value can only be determined experimentally by measuring the bulk density ρ b of the bed and the density ρ s of the grains of the solid phase: ε = 1 -(ρ b /ρ s ).Formulas proposed for porosity calculation as a function of the geometric parameter D/d [1-8] yield substantial differences even for grains of the same shape. For grains in the form of cylinders, therefore, discrepancies may reach 50% in evaluating porosity (Fig. 1).For ratios D/d < 2.4, Pushnov [9] derived the empirical expression ε = A(D/d) m e n(D/d) (where A = 12.6, m = 6.1, and n = -3.6), which makes it possible to calculate the porosity of a bed of spherical grains with sufficient accuracy.Generalization of results of my own experiments and data [1-8, 10] on the average porosity of grains of different shape indicated that for cylindrical vessels with D/d > 2 and FGB height H > 20d, they all can be approximated by the expression (1) where A, B, and n are constants dependent on the shape of the grains (see Table 1). Figure 2 presents graphical relationships between FGB porosity and ratios D/d > 2 for grains of different shape, which were obtained from formula (1). The average deviation of the experimental ε values from those computed on the basis of formula (1) is: ±5.26 % for spherical grains, ±12.9% for cylinders, ±10.47% for lumps, and ±14.23% for Rashing rings. Figure 3 shows the scatter of local average-porosity values (cross-hatched) with respect to experimental data derived by various authors [1-9]. ε = + A D d B n ( / ) ,
Journal of Environmental Engineering and Landscape ManagementPublication details, including instructions for authors and subscription information:Abstract. Authors are analyzing ecological and resource aspects of large-scale chemistry, petrochemistry and power engineering enterprises industrial cooling towers exploitation. Basing on hierarchical approach, the air flow in scales or region, plant and separate shop was examined. Currents of aerial masses at the plant territory are analyzed at presence of one or several sources of harmful gas pressure bumps in atmosphere conditions of interplay of these pressure bumps with steam-air plumes of groups of cooling towers. There are executed the results calculation of bordering zones with increasing of maximum permissible concentrations. The calculations are based on a data of concrete energy object including one or two sources of emission and under condition of one of the sources interaction with a group of cooling towers placed near. There are substantiated the suggestions to organize international ecological monitoring in Baltic region countries.
A new combination packing, which consists of bundles of structured packing and spacers, and which itself is conjoined to the elements of packed film and film-drop assemblies, is examined. The heat-and mass-exchange efficiency, and the dependence of the hydraulic resistance of the new packing on the velocity of the air flow are cited for various irrigation densities.Efficiency of heat and mass exchange with direct contact between the liquid and gaseous phases is ensured, above all, by the developed surface of their contact, which can be created by a packed assembly. Interaction between the phases may be film, film-drop, drop, or jet-drop in nature depending on the type of packed assembly [1][2][3]. A multitude of new types of packings, among which structured packings classed with the family of packed film assemblies, have come into the most widespread use in the past 15 years.Structured packings (SP) are regular packings represented as bundles assembled from flat or crimped sheets, which form a three-dimensional multichannel structure. The sheet components of the packing, which are fashioned from metallic foil, meshes, and polymeric, ceramic, and other materials, may be arranged in a vessel as a set of plates, spirals, cylinders, and may also be assembled into a honeycomb or cellular structure. Coaxial channels, which are formed by the packing elements, may assume different configurations -from simple (a circular or polygonal section) to complex three-dimensional [4].The term structured packings has also been expanded to include film heat-exchange contact assemblies for cooling towers (sprinklers) [5].Despite their high heat-and mass-exchange efficiency, SP have a number of deficiencies, primary among which is nonuniform distribution of the gaseous and liquid flows throughout the cross section of the vessel [1], since the geometric structure of the packing excludes communication between the free channels formed by neighboring sheets. To improve the operating efficiency of SP, it is necessary to ensure uniformity of gas and liquid throughout all channels, even at the inlet to a layer of packing. It is possible to organize sufficiently uniform distribution of flows, however, only on relatively small mock-up installations with a column vessel and in the presence of a liquid distributor, which ensures uniform irrigation over its entire cross section. In practice, it is virtually impossible to provide ideal irrigation of the entire surface of SP in industrial vessels, especially such large-scale vessels as cooling towers.Packings that organize the drop surface of phase contact are distinguished by an appreciably smaller specific surface as compared with packed film assemblies, and, consequently, also by low efficiency [1], although they provide free commu-
Some results of an experimental study of liquid film flow over the surface of a structured packing with a new design for heat and mass exchangers were presented. The influence of the irrigation density on the overflow of liquid from one side of a packing element to the other side was studied. The overflow of liquid from one side of a packing element to the other side was shown to occur abruptly upon attaining a certain irrigation density. A dimensionless quantity, namely, the overflow number, was proposed for estimating uniformity in the distribution of a liquid film over a structured perforated packing bed.
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