High Temperature Water Heat Pipe Life Tests – 150°C to 300°C

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 10 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

Table 4.  Titanium-Water and Titanium-Monel Life Test Pipes – Operating Hours as of May 6, 2013.

Initial

Quantity

Wall Material

Wick

Operating Temperature

Operating Hours

May 6 2013

4

Monel K 500

200×200 Monel 400 Screen

0.064 mm wire

550 & 500 K

72,192 hours

4

CP-2 Ti

150x150CP-Ti Screen

0.069 mm wire

550 & 500 K

72,192 hours

2

CP-2  Ti

Sintered Titanium

-35+60 Mesh CP-2

550 K

60,672 hours

2

CP-2  Ti

100 x100 CP-Ti Screen

0.05 mm wire

550 K

61,064 hours

1

CP-2 Ti

Integral Grooves

550  K

41,345 hours

2

CP-2 Ti

21 S Foil Inside

100 x100 CP-Ti Screen

0.05 mm wire

550 K

62,622 hours

2

Grade 5 Ti

100 x100 CP-Ti Screen

0.05 mm wire

550 K

69,845 hours

2

Grade 7 Ti

100 x100 CP-Ti Screen

0.05 mm wire

550 K

60,672 hours

2

Grade 9 Ti

100 x100 CP-Ti Screen

0.05 mm wire

550 K

60,072 hours

2

Monel 400

120×120 Monel 400 Screen

0.05 mm wire

550K

60,168 hours

2

Monel K 500

120×120 Monel 400 Screen

0.05 mm wire

550K

67,536 hours

2

Monel 400

-100+170 Mesh Monel 400 Powder

550K

58,824 hours

2

Monel K 500

-100+170 Mesh Monel 400 Powder

550K

57,792 hours

Table 4 shows the different life test pipes on test.  Monel 400 is a solid solution alloy with roughly 63% nickel and 30% copper.  It is a single-phase alloy, since the 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.

Figure 19 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 19.  Typical Life Test Heat Pipe and Heater Block.

Figure 19. Typical Life Test Heat Pipe and Heater Block.

 

Figure 20 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 20.

Figure 20.  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.

Figure 20. 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.

 

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 Figure 21.

 

Figure 21.  Titanium/water heat pipes in the test box.  Argon surrounds the heat pipes during testing, to prevent oxidation of the titanium.

Figure 21. Titanium/water heat pipes in the test box. Argon surrounds the heat pipes during testing, to prevent oxidation of the titanium.

 

Recently, 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) as of May 2013.

For example 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 22.  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.

Figure 22. Backscatter Electron Image of CP Ti mesh wire wick in a CP titanium/water heat pipe showed no corrosion.

Figure 22. Backscatter Electron Image of CP Ti mesh wire wick in a CP titanium/water heat pipe showed no corrosion.

 

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