Variable speed drive improves cement plant process control

With an annual production capacity of approximately one million tonnes, Jura Cement is Switzerland’s second largest cement producer and is part of Jura-Holding. Jura Cement’s plants in Wildegg and Cornaux produce high-quality cement for different fields of application.

Recently, Jura Cement replaced a 25-year-old cascade converter with a variable speed drive from ABB. The ACS 2000, rated at 550 kW, controls the clinker cooler exhaust fan of Jura Cement’s plant in Wildegg, Switzerland.

The 630 kW cascade drive was controlling the speed of a slip-ring induction motor on a clinker cooler exhaust fan at Jura Cement’s Wildegg plant in Switzerland. The clinker cooler exhaust fan plays a critical role in the cement-making process. Raw mix enters the kiln and is heated to 1450°C where it is transformed to clinker. Upon exiting the kiln, the clinker is cooled to 100°C by the clinker cooler fans which blow cooling air through the clinker. Accurate energy input is critical to the quality of cement - insufficient heat will result in poorly burned, low-quality clinker, whereas excess heat might damage the kiln shell.

By controlling the clinker cooler exhaust fan, the drive keeps the pressure in the kiln hood constant. It does this by regulating the fan speed that draws cold air through the clinker cooler in relation to the hot air that is being drawn through the kiln.

The cascade drive that controlled the clinker cooler exhaust fan before was built in 1984 by BBC. One of the reasons for changing to a medium-voltage drive was to give a broader speed range from 0 to 1000 rpm. The cascade drive was not operated below 300 rpm, as the cascade’s efficiency dropped considerably at lower speed and the limited speed range restricted some production phases, leading to high energy wastage.

Because of its age, the cascade drive was also proving expensive to operate and maintain, as the carbon brushes needed replacing on the machines every three months. Furthermore, because the technology employed within the drive included discrete components, they were more susceptible to breakdown and spare parts were becoming scarce.

As cement making is a continuous process, the drive needs to operate 24 hours a day and only be taken offline at planned maintenance intervals. With each kiln stop costing several thousand Swiss Francs, maximising the uptime of the process was paramount.

Also, due to the difference in the new technology to the old, there is a great reduction in maintenance time and costs. “The kiln had to be stopped up to 30 times per year. Some stoppages were the result of brush changes for the slip ring motors. Based on experience of the ACS 2000’s performance so far, Jura Cement expects that there will be no kiln stops because of the drive’s low maintenance requirements,” says Hitz.

The design features easily replaceable phase modules which allow the end-user to rapidly replace a module within minutes. This means that, in the event that a phase module needs to be exchanged, the kiln does not have to be stopped, as the drive can be put back into operation quickly.

Jura Cement and ABB are also planning to monitor the energy consumption and compare this against that consumed by the cascade drive. Already Hitz is predicting substantial reduction in energy consumption of about 10 to 20 percent. The ACS 2000 also offers a wider speed range of 0 to 100 percent – compared to the 30 to 100 percent speed range of the cascade drive - resulting in much better process control.

“The wider range of speed control gives us a higher overall efficiency as we have now greater control over the entire process,” says Hitz.