One reason for conducting heat pipe life tests is to demonstrate long life for current products. The goal is to demonstrate 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. Figure 15 shows a number of satellite thermal control Constant Conductance Heat Pipes (CCHPs).
New heat pipes are put on life test whenever a new extrusion is qualified, or the manufacturing process is changed. 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.
Figure 16 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. The results as of April, 2014 are shown in Figure 17. 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.