Basic Computer Thermodynamics

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Basic Computer Thermodynamics

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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