Also the point about pump heat being fairly moot with a radiator may be true for ambient cooling, but it's not for sub ambient cooling, you want the least amount of heat into your system as possible so you can have the coldest temps going to your components.

Of course, and I've even said that myself, in another thread, where I wrote something along the lines of 100 gph at 10 psi, isn't the same as 100 gph at 100 psi.

So I need to know the pressure drop between the pump inlet and outlet, as well as the effective flow rate.

For the same flow rate, yes, but you won't achieve the same flow rate with the same pump, wether its because of the density, or the viscosity. although, I remember reading somewhere though, that most pumps reach the same flow rate, regardless of fluid density, but since a heavier fluid will probably be thicker too, we just can't picture it.

I need a viscosity table! I believe that mercury, although lots denser, might have a similar viscosity as water. Does anyone know?

(It doesn't mean that mercury is a better coolant, we can look into that some other time!)

Mercury is a much better coolant it is about 16 times better than water... just really dangerous and I think pretty hard to pump, but maybe not.

-Sidney

OOOOO! I know the answer to that one!

Actually, there's plenty of examples. Water is more dense than oil, yet has far less viscosity (especially at low temperatures).

Water is actually pretty unique in its cooling abilities. Its heat transfer characteristics are better than darn near everything and its viscosity is also lower than darn near everything else. Too bad it doens't do well below 0°C. But that's where additives come into play.

Ok, fluid viscosity, measured in Pa*s for

Water at 0C=1.8 * 10^-3
Water at 20C=1.0 * 10^-3
Mercury = 1.55 * 10^-3

Note: Liquid viscosities tend to decrease with increasing temperature

From
http://www.phys.virginia.edu/classes...ds2/node2.html

and
http://www.mas.ncl.ac.uk/~sbrooks/bo...01/node13.html

Hmm yeah thats true I forgot about water and oil. Well I guess density doesn't necessarily affect viscosity. I have tried looking up better liquids and the only one that I could find was mercury it's thermal conductivty is 8.something and water is about 0.6

Yeah, mercury is nasty stuff!
http://danpatch.ecn.purdue.edu/~epad...src/poison.htm

http://www.healthyvermonters.info/hp.../mercury.shtml

*sigh* Nothing is ever as cut and dried as you would like. There are two measures of viscosity, dynamic (absolute) and kinematic. The difference between the two is inclusion or exclusion of the material's density. In dynamic terms, the viscosity of mercury and water isn't so different and if water didn't freeze they would be equal at ~ -5 to -10°C. In kinematic terms, mercury's density makes its viscosity much, much lower than water's.

IIRC, its the absolute viscosity that goes into calculating pump flow rates, but don't quote me on that one.

Hmm well if mass doesn't affect the pump then I guess you can use mercury no problem (except for the toxicity problem). Maybe someone should try that, would look really neat in some tygon tubing, just make sure that your tubing won't absorb it (You sure don't want mercury vapor around you).

-Sidney

NO!

Mercury is very, very nasty stuff! If you spill it, it will be released in the form of vapors, and your body can absorb a lot of it, very quickly!!!

You can't even pick it up with a paper towel, because if you touch it, you'll absorb even more!!!

Yeah I know, although I don't think it's as bad as they make it sound because I personally have played with the stuff and didn't get sick or anything (touching it pushing it around messing with it quite a bit), and that was when I was only about 7 or 8 so I should have been affected much quicker than an adult. (What can I say I was a curious kid and found a mercury filled thermometer).

Edit:
Although I don't suggest that anyone do what I did, or that anyone actually mess around with mercury.

-Sidney

It certainly would take care of a few problems though: corrosion, algae, ...

(Note: don't even think about it!)

Hehe, that's very true. And for the people using chillers they wouldn't have to worry about it freezing ;) And you could see really easily if you were leaking any.... or you could just do a self evaluation (am I going mad... am I going mad...) hmm might not work for some people though :D

I imagine it's fairly hard to get a large amount of the stuff though (probably a good thing ;) )

Well, I found an internet link, but I don't want to post it.

They sell it for over $260 per pound, and seeing that a cubic foot of it weighs about 850 (eight hundred and fifty) pounds, I'm sure that it's not a lot, in volume.

Also, since it is hazardous, it'd be VERY expensive to ship.

Yeah... I remember seeing an experiment on PBS where they mixed something like corn starch, or some other kitchen product, and they showed that it looked really liquid, almost like water, but then the guy puts his hand in it, and quickly lifts the plate (an alu pie plate) and them slams it right back down. The plate followed the movement perfectly.

