Is it possiable to cool a 75 watt peltier w/ aircooling?...

[cpr]

Hello,
I have a sick idea. Would it be possiable to cool a peltier running at 75 watts with air cooling? I DO NOT man a 75 watt peltier (because it would run at less w/ only 12v) I mean a PHAT 94 watt peltier running at 20, instead of 24 volts =) I'm just curious if there is any possiable way to air cool these babies (two of 'em, (BP6)). Perhaps a Hedgehog, or a modified Solt1 Hedgehog?

I realize that water cooling is the obvious solution, but I want to look at other possibilities first.

Thank You,
~cpr

[cpr]

Hello,
Muaahaahaahaa!! *evil grin* I do beleive a slot 1 hedgehog would fit on a BP6 http://sysopt.earthweb.com/forum/smile.gif. With teh width of the CPU, cold plate, peltier, and foot on teh hedgehog, it sould clear the capicators.

Would a Slot1 hedgehog have the "umph" the cool such a peltier?

Thank You,
~cpr

[This message has been edited by cpr (edited 09-11-2000).]

[thekingofpain]

Doubt it, folks with T-birds are trying to figure how to pelt them, requires close to a 86watt pelt to overcome the heat these babys put out, the heatsinks we have so far arnt large enough to dissipate the heat the pelt can load into it...38cfm is a minimum for a fan...even on a good Alpha, Global/Win is coming out with a superior (giant) h/s for slot A real soon...we can only wait or add H20 in the meantime...
Your Bp6 is in the same area, doubt your processors will be putting out what a T-bird at 1.1 will but add that big bad pelt to the BIGGEST h/s thatl fit on a BP6 for good results, maybe stick a couple fans that exceed 38cfm with crankin case ventilation for best results, small heatsinks with great demands are our probs at this point...

Im still freaking out about the small area this unused (so far, hehehe) T-bird I have in my hand has to cool thru...unreal...

[This message has been edited by thekingofpain (edited 09-11-2000).]

[krusty the klown]

In theory, yes you can do it. With a large enough heatsink!

Remember that the heatsink has to dissipate the heat generated by the CPU + the electrical energy (volts * amps) used by the peltier - these things don't shift heat for free and the electrical energy consumed by the peltier all ends up as heat - just like a refrigerator or aircon unit.

Find out how much heat is generated by the CPU, add this to the energy consumed by the peltier: this is what your heatsink has to handle. Heatsink performance is rated in ºC/watt (or K/watt, which is the same size - a 1K temperature increase is the same as a 1ºC increase: the temperature scales just start at different points). Say you have 120Watts of heat to dissipate using a 0.25ºC/W heatsink. This means that the heatsink will be 120*0.25=30ºC ABOVE ambient temperature..... ambient temperature is INSIDE the CPU case - that's where the heatsink is!

Now, assuming that the case ventilation can keep the case temperature down to 30ºC, then the heatsink will be sitting at 60ºC. Peltiers have a maximum temperature difference that can exist across the hot and cold junctions - this is somewhere around 60ºC, so the cold plate will be somewhere around 0ºC and the CPU will be somewhere above this, as there will always be a temperature gradient across the thermal junction between the CPU and cold side.

For these modern tiny flip chip designs, a copper cold plate is essential - otherwise the CPU is only in effective thermal contact with a small portion of the peltier cold side.

You will have to have excellent case ventilation, as from my example above, you can see that if the case temperature rises, the heatsink temperature rises hence the CPU temperature rises - inadequate case ventilation could give you some real problems and you might need to invest in (or make) something like this:
http://www.overclockers.co.uk/acatalog/coolercase_tornado.jpg

[cpr]

Hello,
KtK: Aww, thanks for the info. So, lets see, a 75 watt peltier + 45 watts for the CPU (or more) = 120 watts. So if an Alpha is .3, then I need to dissipate 36 C. And if susposedly the Hedgehog is better than an Alpha, then it would have perhaps .25 C/W, and the Slot 1 version sould be able to dissipate more heat correct? Then would I be correct to assume it can dissipate .2 C/W? Or more or less? (I haven't found the actual C/W for any hedgehog) Therefor I need to dissipate ATLEAST between 24 to 36 C.

