Thermal control for Q1 / Q2 in KPA100?

[Darwin, Keith]

As I'm thinking about trying to operate the K2/100 so the fan doesn't
run (or doesn't make noise) I wonder some things.
 
First, blowing air out of a box cools the box better than blowing air
into it.  I'm half surprised the fan blows in.  I suppose it is to
direct the stream of air on the finals.  Still, I wonder about doing it
differently.
 
What about thermal grease?  There is only a non-grease pad between the
transistors and the heat sink.  Would a dab of grease conduct more heat
away from the transistor?  I have some fancy "silver" stuff left over
from my computer assembly activities.
 
What power level should I run if I don't want the fan to come on?  75
Watts?  50, 30, 12?
 
- Keith N1AS -
- K2 5411.ssb.100 -
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[Ron D'Eau Claire-2]

The question of grease vs. pads came up before and Eric answered it as
follows:

We officially recommend that you use the thermal pads supplied with the
KPA100 kit instead of grease. They consist of a very efficient thermally
conductive compound coated on both sides a a very thin aluminum sheet. The
compound reflows when the transistors get hot the first time and fills all
of the small air gaps between the devices and the heatsink. It is very
effective and provides a better thermal bond in most cases than manual
application of thermal grease.

73, Eric  WA6HHQ
Elecraft
=========

You aren't the only one to be bothered by what I call the "whizzer" on the
back of the K2, although I confess I'm getting used to it. Sort of...

The fan doesn't just blow air into the "box", it forces air into the space
between the KPA100 shield and the KPA100 heat sink, which includes the
KPA100 PCB of course. The air exits at the end of the heat sink nearest the
front panel, flowing out through the spaces between the heat sink fins.

One thing some builders have done is to put a large muffin fan on top of the
heat sink. A larger, slower speed fan should be a lot quieter.

Of course, how quickly the fan comes on depends upon the ambient temperature
in the shack and the duty cycle as well as the power level and efficiency of
the finals. I don't know if anyone has run tests to see what the minimum fan
action has to be, but I know that I've run tests with my K2/100 that
involved keying it so much I couldn't leave my hand on the heat sink. No
harm was done to the KPA100 in those tests.  

Ron AC7AC


-----Original Message-----
As I'm thinking about trying to operate the K2/100 so the fan doesn't run
(or doesn't make noise) I wonder some things.
 
First, blowing air out of a box cools the box better than blowing air into
it.  I'm half surprised the fan blows in.  I suppose it is to direct the
stream of air on the finals.  Still, I wonder about doing it differently.
 
What about thermal grease?  There is only a non-grease pad between the
transistors and the heat sink.  Would a dab of grease conduct more heat away
from the transistor?  I have some fancy "silver" stuff left over from my
computer assembly activities.
 
What power level should I run if I don't want the fan to come on?  75 Watts?
50, 30, 12?
 
- Keith N1AS -
- K2 5411.ssb.100 - _______________________________________________

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[Tom Nickisch]

Hi Keith,
   
  The thermal pads actually conduct more heat away than thermal grease.  The pads used these days are quite impressive.  A couple of years ago an engineer Friend of mine and I had this discussion and he then went out and dug up enough data to have me buried for a week.
   
  Tom W8AMZ
 

"Darwin, Keith" <[hidden email]> wrote:
  As I'm thinking about trying to operate the K2/100 so the fan doesn't
run (or doesn't make noise) I wonder some things.

First, blowing air out of a box cools the box better than blowing air
into it. I'm half surprised the fan blows in. I suppose it is to
direct the stream of air on the finals. Still, I wonder about doing it
differently.

What about thermal grease? There is only a non-grease pad between the
transistors and the heat sink. Would a dab of grease conduct more heat
away from the transistor? I have some fancy "silver" stuff left over
from my computer assembly activities.

What power level should I run if I don't want the fan to come on? 75
Watts? 50, 30, 12?

