Heat Pipe Materials, Working Fluids, and Compatibility

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:

  1. Heat pipe working fluid, which transfers heat by evaporation and condensation
  2. Heat pipe envelope which provides a leak-tight pressure vessel to contain the working fluid
  3. 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)
  • Corrosion
  • Materials Transport
Copper/Water Heat Pipes are the most commonly used fluid/envelope pair for electronics cooling.

Figure 1. Copper/Water Heat Pipes are the most commonly used fluid/envelope pair for electronics cooling.

 

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.

 

Figure 2. Grooved aluminum heat pipes with ammonia working fluid are the most commonly used fluid/envelope pair for spacecraft thermal control.

Figure 2. Grooved aluminum heat pipes with ammonia working fluid are the most commonly used fluid/envelope pair for spacecraft thermal control.

 

More details on working fluid selection, and fluid/envelope/wick compatibility are given in the following pages: