When manufacturing chipboards, certain dimensions have to be upheld regarding the thickness of the chips in the individual layers, to ensure the product quality and avoid rejected goods.

Chipboards consist of a middle layer and two covering layers which surround the former. For the chips used for the middle layer the classification limit is 1.2 mm, for the chips used for the covering layers it is 0.35 mm. In order to be able to comply with these values, classifiers are used.

In the classifier the chips are pneumatically separated. The material is distributed equally over a sieve surface by rabble arms, through which a flow of air enters into the suspension chamber from below. Through corresponding air speeds any division required and thus the classification limit of the chips can be attained. The share capable of being suspended is carried away with the flow of air, the coarse particles not capable of being suspended, including sand and foreign bodies are moved by the rabble arms to the side of the classifier and are ejected by means of cellular wheel sluices.

This classification is however subject to certain fluctuations. This occurs for example, if the system is started and stopped or if the material is not equally fed to the system. In order to compensate for these fluctuations, and to maintain a constant classification limit, the airflow is controlled resulting in an equal air speed in the classifier.

At Schenkmann & Piel Verfahrenstechnik in Leverkusen, specialist for supplying various plants and equipment for the wood working industry, Vortex transducers Type VA made by Höntzsch in Waiblingen are being installed in the suspension classifiers. They measure the flow speed of the air. If the speed is too high or too low, the air is accordingly regulated by a bypass valve.

The measuring method is based on an obstacle or rod in a flow triggering a vortex, whereby the vortex separation frequency represents a measurement unit for the flow speed. The vortex separation frequency is independent of te density, pressure and temperature of the medium being measured when operated in the framework of the relevant specifications.

In the suspension classifiers from Schenkmann & Piel, the Vortex transducers are installed in the middle section of the classifier between the upper and lower sieve floor and connected to a measuring transducer type U2a and to a power pack. The output of the measuring transducer is sent to the PLC which controls the motor of the by pass valve. As it is a tendency measurement, the valve should be in the middle setting at the start so that both a higher and lower airflow can be regulated.

In the Höntzsch Vortex transducers, the flow vortexes are scanned by ultrasound which enables them to also measure lower flow speeds in comparison with other scanning methods. Additionally, larger measurement ranges can be covered.