# Thread: The impact of tubing sizes

1. ## The impact of tubing sizes

I've been working on a wholistic guide to designing a water-cooling system of late. Using a mix of real-world test data, and calculating pressure drops, I've been able to put together an analysis of the impact of tubing sizes on CPU temperatures.

Thermochill PA120.2 with 2 x Yate-Loon fans at 12v
Swiftech Apogee GTX
Conroe C2D CPU, overclocked and under load, emitting 100W of heat
2 meters of tubing length

Using 1/2" ID tubing and 1/2" OD barbs, I determined the pressure-drop curve for the system. Using Swiftech's published test data for the Apogee GTX, and a flow-performance curve for the PA120.2, we're able to determine the pumping hydraulic power required to push various flow-rates. Using established typical ratios of hydraulic power to actual power draw and heat dump of known real-world pumps, we're able to throw into the mix the amount of pump heat dump required to push any flow rate. We first establish this independently of an actual pump (i.e. determine the theoretical best pump), and then select an actual real-world pump that best suits the theoretical target, and then using the PQ curve of that pump, determine the final flow rate of the system, and hence the correspondent final CPU temperature.

Now in a wholistic model, we're modelling not just the impact of the water-flow rate on the CPU temperature, but the impact of the total heat dump of the cooling system (CPU, pump, radiator fans) has on the room environment, which in turn raises the temperature of the air in the room, and so in turn raises the water temperature because the air-in temperature into the radiator will have warmed up. The effect is very small, but I still model it.

Global temp = 22C
Room C/W = 0.005
Fan Heat Dump = 2.0W

The proposed tubing sizes and fittings we'll be investigating are:

6.35 (1/4") ID tubing with quick-fit fittings
8mm (5/16") ID tubing over 6mmID|8mmOD barbs
8mm (5/16") ID tubing with quick-fit fittings
9.6mm (3/8") ID tubing over 7.5mmID|3/8"OD barbs
9.6mm (3/8") ID tubing with quick-fit fittings
11.1mm (7/16") ID tubing stretched over 10.5mmID|1/2"OD barbs
12.7mm (1/2") ID tubing over 10.5mmID|1/2"OD barbs

Quick-fit fittings are those similar to those found on the Swiftech MCW50 (http://www.swiftech.com/products/mcw50.asp)

Running the above range of tubing/fitting sizes through the optimal pump power estimator software I wrote, it predicts that the best pump to use is one that's consuming around 10-13W, with optimal pumping efficiency in the ranges of 3-6LPM. I won't go into the intricacies of the pump power estimator. It's not an exact science, suffice to say that it looks at the wholistic scenario given a waterblock, heatload, room C/W, radiator, system restriction, and so on, and puts out a suggestion for where the optimal range of pumping power lies for that setup. This allows us to then pick a real pump that closely matches the suggested pumping characteristics.

Using the Laing data here: http://www.laing.de/file/66 we see that an unmodified DDC1+ (more commonly referred to in forums as the DDC2) is a very good pump fit for our scenario. Another excellent alternative would be the DDC1 with a modded top.

Okay, so our optimised system consists of:
Laing DDC1+ (unmodified)
Thermochill PA120.2 with 2 x Yate-Loon fans at 12v
Conroe C2D CPU, overclocked and under load, emitting 100W of heat
2 meters of tubing length

For the various tubing/fitting sizes, the PQ curves for a full system for each tubing type looks like this:

I overlaid the curves onto the PQ graph for the Laing DDC1+

The flow performance curves for the radiator and waterblock are illustrated on the following graphs:

...and...

The total CPU heat load is 100W. The total system heat load is 114W . We assume a fixed 14W heat dump from pump which was derived from other testing. This does in fact vary a little as we can see by the Laing graph. As flow rates decrease, so does power draw, and therefore the heat-dump as well. For simplicity we'll assume a fixed 14W heat dump for now.