- Keith N1AS -
- K2 5411.ssb.100 -
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[Darwin, Keith]

Whizzer - what a good name.  It actually startled me when it came on and
the noise interfered a bit with copy of the weak station I was working.
I may have to go to closed headphones.

The MEs here at work tell me similar things.  Some of the pads are
better than grease.  Good, grease is a PITN (neck) to work with.

I'm also glad to hear the pad reflows when it gets hot the first time.
That answers a concern I had that maybe things just aren't tight enough
yet.

I have a few muffin fans left over from a PC build-up.  I think I'll set
something up.  The 60 deg air in my basement plus some fan-action should
keep the whizzer under control ...

Thanks Ron & others.

73!

- Keith N1AS -
- K2 5411.ssb.100 -

-----Original Message-----
From: Ron D'Eau Claire [mailto:[hidden email]]

You aren't the only one to be bothered by what I call the "whizzer" on
the back of the K2, although I confess I'm getting used to it. Sort
of...

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[DaveL G3TJP]

I was playing around with the KAT100 a couple of nights ago and was leaving the K2/100 on tune for periods of 15 - 20 seconds at a time (20W) at intervals through several bands.  After a few minutes, the whizzer fired up its engine, so if your average Tx power is 20W then you have maybe 5 mins before the little hovercraft starts up.

I note it blows air INTO the KPA, but there doesn't appear to be much by way of provision for the warmed air to get out, so I guess oscillators are going to move around quite a bit when the warm air circulates around and around.  Maybe a nice, quiet ex-computer muffin fan sitting on top of the heatsink might be a better way of cooling the PA?

73 Dave L  G3TJP
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[Don Wilhelm-3]

Dave,

In heat removal, the name of the game is air circulation, which is different
than just 'blowing air' - there must be an escape path for the heated air.
In the KPA100 that path is from the back to the front of the unit, the air
escapes through the heatsink fins just behind the top of the front panel.
If you have something (even a piece of paper) on top of the KPA100, it will
block this air flow and reduce the cooling capacity - which in turn will
cause the fan to come on earlier than expected.  If you have the KAT100
stacked on top of the KPA100 I suggest changing things to place the KAT100
below the K2/100.

An easy way to add an auxiliary fan to the KPA100 is to place a muffin fan
on top of the heat sink - pulling air away from the heat sink.  Even a slow
moving fan will help, a DC fan salvaged from a computer will do nicely and
you can slow it down with a series resistor.

73,
Don W3FPR

> -----Original Message-----
>
> I was playing around with the KAT100 a couple of nights ago and
> was leaving the K2/100 on tune for periods of 15 - 20 seconds at
> a time (20W) at intervals through several bands.  After a few
> minutes, the whizzer fired up its engine, so if your average Tx
> power is 20W then you have maybe 5 mins before the little
> hovercraft starts up.
>
> I note it blows air INTO the KPA, but there doesn't appear to be
> much by way of provision for the warmed air to get out, so I
> guess oscillators are going to move around quite a bit when the
> warm air circulates around and around.  Maybe a nice, quiet
> ex-computer muffin fan sitting on top of the heatsink might be a
> better way of cooling the PA?
>
> 73 Dave L  G3TJP
>

--
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[Vic K2VCO]

David Lankshear wrote:

> I note it blows air INTO the KPA, but there doesn't appear to be much
> by way of provision for the warmed air to get out, so I guess
> oscillators are going to move around quite a bit when the warm air
> circulates around and around.  Maybe a nice, quiet ex-computer muffin
> fan sitting on top of the heatsink might be a better way of cooling
> the PA?

The air gets out at the front of the heatsink, and blows across the fins
a bit.  I like mine to run extra cool, so I attached a muffin fan to the
right side of the heatsink, sucking air up through the fins.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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[srife]

        From the description of the noise that the fan on your KPA-100 is
making, I'd guess you have a defective fan. I never hear the fan on my
KPA-100. I have a pretty quiet shack, and operate with the volume fairly low
on the K2 and I have only noticed the fan running a few times.


