Vortex or Screw Pumps

In any pumping application, it is always necessary to match the pump type to the application in order to achieve to desired result. One aspect of the selection process is matching the impeller type to the nature of the fluid being pumped. Where large solids or highly viscous media is being handled, then free passage through the pump housing is essential. Two centrifugal pumps that fit the bill and are used across a wide spectrum of industries, including foodstuffs and wastewater, are single screw impeller and vortex impeller centrifugal pumps.

These two very different pumps both meet much of the criteria for pumping solids-laden fluids, so when it comes to choosing which to use a close examination of the application has to be undertaken. It cannot be claimed that one pump is better than the other. It largely comes down to which is more suitable for the application. Issues such as energy efficiency, reliability, gentle pumping action and wear resulting from the nature of the pumped fluid all come into the selection equation.

Screw impellers

The typical application for multi-bladCutThroughe centrifugal pumps is the movement of clean liquids and those of low viscosity. These pumps have a limited suction capacity and are not really suitable for liquids containing solids. The single sealed or open screw impeller overcomes this limitation. The suction stream ahead of the impeller and the design of the inlet blade profile deliver extremely low suction pressures (NPSH). As a result, 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 working life.

The efficiency of the screw impeller pump is significantly 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 most 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.

Vortex pumps

Unlike the screw impeller pump, the vortex pump does not generate a centrifugal force. This is because the impeller is recessed and therefore outside the flow region of the volute. Energy is continuously generated in the fluid surrounding the impeller in what is essential a two-stage pumping process. The outcome of this is the creation of a primary vortex in the fluid which in turn generates a secondary vortex in the volute, producing the fluid flow through the pump.

ImpellersBecause the impeller is recessed it does not come into a significant amount contact with the pumped fluid, it is largely unaffected by the solids contained in the fluid. Those solids that do come into contact with the impeller do not migrate across the vanes. Consequently, the impeller does not tend to become blocked up. There is also a reduction in the radial forces that act on the impeller, so the pump can operate at exceptionally low flow rates.These are all beneficial factors that will appeal to a great many potential users. However, there are downsides, the main one being the lack of hydraulic efficiency. For some applications and industry sectors this may not be too big of a problem, but where there is a cross-over area with the screw impeller it does become a much less desirable option.

Applications

With both screw impeller and vortex pumps being able to offer similar fluid handling capabilities, the issue of which to use within an industry sector or application will generally dictated by economics. Screw impeller pumps are more economical to operate, but represent a higher initial investment than vortex pumps. Therefore, where efficient operation can impact favourably on the unit cost of a product, such as in the food and beverages industries, the screw impeller has the 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 will greatly influence pump selection.

Taking Hidrostal's F-Type pump provides a good example of where the screw impeller pump is used extensively in the food industry. These pumps use 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.

Where food manufacturers handle 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 immediate environment. Operational costs are also reduced with enclosed pipeworks as maintenance is far easier than with conveyors and belts. There is also health and safety advantages as here are fewer exposed moving parts.

Sensitive handling of raw ingredients is not all that the F-Type pump offers users. The pump is ideally suited to the energy efficient circulation and removal of wash water used in the cleaning of raw ingredients. Where brewing is the industry sector, 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

Both vortex pumps and screw impeller pumps have many attributes that enable the pumping of solids and slurries. The availability of several designs of screw impellers enable this pump to take on aggressive and gentle media with high levels of hydraulic efficiency, something that the vortex pump cannot equal. The initial investment may be higher, but this is more than offset by the economics of the pump's hydraulics over the longer term.