AGITATORS-WHAT THEY ARE AND HOW THEY CAN BE MADE BETTER

An agitator is a shaft mounted with impellers which is inserted into a tank. On rotating the shaft using a geared motor or a gearbox, the fluids inside the tank undergo a mixing process. The profile of the blades, the number of blades, the mounting of the shaft and the shaft dimensions are determined by the properties of the fluid to be mixed inside the tank and the batch capacity.

Mixing Tank: It is a closed space of either cylindrical or cuboidal shape with opening ports for entry of fluid to be mixed. Mixing tanks are identified by their batch capacity, noted by V.

Impeller: It is a blade with an aerodynamic profile used for creating turbulence on rotation. This is responsible for mixing of the fluid. The impeller is also referred to as the agitator.

Agitator Shaft: The shaft holding the agitator during operation is termed as the agitator shaft. It is also the output shaft of the gearbox.

Gearbox: It is an arrangement of gears meshed in pairs which used reduces the rpm of the geared motor, increasing torque. The gearbox is connected to a geared motor with a constant input rpm.

Baffles: Baffles are restrictions within the fluid tank. These restrictions increase the turbulence of the mixing process, reducing load on the agitator.

Based on the position of the agitator mounting with respect to the mixing tank, there are three types of agitators:

Top entry agitators: The above diagram shows a top entry agitator. Here the shaft enters the mixing tank from the upper direction.

Bottom entry agitators: opposite to the top entry agitator, the bottom entry agitator enters the mixing tank from the bottom.

Side entry agitators: The shaft enters from the lateral direction of the mixing tank.

SIDE ENTRY AGITATOR

The excessive fluid pressure in an industrial mixer causes wobbling of the impeller shaft which leads to its failure. It also causes damage to the gearbox to which it is connected, leading to frequent need of maintenance. The unwanted effect generates a lot of stress on the mounting of the gearbox. In worse cases, the brackets also fail leading to gearbox dislocation within the mixing chamber. The other factors affecting the longevity of agitator gearboxes are the stiffness experienced by the support bearings and housing bearings and the improper meshing of gear teeth, creating an unbalanced load distribution on the gears. Such factors are to be taken into account while designing a gearbox specifically required for driving the impeller loaded agitator shafts in the industrial mixers.

A number of points have been found out to be given significantly less preference during the design, analysis and optimization of the gearbox components as well the whole assembly.

[1]  Consideration of suitable kinematic arrangement of geartrain. This includes the position of intermediate shafts and their distances from each other and the location of support bearing bearings and gears mounted on the shafts.

[2]  Stress concentration methods have not been fully implemented for the output shaft of the gearbox which holds the impellers. Undercuts and notches have not been considered.

[3]  The orientation of gearbox mounting affects the stress and the weight subjected to the mount. The type of gearbox mounting position and the subsequent stress on the brackets/mounts must be taken into account.

[4]  During static as well as dynamic loading, the housing undergoes forces which may end up leading to cracks in it. Generally the design of the housing is performed based on the static loading distribution. However, when the gearbox is in operation, the dynamic forces are also in effect. Hence they must also be factored into while designing the casing of the gearbox.

[5]  For cylindrical mixers, baffles are used in order to add restrictions to the fluid flow path, thereby creating more turbulence and hence better mixing. Hence the load on the agitator is decreased by a small percentage. This must be subtracted from the net power to be transmitted by the agitator shaft.

On a concluding note, agitators are a vast topic and deal with numerous laws of mixing and agitation. This article serves as a briefing of the mechanical component used in the industry.