High-volume/low-speed (HVLS) fan technology is the unsung hero of un-ducted circulator fan design. Through careful application of aerodynamic design principles and characteristics, un-ducted circulator fans can find large scale applications in manufacturing and warehousing facilities as remarkably effective and energy-efficient HVLS fans.

By Dr. Richard M. Aynsley

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High-volume/low-speed (HVLS) fan technology is the unsung hero of un-ducted circulator fan design. Through careful application of aerodynamic design principles and characteristics, un-ducted circulator fans can find large scale applications in manufacturing and warehousing facilities as remarkably effective and energy-efficient HVLS fans.

Circulator fans, ducted and un-ducted, are used to circulate or mix air within a space. They can also be used to provide local air movement to enhance peoples’ thermal comfort in a space during warm conditions. The fans can make a person feel 11-17 degrees cooler.

Aerodynamic drag

At low speed, increasing the chord of the blades is likely to result in drag. This increases the solidity without increased drag.

Tip vortex losses

Without ducting, circulator fans develop strong vortexes at the tips of their blades. These vortexes reduce the aerodynamic performance of a fan by about 15%.

Solidity ratio

Solidity ratio is one of the most important considerations in the design of HVLS fans. The volume of air flow moved by the fan depends on the solidity ratio. The solidity ratio of a fan is the surface area of the blades relative to the area swept by the blades. Doing so without consideration of other design factors can result in a fan that consumes more energy.

Aerodynamic lift load on blades

Increasing the number of blades on an HVLS fan also lowers the aerodynamic lift load on each blade. Because HVLS fans use more blades, the weight load on each blade is decreased because the load is spread out among the 10 blades. This reduction of lift load increases efficiency and produces less wear and tear on the fan.

Blade stiffness

The aerodynamic lift load increases the bending on a blade, the blade needs to be stiffer. Typically, increased stiffness means increasing the thickness of the blade, which in turn, increases the drag. The design of 10 narrow blades reduces the bending force on each blade and allows thinner, more efficient blade profiles to be used.

Energy efficiency

Large diameter propellers generating low speed air flow have the potential for very high energy efficiency. A typical 50-foot helicopter would have an engine providing 1,500 horsepower to develop 8 lb/ft2, or 0.005 lb/ft2 thrust per horsepower. A 24-foot diameter HVLS fan has a 2 horsepower motor to develop 0.18 lb/ft2, or 0.09 lb/ft2 thrust per horsepower.

Conclusions

When each of these design components is considered, a true HVLS fan should have 10 relatively narrow blades. The number of blades increases the solidity ratio without increasing drag, and lessens the lift load on each blade. HVLS fans also rely on their large size and aerodynamic design principles to move air without using high air velocities. This slow speed creates a quieter, less disruptive air flow, and makes for highly energy efficient fans.

Dr. Richard M. Aynsley is director of research and development at the Lexington, KY-based Big Ass Fan Company. He is chair of the American Society of Civil Engineers Aerodynamics Committee. He has more than 30 years of teaching and consulting experience in the areas of architectural design and aerodynamics. Dr. Aynsley can be contacted by email at dick@bigassfans.com .