Copper is the traditional envelope and wick material for water heat pipes at temperatures below about 150°C. At higher temperatures where the vapor pressure of water increases rapidly, copper is not an acceptable envelope material, due to its low strength and high mass density. Note that the wick and envelope materials should be reasonably close in composition, to avoid gas generation by electrochemical reactions.
ACT has focused on the following materials that have the best chance of being compatible with water at elevated temperatures. These materials also have high yield strengths, low densities and good commercial availability:
- Monel 400, a solid solution alloy with roughly 63% nickel and 30% copper
- Monel K500, a similar nickel-copper alloy with the addition of aluminum and titanium
- Titanium, various grades
Life tests are required to verify that the heat pipe envelope and wick materials and the working fluid are compatible for the long operating life of a heat pipe. The major consequences of incompatibility are corrosion, generation of non-condensable gases, or both. The resulting corrosion products can block portions of the wick, preventing the heat pipe from operating properly. In more extreme cases, the heat pipe can leak.
ACT maintains an extensive life test program in demonstrating the long term operations of various heat pipe materials and fluids. Several examples of life test heat pipes relevant to the high temperature water heat pipe technology include:
- Ti CP-2, with Ti CP screen
- Ti CP-2, with sintered Ti powders
- Ti Grade 5, with Ti CP screen
- Ti Grade 7, with Ti CP screen
- Ti Grade 9, with Ti CP screen
- Monel 400, with Monel 400 screen
- Monel 400, with sintered Monel 400 powders
- Monel K500, with sintered Monel 400 powders
- Monel K500, with Monel 400 screen
These heat pipes currently (November 2006) have been on life test for over 20,000 hours. Figure 1 shows the temperature gradients along a life test Monel/Water heat pipe. This heat pipe operates at 275°C. The stable and low temperature gradients (ΔT) indicate that the materials and fluid in this heat pipe are compatible. The life test continues until a failure (or large ΔT) is detected.
Another significant technical challenge associated with the high temperature water heat pipe technology is the design and manufacture of the wicks for high performances at the elevated temperatures. Water’s surface tension drops off quickly as the temperature increases, placing increasing burdens on the capillary wick design.
Figure 2 shows the photos of various wicks designed, manufactured and tested in high temperature water heat pipes. These include, clockwise, axial grooves, mesh screen, sintered metal powders, sintered metal powder grooves (fine grooves), sintered slab and sintered metal powder grooves (large grooves).
Figure 3 shows the measured performance of two high temperature water heat pipes that are 0.5″ OD and 45″ long. These heat pipes had axial grooves as the wick and were tested at 150°C and 200°C (vapor temperature) and various adverse elevations.
Many applications require integration of the heat pipes with other heat transfer and/or mechanical structures. This often requires making the heat pipes into different geometries or bonding the heat pipes with other materials. Figure 4 shows an example where two titanium/water heat pipes were bent and flattened to allow attachment of fins, while Figure 5 shows three titanium/water heat pipes integrated in a space radiator panel.