Processes, in which several fluid components are to be combined with one another or with solid or gaseous components, play an important role in the chemical, pharmaceutical and food industries and in base and water preparations. The final purpose can be the creation of a homogenous system, acceleration of material or heat transfer or a chemical reaction.

The necessary energy supply is effected mostly through the stirrer rotating in the fluid in containers.

With these arrangements, it is possible to conduct fluid-fluid, fluid-solid and fluid-gaseous reactions, as well as the following basic operations, the classification being accordingly based on the aggregate states of the constituent components:

Fluid + fluid mixture, emulsification, extraction

Fluid + solid suspension, dissolution, extraction, floatation, crystallization, solubility push

Fluid + gaseous absorption, expulsion, frothing, concentration

Heat transfer to the container wall or between the phases is also possible.

Individual agitation mechanisms can be mounted, such as clamp-on or tripod stirrers, with easy-release coupling with containers, or as attached stirrers, mostly with pressure-proof connection.

The widely varying process conditions that can be implemented in stirring containers require a variation in the main characteristics, dimensions and models as listed below:

Volume of the stirring container, ratio of interior height to internal diameter, floor shape, lid type, arrangement and dimensions of the sockets, material, lining, wall thickness, components (e.g. current refractor, heat transfer surfaces)

Stirring mechanism, agitator shape, dimensions and speed, input drive power and the drive motor, transmission, shaft, clutch and bearings; fixed speed input drive, with speed changeable at rest or infinitely variable in operation

The individual parameters have a large variation range. The requisite

holding capacities vary between 10 l for laboratory versions and 50 m3

in case of production systems. The specific power characteristic for stirring processes P/Vis between 0.01 and 4 kW/m³.

The design and construction of a stirring machine takes place in the eight stages described below:

1. Analysis of the operations to be carried out in the stirring container and the process conditions, such as pressure, temperature and aggressiveness of the medium; definition of the material values required for the calculation and the characteristics demanded by the operator.

2. Selection of the container size and shape, selection of the material for container, stirrer and components; definition of a suitable stirrer shape and the assembly conditions

3. Execution of the process technical calculations for all operations running in the stirring container, i.e. determination of the specific power P/V needed for executing the operations. If it is not possible to make any conclusions from the available documents, model trials must be carried out; for versions without current refractor, the cone depth is to be determined.

4. Calculation of the stirrer speed and stirrer diameter needed for the required specific power.

5. Estimate, whether conditions could occur during the process, which would demand over-dimensioning of the drive and the selection of a drive with fixed speed, with speed adjustable at rest or with speed variable during operation.

6. Defining the suitable drive, continuing from stages 4 and 5, taking into consideration the losses in shaft seal and transmission

7. Construction of the sub-assemblies of the stirring machine and selection of the standardized sub-assemblies.

8. Completion of the stirring machine

The processes running in the stirrer machines can be ascribed predominantly to five basic processes - mixing, emulsifying, gassing, suspending and heat transfer.

The mixing, "the unification of molecular dispersion soluble fluids to one system homogenous up to the molecular range", takes place in the stirring container through the various circulation periods of the individual substance ranges on a macroscopic level, through the turbulent oscillatory movements up to the size of the smallest vortexes and subsequently through molecular diffusion.

In case of emulsification, "the unification of fluids non-miscible with each other, where one is distributed in the other in the form of droplets", the intensity of turbulence and the intensity of the shearing fields determines the size of the droplets and thereby the size of the boundary surface of the phase. Sufficient streaming is also additionally needed to prevent tentering or sedimentation of the inner phase.

Similarly, strength of turbulence determines the bubble size in gassing. In many cases, continuous renewal of the surface of the fluid-gas mix is necessary to prevent any enrichment of a foam layer.

For suspending, "the unification of grainy solids and fluids by distribution within each other in a system, which gets dissociated under the effect of gravity", sufficient stream velocity is required to whirl up the solids from the bottom and keep them in the suspension.

Finally, intensification of the heating process requires streaming past the heating surface.

The execution of all operations therefore presumes a streaming involving the entire contents of the container. The operations mixing, emulsification and gassing require, above all, an intensive turbulence.

The stirrer rotating in the stirring medium meets both conditions, the generation of streaming and turbulence. It works in principle like the impeller of a pump, thus representing a working machine.

The relative motion between the stirrer and the fluid, the existing velocity gradients and the flow around the components create vortices, which move through the streams. The viscosity and mutual influences of the vortices generate smaller vortices. The small vortices are first braked quickly and they convert their kinetic energy into heat energy. This creates a turbulence in the stirring container with local variations in intensity and structure. Maximum values of the intensity occur downstream of the stirrer and the components. In case of high viscosity, the generated vortices are dampened heavily. In the container, then, there is no turbulence.

In summary, the following definition can be stated:

Stirring is a process, in which the movement of one or more stirrers create a stream and turbulence or shearing fields within a fluid or a multi-phase system with a continuous fluid phase, where the mix can be a little to very viscous with a viscosity of approximately 5000 P and can have a Newtonic or a non-Newtonic flow behavior. The purpose of stirring is to achieve the most homogenous unification possible, of several components or phases in as small spatial areas as possible as well as to accelerate the processes of reaction, transfer of heat and material.