Positive-displacement pumps screw

The ease with which the separated products leave the bowl determines the richness of the fat. Fluid whole milk enters the separator under pressure from a positive displacement pump or centrifugal pump with flow control (Fig. 1). The fat (cream) is separated and moves toward the center of the bowl, while the skimmed milk passes to the outer space. There are two spouts or oudets, one for cream and one for skimmed milk. Cream leaves the center of the bowl with the percentage of fat ( 30 40%) controlled by the adjustment of a valve, called a cream or skim milk screw, that controls the flow of the product leaving the field of centrifugal force and thus affects the separation. 

Previous tests have shown that centrifugal pumps,6 the most reliable and most widely used pumps in the oil industry, are completely unsuitable for pumping oily-witer mixtures. While not entirely satisfactory, positive-displacement pumps, such as screw and gear pumps, cause less droplet breakup than most. 

Figure 7.4. Performance of several kinds of pumps, (a) Comparison of small centrifugal and turbine pumps (Krista and Annett, 1940). (b) An axial flow pump operating at 880 rpm (Chem. Eng. Handbook, 1973). (c) An external gear pump like that of Figure 7.12(e) (Viking Pump Co.), (d) A screw-type positive displacement pump, (e) NPSH of reciprocating positive displacement pumps.

Next we examine two important continuous positive-displacement pumps the gear pump and the co-rotating fully intermeshing twin-screw pumps. First we show in the following Example how to convert building block 5 into a continuous intermeshing twin-screw pump. 

Rotary positive-displacement pumps with no valve action gear pumps, lobe pumps, screw pumps, eccentric-cam pumps, metering pumps... 

FIGURE 4.3 A screw pump, an example of a positive-displacement pump (left) cutaway view of a deep-well pump (right). 

Positive-displacement pump—A pump that conveys fluid by directly moving it using a suitable mechanism such as a piston, plunger, or screw. 

Liquid additives are mostly added downstream of the plastification section because they tend to lubricate the pellets or cause powders to agglomerate in the feed throat. If a significant amount of liquid is to be incorporated, it can be added at several locations. The most effective method for low viscosity liquid incorporation is to inject into a fully filled distributive mixing section. This requires a pressure injection valve and positive displacement pump. For small amounts of compatible liquid, non-pressurized injection into a low degree of fill area of the screw configuration may also be acceptable. 

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