Stirrer
Stirrers (from Middle High German rüeren, "stir, stir") are the tools of a mixer, stirrer, stirrer kettle or agitator. They are usually interchangeable. Depending on the design and size, the stirrer and shaft are exchanged or the stirrer is mounted on the shaft. Stirrers are - together with baffles - standardized in DIN 28,131th
In different branches ( laboratory , chemical industry and process engineering , construction industry , do-it-yourself , kitchen etc.) and for different stirring tasks / purposes different designs and names are common.
The entire stirrer is often referred to as a stirrer for short. An example of this is the magnetic stirrer, which is widely used in laboratories, or the hand mixer ("mixer") as a kitchen appliance.
Stirrer types
Rotating stirrer
There are numerous types of stirrers made of glass , plastic ( PP , PTFE ) and metal , which are better or worse suited for certain media or tasks. A basic distinction is made between stirrers that convey axially (usually downwards) or radially (to the side).
To the co-rotation of the (low-viscosity) liquid and thereby the formation of thrombi for preventing and higher turbulence to produce, are baffle (or flow distorter, engl. Baffles ) was used. In the case of liquids with a higher viscosity, baffles are not required or they even worsen the stirring result.
Rotating mechanical stirrers can be divided into the following groups according to their stirring speed:
- slow-running stirrers, here further according to the number of their axes
- uniaxial (blade stirrer, bar stirrer, perforated bar stirrer, cross bar stirrer, anchor stirrer, lattice stirrer, screw stirrer, paddle roller)
- multi-axis (planetary stirrer, rotary mixer)
- high-speed stirrer (propeller stirrer, turbine stirrer, disc stirrer, impeller stirrer)
Stirring systems with other active movements
In addition to the stirring systems with rotating stirrers, there are a number of other technologies for mixing media with one another. The systems with non-rotational active movements include, for example
- Vibromixer: A (perforated) disc is moved up and down in a translatory and oscillating manner in a cylindrical vessel .
- Bag mixers: These are mixing systems in which the entire vessel (in this case sterile plastic bags) is fixed on a vibrating or pendulum or Cartesian moving platform
Stirrer forms
A distinction is made between the terms of process engineering and technical chemistry :
- Propeller stirrer : mostly 3 blades , shaped like a ship's propeller , axially conveying, for suspending , homogenizing , heat transfer
- Inclined blade stirrer : inclined, mostly right-angled blades, like propeller stirrers , axially conveying with a radial part, for suspending, homogenising, heat exchange
- Disc stirrer : Circular disc with mostly 6 vertically positioned blades arranged outwards from the shaft, also known as "Rushton turbine", radially conveying, for emulsifying , gassing , generating strong shear forces
- Swashplate stirrer : circular discs, arranged at an angle on a usually vertical shaft, for liquids with foreign bodies and for emulsifying , especially in the kitchen
- Hollow blade stirrer : Circular disc with mostly 6 vertically arranged, semicircular curved blades arranged outwards from the shaft, radially conveying, particularly suitable for gassing , generates strong shear forces
- Impeller stirrer : usually three stirrer blades attached close to the bottom, convexly curved in the direction of rotation, in most cases enamelled and made from a flattened piece of pipe, for suspension, homogenization, heat exchange
- Crossbeam : pitched with mostly four leaves, repeatedly placed one above the other, especially for large stirred tanks, for fermenter
- Anchor stirrer : guided close to the wall for heat exchange, especially viscous media
- Blade mixer : vertical stirring blade, frequently in laboratory scale used
- Gate stirrer : frame structure or paddle stirrer with recesses
- Gas inlet stirrer with holes in the shaft and blade, as well as ground and polished shaft and coupling pin
- Stirrer with interchangeable blades with cut or pointed ends
- Stirrer with movable glass or PTFE blades
- Helical stirrer : close to the wall for viscous media: homogenization and heat exchange
- Toothed disc stirrer : disc with serrated edge; for low circulation but extremely high shear
- Residual quantity stirrer: Floor-mounted stirrer for stirring even small quantities in reactors, high-speed for emulsifying and gassing
Agitator selection (process engineering)
The choice of stirrer depends on the stirring task, the viscosity of the medium, the shear strength (whether the medium has to be spared) or, vice versa, on the stirrer power available or required for the process.
