Heat Pipe Life Tests

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.

Read more about compatibility

Life tests to determine compatibility have been conducted since heat pipes were rediscovered in the 1960s at the Los Alamos National Labs.  In the past, life tests have been conducted on hundreds of different working fluid/envelope/wick combinations, leading to the formation of a long list of compatible envelope/wick/material systems, including copper/water for electronics cooling, and aluminum/ammonia for spacecraft thermal management. Life tests conducted at ACT include Process Control Life Tests, High-Temperature Water Life Tests, and Intermediate Temperature Fluids.

One area of 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 section below for more details.  ACT has been examining different envelope/fluid pairs in this temperature range for the past decade and has demonstrated several new compatible fluid pairs.

Process Control Life Tests

One reason for conducting heat pipe life tests is to demonstrate the long life of current products.  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. The goal is to demonstrate that ACT’s manufacturing processes result in heat pipe products meeting the most stringent reliability requirements that are typically found in critical mission systems, such as satellites or military-embedded computing equipment.

Heat Pipe Life Tests for CCHP Manufacturing Process Control

For example, new aluminum/ammonia Constant Conductance Heat Pipes (CCHPs) are put on life tests whenever a new extrusion is qualified, or the manufacturing process is changed.  In the photo to the right, samples of each of ACT’s extrusions for aluminum/ammonia CCHPs are on life test at elevated temperatures to demonstrate the long-term life required in heat pipes for satellites.  In this test, 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 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). At these low temperatures, the vapor pressure of ammonia is extremely low and any significant NCG would expand and block part of the condenser section. This would show up as a temperature gradient in the condenser section thermocouples.

Fig. 1: Heat Pipe Life Tests for Manufacturing Process Control

Figure 1 shows the thermocouple readings along the condenser section of one of the life test heat pipes during an NCG test at -40°C, -50°C, -60°C and -65°C. As of June 2008, the heat pipe showed no sign of NCG after 17,030 hours of operation.  Most NCG phenomena occur early in the life of a heat pipe. ACT continues to operate the life test heat pipes at elevated temperatures and periodically performs the NCG test to demonstrate real-time, long-life performance.  The results as of April 2014 are shown in Figure 2.  The tests are conducted at 80°C, while the highest operating temperature is 40°C.   Assuming an Arrhenius relationship, the effective life is doubled for every 10°C above the operating temperature, or 16 times.

Fig. 2: Grooved Aluminum/Ammonia Accelerated Life Test Data as of April, 2014

High-Temperature Water Heat Pipes Life Tests

Copper is the traditional envelope and wick material for water at temperatures below about 150°C, with a large experience base. At higher temperatures, where the vapor pressure of water increases rapidly, copper is not acceptable for the envelope material, due to its relatively high mass and low strength.  Titanium, titanium alloys, Monel 400, and Monel K500 have higher yield strength and lower density than copper. Over the past 20 years, ACT has conducted life tests to verify that these materials are 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.  The materials under test include:

• Ti CP-2 Heat Pipe, with CP (Commercially Pure) Titanium Screen
• Monel K500 Heat Pipe, with Monel 400 Screen
• Ti Grade 5 Cylinder (6% Aluminum, 4% Vanadium), with CP Titanium Screen
• Ti Grade 7 Cylinder (0.2% Pd), with CP Titanium Screen
• Ti CP-2 Cylinder, with 21S foil and CP Titanium Screen (21S tubing was not available)
• Ti Grade 9 cylinder (3% Aluminum, 2.5% Vanadium) with CP Titanium Screen
• Ti CP-2 Heat Pipe, with Sintered Cylindrical Wick
• Ti CP-2 Heat Pipe, with Integral Grooves
• Monel 400 Heat Pipe, with Monel 400 Screen
• Monel K500 Heat Pipe, with sintered Monel 400 wick
• Monel 400 Heat Pipe, with sintered Monel 400 wick

The table below shows the different life test pipes on test at ACT.  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.

