Many military and commercial applications specify temperatures ranging from -45˚C to + 70˚C. Water heat pipes are typically used in these applications because of their proven reliability and capability. Water heat pipes operate at full capacity at the higher end of the temperature range, but their maximum power drops off as the temperature is reduced, with 25°C being a rough lower limit for water heat pipe operation; see Water Operating Temperature Range for more details. Below 0°C, the water in the heat pipe is frozen, and heat removal is by conduction only. This is generally not a problem for electronics cooling, since the primary concern is to maintain the electronics below a maximum temperature. When the system starts up from a colder condition, say -40°C, the electronics will warm up until the power is around 25°C, and the heat pipe starts operating. Properly designed heat pipes can operate after thousands of freeze/thaw cycles. If heat pipe operation below 25°C is required, than the thermal designer can switch to a different working fluid such as methanol
Frozen start up can be an issue if the system thermal mass and heat transfer are such that the fluid in the evaporator is thawed and vaporized by the heat input, travels to the condenser and freezes there. This could result in the depletion of fluid in the evaporator, eventually shutting down the heat pipe. This is a system design issue and not typically a heat pipe limitation. There are four ways to address this issue. First, design the system so that frozen start up is not an issue. In other words, the input power and vapor transport are sufficient to thaw the entire system. Second, use active controls such as turning off fans to limit heat transfer in freezing conditions. Third, design in a secondary heat transfer mechanism so that the heat pipes are not needed to prevent device from overheating in freezing ambient conditions. Fourth, add a predetermined amount of non-condensable gas (NCG) to the heat pipe to ensure “orderly” freezing and thawing; see Gas Loaded Heat Pipes for Start Up from a Frozen State for more details. Options one and three are typical in most assemblies by default, but can be assured through analysis and testing.