Mixing is an essential operation in a wide range of food and chemical processing industries.
Converting raw materials into a homogeneous mixture seems simple, but engineers and scientists know that there is a range of variables that need to be just right to ensure the right output and a consistent, high-quality product.
The key factor in getting the desired result from your mixing process is to understand how fluids behave, how equipment is designed, and how variables in the process can be adjusted to result in a uniform product.
The forces at work
All fluids have their own unique properties such as density and viscosity, and when introduced to other fluids, miscibility, solubility and emulsification also come into play.
When you combine the properties of your ingredients with the physical characteristics of your tank or vessel, such as size, geometry and impeller type, you have a bucketload of different forces and properties to contend with.
Furthermore, even within individual batches, there can be slight differences in fluid properties.
So how do you determine the right mixing process and equipment to get the best possible result?
Understanding equipment design
By understanding how mixing equipment is designed and the limits of each unit’s capabilities, you will be better placed to design a process that provides the most uniform mixing results.
When looking to design a process flow for your facility, it can help to sketch a scale drawing of the required mixer with descriptions that provide as much information as possible to convey the type, speed and aggressiveness of mixing required. Include impeller dimensions and placing, shaft length, liquid level and ay other key information.
When it comes to the design stages, some mechanical knowledge is required to ensure the right shaft strength and critical speed is selected, enabling the equipment to handle the necessary load.
- Ensure drawings are accurate
Errors and assumptions in CAD drawings are more common than you think! It’s critical to understand the difference between a ‘sales drawing’ that provides a general overview of the product, and an accurate CAD drawing that has been customised for your individual application. Impellers that might look evenly spaced on a sales drawing might result in something else entirely in a proper dimensional drawing.
- Get the critical dimensions right
One of the most important dimensions for any mixer is the distance between the lower impeller and the bottom surface of the tank or vessel. Another is mixer shaft length – it is critical to ensure this is the correct height for the tank. And yet, because the mixer and tank are often bought from separate suppliers, compatibility issues and mismatched dimensions are common. One solution to this is to use a specialist mixing equipment supplier that offers complete process solutions, meaning the entire unit is designed by one engineer.
- Remember the displacement
It’s easy to overlook the space occupied by the mixer, baffles and other internal components of your tank, which can affect the final liquid level. A good rule of thumb is to allow for your liquid to sit 10% higher than your inside tank dimensions would have you believe.
How much mixing?
With no universally accepted terms to rely on, it can be difficult to describe the amount and intensity of mixing required.
Think about the quantity of material to be mixed in terms of volume or mass, the fluid density or specific gravity of your liquids, and its viscosity, remembering that the more viscous the fluid, the more powerful the mixer must be. As viscosity increases, flow patterns change which means fluid motion becomes more difficult.
The intensity of mixing can be described as mild, medium or aggressive, however, these terms are relative, so a better approach is to use a scale of 1-10. Turbulence is another consideration, with varying degrees from laminar conditions through to high turbulence.
When relating the amount of mixing to your design, remember that blade configuration influences mixing intensity, so the aggressiveness of mixing can be partly controlled by looking at different blade configurations.
Enter the technology
Of course, the most logical way to prototype and validate the above dimensions and calculations is through design software that can simulate the performance of the mixing system.
For example, comparing the performance of a hydrofoil impeller vs a pitched-blade turbine can be easily simulated in design software, allowing the engineer to make informed choices for the final design.
Remember the rule of thumb to ‘measure twice and cut once’: don’t rush the design process and run digital tests with many variables to ensure your final design, once manufactured in its physical form, is going to be the right tool for the job.
Mechanical design and manufacturing
Once you are confident that the process conditions are satisfied, it’s time to consider the mechanical design.
Components such as the shaft are particularly important for more viscous mixtures, ensuring it can handle the necessary torque required to turn the impellers, as well as the bending loads from hydraulic forces within the tank.
The position of the impellers is also important and has a bearing on the mechanical capacity: impellers that operate near the liquid surface can be subject to 3 times the hydraulic load of typical liquid operation. Critical speed is yet another factor that is key to successful mixing, and designing a shaft that can handle the necessary speed and torque at peak operation will lead to better performance in the long term.
Ensure a successful outcome with Mixquip
By understanding the different fluid forces, design considerations and material constraints that go into a mixing process, you can ensure your equipment is designed to provide the best, most consistent outcome.
Contact the engineering team at Mixquip for advice on stand-alone components or complete solutions, all of which can be customised for your particular processing needs. www.mixquip.com
