The water block's job is to remove heat from a source (CPU) and transfer this heat to the fluid. The fluid is then removed from the water block by the pump.

The performance of water blocks is normally measured in resistance to flow and C/W at a specific flow rate. The C/W (Centigrade per Watt) is a measure of how difficult it is for heat to be transferred from the source to the water. The lower the C/W the better. When choosing a water block, you are trying to select one that will produce the lowest C/W as possible in your system.

There are a number of factors affecting the C/W of a water block; the major ones are listed below then discussed

1. Water block material
2. Water block design

Water block material

The water doesn't directly touch the CPU. There is a layer of metal in between the CPU and water. This layer will normally be made from copper but can also be made from aluminum. It's best to use a metal with highest W·m^-1·K^-1 as possible as this result to lower C/W. Here is a list of some metals of relevance:

• Silver (300K) 429 W·m^-1·K^-1
• Copper (300K) 401 W·m^-1·K^-1
• Gold (300K) 318 W·m^-1·K^-1
• Aluminum (300K) 237 W·m^-1·K^-1
• Iron (300K) 80.4 W·m^-1·K^-1

Besides preventing the water from torching the source. The base plate of the water block is used to spread the heat out over a larger area to improve the fluids ability to remove the heat from the source

Water block design

When a Water Block is designed the manufacturers are trying to improve performance by altering the path fluid flows and in doing so transferring more heat to it. Some designs will achieve lower C/W by sacrificing flow though the water block. This makes selecting a water block more complicated than simply selecting one that produces the lowest C/W at specific flow rate. This is because resistance to flow created by the Water Block can effect flow through it and therefore affect the Water Blocks C/W.

To find out how much this resistance will affect a WB's C/W we will need to graph the may factors that effect a WB's performance against each other. They are:

• The pump's pressure / Flow curve
• The water block's backpressure / Flow curve
• The water block's C/W / Flow curve

Here is an example of the above:

We will be using a fictitious pump that has a max pressure of 1,200 cmH2o and a max flow of 1200 Lhr for our example.

We now add two water blocks each has a different flow characteristic. They are plotted against our pump, Which gives us the theoretical flow rate for each water block. As you can see, Block B flows more as it creates less back pressure for any given flow rate.

We can now plot the C/W verses flow rate of our water blocks. As you can see, Block A always produces a lower C/W for any given flow rate than Block B. But because Block A has a higher back pressure it flows less than block B. The result is that both Block A and Block B have the same C/W when compared against our pump. This is because the flow through a water block affects its C/W, more flow means lower C/W.