mixing tank design
Home > News >
DESIGNING A MIXER

 

PRELIMINARY LIMINARY
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 discharge rotors 
- Axial discharge rotors 
- Mix discharge rotors 
- 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 (when viscosity of liquids is directly 
related to the speed of shearing) 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. This is observed in rheoliquidifying liquids (fruit juice, blood), threshold or Bingham liquids 
(paint, varnish, mayonnaise, toothpaste), rheothickening liquids (wet grit, starch suspension, pizza dough) or thixotropic liquids 
(yogurt). 
• 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: In practice, choosing an agitator becomes a compromise In practice, choosing an agitator becomes a compromise: a dominant parameter is dominant parameter is dominant parameter is established and calcul established established and calcul and calculated and and calculated and ated and then ated and then 
the other parameters are checked to insure they the other parameters the other parameters are checked to insure they are checked to insure they are sufficient are checked to insure they are sufficient are sufficient. are sufficient...
VMI recommends and implements the following method: 
 
Step 1 ............................................................... Identification of the type of mixing to perform 1 

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

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

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

Step 5 ................................................... Calculation of the various mixing parameters (tank – rotors) 
STEP 1: Identification 1: Identification Identification of the type of mixing to perform Identification of the type of mixing to perform of the type of mixing to perform
• Solid / liquid Solid / liquid olid / liquid olid / liquid mixtures mixtures
- Soluble powders Soluble powders oluble powders 
 Dissolution 
 Homogenizing 
- Non soluble powder Non soluble Non soluble powders powders
 Placing in and/or maintaining in suspension 
 Homogenizing 
 Dispersion 
• Liquid / l Liquid / liquid mixtures iquid mixtures mixtures
- Miscible liquids Miscible liquid Miscible liquids
 Placing in and/or maintaining in suspension 
 Homogenizing 
 Dilution 
- Immiscible liquids Immiscible liquids Immiscible liquids 
 Emulsion 
• Complex rheol Complex rhe Complex rheology of viscous mixtures gy of viscous mixtures gy of viscous mixtures
 Placing in and/or maintaining in suspension 
 Dissolution 
 Homogenizing 
 Dispersion 
 Heat transfer 
 Grinding 
 
STEP 2: STEP 2: Inventory of the characteristics of mixing materials 2: Inventory of the characteristics of mixing materials Inventory of the characteristics of mixing materials
• Liquids 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: Identifi 3: Identifi Identification of the g Identification of the g cation of the global cation of the global c lobal characteristic characteristics haracteristic haracteristics of mixing of mixing 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 Choice of 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. 
Currently VMI offers the following agitation rotors:


Table I 
Main Flow Main Flow ain Flow Rotor Type Main Function Function Power Powerr 
NPPP
Pumping Pumping Pumping 
NQQQ
Shearing hearing 
Strength Strength trength 
Water propeller (6) Circulation 0.21 to 0.28 0.58 to 0.68 Very low 
Profiled triblade (1) Homogenizing liquid/liquid 0.34 to 0.60 0.84 to 0.87 Very low 
Two way 
profiled triblade (2) 
Dissolution, incorporation 
charges 0.76 to 1.22 1.15 to 1.2 Very low 
PSVB four blade (3) Dilution/Dissolution 1 to 1.95 1 to 1.73 Very low 
PA four blade (5) Dilution/Dissolution 1.8 to 2.2 1 to 1.73 Very low 
AXIAL 
SEVIN with inlets (9) Dissolution/Dispersion 0.4 to 0.55 0.75 to 0.85 Medium 
Centripetal (7) Dilution/Dissolution 1.6 to 2 1.1 to 1.3 Low 
Centrifugal (10) Dissolution 2.5 to 4.5 3 to 3.8 Medium 
Saw teeth (15) Dispersion 0.23 to 0.42 0.19 to 0.31 High 
Deflocculator (8) Dispersion 0.34 to 0.8 0.37 to 0.44 High 
Centri=deflocculator (11) Dispersion 1.1 to 2 0.67 to 0.79 High 
Rotor/Stator wide slots 
(17a) Dispersion/Emulsion 2.1 to 5.9 0.82 to 0.9 Very high 
RADIAL 
Rotor/Stator narrow slots 
(17b) Dispersion/Emulsion 2.3 to 6.2 0.55 to 0.6 Very high 
Note: NP, NQ and shearing strength are expressed for equivalent diameters 
 
• Power: 3 5 N d
P Np  = (P: agitation power; : density; N: rotation speed; d: rotor diameter) is the coefficient 
of drag from the agitator when in the liquid and represents power usage. 
• Pumping: 3 N d
Q N P
Q = (QP: pumping flow rate; N: rotation speed; d: rotor diameter) is the dimensionless 
expression of the pumping flow rate for the agitator. 
• Shearing strength indicates the capacity of the rotor in breaking the friction effect exerted by two infinitesimal 
layers of liquid sliding against one another. Shearing is usually stated as speed of shearing e
V
&= , expressed as 
s
=1
, a value that is very difficult to measure. 
PERFORMANCE MOBILE