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Basic Computer Thermodynamics

Written By: Stephen Bucaro

That desk in front of you and everything else around you is made up of atoms. An atom consists of electrons orbiting around a nucleus. An atom is increadibly tiny. You could line up 10 million of them inside a millimeter. What if we could scale up an atom so that the nucleus was the size of a basketball? The orbits of its electrons would then be 15 miles away.

From this you can understand that atoms are almost all empty space. The nucleus of the atom is composed of quarks. If you could see a quark or an electron up close, it might appear as a tiny vibrating glow of energy. It turns out this world, which is causing us so many problems and so much stress, is mostly an illusion!

The electrons orbit the nucleus at about the speed of light. If you could see them, they might appear as a blur. They do not orbit in a plane like the pictures in books. They create a shell. Sometimes two or more atoms will share electrons, causing them to link together forming a molecule.

Looking at that desk in front of you again, it looks pretty solid. Actually, unless your desk is floating in deep space where the temperature is close to absolute zero, the molecules of your desk are vibrating like crazy. Picture the molecules bouncing around and smacking into each other like balls on a pool table.

If you have ever played pool, you’re very familiar with how when a pool ball hits another pool ball, it transfers it’s energy to the second pool ball. When heat causes molecules to vibrate, they transfer energy between each other in a similar fashion. This action is called “conduction”.

Now picture the CPU of a computer cooking away because the designer wants to push too much power through a small piece of silicon. If we don’t take away that heat as fast as it’s created, that CPU will fry!

The problem is usually solved by mounting a heat sink on the CPU. Conduction causes the heat to move from the hot CPU to the cooler heat sink. Because air doesn’t conduct heat as well as metal, We apply a thin layer of heat sink compound between the CPU and the heat sink to fill in any gaps.

You’ll notice that a heat sink has fins on it. The fins allow the heat sink to conduct the heat to the air adjacent to the fins. The fins provide more surface area to aid in conduction. Eventually the adjacent air will get as hot as the heat sink and conduction will cease.

If we move the air away from the heat sink, it will take the heat energy with it. A fan mounted on the heat sink is used to move the air. This method of heat transfer is called convection. Eventually all the air inside the computer case will get hot, so fans are used to blow the air out of the case of the computer.

The heat has moved from the CPU, to the heat sink, to the air inside the case, to the air in the room where you’re sitting at your computer desk. The room starts to get hot, and eventually the air conditioner turns on.

You can view an air conditioner as a “pipe”. A fan blows the hot air from your room through fins that transfer the heat to a liquid. The liquid is piped to fins outside the house. A fan blows cooler outside air past the fins to remove the heat from the liquid.

The air conditioner has an evaporator valve that changes the liquid to a gas after the heat is removed. In a gas, the molecules are further apart than in a liquid. This causes it to cool down quite a bit more. The gas goes through the fins inside the house, picking up heat. It is then compressed into a liquid to concentrate the heat so the outside fins can remove the heat more efficiently.

Shuttle’s I.C.E. (Integrated Cooling Engine) Heat Pipe uses a very similar method to cool a CPU. The CPU has a heat sink with copper heat pipes. The heat of the CPU causes liquid coolant inside the heat pipe to change to a gas. Convection created by the pressure of the gas moves the coolant to a second heat sink where a fan is used to blow the heat out of the computer’s case. Releasing the heat causes the coolant to change back to a liquid. Gravity then carries the coolant back to the CPU heat sink.

One last method of heat transfer we haven’t discussed yet is radiation. Some of the heat of the CPU and the heat sink is released as infrared radiation. Similar to light (although invisible to human eyes), the radiation strikes the insides of the computer case, causing it to get warm. Ultimately the computer case itself acts as a heat sink conducting heat to the outside air.

This article explains the three ways – conduction, convection, and radiation – that heat is transported from a computer CPU to the air outside the computer case. You now understand the thermodynamics of a computers and why it is important to maintain its various components.

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