# Real world usage; pump flow vs pressure vs pump head, what is more useful in this situation?

I am currently making a water cooling system for my computer and decided to see if I could get some better help here than what people think they know on the computer forums.

So for my cooling system I have three, what are called, water blocks. They have a inlet and an outlet and are the biggest source of restriction. I specifically have three, and they each have 1/4in input and output barbs, I then connect them all with two manifolds.

The manifold is made of 3 PVC T fittings and an end cap. So all the T's are setup such to look like this TTT . One of the ends of the tops on the T's is capped and the other is the input/output, while the bottom of the T's goes to and from the water blocks. (I hope this is very easy to visualize, if you cannot ask me and I will try to explain more)

The input for each manifold is 1/2in tubing and the T's run to x3 1/4in tubing lines. So the water enters in a 1/2in tube, is split to x3 1/4in tubes, then combines back into a 1/2in output tube.

Each waterblock is responsible for, at max, 190watts of heat to dissipate. So the entire loop must remove ~600watts of heat + the pumps heat = ~625 watts. This is where the radiator comes in, I found a nice chart that tells me the size I need to remove X watts from a system so I know I have the right size radiator. The radiator has 1/2in tubing inputs and outputs (but it is restrictive cause of the inside geometry.

So my question is, what should I consider in a pump when trying to find one for this system?

People talk of flow rate, vs system pressure, vs pump head. I know the head is how hard the pump can push with no restriction in its flow. So I figure pressure is combined in the pump head figure.

What no one talks about though is how to find out how much flow and pressure is needed for the water cooling loop to work as it should. Even if I have the right size radiator to remove the amount of heat I have it is worthless if the water is not moving in the right manner.

So can I possibly buy a pump that has too much flow; how can I figure the minimum amount of flow needed for my loop to work?

EDIT: Ok I have found out a little bit more..

So I figure part of what I need to know is the bernoulli equation, which will describe the flow and pressure of the water at a certain point in the system.

What I guess I need to do is equate the amount of heat water can remove at a specific velocity.

So each water block gives off 190watts, and water has a thermal conductivity of .6Watts/m*K at 0C.

How do I equate these two figures to figure out how much flow and head my pump should have?

-
add comment

## 1 Answer

I have no experience with cooling systems or pumps, but I think I can help you with some simple physics here.

The thermal conductivity of water doesn't really matter because the heat is not trasfered by conduction, it's transfered by advection (i.e. by the flow of the water). The avective heat transfer in the tube can be calculated using this formula:

$$Q=F\cdot\rho\cdot c\cdot\Delta T$$

where $Q$ is the heat transfer (watt), $F$ is the volumetric flow rate (m$^3$/s), $\rho$ is the density of water (1000 kg/m$^3$), $c$ is the specific heat capacity of water (4200 J/kgK), and $\Delta T$ is the temperature difference the system should work with (K or °C), i.e. how much the temperature of the water should be allowed to increase when going through the water blocks. To get the volumetric flow needed to achieve a particular heat transfer for a given temperature difference, rewrite the formula:

$$F=\frac{Q}{\rho\cdot c\cdot\Delta T}$$

and insert your values.

-
Ahhh. Ok so for my system Q in watts is 625. Some algrebra leave us with (dT = 6720*Flow)? So then I just choose my delta T and then find a pump with that much flow? What does the Delta T represent in my system, the temprature difference across what? Where does the restritiveness of the system come in at? –  BumSkeeter Apr 2 '13 at 17:29
@BumSkeeter No, the formula would be 1/dT=6720*Flow or dT=1/(6720*Flow). A higher flow gives a lower temperature difference. $\Delta T$ is the difference between the highest and lowest temperatures the water reaches in the system. The lowest temperature depends on the ambient temperature and maybe also on the radiator. I don't know what you mean by "restritiveness of the system", but I guess you mean the dimensions of the tubings and such. I guess they affect what kind of pump you need to be able to achieve a certain flow rate. –  jkej Apr 2 '13 at 17:57
Ahh that made it come together. So if I want delta T to reach from ambient to the top temp the system can achieve then I need flow F. But Flow F is not constant in my system, it changes as the restrictiveness of the path it takes changes. Bernoulli eqn, describes the pressure and velocity of a fluid at a point h in a system right? Can I use that to determine the max flow on the pump for the most restrictive (highest pressure) area of my loop? –  BumSkeeter Apr 2 '13 at 18:11
@BumSkeeter I don't know exactly how you calculate your flow rate, but the flow rate will be the same everywhere in the 1/2 inch tube. Then this flow rate will split among the three 1/4 inch tubes, hopefully rather evenly. These are just fundamental continuity constraints. –  jkej Apr 2 '13 at 18:36
@BumSkeeter You're foregetting that you also only need a heat flow of $Q/3$ (ignoring the heat from the pump) in each of the 1/4 inch tubes, since each water block only gives 190 W. So it all works out, assuming that the flow divides evenly between the three branches. I assume you want to make the three branches as similar as possible to achieve this. –  jkej Apr 2 '13 at 19:00
show 3 more comments