For Better Energy Efficiency, Reduce Fan Motor RPMs

Aug. 28, 2014
Fan applications consume a lot of energy. Here’s how the largest single kiln line cement plant in North America used variable-frequency drives to become one of the most energy-efficient cement plants in the world.

Shaving energy costs and implementing energy-demand control strategies can go a long way toward improving an industrial facility’s bottom line—especially when peak demand charges can be up to 25 percent of your plant’s energy bill.

There are many ways to achieve energy cost reductions, including installation of monitoring units for machines, large energy enterprise management (EEM) software systems, or specifying the right drives and motor combination. Cement maker Holcim (US) identified a small component of its overall manufacturing footprint that could provide big savings: variable-frequency drives for the motors that power its large cooling fans.

Holcim produces large volumes of cement via industrialsized kilns that require constant airflow. The fans that produce that airflow demand a great deal of energy because of the highly stringent production process. Clinker is the critical mineral compound used in making cement. It is produced by mixing and grinding limestone, alumina, iron and silica raw materials into a mixture that is heated in a rotary kiln furnace. At the discharge end of the kiln, hot clinker falls into a cooler, where the temperature is rapidly lowered to about 100 0C by “air quenching”—the use of large fans to blow on the clinker until it is cool.

Holcim’s Ste. Genevieve plant in Bloomsdale, Mo., can produce more than 4 million metric tons of cement per year and 12,000 metric tons of clinker per day. “The kiln must be run with very tight tolerances on speed, temperature and airflow to optimize the clinker manufacturing process,” says Kendall Walden, electrical and process controls manager at Holcim. “Otherwise, energy and operating inefficiencies increase plant operating costs.”

High in-rush currents at startup

Several fan motors, ranging up to 350 hp, are used to create proper airflow in the cooler. When initially installed, the motors used across-the-line starters, with dampers that applied full voltage to the motor terminals at startup. As a result, the motors experienced high startup inrush currents that were five to six times the full-load amps. Additionally, the dampers were operating at less than 100 percent open for a majority of the year.

This combination presented an opportunity to increase overall efficiency, and avoid utility demand charges for excessive amperage draw at motor startup, says Walden.

For this retrofit project, Holcim assigned the engineering to its electrical reliability engineer, Michael Ifurung, who oversaw the implementation of variable-frequency drives (VFDs) from Denmark-based Danfoss VLT Drives.

“It’s well known that VFDs can dramatically cut energy costs in fan applications,” Ifurung says. “For example, if speed can be reduced 20 percent, then kilowatt consumption can be cut up to 50 percent due to the physics of motor affinity laws. The critical engineering decision is to select and configure the right kind of VFD for the application.”

For this industrial setting—Holcim’s is the largest single-kilnline cement plant in North America—the job called for 15 VLT AutomationDrive FC 302 drives, for several 460 VAC motors: five 300 hp, eight 250 hp and two 150 hp.

“These are variable-torque type drives,” Ifurung explains. “At faster speeds, a fan encounters more pressure. That requires more torque to spin the fan. Torque reductions vary as the cube of speed, and every time you can reduce torque, you can reduce RPMs. Also, that means you can cut kilowatts exponentially.”

During operation, the soft-start drives can vary the current frequency from 0 to 32 kHz to deliver a variety of AC motor speeds. Motor speed can then be matched to the required fan RPM to maintain process quality and reduce electric consumption whenever full torque is not required.

PLCs drive the drives

To tap those qualitative and quantitative benefits, the drives use the DeviceNet networking standard to communicate with programmable logic controllers (PLCs). Proprietary processcontrol software crunches the pressure and temperature data and instructs the PLCs to tell the VFDs to change fan RPMs and sequencing as needed.

The PLCs also receive 4-20 mA signals from pressure and temperature transducers as a backup. The drives and PLCs are managed through a human machine interface (HMI) panel in the control room. “The drives operate in an open-loop mode with a speed accuracy of +8 RPM—which makes it easy to fine-tune airflow,” says Walden. “That kind of accuracy gives us reliable fan control to modulate gas flow and the rate at which ambient air is put into the cooler for proper clinker temperature.”

Holcim also used Danfoss’s harmonic distortion calculator to determine the transformers’ K-factor needs. “K-factor is a metric of how the harmonic electric load currents affect heat buildup in the transformer,” says Ifurung. “Knowing the K-factor saved us some money on the transformer.” The VFDs also include a built-in DC coil to ensure very low harmonic disturbance for the power supply.

Another feature of the drive is safe/stop functionality suitable for category 3 installations, in accordance with the EN 954-1 standard.

The big bottom-line benefit of the VFD implementation was the substantial energy savings. “In our case, the results are significant,” says Walden. “This technology has aided our overall plant electrical efficiency. As a result of those savings, our payback for the project is less than two years. And, for the second year in a row, our plant has received the Environmental Protection Agency’s Energy Star rating for super energy efficiency.”