Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes

William G. Anderson, John R. Hartenstine, David B. Sarraf, and Calin Taraun
Advanced Cooling Technologies, Inc.
1046 New Holland Ave.
Lancaster, PA 17601 U.S.A.
717-295-6059, Dave Sarraf
717-295- 6066,


Potential working fluids for heat pipes and loop heat pipes include water, organic fluids, elements, and halides. The paper surveys 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. 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 are believed to make them more stable, this has not yet been demonstrated during heat pipe life tests. Finally, several halides are suitable for temperatures up to 673 K (400°C). Life tests at temperatures up to 400°C were conducted with titanium and three corrosion-resistant superalloys, and six different working fluids: AlBr3, GaCl3, SnCl4, TiCl4, TiBr4, and eutectic diphenyl/diphenyl oxide. (Therminol VP-1/Dowtherm A). Ongoing life tests with superalloys/TiCl4 and AlBr3 have been running for 28,000 hours. Ongoing life tests with up to 45,000 hours demonstrate that titanium/water and Monel/water heat pipes can be used at temperatures up to 550 K (277°C).

KEYWORDS: Heat Pipe, Loop Heat Pipe, Compatible Working Fluids, Life Tests, Water, Halides, Organic Working Fluids


There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 725 K (170 to 450°C), including space nuclear power system radiators, fuel cells, and high-temperature electronics cooling. At temperatures above 725 K (450°C), alkali metal heat pipes start to become effective. As the temperature is lowered, the vapor pressure and vapor density of the alkali metals are decreased. Below about 725 K (450°C), the vapor density is so low that the vapor sonic velocity limits the heat transfer. The heat pipe (or LHP) vapor velocity becomes too large to be practical for alkali metals in the intermediate temperature range.

Historically, water was used at temperatures up to about 425 K (150°C). More recently, it has been shown that water can be used with titanium or Monel envelopes at temperatures up to 550 K (277°C) (Anderson et al., 2006). While water heat pipes can operate at temperatures up to 570 K (300°C), their effectiveness starts to drop off above 500 K (227°C), due to the decrease in the surface tension.

At present, there is no commonly accepted working fluid over the entire intermediate temperature range. Potential working fluids include elemental working fluids such as sulfur, organic compounds, and halides. This paper reviews many of the intermediate temperature heat pipe life tests conducted over the past 40 years and then recommends suitable working fluid/envelope combinations. Anderson (2006) provides more detailed information on these tests.Summary of Sulfur, Sulfur-Iodine, and Mercury life tests

Pure sulfur is not suitable in the intermediate temperature range because of its high liquid viscosity, although it may be useful at higher temperatures. Sulfur has a unique temperature-dependent polymerization property at 470 K, which increases its liquid viscosity peak to approximately 100 Pa-s. This is about three orders of magnitude higher than the maximum level for effective heat pipe operation. The addition of a small percentage of iodine reduces the viscosity to a level that may be acceptable for reasonable heat pipe operation (Polasek and Stulc, 1976, Timrot et al., 1981). A potential problem with both sulfur and sulfur/iodine mixes is that they react strongly with many envelope materials. There are several problems with mercury as a wetting fluid including:

  •  Toxicity
  •  Difficulty in achieving good wetting of the wick and wall material without extensive corrosion
  •  High density, which translates into increased mass.

Additives have been used successfully with mercury to wet coarse, wicks; however, it appears to be very difficult to achieve wetting in finer pore wicks. Heat pipe tests with mercury in a sintered stainless steel wick failed because the mercury did not wet the stainless steel; see Anderson, Rosenfeld, Angirasa, and Mi (2004) for details.

Sulfur, Sulfur-Iodine, and Mercury life tests are summarized in Table 1. Mercury is compatible with 347 SS based on long-term life tests. Sulfur is compatible with pure aluminum based on a short-term life test, as is Sulfur-10% Iodine with 304 SS.


Life tests have been conducted with 19 different organic working fluids. Most of the suitable organic fluids are ring compounds. The reason for this was discussed by Saaski and Owzarski (1977) who pointed out that these types of compounds should be more stable than the long chain hydrocarbons. Saaski and Owzarski also pointed out that replacing some (or all) of the hydrogen atoms with fluorine may make the compound more stable.