The intersections all are:

6.35mm quick fit = 4.45LPM flow, 0.0795 block c/w, 0.0374 rad c/w
8mm barbed = 4.75LPM, 0.0783 block c/w, 0.0373 rad c/w
8mm quick fit = 5.6LPM, 0.0770 block c/w, 0.0369 rad c/w
9.6mm barbed = 5.7LPM, 0.0768 block c/w, 0.0369 rad c/w
9.6mm quick fit = 6.2LPM, 0.0762 block c/w, 0.0367 rad c/w
11.1mm barbed = 6.3LPM, 0.0761 block c/w, 0.0367 rad c/w
12.7mm barbed = 6.35LPM, 0.0760 block c/w, 0.0366 rad c/w

Final CPU temperature is ambient (22C) + system load (114W) * radiator C/W + CPU Load (100W) * block C/W

The final CPU temperatures work out to be:

6.35mm quick fit = 34.21C
8mm barbed = 34.08C
8mm quick fit = 33.91C
9.6mm barbed = 33.89C
9.6mm quick fit = 33.80C
11.1mm barbed = 33.79C
12.7mm barbed = 33.77C

So there we have it. The differences between varying tubing sizes.

Okay, the more astute of you will point out that the block C/W is really the case-to-block C/W, and that the actual CPU-die-to-block C/W is a lot higher. Even if we triple block the C/W (which would be an absolute upper limit based upon older research), we get:

6.35mm quick fit = 50.11C
8mm barbed = 49.74C
8mm quick fit = 49.31
9.6mm barbed = 49.25C
9.6mm quick fit = 49.04C
11.1mm barbed = 49.01C
12.7mm barbed = 49.00C

I'll leave it to everyone's own personal value based judgement to determine the relative importance of the differences seen....

It's certainly not the 5C figure that people bandy about. I never expected that it ever would be myself. In my own testing with arbitrarily choking the flow-rate in a test-system, I've always been amazed at the low flow resilience of many setups. Below 2LPM is where things start getting pear shaped quickly for most systems. My recommendation is that even if you're a low-flow fanatic, always ensure that your flow-rates are above 2LPM at the very least, and preferably above 3LPM if at all possible. Still, even when given 1/4" tubing installed with quick-fits and a decent pump like a DDC2, we can see that flow-rates in excess of 4LPM aren't a problem.

2. Excellent work. Easy to understand to boot.

EDIT: Sticky +1

3. wow

you never cease to amaze me...

great article...

voting for sticky now!

4. Great work.

You can tell that you are feeling better by the amount of interaction of late.

6. Originally Posted by SiGfever

Great work.

You can tell that you are feeling better by the amount of interaction of late.
Or how bored he is ROFL. A few nights ago my phone rang, it was Stew! We had a chat and I conveyed to him (great minds seem to think alike eh? ) that I was considering moving to smaller tubing with push fittings as I'm bored, I think it is neater and will not impact performance much at all. Now to find the proper tubing, g 1/4 and g 3/8 push style fittings - hmmmm.

7. very good

+14 sticky

8. Deserves to be stickied along with all the other guides.

9. Thank you! Very informative.

Cathar, with pumps that have less head pressure than the DDC1+/DDC2 like the D5 or perhaps something even weaker, how much of an impact would tubing size bring upon flow rates and temperatures?

10. WOW! this is truly excellent!! stupid dies....

+3 sticky, no doubt

and can someone please enlighten me on 1/4" "push fittings"?
EDIT: sorry i reread it cause it was so good and saw the push fittings. excellent work again and again

Originally Posted by MetalZone
with pumps that have less head pressure than the DDC1+/DDC2 like the D5 or perhaps something even weaker, how much of an impact would tubing size bring upon flow rates and temperatures?
yes i have this question too.

and also, i still dont get what push fittings are after looking at the link. i am confused, why are they so easy to maintain and stuff? what am i missing??

11. Originally Posted by MetalZone
Cathar, with pumps that have less head pressure than the DDC1+/DDC2 like the D5 or perhaps something even weaker, how much of an impact would tubing size bring upon flow rates and temperatures?
The more restrictive the setup, the more that the setup benefits from a more powerful pump to keep flow-rates up. We're still talking about being slightly more powerful, not massively so. Let's run some figures with a DDC1. It's weaker than the DDC2, so I predict that it'll fall behind the DDC2 a bit for the more restrictive tubing sizes, but close the gap as the tubing opens up. Let's see how it goes eh?

