Positive Displacement Pumps
Positive displacement pumps operate using a completely different mechanism of action from centrifugal pumps. In all positive displacement pumps, a fixed amount of fluid is pulled into the pump with each stroke through creation of a vacuum and then pushed out on the outlet or dis-charge side. Because they are creating a vacuum at their inlet, positive displacement pumps are self-priming and will draw the liquid to the pump. This principle applies to all types of positive displacement pumps whether the pump is a diaphragm, elliptical lobe, flexible impeller, rotary lobe, rotary piston, peristaltic, or piston.
Diaphragm pumps have many applications in a brewery (Figure 24.3). They are operated by compressed air and have many desirable features. Diaphragm pumps work by contracting a rubber diaphragm to pull fluid into a chamber through an inlet valve, then expanding the diaphragm to push fluid out of the chamber through an outlet valve. This isolates most of the moving parts of the pump from the juice or beer. They are very gentle on juice and beer during transfer.
Peristaltic pumps are popular in precision pumping applications that require an exact amount be pumped per shaft rotation (Figure 24.4). The pump works by turning a lobed shaft that progressively squeezes a soft tube filled with fluid, thus pushing the fluid out of the pump on the high-pressure side of the housing. These pumps meet all the brewery-pumping requirements; they are simple, reversible, easy to clean, and very gentle on the fluid being handled.
Flexible Impeller Pumps
Flexible impeller pumps are considered for general use in the beer industry (Figure 24.5). The flexible impeller working principle consists of a rotating rubber impeller with vanes that maintain contact with the pump inner walls. These vanes bend and then straighten as the impeller turns to conform to shape of the pump internal walls. The vacuum caused by the movement of these vanes push the fluid round from the inlet to the discharge pipe. Flexible impeller pumps should never be allowed to run dry as this will cause the impeller to overheat.
Progressive Cavity Pumps
Progressive cavity pumps are also known as Moineau pumps (Figure 24.6). The pumps work by turning a helical rotor within a helix-shaped elastomeric stator. These complex shapes create fixed volumes that move along the axis of the rotor to move fluid from one end of the pump to the other. Progressive cavity pumps are self-priming positive-displacement pumps that minimize the shearing and agitation of the pumped fluid. These pumps are highly tolerant of sediment and solids. Progressive cavity pumps are easy to clean and maintain, and they can be packaged into compact portable packages. The pumps can be expensive but are a good value.
Rotary Lobe Pumps
Rotary lobe pumps are a type of positive displacement pump featuring large, non-contacting chambers that handle must, lees, juice, and beer (Figure 24.7). The internal structure of the pump generally consists of two intermeshed, counter-rotating, gear-like rotors in an ovoid chamber. Typically, the lobes are equilateral trefoils, but bi-lobed, winged bi-lobe, and helical lobe designs are available. The tolerances between the two rotors and the chamber are tight, but usually not airtight. This means that, unlike flexible impeller pumps, most rotary lobe pumps are not self-priming.
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