Choosing the Radiator
If the pump is the heart of our water-cooled system, then the radiator is the skin. In humans, our skin is responsible for dissipating heat. Skin has a large surface area, and sweat glands allow for evaporation cooling. When you run a mile, your skin becomes flushed and red. This is because your body shunts blood away from the internal organs and toward the skin, in order to carry more heat for dissipation. Conversely, when it's cold outside, your body shunts blood away from the skin in order to preserve heat, and you become pale.
The human body, and particularly the brain, is like your CPU: It becomes unstable when overheated. Heatstroke is a condition in which the body's core temperature rises too high. As a result, the internal thermoregulatory feedback system locks up. The body temperature might rise to 107 degrees Fahrenheit, when normal is 98.6 degrees. Disorientation, coma, and massive internal organ failure soon follow. The only remedy is fast cooling. In the emergency room, optimal cooling is done with a fan and sponge baths.
Like the radiator in your car, our system will use a mini-radiator to dissipate heat. As water is pumped through the CPU water block, it's heated. The hot water must shed this heat before returning to circulate again. This is where the radiator comes in.
A good radiator is made of aluminum (or, more expensively, copper) and has hundreds of fine radiating fins (see Figure 1). A good example is the Black Ice series from Hardware Labs. Look for a radiator with half-inch fittings to match our water block and tubing. Traditionally, you use an 80mm or 120mm case fan to cool the radiator fins.
Figure 1 A Black Ice water-cooling radiator from Hardware Labs. A case fan blows air through the radiating fins to cool the water returning to the system.
Fan direction is important. It might seem best to position the radiator fan to blow the radiator-heated air out of the computer case. However, this is not true. Your rate-limiting step in cooling is how fast you can cool the water flowing past the fins. Remember that the ambient temperature inside your case is 510 degrees higher than the air-conditioned room outside it. You diminish your temperature gradient if you're blowing warm air (from inside the case) through the radiator fins. Thus, you need to position the fan to blow fresh room air into the case and through the radiator. As long as you have a second fan at the back of the case blowing the newly heated air out, your ambient case temperature won't rise by much. And your CPU will cool more efficiently.