Heat pipes are designed to operate without maintenance for long periods of time, e.g., for more than twenty years in Spacecraft Thermal Control Systems. One requirement is that the envelope, wick, and working fluid must be compatible. Note that compatibility requirements for heat pipes are more stringent than for many other applications. For example, aluminum and water are not compatible, since an aluminum/water heat pipe will generate large amounts of non-condensable gas in hours or days, stopping the heat pipe from operating.
Life tests to determine compatibility have been conducted since heat pipes were rediscovered in the 1960’s. In the past, life tests have been conducted on hundreds of different working fluid/envelope/wick combinations, e.g., see Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes. Over time a long list of compatible envelope/wick/material systems has been built up, including copper/water for electronics cooling, and aluminum/ammonia for spacecraft thermal management.
Life tests conducted at ACT include:
Most Heat Pipe Life Tests (Process Control Life Tests) today are conducted as a Quality Control measure to confirm that the heat pipe fabrication processes are under control. For example, samples of each of ACT’s extrusions for aluminum/ammonia Constant Conductance Heat Pipes (CCHPs) are on life test at elevated temperature, to demonstrate the long term life required in heat pipes for satellites; see Figure 15. The evaporators of these heat pipes are heated with aluminum heater blocks that have internal cartridge heaters. The adiabatic and condenser sections are exposed to the ambient for heat rejection by natural convection. The heat pipes are operated at elevated temperatures, 24 hours a day for 365 days a year. Periodically, the life test heat pipes are placed back into performance test fixtures, fully insulated and operated at very low temperatures to look for signs of non-condensable gas (NCG)
One area for active research on compatibility is the temperature range from 150 to 400°C, see High Temperature Water Life Tests and Intermediate Temperature Fluids Life Tests for more details. ACT has been examining different envelope/fluid pairs in this temperature range for the past decade, and demonstrated several new compatible fluid pairs.