Re: Cooling Lamps -- Or - Finding direcction when things really suck

["S. Hieber" <shieber at yahoo_com>]

Paulio629 at aol_com said:

> I am getting ready to put some fans in my canopy. Since I
> started overdriving
> things are a BUNCH warmer in there. What is the best way,
> push or pull?
> Do I blow air over the bulbs or draw it out of the
> canopy?


Is a pencil better than a pen.  There are advantages and
disadvantages to each, all of which stem from the fact that
the airflow into a fan is virtually nondirectional, while
the flow out of the fan is very directional.  From a short
distance away form the input side of the fan, you will fell
very little air current but not so on the output side.  If
you try this with a length of ribbon, the ribbon will wave
in the air only on the output side.

So why does this matter?  If you use the fan for suction,
you have to be sure that the air flow through the chamber
has few leaks, or the air will be drawn in from the leaks,
go straight to the fan without passing over the heating
elements (i.e., the bulbs).  If you blow, leaks matter less
because the directional flow tends to force air where you
point it.  Of course it's good to have the exhaust hole at
the opposite of the chamber as the fan, and having no leaks
means all the air passes out the exhaust.  But leaks have
less impact with blowing -- especially if the exhaust hole
is as as large or larger than the air input opening.

Generally, blowing is the easiest to do well, especially if
the chamber wasn't originally designed as an airflow box.

You didn't ask but here's another tip if you're using PCs: 
Either way, suck or blow, you have to decide whether you
want to have the air first flow over the cool or the hot
end of the bulbs.  One way yields more energy efficient
cooling, the other way more efficient lighting.  If the
cool air passes over the hot end first, you will be pushing
heated air over the cool end.  This will tend to even out
the bulb temps end to end and ensure that you don't
overcool the "cool" end of the PC.  It will also mean that
some of the heat form the hot end is not pushed out of the
chamber but "into" the other end of the bulb -- no the the
most efficient cooling.  If you do it the other way, blow
over the cool end first, the cool end might be too cool but
the hottest air will not pass over the full length of the
bulb.  This will provide the most efficient cooling but not
necessarily the most efficient lighting.  Flouresecent are
most efficient within a certain ranges of bulb temps
(around 120 degrees F) and below that, light output
generally decreases.

With a 4" fan and a 55Watt PC, blowing over the cool end
first, you can actually see a difference in light output
and color temp from one end of the bulb to the other, even
more so with a 96 watt bulb.

With T8s and T12s, both ends are hot so you can guess what
that means. ;-)


One final point:  Almost all of the air moved by an axial
or tube-axial fan is near the blade tips, so do not do
anything to obstruct or impede air flow near the edges if
you can avoid it.  This is also part of the reason that
large fans move much more air than slightly smaller fans. 
Large fans moving slowly move more air and do it quieter
than smaller fans, other things being equal.

Axial fans have the best air-flow-volume per decibel ratios
when dealing with the size of fans one might put on an
aquarium.

Cheap bearings are noisier and shorter lived than quality
bearings -- quality counts for more than whether the
bearings are sleeve or ball type.  Having said that,
*really* good ball bearings can be nearly silent and last
almost forever.  Cheap fans are like sunburns, you can get
them quickly but they end up being a pain and are rather
short lived.

Good Luck,
Scott H.

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