Agitation. Mixing - Mechanical aspects

Agitators are at the heart of many manufacturing processes. Without them, few chemical reactions or mixtures would proceed spontaneously. Agitators can be subjected to considerable stress (radial force, axial force, tank pressure, vibratory phenomena, etc.). It is generally part of a heavy investment with a relatively long payback period.

For these reasons, it makes sense to take an interest in this equipment, designed to "last" (some machines have been in continuous operation for over forty years) with minimal maintenance and reduced downtime.

The decision-maker will generally select an agitator based on an analysis of its ability to perform the required operation, and on a study of its mechanical characteristics.

As the design and sizing of a mixer is not standardized (as is the case for tanks), and draws on a variety of sciences, the decision-maker will have some difficulty in distinguishing the important parameters, if he wishes to make a comparison.

In this article, we will describe general methods to help the decision-maker in the difficult task of choosing an agitation system. While the major components of an agitation system are well known (motor, gearbox, seal), the parts built by the agitation designer (turret, shafts, mobiles) are left to the imagination of the mechanical engineer, and can be difficult to analyze. For this reason, we'll go into sufficient detail on the mechanical determination part to enable the decision-maker to select and "check" the mechanical characteristics of the device.

The mechanical design of agitators involves the construction of rotating machines: torque transmission, determination of rotating shafts, evaluation of torsional and bending stresses, and vibration analysis of the system. However, while these phenomena are well "mastered" by mechanical engineers, the task is more difficult than it seems, as the "manifestation" of the forces to be taken into account (in direction, intensity, point of application) is much more random.

It is also difficult for users to select an agitator from a range of proposals, whose characteristics and components have nothing in common. We will therefore look at the different types of agitator assembly, although the manufacturing techniques are not described, as they conform to the traditional mechanical trades (machining, welding, assembly...).

The complete study of the subject includes the articles :

• J 3800 – Agitation. Mixing: basic theoretical concepts ;

• J 3802 – Agitation. Mixing ;

• Doc. J 3803 – Agitation. Mixing ;

• J 3804...