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A few words about pumps and flow rate.

First there's the cooling part of things, with a density of approximately 1000kg/m^3 water is around 830 times denser than air at 1.2041 kg/m^3.
Air has a specific heat capacity of approximately 1 kJ/kg/K whilst water at 25 degrees C is about 4.2 kJ/kg/K. Multiply out these factors, and you end up with a cooling performance of around 3500 times the performance of air by volume (factors such as boundary layer thickness and turbulence excluded). The flow rate of the water is going to make a factional difference to you cooling unless you've got a preposterous amount of things in your loop like 4 gpus, 3 rads, cpu, hard drive coolers etc. We are literally talking 3-4 degrees at most, certainly nothing worth worrying about.

Secondly, there's the way pumps perform. Pumps do NOT create pressure. They create flow. Resistance to flow creates pressure. The more resistance, the more pressure at any given flow rate. If your pump is only capable of sustaining a certain pressure differential across its inlet and outlet, as you increase the resistance, the flow rate will decrease until the system balances.

So for dual pumps...
It really depends on your loop order, if you have a huge number of components in series, they represents a very large resistance to flow. Placing two pumps in series will allow the pumps to provide a much greater effort in terms of pressure, as the maximum pressure they can support for a given flow rate is essentially doubled (think of this as simply adding pressure).
Placing two pumps in parallel, the pumps both produce the same volume of flow, and this flow as you would intuitively think adds together. It gives you no advantage in terms of pressure. You would only use this configuration if you had a vast number of components in parallel. And I mean 4 or more upwards before you start to notice a difference.

Of course, what speed you wish to run your pumps at will introduce sensitivity much much earlier. I barely have my DCP4.0 turned on because its a noisy ****er and I HATE noise. Despite my CPU GPU and GPU all being in parallel, I noticed only about a 6 degree rise in my delta temps between this and when it was just my CPU and radiator and my pump running at full pelt because my fans were too noisy to hear it.

Its really very very marginal and essentially, you shouldn't worry about it. but if you do for any reason, and your loop is entirely off the chain, then it might be worth planning you loop and pump config to get the best noise to flow ratio.

You'll have plenty of water either way.

(My reply to another thread, thought this info could be useful as I see it talked about quite a bit).

EDIT There's a long discussion following this post about the specifics which I urge you to read because they way I've worded some stuff has caused confusion and disagreement. Read both sides for yourselves.

Some significant points from the discussion I could have been clearer about though:
• Pressure IS important in loop design, regardless of what your pump is trying to achieve, there will be pressure in your loop, your pump will be the item exerting force on your coolant and creating that pressure.
• The pressure is a reciprocal system of your blocks and rads creating a resistance to flow which takes pressure to overcome in order to achieve the flow your pump is aiming for.

Will add any that I have forgotten if people would like to point them out.

/EDIT
Edited by YowZ - 2/4/13 at 1:27pm
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So you're saying that as long as the water is moving it will keep a relatively similar peformance in temperature drops?

What about with a coolant instead of water then? Wouldn't the higher heat capacity perform better for temperature drops requiring only a pump strong enough to move it?

Also how do you feel about a pump pushing to res rad to cpu to pump? Is there a noticeable change in pressure where reservoir is located and the size?

What do you mean placed in parallel could you please clarify? I apologize I am new to this... my thought is you have it placed so it goes to 2 pumps simultaneously via a 'y' split then onwards...
Pretty much, I mean, if it slows to a snails pace you will start to notice a rise in temps but generally the limiting factor in a water loop is the TIM between the block and chip.

Coolant and water, coolants are normally 97-99% water with additives, they're not gonna perform significantly better or worse than water.

Res should always come before pump, if you start the loop without coolant/water in teh pump it will run dry and knacker its bearings and it will be noisy and useless. If your flow rate is VERY low, having rads before blocks may make a difference in temps of a couple of degrees, but in most applications the flow rate is so high that it doesn't matter, and reservoir size is entirely aesthetic.

