Solids Conveying Notes

The process steps listed in Table I9-I can sometimes be used to promote mixing. However, they are primarily for funcI ions other than solids mixing. (Note precautions for pneumatic conveying and vibrating in Table I9-I.)... 

Thus, the exit pressure from the solids conveying zone is 3.776 x 103 x 60.9 = 2.3 x 105N/m2 (33 psi). This result indicates that the solids conveying section functions properly and that higher outputs could be obtained at this screw speed before solids conveying limitations (e.g., starving ) were encountered. We should note, however, that the analysis of the solids conveying zone is very sensitive to the values of the coefficient of friction. 

It should be noted, that the effect of the channel depth on the solids conveying rate as determined from a flat plate analysis is different when the curvature of the channel is taken into account. In reality, when the channel depth increases, the area of the flight flanks will increase but the area of the root of the screw will decrease this decrease is not taken into account in the flat plate model. The contact area between the differential element of the solid bed and the barrel is ... 

C.1 Calculation of the Optimum Channel Depth for Solids Conveying. In Problem 8B.2 an expression for the optimum channel depth was obtained. For a 11.4 cm diameter extruder running at 60 rpm with values of fi, = 0.5 and fs = 0.3 and P2/P1 = 200, determine the optimum value of H (i.e.. And the value of H that makes G/p a maximum). It should be noted that in practice it is difficult to obtain accurate values of the friction coefficients, and hence, one must use results of the nature asked for here only as a guideline. 

The energy required for conveying can conveniently be considered in two parts that required for the flow of the air alone, and the additional energy necessitated by the presence of the particles. It should be noted, however, that the fluid friction will itself be somewhat modified for the following reasons die total cross-sectional area will not be available for the flow of fluid the pattern of turbulence will be affected by the solids and the pressure distribution through the pipeline will be different, and hence the gas density at a given point will be affected by the solids. 

A note on good practice Dashed and solid wedge-shaped bonds are commonly used when displaying organic structures to convey a sense of the three-dimensional shapes. The dashed wedge-shaped bonds go into the page and the solid wedge-shaped bonds come toward us. The thin lines are in the plane of the paper. 

Group A powders are the best candidates for dense-phase conveying and can achieve high solids/gas loadings. Note the dense-phase referred to here actually is fluidized dense-phase (Wypych, 1995a). 

Consequently, more elaborate designs had to be tested and/or developed for this application of pneumatic conveying. The following two valves have been found useful in particular areas. Note that in each case it is imperative to ensure fast actuation time (e.g., 1 second for a 100 mm nominal bore NB valve and a few seconds for a 300 mm NB valve, if possible). Large-bore solenoid valves and quick-exhaust valves usually are required for this purpose. Also, note that each one of the following valves provides a full cross-sectional area of flow in the open position and a 100% seal in the closed position. One of the major problems of air-on-sleeve pinch valves is that they do not provide these important features for pneumatic conveying applications (e.g., a small hole in a closed sleeve quickly erodes due to the subsequent high velocities of air and solids). 

Some additional information that can be conveyed in a balanced chemical equation (s) = solid, (g) = gas, (aq) = aqueous, and (1) = liquid. The following equation shows the proper use of these symbols S(s) + 6HN03(aq) —> H2S04(aq) + 6N02(g) + 2H20(1). It should be noted that [aq] means aqueous solution and [1] means in the liquid phase of a pure substance. Use phase-indicating symbols in your balanced equation in no. 3 above. 

Note that even solid fats contain some unsaturated fatty acids, and oils contain saturated fatty acids as well. Besides the degree of unsaturation, the length of the fatty acid chain also influences whether a fat is solid or liquid. Short chain fatty acids, such as found in coconut oil, convey liquid consistency in spite of the low unsaturated fatty acid content. Two of the unsaturated fatty acids, linoleic and linolenic acids, are essential fatty acids because the body cannot synthesize them from precursors they must be included in the diet. 

Editor s Note. The symbolism used in this article is that of A. L. G. Rees of the Australian Commonwealth Scientific Research Industrial Organization, while that used by other authors in the symposium is that of F. A. Kroger and H. J. Vink of Phillips Research Laboratories. This was pointed out to the author, who feels that, of the accepted symbolisms for defect solids, that of Rees is best suited to convey the chemical and structural information required. 

Belt conveyers, screw conveyer, and bucket elevators are used for feeding solids such as rock phosphate, coal, salt, and bauxite in large quantities. This feed system can be calibrated by operating them by feeding known amount from a stack/heap of these materials once a week. Stacks of known amounts (20 tonnes, 50 tonnes, etc.) shall be arranged at the feed point and time required for conveying the entire amount can be noted. Care shall be taken to see that the trough of the screw or the buckets of the bucket elevator are always full. 

The layout of the piping systems has many important factors in pneumatic conveying. One should keep the flow path as the most direct between two points. Bends should be eliminated as much as possible. Care should be taken in the design that the distance after a feed point before the first bend is inserted in a minimum of 3 meters (10 feet) when two-phase conditions are present. If the flow is dilute or dense, this distance is not crucial. The two-phase condition tends to cause a sloshing of the solids in the bend in an unsteady condition. This sloshing behavior causes plugging and other upsets in the operation of the pneumatic conveying systems. As noted before, one should at all costs avoid more than two bends in quick succession. 

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