Does loop order matter? (pressure)

[JonJ678]

I know the water temperature tends to be nearly uniform through a loop, so please don't tell me this.

Is the pressure through the loop also uniform? Since there is generally a reservoir involved I suspect it is not, and will be at a higher pressure closer to the outlet of the pump than partway through the loop. However it is quite possible that it is all the same pressure and I've just missed the point somewhere.

Reason for asking is that some waterblocks are pressure dependent, so putting my ek supreme immediately after a pump would be better than just minimising tubing length.

Cheers

(if interested, the actual motivation for the question is trying to work out where in a loop to put a second ddc. I'm inclined towards just before the cpu, suspended between an sli block and the processor. However it may make more sense to put it just after the other ddc)

 

[JonJ678]

It's good to know I was thinking sensibly after all.

Since I'll be using multiple non-impingement blocks and one which does deliberately do this, and I believe reacts positively to water pressure, does it then make sense to arrange the loop so as to have the restrictive block near the outlet?

I'm a bit unsure on your last paragraph, does it suggest that putting a block near the outlet will help but not significantly, or that adding a second pump is probably a waste of time?

Cheers Jokester

 

[JonJ678]

Ahh. Thank you, one loop with the ek on and one loop with everything else on looks like the way to go then. A common reservoir presumably works fine for this, I don't really want to thermally isolate the two.

I'm a little confused by why this is so. The reasoning behind a second pump in series was that it would roughly double head pressure, so assuming the rest of the loop has similar resistance to the ek supreme, the flow rate through the supreme would be the same as with them on seperate loops. Is the flaw in assuming the rest of the loop has comparable resistance to the supreme?

 

[JonJ678]

All sounds good from here then. I think I'm going to spend a long time testing various combinations, which is probably the best route to go down anyway.

I'm reassured that you also consider this intuitive, I'm starting to lose faith in being able to guess at how fluid dynamics behave. It'll be a while before any results turn up I'm afraid, still waiting on a new ek top at the moment. There are so few results available about high restriction loops, I'm hoping its because they're untried and not because they're invariably rubbish.

I believe that the best results will be achieved in a single loop, arranged as pump -> gpu block -> pump -> cpu block -> everything else, using a T line instead of a reservoir. It's hard to pin down why I want the gpu block between the two pumps, but it 'feels' right to have something between the two to break up the flow a bit.

Cheers man

 

[JonJ678]

That's one of the few things I'm not worried about. The radiator is always full of hot water, it'll cool just fine

 

[JonJ678]

It does not follow from this that pressure is uniform through a loop,

Conservation of mass combined with velocity known to vary throughout the system implies pressure variations. Pressure is going to vary with distance from the ground for that matter. By all means teach me something different, but that quote is not sufficient

 

[JonJ678]

Thanks fornowagain, that's a rather better explanation than I could have offered.

18W maximum isn't so much heat to add to a loop to find out if it cools better though. Martin's flowrate spreadsheet predicts a pretty dire outcome for me with only one pump, though I am certainly going to try with just one before I run out and buy another

 

[JonJ678]

Careful super, looks a bit like you're contradicting someone who's fairly obviously on the ball here. I'll give explaining the source of 'these ideas' a go. The following is not rigorous.

Consider the following.

Tube with water flowing down it at constant veloctiy, fed from an infinite reservoir and going to an infinite drain. It has momentum associated with it proportional to mass of water and velocity.

It then goes through 90 degrees. Momentum is conserved, but now it's all going perpendicularly to the original. Therefore a force acted to change the direction of the water flow.

This force is fairly obviously linked to the bend in the tubing, the tube walls exerting a force on the water which causes the change in direction.

Since the force inevitably carries with it frictional losses, the bend warms up and the kinetic energy of the water decreases. So right there you have energy moving out of the water flowing. These frictional losses are why you don't want 90 degree bends in your loop.

The force to turn the water through 45 degrees is obviously rather less, so frictional losses from a 45 degree bend are less than through a 90 degree bend.

If that isn't sufficient now might be a good time to work out the loss in flow rate for a given volume of water moving through a 90 degree bend, it was part of last terms course but I can't remember the derivation now.


