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Heat pipe cooling

晨怡热管 2008-5-2 22:56:42

THERMAL DESIGN

The equivalent thermal chain of a heat pipe is as follows :

Compared to other cooling systems, an important difference is that the heat exchange coefficient in evaporation (i.e. boiling) depends on the power density at the boiling surface.
As a consequence, heat pipe thermal resistance is not dependent only on the air flow rate but also on the dissipated power as illustrated on the following curve.

MECHANICAL DESIGN

Most of the applications in the field of Power Electronics Cooling are industrial and railway ones.
Most of the heat pipes used are thermosiphons or thermosiphon loops where the liquid returns from the condenser to the evaporator by natural gravitational flow.
This means that the condenser must be at a slightly higher altitude than the evaporator.
For a forced convection heat pipe, the evaporator can be either horizontal or vertical and the condenser vertical or oblique.

For heat pipes working in natural convection, with a rising vertical air flow, typical heat pipe designs are as follows.


Thermosiphon Loop		Cold Wall heat Pipe

WORKING FLUID

For applications using non electrically insulated components (press pack assemblies), where insulation between the evaporator and the condenser (heat exchange surface) is needed, dielectric fluids such as FC72 or HFE7100 are used.
For other applications, methanol or water are commonly used depending on the application temperature range and more particularly temperatures below 0°C for freezing problems.

Limits: During start up at low temperatures, the low vapor pressure and density leads to very high vapor velocity (up to sonic velocity) in the tube: entrainment of the liquid by the vapor stream can occur, with a possible liquid dry out in the evaporator and burn out.
Tube diameters are therefore calculated to evacuate the maximum desired power over the entire working temperature range.

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