Fig. 3: Typical Life Test Heat Pipe and Heater Block

Figure 3, at right, shows a schematic of a typical life test heat pipe set up in a heater block.  The life tests are gravity aided, and cooled by natural convection.  The life test pipes are instrumented with three thermocouples.  One thermocouple is located just above the heater block, while the other two are located in the heat pipe condenser.   During operation, the temperature difference between the evaporator and condenser are monitored to detect non-condensable gas (NCG).  Any NCG is swept by the working fluid to the end of the condenser, where it forms a cold end.

Figure 4, below, shows the titanium/water and Monel/water heat pipes set up in the heater blocks, prior to testing.  One thermocouple is located just above the heater block, the other two are located in the heat pipe condenser (all of the thermocouples are under the hose clamps) in Figure 4.

Fig. 4: CP-Titanium (on left) and Monel 500 heat pipes set up in heater blocks. The fill tubes are much longer than usual, to allow for multiple purge and reseal.

Fig. 5: Titanium/water heat pipes in the test box. Argon surrounds the heat pipes during testing, to prevent oxidation of the titanium.

During the life test, the temperatures of the evaporator and condenser for each heat pipe are monitored, to detect any problems.  It is possible that oxygen and nitrogen can affect the outside of the titanium pipes during the test.  For heat pipes in a space radiator, oxygen/nitrogen will not be a problem.  To prevent this problem during the life tests, the heat pipes are placed inside a box that is purged with argon; see Fi. 5 above.

Fig. 6: Backscatter Electron Image of CP Ti mesh wire wick in a CP titanium/water heat pipe showed no corrosion.

Roughly half of these heat pipes were selected for destructive evaluation.  The working fluids were analyzed, and sections of the heat pipes were examined to determine the type and amount of corrosion in the wicks and heat pipes.  The results showed that Titanium/water and Monel/water heat pipes are compatible at temperatures up to 550 K, based on ongoing life tests that have been running for up to 72,000 hours (8.2 years).

Analysis of the titanium/water heat pipe cross-sections using optical microscopy revealed little if any corrosion when observed at high magnifications.  Even using differential interference contrast, it was difficult to find any corrosion layer; see Figure 6.  When any evidence of corrosion was observed, the layer was typically ~1 micrometer thick.  SEM imaging and Energy Dispersive Spectroscopy (EDS) analysis also did not indicate any substantial corrosion layer.

Intermediate Temperature Fluids

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 ( including 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 (as discussed above) have shown that water can be used with titanium or Monel envelopes at temperatures up to 300ºC.

In a past study, 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.

Lessons Learned from a Series of Halide Life Tests Completed at ACT

An electromotive force difference method was used in research conducted by ACT’s R&D group 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 placed on life-test.  During the life tests, the temperature of the evaporator and condenser for each heat pipe were monitored, to detect any problems.  It is possible that oxygen can affect the outside of the titanium pipes during the test; therefore, the life tests are conducted inside a box that is purged with argon to prevent this.

Possible problems with incompatible fluid/envelope pairs include Non-Condensable Gas (NCG) generation, corrosion, and materials transport.

Three different outcomes were experienced during these exploratory life tests:

1. Non-Condensable Gas Generation

Fig. 7: High ΔT between the evaporator and the condenser after 2000 hours of life testing, indicating the rapid formation of NCG.

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.

Fig. 8: Backscatter Electron Image of a CP Ti/GaCl3 heat pipe shows a Ga-Ti reaction layer, confirming the pair is 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.

Fig. 10: Secondary Electron Image of C-2000 Envelope/ TiCl4 fluid shows a small reaction layer in this compatible heat pipe.

Finally, two envelope/fluid pairs (superalloy/TiCl₄ at 400°C and superalloy/AlBr₃ at 300°C) were found to be compatible, and ran without any problems.  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 10 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.

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