HYGIENIC DESIGN OF PUMPS
Centrifugal Pumps Versus Positive Displacement Pumps
Many types of pump are used in the food industry for applications such as filling, emptying, transferring, and dosing. They are also often an integral part of a CIP system. The choice of pump for any given application depends primarily on the characteristics of the product and liquids to be pumped. Of particular importance are its rheological properties and its sensitivity to shear. Broadly speaking, there are two classes of pumps widely employed in the food industry:
- • Centrifugal pumps used for transferring low viscosity liquids at relatively high flow rates but at comparatively low pressure heads. Liquid is directed into the eye of an impeller rotating at around 3000 rpm, which elevates both fluid pressure and velocity. On leaving the impeller, it is directed into a volute-shaped casing from which it is discharged tangentially.
- • Positive displacement pumps (e.g., lobe pumps, gear pumps, vane impeller pumps, and progressive cavity pumps) capable of handling high-viscosity and shear-sensitive liquids. Several positive displacement pumps can pump liquids containing suspended solids with minimal damage.
Design for cleanability is an important criterion in pump selection, but in many cases the functional requirements of the pump, such as delivery pressure and flowrate, determine the type of pump used, even if they are not the most hygienic option available. Positive-type pumps are usually considered to be less easily cleaned than the other due to the nature of their design.