Well, here's the graph of the curves against an unmodified DDC1, which is the other pump I suggested would closely match the simulator's pump prediction. A fair deal weaker pump than the DDC2. Pressure head is in the ballpark of the D5. I don't have the PQ graph for the DDC1 with a modified top, so I'll run with the stock DDC PQ graph. It'll be interesting.

Pump heat dump is 8.5W (measured by me in the past).

The intersections all are:

6.35mm quick fit = 3.9LPM flow, 0.0812 block c/w, 0.0378 rad c/w
8mm barbed = 4.2LPM, 0.0805 block c/w, 0.0376 rad c/w
8mm quick fit = 4.95LPM, 0.0782 block c/w, 0.0372 rad c/w
9.6mm barbed = 5.0LPM, 0.0781 block c/w, 0.0372 rad c/w
9.6mm quick fit = 5.4LPM, 0.0774 block c/w, 0.0370 rad c/w
11.1mm barbed = 5.55LPM, 0.0771 block c/w, 0.0369 rad c/w
12.7mm barbed = 5.6LPM, 0.0770 block c/w, 0.0369 rad c/w

Final CPU temperature is ambient (22C) + system load (108.5W) * radiator C/W + CPU Load (100W) * block C/W

6.35mm quick fit = 34.22C
8mm barbed = 34.13C
8mm quick fit = 33.86C
9.6mm barbed = 33.85C
9.6mm quick fit = 33.75C
11.1mm barbed = 33.71C
12.7mm barbed = 33.70C

...or tripling the block C/W

6.35mm quick fit = 50.46C
8mm barbed = 50.23C
8mm quick fit = 49.50C
9.6mm barbed = 49.47C
9.6mm quick fit = 49.23C
11.1mm barbed = 49.13C
12.7mm barbed = 49.10C

As you can see, the results are very close to the DDC2. With a modified top, I reckon that the DDC1 would be about equal to the DDC2.

The Laing D5 has about the same pressure as the DDC1, but dumps a fair amount more heat into the loop since the D5 is a more highly flowing pump, but that high flowing nature is wasted with a moderately restrictive setup with a Apogee GTX, and so it'll just dump more heat. Without calculating, a top-of-my-head guess is that you'd see results similar to the DDC1, but add on about +0.25C across the board due to the extra pump heat.

12. wait. was the GTX and PA120.2 pressure drop factored in?

13. Originally Posted by sick_g4m3r
wait. was the GTX and PA120.2 pressure drop factored in?
Yes, of course...

14. really? because you said you pulled those DDC curves from the sight, meaning you didnt alter them according to the pressure drop, or what am i missing?

15. I alwasy wanted such a review to convince me that my next w/cing build will include 3/8" tubes which imho looks much better and takes less space in the case

Thanks for sharing

16. wait i am still not convinced... those DDC curves are stock arent they?

17. Great work

3/8 with quick fit isn't bad after.

18. Originally Posted by sick_g4m3r
really? because you said you pulled those DDC curves from the sight, meaning you didnt alter them according to the pressure drop, or what am i missing?
I'm not sure I'm following what you're trying to say.

The PQ curve for the pump is replicated from the Laing site. There may be some very small variations in the translation from their graph to mine. Laing have more straight-lined some sections, while I applied a measure of smoothing to the curve which is what we would really see if we plotted the PQ curve to 0.1LPM increments.

The separate curves for the system include the pressure drop of the radiator, the waterblock, the tubing, and the fittings. Where the curves intersect with the pump curve is the flow-rate that we will see.

The nature of the curves are such that if there's any errors in transcription that it affects all points equally, so while the results may alter by a few 0.01C points either way (inherent margins of error) the relative differences between each data point should be fairly fixed.

19. Originally Posted by sick_g4m3r
i still dont get what push fittings are after looking at the link. i am confused, why are they so easy to maintain and stuff? what am i missing??
You push the tubing into a hole. There is a collet lock that surrounds and grips the tubing so you can't pull the tubing out unless you release the collet lock, and an O-ring that squishes between the inside of the hole and the outside of the tubing, creating a seal.

