Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!ncar!umigw!rsmas!miller
From: miller@rsmas.miami.edu
Newsgroups: comp.sys.mac.hardware
Subject: Re: Mac temperature, fan, dust.
Message-ID: <8999.26d534ec@rsmas.miami.edu>
Date: 24 Aug 90 18:08:43 GMT
References: <9013@ur-cc.UUCP> <6910N3w162w@tosh.UUCP> <9053@ur-cc.UUCP> <2125@ux.acs.umn.edu>
Lines: 62

In article <2125@ux.acs.umn.edu>, clarson@ux.acs.umn.edu (Chaz Larson) writes:
> In article <9053@ur-cc.UUCP> carlo@eagle.cvs.rochester.edu (Carlo Tiana) writes:
>>In article <6910N3w162w@tosh.UUCP> you write:
>>>> So I decided to turn the fan around, so as to have control over where
>>>> [...]
>>>> Is what I am doing a big no-no?
>>>
>>>Yes. Return the fan to its original position at once. It was designed to
>>>cool the machine. In the configuration you have it in, it is currently
>>>heating it.

This simply cannot be the case unless the outside air is hotter than the inside
air.

>>
>>Now why would that be the case? 

Why indeed. Read on.

> 
> Seems obvious to me.  In the standard configuration, the fan draws outside air
> into the front of the computer, pulls it through the interior of the machine,
> and exhausts the hot air out the back.
> 
> When the fan is reversed, it draws outside air in through the back, blows it 
> across the hot power supply [which heats it] and then into the interior of the
> machine, very likely increasing the internal temperature.
> 
> chaz

Ok, Chaz. Let's consider a little thermodynamics here. The fan, in its normal
orientation, sucks a certain amount of air out of the box in a given period 
of time. If you reverse the fan, it still moves the same amount of air in that
same amount of time only the air moves in the opposite direction. In other
words, since the volume of the box does not change, we get the same volume of
air exchanged with the outside regardless of which way the air moves.

During that time, the Mac generates a certain total amount of heat. That heat 
is transfered from the Mac's components to the flowing air and is removed from
the box either thru the fan or thru the vents depending on the orientation of
the fan. Either way, the same total amount of heat is removed from the box.

Now, if the fan is sucking air out of the box, a component residing near  
vent will be cooler than a component residing near the fan. If the fan blows
air into the box, the opposite will be true. Thus, individual components may be
hotter or cooler depending on which way the wind blows even though the total
amount of heat removed from the box is the same. How much hotter or cooler?
Putting one hand near a vent and the other near my fan I would guess the
difference is less than 10 or 15 degrees. So, unless some component is already
operating right at its heat tolerance, I suspect that there is no effective 
difference in the cooling capability of an inward versus an outward blowing
fan.

HOWEVER, if one puts a filter over the fan resulting in decreased airflow, all
bets are off. I suggest that a filter combined with stronger fan blowing 
into the box might be a better solution to the combined heat & dust problem.

-------------------------------------------------------------------------------
Jerry L. Miller         INTERNET:  miller@rsmas.miami.edu
Rosenstiel School of Marine and Atmospheric Science, University of Miami
Remember: "It's later now than it's ever been; however, vg'f zber yvxr
           vg vf abj guna vg unf rire orra orsber."