Axial fans push high volumes of air at low pressures.
Propeller-style fans are used in many machines. Regardless of their use, the propellers on airplanes, boat motors and furnaces are designed on the same set of physical properties. Fan blades are curved, and grab and push the air around them as the fan spins. As the propellers rotate, they cause a number of predictable effects on air or water.
Pressure vs. Volume
Fans designed with individual fan blades, or axial flow fans, are incapable of developing high fluid pressures. These type fans move high volumes of fluid efficiently, whether the fan is immersed in air or liquid. However, they won't develop a great amount of pressure. In general, these fans are economical to produce and possess average efficiency when comparing the energy used to the volume of air moved. Most of feature large hubs and airfoil-shaped blades that aren't close together.
Fan Shrouds
The efficiency of household fans, such as a freestanding floor fan or a fan inserted into an open window, is affected by the shroud built around the fan blades. Fans that are enclosed in a simple wire mesh lose much of their efficiency because the air is able to roll off the tips of the blades radially. Consequently, these fans create a disturbed, turbulent air flow around the edge of the fan which lessens the amount of air that's propelled directly forward. By adding a shroud around the fan, the fan's efficiency increases significantly. Even the spacing between the tip of the fan blade and a shroud is an important factor when designing the fan's efficiency. By reducing the clearance between the shroud and the fan blade tip by 3 percent to 1 percent, designers can expect to yield about a 10 percent improvement in flow, according to the BASF Corp.
Swirling Turbulent Air
The air propelled by an axial fan is moved forward in a swirled pattern. The air travels out of the fan following a corkscrew motion. This is one of the reasons that a propeller fan moves high volume of air, but doesn't develop high pressure. The fan blades create too much turbulence in the air without any control over the air movement after it leaves the fan blades. This swirled pattern loses pressure quickly and the energy dissipates. The swirling is diminished if the fan blows through a louvered grid.
Sound Waves
Propeller style fans also create a higher noise level than other styles of fans, such as a squirrel cage fan found in most modern furnaces. Noise is the direct result of the blades striking the air and propelling it into the room. If the fan blade was removed from the electric motor that powers the fan, the motor makes very little noise. However, as the blades impact the air, the air is compressed and propelled forward in waves, which generates sound waves.
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