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*******High Flow******* myth or not?

post #1 of 21
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I am active in both german and american forums like ocn.net
I have experienced quite a lot of different preferences and unanimities regarding both sides.
But there is one matter that has always seemed to cause a special amount of debating and stubbornness.

This subject is High Flow and the influence ,impact and actual benefit High Flow really has on a system ,its overall cooling performance and especially the CPU Block´s performance considering the amount of flow applied to it.

As an example I would like to present myself.
I ordered my CPU Block the Aquacomputer Kryos XT quite some time ago.
Now when I mention my WC gear in another thread (PT-Nuke question)charliehorse555 stated that my pump the VPP655 from Alphacool had too little headpressure and the Kryos was too restrictive and that as a result of both the aforementioned disadvantages my system would suffer from a flowrate benath 1 GPM - 227 l/h and would therefore achieve bad temps.
He justified that statement with a review Vapor made abou the Kryos clearly showing the Kryos restrictivness compared to the Supreme HF.
Here the Vapor review:http://www.xtremesystems.org/forums/...d.php?t=254728

Here charliehorse555´s exact statements:
1.)Remember that flow rate scales exponentially. Thus,

1(1.59 ** 2) / (1.23 ** 2) = 1.67 = The Kyros XT is 67% more restrictive than the EK Supreme HF (Plate One).

2.)f you get a pump that has low head pressure for a high restriction loop such as yours the flow rate will be below 1.0 GPM and your temperatures will skyrocket due to laminar flow.

3.)below 1 GPM (227 l/hr) temperatures significantly INCREASE

He also showed me the following pictures to aid his claim(attached)

After giving the matter a tremendous amount of thought I finally decided to return my Kryos and get the EK Supreme HF Full Nickel.

But before I did so ...I consulted with a german forum (hardwareluxx.de) to recieve reasurance of my intended decision.

Here is what they responded:
1.)The performance gain of flow rate over 60 l/h was marginal and not worth mentioning...als

2.)Most of them were very prejudiced and critizised the american obsession with flowrate and that it was rather stupid.

3.)That the "absolute temps" sorted out by Vapor in his review could not possibly be taken seriously.

4.)The americans are still 4 years behind and paralyzed by the highflow hype that is just a myth and the eurpeans luckilty have evolved and confronts the matter of good temps from a more scientific and intelligent way.

5.)The temps Vapor has provided in his review neither stated any boundry conditions like (Ambient tempreture,Water tempreture,Radiator) nor the most important of all Delta T tempreture ...and because of that fact his review can only be refered to as rather shallow , pedantic and could not possibly be taken seriously.

6.)Vapor´s tempreture sensors are of primitive nature and do not supply accurate tempreture monitoring.

7.)Flowrate stands in no direct relation to cooling performance and that the only reason why Vapor did not state any Delta T values was because that would actually proof the common american misconception of (High Flow = good temps ) was just a myth and that he is affraid his audience that mostly consits of a High Flow fixated mob would assault him for it.

8.)Also at the end even myself was attacked by most them ...and they told me to just "go to my US buddies" cause they were tired of me telling them stupid and redundant things about high flow and temps that just aren´t true and are only believed by american watercoolers.



So these are the basic statements of both sides.
I would really much appreciate your oppinions!!!
(Especially the experienced and high ranked watercoolers would be very much appreciated to make a post here)

Cheers and thx in advance!



Edited by Kung Pow - 2/22/11 at 8:29am
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post #2 of 21
it is true that higher flow is pointless BEYOND A CERTAIN POINT. (and I am not referring to the laminar/turbulent transition) what it comes down to is the order of your loop. if you have many blocks in a row without rads between them and you have low flow rates then the water will be significantly warmer by time it reaches the later blocks and therefor temps on those devices will be slightly higher.

whereas if you had higher flow, the deltaT across each individual block will be lower and therefore the temps on later devices in the series will be lower.

this effect is completely negated however if you have a rad between all of your blocks because the water will be cooled by the rad in proportion to how much it is heated by the block.
post #3 of 21
Higher flow rate brings diminishing returns.. So if your flow rate is next to nothing, even a slight boost in flow rate can significantly help.. If your flow rate is pretty good, a bump above that will make almost no difference..

Considering how powerful the pumps are that people typically buy, most people are already well beyond the point where their flowrates could make a significant differences..

I run a MCP655 with a 360 RAD, a 120 RAD, and two low restriction Danger Den GTX 470 waterblocks.. I run the pump at the "1" setting.. If I run it at the "2" setting my temps drop about 2C.. I'm hoping the pump will last longer at the "1" setting so I'm running it at that, I don't really care about that 2C...

I get the same temps at pump settings 2,3, 4 and 5.. So really the only measurable difference is between 1 and 2.. Of course, my loop is very low restriction so I get good flow at the 1 setting.. If I threw any kind of CPU block in there, I'd definitely be running at least "2" on the pump's dial. CPU blocks have a lot more restriction than most GPU blocks and the DD GTX 470 blocks have the least amount of restriction among all GTX 470 blocks..

Also, if you're pumping distilled water like I am and not a bunch of chemicals / anti-corrosives / UV dye / etc, etc.. Your flowrate should be pretty darn good even at lower pump settings..

Edit: Yea, I think they're probably right.. That people are just a little too obsessed about flowrates. At least that's what I gather from what I've seen in my own testing.. On the other hand, some people just don't have pumps like the 655 or they're pumping all this other stuff in their liquid also..

What one person considers "low flow" might be somebody else's definition of "high flow" so it's very subjective and most people don't have the equip to measure flowrate properly.

I've seen some videos of people where they had little hurricanes going on in their res because they have their pump cranked up to max.. I think it might be more of a cosmetic thing.