Stan Rife
W5EWA
Houston, TX
K2 S/N 4216
 

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Darwin, Keith
Sent: Tuesday, December 05, 2006 12:52 PM
To: [hidden email]
Subject: RE: [Elecraft] Thermal control for Q1 / Q2 in KPA100?


Whizzer - what a good name.  It actually startled me when it came on and
the noise interfered a bit with copy of the weak station I was working.
I may have to go to closed headphones.

The MEs here at work tell me similar things.  Some of the pads are
better than grease.  Good, grease is a PITN (neck) to work with.

I'm also glad to hear the pad reflows when it gets hot the first time.
That answers a concern I had that maybe things just aren't tight enough
yet.

I have a few muffin fans left over from a PC build-up.  I think I'll set
something up.  The 60 deg air in my basement plus some fan-action should
keep the whizzer under control ...

Thanks Ron & others.

73!

- Keith N1AS -
- K2 5411.ssb.100 -

-----Original Message-----
From: Ron D'Eau Claire [mailto:[hidden email]]

You aren't the only one to be bothered by what I call the "whizzer" on
the back of the K2, although I confess I'm getting used to it. Sort
of...

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[Darwin, Keith]

-----Original Message-----
From:  Stan Rife

        From the description of the noise that the fan on your KPA-100
is making, I'd guess you have a defective fan. I never hear the fan on
my KPA-100. I have a pretty quiet shack, and operate with the volume
fairly low on the K2 and I have only noticed the fan running a few
times.
--------------------------

On low speed I don't hear the fan at all.  On high it is definitely
heard.  It is louder than some but not all computer muffin fans I've
had.

As a musician I love a noisy fan.  As a ham, I don't.

One of my gripes about the IC-735 was the fan noise.  One of the things
I love about TenTec stuff is that they need no fan.  Well, the one's
I've owned didn't.  Of course the K2 is a small package with a small
heat sink.  Ya gotta get rid of the heat somehow and a fan is the way to
do it given the size constraints of the system.

I'm wondering about using a larger (quieter) fan with some sort of duct
system to get the airflow from the fan to the fan-port on the back of
the rig.  It could be run off the same fan wires that run the
mini-whizzer and would probably move more air.

The first step with be to run a bit reduced power and see what that
gives me.  If it continues to be a annoyance, I'll plop a muffin fan on
top to keep 'er cool & quiet.

- Keith N1AS -
- K2 5411.ssb.100 -

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[Tom Hammond-3]

Hi Dave:

You wrote, regarding the KPA100 cooling fan...
>I note it blows air INTO the KPA, but there doesn't appear to be
>much by way of provision for the warmed air to get out, so I guess
>oscillators are going to move around quite a bit when the warm air
>circulates around and around.

The 'cooling' air exits out the top of the KPA100 assembly, at the
front of the heatsink fins AND out the bottom of the K2 case. Pretty
LOW VOLUME.

>Maybe a nice, quiet ex-computer muffin fan sitting on top of the
>heatsink might be a better way of cooling the PA?

I've made a cooling 'plenum' for my K2, using a very lo-profile cover
and a tiny muffin fan from a CPU cooler. It pulls cool air in from
outside across most of the heatsink fins and exhausts straight up. I
can supply illustrations and some info on the design if you, or
anyone else might be interested.

Cheers,

Tom   N0SS

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[Don Wilhelm-3]

Tom,

Yes, we should all see what you have created.  How about a photo and a short
note on your website (no need for a .pdf file IMHO).  Such an addition is
great for those running digital modes and is nice to have when operating hot
and heavy during a contest.

