Heat Pipe Assemblies
Integrated Heat Pipe Assemblies
Although heat pipes themselves do not actually dissipate significant amounts of heat, they do effectively transfer heat without a large increase in temperature. This unique transfer capability allows them to transport or spread heat to a point remote from the heat generator.
A variety of basic heat sink technologies benefit from integrating heat pipes which improve conduction paths, reduce overall weight and raise thermal performance without increasing volume. The availability of a wide range of heat pipes sizes and power handling capabilities make them suitable for integration in heat sinks for 50W processors to multi kilowatt IGBTs.
Due to the large variety and diverse nature of customer requirement, most heat sinks using integrated heat pipes are developed specifically for the application. Heat Pipe assemblies have been used to solve thermal problems in desktop, notebook, computer servers, telecommunication, motor drives, UPS and transportation applications.
Working With Heat Pipes
How a Heat Pipe Works
A heat pipe is a closed evaporator-condenser system consisting of a sealed, hollow tube whose inside walls are lined with a capillary structure or wick. Thermodynamic working fluid, with substantial vapor pressure at the desired operating temperature, saturates the pores of the wick in a state of equilibrium between liquid and vapor. When heat is applied to the heat pipe, the liquid in the wick heats and evaporates. As the evaporating fluid fills the heat pipe hollow center, it diffuses throughout its length. Condensation of the vapor occurs wherever the temperature is even slightly below that of the evaporation area. As it condenses, the vapor gives up the heat it acquired during evaporation. This effective high thermal conductance helps maintain near constant temperatures along the entire length of the pipe.
Attaching a heat sink to a portion of the heat pipe makes condensation take place at this point of heat transfer and establishes a vapor flow pattern. Capillary action within the wick returns the condensate to the evaporator (heat source) and completes the operating cycle. This system, proven in aerospace applications, transmits thermal energy at rates hundred of times greater and with a far superior energy-to-weight ratio than can be gained from the most efficient solid conductor.
Heat Pipe Assemblies Design Guidelines
Orientation with Respect to Gravity Temperature Limits Heat Removal Reliability Forming or Shaping Effects of Length and Pipe Diameter Wick Structures The wick structure provides a path for liquid to travel from condenser to the evaporator using capillary action. Wick structures have performance advantages and disadvantages depending on the desired characteristics of the heat sink design. Some structure have low capillary limits making them unsuitable for applications where they must work without gravity assist. Thermal Solutions Aavid also designs custom solutions to address specific application needs. We have experience designing solutions for Intel®, AMD, IBM and other RISC and SPARC processors. We solve thermal problems in various form factors including thermally challenging applications such as Blade and modular systems. Contact Aavid with your specific needs and see how our experience can help you.
For the best performance, the application should have gravity working with the system; that is, the evaporator section (heated) should be lower, with respect to gravity, than the condenser (cooling) section. In other orientations where gravity is not aiding the condensed liquid return, the overall performance will be degraded. Performance degradation depends on a number of factors including wick structure, length and working fluid of the heat pipe along with heat flux of the application. Careful design can minimize the performance loss and allow an accurate prediction performance.
Most pipes use water and methanol/alcohol as the working fluids. Depending on the wick structure, pipes will operate in environments with as low as -40癈. Upper temperature limits depend on the fluid, but 60癈 to 80癈 is the average limit.
Heat can be removed from the condenser using air cooling in combination with either extrusion, bonded-fin heat sinks, or flat-fin stock. Enclosing the condenser in a cooling jacket allows liquid cooling.
Heat pipes have no moving parts and have demonstrated life of over 20 yrs. The largest contributor to heat pipe reliability comes from control of the manufacturing process. The seal of the pipe, purity of the materials used in the wick structure and cleanliness of the internal chamber have measurable effect on the long term performance of a heat pipe. Any leakage will eventually render the pipe inoperable. Contamination of the internal chamber and wick structure will contribute to the formation of non condensable gas (NCG) that will degrade performance over time. Well developed processes and rigorous testing are required to ensure reliable heat pipes.
Heat pipes are easily bent or flattened to accommodate the needs of the heat sink design. Forming heat pipes may affect the power handling capability as the bends and flattening will cause a change in fluid movement inside the pipe. Therefore design rules that take into consideration heat pipe configurations and the effect on thermal performance ensure the desired solution performance.
The vapor pressure differential between the condenser end and the evaporator end controls the rate at which the vapor travels from one end to the other. Diameter and length of the heat pipe also affect the speed at which the vapor moves and must be considered when designing with heat pipes. The larger the diameter, the more cross sectional area available to allow vapor to move from the evaporator to the condenser. This allows for greater power carrying capacity. Conversely, length when in opposition to gravity has a negative effect on heat transport as the rate at which the working fluid returns from the condenser end to the evaporator end is controlled by the capillary limit of the wick which is an inverse function of the length of the pipe. Therefore, shorter heat pipes carry more power than longer pipes when used in application not assisted by gravity.
Heat pipe inner walls can be lined with a variety of wick structures. The four most common wicks are:
for Intel® Server CPU
Aavid uses the latest software tools and cooling technology to design thermal solutions for some of the largest OEM server manufactures in the world. As a result we have developed a portfolio of solutions for popular Intel® microprocessors. Our solutions solve thermal problems using the cooling technology best matched for the application.
Cooling Solutions for
Notebook Assemblies
Notebook computers present unique thermal challenges, and heat sinks from Aavid Thermalloy can provide complete solutions to meet these challenges. Selection can be made from off-the-shelf components, custom designed parts, or any combination of these.
Application Specific Designs
Transport Applications Typical Applications Include: Application Specific Designs
Application Specific Designs Vertical Axis Spreading Applications Typical Applications Include:
Transport designs can be useful where space is constrained or airflow is not available directly over the device requiring cooling. Examples of application characteristics where transport heat pipes solutions may solve design issues include:
Vertical heat spreading designs can be useful where horizontal space is constrained or where the use of higher thermal conductive metal like copper cannot be tolerated due to weight. Examples of application characteristics where vertical spreading heat pipe solutions may solve design issues include:
