Vertical or horizontal radiator mounting (passive cooling)?

[Odai]

Hello,

I am looking at purchasing some beefy radiators for my next watercooling setup (maybe the Aquacomputer AMS 840mm, or similar size). With the surface area of these things, I'm wondering what kind of heat transfer power I can expect from purely passive cooling.

Relating to this question, has anyone performed tests to see how much of a gain there would be by mounting the radiators horizontally (so that the convective heat currents rise straight out of the radiator) as opposed to vertically?

Thanks,

Odai.

[OldChap]

So many elements to answering even part of this.

Whether fan assisted or not the main mechanism of heat transfer is dependent on the temperature differential between the hot side and cold side. This leads me to ask what you intend to cool and what you expect/wish the water temperature to be and most importantly what the ambient temperature will be?

My experience of this is with my MOAF cooled 800mm x 800mm rad. Put simply, I expect ambient temperatures to be < 25? for 80% of the year. Even so if I choose to keep the water temperature below 30? even a rad this size needs fan assistance to cool 4 rigs as ambient climbs above 22?. I have thermostatic control of the fan.

What you will see is that there is a steady state in which, for a given power input, the temperature of the coolant rises until the differential between it and the ambient air is enough for it to dissipate the heating load applied by the rig. I put it to you that unless the rig is idling that a small rad is going to allow the water temp to rise above that temperature you will be happy with. That said, a small amount of air passing through a big rad might satisfy your needs. Take a look at a typical setup that uses a Mo-Ra radiator or similar, invariably these run with very low fan revs.

I have not tested the second part of your question but indoors I suspect the may be a small advantage in mounting horizontally .....Outdoors minor wind currents would likely have more effect on a vertically mounted rad (even though my rad is in a sheltered spot, a windy day is great for passive cooling).

If you want a mainly passivly cooled rig I think you may need to be thinking in terms of 2 rads of a size greater than 360 x 360 unless those rads are situated in a permanently cool place. Your AMS 840? In my view maybe two or three would do it

Hope this gives you an idea or two.

[Martinm210]

Don't bother.

I couldn't even complete a 100 watt test with a 480 radiator mounted horizontally. Perhaps with some ducting and drafting assistance it could do better, but you are much better off running silent 500RPM fans than wasting your money on a room full of rads to go completely passive.

http://martinsliquidlab.files.wordpress.com/2012/02/passivetfc480failed1.png?w=614

http://martinsliquidlab.org/2012/02/04/passive-0-rpm-fans-radiator-test/

Note that it does take an EXTRAORDINARLY long time for the system to reach temps, I was logging 100W for almost 3 hours and temps continued to climb.

[stren]

+1 - I think the huge Mora 3 140x9 radiator included a note with it saying that passively it it could only dissipate something like 180-200W max. After Martin's test I'm not sure whether to even believe that. They did claim though that orientation didn't matter which was a surprise. I assumed for passive that horizontal would be better.

[Odai]

Thanks a lot for all the help guys.

Just to clarify, it wouldn't be my intention to always run them passively. Rather, I would just have the fans switched off only when the system is in a low power state. I think we're talking in the region of about 50W heat being dumped into the water.

Martin, any chance you tested the same scenario but with vertical orientation? I am keen to know what the real world difference would be between the two. As mounting vertically would be a bit easier in my case, I'm just trying to figure out if it's worth going through the hassle of mounting horizontally.

Thanks again!

Odai.

[Martinm210]

No I didn't, but I don't know how it could possibly be any better. If the delta for a quad rad at 100 Watt is 25C, to get a 10C delta you could only run about 40W or 10W per 120mm radiator section as a ballpark. I just didn't feel comfortable approaching 45C water temps in the test as tubing and everything starts getting uncomfortably hot. I'm sure a low watt system could be done with conventional rads, but it's got to be really oversized. I just question the value (unless it's just for the novelty) when 500RPM fans are less audible any any power supply unit.

If you really want dead silent, build a server room and keep the noise behind a wall..:)

[Church]

lol, noise of server room's backup power diesel and half faulty conditioning will get through walls! :)
IIRC when designing heatspreaders for electronic elements, difference of heat dissipation area of active vs passive was somewhere within 4x-10x. +If one looks at generic trend - slower the fans (less pressure), less FPI rad should have (for passive cooling even low fpi rads like tested TFC one seem too densely finned. Good example for passive heat dissipation would be room heating rads, with 1-3FPI). +Apart from mentioned by martin fan in PSU you also have noise of pump. +1 for passive LC not being worth it. Oversized rads, not absolutely quiet due other fans and diff. noise sources (pump/hdd/keyboard/mouse), when reasonably sized/weighting/priced and better looking rad (as in - mounted internally) could do same? Seeing only drawbacks in going passive.

[stren]

So I tried it on my workstation by just unpowering the fans (yate loon low speed). Loop is a 3930K at 4.7GHz (all the time even when idle), Mora3 140.9 with yates mounted all internal to a TX10-D (rad is somewhat dusty), 2x D5 Strong @ 12V and 4 pairs of VL4N QDCs. After an hour of pretty much idling the CPU temps had risen 20C and were still slowly going up. So yes you can get away with it for a while if you want to only turn fans on now and again, but certainly you're going to need to automate that otherwise you'll have problems. Don't just expect to be able to let them run passively forever even when idle and play every trick in the book to reduce your idle power. As martin said higher coolant temps means your tube gets squishier and looser. Make sure your clamps/compression fittings are tight if you attempt it. +1 on very low speed fans instead.

[Odai]

Many thanks for all the help guys.

I probably will just end up running fans at a very low RPM, I was just hoping to get some idea of the difference in cooling power when taking into account the additional convective effect of having a conventional radiator mounted horizontally. Obviously the difference would be a negligible amount proportionally with a high airflow system, but it could be significant in systems that are completely passive or using very low airflow rates. I'm thinking the only way to actually find out is to experiment myself, it seems what I need to know is too specific for someone to have already tested it. :D