Laboratory stirrer
Laboratory stirrers consist of a stirrer drive with a clamping device for the stirrer shaft (similar to a drill) and a stirrer shaft with a suitable stirring blade . They are used to carry out basic procedural operations such as
- Dissolving (of soluble substances in liquid),
- Homogenization (of mutually soluble liquids),
- Suspending (of insoluble solids in liquid),
- Gassing (gassing stirrer)
and to improve the heat transfer to heat exchange surfaces (e.g. in the laboratory reactor temperature controlled by the jacket or in the vessel on a hot plate ).
The speed of the agitator drive is usually continuously adjustable, e.g. B. in the range from 50 to 2000 / min. The speed of modern laboratory stirrers is electronically regulated.
Stirrer drives that have a torque measurement are referred to as measuring stirrers or torque measuring stirrers . With the help of the torque measurement, the torque transmitted from the stirring blade to the shaft by the internal friction of the liquid (depending on the viscosity , the elastic properties, the speed and the stirrer geometry) can be recorded. In many experiments, the torque is an easy-to-obtain and meaningful measurement signal.
Examples:
- Polymerization: e.g. B. Recognition of the "start" of the reaction, qualitative tracking of the course
- Crystallization: Recognition of the onset of crystal formation and conclusions about the crystal concentration
- Reaction calorimetry : Determination of the amount of heat introduced by stirring, dissipation
The choice of a suitable stirring blade depends on the respective application:
| Type | Preferred application |
|---|---|
| Propeller stirrer | Fluidizing solids |
| Anchor stirrer | Opt. Heat transfer |
| Disk stirrer | Dispersing |
| MIG stirrer | Optimal mixing |
| Toothed disk stirrer | Crushing v. Particle |
| Spiral stirrer | Highly viscous liquids |
Since the torque depends on the viscosity, conclusions can be drawn about the viscosity from the measured torque. However, this is not trivial, as other factors such as stirrer geometry, speed, level and temperature must be taken into account.
In addition to the actual stirring task, measuring stirrers are used for scale-up tests. Here, design data for a production reactor are obtained in a scaled-down model reactor .
Modern laboratory stirrers offer the option of limiting the torque (e.g. to protect a glass stirring blade) and defining the restart behavior after a power failure.
literature
- aprentas (ed.): Laborpraxis Volume 1: Introduction, General Methods . 6th edition. Springer International Publishing, Cham 2017, ISBN 978-3-0348-0965-8 , Chapter 8: Grinding, mixing, stirring , doi : 10.1007 / 978-3-0348-0966-5_8 .