What makes push-fit fittings so good is that there's nothing blocking the entrance/exit of the tubing. It's not like tubing squished over a barb where the liquid has to squish through the smaller barb orifice. With push-fit/quick-fit fittings there's nothing obstructing the flow at all.

+sticky

21. Sticky plz

22. Wow... Never expected that

At least we can still say small tubes are ugly!

23. Originally Posted by nikhsub1
Or how bored he is ROFL.
LOL. Consider it an act of penance.

Way back in the day, when many w/c setups were 3/8", and only some were 1/2", we had the super-hot AMD T'bird CPU's, and poorly designed open-flow waterblocks with little internal furniture that demanded that as much flow be rammed through them as possible to perform. Further, most pumps that were available were like the Eheim 1250. High-flow, low-pressure pumps that dumped a fair amount of heat, and radiators taken from cars that also demanded flow rates in excess of 10lpm to get past their performance knee.

i.e. 1/2" ID made sense then. The benefit still wasn't huge over 3/8", but it was noticeable. Tests back then showed that the move from 3/8" to 1/2" meant anything from a 0.2-1.0C improvement, depending on various factors. A 1.0C improvement was enough for most people to make the jump, and so 1/2" tubing got its following. I also feel responsible in part for the jump to the 1/2" band-wagon, since I conducted a fair few tests back then to justify it.

Fast-forwards to today, and we have well designed middling restriction blocks which are more flow agnostic. We have well designed radiators that are more flow-agnostic (both in terms of fan power and liquid flow-rate). We have pumps that are near optimal for PC water-cooling that strike a good balance between pressure, peak-flow, noise, and heat. In short, everything has, quite rightfully, been pegged back from old-school high-flow excess, and tempered with a more balanced approach.

The one thing remaining is tubing size. It's also the thing that seems to cause massive grief. Years back when running tests with the Cascade/Storm designs, I realised that flow rates didn't have to be that high. When I started factoring in pump heat coupled with radiator performance, we learned that there is such a thing as "too much pump". There really is a happy middle ground.

I saw that people loved their low-restriction 1/2" tubing, but people also complained about how big it was. I agreed. 3/4" OD tubing is rather large. People didn't want to give up the idea of the benefits of 1/2" ID tubing, so that's when I investigated and came up with the idea for 7/16"ID|5/8"OD tubing. I ran the tests and the maths. No net difference. The amount of people who adopted the smaller tubing showed just how many people were unhappy with huge tubing. People want small tubing, but that don't want to lose the performance benefits of larger tubing.

Came now to today, with me stuck in the house with injuries, and with all the blocks, pumps, and radiators getting pretty darn close to optimal, I thought it time to revisit the issue of tubing sizes in light of modern developments. Maybe I can atone for convincing so many people to go with 1/2" ID all those years ago, who are still using it, and don't realise that they don't still have to.

Also, I'm just sick of small-bore/big-bore bickering. I've always been a "middle-ground is best and cut-the-crap" kind of guy. Been running tests and for some time now I've even considered converting to 8mm ID tubing with quick-fittings because the differences are so small on modern hardware.

I just wanted to share what I've been seeing in private tests, and provide the theory behind it as well. After running the maths I've decided:

3/8" ID | 1/2" OD into 1/2" ID push-fittings is ideal
Caveat: Where very tight radii are needed, can use 5/16" ID tubing instead for those short sections. 5/16"ID|1/2"OD tubing has a 1" bend radius.

It's not small-bore, and it's not big-bore. It's the middle-ground and for the loss of ~0.05C, it's perfectly acceptable. As we add the extra restriction of GPU blocks it becomes even more justifiable.

Yeah, Swiftech have had 3/8" systems for ages now and had stuck by it in the face of a rampant 1/2" market. They were right to do so.

24. Great guide cathar. This outta settle anyone who comes asking "What size tubing should I get?".
Sticky +(whatever up we are to). lol

25. thanks for taking the time

i will certainly not change my 3/8" fittings to 1/2" after seeing this thread

i have a similar setup with DDC-2 and also a GPU block (MCW60) in the loop

3/8" tubing is easier to maneuver around that's for sure