"Made in VMI-RAYNERI" Journal

Designing a mixer

Deciding on and choosing the size of a type of a mixer consists in finding the optimum parameters for the implementation of the desired procedure. Frequently, optimization is limited by constraints such as costs, bulk or physical limits. This approach consists in choosing a certain number of parameters:

• Type of agitators and position (radial / axial / angled discharge rotors, dispersion/emulsification rotors)
• Geometry of the tank (size, shape)
• Rotation of the rotor (speed, rate of discharge)
• Length of mixing
• Imposed physical conditions (pressure, temperature)

The people who make these choices rely on their knowledge and experience to make them and choices become additionally complex because of a certain number of factors of which the most frequent follow:

• The nature and rheology of products can lead to complicated expressions of a certain number of parameters and specifically of their respective progress during the mixing process. More precisely in the case of non Newtonian liquids for which is observed non linear progress of the required power and the rate of flow of circulation in respect to the rotation speed of the agitator.
• Constraints regarding some parameters because of experience or technologic and economic reasons, such as the peripheral speed   return from one type of mixer to another, shearing rate, speed of flow or pumping limit the margin of action for the calculation of the other mixing parameters.  It is a limiting factor but we must consider that these constraints, in the end, lead to a more rapid result by minimizing choices.

In practice, choosing an agitator becomes a compromise:  a dominant parameter is established and calculated and then the other parameters are checked to insure they are sufficient. 

VMI recommends and implements the following method:

Step 1................................................................... Identification of the type of mixing to perform

• Solid / liquid mixtures (dissolution, homogenizing, placing in and/or maintaining in suspension, dispersion)
• Liquid / liquid mixtures (placing in and/or maintaining in suspension, homogenizing, dilution, emulsion)
• Complex rheology of viscous mixtures (placing in and/or maintaining in suspension, dissolution, homogenizing, dispersion, heat transfer, grinding)

Step 2...................................................... Inventory of the characteristics of mixing materials

• Liquids (density, viscosity, percentage, initial and final temperature, type of discharge)
• Solids (nature, percentage, density, granulometric dimensions and distribution, settling speed, wettability, solubility)
• Gas (nature, flow, pressure, solubility)

Step 3................................................................ Identification of the global characteristics of mixing rotors

• Flow mainly generated (axial or radial)
• Importance of the pumping effect (high, medium, low)
• Importance of the shearing effect (high, medium, low)
• Capacity of generating turbulence (high, medium, low)

Step 4................................................................ Choice of the rotors

You must then chose between the varieties of rotors offered by VMI the one that is the best adapted to the mixture you want to produce. Your choice should be based on the following:

• Intrinsic characteristics of the rotors taking into account the preferred type of flow, knowing that frequently a compromise must be made between the type of discharge (axial, radial, turbulent…) and mechanical effect generated (circulation, shearing, …),
• Laboratory tests,
• Financial criteria: example - choice in order to achieve the best  Nq/Np performance to minimize installed capacity,
• Functional criteria: example - choice of a rotor that is the easiest to clean.

Step 5................................................................ Calculation of the various mixing parameters (tank – rotors)

These calculations are performed taking into account as main parameters one or several criteria for a precise mixture:

• Criteria for mixing efficiency (peripheral speed, recirculation rate therefore capacity of the turbine, length of the mixing process)
• Criteria linked to the rheology of the product (the higher the viscosity of the product, the higher the diameter of the rotor at low speed)
• Economic criteria

Calculation of the Reynolds number Re, the corrected capacity number NP, the absorbed pump power Pab, the turbine flow rate Q, the drag flow rate Qe (the viscosity of the liquid is taken into account), the recirculation rate TRC, the mixing time Tm, the peripheral speeds  (VP), flow speeds (or transversal) (VF), and rising speed (VR).

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