There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 180 to 430ºC (450 to 700 K), including space nuclear power system radiators, fuel cells, geothermal power, waste heat recovery systems, and high temperature electronics cooling. At temperatures above 400 to 430ºC, alkali metal (cesium) heat pipes start to become effective. Below about 430ºC, the vapor density for cesium is so low that the vapor sonic velocity limits the heat transfer. Historically, water was used at temperatures up to about 150ºC. More recently, long term life tests have shown that water can be used with titanium or Monel envelopes at temperatures up to 300ºC.
A recent survey found that researchers had conducted life tests 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. 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.
Another family of potential intermediate temperature fluids are the halides. A halide is a compound of the type MX, where M may be another element or organic compound, and X may be fluorine, chlorine, bromine, iodine, or astatine. Starting with Saaski and Owarski1 (1977), a number of researchers have suggested that halides are potential heat pipe fluids. They are attractive because they are more stable at high temperatures than organic working fluids, and because their Merit numbers peak in the intermediate temperature range.
A series of halide life tests are ongoing at ACT. An electromotive force difference method was used to select four halides that were believed to be most compatible with superalloy envelopes: AlBr3, GaCl3, SnCl4, and TiCl4.
Heat pipes with several different superalloy envelopes were fabricated, and place on life test. During the life tests, the temperature of the evaporator and condenser for each heat pipe are monitored, to detect any problems. It is possible that oxygen can affect the outside of the titanium pipes during the test. To prevent this problem, the life tests are conducted inside a box that is purged with argon.
Possible problems with incompatible fluid/envelope pairs include:
- Non-Condensable Gas Generation
- Materials Transport
Three different outcomes were experienced during these exploratory life tests:
- The heat pipe generated non-condensable gas
- The heat pipe was corroded and failed
- The heat pipe demonstrated good long term life
Non-Condensable Gas Generation
The temperature difference between the evaporator and condenser were monitored, to detect non-condensable gas; see Figure 20. The SnCl4/superalloy, GaCl3/titanium pipes, and Therminol/titanium pipes all have a high ΔT, indicating that these envelope/working fluid pairs are incompatible.
The envelope/working fluid incompatibilities were verified by sectioning and examining the heat pipes. Figure 21 shows that the heat pipe with a CP Ti envelope and GaCl3 working fluid, underwent extensive corrosion. Some of the corrosion layer was observed to crack and chip during polishing. The fracture surfaces were indicative of a brittle failure mode. In combination with the extensive cracking of the corrosion layer, this seems to indicate that the corrosion layer is quite brittle.
All of the heat pipes with large amounts of gas generation showed corrosion when examined internally. The GaCl3/superalloy pipes were so imcompatible that they all leaked at the pinch-off weld after roughly one week of operation at 360°C (633K); see Figure 22.
Successful Long Term Life Tests
Finally, two envelope/fluid pairs (superalloy/TiCl4 at 400°C and superalloy/AlBr3 at 300°C) were found to be compatible, and continue to run without any problem. These pipes have currently been running for 57,000 hours (6.7 years). The AlBr3 pipe is of particular interest, since it is running at 673 K (400°C). This is close to the temperature at which cesium starts to work. For example, sectioning and analysis confirmed the compatibility of these envelope/fluid pairs. Figure 23 revealed a 1 to 2 micrometer thick corrosion layer on the surface of a heat pipe that paired Hastelloy C-2000 with TiCl4 as the working fluid.
 Saaski, E.W., and Owzarski, P.C., “Two-Phase Working Fluids for the Temperature Range 50° to 350°C,” Sigma Research, Inc., Final Report, Contract NAS3-20222, NASA Lewis Research Center, June 1977.