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The importance of the "order" of your loop? - Page 5

post #41 of 110
Thread Starter 
Quote:
Originally Posted by Nordar View Post
I think most people agree that there is a difference in temperature between the input and output of radiator. The question is just how much of a difference is there and how does it affect our system? That is why I in my previous post in this thread wrote that there is hardly any difference because my stance on the subject is that the difference is small and in fact so small that it should not be the dictating factor when deciding loop order.


The following link is to a test that to the best of my understanding empericaly shows that loop order is insignificant:

http://www.xtremesystems.org/forums/...ght=Loop+order


/Nordar
+1 rep for the time and the link.
post #42 of 110
Quote:
Originally Posted by Nordar View Post
I think most people agree that there is a difference in temperature between the input and output of radiator. The question is just how much of a difference is there and how does it affect our system? That is why I in my previous post in this thread wrote that there is hardly any difference because my stance on the subject is that the difference is small and in fact so small that it should not be the dictating factor when deciding loop order.


I have also tried to look for help in the theoretical world of physics (in which I am certainly no expert so excuse me if I make a mistake in my calculations). I have tried to make some assumptions about a typical system and then tried to calculate the theoretical difference between input and output temperature of the radiator.


Assumptions:
  1. All components water cooled is daisy chained and the readiator is daisy chained as well. The radiator is placed after the components (worst case scenario)
  2. The water cooled components produce 500W of energy that is dumped (might correspond to one serious GFX card, CPU, mother board cooling and pump heat dump)
  3. Flow rate: 2 GPM (this is a flow many aim at or want to exceed)
  4. The system is fully loaded and water temperature has reached equilibrium


Theory:
Equilibrium:
When the computer is turned on the water is at room temperature. Since there is no difference between radiator temperature and ambient there will be no heat removed by the radiator. However, there is a big difference between CPU temperature and water temperature so a lot of the CPU heat will be removed (and the CPU runs at it coolest possible). Since heat is added and none removed (only at the start) the water temperature will rise. As soon as it does the radiator will start to remove heat. In the beginning less heat is removed than added by the components because heat transfer is liniar to the difference between radiator temperature and ambient and that difference is still small. So water temperature continues to rise. At some point the radiator will remove exactly the same amount of heat as add by the components. If this where not the case the water would eventually start to boil (assuming that heat is only removed at the radiator which I think is fair and conservative assumptions). This is the point I call equilibrium. If the workload changes the equilibrium will change as well. For this discussion the worst case is when the CPU and GPU working out to the max because this will yield the biggest heat dump into the water and hence mean that the biggest heat removal by the radiator which again means the biggest temperature difference between input and ouput.


The heat flow equation:

Q = m * c * dT

Where
  • Q = Heat (measured in Joules. 1 joules = 1 Watt second)
  • M = mass (in this case the mass of the water going through the radiator in the timeframe we calculate over)
  • c = Specific heat or heat capacity per unit mass of a body. This variable is dependent on: the material used (which in this case is water), its temperature and the presure. For water it is close to 4 J/G*C at 15C and 101.325 kPa). I dont think it changes much at other temperatures and preasures but this is where my physics knowledge ends.
  • dT = Change in temperature.


We can use this formula to calculate the theoretical change in temperature between input and output of the radiator.


Calculations
What we know:
  • 2 gallons of water passes through the radiator each minute. This corresponds to about 7.5 Liters of water/minute which equals 125ml/second. 125ml of water weights (approximately) 125g. so M = 125g in our formula above
  • 500W needs to be removed by the radiator (since 500W is added by the components we know 500W must be removed as well since we are in equilibrium). We know the amount of heat removed is Q = 500 W * seconds
  • c = (approx) 4 J/(g*C) = 4 Ws/(g*C) , C = celcius, remember 1 Joules = 1 Ws per definition
  • dT = temperature difference we want to compute.

Entering these values into the heat flow equation yields:

500 Ws = 125g * 4 Ws/g*C *dT

==>

500 Ws = 500 Ws/C * dT

==>

dT = 1.0 C


So theoretically there should be a 1 degree difference in temperature between input and output of the radiator. If 1000W of energy is added the difference would be 2C and so forth.



