Agitation

Mixers are commonly used in sanitary vessels to aid in heat transfer, mix ingredients, or alter the product in other ways, such as gas dispersion or emulsification. Agitation is a separate discussion, however the geometry of a tank designed for mixing is important. The tank manufacturer must be aware of the entire process the customer desires to specify the correct mixer. The rheology of the fluid is important, as well as the desired mixing levels, utilities available, time constraints, and any unusual environmental considerations such as explosive atmospheres, foaming, burn on, etc. The power draw of any mixing impeller is proportional to the fluid properties, the impeller diameter to the 5th power, and the speed cubed. Many fluids exhibit viscosity changes with temperature, this must be taken into consideration. Before entering any vessel with an agitation system, proper lockout-tagout rules must be followed. Pressure vessels may require a mechanical seal for proper operation. Mixer seals are available that allow for the large shaft deflections and runout conditions mixers are subject to. It is generally inadvisable to run mixers right at the impeller level. For most impellers, it is recommended that there be at least 1 impeller diameter of fluid above and below the impeller for proper operation.

A mixer can be best understood as a inefficient pump. The 2 main parameters that need to be known to properly size a pump are flow and pressure. Like a pump, mixers generate flow and pressure, but the pressure is generally referred to as shear. Different processes require varying amounts of flow and shear, simple fluid mixing is usually a flow dependent operation while emulsification is more shear dependent. Several processes require different amounts of flow and shear at different times in the process, multiple mixers are specified to address varying flow and shear requirements. For sanitary service, the mixer is often designed to be CIP cleaned and may be of welded, non-removable construction.