Potential problems with the organic working fluids include the possibility of polymerization and/or dissociation. Polymerized fluids generally undergo an increase in liquid viscosity which, in a heat pipe, will decrease the circulation of the working fluid and therefore its heat transport capacity. Dissociation normally generates noncondensable gases (NCG), which over time will build up in the heat pipe condenser. The presence of NCG reduces the effective length of the heat pipe condenser and therefore the area available for heat radiation. This will either cause the temperature to rise at a given power level, or the power level to be decreased at a given temperature.

 Summary of Diphenyl, Diphenyl Oxide and Eutectic Diphenyl/Diphenyl Oxide Life Tests

Typically, organic fluids develop problems more quickly as the temperature is increased. The maximum operating temperature for an organic fluid depends both on the operating temperature, and how long the heat pipe needs to operate. For example, Anderson tested 304 stainless steel heat pipes with Dowtherm A working fluid. Heat pipes operating at 723 K (450°C) gassed up in ~180 hours, while pipes operating at 673 K (400°C) took roughly 1,500 hours for NCG gas generation to start affecting their behavior.

The most commonly tested organic fluids have been diphenyl, diphenyl oxide, and a eutectic mixture of diphenyl/diphenyl oxide (Trade Names Dowtherm A, Therminol, and Diphyl). Eutectic diphenyl/diphenyl oxide is nearly an azeotrope (Basilius and Prager, 1975), so the liquid and vapor have almost the same composition. This avoids the problems encountered with other mixtures such as NaK, where fractional distillation can occur (Anderson, 1993). Life test results for these three fluids are summarized in Table 2.

Summary of Organic Fluid Life Tests Other Than Diphenyl and Diphenyl Oxide.

Summary of Organic Fluid Life Tests Other Than Diphenyl and Diphenyl Oxide.

When using diphenyl, diphenyl oxide, or diphenyl/diphenyl oxide at temperatures over 673 K (400°C), non-condensable gas is generated in a relatively short time period; see Table 2. The exact period depends on the fluid and material and decreases as the temperature increases. For example, Kenney and Feldman found that their diphenyl pipes took less than 72 hours to gas up at 748 K (475°C), and 366 hours to gas up at 695 K (422°C). Between 300 and 400°C, these fluids are generally suitable, for short-duration tests near 400 C, and long-duration tests near 300 C (For example, Groll et al. found that 321 SS was compatible for ~40,000 hours at 573 K (300°C), but not at 623 K (350°C).

Life tests results for organic fluids (other than diphenyl and diphenyl oxide) are summarized in Table 3. Fluids have been ranked by the highest temperature for a compatible life test with any envelope material.

Since all of their life tests to date have been compatible, two fluids stand out in Table 3: toluene and naphthalene. Toluene was compatible with a copper-nickel alloy at 553 K (280°C), as well as with aluminum, mild steel, stainless steel, and titanium at lower temperatures. This is probably close to the maximum useful range of toluene, since the critical point of toluene is 592 K (319°C).

Halide Life Test Summary

Water is generally a better working fluid, since it can also be used in this temperature range, and has a Merit number that is roughly 50 times higher than toluene. However, toluene has three advantages over water, which may make it a suitable choice for certain conditions. The advantages include:

  •  Compatibility with a larger number of envelope/wick materials
  •  Melting temperature of 178 K (-95°C) versus 273 K (0°C)
  •  Lower saturation pressure (e.g., 23.4 atm. at 550 K versus 60.4 atm. for water)

Naphthalene is compatible with stainless steel, copper-nickel, and titanium, based on long-term life tests at 593 K (320°C) and above. It has also been shown to be compatible at lower temperatures with aluminum and mild steel. It was compatible with stainless for short-term tests at 380°C.

While fluorinated compounds are believed to be more stable than the same compound without fluorine, this has not been verified in life tests date. Gryzll, Back, Ramos, and Samad, (1994) found that Decafluorobyphenyl (C12F10) was less stable than Diphenyl (C12H10) under the same test conditions. Perfluoro-1,3,5-triphenylbenzene underwent severe thermal decomposition. Naphthalene was compatible with mild steel at 623 K (350°C) for 5,520 hours, while Monochloronaphthalene was found to be unsuitable after 642 hours at 560 K (287°C), and Octafluoronaphthalene had NCG gas generation at 488 K (215°C). Other stable, fluorinated life tests have been conducted at temperatures of 530 K (257°C) and below.

Halide Life Test Pipes

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 Owarski (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 number peaks in the intermediate temperature range. Information on halide properties can be found in Anderson, Rosenfeld, Angirasa, and Mi (2004) and Devarakonda and Anderson (2005).

Saaski and Owzarsky (1977) proposed an electrochemical method to predict the compatibility of halide working fluids with envelope materials. Tarau, Sarraf, Locci and Anderson (2007) found that this procedure had good agreement with the halide life tests discussed above.

Halide life tests are summarized in Table 4. Some halides appear to be suitable for temperatures up to 673 K (400°C), and possibly at higher temperatures. Tests are ongoing with TiBr4/titanium at 653K (380°C), and with AlBr3/Superalloys at 673K (400°C). Very long-term life tests show that TiCl4 and SnCl4 are both compatible with mild steel. No tests to date with an aluminum envelope have been successful. This is due to the very high decomposition potential of aluminum when compared to other metals (Tarau, 2007).

A series of halide life tests are ongoing at ACT. The selection criteria were discussed in Anderson et al. (2007). The fluid/envelope combinations tested are shown in Table 5. The titanium pipes had a 50 x 50 mesh titanium screen wick, and the C-22 pipes had an 80 x 80 mesh C-22 wick. The other two types of pipes were bare. Note that all of the superalloy pipes had C-22 endcaps and fill tubes (due to availability).

Operating temperatures for the heat pipes are shown in Table 7. The operating temperature was set based on the vapor pressure, and the allowable stresses in each heat pipe as a function of temperature. 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. During the life test, heat pipe temperatures are monitored to detect the formation of noncondensable gas.

The GaCl3/superalloy pipes all leaked at the pinch-off weld after roughly one week of operation at 360°C (633K). Note that all of the superalloy pipes used a C-22 fill tube, since that was more readily available.

After roughly 11,000 hours, the Hastelloy B3 pipe with AlBr3 failed After roughly 11,000 hours, apparently at a weld. Note that this failure is probably due to the pipe being severely overheated after the first 300 hours of testing when the heater shorted out. The maximum temperature is not known, however, it was sufficient to bubble the aluminum heater block.

The GaCl3/Titanium, TiBr4/titanium, and SnCl4/ superalloy pipes all developed large amounts of noncondensable gas. After roughly 20,000 hours of operation, the pipes were taken off life test, and stored for later analysis.

The TiCl4/superalloy and AlBr3/superalloy pipes continue to run without any problem. These pipes have currently been running for 3.2 years. The AlBr3 pipes are of particular interest, since they are running at 673 K (400°C). This is close to the temperature at which cesium starts to work.

Titanium, titanium alloys, Monel 400, and Monel K500 have higher yield strength and lower density than copper. As discussed below, they have been shown to be compatible with water, hence can be used in thinner and lighter weight heat pipes than copper at a given operating temperature and working fluid vapor pressure. Anderson, Dussinger, Bonner, and Sarraf (2006) started a series of life tests with commercially pure (CP) titanium, titanium alloys, Monel 400, and Monel K-500. The life test results are updated below. The materials under test include:

  • Ti CP-2 Heat Pipe, with CP Titanium Screen
  •  Ti CP-2 Heat Pipe, with Sintered CP TitaniumWick
  •  Ti CP-2 Heat Pipe, with Integral Grooves
  • Ti Grade 5 Cylinder (6% Aluminum, 4%Vanadium), with CP Titanium Screen
  •  Ti Grade 7 Cylinder (0.2% Pd), with CPTitanium Screen
  •  Ti CP-2 Cylinder, with 21S foil and CP Titanium Screen
  • Ti Grade 9 cylinder (3% Aluminum, 2.5%Vanadium) with CP Titanium Screen
  • Monel K500 Heat Pipe, with Monel 400 Screen
  • Monel K500 Heat Pipe, with Sintered Monel400 Wick
  •  Monel 400 Heat Pipe, with Monel 400 Screen
  •  Monel 400 Heat Pipe, with Sintered Monel 400 wick

The heat pipes with integral titanium grooves are intended for spacecraft thermal control. A typical cross-section is shown in Figure 1.