If this is too difficult to grasp, have a play with this zipped excel file and trust that the author knows exactly what he is doing.

Out of curiousity, do you have a water cooled system or is this conjecture on your part?

Well beaten to it by Mike, serves me right for writing too much



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How terrible is inadequate flow for a loop? Martins sheet had a guess at around 1/3 gph for me with one pump iirc

 

[JonJ678]

I fear you are very confused Superewza.

Firstly friction is never irrelevent. Ever.
Secondly that is centripetal force, which is a psuedo force anyway and nothing like friction.
Thirdly even if you chose a bend with a smaller cross section than the rest of the pipe, and so succeed in water velocity increasing, mass flow rate through any cross section is going to be constant because mass is conserved and the speed after the bend is going to be the same as before.

The reason this is a stupid way to design bends is that it exaggerates the effects of friction so you lose even more kinetic energy to the bend, and as velocity is a function of kinetic energy you then lose flow rate. One bend slows down the entire loop by a little bit.


Water will not flow through your loop without a force. It just won't, that's friction again. You can supply this using a pump, or through care with convection systems, and it doesn't change that more resistance in the loop means more force is needed to maintain a given flow rate.

The reason you are convinced of this is that your reasoning is poor and you are unwilling to accept different viewpoints than your own. Bends and restrictions in the loop are not meaningless, you can at best make the case that if the pump head is sufficient then you can ignore a large amount of restriction. Which is kind of what this thread is about, how to overcome a large peripheral resistance.

Your final point of if the water moves too fast it wont be able to take heat away from compenents is also complete rubbish. There is always water in the block, it is always cooler than the block, so heat always flows into the water.

A large part of your argument relies on breaking the principle of conservation of mass, and the rest on 'there is no friction' yet you have faith that you are correct regardless.

gah

 

[JonJ678]

What do you mean by "the twice as fast, twice as many times in the radiator. "?

For a given water temperature, at anything above a minimum flow rate through the radiator i'd expect heat dissipated to be near constant. The minimum flow rate being the point where diffusion is no longer the predominant means of heat transfer through the water. However below this threshold I'd expect it to suffer, heat transfer through near static liquid is a lot less efficient than through a mixed one.

I was fairly certain that flow rate didn't matter as long as it is enough. This is supported by turning the pump off. However from zero flow rate up to some value (which I think is around 1gph for most loops) the system performs better, anything above this shows negligible improvements until excess heat dumped by the pumps starts to lower performance.

Similarly, at low flow rate the ek block I have in mind underperforms because it's impingement mechanism cannot function, although I believe this to be a seperate issue to the above it was what started this thread.

This is not me agreeing with you Superwza. I'm disregarding your arguments until I see a response to either my own, D D Danneh, fornowagain or MikeTimbers posts which make the flaws in your theory very clear. At best you just don't know what the terms you're using are, despite fornowagain underlining mass flow rate for you. You still haven't said if you actually have a water cooled computer, I'm starting to suspect that this is educated trolling on your part.

 

[JonJ678]

Jokester isn't agreeing with you, he's saying that flow rate doesn't make a significant difference.

Water velocity is not the same thing as flow rate, and bends definitely do reduce flow rate. As does tubing, blocks, reservoirs and basically everything that isn't a pump.

Thanks D D Danneh, that would be the problem with excessively low flow rate. i agree that it would take a hell of a lot of 90 degree bends to drop it that low, but what about an ek supreme, five chipset blocks and three radiators? Becomes a bit harder to guess

Got you with regards time spent in the radiator, I agree that doubling flow rate is hardly going to double heat extracted. However I remain certain that the flow rate has to be sufficient for the water in the reservoir to be mixing, otherwise it'll set up an internal gradient with hot water in the middle and cold beside the copper/brass surface. Flow rate needs to be sufficient to prevent the stagnent layer of water next to the inner surface of the pipes being too thick or performance will suffer

 

[JonJ678]

Only if pressure is constant Superewza, and it isn't.

Jon has that many things to cool, though it can be argued that I've lost track of why I started overclocking in the first place when the watercooling costs more than the hardware.

