Thread: 10mm vs 12mm ?
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Unread 02-22-2005, 12:57 AM   #9
Cathar
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Join Date: Sep 2002
Location: Melbourne, Australia
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When considering the tubing size, one must also consider the level of restriction of the fittings that the hose attach to, when deciding how much restriction it all offers.

One important aspect of tubing size is not so much the performance, but the level of restriction that is affecting the pump inlet. Centrifugal pumps don't much like having to suck water into them, and excessive restriction at the pump inlet can greatly shorten the life-span of the pump. It should be noted that some specific centrifgual pump designs are more susceptible to this sort of failure than others. Regardless of what your final tubing size choice is for the loop, the pump's inlet should not be of a tubing size that is less than the pump is designed for, and the path of water into the pump inlet should be as unrestricted as possible.

Restricting the outlet of the pump generally has little ill-effect though other than lowering flow rates.

Rather than blindly saying that 1/2" ID is always superior, it makes more sense to have a balanced approach to the issue. If we instead say that in a "well balanced system" that the tubing plus fittings should not account for more than 20% of the total system's resistance, then this is perhaps a better approach to take when evaluating the significance of a tubing size choice.

Doing so presents the uncomfortable issue for many that this needs to then be balanced with the pump and block choice.

Up to about 18 months ago almost all pumps that you could buy were quite low pressure and high flow by the standards of some of the pumps available today. Also there were a number of waterblocks about that relied on very high (again, by today's standards) flow rates to achieve close to their peak levels of performance. This combination of pumps and blocks almost necessitated the use of 1/2" ID tubing, and to this day this belief is still largely followed.

A large amount of research has found that flow rates in the 1-1.5gpm (4-6LPM) range is however quite desirable for various radiators, as well as for many of the high-end waterblocks on the market. Flow rates much above 6LPM are largely unneeded any more. Because of this, 7/16" ID tubing is perhaps the best balanced tubing size to use if one were picky about performance, and 3/8" or 10mm ID is also perfectly acceptable if targetting 4-5LPM (1-1.25gpm) flow rates.

8mm ID tubing even with a modern pump largely limits oneself to 1gpm or less for achievable flow rates in a full system. This is getting towards the start of the "knee" below which many common blocks and/or radiators will start to noticably lose cooling performance.

In rough summary:

8mm ID = borderline acceptable balance of tubing size vs performance
3/8" ID or 10mm ID = good balance of performance with a manageable tubing size
7/16" ID = excellent performance with a still somewhat easily manageable tubing size
1/2" ID = pure overkill for most any modern waterblock/pump scenario with unwieldly large and difficult to route tubing

There are however certain exceptions to that summary, for which there is no substitute for doing a bit of good research with respect the to any specific setup that you have planned.

Last edited by Cathar; 02-22-2005 at 01:02 AM.
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