Waiblingen, December 16,1997 --- Due to economical and ecological reasons, the process parameters must be constantly monitored and accurately coordinated when producing cement.

At its location in Allmendingen, E. Schwenk KG produces a number of high quality types of cement. The sophisticated technology used and the high level of automation make a major contribution to achieving this exacting standard. Thus for example, the quantity of intake air to the furnace which has a decisive influence on the qualities of the final product is automatically controlled. Vortex VA flow sensors from HÖNTZSCH are used to measure the intake air volume flow rates.

The cement manufacturing process starts with the surface mining of lime stone and marl in a nearby quarry. The rock is taken to the mills with loading shovels and heavy goods vehicles where it is crushed to fist size. Next, supported by a computer, the basic substances, limestone, marl, sand and iron oxide are fed to the milling/drying plants. The material mixture is ground and simultaneously dried by exploiting the waste heat from the rotary furnace.

The raw meals produced in this way are chemically converted into various cement clinkers in a rotary furnace at a temperature of approximately 1,450 °C. This process runs continuously. The raw mixture is constantly added and transported downwards by the rotation of the 120 m long rotary furnace which is constructed on a slight slant. Once it has been cooled down, the mixture then leaves the furnace again as a granular cement clinker. Following this, the clinker is milled with limestone meal, flue dust and gypsum. In this way approximately 4,000 tonnes cement are produced per day at the Allmendingen plant.

The rotary furnace is heated by a burner lance which protrudes 20 m into the furnace. At the peak of the lance, with the help of a strong axial air flow coal dust is blown in and ignited. In order to attain a good heat distribution, the coal dust is swirled around by a radial flow of air. The ratio between axial and radial air flow is approximately 3:1. An optimal product quality can only be guaranteed when the ratio of both air flows and coal dust are perfectly matched to each other.

A further important aspect is the high safety requirements. For example, if combustion is not optimal, explosive carbon monoxide could form. As a result there is a risk that the gas in the electric filters which clean the furnace exhaust with a voltage of approximately

70 kV, could ignite due to the formation of sparks. For this reason, in addition to the furnace intake air, the CO concentration is also measured at various points.

As the continuous operation of the rotary furnace is essential, a robust, reliable and accurate volume flow measurement system had to be installed for measuring the furnace intake air flows. Schwenk therefore chose two Vortex flow sensors type VA40/20 which have been installed in the pipelines of the axial and radial air intake. The major advantage of the Vortex flow sensor VA is its high reliability, as well as its resistance to dust loads and other impurities.

Particularly worth mentioning is the self-cleaning effect of the sensor which can be explained by the formation of vortices and the pressure fluctuations in the sensor head connected with this.

The evaluation of the frequency signals of the two VA vortex measurement recorders is attended to by two U2a transducers. There the sensor information is converted into flow proportional current signals which are visualised on the main operation centre and monitored to ensure there is no drop below a volume flow alarm value.

The vortex measurement principle is based on a Karman vortex street being formed in the vortex sensor head on a vortex shedding element e.g. a triangular fin. The vortices modulate an ultrasound field located downstream after the fin which is generated by a piezo oscillator. The modulated ultrasound signal is recorded by another piezo ceramic acting as a microphone. As there is a constant ratio between vortex shedding frequency and flow speed over a large speed range, the flow speed can be determined from the frequency signal.

The major advantage of ultrasound scanning of the vortex street compared with other scanning processes which for example measure the dynamic effect on the disturbing body, is that very small flow speeds as low as 0.5 m/s can be recorded.

As the sensor has to be intrinsically safe for the application described, the frequency signal proportional to the flow speed is converted into a DC decoupled signal by an LDX Ex transducer. Then in a U2a transducer this signal is transformed into a flow proportional analogue signal which is available as a measurement parameter to control and monitor the plant.