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Old 02-02-2010, 04:15 AM
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Wink [DIY] Easy subzero for beginners.

Today we discuss subzero unit which can be built by anybody for low cost.

Difficulty: easy
Cost of parts: ~50-250$
Time needed: one to few days.
Specs: 0 to -30C on CPU/GPU's.
Skills: Just watercooling assembly, insulation. nothing more

First we need to get one pcs of air window conditioner unit (not split one, just those which install in windows) - price ~20-150$. 7000-9000BTU power rating are just ok. It needs to be in good condition, working and cooling.

The principle of aircon unit cooling is very similar to stock phasechange CPU units, fridges and such stuff. Compressor pumps R22 gas (freon) to condenser under high pressure, where it loses heat and condensing into liquid form. R22 boiling point at 1 atm is near -41C, so after pressure drop device (few meters of captube usually) it's injected into evaporator placed inside room and divided from hot chamber by insulation. Pressure in evaporator is low, and R22 boils there taking loads of heat from air. Fans circulate air thus cooling of such unit is permanent, until selected temperature reached.

The idea is simple. We don't need to cool air (coz it's low heat conductive thing). But we can make sealed tank with evaporator, and put some type of glycole liquid into it, like those used in cooling loop of cars.
This task don't require any phase-change loops knowledge, don't need to have tools for pipe work/brazing and freon tanks to work with. All cooling stuff is already assembled and ready to go in these a/c units as described earlier.

To be more practical, I will describe unit made by myself few weeks ago.

It's some noname 7000BTU unit, bought for ~$150 locally. 99.9% of such units have nice rotary compressor, and quite large condensors, with R22 gas fill near 1kg. Most of them tuned to max heat power near 2-5kW at near zero temps.

First we remove fan from evaporator side. On pic above it's smaller radiator on bottom of pic. We don't need fan here. Also we need to remove electrical curcuits from control box, which set temp point to shutdown compressor. We need compressor working nonstop, so just wired it to power outlet directly via ON/OFF switch.

In result we get almost -20C on evaporator, without any insulation, any gas/brazing works. Next thing - just put evap into sealed tank, and cool liquid down. Smart people don't need further instructions from this point, but I will describe all to the end

Next task - measure size of evaporator, and go to market to get suitable tank
to fill all evaporator. I got only bit smaller one, but still show must go on

Then got glycol liquid, which don't freeze at -40C (or better). If we use water or watercooling liquid - we will have just ice on evaporator, and no subzero liquid available.

Next I made a paper box from TV package (I hate TV's and never see it ) put tank inside it and fill all air space between cage and tank with foam used in building insulation. It's sold in spay bottles and cost low (~10$ here).
On top I've put some plates of 20mm thick insulation sheets.

Next we just take usual watercooling system with CPU/GPU waterblocks and remove it's pump to bigger and more durable pump.

I've used WILO R-star serie pump, 30W 220VAC, with 3 speed selector. It's working noiseless, and have metal iron body. Need to insulate it too, but I was too lazy

We DON'T need any radiator of this waterchilled setup, coz it only catch heat from warmer ambient air, than chilled evaporator from a/c unit. No need to point lots of condensation, so insulate all tubing and waterblocks stuff with K-flex and similar things.

For first try I've set all up and fire.

First frost arrive after 5-7min's of a/c unit working. Tank was filled with 10L of glycole, so it need some time to cool down all that weight.

Then time to insulate all cold side piping in a/c, to prevent ice frosting for prolongued runs

Here is what happens if we mix glycole with water:

Water made ice on rad, thus reducing heat exchange area very fast.
Next time after 1 hour of working, i've got -27C liquid in unit

One more thing - more liquid in tank - less response from system in general. I mean if we use 1L of cold liquid pumping thru waterblocks - we will get fast temperature changes, during CPU/GPU intensive loads. But a/c will also chill heated liquid faster. More liquid - all will be more smoothy. Optimal is 10-15L, makin possible even 32M superpi and few 3dmark06 runs with only 2-5C raise of liquid. And better to cover as much evaporator as possible, to reduce show formation on opened areas. Show >>> Water >>> Less frosting point of glycole.

Remember, All we used to built unit - is basic things known by all watercooling builders. No phasechange knowledge needed

Lowest temp I've got with +2C ambient temp is -33.8C in liquid, Not bad for watercooling, huh?

On such temps show arrive on every tube and waterblock, so take care of insulation

View of testrig with opened top of evaporator tank

So it's really easy to build, and only bit less comfortable to use comparing phasechange unit.

In final I spend days benching 8600GT at -15C by Rivatuner monitoring, 9500GT at -10C, 7800GTX with -12C, QX9650 with -5C, Xeon 3470 with -12C. Unit was placed on balcony, with monitor placed toward room window, with keyboard,mouse and RESET wired to warm room, so I've benched that stuff in quiet and comfort, without lurking with LN2 pouring every minute

And to cool more CPU's/GPU's only usual waterblocks needed to add. No need of custom evaporators/pots and such milling stuff.

I've had runs with 3x8800Ultra vmodded + Xeon3470 on this chiller, and all time liquid in tank don't raise above -20C during test runs. It's about of 1000W of power total removed at -20C

Pros :

* Ability to easily cool all parts of PC, like CPU and multi-GPU setups. Only usual waterblocks are needed!
* No phase-change cooling knowledge needed
* Available to any builder for low cost (~200$ for all parts possible)
* Good temps for subambient benching, -30C is easy
* All parts stock available, no handmade stuff needed.
* Possible to run 24/7, no need special man to pour LN every minute

Cons :

* Consume lot of space, comparing SS or usual watercooling, if using only 1 point cooling (1CPU/1GPU e.g).
* Produce noise from high-CFM fan and compressor
* Need of insulation all stuff
* Less power cooling comparing to SS.

Questions? Remarks? Found errors?

Stay tuned.

P.S. sorry for bad english, please report correct spelling, wanna improve my english a lot.

Last edited by TiN; 02-02-2010 at 08:44 AM.
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