Heat Pipe Structure

Heat pipes may be described as having two sections: precool and reheat. The first section is located in the incoming air stream. When warm air passes over the heat pipes, the refrigerant vaporizes, carrying heat to the second section of heat pipes, placed downstream. Because some heat has been removed from the air before encountering the evaporator coil, the incoming air stream section is called the precool heat pipe.

Air passing through the evaporator coil is assisted to a lower temperature, resulting in greater condensate removal. The "overcooled" air is then reheated to a comfortable temperature by the reheat heat pipe section, using the heat transferred from the precool heat pipe.

A. Heat is absorbed in the evaporating section.

B. Fluid boils to vapor phase.

C. Heat is released from the upper part of cylinder to the environment; vapor condenses to liquid phase.

D. Liquid returns by gravity to the lower part of cylinder (evaporating section).

As the density of transitors in a microprocessor increases, the amount of heat disipated increases. A Pentium 4 processor (180 nm running at 2GHz) disipates, 55 Watts of power as heat. Its area is just 131 mm². This gives a 55 W/(131/(10²)) = 42 W cm^-2. In comparison a steam iron is 5 Wcm^-2.

One solution is the heat pipe. As its name suggests, it transfers heat from high temperature regions to lower temperature regions where there is more space for

heat sinks or cooling fans.

Although it just looks like a sealed metal pipe, there is a wick or porous material and a liquid with a high latent heat of vaporisation. When the pipe is heated the liquid uses the heat to evaporate and changes into a gas, the gas moves to a colder region of the heat pipe where is condenses and uses the latent heat to change back into a liquid. Heat pipes are a reliable and cost effective solution for laptop computers where fans would reduce battery life