The heat pipe functioning

Among classical cooling systems, the heat pipe functioning hold a particular position because of it is a passive solution. Patented in 1942, the heat pipe technology has been widely used during the sixties for space applications. It is now more and more used for industrial applications.

Thermal management for electronic components is a main issue in the performance of power converters and computers. Performance of electronics components is currently improving, leading to huge amount and high density of power.

How it works ?

A heat pipe is made by a sealed enclosure, a working fluid and a capillary network. During the manufacturing process, all the air enclosed in the pipe is removed. We insert the suitable quantity of fluid in order to fill up the capillary network. A balance between liquid phase and vapor phase is created.

When the heat is applied to one of the end of the pipe (evaporator area), the working fluid is transforming into vapor. This leads to a light increase of pressure that moves the vapor to the other end of the pipe (condenser area). In the condenser area, vapor is transforming back into liquid. The liquid is flowing through the capillary networks and then come back to the evaporator side with the help of gravity (thermosiphon) or the capillary pumping effect (heat pipe). The cycle can be renewed indefinitely, it is autonomous and without any maintenance.

During the evaporation of the liquid the fluid absorbs energy. In return, it gives back energy during condensation of the vapor. Thus there is an overall thermal transfer between evaporator and condenser with a very small difference of temperature. The middle area between evaporator and condenser is adiabatic. This area can reach few meters, that allows heat transfer over high distances.

If you compare a heat pipe, with a full copper bar having the same dimensions, subjected to the same difference of temperature, the heat pipe has about 1000 time more thermal conductivity.

Position and size of the hot/cold sources can be variable. It is also possible to have several hot/cold sources. However accurate characteristics, are depending from the positions of these sources and pipe diameter.

The heat pipe is only a way to convey heat. If it is requested to dissipate the power to the air, it is necessary to set fins in order to increase exchange surface. A base plate is generally set up on evaporator in order to collect calories from the hot source (electronic components). As usual for thermal management, all the interfaces have to be flat and continuous for a good overall performance.

Characteristics

Working fluid

Enclosure/Fluid classicaly used, and temperature ranges:

  • Copper / Water: [+5°C ;+250°C] – Standard solution for electronic
  • Copper / Ethanol: [-45°C ;+130°C] – Cold start applications
  • Aluminum / Ammonia: [-60°C ; +70°C]
  • Stainless steel / Ammonia: [-60°C ; +70°C]

Technology

  • Gravity heat pipe or Thermosiphon: pipe working with the help of gravity. It is requested that the hot source is below the cold source, with a minimum angle of 3° / horizontal
  • Heat pipe with capillary structure: pipe working with capillary pumping. The negative inclination compared to horizontal can be allowed (hot source higher than cold one)

Power

  • From 5W to 1000W per pipe
  • Pipes can be added in parallel to achieve higher thermal transfer

Thermal resistance and delta T°

  • Depending from functioning point

Dimensions

  • Diameter: 3mm to 16mm
  • Length : 50mm to 5000mm

Applications

Type of cooler we can provide

  • Surface / Air or Air / Surface
  • Surface / Surface
  • Air / Air
  • Spreader

Capillary pumped loop (CPL)

Capillary pumped loop, have been derivated from heat pipe principle. Liquide phase is separated from vapor in the same closed loop.

CPLs are built with an evaporator area, a capillary structure and a condenser area. Heat flux on the evaporator is leading to a pressure increase in the capillary structure. The pressure increase push the working fluid to flow in the loop.

CPLs are interesting because of their resistance against high accelerations. Most of CPLs are used for spatial applications.

They are however hard to set up, because of the transitory cycles that may happen during start and stop.

The heat pipe functioning

Heat pipes coolers are like the Loch Ness monster... lot's of people are speaking about, but few people have really tested them! This technology sounds a little bit mysterious for people coming from a pure electronics background ! How can such a thing (looking like a basic copper tube), would be able to convey something like 1000 time more calories than the best typical thermal conductor in electronics : copper ! You will certainly find the answer in our dedicated page to the heat pipe functioning, or to heat pipe page on wikipedia, if you want to go deeper in the subject. [caption id="attachment_1877" align="aligncenter" width="685"]Heat Pipe functionning Heat Pipe functionning[/caption]

Thermal management challenges in power electronics

As it was explained in our market analysis article about SIC semiconductors, power electronics is currently at the beginning of a revolution. Semiconductors are now able to work up to incredibly high frequency (50 to 100kHz), with excellent efficiency, and low losses. Because working at high frequency leads to reducing size and weight, this revolution increasesthe density of power losses. New thermal management challenges are in front of us. But good news, heat pipes are here to help solving them ! Here are the main reasons why ?

Heat pipe advantages

  • High thermal conductivity. Because heat pipes are using one of the most efficient way to convey calories : phase change, their high thermal conductivity will be the best advantage to solve your thermal management issues.
[caption id="attachment_1880" align="aligncenter" width="300"]Heat Pipe thermal conductivity simulation Heat Pipe thermal conductivity simulation[/caption]
  • Temperature range. It is mainly depending from the working fluid inside the heat pipe. Considering most of the power electronics application going from 50 to 200 degC, you will not find a better fluid than water. Good news, water is not dangerous and not explosive as you may know... Bad news, water is electrically conductor as you may know too. Electronics designer never sleeps soundly when water flows close to their components ! If you feel a bit of apprehension, read our next point.
 
  • Reliability. Heat pipes are excellent in reliability because they use no mooving mechanical parts and need no energy like electricity. No worry about maintaining the motor or the filter of your pump like in a cooling loop. The difference of temperature is the only "motor" which garanty an endless  functionning. Heat pipes are commonly used in aerospace application and railway, where life cycle of the product can be up to 50 years !
[caption id="attachment_1861" align="aligncenter" width="300"]Railway Heat Pipe Cooler Railway Heat Pipe Cooler[/caption] We hope this article have contributed to better understanding of heat pipes.

Contact us

For any question, enquiry, contact COUGAR ELECTRONICS here. Our thermal engineers will be happy to help :-)
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