|
|
 |
| |
| 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. |
| |
|
| |
|
