Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes

There are a number of applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 750 K, including space nuclear power system radiators, fuel cells, geothermal power, waste heat recovery systems, and high temperature electronics cooling. Potential working fluids include organic fluids, elements, and halides. The paper reviews previous life tests conducted with 30 different intermediate temperature working fluids, and over 60 different working fluid/envelope combinations. Life tests have been run with three elemental working fluids: sulfur, sulfur-iodine mixtures, and mercury. Other fluids offer benefits over these three liquids in this temperature range. Life tests have been conducted with 19 different organic working fluids. As the temperature is increased, all of the organics start to decompose. Typically they generate non-condensable gas, and often the viscosity increases. The maximum operating temperature is a function of how much NCG can be tolerated, and the heat pipe operating lifetime. The highest long term life tests were run at 623 K (350°C), with short term tests at temperatures up to 653 K (380°C). Three sets of organic fluids stand out as good intermediate temperature fluids: (1) Diphenyl, Diphenyl Oxide, and Eutectic Diphenyl/Diphenyl Oxide, (2) Naphthalene, and (3) Toluene. While fluorinating organic compounds is believed to make them more stable, this has not yet been demonstrated during heat pipe life tests. Ongoing life tests suggest that the halides may be suitable for temperatures up to 673 K (400°C). However, property data for the halides is incomplete.