[Amps] Muffin Fans and hot exhaust

[Ian White, G3SEK]

Expanding on some of Tom's very sensible comments:

>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.
>
So the question is: does your amp have significant back-pressure?

If it has a ceramic tube where all the air has to be forced through the 
restriction of the cooler, the answer is YES. You need a centrifugal 
blower.

If it has a big glass tube where the air can flow freely around the 
outside, then the answer is probably not - you can use a fan, so long as 
you don't build in extra pressure drop by making the holes through the 
shielding too small and/or too few.

>
>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.
>
Especially bearing in mind that the fan motor is in the hub, right in 
the hot air flow. Most blower motors hang out the side in much cooler 
air.

>
>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,

It depends how close the blower is to "stalling", which has a different 
meaning for a blower than it does for a car engine. When a blower 
"stalls" it continues to run, but it fails to move air - it just sits 
there and churns.

Bear in mind that an amp designer doesn't use a custom-designed blower. 
He can only choose a model from the available range, and maybe the next 
size up won't fit inside the amp. This means that some amps are 
generously provided, but others may be marginal. If the blower is a good 
size, operating well away from the stall point, an extra fan may not 
increase the total air flow at all. On the other hand, if the blower is 
perilously close to stall (especially a few hundred hours in, when the 
tube cooler is partly blocked with dust) then an extra fan will work 
wonders.


>plus it ensures the cool inlet area has a positive
>pressure keeping hot exhaust air from leaking back into the cool
>inlet area.
>
Another important factor is the efficiency of heat transfer. For highest 
efficiency the air needs to be turbulent, so that it scrubs away the 
insulating layer of hot air at the surface of whatever you're cooling, 
and mixes it into the bulk of the airflow.

Many amps these days have the whole-box air intake right behind the 
transformer, so that all the inflowing air passes over it. That's good, 
but it isn't perfect because the air is flowing quite gently over the 
transformer with little turbulence. When you use the option of 
installing a fan over the air intake, you probably aren't adding much 
air flow, but now you're creating lots of turbulence which cools the 
transformer and the whole power supply compartment much more 
effectively.


-- 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)