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Open screw impeller pump benefits

12 March 2018

In any pumping application, it is necessary to match the pump type to the application to achieve to desired result. Often a screw impeller pump is chosen. Food Processing finds out why. 

Before deciding on the type of pump to specify, a close examination of the application should be undertaken. Issues such as energy efficiency, reliability, gentle pumping action and wear resulting from the pumped fluid will all come into the equation. Where the requirement is to move clean liquids and those of low viscosity, multi-blade centrifugal pumps are typically selected. However, these pumps have a limited suction capacity and are not usually suitable for liquids containing solids. The single sealed or open screw impeller overcomes these limitations.

It is important to match the impeller type to the nature of the fluid being pumped and, where large solids or highly viscous media are being handled, free passage through the pump housing is essential. A centrifugal pump used across a wide spectrum of applications in food processing and beverage production is the single screw impeller pump.

Low suction pressures
With the open screw or single sealed impeller the suction stream ahead of the impeller and the design of the inlet blade profile deliver very low suction pressures so high flow rates can be achieved at high pump speeds and high temperatures. Furthermore, because there is limited fluid contact with the impeller, shear rates are low and free passage of solids and slurries is achieved. These all contribute to less wear on the impeller and pump housing – factors that contribute to lower maintenance and longer life The efficiency of the screw impeller pump is higher than other types of pumps used for similar applications. The pump head curves are continuous and steep, making it cost-efficient to adjust the pump operation to the specific pumping requirement and in particular speed-controlled operation.

The impellers come in different profiles, each designed for the nature of the fluid being pumped. Open impellers are typically employed for handling abrasives, coarse suspended solids, fibrous materials and viscous fluids, whereas closed impellers are associated with highly abrasive media or the gentle movement of sensitive media.

Screw impeller pumps are economical to operate, but represent a higher initial investment than other types of centrifugal pumps. Therefore, where efficient operation can impact favourably on the unit cost of a product, the screw impeller has the competitive edge. Where energy efficiency is less of a consideration and resistance to wear and abrasion from the pumped fluid is more highly prized, then the lower investment cost of the vortex pump can influence pump selection.

Hidrostal’s F-Type pump is a good example of where the screw impeller pump is used in the food industry. It uses a shrouded single spiral vane impeller that is axially extended for smoother operation. Media entering the pump undertakes a long slow turn around the long radius, low angled vane. The hydraulic gradient within the pump generates a gradual pressure and slow change from radial to axial flow.

For handling raw ingredients this type of pump offers the versatility to operate closed pipe transfer systems, as opposed to being restricted to conveyors, elevators or containers. The enclosed pipework system has the benefit of being more hygienic and cleaner as it is protected from any potential harmful elements in the environment. Operational costs are also reduced as maintenance is far easier on enclosed pipework than with conveyors and belts. There is also health and safety advantages as there are fewer exposed moving parts.

The F-type pump is also suited to the energy efficient circulation and removal of wash water used to clean raw ingredients. In a brewing, for example, the pump can be used in malt production for carrying grain in water without any damage to the germ, as well as for mashing, lautering, wort circulation and trubbing.

Summary
The availability of different designs of screw impellers enable this pump to take on aggressive and gentle media with high levels of hydraulic efficiency. The initial investment may be higher than for other pump types, but this is more than offset by the economics of the pump’s hydraulics over the longer term.


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