You could use two Y splitters, or you could use a dual pump block, which essentially acts a manifold.
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dual pump blocks come in parallel and series btw
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A couple of notes:

Part of the water cooling "hobby" is extreme cooling and despite the practical viewpoint of a few degrees doesn't matter, many users do enjoy the challenge and reward of those few degrees. Since most products perform within a few degrees the practical view could and probably should simply install a set of products with adequate capacity and move on...never to join in these forums for further discussions.

However, those few degrees no matter how practically insignifcant are desired by most participating here, so it is important to the users here in these forums...no?

With that said, the root of the performance discussion almost always results in comparing and evaluating degrees and even fractions of degree because that is what we have. Once you have exhausted the 5-15 degree gains within water/air deltas by going with really large radiators, what is left are the blocks and pumps. The blocks are all just a few degrees apart and so is the result with varied pumping powers. I have waffled myself going back and forth between "it doesn't matter" and back to normal and all I can say I agree that practically it does not, but personally yes it does and everyone here wants the absolute best in performance AND aesthetics because it is what makes our little niche of a hobby fun.

While I agree that it is restriction that relates to pressure conditions, saying a pump only creates flow isn't correct either. A pump converts mechanical energy into both flow rate energy and pressure energy and these two resulting energy forms are equally important. Many industrial grade pumps such as the Iwaki series have multiple impeller options to basically tune the pump performance toward one or the other depending on the application. There is much more to pumps than flow rate. Actually in watercooling we tend to have loops that are more restrictive and favor pumps with high pressure and head capabilities over flow capabilities. A D5 vs DDC is a perfect example. The D5 has a higher max flow rate, yet in most watercooling loops the DDC will net more flow because of it's better pressure capabilities in areas (1-2GPM) where it matters.

Also pump head is critical in non closed loops such as water supply systems and all sorts of industrial conditions. In a well water is pumped UP out of a well and then lifted UP to a water tower. Those pumps even without any restriction need to overcome the static head differentials of lifting water a significant amount where pump pressure head is critical. So pressure drop or restriction is just one of several forms of pressure resistance outside of the closed loop.

IMO, a few degrees are important to most users here and a pump's pressure performance is generally MORE important than it's flow rate capabilities. It is true that flow rate is all that matter in the result, but pressure capabilities is critical in overcoming the restriction and producing that desired flow rate result. I do completely agree that it is overemphasized, but so is pretty much everything else in watercooling.
Edited by Martinm210 - 2/2/13 at 9:36am
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Is there a simple guide t calculate pressure needed for a setup or do I need to find the product then check statistics on that particular model once I've determined its what will be used? Aiming for the lowest temperature possible, every little degrees dropped helps.
Quote:
Originally Posted by 111ch1

Is there a simple guide t calculate pressure needed for a setup or do I need to find the product then check statistics on that particular model once I've determined its what will be used? Aiming for the lowest temperature possible, every little degrees dropped helps.

Martin's Pump Planning Guide

Check out his archives too for the pump planning tool but that's probably outdated.
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Quote:
Originally Posted by Martinm210

A couple of notes:

Part of the water cooling "hobby" is extreme cooling and despite the practical viewpoint of a few degrees doesn't matter, many users do enjoy the challenge and reward of those few degrees. Since most products perform within a few degrees the practical view could and probably should simply install a set of products with adequate capacity and move on...never to join in these forums for further discussions.

However, those few degrees no matter how practically insignifcant are desired by most participating here, so it is important to the users here in these forums...no?

With that said, the root of the performance discussion almost always results in comparing and evaluating degrees and even fractions of degree because that is what we have. Once you have exhausted the 5-15 degree gains within water/air deltas by going with really large radiators, what is left are the blocks and pumps. The blocks are all just a few degrees apart and so is the result with varied pumping powers. I have waffled myself going back and forth between "it doesn't matter" and back to normal and all I can say I agree that practically it does not, but personally yes it does and everyone here wants the absolute best in performance AND aesthetics because it is what makes our little niche of a hobby fun.

