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Unformatted text preview: process in this temperature range only, and thus it will not be
a suitable fluid for applications involving temperatures beyond this range.
Furthermore, water will undergo a phase-change process at a specified temperature only if its pressure equals the saturation pressure at that temperature. For example, if a heat pipe with water as the working fluid is designed
to remove heat at 70°C, the pressure inside the heat pipe must be maintained at 31.2 kPa, which is the boiling pressure of water at this temperature. Note that this value is well below the atmospheric pressure of
101 kPa, and thus the heat pipe will operate in a vacuum environment in
this case. If the pressure inside is maintained at the local atmospheric pressure instead, heat transfer would result in an increase in the temperature of
the water instead of evaporation.
Although water is a suitable fluid to use in the moderate temperature
range encountered in electronic equipment, several other fluids can be used
in the construction of heat pipes to enable them to be used in cryogenic as
well as high-temperature applications. The suitable temperature ranges for
some common heat pipe fluids are given in Table 10–5. Note that the overall temperature range extends from almost absolute zero for cryogenic fluids such as helium to over 1600°C for liquid metals such as lithium. The
ultimate temperature limits for a fluid are the triple- and critical-point temperatures. However, a narrower temperature range is used in practice to
avoid the extreme pressures and low heats of vaporization that occur near
the critical point. Other desirable characteristics of the candidate fluids are
having a high surface tension to enhance the capillary effect and being
compatible with the wick material, as well as being readily available,
chemically stable, nontoxic, and inexpensive.
The concept of heat pipe was originally conceived by R. S. Gaugler of
the General Motors Corporation, who filed a patent application for it in TABLE 10–5
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- Spring '10