Conveying characteristics

Conveying characteristics also will provide useful information when an existing plant needs to be upgraded to achieve say, a higher conveying rate of solids. For example, it will be possible to determine whether the system and the material will be able to cope with the increased pressure and/or air flow requirements (i.e., whether the combination of pipe size and blower/compressor rating will be sufficient). 

The determination of steady-state conveying characteristics for a given product and test rig has been the subject of a number of earlier investigations such as Mason et al. (1980) and Mills et al. (1982). A standardized test procedure also has been developed and presented by 

Wypych and Arnold (1985b) and hence, only a brief description is presented here. The test procedure basically consists of three different types of experiments which are applied to the material until sufficient data have been collected for the determination of conveying characteristics. The steady-state parameters generated specifically for this purpose are 

Some typical examples of pneumatic conveying characteristics for three different fly ash samples conveyed on the same long-distance test rig are presented in Figs 11,12 and 13. Some important information regarding these materials and results is summarized below. 

1175 kg m3, Geldart (1973) Group A material but Dixon (1981) Group B material. The latter was confirmed by the nature of the blockages obtained on the test rig (e.g., severe pipe vibrations), as well as the rapid observed deaeration rate of the material. 

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Power to Operate a Screw Conveyor. The power required to operate a screw conveyor is dependent, to a large extent, on the handling characteristics of the material to be transported. Formulas for calculating power use empirically derived factors to account for the conveying characteristics of specific materials, the configuration of the screw, and the beating friction. These formulas have been developed by CEMA and can be found ia the hterature (24,25) and ia engineering handbooks. It is assumed that the total power is equal to the sum of the power required to overcome friction and the power required to transport the material. 

The following comments are based on the findings of Wypych (1989b) who compared the actual conveying characteristics of several materials with the slugging classifications of Dixon (1979) and Dixon (1981). 

The above three conveying characteristics demonstrate the wide range of performances that can occur for fly ash and hence, the importance of employing such information in the design and optimization of longdistance systems. 

Using a standardized test procedure (Wypych and Arnold, 1985b), determine test rig conveying characteristics, similar to those shown previously in Figs. 11-13. 

Predict conveying characteristics for a wide range of pipeline configurations (Pan and Wypych, 1994)... 

The main problem with a feed controlled condition is that throughput is determined entirely by what the feed end manages to pull in and this is very sensitive to the conditions in the feed area. For this reason it is better to keep the machine running full so that the conveying characteristics of the screw determine output rather than the combined characteristics of screw, feed roll, feed opening and feed strip. The compression designed into a screw should therefore be sufficient to achieve complete filling at the lowest likely head pressure. 

The physical properties of granular materials that bear particularly on their conveying characteristics include size distribution, true and bulk densities, and angle of repose or coefficient of sliding friction, but other less precisely measured or described properties are also of concern. A list of pertinent properties appears in Table 5.2. The elaborate classification given there is applied to about 500 materials in the FMC Corporation Catalog 100 (1983, pp. B.27-B.35) but is too extensive for reproduction here. For each material the table also identifies the most suitable design of screw conveyor of this... 

Data was recorded from the manometer banks from each test and this was analysed in terms of a pressure gradient in mbar/m. The resulting data was also plotted in the form of conveying characteristics, and representative sets of data obtained are presented in Figs. 7-10. In Figs. 7 and 8, data obtained for vertically upward flow are shown, with cement in the 53 mm bore pipeline in Fig. 7 and barytes in the 81 mm... 

Fig. 6. Conveying characteristics for pulverised fuel ash in 81mm bore pipeline.

The conveying characteristics for the oil well cement conveyed through the steel pipeline are presented in Fig. 15. Once again as wide a range of conveying conditions were investigated as possible. Similar sets of conveying characteristics were obtained for the barytes in the steel pipeline and for both materials in the rubber hose pipeline. The rubber hose was rated to a lObar capability. 

A rectangular grid was constructed on each set of conveying characteristics, as in the above analysis, and the ratio of pressure drops was evaluated at corresponding grid points of material and air flow rates. In Fig. 16, a comparison of the pressure drop required to convey the cement through the rubber hose, compared with that to convey the cement through the steel pipeline, is presented. It will be seen that... 

Fig. 15. Conveying characteristics for cement in 40 m long steel pipeline.

Fig. 22. Conveying characteristics for coal at different back pressures.

Single screw extruder operating curves. The conveying characteristics of a single screw extruder can also be analyzed by use of dimensional analysis. Pawlowski [6,7] used dimensional analysis and extensive experimental work to fully characterize the conveying and heat transfer characteristics of single screw extruders, schematically depicted in Fig. 4.7. 

The power law equation was used for the viscosity for the other curves in the two diagrams. The flow exponent n was varied between 0.4 and 0.9. The choice of the power law equation provides a non-linear relationship between the flow rate and the pressure and the flow rate and the power, respectively. We note that the flow exponent has significant influence both on the conveying characteristic and on the power characteristic. 

Figure 8.10 Conveying characteristic of a screw element with Newtonian and shear thinning materials...

The power characteristic is also strongly dependent on the flow exponents (Fig. 8.11). The more shear thinning the polymer, the less power is required to convey the polymer melt. Unlike the conveying characteristic, the intersections of the individual curves with the x-axis lie far apart and are located outside the figure. 

Assuming that the product characteristics and the conveying characteristics of the two extruders are identical, the throughput can be converted assuming a constant fill rate of the screw channels. This calculation uses only the known values of screw speed , available free volume , and screw pitch . 

Twin-screw machines in which the screws are counter-rotating have particularly positive feed and conveying characteristics residence times and temperatures should be more even throughout, so configurations of this kind are satisfactory for unplasticized PVC. The main disadvantages are the possibility of entrapment of air in the material, the generation of high pressures, comparatively low maximum speeds, and low rates of output. 

Example 26 Conveying characteristics of single-screw machines... 

In scrutinizing the conveying characteristics in Fig. 39, one discovers three typical ranges in which the screw machine can function. They are outlined in Fig. 40. In this representation, the throughput number Q is standardized by the intercept Ax. In other words, this is the numerical value of Q where the screw machine is conveying without pressure formation. With this flow-kinematic parameter, A = Q/ A1 the state of flow of a screw machine can be outlined more distinctly. 

Fig. 39 Conveying characteristics of a single-screw machine of given screw geometry, taken from [5],...

A dimensionless formulation of this relationship is possible using the conveying characteristics of the screw machine in question, see Fig. 39. In the active conveying range, 0 < A = 1, the dissipation characteristic passes through a minimum in which the lowest power dissipation H occurs for the given values of q and Ap, this corresponding to H/q = min. 

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