Heat Pipe Life Tests
Heat pipe life test (click to enlarge)
ACT maintains an extensive, ongoing heat pipe life test program which serves two purposes. First, it demonstrates that ACT's manufacturing processes result in heat pipe products meeting even the most stringent reliability requirements typically found in critical mission systems such as satellites. The figure to the right shows a number of satellite thermal control Constant Conductance Heat Pipes (CCHPs) on life 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 for 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.
The figure below 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 perform the NCG test to demonstrate real-time, long life performance.
Heat pipe life test (click to enlarge)
A second reason for running life tests is to demonstrate chemical compatibility between the heat pipe materials and working fluids for new applications. An example includes the space nuclear power system thermal control heat pipes that operate at higher temperatures (500K-700K) where common heat pipe material/fluid combinations are not applicable.
In 2004, ACT started a series of life tests that successfully demonstrated that titanium and Monel are compatible with water at temperatures up to 280°C (550 K). Previously, most water heat pipes were made of copper, and could not be used at temperatures over 180°C. The figure to the right shows a photo of a typical life test setup for high temperature titanium/water heat pipes. These pipes are intended to operate in the vacuum of space. Therefore, the titanium heat pipes are surrounded by an argon cover gas during testing to prevent any reaction of the titanium with the ambient air.
Life tests of the titanium and Monel heat pipes are continuing, with over five years of compatible operation demonstrated. ACT's high temperature titanium/water and Monel/water heat pipes have been incorporated into an advanced radiator panel design for NASA as well as into heat pipe heat exchangers for commercial applications.
After successfully extending the water heat pipe operating range to 270°C, ACT has been working to develop working fluids in the higher temperature range, between water and the alkali metals. This temperature range is referred to as the intermediate temperature range. Currently, this temperature range has very few potential working fluids. A survey of previous work was conducted that identified several organic fluids, as well as halides as potential working fluids. Halides are compounds of a metal and either fluorine, chlorine, bromine, iodine or astatine. They are attractive because they are more stable at high temperatures than organic working fluids and their heat transfer properties peak in the intermediate temperature range.
Life tests are ongoing with various titanium and nickel super alloys as the envelope material, and halide and organic working fluids. Several attractive fluids have been identified, including AlBr3 and TiCl4.