Written by George Nikolopoulos
When our customers use our inline high shear mixers, we recommend that they use a positive displacement (PD) pump for feeding product into the unit. Examples would be lobe-type pumps, progressive cavity pumps or twin-screw pumps. Some customers would ask, “What about using a centrifugal pump, or a diaphragm pump?”
Our preference for a PD pump is twofold. First, a PD pump will feed the inline mixer with a constant and consistent flow rate; this ensures the product is processed uniformly from start to finish. Variations in feed rates will change the residence time of the product, so that some of the stream is processed less, and some processed more – a variable flow rate will give you variable process results, such as non-uniform particle size. Second, a PD pump can more readily handle a wider range of product viscosities. These viscosities can vary from recipe to recipe, or there can be an increase in viscosity in the fluid stream resulting from processing through some of our inline mixers, like the Boston Shearmill®.
Other types of pumps: One of the limitations of a centrifugal pump is viscosity. It is the pump of choice for liquid transfer, but a centrifugal pump may have different flow rates based on the head feeding it (height of liquid level in the tank as well as height of tank above the pump). As for diaphragm pumps, the flow is intermittent. There is an impulse of flow, followed by an interruption of flow as the diaphragm switches sides, then the flow continues. This cycle of flow-stop-flow-stop is fine for discharging a tank or transferring from Vessel A to Vessel B, but the interrupted flow does not allow for uniform process results. You could have undesirable variations in average particles size of an emulsion, or the level of de-agglomeration in a dispersion, due to over-milling and under-milling.
Therefore, a PD-type pump is preferred to feed product to our inline high shear equipment (i.e., our Dynashear®, Boston Shearpump®, Boston Shearmill® and Mayomill®), because of its ability to provide a constant and consistent flow rate.
Contact us for more information or to talk to our experts about a mixing solution for your process!