Articles
ISIS research facility
The Science and Technology Facilities Council (STFC) has reduced energy costs at its ISIS research facility at Harwell near Oxford through the installation of three direct-drive cooling tower motor and drive systems to the cooling towers of the neutron source’s proton accelerator.
The ISIS neutron source uses muon spectroscopy and neutron scattering to probe the structure and dynamics of condensed matter on a microscopic scale ranging from the subatomic to the macromolecular. The three towers form part of the accelerator’s primary water-based cooling system.
Previously, the fans in the cooling towers were run at full speed using a conventional system consisting of a fixed-speed AC induction motor connected by a drive shaft and disc coupling into a right angle gearbox. This reduced the motor's speed to that required by the fan, typically around 6:1.
With this arrangement, the gearbox required regular inspection and maintenance of the lubrication and seals. The possibilities of gearbox oil leaks were also potentially harmful to the environment. Misalignments in the power transmission system could cause vibration, wear and noise, while the mechanically-complex power transmission system introduced significant energy losses.
The system provided no opportunity to vary the speed of the fans as the load on the cooling system changed. Engineers decided that the motors, installed some 15 years ago, were approaching the end of their lives and needed replacing.
The upgrade saw the installation of three ABB-Baldor direct-drive cooing systems incorporating permanent magnet (PM) motors. These are a viable alternative to conventional AC induction motors and can deliver significant energy efficiency advantages, said ABB, even compared with the latest premium-efficiency types of motors. This higher efficiency is more pronounced when run at low frequencies which is the case for these direct drive cooling tower motors, said the firm.
Using a variable-speed drive with a special control algorithm, the PM motor can achieve the same low output speed as that previously attained by using the gearbox. As such, the gearbox has been eliminated and the PM motor has taken its place on the base. This allows the motor to directly drive the fan, thereby removing the need for the driveshaft and coupling used in the previous arrangement. This further increases efficiency and reduces noise while substantially reducing drive system maintenance, said ABB.
The three drives were installed in about a week. Two duty fan units are run 24/7, with one in standby mode. Compared to the previous situation, in which the motors were run at 45 Hz, giving a fan speed of 220 rpm, the ABB-Baldor drives now run the motors at 9 Hz, giving a fan speed of 140 rpm. This gives a normal current load of only 10 A, while even the highest ambient temperatures and relative humidity need around 17 A.
The ability to vary the speed of the fans means that the water in the cooling towers can be evaporated at the rate that matches demand. This means that lower amounts of chemicals can be added to the water to combat the threat of Legionella and other pathogens, saving more expense.