A distinction is made between the following essential stirring tasks, which in practice have to be carried out individually, simultaneously or in a chronological sequence in a stirred tank:
- Homogenization: The equalization of differences in concentration of different, mixable media
- Dispersing liquid / liquid: The stirring of insoluble media into another fluid
- Dispersing liquid / gaseous: The stirring of a gas phase into a liquid phase, e.g. B. in hydrogenation
- Heat transfer, d. H. the targeted introduction of large amounts of heat into a fluid
- Suspending : The fluidizing and mixing of solids in a liquid phase
- Emulsification : The stirring of one liquid phase into a second liquid that is not soluble in one another
For each of these stirring tasks, certain types of stirrers are better or worse suited. As in practice, as described above, stirring tasks rarely occur in isolation, a compromise must often be made between the "optimal" stirring element for the most important process step and the general suitability for process steps that also occur but are less relevant for the yield and quality of the product . In some cases, several agitators that can be operated independently of one another are installed in one agitator tank in order to operate the best agitator for several process steps.
Multi-stage stirrers are often used in practice, as shown in the illustration on the right. While the lower level realizes a high power input with high shear and a residual quantity stirring function, the upper stirring element is suitable as an axial conveyor to achieve a good homogenization of the container contents.
Installation positions of agitators
Different installation positions can be distinguished for an agitator, depending on the nature of the agitator and the container construction :
- centric installation
- from above
- from underneath
- eccentric installation
- from above
- from underneath
- side installation
With the centric installation of agitators, the use of baffles is necessary when agitating low viscosity media in order to suppress the entrainment of the liquid and thus improve the mixing characteristics (see above). With eccentric installation this can be dispensed with, depending on the container construction and the fluid properties. In practice, however, very high transverse forces occur on the stirrer shaft, so that it usually has to be made stronger than with centric installation. Particularly short stirrer shafts can be realized if stirrers are built into the tank from below. However, special attention must then be paid to the choice and design of the mechanical seal or the shaft seal.
Types of drive for agitators
The drive types direct drive or indirect drive are basically possible for an agitator.
- direct drive
The drive takes place via a shaft with a corresponding sealing of the shaft, if this has to be inserted into a container. The shaft can be coupled directly or indirectly (via a belt) to a motor with or without a gearbox. The motor can in principle be designed with or without a frequency converter. Directly driven agitators can be used for all agitating tasks. This includes in particular the range of liquids with high densities and / or viscosities .
- indirect drive
The drive takes place via a magnetic coupling with a magnet each in the agitator located on the inside of the container and the drive located on the outside of the container. This type of drive is limited in its drive force by the fluid properties, in particular density and viscosity . A coupling torque higher than approx. 400 kN can only be achieved with great effort.
Laboratory names
In the laboratory are u. a. the following terms are common:
- Magnetic stirrer : stirring rods, "stirring fish" in rod, dumbbell, triangular, egg or cross shape
- Propeller stirrer (see above)
- Stirrer with interchangeable blades (cut off or pointed ends)
- Turbine stirrer (different designs from inclined blade stirrer to disc stirrer)
- Jet stream mixer (maximum speed targeted on the ground)
- Centrifugal stirrer (blade stirrer with blades which can be folded in towards the shaft and which strive outwards due to centrifugal force . Allow the introduction into narrow openings of pistons)
- Anchor stirrer (see above)
- Half-moon stirrer: blade stirrer close to the floor for round-bottom flasks (also in foldable version)
- Fan stirrer (like grid stirrer)
- Dissolver (like toothed disc stirrer)
- Agitator coil (spring spiral instead of leaf, radially conveying like disc agitator)
- Visco Jet (2 or 3 tangentially arranged , "bottomless" cups functioning as nozzles )
- ruvastar (3 tangentially arranged deflection bends)
Household stirrer
In the kitchen, the following are common for so-called mixers:
construction industry
- Propeller stirrer (especially for mixing colors)
- Spiral agitator
- Circular stirrer, a simplified spiral stirrer for lower resistance e.g. B. as
- Mortar stirrer