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Drives replace dampers to achieve immediate energy savings

01 June 2009

Two modular ac drives from Control Techniques have not only produced immediate energy-savings on a previously mechanically damped fan application, but also paid for themselves in under a year

Civil and Marine’s Middlesbrough plant imports blast furnace slag from the nearby Corus steelworks and grinds it into a fine powder to produce ground granulated blast-furnace slag (GGBS), a very useful by-product that is used to enhance the properties of concrete, replacing between 20 and 80% of the cement content. Concrete containing GGBS is less permeable and chemically more stable than conventional concrete, and offers superior resistance to sulphate attack and cracking caused by alkali-silica reaction.
However, GGBS is a highly abrasive material and excessive wear and tear on dampers prompted engineers at the site to find an alternative means of controlling the airflow that extracts the product from the ball mill and Sepol separator. Site electrical engineer and assistant works manager, Rob Thwaites takes up the story.

“As neither fan operated at full speed, we also saw an opportunity for energy-savings. We brought in Control Techniques who calculated likely savings and predicted that the drive for the Sepol separator would have a payback of just seven months, whilst the ball mill would pay for itself in nine. The calculations have proved to be very accurate and we have made precisely the savings that Control Techniques predicted.”

The Sepol separator is driven by a 200kW motor running at between 71 and 75% of full speed. With the damper in place, its power consumption was 146kWh. Now, under inverter control, this has been reduced to just 61kWh, resulting ing savings of 85kWh. Meanwhile, the roll mill extract fan is driven by a 160kW motor, and with the damper in place, the air flow was restricted to 50%, resulting in a power consumption of 105kWh. With the drive, this has now been reduced to 45kWh - a saving 60kWh on this machine. The total savings of 145kWhr, over a 100-hour production period per week, are around £700 – and that is now achieved every week!

The 200kW and 132kW Unidrive SPM drives were retrofitted into existing cubicles by Rob Thwaites’ own team. Thanks to their modular design, the power circuits were easily fitted into non-standard enclosures, and the drives integrated into the existing PLC-based control system using the software tools used for both commissioning and programming.

Unidrive SPM is a versatile platform, configurable into open- and closed-loop modes, paralleled and load sharing, with active input and regeneration, in multi-pulse configuration (12, 18, 24 etc.) or fitted on a common bus for energy sharing.

Apart from these welcome energy savings, Mr Thwaites says that a further benefit his team hadn’t anticipated was a reduction in noise, which has made things a lot more comfortable for the operators. “We also anticipate that fan motors and bearings will last longer and require less maintenance,” he adds.

Additional Unidrive SP units have been installed on two 450kW pre-grinders, with four drives on each, working in load-sharing mode. With the drives in place, this section (which takes a mixture of the coarse pelletised and granulated GGBS) has seen an overall increase in throughput from 50 to 70 tonnes per hour - further enhancing the energy efficiency of the plant by reducing the energy consumed per ton of product processed.

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