Gah. this took quite a while to figure out and write. I hope it will be useful reading . This is why water cooling is fun... .


/Nordar
Your pretty well right on for a theoretical loop, using my cheat I came out about the same....Heat Transfer (Q = m * c * dT) Equation Solver/Calculator

That would be a static loop with no other variables that would dissipate a set amount of heat dump, with a static heat source.

A real loop would have a lot of variables, convective heat ransfer, thermal coefficients in the heat exchanger and blocks (WMK of materials and thickness) pressure and viscosity changes with temp's, coolants...etc...

But I pretty well agree with what you have there.


Quote:
Conclusion:

From the above (and assuming I made no mistakes in my application of the formula) I personally conclude:
  1. There is a difference. However, it is small.
  2. The difference in input/output is not enough for me (even for a big system) to dictate loop order. I would personally prefer tidyness, simplicty and reduced tubing (easier to bleed, fill, drain...).
  3. Also look at what happens if you spread out the radiators (typically 1 before CPU, 1 after CPU) between each block. Then the radiators share the work of removing the heat and the difference in input/output temperature of EACH radiator will be less.
  4. A good question now is: What does a 1C improvement in water temperature do to the CPU tempeature. My guess is very little - a guess that is backed up by the people who says they tried different loop orders without any change in CPU temperature.
  5. If one insists on the importance of loop order the above indicates you should put all radiators in series and place them right in front of the CPU to get biggest temperature drop before the water hits the CPU block.
1- A small difference, as I would agree I wouldn't run much extra tubing to get one in that config, although tubing frictional losses are almost nill till you get ridiculous amounts of tubing strung out around the room.

2- Your opinion, which is fine, it's a free country. But opinion is opinion, it may be a fact to the individual that gives their opinion, but that does not make it fact.

3-Agree entirely, placing rad's between components is a waste of time and effort.

4-Not really a lot, I don't think anyone here has said it does. But then again your talking about the most critical measurement, other then actual CPU temp, in a loop.

That would be your rad out to ambient DeltaT. A 10C rad out is close to being no better then a HSF. A 5C rad out delta will let yo make some pretty good OC'es. So 1C in my estimation would be 20% of the cooling potential between that 5C rad out and 10C rad out.

My opinion, I like to OC so I take any advantage I can, especially free ones. There are a lot of little tweaks to optimize a loop. I actually try to stay well under a 5C rad out DeltaT.

But everyone is different and WC for various reasons. If I was doing it for quiet I wouldn't worry about even a 10C rad out to much.

I've set up 100's of loops in many configurations and I always got a little better temp's with the CPU right after the rad. That includes extremely restrictive jet blocks which I thought should cool better if I put them right after the pump from the increased head and flow.




Quote:
The following link is to a test that to the best of my understanding empericaly shows that loop order is insignificant:

http://www.xtremesystems.org/forums/...ght=Loop+order

I would hardly call that empirical at all, there are a couple of flaws in it.

First he's using the Swiftech rad, which is a low fin per inch rad designed for low cfm fans 50-60cfm, they will keep cooling with more cfm but the cooling curve will flatten out big time. He's using 100cfm fans on it, which the rad was never designed to be used with.

The second flaw is stacking his rad's, that has never worked as well as having the rad's separate, the second rad will be pulling several C hotter air through it then the first.

If he had used a rad that is designed for higher pressure and cfm fans he may of gotten a little better temp's.

Did he get banned for trying it rip off the HWBot along with Skinnee in the MSI Lords of Overclocking OC'ing competition? Someone said he did but I've not seen it myself.

Skinnee is in this...
HWBOT response to FM LOC incident
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post #43 of 110
Quote:
Originally Posted by ira-k View Post
The second flaw is stacking his rad's, that has never worked as well as having the rad's separate, the second rad will be pulling several C hotter air through it then the first.
This is pretty much the exact argument I was making against having your devices daisy chained together in the loop like... CPU-GPU1-GPU2-GPU3 without a rad in between somewhere.

What is the difference between "several C hotter air" and "several C hotter water" ?
post #44 of 110
Quote:
Originally Posted by Lucretius View Post
This is pretty much the exact argument I was making against having your devices daisy chained together in the loop like... CPU-GPU1-GPU2-GPU3 without a rad in between somewhere.

What is the difference between "several C hotter air" and "several C hotter water" ?
With all else being equal the radiators will remove exactly the same amount of heat as added by the components being cooled (in the equilibrium state). This is independent on where in the loop they are placed.

The heat transfer capacity of the radiators will of course depent on the difference between the water temperature and the air pull through them (and hence potential be able to lower the equilibrium temperature). However, as shown above the temperature drop over the radiator is very little so I doubt it will be of great influence. The air pulled through a set of stacked radiators will change temperature much faster because the specific heat (heat capacity) of air is about 1/4 of that of water but more importantly because air weighs much less than water (however, a higher air flow work in the other direction).

So the air in stacked radiators will change temperature a good bit while the water over a series of radiators won't. That is why stacked radiators loose efficiency but radiators in series do not.

For the very OC fixated benchmarker the best bet is to run all the radiators in series in order to maximize the temperature drop over the radiators and then place the CPU after the radiator because it will be the most heat critical part. However, as proven above the effect is small.


I hope I made sense .

/Nordar
Edited by Nordar - 8/23/10 at 5:01am
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post #45 of 110
Quote:
Originally Posted by ira-k View Post
Your pretty well right on for a theoretical loop, using my cheat I came out about the same....Heat Transfer (Q = m * c * dT) Equation Solver/Calculator

That would be a static loop with no other variables that would dissipate a set amount of heat dump, with a static heat source.

A real loop would have a lot of variables, convective heat ransfer, thermal coefficients in the heat exchanger and blocks (WMK of materials and thickness) pressure and viscosity changes with temp's, coolants...etc...

But I pretty well agree with what you have there.
I agree with you as well that the calculations based on the heat transfer equation is simplified. However, the other factors you mention only have little effect and the effect they have will all work towards reducing the temperature drop (in equilbrium). For instance the tubing and resevoir will work as passive radiators.



Quote:
Originally Posted by ira-k View Post
That would be your rad out to ambient DeltaT. A 10C rad out is close to being no better then a HSF. A 5C rad out delta will let yo make some pretty good OC'es. So 1C in my estimation would be 20% of the cooling potential between that 5C rad out and 10C rad out.

My opinion, I like to OC so I take any advantage I can, especially free ones. There are a lot of little tweaks to optimize a loop. I actually try to stay well under a 5C rad out DeltaT.

But everyone is different and WC for various reasons. If I was doing it for quiet I wouldn't worry about even a 10C rad out to much.
I admit that I am borderlining my understanding of physics (but that just makes it more interesting). So do take my posts as an attempt to learn.

I was under the assumption that for most radiator tests the water to air delta shown are based on average temperature of the water in the loop (and not rad out water temp). If this is the case is it then possible to compare the 1C temperature drop presented in my calculations above with a 5C delta (for instance)? The average water temperature in the loop is pretty much independent on the radiator placement in the loop.

Quote:
Originally Posted by ira-k View Post
I've set up 100's of loops in many configurations and I always got a little better temp's with the CPU right after the rad. That includes extremely restrictive jet blocks which I thought should cool better if I put them right after the pump from the increased head and flow.

This is very interesting (especially for the serious benchmarker) and is a different experience than posted by others in this thread (that state they do not experience any difference in CPU temp using different loop orders). Maybe it's only measurable at extreme overclocks? or so small an effect that it is hard to meassure? Anyways it encourages one to experiment. .


Quote:
Originally Posted by ira-k View Post
I would hardly call that empirical at all, there are a couple of flaws in it.

First he's using the Swiftech rad, which is a low fin per inch rad designed for low cfm fans 50-60cfm, they will keep cooling with more cfm but the cooling curve will flatten out big time. He's using 100cfm fans on it, which the rad was never designed to be used with.

