Screw pumps, viscous material

Screw pumps, viscous material 329 Seals, mechanical 339 Sears. F, W. 172.179... 

When it is deleterious, laminar flow can be avoided by mixing over the cross section. For this purpose static mixers in line can be provided. For very viscous materi s and pastes, screws of the type used for pumping and extrusion are used as reactors. 

A most important class of pump for dealing with highly viscous material is represented by the screw extruder used in the polymer industry. Extruders find their main application in the manufacture of simple and complex sections (rods, tubes, headings, curtain rails, rainwater gutterings and a multitude of other shapes). However, the shape of section produced in a given material is dependent only on the profile of the hole through which the fluid is pushed just before it cools and solidifies. The screw pump is of more general application and will be considered first. 

There are a number of machines that do not utilize Archimedean screw for transport of the material, but still fall within the category of continuous extruders. Sometimes these machines are referred to as screwless extruders or elastic melt extruder. These machines employ disks or drums to melt, mix, pump, and extrude the material and are based on viscous drag transport principle. They are not widely used in industry as forming machines, and will not be discussed here. 

It is also possible (as mentioned earlier) to have extruders that use twin or multiple screws. Such extruders are utilized in certain operations because they offer specific advantages. These include (1) increased output at low screw speed, (2) improved pumping control over a wide range of operating conditions, (3) decreased viscous dissipation and internal heat generation, (4) ability to handle materials that are difficult to feed, and (5) lower power requirements (see Fig. 7-9). 

The complex interplay between the rate of viscous energy dissipation, temperature, and material viscosity in an extruder sometimes results in a variation in output (surging) even at constant screw speed. Where it is necessary to minimize surging, positive-displacement gear pumps are often added after the extruder. 

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