REFLECTOR: What is a fan

[Brian Michalk]

There seems to be some confusion about the types of fans we are talking 
about.  There is an article on Wikipedia:
http://en.wikipedia.org/wiki/Mechanical_fan

Axial fan, like the cheap box fan from WalMart:
The axial-flow fans have blades that force air to move parallel 
<http://en.wikipedia.org/wiki/Parallel_%28geometry%29> to the shaft 
about which the blades rotate. Axial fans blow air along the axis of the 
fan, linearly, hence their name. This type of fan is used in a wide 
variety of applications, ranging from small cooling fans for electronics 
to the giant fans used in wind tunnels 
<http://en.wikipedia.org/wiki/Wind_tunnel>. Axial flow fans are applied 
for air conditioning and industrial process applications. Standard axial 
flow fans have diameters from 300--400 mm or 1800 to 2000 mm and work 
under pressures up to 800 Pa.

Centrifugal fan: works on the principle that air has mass, like the 
compressor turbine in a turbocharger.

Often called a "squirrel cage" (because of its similarity in appearance 
to exercise wheels <http://en.wikipedia.org/wiki/Hamster_wheel> for pet 
rodents) or "scroll fan", the centrifugal fan has a moving component 
(called an impeller <http://en.wikipedia.org/wiki/Impeller>) that 
consists of a central shaft about which a set of blades, or ribs, are 
positioned. Centrifugal fans blow air at right angles to the intake of 
the fan, and spin the air outwards to the outlet (by deflection and 
centrifugal force <http://en.wikipedia.org/wiki/Centrifugal_force>). The 
impeller rotates, causing air to enter the fan near the shaft and move 
perpendicularly <http://en.wikipedia.org/wiki/Perpendicular> from the 
shaft to the opening in the scroll-shaped fan casing. A centrifugal fan 
produces more pressure for a given air volume, and is used where this is 
desirable such as in leaf blowers 
<http://en.wikipedia.org/wiki/Leaf_blower>, blowdryers 
<http://en.wikipedia.org/wiki/Blowdryer>, air mattress inflators, 
inflatable structures 
<http://en.wikipedia.org/wiki/Inflatable_structure>, climate control 
<http://en.wikipedia.org/wiki/Climate_control>, and various industrial 
purposes. They are typically quieter than comparable axial fans.


      Crossflow fan

<http://en.wikipedia.org/wiki/File:Tangential-flow-coloured-labels.png>
<http://en.wikipedia.org/wiki/File:Tangential-flow-coloured-labels.png>
Crossflow fan

The /crossflow/ or /tangential/ fan, sometimes known as a /tubular/ fan 
was patented in 1893 by Mortier, and is used extensively in the HVAC 
<http://en.wikipedia.org/wiki/HVAC> industry. The fan is usually long in 
relation to the diameter, so the flow approximately remains 
two-dimensional away from the ends. The CFF uses an impeller with 
forward curved blades, placed in a housing consisting of a rear wall and 
vortex wall. Unlike radial machines, the main flow moves transversely 
across the impeller, passing the blading twice.


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