The second flaw is stacking his rad's, that has never worked as well as having the rad's separate, the second rad will be pulling several C hotter air through it then the first.

If he had used a rad that is designed for higher pressure and cfm fans he may of gotten a little better temp's.
I understand your points but it is I believe the only test available that looks into this matter and even though not conducted using an ideal test setup it still points in the direction that the temperature drop over a radiator is small which the theory backs up.


/Nordar
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post #46 of 110
Quote:
Originally Posted by Nordar View Post

This is very interesting (especially for the serious benchmarker) and is a different experience than posted by others in this thread (that state they do not experience any difference in CPU temp using different loop orders). Maybe it's only measurable at extreme overclocks? or so small an effect that it is hard to meassure? Anyways it encourages one to experiment. .


/Nordar
$20 says Ira had a probe hooked up to the CPU and wasn't using some type of software monitoring.

My understanding of physics is also limited but if I am understanding everything, the bottom line is, there is a difference and it is beneficial, no matter how small the benefit is. Just because it can't be measured in units of 1C or larger doesn't mean anything. .01C could be an extra 10mhz out of my CPU.

IIRC there is something somewhere that describes what you are supposed to do to accurately measure your CPU temp. It involves carving a channel in the CPU IHS and placing a probe there.
Edited by Shrimpykins - 8/23/10 at 9:12am
 
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post #47 of 110
Thread Starter 
Quote:
Originally Posted by Shrimpykins View Post
$20 says Ira had a probe hooked up to the CPU and wasn't using some type of software monitoring.

My understanding of physics is also limited but if I am understanding everything, the bottom line is, there is a difference and it is beneficial, no matter how small the benefit is. Just because it can't be measured in units of 1C or larger doesn't mean anything. .01C could be an extra 10mhz out of my CPU.

IIRC there is something somewhere that describes what you are supposed to do to accurately measure your CPU temp. It involves carving a channel in the CPU IHS and placing a probe there.
If it can't be measured in 1C or larger, is it really worth it? If it requires extra tubing and more bending (possibly rotary fittings), wouldn't it be better to just make the simplest loop possible? If we are talking about less than 1C, then I'm certainly not going to make my loop more complicated to ascertain it.
post #48 of 110
Quote:
Originally Posted by Corrupted View Post
If it can't be measured in 1C or larger, is it really worth it? If it requires extra tubing and more bending (possibly rotary fittings), wouldn't it be better to just make the simplest loop possible? If we are talking about less than 1C, then I'm certainly not going to make my loop more complicated to ascertain it.
That completely up to opinion. I have my rad right before my CPU and will have another radiator right before my GPU's and the tubing isn't that complicated IMO.
 
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post #49 of 110
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Quote:
Originally Posted by Shrimpykins View Post
That completely up to opinion. I have my rad right before my CPU and will have another radiator right before my GPU's and the tubing isn't that complicated IMO.
I'm going to have it the same way...however getting the better rad to be before the CPU rather than GPU would require a lot more length and a much less clean look in the case.
post #50 of 110
As mentioned before each to his own. Some see benefit in maximum performance at the potential cost in looks and increased complexity of the tubing. Others see benefit in clean looks and potential minimal tubing which could provide easier filling and bleeding. Either way the difference is small and down to preference.

Personally I will go for clean looks. In my current build I think I actually might end up with res->pump->gfx 1 + gfx 2 (parallel) -> Mostfet 1 -> NB/SB -> CPU -> Mosfet 2 -> radiator 360 -> radiator 240 -> res. I think it will do well anyways even with a decent 4.0-4.2 24/7 OC on my I7 930. I am not into benchmarking.

IF I where to go for maximum performance I would go res -> pump -> radiator 1 -> radiator 2 -> cpu -> the rest...

This would provide maximum temperature drop over the radiators before the water hits the CPU. The CPU with a 150 Watt output or so would only heat the water by about 0.25 degrees with a 2 GPM flow so no need for radiator after CPU.

Thats what I (currently) believe.


/Nordar
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