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What can my pump handle? A guide.

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post #1 of 114 (permalink) Old 09-03-2011, 09:11 PM - Thread Starter
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Choosing the Correct Water Cooling Pump
A guide by charliehorse55




What flow rate do I need in my loop for the best performance?
Most loops achieve maximal efficiency around 1.0 GPM. Increasing the flow rate beyond this point as little to no effect on temperature for most modern blocks, and the extra heat dump from the added pumps required to reach higher flow rates can actually have a larger impact than the increase in flow. This is because the heat capacity of water is incredibly high. At 1.0 GPM of flow, it takes over 250W of heat to raise the water a single degree.
Math (Click to show)

Heat capacity of water = 4187j per kg
1.0 GPM of flow in litters per second = 0.063 kg/second
4187j*0.063 kg/second

A joule is 1 watt for 1 second, so the units of time cancel out:

4187W * 0.063 kg = 264W

264W to raise a 1.0 GPM flow of water by a single degree

Let's assume you are running a CPU + dual GPU loop. With a heat dump of around 200W per component, the last component in the loop will have the water entering the block at 1.5C higher than the first. If you increased your flow rate to 2.0 GPM, this would drop to 0.75C, lowering the GPU temperature by 0.75C. This is not a huge gain, but it is appreciable, until you factor in the added heat dump from the pump. To get to 2.0 GPM on a dual GPU + CPU loop you would need to add another 30W of pump at a minimum. Even with a quad radiator, this is going to increase your water-air delta by about 0.2-0.5C, effectively raising the temperature of all of your blocks by that much. All of a sudden these small gains become absolutely meaningless.

If the heat capacity of water is so high, why can't I have less than 1.0 GPM?
At low flow rates, the performance of your water blocks drops dramatically as the flow inside of them switches from turbulent to laminar. Turbulent flow occurs when a lot of water is going through a small pipe. When water is flowing this way, it creates random vortices that ensure the water is constantly mixing, and that fresh, cold water is constantly touching the waterblock. If the flow rate is low enough, the water switches to flowing in a laminar way. When water flows in this way, it does not mix as much, and the water on the edges may be significantly warmer than the water around it. This is bad for performance, as heat is not transferred away from the block as quickly.



Older water-cooling equipment used very open blocks with large openings for water passing through, making it very easy for a laminar flow to develop. This is why so many older blocks are incredibly flow dependent, requiring high flow rates to get good results. Modern blocks use very small channels, so that turbulent flow occurs at much lower flow. Almost all waterblocks today are designed to have turbulent flow at low flow rates, and as such the temperature gains from higher flow rates have largely disappeared. Most blocks will continue to perform down to about 0.6-0.7 GPM, but it's a good idea to have around 1.0 GPM to have some headroom.

How do I know which pump I need to achieve 1 GPM?
Pumps are rated on a pressure/flow curve. At lower flow rates, they can produce more pressure, and at higher flow rates, less pressure. The blocks in your loop provide restriction that scales in the opposite direction, as higher flow rates lead to a greater pressure difference being required. The flow rate of your loop is determined by the intersection of these two lines:



A more detailed explanation of this graph can be found on xtremerigs.

Can't you simplify this?
Yes. You don't have to find the exact intersection of those two lines because you don't need to know the exact flow rate of your loop. You just have to make sure that is greater than 1 GPM. You can add up the pressure drop created by our components at 1 GPM, and then make sure you have a pump capable of that pressure at 1 GPM. Finding the right pump becomes as simple as adding up a few numbers!

I've made a rough guide to get a general idea of how much pumping power you need. The data on the pumps is deadly accurate, however as there are many, many different types of blocks it would be mundane to list all of the pressure drops (and there isn't data available for everything!). Feel free to substitute any value in the first chart with ones specific to your parts. (You can find this data on review websites).

First, add up the restriction of your loop (these values are safe averages)

Pressure Drop figures for components
Radiators: 0.25 PSI each (the size of the radiator has no real effect)
CPU Block: 0.9 PSI
GPU block: 0.9 PSI
Motherboard block: 2.0 PSI (fullcover)
Fittings: Negligible (An extreme setup with 10+ 90 deg fittings might add 0.5 PSI in total)
Tubing: 0.5 PSI per 10 feet for 3/8" ID, negligible for 1/2" ID.
Reservoir: Negligible

If you run your GPU blocks in parallel, divide the PSI drop by the number of blocks, so if you have 3 blocks in parallel you would only add 0.3 PSI to the total.

*Exceptions*
HW Labs GTX Radiators are around 0.7 PSI each
Anything by aquacomputer is about double the normal pressure drop

Now that you have the total pressure drop for your loop, find a pump that can handle at least that much pressure at 1 GPM of flow.

Maximum Head Pressure at 1.0 GPM

Iwaki RD-30: 13.72 PSI
MCP50X Stock: 6.54 PSI
MCP35X Stock: 6.52 PSI
MCP355 + Top: 6.14 PSI
MCP655 + Top: 4.95 PSI
MCP655 Stock: 4.72 PSI
MCP355 Stock: 4.63 PSI
MCP350 + Top: 4.48 PSI
DCP 4.0 Stock: 3.83 PSI
XSPC 750: 3.59 PSI
655-B + EK Top: 3.38 PSI
MCP350: 3.25 PSI
655-B Stock: 3.23 PSI
DCP 2.6: 2.55 PSI
Eheim 1250: 2.41 PSI
DCP 2.2: 2.08 PSI
Eheim 1048: 1.78
Eheim 1046: 1.02 PSI


Double pump setups are usually almost exactly double the performance of a single pump. Keep in mind, you should also choose your pump based on how loud it is, how much power it uses and how it will fit into your case. This is merely a guide to help you choose a pump that is powerful enough to handle your loop.

Feel free to ask anything, I will clear up any questions you may have and potentially add it to this post.
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post #2 of 114 (permalink) Old 09-03-2011, 09:16 PM
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post #3 of 114 (permalink) Old 09-03-2011, 09:40 PM
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post #4 of 114 (permalink) Old 09-03-2011, 09:42 PM
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Can't wait to read this, sub'd also.

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post #5 of 114 (permalink) Old 09-03-2011, 09:47 PM
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Well, now you've let the hype set into people's hearts... make sure you deliver biggrin.gif


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post #6 of 114 (permalink) Old 09-03-2011, 09:56 PM - Thread Starter
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post #7 of 114 (permalink) Old 09-03-2011, 09:59 PM
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Great guide, nice sunday read!

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post #8 of 114 (permalink) Old 09-03-2011, 10:48 PM
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This is exactly what everyone just getting into watercooling needs to read(me). I didn't feel confident with my setup up until this very moment.

Great work! this needs to be stickied

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post #9 of 114 (permalink) Old 09-03-2011, 11:32 PM
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Super. You should add the Aquacomputer/Eheim and Iwaki pumps to the list, since some enthusiasts do use those. wink.gif I have also asked Juggalo23451 to determine sticky eligibility; he will be responsible for making it so in that case.


Combined air and water cooling tips: Fan database | My San Ace 9G1212H1011 review | Don't mix metals or use premixes / dyes | Distilled vs. tap water | Good radiator brands | Water (almost) can't flow too fast | Which PT Nuke to use | Regarding thermal paste spreading
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post #10 of 114 (permalink) Old 09-03-2011, 11:36 PM
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What are the restrictions on RAM and Mobo blocks?
This is very informative. Awesome work

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