Also like to point out at the "1" setting my flowrate is probably below 1GPM, and the 2C higher temps because of it might be considered "significant" to many people.
Edited by Sidicas - 2/22/11 at 9:20am
post #4 of 21
I'm laughing that they called Vapor a troll.... lol.

Also, most of the information they said his report "lacks" can actually be found on the methodology page: http://skinneelabs.com/aquacomputer-cuplex-kryos-xt/3/

Any review that tests flow rates vs. temperatures shows a large spike in temperatures as the flow rate is reduced below 1 GPM (or 200 l/hr for metric folks).

Link to the thread on the German forums please.
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post #5 of 21
If you increase the flow rate you are almost always going to improve the heat transfer from your block to the water, the question is by how much.

The reason for the improved heat transfer is to do with the temperature difference between water and block (along with the thickness of the boundary layer etc). The higher the temperature difference the higher the heat transfer rate. Pushing more water though the block will mean that the water heats up less, maintaining a higher temperature difference throughout the block and therefore maintaining a higher heat transfer rate. It also reduces the thickness of the lamina boundary layer on the component (present even in turbulent flow) which improves heat transfer. This will continue to have an effect as your flow rate increases, but the effect will become smaller and smaller.

Looking at other reviews, like Vapor's review of the EK Supreme HF, you can see that at increased flow rates, the temperatures do indeed drop with higher flow rates, however the effect decreases as the flow rate increases:



As long as you keep the flow rate above, say, 1GPM, you should be fine, as further increases in flow rate will not lead to significantly lower temperatures. It is worth noting that the increased flow rate also improves the heat transfer of the radiators too, leading to lower water temperatures:


(higher is better)

Adding a second pump might seem like a good idea at this point, as the increased heat transfer rates of both the radiator and block will surely reduce your temperatures, however you also have to take into account the effect of the pump heat dump. Although this is quite small, so is the increase in cooling capacity. For most people a second pump will not lead a significant increase in cooling performance, and may even be counter productive.

Tl:dr

Increasing flow rate does lead to lower temperatures
After ~1GPM the increase in performance is very small
Adding more pumps will normally not help much, and may even be counter porductive

Edit:

Have a look at this short article on DazMode for another, slightly less scientific, review.
Edited by GingerJohn - 2/22/11 at 9:40am
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post #6 of 21
I was looking at some of those charts and it looks like the biggest difference between the lowest pumping power and the highest pumping power is only about 5C.. I wouldn't consider that significant, but I'm sure some people would though..

Most people I'd imagine would be somewhere in the middle faced with the potential of gaining 2.5C better temps if they really cranked their pump up hard and doubled their flow rate.. Of course that comes with the price of all the vibrations and noise of a pump running at 3+GPM..

Or faced with the potential of losing 2.5C (such as I did) running the pump at a setting or two below what most people would typically recommend.

Most people would end up right smack in the middle of the curve (between 1 and 1.5GPM) where it doesn't really make that much of a difference (+/- 2.5C) in my opinion, whether they had higher flow or less flow..
Edited by Sidicas - 2/22/11 at 9:39am
post #7 of 21
Sidicas even with the pump at setting one you are still getting above 1 GPM. Your radiator is about 0.2 PSI of restriction, the block is around 0.3 and at setting one one the 655 you have 0.75 PSI of head pressure. Since I doubt the tubing and fittings are more restrictive than 0.25 PSI I think you are good to go.
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post #8 of 21
I compare all this to overclocking and voltages. To what extent do you stop increasing voltages versus the performance gain. It isnt bad to have too high of flowrate, but if water isnt moving fast enough, thats why temperatures increase.

Its almost the same concept with radiators. with a low FPI rad, having faster rpm/higher cfm fans are kind of pointless. But with a high FPI, using low rpm/cfm fans are alright, but not suggested for the best performance. As more air moves across the fins, the more heat can be dissipated. But as less air moves, more heat builds up on the fins and therefore keeping the water warmer.
- This can be applied to flowrate (air) and cpu block (rad). More water flowing will draw more heat off the block's pins.

Sorry if thats confusing. But soon there is marginal benefit, but too low can cause problems.

Someone correct me if im wrong
Edited by tlxxxsracer - 2/22/11 at 10:00am
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post #9 of 21
Thread Starter 
http://www.hardwareluxx.de/community...788493-10.html

starting from page 8 they said a lot about the uselessnes of PT-Nuke etc....and beginning with page 9 starting with my question (thegooch12) debates starts and ends at page 14.
But it is all in german be aware of that
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post #10 of 21
In my latest estimator, you can experiment with these concepts. I do have some approximate pump heat dump values entered for the D5 and DDc series pumps.

Worth is user defined leaving this discussion opinionated. For one guy running extreme radiator capacities where cost is of little interest and fractions of a degree are important, he might very well justify extreme flow rates. On the flip side, someone a tight budget and/or with a smaller radiator/low speed fans would obviously be better off spending money elsewhere.

To understand the topic you need to clearly understand the effects of flow rate on cpu blocks as well as pump heat dump impacts which varies depend on the radiator fan combo. The less rad capacity you have the more critical pump heat dump is, etc.

Bottom line we are talking small numbers so don't let anyone sink your fruity pebbles..

Water cooling parts combinations are pretty hard to really mess up. Low flow rates usually become a bleeding problem moreso than a block efficiency problem.

But any gains are small, so to ignore fractions of a degree might be practical for some, but a piece of the challenge and fun for others.

We all have different priorities, no need to make everyone agree on personal preferences. We could have the same argument over air vs watercooling.. Just have fun with it...

Good technical discussion though..
Cheers!
Martin
Edited by Martinm210 - 2/22/11 at 12:42pm
    
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