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Re: Apple IIGS Cooling

Paul G. wrote:

On Apr 11, 8:38 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:


Paul G. wrote:


On Apr 10, 11:27 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:



No, not a loop.



The "laptop coolers" are about 10"x9"x3/8" with two very thin, quiet,
medium-speed fans that draw air into the narrow case and vent it out
one of the long sides.



When one is placed upside down on top of a IIgs case, hot air is drawn
upward through the case lid and then exhausted gently backward (or
forward if you are cold ;-), where natural convection ensures that it
will not reenter the IIgs.  The hot air withdrawn is replaced by cool
outside air coming in from all the other holes in the IIgs case (most
of which are on the bottom).  The power supply will be in a sea of
cooler air, with the slight pressure differential providing assisted
convection cooling.



Ah, OK, so the "laptop cooler" exhaust is at 90 degrees to the intake,
that makes all the difference. My mistake, I thought they had axial
flow.  I have to disagree on the utility of the bottom vent holes,
though- they are there, but effectively negated by the metal RF
shield.  Someone wasn't clear on the concept of convective cooling-
the big vents need to be on top, as the volume of the hot exhaust air
is greater than the volume of the cool air coming in the bottom...


Actually, the volume of air entering the case--mostly from the bottom--
is *exactly* the same as the volume of air leaving via the top.

Otherwise, we'd soon have a vacuum in the IIgs and it would implode.
;-)

Air can quite easily flow around obstacles at low speed, and that is
quite enough to change the air often enough to keep a IIgs cool.

If you don't believe it, allow some smoke to waft around the case
with a cooling fan on and watch the flows and rates.

It is undeniable that the primary place where hot air collects is
at the top of the case, and that's exactly the air that is removed
by an "exhaust" fan on top of the case.



Heh heh!  Now this is fun.


;-)


I disagree- cold air is denser than hot, therefore the volume of hot
air exiting at the top is greater than the volume of cold air entering
at the bottom- air expands when heated. Therefore the holes at the top
should be larger than those at the bottom.


Fair enough.  I should have said the *mass* of air.

The volume of heated air exiting is a few percent more than the volume
of cooler air entering the case.  For our purposes--determining the
effective air flow--that effect is minimal.


I also say the "primary place where hot air collects" is inside the
powersupply. ;-)  That's why I mounted my fan on the right front side
of the powersupply, where Apple helpfully supplied fan mounting
holes.


That fan not only compensates for insufficient convection cooling of
the power supply but it also creates an upward stream of air directed
at the left front quadrant of the top cover, and so assists overall
cooling.

Lowering the temperature of the air surrounding the power supply
will also significantly improve the transfer of heat from the power
supply to the outside air, even without a fan directed into the
power supply.


I just took a careful look at one of my IIgs's. There is a nearly
solid piece of sheetmetal across the bottom. At first glance it
appeared that there were some vent holes directly under the P/S, but
it turned out that the speaker is mounted over them, so they are for
sound.  The motherboard is also nearly solid. Lastly, the lid has
about 1/3 of the surface area covered with a solid RF shield.  It's a
very poor design for cooling, even with a fan.  Of course a fan is
going to help a lot over simple convection.


Well, theory has just yielded to practice...  ;-)

After a closer inspection of the IIgs case (but without removing the
main board), it appears that no clearances or slots were designed for
getting air from under the machine into the machine!  Amazing, for a
supposedly convection-cooled design.

If the only perforations in the case that can be counted on are in the
top, then, considering the lineup of perforations in the power supply
case, the optimal place for an external exhaust fan is over the left
front corner of the top of the IIgs case.

That will assist the convective flow from the power supply, and will
cause cool external air to flow *into* the case from the back of the case on both sides. This looks like the best that could be achieved 
with airflow only from the top.

I've never had a heat problem in my IIgs, but if I did, I would remove
the metal shield from the bottom of the case to allow air to flow in
better from the bottom.  I know several people have suggested removing
the metal shield from the top of the case, but *if* warm air is being
exhausted from the left front, then assisting flow of outside air from
the right front might simply reduce airflow elsewhere, where the heat
density is greater.

I also note that installing the "IIgs cooling fan" on the side of the
power supply would effectively force a stream of air up from the power
supply toward the left front quadrant of the case.  The combination of
such a fan and an external exhaust fan in that quadrant should produce
very effective cooling.

I surrender to the data.  Does anyone have temperature-measuring
methods that could be used to find the best cooling solution for
various situations?  Unfortunately, there are a lot of variables
for precisely the cases that matter most: several high-powered
cards.  (But, in general, it seems that exhausting from the left
front of the top, to draw external air into the back of the top,
would result in pretty effective heat removal from cards.)

And a exhaust fan would be most effective if blows the warm
air out in such a way that it does not reenter the top rear of
the case--the left side would appear to be the best choice.

-michael

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