Description

Agitators are devices that are used to stir or mix fluids, especially liquids, which is one of the basic mechanical process engineering operations. Essentially, agitators are used for the homogenization of liquids or liquid-solid mixtures by generating horizontal and vertical flows. These flows are generated by rotating agitator blades.

The Agitator Process
 A rotating agitator generates high speed streams of liquid which in turn entrain stagnant or slower moving regions of liquid resulting in uniform mixing by momentum transfer. When it is necessary to mix materials of different densities, it is better to use vertical flow agitators because of the tendency of sedimentation of the denser component. Wherever possible, movement of the entire amount of material together in the mixing vessel, which occurs frequently during the agitation process, should be avoided because of the unwanted separation caused by centrifugal forces.
 
 
Agitator Parts
 At the simplest level, agitators consist of an impeller and a shaft. An impeller is a rotor inside a tube or conduit attached to the shaft. It is used to increase the pressure and flow of a fluid. Modern agitators use sophisticated process control electronic devices to regulate the mixing process.

Agitators are typically used in 

• Municipal Waste Water Plant
• Industrial Waste Water Treatment Plant
• Chemical Process Industry
• Pharmaceutical Industry
• Raw Water and Drinking water process plant
• Power Plant
• Food Industry
• Paint Industry 

Principles of Process Agitators

The Science behind Process Agitators
 
Technically speaking, for a liquid-liquid mixed process, shear rate is the greatest consideration during agitator selection and design. The shear rate decreases exponentially with distance from the agitator. Thus the shear stresses and strain rates vary greatly throughout an agitated liquid in a tank. As the viscosity of the fluids to be mixed increases, the physics fluid mechanics change from that of turbulent flow (like in liquid agitators) to that in which viscous drag forces dominate. Additionally, some fluids exhibit Non-Newtonian behavior – their viscosity cannot be designated by a single coefficient. Mixing of such fluids requires special heavy duty agitators. As the dynamic viscosity of a Newtonian liquid is independent of shear at a given temperature, its viscosity will be the same at all points in the tank. In contrast the apparent viscosity of a non-Newtonian liquid varies throughout the tank. This in turn significantly influences the mixing process. For shear thinning liquids, the apparent viscosity is at a minimum in the immediate vicinity of the agitator. The progressive increase in the apparent viscosity of a shear thinning liquid with distance away from the agitator tends to dampen eddy currents in the mixing tank. In contrast, for shear thickening liquids, the apparent viscosity is at a maximum in the immediate vicinity of the agitator. In general shear thinning and shear thickening liquids should be mixed using high and low speed agitators respectively.
 
 
Fluid Movement in Process Agitators
 
Wherever possible, the movement of the entire contents of the agitator vessel should be avoided as rapid movement tends to segregate the components due to centrifugal forces.
 
The mixing time is short if the components to be mixed undergo a large number of changes of location. This can take the form of movement of the agitator itself or of material flows generated by the agitator. They can be achieved by impact, flow around obstacles, crossing directions of flow and speed differentials at the interfaces of parallel flows.
 
 
Process Agitators – Design
 
The design and the operation of the agitator are crucial for efficient operation. However, there is no strict science that governs the best design. Rather, the designing of good process agitators is more a matter of experience. A core knowledge of fluid dynamics is must for good agitator design.

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
 
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 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.
 
 
Turbine Agitators
 
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.
 
 
Helical Agitators
 
hese 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.