Heat Sinks with Heat Pipes
Heat sinks with heat pipes are used to remove heat from a concentrated source and transport it to a larger surface area, improving heat transfer and dissipation efficiency. Heat pipes are available in various diameters, lengths, and power handling capacities. Heat sinks with heat pipe are used to handle power from 40W to multiple KW.
How Heat Pipes Work:
A heat pipe is a simple device that can quickly transfer heat from one point to another. It consists of a sealed copper container whose inner surfaces have a capillary wicking material. A heat pipe is similar to a thermo siphon. It differs from a thermo siphon by virtue of its ability to transport heat against gravity by an evaporation-condensation cycle with the help of porous capillaries that form the wick. The wick provides the capillary driving force to return the condensate to the evaporator. 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.
When heat is applied at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe.
Heat pipes can be built in almost any size and shape, and have an effective thermal conductivity orders of magnitude higher than that of copper. The heat transfer or transport capacity of a heat pipe is specified by its axial power rating (APR) - the energy moving axially along the pipe. The larger the heat pipe diameter, the greater the APR. Similarly, the longer the heat pipe, the lower the APR. Below, we identify several of the key considerations when designing a heat pipe:
Heat pipes normally perform best when they are used in a vertical position; they can be used in other positions but their performance will be degraded. Many other factors can influence performance, such as heat flux, working fluids, wick structure, the length of heat pipe, and others. We recommend specifying the area(s) of heat removal so that the heat pipe can be oriented accordingly to remove heat from the condenser area of the heat pipe.
Most often, heat pipes need to be bent to address the intended use, which has an impact on the performance. The radius of the bend, its location, and the amount of flattening present can all affect a heat pipe's performance, but careful design can minimize the performance loss while maximizing the predictability of the performance.
The operating temperature limits for heat pipes depend on working fluid used. The chart below outlines the useful operating range for various commonly used fluids.
Medium Melting Point Boiling Point Operating Range Water 0 100 30 to 200 Ammonia -78 -33 -60 to 100 Methanol -98 64 10 to 130 Ethanol -112 78 0 to 130 Sodium -98 892 600 to 1200
Wick or Capillary Structure
The primary purpose of the wick is to generate capillary pressure to the working fluid, from the condenser to the evaporator. It must also be able to distribute the liquid around the evaporator to any area where heat is likely to be received by the heat pipe. Often, these two functions require wicks of different forms. The selection of the wick for a heat pipe depends on many factors, several of which are closely linked to the properties of the working fluid. Groove tube surfaces, screened mesh, and sintered powder are the most commonly used wick structures.
Heat pipes have no moving parts and thus can have high reliability with long lifespans, but this largely depends on the manufacturing method, purity of the working fluid, sealing method, and cleanliness of the internal surfaces before sealing. We recommend specifying the reliability needed when submitting a quote request for heat pipes.