By Nina Wolgelenter
Reinvented, resized, and re-engineered, contemporary ceiling fans should no longer be an afterthought when it comes to design and occupant comfort. Innovative modern engineering has re-established ceiling fans as an integral part of comfort and energy conservation. Whether used on their own, employed with natural ventilation, or specified in tandem with air-conditioning systems, achieving proper air circulation with ceiling fans is no longer up in the air.
Taking into consideration regional climate, function, and design, conditioned spaces and those using natural ventilation need a way to maintain a certain ambiance by circulating the air within a space to establish a high comfort level. Fans effectively work in two ways—providing a feeling of coolness in the summer and destratifying warm air in the winter.
The effectiveness of a large-diameter, low-speed fan lies in its ability to move great volumes of air slowly and gently without disrupting the atmosphere of the facility. During the cooling season, with fans operating between 60 to 100 per cent of capacity, the additional air movement does not cool the air, but rather creates a cooling sensation.
Along with summer comfort, properly engineered fans are capable of destratifying a space in the winter, reducing heat-energy consumption by as much as 30 per cent. Considering hot air naturally rises as cold air falls, large-diameter, low-speed fans mix these two extremes, creating a more uniform temperature.
Contrary to the typical method of reversing a high-speed fan, a large-diameter fan is simply slowed to 10 to 30 per cent of its maximum speed, redirecting warm air from the ceiling to the occupant level to further increase comfort. With the ease of simply slowing the fan speed in lieu of changing its direction, users are able to avoid drafts. According to American Society of Heating, Refrigerating, and Air–conditioning Engineers (ASHRAE) 55–2010, Thermal Environmental Conditions for Human Occupancy, drafts occur at approximately 12.2 m/minute (40 fpm) of air velocity.
Specifying appropriate products
Ceiling fans have gone through a transformation over the past few years—technology and design are now on an equal playing field. While ceiling fans offer enhanced thermal comfort to a space’s occupants, it is also important to understand how new engineering practices have added to product energy efficiency and quieter operation. Magnetic motors and high-efficiency airfoils yield maximum efficiency without disrupting ambiance, making contemporary ceiling fans far more appealing than their predecessors.
Ceiling height, square footage, and space use are all important factors when solving the comfort needs of the occupants. Fans for lower ceilings differ in design and engineering from those designed specifically for high-ceilinged spaces. However, with the advent of more advanced technology, fans now operate nearly silently in sound-sensitive environments that benefit from elevated air speed, regardless of their dimensions. Examples include:
- offices;
- galleries;
- churches;
- retail stores; and
- residences.
One of the technologies involves permanent toroid magnets, which are more efficient than electromagnets. Shaped like a doughnut, their polarity alternates north to south at multiple points along the circumference, eliminating the jerking seen with other magnet-based motors that jump ahead to meet the next pole. As the charge travels along the motor’s copper windings, the magnet follows, smoothly spinning the airfoils.
Another important new technology is sensorless control, which means the electronic controller detects the position of the permanent magnets relative to the windings. The voltage generated in the windings are directly measured, resulting in smooth, quiet operation.
Highs and lows
High ceilings and voluminous spaces such as warehouses with 6- to 15-m (20- to 50-ft) high ceilings generally call for a larger fan, from 2.4 to 7.3 m (8 to 24 ft) in diameter. The concept is similar to regular ceiling fans, but large-diameter, low-speed fans can be significantly more effective at circulating the air. (For more, see Richard Ansley’s article, “Fan Size and Energy Efficiency,” in International Journal of Ventilation [vol. 1, no. 1]). To achieve the desired effect, it is important to understand how the right fan can improve the space—a hypothetical situation involving theoretical calculations performed by a consulting engineer can provide more information.
A 950-m2 (about 10,000-sf) facility in a northern climate could require a 30-ton constant volume rooftop unit with a 10-horsepower supply fan furnishing conditioned air to the space through metal ductwork and diffusers. The total design, material, and installation cost of the ductwork and diffusers is around $20,000, with an additional annual energy expense of operating the HVAC system at roughly $7000. Conditioned air is thus pumped into the building, but it is often not effectively circulated.
These fans work in tandem with the HVAC to provide very efficient air distribution. Since the combined system of HVAC working with a fan does not rely on the former’s own supply fan to push the conditioned air through an intricate ductwork design, the designer can also decrease the HVAC fan’s motor size. In this scenario, the motor size is reduced from 10 to 7.5 horsepower, providing an initial savings of $200. The big savings come when the user forgoes a majority of ductwork, saving up to $15,000 on duct design, material, and labour.
Ranging in height from 2.4 to 4 m (8 to 13 ft), lower ceilings require a different set of parameters, but can also benefit from properly designed fans while complementing the HVAC system for optimal energy efficiency. In both small and large settings, it is important to consider the acoustics of the environment.
Airfoil 101
Taking the lead from micro-airborne vehicles and airplane wings, high-efficiency airfoils move large amounts of air with minimum resistance for maximum efficiency. Unlike the common fan blade, airfoils are carefully engineered for each specific fan model.
Well-designed small-diameter fans maximize the amount of air moved by each part of a shorter airfoil at every speed. The tips spin much more quickly than the portion near the motor, so optimizing air movement for every part of the airfoil and potential speed setting further increases fan efficiency.
Understanding the pitch (i.e. angle of attack) at which the airfoil is positioned also helps illustrate the effectiveness of fans. An exaggerated pitch (i.e. almost vertical angle of attack) will either create increased drag or require more energy due to the increased drag; a flat, a mostly horizontal airfoil will not move much air at all. The most efficient angle lies somewhere in between. Operational speed, along with airfoil size and shape, also affects the performance of the fans. The optimal combination produces the most air movement with the least amount of energy.
As the fan diameter increases, it is important to observe airfoil width and pitch with the discerning knowledge a narrow blade coupled with a moderate pitch will deliver far less drag than wider, improperly pitched blades.