As you have pointed out, it is important that the fan moves air upward so it
aids the flow from the internal fan rather than blowing air onto the heat
sink. Just laying a muffin fan on top of the heat sink is effective as a
'quick and dirty' solution, your plenum sounds like a great permanent
addition for those instances where there is not a lot of free-air space
above the K2/100.

The fact that an exhaust fan flow cools objects better than blowing air onto
a surface is a bit counter-intuitive because standing in the downstream side
of a fan feels cooler to the skin - but that is only because of evaporation.
Careful measurement will show that the cooling effectiveness for an object
like a heat sink is more effective by moving the heated air away from the
object.  Getting the heat away is the objective, and blowing on it is less
effective than exhausting the heated air.

73,
Don W3FPR

> -----Original Message-----
>
> I've made a cooling 'plenum' for my K2, using a very lo-profile cover
> and a tiny muffin fan from a CPU cooler. It pulls cool air in from
> outside across most of the heatsink fins and exhausts straight up. I
> can supply illustrations and some info on the design if you, or
> anyone else might be interested.
>
> Cheers,
>
> Tom   N0SS
>
>

--
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[Darwin, Keith]

Very key point.  If you want to take heat out of a box, you're far
better to point the fans out and let them draw in cool air from the
other side than to point the fan in and "blow cool air in".  Here's why.

When you blow a fan into a box (K2, PC, or your house in the summer)
you're blowing into a confined space and the pressure goes up.  That
increase in pressure fights against the fan and reduces the amount of
air that flows.  Mechanical efficiency and air transfer go down.  Turn
the fan around and blow it out of the box into the "universe".  The
outside world cannot be pressurized by the fan so the fan moves air more
effectively.

And I believe the air being blown into the box helps to set up dead
zones where hot air can pool up.  The flow through the box is better in
the exhaust case.

I've tested the theory both with my computer and with using fans to cool
my house in the summer.  In both cases, better cooling is achieved with
fans blowing out than in.

Makes me wonder about reversing the fan in the K2.  It just might make
it more effective and allow it to say on low longer.  I have to open it
up to tighten the HW and insert one more shield spring so I'll get
another chance to give it a good lookin' over.

- Keith N1AS -
- K2 5411.ssb.100 -


-----Original Message-----
From: Don Wilhelm

The fact that an exhaust fan flow cools objects better than blowing air
onto a surface is a bit counter-intuitive because standing in the
downstream side of a fan feels cooler to the skin - but that is only
because of evaporation.
Careful measurement will show that the cooling effectiveness for an
object like a heat sink is more effective by moving the heated air away
from the object.  Getting the heat away is the objective, and blowing on
it is less effective than exhausting the heated air.

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[Mike Harris-9]

G'day,

| The fact that an exhaust fan flow cools objects better than blowing air
onto
| a surface is a bit counter-intuitive because standing in the downstream
side
| of a fan feels cooler to the skin - but that is only because of
evaporation.
| Careful measurement will show that the cooling effectiveness for an
object
| like a heat sink is more effective by moving the heated air away from
the
| object.  Getting the heat away is the objective, and blowing on it is
less
| effective than exhausting the heated air.

Never seen a suction cooled external anode HPA, from single 4CX250's to
10kW Klystrons.  I've used a small PC fan blowing onto the right hand side
of the heatsink, front to rear, inclined about 45 degrees.  During 10-12
hour QRSS beacon runs at 40W o/p the little fan never came on and the
heatsink was room temperature.

A bladed fan is easy to stall if there is any restriction to throughput.

Regards,

Mike VP8NO

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[Tom Hammond-3]

Don, et al:

I've now placed the PDF of my KPA100 Cooling Fan Enclosure on my web
site (www.n0ss.net). The PDF is located in the "K2 Specific Files"
section of the page, about 3/4 of the way down the page.

The fan shown in the illustrations came from a CPU cooler.. It's much
smaller than most PC cooling fans, but just about anything will work
pretty well is seems. The PDF contains hole cutting and drilling info
for both 92mm and 80mm square muffin fans. I originally had an 80mm
square fan installed, but when I scavenged the tiny CPU cooler fan, I
installed it using a cover plate which mounted in the 80mm fan mounting holes.

