Types of Agitators – Agitator’s Design and Significance

Agitator Mixer

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Agitator Types

Agitators come in many sizes and shapes. There are two types of agitators – mechanical and electronically controlled. In the first article, we will cover mechanical process agitators.

The basic types of mechanical agitators are:

Paddle Agitators

paddle agitator

This is one of the most primary types of agitators with blades that reach up to the tank walls. Paddle agitators are used where an uniform laminar flow of liquids is desired.

 

Anchor type Agitators

 

This simple agitator consists of a shaft and an anchor type propeller and can be mounted centrally or at an angle. It is mainly used in reactors.

 

Radial Propeller Agitators

Radial agitators consist of propellers that are similar to marine propellers. They consist of two to four blades that move in a screw like motion, propelling the material to be agitated parallel to the shaft.

 

Propeller Agitators

A propeller agitator is shaped with blades tapering towards the shaft to minimize centrifugal force and produce maximum axial flow. Propeller agitators are popular for simple mixing jobs.

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Turbine Agitator

 

   

Yet another type of process agitator is the turbine agitator. Turbine agitators can create a turbulent movement of the fluids due to the combination of centrifugal and rotational motion.

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Helical  ribbon Agitators

These agitators have blades with a twisted mechanism, just like the threads of a screw. The curves result in a vigorous motion of the fluids to be agitated. Helical agitators are most useful for mixing viscous liquids.

Choosing an agitator depends upon the specific gravity and viscosity of the products to be mixed. Agitators need to be designed, engineered and manufactured to suit individual applications. Core knowledge of fluid mechanics is essential for choosing the right type of agitators.

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A simplified brief description has been tabulated below.

 

Agitator Type Applications Advantages Disadvantages
Paddle * Mixing of Solids,
* Slurry Mixing,
* Used during Crystals
forming phase during
Super saturated Cooling
* Heavy duty,
* Apt for Slow operation,
* Can have 2 or 4 blades
* Power Consumption is very high,
* Inefficient Mixing
Turbine
*Straight Blade
*Pitched Blade
*Curved Blade
*Disk Blade
* Liquids and Gas reactions,
* Highly used during Reaction and Extraction Operations.
* Generates high  Radial Flow,
* Highly used for  dispersion operations
* Not preferred for solvents with high viscosity[NMT 20 cP]
Screw type * Have to use in addition  to other agitators.
* Mostly used in Food processing.
* Uniform mixing of High viscous masses. * Not preferred for im-miscible solvents.
Helical Blade
*Ribbon Type
*Helical Screw
* Most Probably used in Paint industry. * Can handle Visco-elastic liquids efficiently * Low possibility for Radial mixing
Anchor * Highly used in Pharma Industry for Several Operations * Increase possible heat transfer rate in reactors, from reactor heat transfer surface to Mass. * Required high Efficiency Gearbox,
* Required high Power.
Gate * Highly used for blending Operations * Provides efficient Mixing and agitation control,
* Can handle Psuedo-plastic liquids.
* Not preferred when both liquids and gases combine involves
Propeller * Suitable for GLR’s, ANFD’s, can handle Corrosive materials with Glass lining. * Will increase the homogeneity,
* Can be used in two different patterns for drying and pressing.
* Need to be operated at high speed to avoid solid settlings in reactors.
* Need to be operated at low speeds in drying operations.

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Diameter of Agitator: Usually the diameter of agitator depends on the Diameter of the vessel,

It is generally 1/3rd the diameter of the tank for Marine Propeller, Axial flow turbine, Turbo Propeller, Disc Blade turbine, Flat Blade turbine, Backward Blade turbine. it is generally 4/5th of the diameter of the tank for Paddle, Anchor, Gate.

Type of Agitators and their functions:

Marine Propeller is generally an item produced by casting process in a foundry and it has 3 or 4 blades. Cast agitators have two basic advantages, uniformity of material and hard surface. These have tapering blades, and angle of blade varies from root to tip. This produces maximum axial flow. The diameter of Marine Propeller impeller is 15% to 30% of diameter of tank. These have tip speeds between 300 to 500 meters per minute.

Axial Flow Turbine, Turbo Propeller and Flat Blade Turbine have blades ranging from 3 to 6. These have tip speeds between 200 to 300 meters per minute. The diameter of impeller is 25% to 60% of tank diameter. For Axial Flow Turbine and Turbo Propeller, the angle of blade varies from 30 degrees (for less viscous liquids) to 60 degrees (for more viscous liquids). Standard angle is 45 degrees. Power requirement increases with higher pitch angle. For Flat Blade Turbine, the length of blade is 25% of diameter, and disk diameter is 60% to 70% of the diameter of impeller.

Paddle, Anchor, or Gate have only 2 blades. These extend close to the tank wall and have tip speeds between 80 to 150 meters per minute. These push and rotate the liquid in a laminar flow. There is no axial or radial mixing. The width of blade is 1/8th or 1/10th of the agitator diameter.

Number of Agitators:

There can be many number of agitators connected to a shaft, and it can be calculated as

Number of agitators = (Maximum liquid height x specific gravity) / Diameter of tank. Gap between two Agitators = Liquid height / (Number of impellers – 0.5).  Basic Design Equations for a Vertical Vessel and Agitator:

Vessel Volume = (pi x vessel dia x vessel dia x vessel length) / 4.0

Agitator Shaft length = vessel length + (vessel dia x 0.25)

For Rectangular Tanks,

Tank Volume = Vessel Length x Vessel Breadth x Vessel Height

Agitator Shaft length = Vessel Height

Based on geometry of vessel, many factors for impeller can be decided.

 

Agitator Agitator Speed Power Number Pumping Factor Impeller Diameter
Marine Propeller 100 – 300 RPM * 0.3 for 3 bladed impeller,
* 0.33 for 4 bladed impeller
* 0.33 for 3 bladed,
* 0.34 for 4 bladed.
1/3rd of tank dia
Axial Flow Turbine 100 – 300 RPM * 1.35 for 3 bladed,
* 1.4 for 4 bladed,
* 1.45 for 5 bladed,
* 1.6 for 6 bladed.
* 0.6 for 3 bladed,
* 0.69 for 4 bladed,
* 0.78 for 5 bladed,
* 0.87 for 6 bladed.
1/3rd of tank dia
Turbo Propeller 100 – 300 RPM * 1.35 for 3 bladed,
* 1.4 for 4 bladed,
* 1.45 for 5 bladed,
* 1.6 for 6 bladed.
* 0.6 for 3 bladed,
* 0.69 for 4 bladed,
* 0.78 for 5 bladed,
* 0.87 for 6 bladed.
1/3rd of tank dia
Disc Blade Turbine 100 – 300 RPM 5.0 0.7 – 0.8 1/3rd of tank dia
Flat Blade Turbine 100 – 300 RPM 5 0.7 – 0.85 1/3rd of tank dia
Backward Blade Turbine 100 – 300 RPM 5 0.65 – 0.85 1/3rd of tank dia
Paddle 50 RPM 5 0.03 80% of tank dia
Anchor 50 RPM 5 0.07 80% of tank dia
Gate 50 RPM 5 1 80% of tank dia