Imagine there was a way to remove heat remotely from your enclosure, and the cooling technology had no moving parts to fail, wear out, replace or lubricate.

Guess what? That technology exists: heat pipes and loop heat pipes can do just that.

A heat pipe is a heat-transfer device with a highly effective thermal conductivity. Heat pipes are evacuated vessels — typically circular in cross-section — that are back-filled with a small quantity of a working fluid. The passive devices are used to transfer heat from a heat source to a heat sink with minimal temperature gradients, or to iso-thermalize surfaces.

Heat pipes move heat over distances — from less than 2" to greater than 3'. The heat from a heat source enters the evaporator end of the heat pipe, causing the working fluid to change phase from liquid to vapor. The vapor travels through the vapor space within the heat pipe to the other end — the condenser end — where a heat sink or other secondary heat-dissipation device removes the heat energy. The release of heat in the condenser end causes the vapor to condense back to liquid, which is absorbed into a capillary-wick structure. Wick structures incorporated into the internal walls of a heat pipe allow the liquid condensate inside the heat pipe to return from the condenser section to the evaporator section via capillary action. The heat-moving efficiency of this type of thermal solution is determined by factors such as wick materials, working fluid, diameter, length, bending, flattening and orientation.

The four common, commercially produced heat-pipe wick structures are:

•    Grooves in the internal tube wall.

•    Wire or screen mesh.

•    Sintered powder metal.

•    Fiber/spring.

Different wicks have varying capillary limits, which are defined as the capillary pumping rate at which the working fluid travels from condenser to evaporator.

The loop heat pipe also is a two-phase heat-transfer device using capillary action to remove heat from a source and passively move it to a condenser or radiator. Loop heat pipes are similar to heat pipes but are able to provide reliable operation over longer distances — up to approximately 246' — with the ability to operate against gravity (high G environments).

In a loop heat pipe, the wick structure is only in the evaporator. The vaporized fluid is separated from the liquid and travels in a loop, through the condenser, back to the evaporator. One heat-pipe manufacturer has developed and manufactured different designs of loop heat pipes ranging from sizes greater than 2,000 W to miniature units less than 100 W. Loop heat pipes have been successfully employed in a range of aerospace and ground-based applications.

Working-Fluid Effects on Performance

The type of working fluid used influences heat-pipe performance. A heat pipe or loop heat pipe only functions when the working-fluid temperature is above its freezing point. When the temperature is above the vapor-condensation point of the working fluid, the vapor will not condense back to liquid phase, and no fluid circulation or cooling occurs.

Fluid selection is based on the operating temperature range of the application. Some heat pipes and loop heat pipes work in operating temperatures from cryogenic levels below -418 to greater than 3,632°F (-250 to greater than 2,000°C). Water is the most common working fluid due to its favorable thermal properties and operating temperature range of 41 to 482°F (5 to 250°C).

The orientation of a heat pipe relative to gravity, combined with its wick structure, play an important role in its performance. For example, the groove wick has the lowest capillary limit but works best under gravity-assisted conditions in which the evaporator is located below the condenser. Loop heat pipes are less sensitive to orientation and rely on a high capillary-pumping wick in the evaporator to drive performance.

Heat pipes can be formed (flattened or bent) for integration into an assembly but doing so reduces the maximum amount of heat that can be transported. Avoiding this limitation is a design consideration.

Applications for Heat Pipes

For moving heat in industrial, electronics and other applications, heat pipes and loop heat pipes generally are integrated into a thermal subsystem to transport

heat from the heat source to remote areas. Heat pipes are effective in carrying heat away from heat sources and heat-sensitive components to a finned array or a heat sink in another location.

A high-capacity power-electronics cooler is an example of a thermal solution where space often is insufficient for mounting a finned heat sink directly adjacent to the heat source. Instead, high-capacity heat pipes move the heat to the finned array, which dissipates heat energy using forced convection. Hundreds of watts can be dissipated this way.

The integration of heat pipes and loop heat pipes into a thermal solution delivers many benefits, including:

•    High effective thermal conductivity.

•    Long distance heat transport.

•    High reliability.

•    No moving parts.

•    Passivity. The pipes do not require fans, chemicals, gears or other devices with potential maintenance issues.

 Heat-dissipating pipes can be designed for external environmental factors such as mechanical shock, vibration, force impact, thermal shock or cycling, and corrosive environments that can affect heat pipe life. Heat pipes and loop heat pipes are widespread thermal solutions used in computer, electronics, power electronics and other applications.