Page 363 - Mechanical Engineers' Handbook (Volume 4)

352 Heat Pipes

Table 5 Experimental Compatibility Tests
Stainless
Aluminum Brass Copper Inconel Iron Nickel Niobium Silica steel Tantalum Titanium Tungsten
Acetone C C C C C
Ammonia C I C C C
Cesium C C
Dowtherm C C C
Freon-11 C
Heptane C
Lead I I I I C I C
Lithium I I C I C I C
Mercury I I I C I I
Methanol C C C C C
Silver I I I I C I C
Sodium C C C C I
Water I C I C C C C
Note. C, compatible; I, incompatible.

caps, wick, and working fluid) is necessary for maximum reliability and performance. Several
steps are needed to properly clean the heat-pipe container and wick structure, such as solvent
cleaning, vapor degreasing, alkaline cleaning, acid cleaning, passivation, pickling, ultrasonic
cleaning, and vacuum baking. Many of these steps are used in a single cleaning operation. 6,8
It is necessary to treat the working fluid used in a heat pipe with the same care as that
given to the wick and container. The working fluid should be the most highly pure available,
and further purification may be necessary following purchase. This may be carried out by
distillation. The case of low-temperature working fluids, such as acetone, methanol, and
ammonia, in the presence of water can lead to incompatibilities, and the minimum possible
water content should be achieved. The amount of working fluid required for a heat pipe can
be approximately calculated by estimating the volume occupied by the working fluid in the
wicking structure, including the volume of any arteries, grooves, cornered regions, and so
on. The amount of working fluid charged to a heat pipe significantly affects the heat-transfer
performance of the heat pipe. For example, the heat-transfer performance of a grooved heat
pipe currently being used in the laptop computer is very sensitive to the charging amount
of working fluid.
Once the amount of working fluid required is determined, the working fluid can be
introduced into the heat pipe by an evacuation and backfilling technique, a liquid fill and
vapor generation technique, a solid fill and sublimation technique, or a supercritical vapor
technique . The most common among those for the low- or moderate-temperature working
6
fluids currently being used for the electronics cooling is the evacuation and backfill tech-
niques. All these charging methods are to prevent noncondensable gases from entering the
heat pipe during the charging process. To charge the working fluid in, a suitable evacuation/
filling rig must be applied. The rig must be able to evacuate the container to 10 4 torr or
less. The filling rig is used to evacuate the pipe and charge it with the proper amount of
working fluid. Details of the charging process depend on the state of the working fluid at
the ambient temperature. The material of construction is generally glass, stainless steel, or
plastic materials. Glass has advantages when handling liquids in that the presence of liquid