The City of Tampa (Fla.) Advanced Wastewater Treatment Plant is a belowground facility that treats more than 96 mgd. Not long ago, employees were drawing unflattering analogies between their underground work site and a shoebox. "Once you dropped below grade, it got hotter and there was no breeze," says plant team leader Eddy Drovie. "It was just real stagnant and nasty."
Research has shown that people do their best work within a surprisingly narrow range of thermal conditions: Uncomfortable work environments are shown to decrease employee morale and productivity. Large spaces, as in many wastewater treatment plants, present a challenge to efficiently maintaining air movement and comfortable, consistent conditions using traditional heating, ventilating and air conditioning (HVAC) systems.
However, there is a middle ground between surrendering to the expense of air conditioning and losing productivity to comfort-related problems.
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Bigger is better
It's common knowledge that feeling too hot makes people less effective at doing their jobs, but researchers have put science behind the observation. They have shown that worker productivity steadily decreases as temperatures rise above 77 degrees F.
OSHA considers temperatures of 100.4 degrees F and above to be dangerous for workers, while air temperatures that exceed 95 degrees F significantly increase the heat load on the body,[ii] decreasing the body's ability to cool itself. In this instance, nondisruptive and nonturbulent airflow can augment natural evaporative cooling to make people feel more comfortable, boosting productivity.
Thus, when it comes to cooling workspaces, bigger low-speed fans are actually quite a bit better than small high-speed fans. Large-diameter, low-speed overhead fans use efficient motors to move massive amounts of air silently and gently. Their effect is somewhat similar to that of a traditional ceiling fan that moves air quietly to help cool occupants of an otherwise stuffy house on a summer day.
The key to the effectiveness of large-diameter, low-speed fans is the huge volume of air generated by their airfoils with winglets, which generate air movement at floor level. Upturned winglets and precision airfoils direct the flow of air up and over obstructions on the floor, providing elevated air speed throughout the space, including directly underneath the fan.
Because the fans' airfoil systems are engineered for maximum air movement with minimum resistance, large-diameter, low-speed fans create dramatic airflow with great efficiency. A facility's cooling costs are directly related to the energy consumption of the equipment used to provide comfort, where energy usage depends on the size and number of fans, the horsepower ratings of the motors, and the hours of operation per week.
For example, with typical summer hours of operation, a 20-foot-diameter low-speed fan uses, on average, 5 to 10 kWh of energy daily, which equates to an energy cost of about 48 to 96 cents — a fraction of what air conditioners can require.
As with many other products, the quality of today's large-diameter, low-speed fans bears little resemblance to predecessors. Recent innovations have led to simpler installation, quieter operation and longer life. Advancements have led to fans with on-board controls that significantly reduce the cabling required between the motor and the variable-frequency drive (VFD). This means minimal electrical noise and feedback.
As the distance between the motor and the VFD increases, so does the likelihood that electrical noise will be a problem. Placing the motor and VFD close to each other greatly reduces electromagnetic interference (EMI) and radio frequency interference (RFI). In addition, precision gearing provided through advanced gearboxes reduces friction and noise, allowing for cooler operation and longer service.
Caution: Low clearance
These very large overhead fans are excellent air movers in large spaces, but what about smaller spaces or areas with numerous overhead obstructions, such as the City of Tampa Advanced Wastewater Treatment Plant?
For such cases, large-diameter fans are available in vertical, mobile packages. Mobility allows for flexibility in any environment, from open-air treatment plants to accompanying storage and maintenance facilities.
"We tried several small fans, and they were just not good enough," says Tampa's Drovie. The fans either took up too much space, were too noisy, or simply fell apart. The 6,900-square-foot Tampa facility now uses an 8-foot-diameter mobile vertical fan that workers can easily roll into different bays to provide large volumes of quiet, nondisruptive air movement in any direction, delivering extremely efficient cooling.
"The fact that you can move those fans around is a plus," said Drovie. "I think that's a big part of the reason why we chose this, along with its ability to cool."
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Cold climates, too
The usefulness of large-diameter, low-speed fans isn't limited to summer: Many large facilities in cooler climates install fans specifically for winter. Large-diameter, low-speed ceiling fans can destratify heat by moving large volumes of air without creating a draft. The steady mixing of air creates a uniform temperature throughout the space, including near the thermostat.
The thermostat setpoint remains the same, and the heating system simply works less hard to maintain that temperature. The energy saving achieved by reducing the heat escaping through the roof is similar to turning the thermostat down three to five degrees, and that can translate to significantly lower operating costs.
People recognized the cooling potential of air movement as early as the second century B.C. With electricity came the first electric ceiling fan in the 1880s. Fast forward 130 years, and precisely engineered large-diameter, low-speed fans have improved the science of air movement to bring year-round, energy-efficient comfort to wastewater treatment facilities and indoor workspaces of any size and type.
About the authors
Christian Taber leads the applications engineering team at Big Ass Fans, a manufacturer of large-diameter, low-speed fans based in Lexington, Ky. Erin Hsu is a copywriter for Big Ass Fans.
Seppänen O, Fisk WJ, Faulkner D. 2003. Cost benefits analysis of the nighttime ventilative cooling. In: Proceedings of the Healthy Buildings 2003 Conference, Singapore 2003.
[ii] Occupational Health and Safety Administration Technical Manual, Section III: Ch. 4; Heat Stress. 1999.