Proton accelerator cuts cooling costs with ABB drives 28 November 2013

The ISIS neutron source for the proton accelerator at Harwell, near Oxford, has reduced its energy costs following fitment of three ABB-Baldor direct-drive motor and drive systems for its cooling towers.

The news comes from the STFC (the Science and Technology Facilities Council), which explains that the ISIS plant had been running its cooling tower fans at full speed, using fixed-speed ac induction motors installed at its inception 15 years ago.

From that, power transmission was via a conventional connection to a drive shaft and disc coupling into a right angle gearbox, which reduced the speed to that required by the fan, typically 6:1.

STFC points out that the penalties were not only higher than necessary energy consumption, but also energy losses through the complex drive chain – and a gearbox that requires regular inspection and maintenance. Issues included the usual mix of lubrication and seals, but also alignment, vibration and wear.

The new ABB-Baldor system incorporates a PM (permanent magnet) motor, delivering significant energy efficiency advantages even over premium-efficiency ac induction motors. Just as important, they run at low frequencies, being designed as direct-drive cooling tower motors.

In fact, according to STFC, using a variable-speed drive, the PM motor can achieve the same low output speed as that previously achieved via the gearbox – making the latter, and the drive shaft and coupling, all redundant.

ABB explains that its cooling tower control algorithm provides sensorless feedback for the PM motor, optimised to manage the large inertia of fans, with a low starting current.

In brief detail, two duty fan units now run 24/7, with the third in standby mode. And whereas the previous configuration involved running the motors at 45 Hz – giving a fan speed of 220 rpm – the new direct drives' motor frequency is 9 Hz, resulting in a fan speed of 140 rpm.

The ability to vary the speed of the fans also means that water in the cooling towers can be evaporated at a rate that matches demand. This reduces energy consumption but also means that lower quantities of chemicals can be added to the water to combat the threat of Legionella. =

Brian Tinham

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