What is a heat pipe?

A heat pipe is a two-phase heat transfer device that consists of a vacuum tight envelope, a wick structure and a working fluid. Heat pipes circulate a working fluid, such as ammonia, that evaporates and condenses to transport waste heat. They provide an energy efficient means to transport 100s of watts of heat across several meters. They can be embedded within a honeycomb panel and bonded to the interior side of the facesheets. Surface-mounting heat pipes with flanges attach to heat pipe panels to transfer heat from internal equipment panels to external radiator panels.

A heat pipe is a passive device that transports heat efficiently from one point to another. It is made of a sealed contained with a working fluid and wick, or grooves, inside. As heat is applied to the evaporator section, the working fluid vaporizes and flows to the cool or condenser section of the heat pipe. There the working fluid condenses, releasing the heat of vaporization.

Figure 1. Heat Pipe

How does a heat pipe work?

The heat pipe is evacuated and then back-filled with a small quantity of working fluid, just enough to saturate the wick. The atmosphere inside the heat pipe is set by an equilibrium of liquid and vapor. As heat enters at the evaporator, this equilibrium is upset generating vapor at a slightly higher pressure. This higher pressure vapor travels to the condenser end where the slightly lower temperatures cause the vapor to condense giving up its latent heat of vaporization. The condensed fluid is then pumped back to the evaporator by the capillary forces developed in the wick structure.

This continuous cycle transfers large quantities of heat with very low thermal gradients. A heat pipe's operation is passive being driven only by the heat that is transferred. This passive operation results in high reliability and long life.

Figure 2. Heat Pipe Cutaway

Heat Pipe Thermal Modeling Techniques