Evaporation and condensation of the working fluid is what gives heat pipes their high effective thermal conductivity, which can be as high as 100,000 W/m K. During heat pipe operation, working fluid is vaporized in the evaporator, and then condensed in the condenser, transferring heat.
The first step in selecting a heat pipe working fluid and envelope/wick material is to determine the operating temperature range. For a heat pipe to operate, it must be at saturated conditions, where the heat pipe contains both liquid and vapor. Because of this, fluids can only operate (theoretically) between the triple (freezing) point and the critical point, where vapor and liquid phases have the same properties. In reality, the operating temperature range for any given fluid is smaller, since the power that the heat pipe can carry drops off sharply near the freezing and critical temperatures.
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). The practical operating temperature range for a copper/water heat pipe is roughly 25° to 150°C. At lower temperature, fluid properties limit the heat transfer. At higher temperatures, a copper envelope to withstand the vapor pressure becomes too thick to be practical. Titanium and Monel envelopes extend the upper temperature limit to 300°C.
The most commonly used envelope/fluid pairs include:
- Copper/Water for Electronics Cooling
- Copper or Steel/Refrigerant R134a for Energy Recovery
- Aluminum/Ammonia Spacecraft Thermal Control
- Superalloys/Alkali Metals (Cesium, Potassium, Sodium) for High Temperature Heat Pipes
As discussed in Compatible Fluids and Materials there are a large number of other compatible envelope/fluid pairs that are used at other temperature ranges, or when additional factors must be considered. For example, copper/methanol is often used for electronics cooling when the heat pipe needs to operate near or below 0°C, when water freezes. Fluid choices in a given temperature range are ranked by the Merit Number.