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As any seasoned facility manager knows, weighing lowest first cost against lowest lifetime cost can be a difficult conversation to have with management, especially in government organizations, where one might be bound by low-bid stipulations. Nevertheless, life-cycle cost considerations are crucial for justifying investments in energy-efficient fans.
"One design trend to bear in mind is that fan sizing/selection software often gives a range of fan sizes to meet a given project requirement," says Michael Ivanovich, director of strategic energy initiatives for the Air Movement Control Association (AMCA) International.
Obviously, all of the fans in such a selection will meet airflow and pressure requirements of a system, but, Ivanovich notes, the smaller fans will have to work a lot harder to do so.
"Efficient products that are selected during the design and specification process may be 'value engineered' to smaller, less costly, lower efficient products during the bidding phase," says Tim Mathson, principal engineer for Greenheck and member of AMCA's air control task force. "For a typical fan performance requirement, there are almost always multiple fan sizes that can do the same duty. The smaller sizes almost always come with a lower price tag, but they also almost always perform at a lower efficiency."
And with the harder-working nature of smaller fans also comes the potential for decreased fan longevity and increased system noise, which, apart from the system efficiency, potentially affects productivity.
To minimize energy use, facility managers also have to be mindful of "system effects," says Mathson. System effects happen when ductwork and fittings slow the movement of air. That causes fans to work harder and be less efficient, and also often increases system noise and vibration. One way of preventing this inefficiency is to minimize static pressure in the system by providing proper inlet and outlet conditions at air handlers and other equipment where fans are installed, says Ivanovich.
He also cites two other practices that lead to less energy use by fans. First, regularly check the fan schedules in controllers and automation systems. Fans that are off when they should be use the least amount of energy. Secondly, regularly check VFD settings and belt drives to ensure they are properly set and maintained.
Mathson warns that building owners can't solely focus on fan efficiency; rather, they need to keep a focus on how much energy is being consumed by the fans in the system, he says.
"It's safe to say that the most important factor in fan energy consumption is how much pressure the fan must overcome," Mathson says. "Look for ways to reduce the system pressure by removing or opening up dampers, sealing leaks in ducts, or improving airflow into and out of the fan."
He also notes that owners could consider replacing constant volume systems with variable volume. Then, once the system requirements are minimized, compare fan the various fan options based on energy consumed.
"Look at the total cost of ownership for a period of time, say five to 10 years," Mathson says. "More efficient fans can often pay for themselves in reduced energy costs just one to two years."