Variable speed drives (VSDs) have been around for a long time now. For a while, they were seen as the perfect quick and easy way to save money. Particularly on large ventilation fans, where the reduction in electrical power consumption is considerable. Applying the ‘Cube Law’ for centrifugal fans, it can be calculated that a 10% reduction in motor speed results in a 27% reduction in power consumption (0.9 x 0.9 x 0.9 = 0.73).
Using the simple calculation above, it appears that installing VSDs would be a great decision. And for a while, quite a few companies made a great deal of money by indiscriminately installing drives to existing fan motors. The VSD would be typically set to run to a fixed speed (somewhere between 30-45Hz) and an instant, ongoing saving would be achieved.
There are, however, two problems with this approach. Number one is that the plant will have been commissioned and balanced to achieve the designed number of air changes, for all served areas. By simply reducing the fan running speed, the original balancing and supply volumes are jeopardised. The second problem is that, as with running at full speed, varying volumes are achieved over the life cycle of the installed filters. When clean filters are fitted, maximum air flow is achieved, as the filter collects particles delivered volumes decrease.
A slightly more sensible approach would be to employ an occupied/setback profile (where appropriate). Certain areas may require a controlled air supply twenty-four hours a day, but do not need full control when areas are unoccupied. Examples of this application would be hospital theatres and museum exhibition spaces. In this scenario, the building management system would run at a fixed speed during the occupied period and at a reduced speed outside of occupation. Although this would offer an energy reduction, the two problems described in the paragraph above would still be present.
A far more effective way to obtain energy savings, from variable speed drives, is to control them to a discharge or index indicator. This would typically be either pressure or velocity. By measuring the volume of air being delivered, we can command the fan to run at a higher speed when air delivery is low and lower when air delivery is high. This approach takes account of the filter(s) status, providing the correct air changes across the life of the filter(s).
Adding automatic monitoring and control to VSD driven fans is relatively simple. Most modern variable speed drives have the inbuilt capability to receive a signal from a monitoring device such as a static discharge pressure transducer. This means that a VSD and transducer can be installed, using the original fan supply, with no requirement for a link to a separate control system. In built time schedules, also mean that occupied/setback regimes can also be implanted, compounding the possibility for potential savings.
With the above easily achievable, it is somewhat surprising that variable speed drives are still being installed without effective control. Hopefully, with information available, this should change and we will start to see easy win reduction gains achieved for all large fan application.
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