Mosfets x2, northbridge/southbridge, ram, 8800gt, cpu. Considering the wisdom of water cooling the psu as well. This is why I've developed an acute interest in water pressure

 

[JonJ678]

Reason for asking is that some waterblocks are pressure dependent, so putting my ek supreme immediately after a pump would be better than just minimising tubing length.

This is why it is generally a good idea to have the cpu block as soon after the pump as possible as this is where the greatest pressure is which helps overcome the extremely restrictive nature of impingement blocks.

Happy days! I am thrilled to have you agreeing with me, even if it has taken 40 posts for anyone to do so

I'm pretty confident in this loop order being an improvement. I don't want to hazard a guess at how much of one. But its a million small things which get you to the result you want, so no 90 degree barbs for me either.


Anyone want to hazard a guess on pump->pump-> system vs pump->block->pump->system?

The former is the only way I've ever seen serial pumps done. But the water coming out of a ddc is not going to be beautifully laminar, nor will it be predictably turbulent (at least not by me). So the idea to feeding it through a block before the second pump is to break up the flow, isolating the pumps from one another.

If the block in question is the gpu one, then between this and the cpu looks like the ideal place to put a second ddc. Mount it by using compression fittings and tubes, and if needs be running a wire harness around it which I tie to the top of the case if it sags when warm. Shame it can't be run upside down, perhaps between a mosfet and the processor would work better.

 

[JonJ678]

And the idea is down again. Damn the analogy with electricty.

Your first reply says this though

Water pressure decreases round the loop with the most pressure at the pump outlet and least pressure at the pump inlet

which certainly suggests that for a pressure sensitive pump it should be near the outlet, no?

Sorry fornowagain, I wrote this before I saw your post. I agree completely, but question the range over which your equations are valid. Would they still be applicable at a flow rate of 0.2gpm for example?

Quite curious about what you do for a living, this isn't the first time you've produced mathematics on request for me. Thank you

edit: 0.2gph really would have been slow...

 

[JonJ678]

I feel slow on the uptake today, thank you for sticking with me. I'm studying to become a mechanical engineer, which perhaps explains the interest in the theory behind this. I did not do well in thermofluids.

Non-compressible was when the penny dropped I think. At least that spares me the difficulty of obscure pump locations, though I still like the idea of hanging them above the motherboard.

So. The conclusion is that we return to the model of 'loop order doesn't matter, reservoir before pump is helpful for filling and try to minimise tubing length.' I'm happy with this.

However one issue remains, that of the ek supreme. I think I'm convinced that the rest of the loop wont care one jot if there's one ddc or two running it, but the cpu block might do. Is there any way to judge this except for getting another pump and trying it myself?

Can I even say that two ddcs in series running everything would give the same cpu temperature as one for the cpu, one for everything else with a common reservoir (in which perfect mixing conveniently occurs)? The EK will be designed to expect a certain head pressure, I don't know what this is but it may well be over what one ddc can offer it if it's all in one loop

 

[JonJ678]

I'm very much in favour of mathematics as a means of defending your point, its a language which is difficult to misinterpret. Especial thanks for the graph above.

The graph above for the EK suggests that the difference between 60lph and 240lph is a few degrees. Which I consider significant. However, a seperate loop is unsuitable as it wastes cooling capacity of the second radiator (an 8800t and chipset blocks is far less than a 240 can cope with). So I want both 240 radiators cooling the i7.

For your four points, the first I have no issue with at all
I also think it has to be above a minimum, why have you set this value at 1gpm?
This one is rather included in point 2
And the forth is good, though I might amend to buy second hand quality parts (for some, I'm not sure about second hand pumps)


I think the choice for me is between
Using two ddcs in series, everything on one loop, no reservoir
Using one loop with pump- ek supreme - 240 radiator - reservoir and the other drawing from the same reservoir and running everything else.

The former probably offers less flow rate to the ek supreme, but the latter introduces all form of inefficiency with regard to the combined radiator. So it's a difficult decision.

One more thing to try is taking the steel plate out of the ek. This will cut down resistance enormously but kills the impingement mechanism, could be better or worse. I suspect better with low flow rate.