# Series Versus Parallel Liquid Cooling Loops

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Which cools better?

There are many overclocking sites that overclock and provide benchmark results. Some run single loops between the CPU and GPU cooling blocks. Some run two loops between the CPU and GPUs. The benefit of the two loop set-up allows for the hot water exiting the cooling block to go directly to the radiator. Single loops send the hot water exiting the CPU block to the GPU blocks. Therefore, the GPUs are getting hot water which is used to transfer the heat off them.

Now lets look at the GPU cooling, regardless of the inlet water source. With many multi-GPU configurations, the graphics cards are also placed in a single loop for liquid cooling. This means that the hot water exiting card 1 is sent to card 2 to cool it. In 3-way SLI, the 3rd card is getting hot water from BOTH card 1 and 2.

Overclocking a processor causes more heat to be generated. As the heat increases, the processor's stability decreases. Therefore transfering the heat away from the surface of the processor allows for stable overclocks. So it is important to get the heat transfered efficiently. Water cooling does this well and provides good temperatures at increased loads.

So my theory is:

To get the best heat transfer and coolest operating temperatures, you need to have the lowest inlet temprature for EACH device using liquid cooling. Thus you can overclock your processor HIGHER with better stability.

So I got my old Thermodynamics book from college. I found the equations used for conductive and radiant heat transfer.

Q=-kA(Ts-T)

Q: heat transferred (the higher the more heat removed)

h: heat transfer coefficient (constant for H20)

A: area (surface area of block)

Ts: temperature of the surface

T: temperature of the medium (water)

So without doing any math, we can see the relationship between the amount of heat transfered to the difference of the surface and medium temperatures. The higher (Ts-T) is, the greater the Q will be. So if we have a CPU overclocked to a set frequency, the amount and temprature of heat needing to be transfered is constant. So the variable we can control to increase the Heat Transfer (Q) is T, the temperature of the inlet medium (water).

Let's do some math to see how this theory would factor against the heat transferred (Q). A and -k are constant so we don't need to analyze their effect.

Series

(Ts-T): 50C-40C=10C

Parallel or indepedent water source per device

(Ts-T): 50C-35C=15C

With a simple 5C variance of the inlet water temperature, the Heat Transferred (Q) increases by a factor of 10 to 15. We can derive a formula that calculates the factor in Series versus Parallel Heat Transfer since A and -k are constant.

Qp: parallel heat transfer

Qs: series heat transfer

added s and p for the corresponding temperatures of the parallel versus series quantities

Qp/(Tsp-Tp) = Qs/(Tss-Ts)

thus,

Qp/Qs = (Tsp-Tp)/(Tss-Ts)

So using our 5C temperature variance from above we get:

Qp/Qs= 15C/10C

thus,

Qp = Qs(3/2)

Hence, the amount of heat transfered in the parallel loop is 3/2 times the series loop. With a 5C temperature drop in the inlet temperature, the parallel solution gets you 1.5 times the heat transfer. If we had lowered it another 5C the factor would be 2.

In conclusion, running series cooling loops lowers your heat transfer ability and limits your maximum overclock with stability.

It would be interesting to see if someone would try an experiment on graphic cards using this theory. They would have to split the tube coming out of his resorator and run seprate inlet lines to the cards. Then bring them back together as the water returns to the resorator.

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What exactly do you mean by "resorator"? The Thermaltake hybrid radiator/reservoir?

In practise, the water temperature before and after any component will only have a very small delta of a few degrees C at most, passing water over a 150W heat source at high speed isn't going to make it boil...

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In practise, the water temperature before and after any component will only have a very small delta of a few degrees C at most, passing water over a 150W heat source at high speed isn't going to make it boil...

That, and I imagine running a couple graphic cards and CPU in parallel would make a tubing mess.

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Have tried both parallel only yielded a 2C difference.

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That, and I imagine running a couple graphic cards and CPU in parallel would make a tubing mess.

I run two separate loops (CPU; Everything else)... and it's not exactly easy to work on my main rig lol

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But the more loops you have, the more flow is reduced.

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NOt necessarily. He has 2 different loops so the flow reduction is not there. Now when it comes to adding more devices into a specific loop you end up with a flow reduction. I consider a "loop" 1 sealed system not a jump from the CPU to the NB to the VRM to the res and back to the pump. That would be 1 loop not 4.

The benefits of a parallel system could be there and has avidly been discussed. With the hard pipe adapters available now as well as the parallel flow tops available from som manufacturers of full cover blocks its easier to plumb(1in and 1 out). But overall volume to each card is reduced. The last time I ran 2 cards with full cover blocks they were run in series and I really did not see more than a 2 to 3 degree difference in the cards. This was significantly less than what I saw with air cooling so it was a win.

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The last time I ran 2 cards with full cover blocks they were run in series and I really did not see more than a 2 to 3 degree difference in the cards.

This has been the case for me for:

2x 7800GT DD Maze4 low-profile (GPU only)

2x 8800GTS DD NV-88GTS (full coverage)

2x 8800GTX DD Maze4 (GPU only)

I can't see where you got 5

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Pretty simple fix this is what im doing cpu-radiator-gpu1-gpu2-second radiator-pump just run a radiator inbetween and ur temps go back down

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If you know the flow rate and the GPU's TDP (estimated, calculated, whatever), you can figure out what the temperature increase the water will undergo as it passes through a GPU waterblock.

delta u = m c_v delta T

u being internal energy...Now, TDP is in watts (power)...so...

delta u / t = m_dot c_v delta T

where delta u / t would be the power and m_dot is mass flow (pumps are in volumetric flow, multiply this water's density to get mass flow rate).

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That is exactly how I had my old box setup

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