Hoppers, design

FIGURE 8.7 Corotating screw extruder, (a) self-wiping intermeshing screw elements, (b) selfwiping intermeshing kneading block elements. 

The flow of material in twin-screw extruders is very complex, and the flow patterns are difficult to predict mathematically. For this reason the simulation of processes in twin-screw extruders is not as well developed as it is for single-screw extruders. It is therefore difficult to predict the performance of a twin-screw extruder based on geometrical features, polymer properties, and processing conditions. Hence, it is difficult to carry out accurate design calculations. For this reason twin-screw extruders are constructed 

The sizes of twin-screw extruders range from 25 to 244 mm (this is the diameter of one of the barrels). The barrel-length-to-diameter ratio, UD, ranges from 39 to 48. The length can be altered as required for most twin-screw extruders because of the modular construction. 

Most extruders are of the plasticating type in which solid pellets are fed to the extruder where they are converted to melt and pressurized. The extruder is fed by solids that enter the extruder from a hopper (which is a metallic cylinder with a converging section as shown in Fig. 8.9) or are metered in. The flow patterns in the hopper are complex and are still the subject of research. Our intentions here are to estimate the pressure at the base of the hopper as this value is needed to calculate the pressure rise in the exttuder. 

FIGURE 8.8 Photograph of various types of screw elements commonly used in corotating twin-screw elements. (Courtesy of Berstorff Corp., Charlotte, NC.) 

---

Conica.1 Hoppers. Design charts for conical hoppers typically are plots of wall friction angles, ( ), vs hopper angle, 9. Charts such as that in... 

Sludge-blanket clarifiers are difficult to start up because the first blanket must be estabUshed, and large-scale units require extensive excavation. Sizes range from 600 x 600 mm to 50 x 50 m. Precipitation and crystallization can be carried out in similar hopper-designed units, having overflow rates of 80 m/h or higher. 

Dust lood Plote design Hopper design Gos velocity Gos distribution Rapping force Popping intervol... 

Although a mass-flow bin is obviously preferable to a funnel-flow vessel, the additional investment generally required must be justified. Often, this can be done by the reduced operating costs. But when installation space is limited, a compromise must be made, such as providing a special hopper design and sometimes even a feeder. Certainly, with mass-flow bins the feeder is not required for flow, but it might still be used for other reasons, such as conveying the material to the next process step. 

The method makes use of the principle that a constant ratio of induced stress. s in the stored contents to the consolidating pressure p exists. Thus, for any hopper design for which th.eff curve is available, the shear-tester results can be potted, and the point where/= s is located. Since the distance at which this occurs above the hopper vertex is also known, these values become the hopper dimensions at that point. 

In an effort to put powder flow studies and hopper design on a more fundamental basis, Jenike [45] developed a powder shear tester and methodology that permits an assessment of powder flow properties as a function of consolidation load and time as well as powder-hopper material interactions. The methodology has been used extensively in the study of pharmaceutical materials [39,58-61]. From the yield loci obtained using this method, several parameters can be determined that influence powder flow, and discussions of these points are well documented in the literature [49,62,63]. 

Dust lood Plate design Hopper design Gas velocity Gas distribution Rapping force Rapping interval... 

Keywords Fluid mechanics Hopper design Vertical pipeline Pneumatic conveying Inclined pipeline Pipeline bends Rubber hose Dense phase... 

In this chapter, the mechanics of hopper flow and standpipe flow along with their operational characteristics are described. Problems such as segregation, inconsistent flow rate, arching, and piping that disrupt and obstruct the flow of bulk solids in hoppers are discussed. Remedies with respect to the use of flow-promoting devices such as vibrators and aerating jets to reinitiate the flow are presented. The importance of the flowability of the solids to be handled in relation to the hopper design is emphasized. 

The weaknesses associated with the flow through an orifice and angle of repose measurements limit their application for powder flow studies and hopper designs. Consequently, several powder shear testers and methods that permit a more thorough and precisely defined assessment of powder flow characteristics were developed. Shear testers that measure the frictional characteristics of a powder bed under load yield valuable information with regard to powder flow in high-speed tablet equipment. A number of types of shear cell testers are available, but the most common types used in the pharmaceutical industry are the Jenike shear cell and the Schulze ring shear tester.61,62... 

FMC Technologies recommends that to obtain a uniform material flow pattern, the ratio of the throat (T) to the hopper gate height H) be 0.6 for an ideal hopper design. The material at the front and the rear of the hopper will then move at nearly the same velocity. An acceptable design may be obtained if the ratio of T/H is between 0.5 and 1.0 however, a ratio outside these limits may distort the material flow patterns and reduce the feed rates (FMC Technologies, 2000). See Figure 18.18. 

A history of hopper and feed screw designs showed that each design evolved to facilitate and improve powder flow to the compactor feed screw conveyance system. Feed hopper designs depicted in Fig. 4 show designs incorporated in older compactor models [examples (A)-(C)]. Dehont et al. described the powder compaction feeding systems that were in use up to 1989. ... 

Fig. 4 Feed hopper designs (A) simple hopper (B) simple flap hopper (C) flap distribution box and (D) force-feeder. (From Ref., courtesy of Ellis Horwood Limited.)...

As with other unit operations, the scale-up success for roller compaction is dependent on understanding and identifying the process parameters that will influence the quality of the final product. The typical parameters that can influence compacted ribbon characteristics and the resultant final product qualities are hopper design, feed screw design, and roll configuration. 

---