Now, would my current cooling system be sufficient? I have 2x 120 mm in the front I can use to A) Blow air onto the heatsinks, or B) suck air off of them. And near the heatsinks (by the serial ports) I have 2x 60mm that can A) suck air off the HS, or B) blow outside air onto 'em.

And of course, since this is on a BP6, I have twice to worry about http://sysopt.earthweb.com/forum/smile.gif

Thank You,
~cpr

[krusty the klown]

Hmmm.... I don't know whether the ventilation will be adequate or not - there are two things to consider: firstly the temperature rise of the heatsink above its surroundings: this is what we've previously estimated as being around 35ºC - you'd be safer doing the calculations using 0.3ºC/W - this will give you a 'worst case' estim8. The temperature inside the case will be dependent on the temperature of the incoming air, the total heat dissipated by all heat sources inside the box (OK, so the main heat sources are the peltiers+CPUs - you could take a best case as 250Watts and worst as 300) and the mass flow rate of air through the box. Kinda think of it like one of those electric showers - in the winter when the inlet water is colder, the outlet water will be colder. You can increase the temperature of the outlet water by increasing the power to the heating element or by reducing the flow rate.

I could make a 'back of cigarette packet' calculation if I knew the flow rate through the case - do you know the cfm rating of the fans?

Otherwise, short of building it and seeing what happens, the only way to pre-test the case ventilation would be to rip the insides out of your computer, replace all PCI/AGP/ISA card outlet blanking plates and all disk drive blanking plates, place a 250Watt heat source inside the computer, run the fans and measure the outlet temperature of the air.

250W heat source.....? Filament light bulbs convert almost all of their input energy into heat, so that could be a possibility - but it requires mains electricity!

The whole thing's venturing into the unknown a bit (or should that be Abit http://sysopt.earthweb.com/forum/wink.gif ) using approximations and assumptions, so if you do go ahead, try to build in some 'slack' and be prepared to have to make modifications if it doesn't work exactly as planned first time!

[cpr]

Hello,
KTK: Since I don't have much to loose, I'll try the setup anyways, just to see what happens, if its to powerful, I'll have to find lesser peltiers. (Noting ventured, nothing gained... (of course, nothing lost either =))

In worst case, I'll have ~40 degrees to dissipate =/...

As for fans, I have 2x 120mm Sunon 108cfm (Loud =) and in rear I have 2x 60mm Sunnon HO 23.5 CFM...

Thank You,
~cpr

[krusty the klown]

Back of cigarette packet time:

assuming that the fans are set up with one of each type admitting air to the case and the other two expelling air from the case, then allowing for losses, let's assume that the total throughput is 100cfm.

there are 36cubic feet in 1 cubic metre (I do physics in metric - SI - units http://sysopt.earthweb.com/forum/smile.gif )
so the air flow is 0.046 cubic metres/second

The density of air is approximately 1kg/m^3 and the specific heat capacity is about 1000J/kg/ºC

We have 300Watts = 300Joules/second (maximum) and 0.046kg/s air flow.

Therefore, the temperature rise (inlet air temp - outlet air temp) is 300/(0.046*1000) = 6.5ºC

For a room air temperature of 20ºC, the temperature inside the case will be about 26-27ºC and the temperature of the heatsink will be 40+26 = 66ºC. If the peltier can maintain a 60ºC temperature difference between hot and cold sides, the cold side will be about 6ºC.

The temperature of the CPU will be above this, as the thermal junction between CPU and cold plate will have some 'thermal resistance' (this thermal resistance is what thermal paste and lapping aims to reduce).

Note that these are approximate calculations and assumptions have been made just to get the calculations simple!

If there's anything you don't understand about the calculations feel free to ask! (may not be able to reply till Monday)

[cpr]

Hello,
KTK: Thanks a Million man. If I could get the cold side of the peltier at 6C, I'll be VERY happy. I'll be sure to tell you how it works out when I get it setup. Althought that might be for a few months.

Thank You,
~cpr