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, which transfers heat by evaporation and condensation
- Heat pipe envelope which provides a leak-tight pressure vessel to contain the working fluid
- Heat pipe wick, 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 problems include:
- Gas Generation (most Common)
- Materials Transport
Since heat pipes were rediscovered by George Grover in 1963, extensive life tests have been conducted to determine compatible fluid-envelope pairs, and a large number have been found. Some of these life tests have been conducted for decades.
Compatible envelope/fluid pairs include copper/water for electronics cooling, aluminum/ammonia for spacecraft thermal control, superalloys/alkali metals for high temperature applications such as isothermal furnace liners, and copper or steel/Refrigerant R134a in Energy Recovery Heat Pipe Heat Exchangers. See Fluid/Envelope/Wick Compatibility for more details.
More details on working fluid selection, and fluid/envelope/wick compatibility are given in the following pages:
- Working Fluids, which surveys the working fluids suitable at different temperatures
- Fluid/Envelope/Wick Compatibility, which discusses the most commonly fluid/material pairs
- Compatible Fluids and Materials, a more detailed discussion of fluids and materials known to be compatible
- Life Tests, discussing how compatibility is demonstrated
- Process Control Life Tests, for verifying that the heat pipe manufacturing process is under control to produce long-life pipes
- High Temperature Water Life Tests, work by ACT to increase the water upper temperature range from 150 to 300ºC
- Intermediate Temperature Fluids, work by ACT and others on fluids in the temperature range from 250 to 400ºC
- Examples of Incompatible Fluids/Envelope Pairs, demonstrating problems that can occur when exploring potential fluid/envelope pairs