He was demonstrating that the viscosity changes, so I guess that's an example of dynamic (absolute) viscosity, versus kinematic viscosity. Sound about right, myv65?

(In this case though, I think the guy was demonstrating that some kind of starch has a low dynamic viscosity, but a high kinematic viscosity)

In one of the science links (above) it says this:
"Kinematic viscosity is defined as the ratio of the viscosity to the density."

Does that make any sense?

The most important reason for not using mercury vs. water is not visc. its Specific gravity. Mercury is by weight to volume inefficient. On the other hand if you take water and add a gylcol mixture to a 20% ratio you enable a better transfer of heat from the medium to the radiator. While inhibiting thermal gains from the pump.

To further improve Delta T across a radiator you should have the pump after the radiator with a larger inlet than discharge.

I do this for a living and have just started to look at trying my hand at cooling my PC this way. I'll post some Ideas soon.

Looking forward to it!:D

I got into this thread becasue it's a topic of interest. For me (maybe just a few of us), I have a need to understand the physics behind it all.

Otherwise I agree, everything is going to come down to the rad.

So far, I think I've explored every component individually (or at least tried!) so that eventually, I'm able to understand how it all comes together.

It's clear to me though, that the rad is going to be the most difficult of all, since there really isn't any kind of standard for them. Fans have standards, pumps have standards, but rads are a whole different story.

So far, I've established that a higher pressure (higher velocity) can dissipate more heat. Good for the block, good for the rad. So I'm thinking of trying to build a rig with an effective flow rate of about 200 gph.

I've also spent a good deal of time looking into phase change, and it seems that the easiest thing to do, is to replace the rad with the bucket inside a humidifier, where you turn the cold coils inside the bucket.

Again, shooting for 200 gph (effective), it would be a fair bit easier to achieve, since there's no restriction from a rad, but as this thread indicates, I'd hit a wall because my pump would induce a fair amount of heat.

At this point, the block design matters a lot more, because I want a high pressure, but not so much that a pump will induce a lot more heat.

So I'm down to a cross-drilled design (which I believe to be of the best), but I'm still looking...

Interesting... but even if mercury is "by weight" not as efficient as water, isn't it more efficient, for the same volume?

Rad>pump huh? I've been recommending rad>block (with the pump anywhere) so that the coolant entering the block is at its lowest temp. Am I wrong?

"...with a larger inlet than discharge." That makes good sense, but I think that most of us are stuck with what the pump manufacturer dictates. I take it that the reason for this is to minimize the pressure at the pump inlet, so that the pump can be a little more effective? (I'm stretching here, I can almost see it, but I'm not there yet!) Which would mean that my rad>block suggestion is still good, but the tubing size to the pump inlet should be bigger (biggest of all tubing/channels)?

(That would be consistent with a lot of high power pump setups I've seen)

I agree that it is a very interesting thread. I enjoy learning from those who know a lot more than I do about the physics of heat and water.

Very interesting to see the science of PC watercooling mature.

Myrd, I am confused by your statement. Pump adds heat to water, water flows into radiator where heat is dissipated. The only thing I can think of is that you are indicating that once the water has gainied its maximum amount of heat(after WB and pump) that the radiator will be at its maximum effectiveness. Do you agree?

Here is a link to a resource for liquid weights and densities. This page shows the effective increase in work required by a pump to move a liquid.

Specific gravity chart

A normal impeller pump is limited in its ability to move liquid beyond a certain head pressure. As specific gravity increases losses in the impeller due to 'Slip' cause the effective losses to entropy to compound. Water at a specific gravity of 1 is roughly 16 times easier to move. If you were to replace the impeller type pump with a positive displacement pump you would gain a higher head capacity. But this would still not allow you to achieve the same efective cooling as with a chilled water medium.

This is the direct heat gain profile of most water cooled systems
[list=1][*]System pump imparts energy to water in the forms of Flow, Heat, entropy[*]Water passes to tubing which will remove heat from the enclosure to due Entropy. Heat always migrates to cold.[*]Water now slightly warmer 'Calories in gain from the tubing more dependent of Flow rate' moves to cooling block of component.[*]Based on temp. difference between the water and the cooling block and again the flow rate. Heat migrates to the liquid medium.[*]Water enters tubing where gains from the enclosure are now almost nonexistant.[*]Water enters radiator where in an ideal system it should reduce in velocity to enable a greater thermal transfer. [*]Water returns to the pump where velocity increases again.[/list=1]

The heat gain from the pump is not worth the reduced capacity caused by it preceeding the radiator. You want to have the least possible loss to entropy at the cooling block. Entropy caused by the tubing and fittings is from 'Laminar Flow'.

