Gravity-Induced Region

In most cases, single screw extruders are flood fed. The hopper is full of pellets that are drawn down toward the base of the hopper via gravity as the screw takes pellets away from the base. With respect to the efficiency of feeding in this region, the primary concern is the ability of the solids to flow freely. Several variables affect this flow. 

Bulk density is the measure of the density of the feed material, including the air between particles. The bulk density value can provide one measure of the solid s ability to flow freely. Generally, materials with higher bulk density flow more freely than those with low bulk density. Pellets resembling ball bearings that flow easily may have a bulk density of about 0.7 g/cnf. Very fluffy regrind may have a bulk density as low as 0.3 g/cm and is more difficult to feed, perhaps requiring special hardware such as a crammer feeder. 

A simple test of bulk density is the hand squeeze test. After squeezing a handful of feed material and then opening the fingers, material that feeds well will probably spill out easily. However, material that is difficult to feed will often stay clumped together. 

Another condition that promotes good hopper flow is low friction. When friction between the pellets (internal friction) is low and friction between the pellets and the hopper (external friction) is low, solids are more likely to flow freely. Factors that may increase internal and external friction include rough surfaces, poor hopper design, moisture, heat, and static electricity. 

Therefore, the degree to which solids flow freely depends on both the material and the hopper design. Under proper conditions, solids move in plug (mass) flow, where the first in the top are the first out the bottom, and no mixing takes place. 

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Toth, J., Gravity-induced cross-formational flow of formation fluids, red earth region, Alberta Canada Analysis, patterns, and evolution. Water Resour. Res.. 14(5). 805-844 (1978). 

After the probable depth of penetration of the regional gravity-induced groundwater flow system has been identified, it should be decided whether or not the gravity-induced flow system should be taken into accoimt in the further evaluation of the potential paths of hydrocarbon migration. 

The hydrodynamic condition in a stable subaerial region is given by the characteristics of the prevailing gravity-induced groundwater flow systems. 

Proceedings 24th Int. Geol. Congr. Section 11, pp. 153-163 Tdth, J., 1978. Gravity-induced cross-formational flow of formation fluids. Red Earth Region, Alberta, Canada Analysis, Patterns and Evolution. Water Resources Research, Vol. 14, no. 5, pp. 805-843... 

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