April 3, 2012
Air movement is a cost-effective way to reduce condensation, create a more uniform temperature from floor to ceiling, and increase employee comfort and productivity. Large-diameter, low-speed fans, up to 24 feet in diameter, provide efficient air movement while using less energy than smaller fans running at faster speeds.
Corrosion and attempts to prevent it cost the metal fabrication industry millions of dollars each year. Rust, which causes massive amounts of product loss for those in the metal industry, is the result of condensation.
High humidity and wildly fluctuating temperatures plague facilities throughout the country, particularly in the spring and fall, leading to condensation on any surface with significant heat-producing or heat-retention capacity. For most facilities this means water on concrete floors, mold growth on moist surfaces, and poor indoor air quality.
Although condensation is more probable in areas of high relative humidity,1 industrial air movement systems can help to reduce condensation buildup in any facility, regardless of climate. Some successful tactics to minimize condensation are:
When stacked sheet or coil is delivered during cold temperatures, many warehouse spaces develop condensation problems when the transported metal is brought into a warmer space. It may take hours for the temperature of the delivered product to approach the inside air temperature.
During this warm-up period, the metal’s surface temperature is initially below the dew point temperature of the air, leading to condensation on the surface of the metal and potential corrosion.
Heat transfer from the air to the metal surface can occur up to 2.5 times faster by increasing the air speed across the metal by 100 feet per minute (1.2 miles per hour).2 A faster warm-up time coincides with less condensation and less chance for corrosion.
Once the temperature of the metal is above the dew point, air movement produced by the fan speeds the evaporation rate of the moisture on the metal’s surface by one-third, decreasing the time in which corrosion has to form (Figure 1).
Stratification, layers of air that vary in temperature from the floor to the ceiling, occurs because hot air is less dense (lighter) than cold air. The air coming out of a forced-air heater is approximately 5 to 7 percent lighter than the air in the space being heated. Since the hot air is lighter, it tends to rise. This can lead to a significant temperature difference between the floor and the ceiling.
Figure 2 (top) depicts a space with a stratification problem. The thermostat is set to 65 degrees F, and the heaters operate to maintain that temperature at the height of the thermostat. The hot air supplied by the heaters rises and accumulates at the ceiling, resulting in 10 degrees F of stratification.
A destratified space has only a slight temperature difference from floor to ceiling (Figure 2, bottom). Heating air that rises to the ceiling wastes energy. When that heated air is lost through the roof, even more energy is wasted.
Figure 3 shows the fluctuating temperature profile of a typical stratified space (A) and the consistent profile of a destratified space (B).
Air movement significantly influences both comfort and productivity. While high-speed air movement can have a negative effect on comfort in cold temperatures, more air movement is considered beneficial as temperatures rise. When the operative temperature, the temperature that occupants actually feel inside a space, reaches about 75 degrees F, more air movement adds to comfort (see Figure 4). Above 77 degrees F, each degree increase in operative temperature equates to a 1 percent decrease in productivity.3
Operating at a high speed setting, large-diameter, low-speed fans can offer 4 to 8 degrees of cooling effect, which will increase employees’ comfort and productivity (see Figure 5).
Airfoil design directly influences the way air is moved and should be closely scrutinized when investigating fan systems.
The blades of a conventional ceiling fan generally consist of a flat plate, set at a 10-degree angle. While this design is simple and inexpensive, disadvantages arise as the size and power of the fan increase.
Structurally, thin, flat blades of some ceiling fans are more than adequate to create airflow for 2 or 3 feet. As blade length increases, however, the blades may no longer be rigid enough to support their own weight, and they may begin to droop downward.
Blades may also flex upward because of air pressure on the lower surface, which increases the risk of striking an object during operation. Repeated flexing also increases the likelihood of blade fatigue and failure.
Some ceiling fan blades may have aerodynamics issues as well. As a flat blade moves through the air, its angled lower surface pushes the air downward and creates a vacuum on its upper leading edge. The vacuum pulls air in all directions and results in turbulent, low-pressure air that causes the blades to operate inefficiently and reduces the effectiveness of the fan. This situation becomes even more difficult as the blade gets thicker, because the flat front blade edge increases its drag and the turbulence in its wake.
The blades of a typical large-diameter, low-speed fan are made of extruded aluminum and formed in the shape of a hollow airfoil. The substantial vertical thickness of this shape lends rigidity to the blade—enough to support blade lengths up to 12 ft. without significant deflection either at rest or under load. The hollow cross section makes the blades lighter than a blade made out of solid wood.
With a rounded nose and smoothly curving upper and lower surfaces, the blade allows air to flow smoothly over it and generate less turbulence in the wake, allowing the other blades to operate with greater efficiency.
Unlike a flat blade that runs forward or in reverse, an airfoil is designed to pass through the air in one direction. It loses its efficiency when reversed.
Many people believe that an industrial fan should be run backward for destratification in the winter, but this is not necessarily the case. This approach is used for household ceiling fans, which have flat blades instead of airfoils and motors that cannot run slowly enough to avoid creating drafts when run in the forward direction.
With airfoils and the sophisticated electronic motor control systems used on modern large-diameter, low-speed fans, draft-free destratification is achieved more efficiently by running the fan forward at a very slow speed. In addition to the greater airfoil blade efficiency, the slower motor speed uses less energy than a flat-bladed fan running faster in reverse.
1. R. Mason et al., “Advanced Coatings and Processes for Field and Depot Corrosion Repair of Military Hydraulic Components,” in proceedings from NACExpo 2006, 61st Annual Conference and Exposition, March 2006.
2. 2003 ASHRAE Applications Handbook, pp. 4-6.
3. Olli Seppanen, William J. Fisk, David Faulkner, “Cost Benefit Analysis of the Night-time Ventilative Cooling in Office Building,” Lawrence Berkeley National Laboratory, LBNL Paper LBNL-53191, 2003.
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