Selecting high-volume low-speed fans

When choosing a fan system for a facility, a number of key factors must be considered, including the system's effectiveness, cost efficiency, the size of the facility, and any obstructions to air flow. Box fans, ceiling fans, axial and propeller fans, centrifugal fans, or mixed and cross flow fans may come into consideration.

02/10/2005


Key Concepts

 

  • HVLS fans are efficient at ventilating and circulating air in large plants.

  • Fans can reduce heat loss through the roof and cool areas where air conditioning isn't an option.

  • Minimizing the amount of ventilation required to achieve adequate air quality is also a benefit.

    Sections:
    HVLS fans
    Uses
    Benefits
    Cooling
    Features
    Maintenance
    Size and number
    Cost
    More Info:


    When choosing a fan system for a facility, a number of key factors must be considered, including the system's effectiveness, cost efficiency, the size of the facility, and any obstructions to air flow. Box fans, ceiling fans, axial and propeller fans, centrifugal fans, or mixed and cross flow fans may come into consideration. For air circulation, heating, and cooling in a large space with a high ceiling, those fans may not meet a facility's needs.

    High-volume low-speed (HVLS) fans have evolved out of the need for greater air movement as a solution for the ventilation and circulation of air in large spaces such as plants. The technology has proven to be beneficial from a work environment perspective as well as from an energy efficiency and cost savings perspective.

    At some point in the evolution of fan technology, engineers became focused on using speed to increase the air displacement of a fan - the cubic feet of air per minute moved through the fan - and neglected some important physics issues such as:

    • High-velocity air movement is both unpleasant and disruptive

    • Air speed greater than four or five miles per hour usually offers little, if any, additional cooling benefit

    • In very hot air conditions, very slow moving air actually cools best

    • A high-velocity, narrow air jet dissipates very quickly

    • A large column of air will travel farther than a small one because the amount of friction per cubic foot of air or friction interface is significantly less than that of a small column of air. A fan that is 3 feet in diameter creates an air column with more than six times as much friction interface per cubic foot as the air column from a fan with a 20-foot diameter

    • Large, low-speed fans, properly arranged, are capable of establishing continuously circulating air currents that transport vastly more air than smaller high-speed fans.

      • HVLS fans

        By taking advantage of the physics of fluid dynamics, NASA-inspired airfoil designs, and the latest manufacturing technologies, these highly efficient air movement systems emerged in the late 1990s. Ten-bladed ceiling fans (Fig. 1) range in size from eight to 24 feet in diameter and typically require one horsepower or less to run at low speeds.


        Uses

        Originally designed to create an evaporative cooling effect over large areas (up to 20,000 square feet), these systems are also used to combat wasted heat during colder seasons; reduce installation, tonnage, and operating costs of air conditioning; provide healthier air mixing and ventilation; and reduce damaging humidity and moisture.

        Benefits

        Heating

        • HVLS fans achieve greater comfort by bringing trapped warm air from the ceiling down to floor level while reducing heat loss through roof vents and skylights

        • Vastly more efficient than the high-speed fans that typically push warm air down to the floor, HVLS fans result in significant energy savings (20% and greater) due to the de-stratification of heated air in a facility

        • Air is mixed to eliminate hot and cold spots on the floor and at varying levels of elevation in the building

        • The amount of air that must be continuously replaced with cold outside air to achieve adequate ventilation is minimized

        • Quiet and non-irritating, the fans contain no loud high-speed air jets.

          • Cooling

            • HVLS fans provide an efficient and cost-effective alternative to cool environments where air conditioning is not feasible, such as in un-insulated buildings or buildings with large open doors or sides

            • Non-disruptive cooling improves the comfort and productivity of the work environment

            • Used in conjunction with conventional evaporative coolers or air conditioners, HVLS fans evenly distribute cool air

            • HVLS fans cut the costs of operating air conditioners by decreasing the amount of output needed and providing an equivalent cooling effect at a lower cost.

              • Ventilating

                • Fresh incoming air is continuously mixed with indoor air, providing efficient ventilation and minimizing the total amount of ventilation required to achieve adequate air quality

                • Reducing the volume of air needed for ventilation reduces the number of high-speed exhaust fans needed, in some cases eliminating them altogether, cutting fan power consumption by up to 90%.

                  • Massive air mixing also results in greater absorption and dispersion of noxious fumes and damaging moisture and humidity, resulting in a healthier, drier, and safer work environment. Other benefits include elimination of birds and bugs, as well as avoiding the noise and disruption from other air flow solutions.

                    These easily installed fans were designed to work in conjunction with existing environmental, fire, and other safety control systems. They supplement existing HVAC systems by making them more effective while reducing or eliminating many of their inefficiencies.

                    Features

                    The typical industrial high-volume low-speed fan system is 20 feet or 24 feet in diameter. The blades are easily bolted to a cast aluminum hub. The power unit is comprised of a one horsepower motor specifically matched with a gear reducer. A single motor control, with variable frequency drive, allows for blade speed adjustment, including reversing.

                    Maintenance

                    Periodic maintenance on the HVLS fans involves cleaning the anodized blades, minor lubrication of components, occasional bolt tightening, and inspection of built-in safety features.

                    Size and number

                    A fan system for a 100,000 square foot facility, with a typical layout of racking and equipment, would probably require eight to 12 fans to obtain the optimal benefits of a HVLS fan system. The key factors are location, height, size, and density of airflow obstructions (Fig. 2).


                    For size selection, bigger is usually better for large areas. Size options allow for custom solutions to unique location challenges. Ceiling heights are ideally more than 15 feet. Clearance needed above the fans should be at least three to five feet for best airflow; nominal clearances of blades from nearby equipment and structures are recommended.

                    Cost

                    Each fan costs about a nickel an hour to operate, using about as much energy as three or four light bulbs.

                    When heat de-stratification (and/or as a supplement to A/C) is a benefit, rapid payback and a high return on investment is attainable. For example, with a 15-degree gradient from floor to ceiling, one recent installation analysis estimated an annual heat savings of 40%, a payback period of 1.4 years, and a two-year ROI of more than 40%.

                    More Info:

                    The author can be reached for questions at 866-668-3247. Article edited by Jim Silvestri, Managing Editor, 630-288-8777, jsilvestri@reedbusiness.com .





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