A heat pipe is a simple device that depending on the application can quickly transfer heat from one point to another or it can be used as a means to spread heat, to isothermalize an uneven temperature distribution.
Heat pipes are often referred to as the “superconductors” of heat as they possess an extra ordinary heat transfer capacity and rate with almost no heat loss.
Heat Pipe Construction
In its most common form a heat pipe consists of a sealed hollow copper tube whose inside walls are lined with a capillary structure or wick typically a sintered metal powder, sintering is a method for making objects from powder, by heating the material until its particles adhere to each other. The wick lined tube contains a relatively small quantity of ‘working fluid’ such as water, which is soaked into the wick structure. Air is removed from the pipe forming a vacuum so the only gas that the Heat pipe contains is the gaseous form of the working fluid finally the pipe is sealed.
The vacuum dramatically lowers the temperature at which the working fluid will evaporate into a gas. If part of the Heat pipe is in contact with a heat source that requires cooling the liquid adjacent to the heat source will absorb the heat and evaporate into a gas.
As the gas fills the heat pipes hollow centre, it diffuses throughout the length of the heat pipe.
Condensation of this gas back into a liquid will occur wherever the temperature is even slightly below that of the evaporation area. As it condenses and becomes a liquid again, the gas gives up the heat it absorbed during evaporation process.
The wick structure on the inner wall of the Heat pipe provides the path to return the liquid back to the evaporation area. The driving force is the capillary action of the wick, the quality and type of wick usually determines the performance of the heat pipe, for this is the heart of the product. Different types of wicks are used depending on the application for which the heat pipe is being used.
After a brief start-up period, the heat pipe functions smoothly as a rapid conveyor of heat. The working fluid cycles around the pipe, evaporating from the wick at the hot end of the pipe, traveling as a gas to the cold end of the pipe, condensing on the wick, and then traveling as a liquid back to the hot end of the pipe via capillary pumping within the wick structure.
For continuous operation the working fluid must cycle from the evaporation zone to condensation zone. The Wick structure uses capillary pumping and is the mechanism used to move the working fluid from the condenser back to the evaporation zone.
There are several common Heat pipe wick structures. The most common heat pipe wick structures include:
- No structure at all (Wickless)
- Axial grooves on the inner heat pipe wall
- Screen/wire mesh
- Sintered powder metal
- Fibre Wick
|Wickless – Find out more about Columbia-Stavers Wickless technology
|Grooved – Find out more about Columbia-Stavers Grooved technology|
|Screen Mesh – Find out more about Screen Mesh technology|
|Sintered Powder – Find out more about Sintered Powder technology|
|Fibre Wick – Find out more about Fibre Wick technology|
Heat Pipe Systems
Heat pipes have many advantages and applications when used as part of a thermal solution. Integrating heat pipes into a system can solve problems in systems that might have limited space at the power source to fit a heat sink, may have a weight restriction, high density power application or limited airflow. There many ways to integrate heat pipes but most applications fall into two main categories.
Heat Pipe Assemblies
One of the fundamental properties of a heat pipe is the unique heat transfer capability that allows them to transport heat to a point remote from the heat generator, often to a location better suited to deal with the thermal dissipation.
One end of the heat pipe is attached to the heat source often through a metal plate called the evaporator. The other end is located in a cooler area and often has fins attached to the heat pipe, the condenser. Evaporator/condenser heat pipe assemblies can be used to transport heat from a high power component located in a constrained area with no room for a more traditional heat sink solution, to a region where fin array can be placed. Fins can be designed for both natural and forced convection. The heat pipes are malleable enough to be formed into complex geometry in order to avoid potential obstacles.
Embedded Heat Pipes
A heat sink operates by spreading the heat from the heat source/sources through its base and into the fins for dissipation. The efficiency of the Heat sink is constrained by the spreading resistance in the base material. By embedding heat pipes into the base of a heat sink, the spreading capability of the base, and therefore the overall heat sink performance can be greatly improved. This means that a heat sink can often be reduced in size or the system can be upgraded to a higher power.
|Embedded Heat pipes – Find out more about Columbia-Stavers Embedded Heat pipes
A vapour chamber is a planar 2-phase device that works in a similar way to a heat pipe. The most common vapour chambers are square or rectangular but they could be manufactured in almost any shape. In the most basic configuration, the vapour chamber consists of a copper sealed container. As in a heat pipe a wick structure is formed on the inside wall of the container and a small amount of working fluid commonly water is added (to soak the wick), a vacuum is pulled and the container sealed.
The low pressure existing inside the chamber (created by pulling the vacuum) allows the working fluid to evaporate at a temperature much lower than its normal boiling point.
When heat is applied to the vapour chamber, the working fluid in the wick structure near that location evaporates and rushes to fill the entire volume of the chamber (driven by pressure difference). When the vapour comes into contact with a cooler surface, it condenses, and gives up its latent heat of vaporization. The condensed fluid is returned to the heat source by the capillary action of the wick structure. As the vaporization and condensation cycle repeats, heat is moved from the heat source to the entire volume of the chamber, resulting in a uniform temperature distribution on its surface.
|Vapour Chamber Assembly – Find out more about Columbia-Stavers Vapour Chamber Assemblies