I have two fan speeds... The low speed suffices for about 90% of my
operation and the higher speed is used when I'm contesting and want a
bit more cooling. The KPA100's fan seldom comes on, and when it does,
it's on for a very short length of time... and everything(!) stays
nice 'n cool.

The cooling enclosure is low profile and extends all the way to the
back of the KPA100 heatsink, so cool air is drawn over a large area
of the fins. Additionally, the enclosure extends just ahead of the
front of the heatsink, so it draws air OUT of the K2 case via the
vents in front of the heatsink (vents actually 'made' by the heatsink
and the cover plate just ahead of it).

73,

Tom   N0SS


At 08:38 AM 12/6/2006, Don Wilhelm wrote:

>Tom,
>
>Yes, we should all see what you have created.  How about a photo and a short
>note on your website (no need for a .pdf file IMHO).  Such an addition is
>great for those running digital modes and is nice to have when operating hot
>and heavy during a contest.
>
>As you have pointed out, it is important that the fan moves air upward so it
>aids the flow from the internal fan rather than blowing air onto the heat
>sink. Just laying a muffin fan on top of the heat sink is effective as a
>'quick and dirty' solution, your plenum sounds like a great permanent
>addition for those instances where there is not a lot of free-air space
>above the K2/100.
>
>The fact that an exhaust fan flow cools objects better than blowing air onto
>a surface is a bit counter-intuitive because standing in the downstream side
>of a fan feels cooler to the skin - but that is only because of evaporation.
>Careful measurement will show that the cooling effectiveness for an object
>like a heat sink is more effective by moving the heated air away from the
>object.  Getting the heat away is the objective, and blowing on it is less
>effective than exhausting the heated air.
>
>73,
>Don W3FPR
>
> > -----Original Message-----
> >
> > I've made a cooling 'plenum' for my K2, using a very lo-profile cover
> > and a tiny muffin fan from a CPU cooler. It pulls cool air in from
> > outside across most of the heatsink fins and exhausts straight up. I
> > can supply illustrations and some info on the design if you, or
> > anyone else might be interested.
> >
> > Cheers,
> >
> > Tom   N0SS
> >
> >
>--
>No virus found in this outgoing message.
>Checked by AVG Free Edition.
>Version: 7.5.432 / Virus Database: 268.15.9/571 - Release Date: 12/5/2006
>11:50 AM

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[David Cutter]

I'm always a little wary about changing a design.  I ask myself: what was in the designer's mind when he did this?  What characteristic was he trying to achieve and what information did he have that I do not?

Consult the fan design and see how its rating varies with pressure on one side or the other.  The air flow rate drops dramatically from its headline advertised value as soon as it encounters any back pressure in front of it and with restriction on the inlet side.  The manufacturer should know which is best for his product.

There are lots of fan types about and some are better at sucking than blowing.  

