Basics of Industrial Agitators or Mixers

Agitation is a means mixing of phases can be accomplished and by which mass and heat transfer can be enhanced between phases or with external surfaces. the process of mixing is concerned with combinations of phases.

1. Gases with gases.
2. Gases into liquids such as dispersion.
3. Gases with Granular solids such as fluidization, pneumatic conveying, drying.
4. Liquids into gases such as spraying and atomization.
5. Liquids with liquids such as dissolution, emulsification, dispersion.
6. Liquids with granular solids such as suspension.
7. Pastes with each other and with solids. 8. Solids with solids such as mixing of powders.

Hydraulically, mixers behave like pumps. Mixing applications can be either a batch or a continuous process. Although the terms agitation and mixing are often used interchangeably, there is a technical difference between the two.

Agitation creates a flow or turbulence as follows:

1. Mild agitation performs a blending action.
2. Medium agitation involves a turbulence that may some gas absorption.
3. Violent agitation creates emulsification.

Mechanical mixers are used as follows:

1. To mix two or more no homogeneous materials.
2. To maintain a mixture of materials that would separate if not agitated.
3. To increase the rate of heat transfer between materials.

Mixers are Designed to achieve one of the following.

• Blending: combines miscible materials to form a homogeneous Mixture.
• Dissolving: the dissipation of a solid into a liquid.
• Dispersion: the mixing of two or more non-miscible materials.
• Solid Suspension: suspends insoluble solids within a liquid.
• Heat Exchange: promotes heat transfer through forced convection.
• Extraction: separation of a component through solvent Extraction.

Agitator or Mechanical Mixer usually consists of a shaft-mounted Impeller connected to a drive unit.

General Arrangement of Industrial Agitators or Mixers:

Typical arrangement of agitator is shown in above has consist of Dished Vessel, Stirrer, Cooling Coils, Baffle plate, Foam breaker, Seal, Bearing assembly, Gear Box, Motor Drive.

Selection of an efficient agitation depends on Nature of Liquids, Operating Condition, intensity of circulation and shear. Factor to be considered are Type of agitator, Circulation pattern, Location of agitator, Shape and size of the tank, Diameter and Width of agitator, Method of baffling, Power required for Agitation, Shaft Over hang, Types of Stuffing box,, Seal, Bearing, Drive System etc.

Terminology Used in Industrial Agitator or Mixer:

• Agitator: The assembly consisting of impeller, impeller shaft and drive including other parts such as gland, and bearing used in conjunction with the above.
• Impeller: The actual element which imparts movement to the charge (fluid).
• Propeller: A high speed impeller which essentially imparts axial thrust to the fluid.
• Turbine: An impeller with essentially constant blade angle with respect to a vertical plane, over its entire length or over finite sections, having blades either vertical or set at an angle less than 90” with the vertical.
• Paddle: – An impeller with four or fewer blades, horizontal or vertical, and essentially having a high impeller to vessel diameter ratio.
• Anchor: Basically a paddle type impeller which is profiled to sweep the wall of the containing vessel with a small clearance.
• Baffle: An element fixed inside the vessel to impede swirl.
• Draught Tube: A tubular fitting which is arranged to direct the liquid flow produced by the impeller.
• Filling Ratio: The ratio of liquid depth in the Vessel to Vessel Diameter.
• Swirling: The Continuous Rotation of liquid about a Fixed Axis.
• Vortex: A depression in the surface of a liquid produced by swirling.
• Fully Baffled Condition: A condition when any further increase in baffling causes no significant increase in power consumption, this may be considered as a state where the liquid swirl in the vessel has become negligibly small and when all the power input to the impeller expended to create turbulence.

General Guideline of Mixers or Agitators

1. All mixers/agitators rotate clockwise.
2. 2. In general, agitators are sized on the basis of the required torque per unit volume. Other factors that affect size and torque are:

• Viscosity > 100 cP (viscosity can affect blend times).
• Critical speeds.
• Tip speed.
• Impeller diameter.
• The required degree of agitation

3. Each shaft is designed for mechanical loads and critical shaft speed. Motor size and shaft design are related. A larger shaft to take the torque will require more horsepower to eliminate wobble.

4. To prevent solid buildup on the bottom, a radial-blade impeller may be used. If elected, then place the blade one blade width off the bottom.

5. Power consumption:

• Operating speed is back-calculated to ensure delivery of the proper power for a given impeller diameter.
• The speed and horsepower define the torque required for the system. The torque in turn sets
• the shaft size and gear box size.
• Impeller power consumption determines the horsepower and impeller diameter required for a given mixing process.

6. Mixing parameters:

• Shaft angle.
• Time.
• Impeller type and diameter.
• RPM (pumping capacity).
• Power.
• Viscosity, specific gravity.

7. A steady rest bearing may be utilized at the bottom of the tank if the mixing application allows.

8. Other applicable data:

• Types of seals or packing.
• Metallurgy.
• Drain location.
• Man way size.
• Indoor/outdoor.
• Mixed agitator run times.
• Head room required above tank.

This all above are the basics of Industrial Agitator or Mixer.

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