lower heat losses and greater reliability. Less heat to dissipate also means that the devices need less material for the heat sinks that keep it cool, a fact that allows engineers to design them in smaller packages.

For models rated at 30 horsepower and greater, Schneider Electric’s engineers have replaced the mechanical pre-charge resistor and contactor for charging the DC bus with an input bridge controlled by a microprocessor that converts incoming AC power to DC. This combination is both smaller and a solid-state package. “By replacing mechanical components with electronic ones, once again designers were able to get a smaller, more reliable drive and eliminate heat-producing connections,” says Spronk.

Modern variable-frequency drives can do more than just reduce the electricity required for running the motors that they oversee. They also can generate huge energy savings in the process of using them, as managers at the on-campus power plant discovered at the University of Texas, in Austin. By retrofitting the fans on one of the boilers there with ABB’s ACS800 drives, the university expects to save between $500,000 and $1 million a year—by reducing its annual need for fuel and electricity by 320,000 million British thermal units (Btu) and 746,000 kilowatt-hours (kwh).

Despite their magnitude, the savings were not the catalyst for the retrofit. Rather, they were a welcome byproduct of an emissions control project initiated to comply with the stricter state regulations that will take effect next year. Since the project ended, the 150,000-pound Boiler Three emits only 21 tons of nitrogen oxides per year, a fraction of the 151.7 tons it spewed before. Plant management expects similar savings and reductions on Boiler Seven this spring, when it plans to complete the retrofit of this 500,000-pound unit.

Fan speed interpolation

The variable-frequency drives are an essential component of Compu-NOx, a system for controlling emissions of nitrogen oxide. The system controls combustion through a linear relationship between the speed of the fan and the airflow that it creates. “The system measures fuel flow and then interpolates the correct fan speed to give the desired flow,” explains Robert Benz, president of Benz Air Engineering Co., of Las Vegas, the supplier of the university’s new emissions-control system.

When dampers controlled the airflow before, the fans had to run at full speed all the time, which not only wasted energy, but also made control imprecise. “When dampers move up and down, even slightly, that generates a lot of airflow,” says Ryan Thompson, a project engineer for the university’s Utilities and Energy Management Department. “With the ABB drives, we have very precise control over the airflow, which makes all the difference in the world for fuel efficiencies and emissions.”

An unexpected consequence of the precise control over airflow is the ability to bank the auxiliary boilers—that is, to keep them in hot standby mode in case they are needed. “It allows us to keep one boiler at the ready without using any measurable amount of gas,” says Juan Ontiveros, director of the university’s Utility and Energy Management Dept. “It’s a big advantage, saving us 30,000 pounds of steam per hour.” The ability to maintain a stable and precise fan speed regardless of the load on the boiler means that Boiler Three can operate at less than 1 million Btu of input, a remarkable feat considering that its input rating is 183 million Btu.

Choosing an emissions-control system based on variable-speed drives accrued savings in other ways too, such as not having to invest $2 million in new burners for Boilers Three and Seven. “The power factor for the drive is near unity, regardless of operating speed or load,” adds Benz. “By putting on the drive, we are able to increase the capability of the transformer feeding it, so it saves the university from having to make power factor corrections.” And given reduction in energy consumption and the soaring costs of electricity and fuel, plant management expects the payback of the retrofit to be within 12 months. So a much more efficient system has stanched the bleeding in its energy budget.

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