First, the difference between a fan and a blower is in the
construction. Fans have blades that work by pushing air with a small
pressure drop, blowers generally are centrifugal that spin an
impeller. Some blowers can be pretty puny in pressure differential,
smaller than some fans, but as a general rule they work better
against back pressure if the impeller is designed for pressure rather
than maximum flow volume.
> Am I correct in thinking that hot air is less dense? How does this
> affect the ability of a given fan at a given rpm to move the weight
> of air?
Hot air is less dense. But we have to view heat in absolute
temperature, where a doubling of temperatures halves the density.
An increase in temperature from 40F to 80F is only an increase from
about 490 to 530 degrees absolute, so it is a small change in
density.
The real problem with heat is it can damage the air mover. The
question was about 4-400A's which means the dissipated power would
likely be several hundred watts. Without a substantial volume of air,
the temperature would be hundreds of degrees F. Probably well beyond
the design limits of most fans.
> I am assuming that it's the weight of the air that matters in cooling,
> on the basis that the amount of heat needed to raise the air
> temperature is the mass of the air times the specific heat of the air.
> Which specific heat I'm not sure - constant pressure or constant
> volume. I suspect the former.
The problem with putting a fan on the outlet is most fans do not have
substantial pressure differential. The internal blower or fan, as a
general rule, is operated nearly at a stall against the back pressure
of the air system. Adding a fan on the outlet generally does almost
nothing to improve airflow, unless that fan or blower is huge
compared to the blower or fan in the unit (which also means very
noisy) and develops a significant reduction in inlet pressure. Most
blower and fans are not designed that way because most are designed
to operate "pushing" air with the inlet at about one atmosphere,
although I suppose we could find a "puller" if we looked.
It is much more efficient to force air into the PA as long as the
blower or fan has more volume than the internal fan or blower because
this increases the blower or fan inlet pressure on the fan or blower
that is almost stalled against the system back pressure.
This translates to a much higher outlet pressure than you might
expect from the fan or blower that is dead headed against the
restriction, plus it ensures the cool inlet area has a positive
pressure keeping hot exhaust air from leaking back into the cool
inlet area.
This entire thing is much different than a 100 watt radio with a
large low-restriction large physical surface area heatsink.
73, Tom W8JI
W8JI@contesting.com
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