Many valuable products are viscous fluids.? These include paints and coatings, adhesives, high solids concentration slurries and personal care products such as shampoo and toothpaste.? The power input by an agitator and the time to blend ingredients added to a batch are directly dependent on the viscosity of the fluid.
An added complication that must be considered is that most viscous fluids are also non-Newtonian.? This means that the viscosity that the agitator impeller “feels” is dependent on the rotational speed at which it operates.? The majority of fluids exhibiting non-Newtonian properties are “shear-thinning” so, the faster the impeller rotates, the lower the apparent viscosity of the fluid.? Since the viscosity changes with impeller speed, the design of an agitator (and pump) cannot be reliably accomplished using a single value of viscosity.
Incorrect understating of the effect of non-Newtonian behavior on agitation can lead to problems such as:
- Longer than anticipated batch times (longer time to achieve desired homogeneity) due to underestimating the actual viscosity of the fluid.
- Slow incorporation of added ingredients due to minimal motion at the surface of the batch.
- Stagnation / dead zones created because the impellers simply cannot overcome the viscosity of the fluid to generate motion.
- Poorer than expected heat transfer due to underestimating the apparent viscosity at the wall of the vessel.
Philadelphia Mixing Solutions and Mixing Solutions Limited have a state-of-the-art laboratory where the appropriate properties of a non-Newtonian fluid can be measured.? We also have a long and successful track record of correctly interpreting these measurements and designing equipment for these processes.