Heat pipes are passive heat transfer devices, which have very long lives when properly designed and fabricated. Long life is critical in applications such as spacecraft thermal control, where heat pipes in satellites can operate for decades, with no opportunity for repair or replacement. Most of the problems with long term operation of heat pipes are caused by material compatibility between the working fluid and the heat pipe materials.
Every heat pipe has three components
- Heat pipe working fluid, the fluid material within the envelope which transfers heat by evaporation and condensation (learn how to select the working fluid)
- Heat pipe envelope, the outer “shell” material which provides a leak-tight pressure vessel to contain the working fluid
- Heat pipe wick, the material within the envelope used to return liquid from the condenser to the evaporator using capillary forces.
Once the operating temperature is known, the heat pipe designer selects the working fluid. The envelope and wick materials are then selected from materials known to be compatible with the working fluid. If the materials are not compatible, possible incompatibility problems include:
- Gas Generation (Most Common)
- Materials Transport
More details on working fluid selection, and fluid/envelope/wick compatibility are given on the following pages:
- Working Fluids, which surveys the working fluids suitable at different temperatures
- Merit Number and Fluid Selection, which discusses how fluids at a given operating temperature are ranked against each other
- Fluid Operating Temperature Range, which discusses how the upper and lower operating temperatures for a working fluid are set
- Fluid/Envelope/Wick Compatibility, which discusses the most commonly used fluid/material pairs
- Life Tests, discussing how compatibility is demonstrated