VSD control of air handling units slashes energy bill

A company's plan to save energy by upgrading from v-belts to timing belts on air handling drives has been expanded to include a wide range of automation equipment. This has cut EUR140,000 from the annual energy bill, improved the work environment and suggested opportunities for further savings.

Stryker Orthopaedics manufactures a range of orthopaedic implants, bone cements and bone substitute materials at its factory in Limerick, Ireland. The facility has been in operation for over 35 years and employs 500 people. Like many other companies it has become increasingly aware of energy cost increases in recent years and has undertaken several initiatives to reduce energy usage/costs and to protect the environment.

In late 2006 maintenance manager Robert McKillican calculated that changing from v-belts to timing belts would reduce energy consumption by about 10 per cent, so he called in local industrial factor Michael Dineen of Dickson Bearings to discuss the idea. However, it soon became clear that slippage of the v-belts gave a soft starting effect, which would be lost with a toothed belt.

VSD advantages over soft starts

McKillican and Dineen briefly considered the use of electronic soft starts, but decided that variable speed drives would be better because they could be used to tune the air handling to match demand exactly.

Stryker's plant has eight extraction fans, all of which are worked quite hard to ensure complete extraction of dust particles. Initial calculations suggested that significant energy savings could be made if each were tuned to suit the exact local demand, rather than run faster than necessary. Additionally, staff numbers and production levels vary during Stryker's two-shift day, so there is potential to slow down the air handling even further at different times throughout the shifts.

There were a number of practical issues to be addressed, not least which drive to use. Dineen undertook some research and recommended the L300P Series from Hitachi, three at 55kW and five at 37kW, which are particularly appropriate for fan drive applications and have an enviable track record on energy-saving applications.

Meanwhile McKillican was unearthing some practical considerations. The control panels, for instance, had to be designed to fit into a smaller than ideal control room. And they would be close to some sensitive equipment, so chokes would be required on every input and output to minimise noise generation and harmonics on the mains. With the Hitachi drives it was also possible to set a maximum total output, so that penalty charges from the electricity supplier could be avoided.

PLC adds intelligence

A PLC would be required to control the system, and it seemed logical to stick with Hitachi and use an EH150 series unit. This communicates with the plant's existing building management system (BMS) to make the whole installation intelligent and interactive.

Finally it was realised that security staff as well as control engineers would need access to the system. It was therefore decide to install two touch screen HMIs, again Hitachi, one in the control room, the other in the security office, and to use clear graphical displays to make the system's use as intuitive as possible.

Within days of completing the installation, energy meters strategically located around the system were confirming that considerable savings were being made. No detrimental effect on machine performance or other production compromises were identified. But McKillican was not ready to rest on his laurels; he knew that there were further improvements to be made, although he had to introduce them slowly so that production was never disrupted.

Initially the drives were configured to give the fans three set speeds, but the intention is to upgrade this with PID control and pressure sensor feedback so that load matching becomes exact. With the PLC and BMS working together, it has been possible to programme in timings for tea and lunch breaks so that the air handling is not running unnecessarily when machines are idle for two hours each day.

Additional savings of 15 per cent

McKillican says that there were other gains too: "Simply putting drives into our system improved our electrical usage profile, so it is saving us an extra 15 per cent above expectations. Also by load matching we have eliminated draughts from around the shop floor, so people are more comfortable due to the improved work environment.

"But the biggest unexpected saving – and it is massive - is in the gas bill for space heating. Not only are we pulling in less outside air, but we are now able to recycle half of our heated air. We are saving EUR60,000 on gas, nearly 30 per cent, and there is scope to further increase the recycling!" This is on top of a EUR80,000 saving that has been made on electricity, to run the dust extraction system.

With the bit between his teeth, McKillican is now working on several more energy-saving projects: motors throughout the site are being audited for efficiency; lighting has been modernised, generating a EUR60,000 saving; those air handling units without recirculation capabilities are being upgraded.

McKillican states: "We have written a new module for our SCADA software so that we can monitor energy consumption in each manufacturing cell. Soon each machine manager will be accountable for his own measured energy consumption; I expect this will lead to a significant change in thinking and that a great multitude of minor energy losses will be brought under strict control.

"For many years we senior managers were able to ignore energy issues, but once we started monitoring usage our attitude changed completely. This is now filtering out to the whole workforce. And the lesson is that monitoring energy consumption leads to a constant drive for more savings."