David
G3UNA

>
> From: "Darwin, Keith" <[hidden email]>
> Date: 2006/12/06 Wed PM 03:08:22 GMT
> To: <[hidden email]>
> Subject: RE: [Elecraft] Thermal control for Q1 / Q2 in KPA100?
>
> Very key point.  If you want to take heat out of a box, you're far
> better to point the fans out and let them draw in cool air from the
> other side than to point the fan in and "blow cool air in".  Here's why.
>
> When you blow a fan into a box (K2, PC, or your house in the summer)
> you're blowing into a confined space and the pressure goes up.  That
> increase in pressure fights against the fan and reduces the amount of
> air that flows.  Mechanical efficiency and air transfer go down.  Turn
> the fan around and blow it out of the box into the "universe".  The
> outside world cannot be pressurized by the fan so the fan moves air more
> effectively.
>
> And I believe the air being blown into the box helps to set up dead
> zones where hot air can pool up.  The flow through the box is better in
> the exhaust case.
>
> I've tested the theory both with my computer and with using fans to cool
> my house in the summer.  In both cases, better cooling is achieved with
> fans blowing out than in.
>
> Makes me wonder about reversing the fan in the K2.  It just might make
> it more effective and allow it to say on low longer.  I have to open it
> up to tighten the HW and insert one more shield spring so I'll get
> another chance to give it a good lookin' over.
>
> - Keith N1AS -
> - K2 5411.ssb.100 -
>
>
> -----Original Message-----
> From: Don Wilhelm
>
> The fact that an exhaust fan flow cools objects better than blowing air
> onto a surface is a bit counter-intuitive because standing in the
> downstream side of a fan feels cooler to the skin - but that is only
> because of evaporation.
> Careful measurement will show that the cooling effectiveness for an
> object like a heat sink is more effective by moving the heated air away
> from the object.  Getting the heat away is the objective, and blowing on
> it is less effective than exhausting the heated air.
>
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[Mike S-8]

At 10:08 AM 12/6/2006, Darwin, Keith wrote...
>Very key point.  If you want to take heat out of a box, you're far
>better to point the fans out and let them draw in cool air from the
>other side than to point the fan in and "blow cool air in".  Here's
>why.
>
>When you blow a fan into a box (K2, PC, or your house in the summer)
>you're blowing into a confined space and the pressure goes up.  That
>increase in pressure fights against the fan and reduces the amount of
>air that flows.

It's not that simple. How does the fan know which side is confined?
It's not a matter of "pressure fights" the fan in only one case - it's
equally valid to say that "vacuum fights" in the other. A fan is a
pump, which creates a pressure differential. At the extreme, one can
only achieve a suction equal to atmospheric pressure, while the amount
of pressure can be virtually unlimited. So, in the extreme case, more
flow can be achieved by blowing than by sucking.

For the application at hand, it doesn't make much difference to total
airflow whether the fan is sucking or blowing - the fan is limited by
the pressure differential it can create. Whether that differential is
between atmospheric and a (very) slight vacuum, or atmospheric and a
(very) slight pressure simply doesn't make any practical difference.

For electronics, airflow direction may be determined by the available
fan location(s), and a desire to have the most heat sensitive
components receive the coolest airflow. Another consideration is that
by having the fan blow into the device, all airflow through the device
travels through a well defined area - so it can be easily filtered.

"Pressurizing the enclosure is the preferred method, since incoming air
can be readily filtered. In addition, a pressurized enclosure will
prevent dust entering through cracks or crevices. The fan is also
handling cooler, denser air, and it will therefore have a slightly
higher pressure capability (this may be a very slight advantage for low
heat dissipating systems). An important feature of a pressurized system
is that the fan life and reliability are increased due to the fan
ambient temperature being lower. The disadvantage of pressurization is
that heat generated by the fan is dissipated into the enclosure." -
Mike Turner, Comar Rotron (a major fan manufacturer) -
http://www.electronics-cooling.com/Resources/EC_Articles/MAY96/may96_01.htm




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[Geoffrey Mackenzie-Kennedy-2]

David G3UNA wrote:

> Consult the fan design and see how its rating varies with pressure on one
> side or the other.  The air flow rate drops dramatically from its headline
> advertised value as soon as it encounters any back pressure in front of it
> and with restriction on the inlet side.

-------------------------------------------------------------------------------------

The restriction on the inlet side that David mentions would include the
effect of any air filter. This might appear to be obvious, but it does mean
that the filter has has to be cleaned when necessary to maintain proper air
flow, a task often forgotten. I believe that air cooling systems should
include air filters whenever possible.

73,
Geoff
GM4ESD



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[David Cutter]

Also finger grills and panel slots create turbulence which can drastically
reduce flow.  For a common or garden 'computer' type of fan, a dust filter
can practically stop air flow all together, so, choose with care.