:) For once my job actually is fun!

Myrd,

I appreciate your enthusiasm, but a couple of your statements are either misleading or altogether incorrect. The two that particularly stick out regard heat moving from the enclosure through the tubing to the fluid and the second regards slowing flow in the radiator.

For starters, our plastic tubing is a pretty good insulator. Even if the fluid was 10°C cooler than the air within the case (extreme delta T) there would be little energy exchange across the tubing. As to the radiator, one only needs to consider heat flux between the tubing within it and the fluid. Heat flux integrated over the total surface area equals the total heat transfer. Heat flux depends on many factors, one of which is velocity. Higher velocity equals higher heat transfer coefficient. Higher heat transfer coefficient equals greater heat transfer for a given delta T or lower required delta T for a given amount of heat transfer. The differences are pretty minor and overall results are impacted a lot more by air flow, but nonetheless it is a fact that heat dissipation in a radiator will be more efficient at higher velocity. Only when the energy required to drive the higher flow rate exceeds the incremental gain in convection coeffient will reducing flow improve heat exchanger efficiency.

If you want the coolest fluid to strike the block, there can be no doubt that the radiator should preceed the block. Again, for "typical" flow rates, the differences are all but immeasurable, but it does not change the fact about which location is "best". On a more practical note, what really ought to dictate radiator position is which option yields the coolest air to the fans. A 2°C drop in air temperature will have a larger effect than moving the radiator ahead of vs after the pump.

I have a made a few changes from this photo. I have added a new TC-4 and I have removed all the PCI case covers. You will see in the photo air is pushed over the pump and out the back of the case. Now for heat being added to the water from the pump is entirely true.

I shut down the computer, not one fan was running in the entire system. Only the Ehiem 1250pump was running. I ran the pump over night. The ambient air temp inside the house was 27c and the water temp was a cool 46c.

Bump.

I've got 2 follow up questions, as I'm selecting my pump.

1- If the pump induces heat (and we've shown that it does), WHERE does this heat appear? I mean, if the heat comes from water friction on itself/tube walls, wouldn't the heat appear there? Wouldn't that mean that the heat appears mostly where the restrictions are, i.e. the rad and waterblock?


2-I'm shooting for a high flow rate, but I'm not willing to spend $100 on a pump, so I'm thinking about using two Via 1300 pumps in series. I realize that they probably won't be running in a very efficient range, but I'll get the higher head. Comments?

Hey Ben,

It's all about energy. You get heat as a by-product of energy dissipation in the flow. Energy dissipation in the flow is nothing more than pressure drop. So yeah, you get some small heat generation simply flowing through tubing, but the majority comes where the big pressure drops are. This will be any restrictive fitting, the block, and to varying degrees within the radiator.

And to answer #2, yes, you'll see higher total head with two pumps in series. This will result in higher total system flow, but probably lower overall pumping efficiency unless everything in your system is very low resistance.

That's what I thought. Thanks Dave!

Anyone care to comment on using two pumps in series for more head?

Before i got the last pieces to my system (CPU and MB), i made a little (and somewhat unscientific) test of the amount of heat, the pump puts into the water.
test setup:
Ambient: 22,6
Eheim 1048 (600Ltrs/h @head)
radiator: BlackIce (classic i think)
block: DD Maze2
Senfu temp-diode (which i think i pretty accurate)
hooked it up like this:
pump, radiator, WB, res, pump.

filled and bleed the system, and put the probe between the fins of the rad. I let it sit still overnight, so the watertemp would reach ambient. startingtemp was 22,6C measured between the fins in the rad. fired up the pump, and had it run overnight: the following morning, the temp was at 28,4C, and ambient still 22.6C
made the same test again, but measured the temp on the WB instead - watertemp this time (measured on WB) 29,4C.
So, although not all that much, the pump does add somewhat heat to the loop, but nothing significant (at least for the Eheim 1048).

Hum... I was considering 2 Via Aqua's in series, until I came across the Rio 2500HP:

flow @ 0head: 782 gph
max head: 10 ft
Power: 55 Watts
3/4 connections
found it at $34.

The Via pump
flow @0: 370 gph
max head: 6 ft.
Power: 20.5 Watts
1/2 connections
Can be found at $18.

What to do, what to do...