>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.
>
Good point. Also. every 1000' elevation decreases air density c. 3%.
>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.
>
The air that is pushed through a SB-220 has a temp rise of less than 40ºF
on SSB. The TL-922 uses an exhaust fan config. to remove c. the same
amount of heat, and I have heard of no fan failures.
>> 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 TL-922 has a cooling system that pulls air through the cabinet and
the 3-500Z temps are quite safe.
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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