tics for cement were presented in Figure 4.5b and are reproduced here in Figure ... All three materials were conveyed through the Figure 4.2 pipeline which.
Although the performance of a pipeline with air only can be predicted reliably, the addition of material to the flow of air changes the situation entirely. This was illustrated in Chapter 5 where the conveying characteristics of a number of different materials were presented. These were used to illustrate the differences in conveying capability between different materials, and the very wide differences that can exist between materials that can be conveyed in dense phase and those that can not. In this chapter these conveying characteristics are developed further to illustrate the influence of conveying air velocity, and hence air flow rate, in more detail. Power requirements and specific energy are also considered, so that the influence of velocity can be considered in more meaningful terms. This will also provide a better basis for comparison between dilute and dense phase conveying capability and provide a basis on which pneumatic conveying can be compared with alternative methods of conveying. Pipeline bore and conveying distance are then considered. Pipeline bore is important because of the major influence that it has on the conveying capability of a pipeline. Conveying distance is generally the most problematical of all the variables. Conveying distance will nearly always be different from one situation to the next, and hence the pressure gradient will also be different. It is essentially the
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Chapter 7
210
pressure gradient that will dictate the solids loading ratio at which a material can be conveyed through a pipeline, as was illustrated in Figures 4.23 and 24. Then for materials that have very good air retention properties, such as cement, the minimum conveying air velocity varies with solids loading ratio, as was illustrated in Figure 4.6. 2
MATERIAL COMPARISONS
Various materials were compared in Chapter 4 in terms of their conveying capability and the broad divisions that result between materials that can be conveyed in dense phase and those that can not. In this section the differences are examined in terms of conveying air velocities, power requirements and specific energy. For continuity the three materials considered earlier are examined further. The materials were cement, sandy alumina and polyethylene pellets. Conveying characteristics for cement were presented in Figure 4.5b and are reproduced here in Figure 7.1 for reference. All three materials were conveyed through the Figure 4.2 pipeline which was 165 ft long of two inch nominal bore and included nine 90° bends. Similar data for the alumina and polyethylene pellets from Figures 4.8b and 12b are similarly reproduced in Figures 7.2 and 3. To allow visual comparisons to be made the same axes have been used for all three materials and conveying line pressure drop values up to 25 lbf/in2 have been considered in each case. Pressure Drop
160
120
Solids Loading Ratio
20
10
40
80
120
160
Free Air Flow Rate - itVmin Figure 7.1
Conveying characteristics for cement.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
211
Material Conveying
Solids Loading Ratio
30
Conveying Line Pressure Drop - lbf/in 2
20
NO GO AREA t3 10
Conveying Limit
40
Figure 7.2
160 120 80 Free Air Flow Rate - ft/min
200
Conveying characteristics for sandy alumina.
The data, therefore, relates to positive pressure conveying. A relatively high pressure has been used in order to accentuate the differences between the materials considered. The same differences, however, will exist in negative pressure conveying and so the analysis undertaken, and the results obtained, will differ little between positive pressure and vacuum conveying.
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