Operating Temperature Range

Working Fluids Theoretically Operate from the Triple Point to the Critical Point

Heat pipes are two-phase heat transfer devices.  For a heat pipe to operate, a saturated working fluid is required, with both liquid and vapor in the heat pipe.  The working fluid latent heat is transferred by vaporizing the liquid in the evaporator, and condensing the vapor back to liquid in the condenser.  Theoretically, the heat pipe will operate at temperature just above the triple point (the unique temperature and pressure where the working fluid can be in liquid, vapor, and solid form), to just below the critical point (vapor and liquid have the same properties).  As discussed below, there are other constraints that shrink the practical temperature range.

Figure (1)

The triple point and critical point for a number of common heat pipe working fluids are shown in Figure (1) and Table 1.  Two points should be noted.  First, there are gaps in the cryogenic heat pipe temperature range (below about 100 K), where there is no currently known working fluid.

Second, there are many potential working fluids at a given temperature, for temperatures above 200 K.  The fluid selected is normally the fluid with the highest Merit Number when a compatible heat pipe envelope are acceptable.  For example, while ammonia is a better-working fluid than methanol, methanol must be chosen when a copper wick and envelope are used.  For large geothermal thermosyphons a fluid with a low global warming potential may be selected.

Table 1.  Selected Heat Pipe Working Fluids, Triple Point and Critical Point.  Freezing point is used for the halides, cesium, and lithium, since the triple point is unavailable.

Practical Temperature Limits for Working Fluids

In practice, the fluid range is smaller, at both the low and high end of the temperature range.   For example, a water heat pipe will carry some power between the water triple point (0.01°C) and the critical point (373.9°C).  Maximum power calculations for a typical water heat pipe are shown in Figure 5.  The peak power occurs at a temperature near 150°C), and drops off at lower and higher temperatures.  Practically, most water heat pipes are designed to operate above ~25°C).  At lower temperatures, the vapor pressure decreases, as well as the vapor density, so the vapor velocity for a given amount of power increases.  At temperatures below about 25°C, the viscous and sonic limits become important, limiting the heat pipe power.

Figure (2) Heat pipe performance typically peaks somewhere in the middle of the temperature range between the triple point and the critical point.

Practical Operating Temperatures for Water

The practical upper temperature limit for copper/water heat pipes is roughly 150°C, and is set by the maximum allowable stresses in the copper envelope; see Figure 6.  At 150°C, the saturated water vapor pressure is 69 psia (477 kPa).  Since copper is relatively soft, the required diameter at wall thickness above 150°C C becomes impractical.

Figure (3) Saturated Water Vapor Pressure as a Function of Temperature.

Titanium or Monel envelopes increase the maximum operating temperature range for water to 300°C.  In this case, the upper temperature limit is set by the fluid properties.  As with any saturated fluid, the saturated vapor and liquid properties become more and more similar as the critical point is approached.   A good heat pipe working fluid has a large latent heat and a large surface tension.  As shown in Figure 7 and Figure 8, both the latent heat and the surface tension approach zero near the critical point (373.9°C).

Figure (4) Water Surface Tension as a Function of Temperature.

Figure (4) Water Latent Heat as a Function of Temperature.

Practical Temperature Limits

Table 2 lists practical temperature limits.  Note that the upper-temperature range for some of these fluids is set by the fact that a superior fluid can be used at higher temperatures.  This is discussed in more detail in Compatible Fluids and Materials.

Table 2.

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