Titanium Heat Pipe with Integral Grooves for spacecraft thermal control.

Monel 400 is a solid solution alloy with roughly 63% nickel and 30% copper. It is a single-phase alloy, since copper and nickel are mutually soluble in all proportions. It can only be hardened by cold working. Monel K500 is a similar nickel/copper alloy, with the addition of small amounts of aluminum and titanium that give greater strength and hardness. The system is age-hardened by heating so that small particles of Ni3(Ti, Al) are precipitated throughout the matrix, increasing the strength of the material. The advantage of Monel K-500 is that the strength can be partially recovered after a wick is sintered inside.

Table 2 shows the different life test pipes on test. Note that the heat pipe operating temperature is dropped to 340 K (70°C), before the measurements. With this much lower temperature and vapor pressure, the non-condensable gas expands, making it easier to detect. The Monel pipes have shown almost no signs of gas generation. The titanium heat pipes all generated gas initially. This was believed to be a result of a passivation process that produced titanium oxide on the surface of the heat pipe.

Titanium-Water and Titanium-Monel Life Test Pipes

The gas was removed from all of these pipes by heating to about 115°C and venting them. The thermocouples were monitored to verify that the noncondensable gas has been forced out of the condenser by the pressure difference. The heat pipe fill tube is then resealed. Since they have been resealed, they have all generated a small amount of gas.

Some of the CP-Ti and Monel pipes currently have operated for 5.2 years. Life test pipes with titanium alloys are also underway, with operating times of up to 4.2 years. Titanium/water and Monel/water heat pipes can be considered a mature or mainstream technology that is ready for widespread application.

A survey was conducted for intermediate temperature life tests. Life tests have been 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. Mercury is toxic, has a high density, and problems have been observed with getting the mercury to wet the heat pipe wick. Sulfur and Sulfur/Iodine have high viscosities, low thermal conductivities, and are chemically aggressive.

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. At high enough temperatures, carbon deposits can be generated. Themaximum operating temperature is a function of how much NCG can be tolerated, and the heat pipe operating lifetime. Three sets of organic fluids stand out as good intermediate temperature fluids: 1. Diphenyl, Diphenyl Oxide, and Eutectic Diphenyl/Diphenyl Oxide (Dowtherm A, Therminol VP, Diphyl) 2. Naphthalene 3. Toluene

The halides appear to be suitable for temperatures up to 673 K (400°C), and possibly at higher temperatures. Long-term life tests are ongoing with TiBr4/titanium at 653K (380°C), and with AlBr3/Superalloys at 673K (400°C). Long-term life tests show that TiCl4 and SnCl4 are both compatible with mild steel. No tests to date with an aluminum envelope have been successful. This is due to the very high decomposition potential of aluminum when compared to other metals. However, property data for the halides is incomplete.

Water life test pipes have been fabricated with commercially pure (CP) titanium, Monel K-500, Monel 400, and various titanium alloys, and run at 500 K (227°C) and 550 K (277°C). Heat pipes have been tested with screen and sintered powder wicks, as well as grooved CP-Ti pipes. Some of the CP-Ti and Monel pipes currently have operated for 5.2 years. These pipes continue to operate successfully, with a small amount of gas generation in the CP-Ti pipes. Life test pipes with titanium alloys are also underway, with operating times of up to 4.2 years. Titanium/water and Monel/water heat pipes can be considered mature or mainstream technology that is ready for widespread application.


The water and halide life tests were sponsored by NASA Glenn Research Center under Contracts NNC05TA36T, and NNC06CA74C. Duane Beach and then David Ellis were the Technical Monitors. Chris Stover and Rod McClellan were the technicians on the program. We would like to thank Cheryl Bowman and Ivan Locci of NASA Glenn Research Center for helpful discussions about the fluids and materials. We would also like to thank Laurie Anderson, Al Basiulis, Claus Busse, Don Ernst, Manfred Groll, Larry Grzyll, and Bob Reid for their generous help in locating and supplying references.


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