It is very common in the world of static pressure equipment to utilize processing equipment installed in vessels and tanks to help stir the contents. Typically, these are called agitators or mixers. Generally speaking, these consist of a motor or motor and gearbox that sit on top of the vessel or tank, which drives a long shaft in the vessel with various types of mixing blades or vanes. While this seems straightforward, there are many different factors that go into the design and specification of this equipment, such as:
- Motor Horsepower
- Gearbox/Speed Reducer Need and Ratio to Determine RPM
- Shaft/Blades/Vanes Materials
- Hardware Materials
- Shaft Size
- Blade or Vane Shape/Size/Configuration
- Distance from Shell/Head
- Fabrication Tolerances of Shell/Head
- Seal Type (Gasket vs Mechanical Seal vs Liquid Seal)
- Vessel/Tank Head Design Aspects for Reactionary Forces and Need for Reinforcement Pads/Gussets
The process conditions inside the equipment will determine how it’s designed. Details such as temperature, viscosity, phase change (or all of these!) factor in. In short, it is a very complex and often iterative practice to design a proper mixing system and in many cases, the vessel/tank design can change considerably based on changes in the design. Mixer design can even affect details like the heat transfer to a set of internal coils or half pipe, so it’s important to understand how those systems react to each other. Remember, if a vessel is already designed and in fabrication and the process changes one or more details, it could create a significant change in the tank or vessel that will likely result in delays and will certainly result in often considerable cost increases. In some cases, a complete redesign of the vessel/tank may be required and could result in scrapping all or most of the work completed to date.
If your process depends on proper mixing to stay homogeneous or to make your reaction work as designed, it’s paramount to understand all the ins and outs of Agitator/Mixer design.