Magnetic control of process load speeds saves energy at mills

Motors drive the pumps at Daishowa America (Port Angeles, Wa.) at 7,000 gallons per minute (gpm), while the paper mill's treatment process averages 4,800 gpm. To achieve the lesser flow rate, the system previously used bypass and discharge valves to restrict and regulate flow. Consequently, the bypass valve returned 2,200 gpm to the pump station, wasting significant energy.

08/01/2001


Motors drive the pumps at Daishowa America (Port Angeles, Wa.) at 7,000 gallons per minute (gpm), while the paper mill's treatment process averages 4,800 gpm. To achieve the lesser flow rate, the system previously used bypass and discharge valves to restrict and regulate flow. Consequently, the bypass valve returned 2,200 gpm to the pump station, wasting significant energy.

In addition, starting the motors while connected to the pumps created locked rotor currents, causing the motors to overheat. Starting and stopping the pumps also created flow surges and water hammer that damaged the pipes. And, because the pumps always operated at top speed, the equipment suffered from vibration and cavitation.

To alleviate these problems, mill managers recently installed adjustable-speed drives (ASDs) from MagnaDrive (Seattle, Wa.) on two 100-hp, 1,175-rpm motors that power pumps in a wastewater treatment process. The ASDs use rare-earth magnets to precisely control pump load speed, which reduces energy used, while maintaining required flow rates.

This eliminated use of the energy-wasting bypass and discharge valves, reducing energy demand 56% from 142 kW to 62 kW.

Also, the motors can be started uncoupled from the pumps, avoiding flow surges, water hammer and overheating. Further, the ASD eliminated system vibration, which allows the mill to save $15,000 in maintenance costs.

Torque transmitted across gap

In a typical application, MagnaDrive's ASD is installed between a motor and the driven load, usually a pump, fan or blower. The drive is a non-electronic, mechanical device. It consists of two independent components with no physical contact. One component, a precision rotor assembly containing high-energy permanent magnets, is mounted on the load shaft. The second component, a copper conductor assembly, is connected to the motor shaft. Relative motion between the magnets and the copper creates a magnetic field that transmits torque through the air gap between the components.

Varying the width of the gap changes the coupling force, so the amount of torque transmitted between the motor and load can be continuously adjusted. This allows precise and efficient speed control for optimum performance, including "soft" starts and stops.

Alternating flow rates

Similar to Daishowa, plant managers at Ponderay Newsprint mill (Usk, Wa.) installed MagnaDrive's ASD to increase energy efficiency of a two-stage pulping and de-inking process. In this application, a 250-hp, 1,800-rpm motor-driven system pumps water under constant 50-psi pressure.

However, during each one-hour cycle, only the 12-minute de-inking stage requires maximum flow of 3,200 gpm, while the remaining 48 minutes requires only 1,610 gpm. To reduce flow to 1,610 gpm, while maintaining 50-psi, the system previously used a bypass valve to return 1,590 gpm to the pump. Because the pump always ran at full capacity, this system also suffered damage from vibration and cavitation.

Ponderay's managers used the ASD to vary pump speed as needed, while maintaining a constant 50-psi. This allowed them to close the wasteful bypass valve. Energy demand dropped 62% from 173 kW to 65 kW. The ASD also reduced vibration and eliminated the cavitation problem, again reducing maintenance costs.

For more information, Circle 496 or visit www.magnadrive.com .


Author Information

Jim Montague, news editor jmontague@cahners.com




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