While I agree that it is restriction that relates to pressure conditions, saying a pump only creates flow isn't correct either. A pump converts mechanical energy into both flow rate energy and pressure energy and these two resulting energy forms are equally important. Many industrial grade pumps such as the Iwaki series have multiple impeller options to basically tune the pump performance toward one or the other depending on the application. There is much more to pumps than flow rate. Actually in watercooling we tend to have loops that are more restrictive and favor pumps with high pressure and head capabilities over flow capabilities. A D5 vs DDC is a perfect example. The D5 has a higher max flow rate, yet in most watercooling loops the DDC will net more flow because of it's better pressure capabilities in areas (1-2GPM) where it matters.

Also pump head is critical in non closed loops such as water supply systems and all sorts of industrial conditions. In a well water is pumped UP out of a well and then lifted UP to a water tower. Those pumps even without any restriction need to overcome the static head differentials of lifting water a significant amount where pump pressure head is critical. So pressure drop or restriction is just one of several forms of pressure resistance outside of the closed loop.

IMO, a few degrees are important to most users here and a pump's pressure performance is generally MORE important than it's flow rate capabilities. It is true that flow rate is all that matter in the result, but pressure capabilities is critical in overcoming the restriction and producing that desired flow rate result. I do completely agree that it is overemphasized, but so is pretty much everything else in watercooling.

Oh yeah, it is an enthusiast hobby, and if you can net 15 degrees then you're on to a winner, but its only really worth it if you are doing some seriously extreme overclocking IMO. Its when you have a guy with 2 rads, a cpu and a gpu worrying about how many pumps he needs that you wanna turn round and say, it doesn't matter, picking the right pump in the first place will have much more of an effect.

I must correct you on the pressure thing though, and there is no such thing as pressure "energy". The only places energy goes in a loop that is in a steady state is electrical to thermal (motor) and kinetic (fluid). It then goes from kinetic to thermal via fluid friction as it passes through the loop. The geometry and speed of an impeller will effect its flow and pressure characteristic but ultimately the pressure only exists while there is resistance to flow.
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Quote:
Originally Posted by YowZ

Oh yeah, it is an enthusiast hobby, and if you can net 15 degrees then you're on to a winner, but its only really worth it if you are doing some seriously extreme overclocking IMO. Its when you have a guy with 2 rads, a cpu and a gpu worrying about how many pumps he needs that you wanna turn round and say, it doesn't matter, picking the right pump in the first place will have much more of an effect.

I must correct you on the pressure thing though, and there is no such thing as pressure "energy". The only places energy goes in a loop that is in a steady state is electrical to thermal (motor) and kinetic (fluid). It then goes from kinetic to thermal via fluid friction as it passes through the loop. The geometry and speed of an impeller will effect its flow and pressure characteristic but ultimately the pressure only exists while there is resistance to flow.

Like I said, I flop back and forth, but I and a lot of people in these forums do care about a few degrees or even one degree. It is a personal value and yours is different, that's fine..

If you disagree about pressure being a form of energy or power, then why is it a critical measure in determining water horsepower.

http://www.gemi.org/waterplanner/calc-horsepower.asp

To calculate pump efficiency you measure water horsepower vs brake horespower and the water horsepower calculation includes both flow rate and pressure head energy in the equation.

If water pressure head didn't have any energy, then we should not worry about dam failure as it would not have any energy to cause any damage correct? If pressure energy didn't exist, we also wouldn't have hydro electrical power generation, these are all very much a form of water pressure potential energy.

Water under pressure, or water lifted up in elevation are the same in regards to pressure head and are a form of stored potential energy that can create work. The pump converts electrical energy into thermal, and mechanical energy. The mechanical energy is then transfered into forms of kinetic, potential, and thermal energy. A good 75-85% of the electrical energy is lost in the process, but several forms of energy are created...kinetic is only one of many.

Perhaps pressure head and lift are different, but they produce the same pressure head number. My point was that ignoring pressure in pumps is missing a huge part of the picture, without pressure energy, you could not overcome the restriction and pumps vary quite a bit in there pressure vs flow rate performance and tuning.
Edited by Martinm210 - 2/2/13 at 6:10pm
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That's because to calculate power (or energy) in a dynamic application you typically need force and velocity (or displacement for energy). Pressure is force per unit area and flow rate is velocity times unit area. Pressure is just force over an area, with nothing to push against, there is no pressure.

Head of water is density x gravitational acceleration x height, its weight creates pressure.

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