In the axial flow
ventilator the air must blow in line with the axis of rotation and
there must be blades with space between them. With few
exceptions, the axial blower or fan must not have a lot of back
pressure and will work a lot better if the blade is shrouded. One
obvious exception is the turbine compressor used in jet engines, but
this requires very tight fittings and very high speed so the air is
being rammed in against the outgoing air much faster than it can move -
it uses a lot of power and makes a lot of noise. Shrouding works
better because the greatest loss in an unshrouded fan is the air coming
off near the tips and immediately turning out and back circulating only
through the fan and not through the room. The most common shrouded fan
people see is muffin style fan in computers. An axial blower can
have multiple sets of blades and the blades can even over lap so light
is not visible through the fan. Fans are used most often for
moving large quantities of air between spaces at atmospheric pressure.
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