1.1) Working fluids
The working fluid is the heat transfer agent in thermosyphons and heat pipes. The ideal working
fluid for a given application depends on the operational temperature range and the material of
the heat pipe or the thermosyphon. Since the operation of a heat pipe is mostly based on working
fluid vaporization and condensation, the working fluid must be carefully selected, and it is
considered an important factor to be taken under consideration in both project and manufacturing
stages. The operational temperature levels range from 100K (cryogenic) to 1000K (liquid metals).
A few important criteria on the definition of the working fluid are the thermophysical properties
and its thermal and chemical compatibilities with the tube material. The working fluid is selected
according to the following: operation temperature and pressure; maximum heat transfer; thermophysical
properties; toxicity, ignition, and explosion risks; and equipment conditions over time.
Depending on the working fluid, non-condensable gases may be generated as the fluid degradates,
which lowers efficiency in the thermosyphon. Organic fluids, which work in intermediate temperatures,
may not only generate non-condensable gases but also polymerize, changing their thermophysical properties.
Currently, research on working fluids involves the study of non-condensable gas generation, working
fluid and tube material compatibility, and product life cycle.
a. High temperature thermosyphons
b. Intermediate temperature thermosyphons
c. Low temperature thermosyphons
d. Cryogenic temperature thermosyphons