David
G3UNA

----- Original Message -----
From: "Geoffrey Mackenzie-Kennedy" <[hidden email]>
To: "Elecraft Discussion List" <[hidden email]>
Sent: Thursday, December 07, 2006 1:05 PM
Subject: Re: RE: [Elecraft] Thermal control for Q1 / Q2 in KPA100?

> David G3UNA wrote:
>
>> Consult the fan design and see how its rating varies with pressure on one
>> side or the other.  The air flow rate drops dramatically from its
>> headline advertised value as soon as it encounters any back pressure in
>> front of it and with restriction on the inlet side.
>
> -------------------------------------------------------------------------------------
>
> The restriction on the inlet side that David mentions would include the
> effect of any air filter. This might appear to be obvious, but it does
> mean that the filter has has to be cleaned when necessary to maintain
> proper air flow, a task often forgotten. I believe that air cooling
> systems should include air filters whenever possible.
>
> 73,
> Geoff
> GM4ESD
>
>
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[Geoffrey Mackenzie-Kennedy-2]

Indeed, turbulence can reduce flow. I must confess that I have no experience
with using a 'computer' type of fan to cool rigs.

73,
Geoff
GM4ESD

----- Original Message -----
From: "David Cutter" <[hidden email]>
To: "Geoffrey Mackenzie-Kennedy" <[hidden email]>; "Elecraft
Discussion List" <[hidden email]>
Sent: Thursday, December 07, 2006 1:43 PM
Subject: Re: RE: [Elecraft] Thermal control for Q1 / Q2 in KPA100?


> Also finger grills and panel slots create turbulence which can drastically
> reduce flow.  For a common or garden 'computer' type of fan, a dust filter
> can practically stop air flow all together, so, choose with care.
>
> David
> G3UNA



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[David Cutter]

A designer will take into account which parts he wants to cool first.  For
instance, if a sensitive component (vfo?) is on the cool side of the draught
then it will not be affected by a hot component down stream.  However, if
the direction is reversed you could be creating a problem that wasn't there
before.

David
G3UNA


----- Original Message -----
From: "Darwin, Keith" <[hidden email]>
To: <[hidden email]>
Sent: Wednesday, December 06, 2006 3:08 PM
Subject: RE: [Elecraft] Thermal control for Q1 / Q2 in KPA100?


Very key point.  If you want to take heat out of a box, you're far
better to point the fans out and let them draw in cool air from the
other side than to point the fan in and "blow cool air in".  Here's why.

When you blow a fan into a box (K2, PC, or your house in the summer)
you're blowing into a confined space and the pressure goes up.  That
increase in pressure fights against the fan and reduces the amount of
air that flows.  Mechanical efficiency and air transfer go down.  Turn
the fan around and blow it out of the box into the "universe".  The
outside world cannot be pressurized by the fan so the fan moves air more
effectively.

And I believe the air being blown into the box helps to set up dead
zones where hot air can pool up.  The flow through the box is better in
the exhaust case.

I've tested the theory both with my computer and with using fans to cool
my house in the summer.  In both cases, better cooling is achieved with
fans blowing out than in.

Makes me wonder about reversing the fan in the K2.  It just might make
it more effective and allow it to say on low longer.  I have to open it
up to tighten the HW and insert one more shield spring so I'll get
another chance to give it a good lookin' over.

- Keith N1AS -
- K2 5411.ssb.100 -


-----Original Message-----
From: Don Wilhelm

The fact that an exhaust fan flow cools objects better than blowing air
onto a surface is a bit counter-intuitive because standing in the
downstream side of a fan feels cooler to the skin - but that is only
because of evaporation.
Careful measurement will show that the cooling effectiveness for an
object like a heat sink is more effective by moving the heated air away
from the object.  Getting the heat away is the objective, and blowing on
it is less effective than exhausting the heated air.

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