Heat Pipe Design Guide

Copper/water heat pipes have a copper envelope, use water as a working fluid and typically operate in the temperature range of 20 to 150°C. Heat pipes are an extremely effective method of reducing hot spot temperatures and increasing allowable power in a system by moving heat to an external sink or spreading heat along a surface.  ACT manufactures copper/water heat pipes from the following standard tube sizes (outside diameters) which are readily available:

  • 3, 4, 5, 6 and 8 mm
  • 1/8, 1/4, 3/8 and 1/2 inch

ACT can also create custom heat pipes outside of these diameters. ACT can bend and flatten to the following specifications. Please note that these values are guides, not limits. ACT has designed heat pipes below these values.

  • Bend Radius > 3 x OD of the heat pipe
  • Flatten ~ 2/3 x OD of the heat pipe
Figure 1. Typical bend radius is greater than 3 times the heat pipe O.D.

Figure 1. Typical bend radius is greater than 3 times the heat pipe O.D.

 

ACT very rarely provides customers with stand-alone heat pipes. With the increased demands for system ready solutions, ACT manufactures custom assemblies developed specifically for each customer’s design requirements. Heat Pipes are easily integrated into system designs because of their ability to form to intricate geometries; see Figure 2.   Heat pipes are not structural elements, but can be embedded in assemblies to form a high-conductivity, or HiK™ plate.

Heat pipes used in an assembly

Figure 2. (a) Heat pipes can be used in an assembly to cool discrete components by moving heat off the chip to a remote heat sink. (b) Heat pipes can also be used in “block-pipe-block” configurations, where the evaporator and condenser of the heat pipe are embedded in blocks attached to the electronics and the heat sink. (c) Another example of heat pipe bending.

 

ACT engineers will work with customers to properly integrate heat pipes into a system to minimize thermal resistance and maintain system integrity. ACT is experienced in soldering heat pipes as well as using thermal epoxies as an attachment technique. ACT Engineers are available to advise on which solution is best for your application.

At ACT, we guarantee quality and reliability for our heat pipes by our certified manufacturing procedures. All our heat pipes are subject to thermal testing before being delivered to the customer. ACT also has capability to validate our heat pipe performance under the following conditions:

For more information, please see the Heat Pipe Product Reliability Guide, and in Water Heat Pipe Parameters and Limitations.

The total heat load a heat pipe can carry is a function of total length, evaporator and condenser length, diameter, and orientation with respect to gravity. There are several limits that govern heat pipe theory, however in terrestrial applications the capillary limit is the most limiting factor.  This occurs when the capillary pumping capability is not efficient to provide enough liquid to the evaporator from the condenser. This will lead to dryout in the evaporator. Dryout prevents the thermodynamic cycle from continuing and the heat pipe no longer functions properly. Heat pipes are most capable when the evaporator is below the condenser creating a liquid return path that is gravity aided, and the maximum power decreases as the adverse evaporator elevation is increased; see Figure 3.  The maximum adverse elevation for a water heat pipe is roughly 25 cm (10 inch).  ACT will use worst case orientation in its heat pipe designs to assure the design will operate.

Elevating the evaporator above the condenser

Figure 3. Elevating the evaporator above the condenser reduces the maximum power that the heat pipe can carry. It does not affect the heat pipe resistance.

 

Though the total power is affected by orientation, thermal heat pipe characteristics are not. For instance the effective conductivity and thermal resistance of a heat pipe are simply a function of diameter and overall length. As long as the heat pipe can operate at a given temperature and power, it should see effective conductivities of about 50,000 – 200,000 W/mK. This range is a function of overall length with longer heat pipes having the higher effective conductivities. Heat pipes are convection devices, however through modeling and testing, ACT has proven these effective conductivities.

For a guide to how much power a heat pipe can carry please see our Heat Pipe Calculator

ACT has a proven track record of designing and manufacturing heat pipes for any suitable application. For additional help in designing with heat pipes please email one of our capable engineers at heatpipesolution@1-ACT.com or call us at (717)-295-6061.