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The Basics of Heat Transfer


Last updated Dec 21, 2023 | Published on Jun 10, 2019

Heat Spreading

Heat spreaders offer cost-effective, reliable high thermal conductivity and efficacy with almost no moving parts. A heat spreader is an effective solution for dealing with heat sources that have a high heat-flux density (high heat flow per unit area) and where the secondary heat exchanger in itself is not an effective method of dispersing heat (due to space limitations, energy use, cost, etc.). Heat spreaders can allow designers to use an air-cooled, rather than liquid-cooled, secondary heat exchanger. Most heat spreaders are copper plates that function as heat exchangers. Heat spreaders transfer heat between its source and (generally) a secondary heat exchanger. The heat “spreads” from the heat source through the heat spreader, thus moving from a smaller to a larger cross sectional area (the secondary heat exchanger). While the heat flow is the same in the heat spreader as in the secondary heat exchanger, the heat flux density is reduced, making it easier to dissipate the heat via air cooling. The lower heat flux density also allows the secondary heat exchanger to be made of less expensive materials. Boyd offers a variety of heat spreading technologies that provide significant improvement in solution effective thermal conductivities. These heat spreading technologies include:
  • Advanced Solid Conduction (k-Core®/ Graphite Technologies)
  • Embedded Heat Pipe Cold Plates
  • Vapor Chamber Assemblies

Heat Transfer

Heat Pipes

Heat 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. Boyd 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. Boyd 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. Boyd 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 moving parts and other similar 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.

Dispersing Heat

Using thermal solution technologies from Boyd such as heat sinks, heat pipes, vapor chambers, loop heat pipes, k-Core®, liquid cold plates, and heat exchangers, designers can choose to dissipate waste heat to air (natural or forced convection), to liquid (water, water/glycol, PAO), or radiate to space.

Dissipating Heat to Air

In many applications, the preferred method of thermal management is convection cooling to air, especially in electronics cooling applications. With Boyd’s heat sink, heat pipe assemblies, and heat spreader technologies, waste heat is typically absorbed from a heat generating device (e.g., an electrical component within an electronics system — i.e. computers and data centers) and then moved or spread for dissipation into the ambient air through either natural or forced (using a fan air mover). Thermal technologies from Boyd such as remote heat pipe assemblies and vapor chambers allow the designer to move heat from high heat flux components to a location with a larger surface area (typically plate fins or folded fins) and lower heat flux for dissipation into the ambient air.

Dissipating Heat to Liquid

Applications with large heat loads such as military radars or power electronics often require waste heat to be dissipated into the liquid coolants (water, water/glycol, PAO) of a secondary system for ultimate heat dissipation. Boyd’s heat pipe cold plates and liquid cold plates allow designers to move heat from a heat generating device into a coolant being circulated from a secondary system.

Dissipating Heat through Radiation

As satellites are packaged with more electronics, the challenge of rejecting heat through the limited surface area becomes greater. Boyd’s low temperature, axially grooved heat pipes (ammonia/aluminum, ethane/aluminum) and loop heat pipe technology make it possible to reject heat through radiator panels that are stored for launch, then deployed from the satellite when the satellite achieves orbit. Our low temperature axially grooved heat pipes spread heat out from the satellite electronics to the radiator panels, dissipating waste heat to space. And our loop heat pipe technology is capable of transporting and rejecting heat loads from hundreds of W to greater than 2,000 W.

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