Heat Pipes are passive two-phase heat transfer devices that transfer heat by evaporation and condensation. The heat pipes used in electronics thermal management, Spot Cooling Heat Pipes, HiK™ Plates., and Vapor Chambers, all use water as the working fluid. This web page discusses the parameters and limitation of water heat pipes. Heat pipes with other working fluids have different limitation, for example, their operating temperature ranges are different.
There are three general limitations for passive two phase devices, including heat pipes, HiK™ plates, and vapor chambers:
All heat pipes have a temperature range over which they have the best performance. Maximum power versus operating temperature for typical water heat pipes is shown in Figure 5, calculated with ACT’s heat pipe calculator. The maximum power is high from roughly 60 to 200°C, falling off gradually at lower temperatures (the power also falls off at higher temperatures, but this is typically not a concern for electronics cooling).
The drop off in power as the temperature is reduced is set by the fluid properties of the working fluid. As the temperature (and associated saturation pressure) of the water is reduced, the water vapor density is also reduced. To carry a given amount of power, the vapor velocity in the heat pipe must increase, which in turn increases the pressure drop in the heat pipe while reducing the power that can be carried.
ACT generally designs heat pipes to operate at temperatures above ~25°C. At temperatures below 0°C, the water is frozen in the heat pipe, and conduction through the heat pipe wall is the primary method of heat removal. It is important to note that this is not generally 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 temperature is around 25°C, and the heat pipe starts operating. Properly designed heat pipes can operate after thousands of freeze/thaw cycles, see Figure 6.
If operation below 25°C is required, than the thermal designer can switch to a different working fluid such as methanol, or use an encapsulated conduction card.
Adverse Vertical Height
Heat pipes return liquid from the condenser to the evaporator through a wick, which allows them to operate in any orientation. During operation, capillary forces in the wick must overcome the sum of liquid and vapor pressure drops as well as the adverse gravity head and acceleration. As the adverse elevation is increased (evaporator located above condenser), more of the wick pumping capability is used to counteract the adverse gravity head, and the maximum heat pipe power is reduced. This can be seen when comparing Figure 5 (a) and (b), showing power with a level heat pipe, and a heat pipe with an adverse 4 inch (10 cm) elevation, respectively. It can be seen that the maximum power is reduced significantly. Other adverse elevations can be examined with calculated with ACT’s heat pipe calculator.
In general, water heat pipes can operate with the evaporator elevated a maximum of 9-10 inches (23-25 cm) above the condenser. This sets the maximum elevation for spot cooling heat pipes and vapor chambers. This elevation is doubled for HiK™ plates to 18-20 inches (46-50 cm), when the HiK™ plates are cooled on both the top and the bottom, and a double set of heat pipes is embedded. Note that if the electronics and sink can be arranged so that the electronics are lower than the heat sink, then the heat pipe behaves as a thermosyphon. In this orientation, gravity returns the liquid to the evaporator rather than capillary forces, and the heat pipe length is essentially unlimited.
During operation, capillary forces in the wick must overcome the sum of liquid and vapor pressure drops as well as the adverse gravity head and acceleration. Water heat pipes will stop operating under high adverse acceleration, when the wick can no longer return condensate to the evaporator, and the heat pipe deprimes, or dries out. The wick will quickly reprime after the acceleration stops. Heat is stored by a rise in temperature during the acceleration. Most accelerations are relatively short, and this temperature rise is acceptable.
If the acceleration is of longer duration, then the thermal designer has three choices:
- Design the heat pipes “gravity aided” under acceleration if the axis and direction of the acceleration are known.
- Arrange heat pipes in pairs, so that one pipe is always “gravity aided”; see Figure 7.
- Use an encapsulated conduction card.
More information on When to Use Heat Pipes, HiK™ Plates, Vapor Chambers, and Conduction Cooling: