In alt.comp.hardware.pc-homebuilt rickman said:
Water is a pretty good coolant with a high specific heat and ready
availability.
How would a hobbiest work with heat pipes other than using a stock
heat sink incorporation heat pipes? I was not aware that you could
construct your own heat pipes in any practical way.
Heat-pipes are relatively simple devices.
A source of heat, a heat-sink, and a pipe or other connection between
them, a volatile liquid/gas in the device, with a capillary cloth or
material running from the sink to the source.
Water, while usable, is a fairly poor "volatile liquid" to use for a
heat-pipe. You'd prefer a liquid that at "normal" pressure would easily
vaporize at the heat-source temperature; yet condense at the heat-sink
temperature (usually slightly above room-temperature).
There are many liquids that fill the bill:
Gasoline
Propane
Alcohol
Ammonia in water
Various refrigerant liquids.
People tend to shy away from the first three, even when used in a
completely closed device, because of their flammability, and the fourth
because of it's toxicity. That's why Freons were invented (and used
today).
The basic design of a heat-pipe is very simple:
Heat boils a volatile liquid at one end, and it's condensed at the
other. A relatively small cloth, rope, or other such item returns the
now-condensed liquid back to the heated end by capillary action. No
moving parts except the liquid itself.
Yes, water *can* be used; as water *will* boil at reduced pressures.
The common and easiest way to do so is to partially fill the device with
the desired liquid (again, yes, water will do) and bring the *whole
device* to the boiling-point; expelling liquid, air, steam, and other
gas until only the desired liquid and it's gas (steam) is left in the
device; then seal it while still hot and before it starts to suck air
back in.
This can be used to produce "hand boilers" that boil water in the palm
of your hand from the heat therein. Like I said, water *can* be used.
A person can make a demo of this with an old clear lightbulb and a
bunsen-burner. Carefully remove the screw-base without breaking the
bulb. Carefully either nip off, break off, or melt open the seal. Heat
the bulb with a flame for a few seconds to drive out some of the air
inside; then stick the tip into a cup of distilled water; holding it
with a pot-holder so you hands don't get burned. Take the bulb, now
about 10% filled with water, and place over the bunsen-burner in a rack.
Heat until the water is boiling furiously and you figure only steam is
coming out the nib. Reduce heat, and seal. To seal, the *preferred*
way is to melt the original nib closed with the bunsen-burner. However,
wax or epoxy *can* be used if the demo isn't going to be permanent.
Let cool. You'll now have a globe with a small amount of water in the
bottom that will actually *boil* in the palm of your hand from the heat
of your body, until the temperature equalizes.
A heat-pipe works the same way; except it has a heat-sink on the other
end taking away heat as fast as produced on the boiling end; with some
method of returning the liquid to the boiling end ... usually a small
bit of cloth or water-loving fabric that easily wets in the working
fluid. However, in *some* systems where the heatsink is *always* above
the heat-source, gravity also works quite well.
http://www.cheresources.com/htpipes.shtml
http://technology.grc.nasa.gov/tops/TOP300155.pdf
http://electronics-cooling.com/articles/1996/sep/sep96_02.php
Actually, wouldn't the design I described using convective movement of
water be pretty much like a heat pipe? I guess the difference is that
the water never goes to the liquid phase... at least I hope not!
The transfer is nowhere near as efficient as a heat-pipe.
In a really good heatpipe design, the temperature difference between
heatsink end and heat-source rivals or even sometimes beats pumped
liquid designs; but without the problem of pumps, seals, and extra heat
input by the pump itself. Also not energy source other than the heat
differential itself is needed to power the device.
I did a little reading, and it looks like you could use Butane to form
a closed loop, passive two phase cooling system at just a little over
2 bar. This would give a vapor temperature of about 25C. Of course
it might be necessary to have a slightly higher working temperature to
make the radiator work effectively. Pentane has a boiling point of 36
C which is higher than desired for cooling the CPU, but will make the
radiator more effective and not require a pressurized system. Of
course both of these are flammable, but with Butane a leak would
quickly depressurize the system and the computer would alarm from an
over heat condition. But clearly, this requires a lot more thought...
Butane, pentane, propane, all make good working liquids for that
temperature-range. Water *can* be used (see above); but it's "normal"
heat-of-vaporization isn't ideal for the job. Some people though get
scared when working with flammables like those.
OTOH, the machinery to work constantly with freons these days and not
let waste escape to the atmosphere, get rather expensive. (Ask any
air-conditioning auto-mechanic.)
As for a "pressurized system": ALL such systems are pressurized, or at
least SEALED so that there's nothing inside but the desired liquid and
its gas. At whatever temperature you work it at, the liquid/gas *will*
reach equilibrium, where additional heat will cause some of it to boil,
while reducing or removing heat will cause some to condense. That's how
the system works.
And work it will, no matter what the working-fluid, over a *wide* range
of temperatures. Even water. The only worry being:
A. Freezing of the working liquid.
B. Not enough liquid vaporizing fast enough.
C. The working pressure getting too high for the design.
A. In this design is something you won't have to worry about.
(If it freezes, nothing is damaged.)
B. This is where your choice of liquid/gas is chosen.
C. You need to make sure you have a big enough heatsink to keep the
temperature low enough that this doesn't occur.
Finally, you want to make sure your wick and distance run *can* return
enough liquid to the high-end fast enough; or you'll have the source
boiling away all the liquid faster than it can return; with all the
liquid at the heatsink end and only gas at the source (back to your
original convective design) ... a definite no-no for a CPU cooler.
Gravity as a backup helps a lot.
However, when properly designed, heat transfers from source to sink of a
heatpipe with almost unbelievable efficiency; and pretty much at the
speed of sound. It's *almost* a heat-superconductor. Almost.
Like stated above though, that does you no good if your heatsink isn't
big enough to cool the device. All a heatpipe does is *transfer* heat
(like a watercooled device) not get rid of it. You still need a nice
large external heatsink for that. The bigger the better.
You just don't need to have it sitting right on top of the CPU any more.
