Heat Pipe
Heat pipes, or thermal pipes, and their specialized designs are capable of transporting heat very efficiently from one location to another. They use the evaporation heat of a specific medium to create an internal heat flow. With capillary action or gravity, auxiliary devices like circulation pumps become unnecessary. This type of thermal management, especially with high operating temperatures up to 3,000K, is suitable for a wide range of applications. Heat pipes are used in satellites, solar collectors, computers, rail systems for heating switches, as well as in smartphones and electronic devices.
Operating Principle
Inside heat pipes is a capillary structure that enables fluid transport without additional energy input, even against gravity. This allows the heat pipe to transfer heat both against gravity and horizontally, as the cycle is maintained in all orientations due to the capillary structure. Additionally, heat pipes can be bent to optimally fit specific applications.
Loop heat pipes feature a capillary structure only in the evaporator. In the cycle itself, vapor and liquid phases are separated, allowing distances of up to 10 meters between the evaporator and condenser. A loop heat pipe is resistant to vibrations, offers flexible installation, requires no gravity for operation, and can even function in multi-g environments.
Vapor chambers operate based on the same principle as heat pipes. With their flat design, often only a few millimeters thick, they are typically used to absorb heat from a small area and distribute it across a larger surface. Inside, a working fluid and a wick structure operate under vacuum. The wick transports the liquid from the hot region to cooler areas within the chamber. Depending on the application, the heat is then transferred either directly to an attached heat sink or further dissipated via heat pipes.
The choice of working fluid should match the operating temperature range for all types of heat pipes. If the fluid is not suited, heat conductivity is limited to that of the material, and efficient heat transfer is no longer guaranteed. By using various working fluids, materials, and sizes, heat pipes can reliably operate across a wide temperature range.
Rules for Heat Pipes
- The heat pipe is a closed system in which, upon heat input, liquid evaporates at any given point (heat source) and condenses at the cooler end. The vapor of the liquid serves as the actual transport medium.
- The temperature distribution within the system is relatively low. The temperature gradient is approximately determined by the thermal resistance of the heat pipe.
- Heat pipes allow heat to be transported quickly and efficiently from a warm location to a cooler one. Compared to pure copper, heat pipes enable heat transfer capabilities that are 100 to 10,000 times greater.
- Heat pipes can be used to create uniformly tempered workspaces and surfaces.
- The heat transfer between the heat source, heat pipe, and cooling system is crucial for maximum performance.
- For this reason, it is essential to optimize connection and transition points or use heat pipe systems with integrated connection surfaces.
- The heat transport process can only be ensured if the heat pipe operates within its power and temperature range.
- Even with capillary heat pipes, installation orientation affects performance, though to a lesser degree than with other types.
- In non-capillary variants, approaching a horizontal orientation reduces efficiency.
- Too small bending radii can damage the internal structure of heat pipes, while excessive angles can impair functionality up to the point of ineffectiveness.
Installation of Heat Pipes
Heat pipes in technical applications can be integrated in several ways. The simplest, but also the least effective, method is mounting within a borehole. In this case, a vent hole and thermal paste or adhesive are necessary, along with careful attention to tolerance measurements.
An alternative method is pressing a heat pipe into a groove, resulting in a D-shaped cross-section (round with a flat top). This provides flat, high-performance contact surfaces. Additionally, installation in two symmetrical half-shells is possible. Soldering offers the best thermal option, although continuous temperature monitoring is required. At Quick-Ohm, heat pipes are soldered with cooling fins and mounting parts.
Heat Transfer Performance and Limitations of Heat Pipes
The performance of heat pipes is determined by a range of factors, including both the working fluid and the material of the heat pipe.
Performance-Influencing Factors, Working Fluid
- The enthalpy of vaporization of a working fluid defines the energy required to evaporate a specific amount of liquid at a given temperature. Conversely, the enthalpy of condensation indicates the energy released when the gas condenses at a certain temperature.
- The melting point refers to the temperature at which a substance transitions to a liquid state.
- The surface tension of a liquid, along with the pipe's radius, affects capillary forces. Smaller radii or finer mesh or grain sizes in capillary structures (specific to sintered heat pipes) are advantageous here.
- The viscosity limit determines the extent of condensate flow within the heat pipe and generally limits heat flux density at low operating temperatures.
- The pressure resistance of a heat pipe increases the possible vapor pressure of the working fluid. For example, water can reach temperatures up to 374°C, but this results in a pressure of 210 bar.
- The density of the working fluid also impacts the performance of a heat pipe. Depending on the substance, a specific capillary size is needed to maintain the fluid cycle.
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