I agree with Peter's reasons. As far as a vacuum moving more air than a
positive displacement system (minor math ahead):
In a duct, the pressure drop (corrected to standard air density and pressure)
is equal to the resistance of the duct times the flow squared (sigma_delta_p =
z * w^2). If you solve for flow (w), the flow is equal to the square root of
the pressure drop divided by the resistance (meaning the flow is proportional
to the pressure differential). The air doesn't care whether it is being
sucked or blown (woops), it is the pressure differential that drives the flow.
In applications where a high powered electric fan is used (got one here that
is 27 kilowatts, for instance), the fan will be "behind" the component as a
draw thru system because the heat of the fan motor will enter the airstream
and heat up the component. The 27 kW unit I mentioned adds 21 degrees to the
airstream at its design point.
|