For members of the personal computing enthusiast community, adding water cooling to a setup is a desirable thing to do. It not only looks cool, but it also helps to lower the temperatures of the CPU and GPU more than a traditional air cooler could. However, this is only worthwhile for components that output considerable amounts of heat, such as an AMD Threadripper or Intel Core i9 10900K. But this didn't discourage Michael Klements from going ahead and adding a fully custom water cooling loop to the top of this Raspberry Pi 4, even though it only has a relatively small quad core Arm Cortex-A72 SoC clocked at 1.5GHz stock.
Traditional water cooling loops consist of three main parts: a water block that attaches to the integrated heat spreader (IHS) and transfers heat into the cooling fluid (normally water), a pump for moving that coolant, and a radiator for transferring the heat from the water into the surrounding air. There is almost always a fan or two on the radiator that helps move cooler air past the fins at a faster rate for better cooling performance.
According to physics, when there exists an energy gradient in a system, e.g. there is a side with greater energy and a side with a lower one, the entire system wants to have the same amount of energy, so energy goes from the higher potential to the lower one until an equilibrium is reached. Think of how leaving a bowl of hot soup on the counter will eventually reach room temperature. This is the same way a cooling loop functions. The higher energy potential in the CPU must be moved to the outside air somehow. Air coolers achieve this by passing air across many small metal fins to let the ambient-temperature air pick up heat from the cooler.
However, this can only handle a certain amount of heat, and since air has a higher thermal resistance than water, it makes for a poor coolant. That's why liquid coolers are better at drawing heat away from the CPU, as the metal block picks up heat, transfers it to the water, and then that water flows to a radiator where the heat can be exhausted away. One downside to this approach is the complexity, since many more parts are needed to make this happen. And for things that don't output that much heat, the law of diminishing returns applies, since a bigger water cooler provides a marginal benefit over the air cooler.
Klements began putting together his loop by first designing a custom acrylic mounting bracket that goes over the CPU. It has holes on each of its four corners that line up with the mounting holes on the Raspberry Pi 4. There is a 30x30mm cooling block that gets held down onto the IHS, and this is what transfers heat from the CPU into the coolant.
Next, he cut out a mounting bracket for the single 120mm radiator, along with the water pump/reservoir combination. Finally, Klements attached plastic tubing from the water block to the pump and radiator and filled the loop with water.
The temperature test began with running the Pi at 100% while clocked at 1.5GHz for five minutes in a room with an ambient air temperature of 25C. It started out at 28C and then spiked up to 31C, where it remained for the rest of the test. Then he ran a second test, but this time with an overclock of 33%, from 1.5GHz all the way to 2GHz. Quite impressively, the Pi was able to run at this speed while at only 36C.
All-in-all, water cooling the Raspberry Pi 4 can result in some very low temperatures for some great overclocks, but it's probably not very practical for everyday use.