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Thermal Management: Moving Heat

Moving heat by remote heat dissipation with passive devices like heat pipes or loop heat pipes provides an efficient thermal solution with high performance, low weight, low cost and high reliability – there are no moving parts to fail, wear out, replace or lubricate.

Heat Pipes

therma-sink-moving heatHeat pipes can be used to move heat over distances ranges from a few inches (>50mm) to greater than 3 feet (> 1 meter). In a heat pipe, 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. The capillary 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 of the heat pipe to the evaporator section via capillary action.

The heat-moving efficiency of this thermal solution is determined by factors such as wick, 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 and fiber/spring. Different wicks have varying capillary limits (the capillary pumping rate at which the working fluid travels from condenser to evaporator).

Loop Heat Pipes

A loop heat pipe (LHP) is also a two-phase heat transfer device that uses capillary action to remove heat from a source and passively move it to a condenser or radiator. LHPs are similar to heat pipes but have the advantage of being able to provide reliable operation over long distance (up to 75 meters) and the ability to operate against gravity (high g environments).

In a loop heat pipe, the wick structure is only in the evaporator and the vaporized fluid is separated from the liquid and travels in a loop through the condenser back to the evaporator. Aavid Thermacore has developed and manufactured different designs of LHPs ranging from powerful, large size LHPs (>2000W) to miniature LHPs (<100W) that have been successfully employed in a wide range of aerospace and ground based applications.

Working Fluids, Operating Temperature Ranges, Orientation and Forming

The type of working fluid also 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 – and no cooling – occurs. Working Fluid selection is based on the operating temperature range of the application. Aavid Thermacore has designed and developed heat pipes and loop heat pipes for operating temperature ranges from Cryogenic (<-250°C) to High Temperature (>2000°C). Water is the most common working fluid due to its favorable thermal properties and operating temperature range of 5°C to 250°C .

Aavid Thermacore has designed, developed and manufactured heat pipes using over 27 different working fluids.

The orientation of a heat pipe relative to gravity, combined with its wick structure, also plays an important role in its performance. For example, the groove wick has the lowest capillary limit but works best under gravity-assisted conditions, where 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. If a heat pipe is flattened or bent, it will reduce the maximum amount of heat that can be transported. Avoiding this limitation is a design consideration.

Heat Pipe Applications

For moving heat in industrial, electronic, aerospace and other applications, heat pipes and loop heat pipes are generally 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 is often 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.

Benefits of Heat Pipes and Loop Heat Pipes
The integration of heat pipes and loop heat pipes into a thermal solution delivers many benefits, including.

  • High effective thermal conductivity (>5000 W/m•K)
  • Long distance heat transport
  • High reliability
  • No moving parts
  • Cost-effective
  • Passive — do not require fans, chemicals gears and other potential maintenance challenges

In addition, heat pipes and loop heat pipes can be designed for a variety of external environmental factors such as mechanical shock, vibration, force impact, thermal shock/cycling, and corrosive environment that can affect heat pipe life.

Heat-moving heat pipes and loop heat pipes are widespread thermal solutions used in computer, electronics, power electronics, radar electronics and aerospace/avionics applications.
Other cost-effective, reliable thermal management approaches available from Aavid Thermacore include technologies for spreading heat and for dispersing heat.


 

Thermal management challenge? Take the first step towards the solution for you. Please contact an Aavid Thermacore technical representative or e-mail info.thermacore@aavid.com for more information on our thermal solution technologies.

 

 
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