Heat Pipe, A Two Phase Heat Transfer Device, DESIGN CONSIDERATIONS

Heat Pipe Design Guide

Heat Pipe

A Two Phase Heat Transfer Device


A Highly Effective, Thermally Conductive Solution

Many factors go into the custom design of heat pipes including:

  • Orientation of heat pipe,
  • The heat pipe geometry,
  • To spread heat or exchange heat,
  • The length of the heat pipe,
  • The heating load from the heat source,
  • Potential g loading on heat pipe,
  • The designed purpose of the heat pipe,
  • The interface and attachment methods.
  • The evaporator /condenser section lengths,
  • The number of bends and/or flattening, etc.

Once the inputs are known, then a new design can be created or an existing design could be validated.

  • First using 3-D modeling techniques,
  • Second using prototypes / thermal testing

After these steps are carried out, the design can go into production.

For a given heat pipe design, the maximum heat pipe power is a function of length, where the evaporator is located above the condenser and temperature.

A heat pipe can only carry a certain amount of energy per second. Heat loads larger than 30 or 50 watts may require multiple heat pipes or a single heat pipe of larger diameter to move the heat from the heat source.

A heat pipe used to spread heat from a small heat source to a large heat exchanger will be designed differently than a heat pipe used to exchange the heat from the heat source to a dissipation point.

Heat pipe geometry can greatly affect the function of the heat pipe. Heat pipes with one or two large radius bends may have a small reduction of 5% or less of the heat pipe’s normal thermal capacity, but smaller radius bends will further reduce that capacity.

Flattening heat pipes will also affect the thermal performance, as flattened heat pipes have greater surface area to accept heat and create better thermal contact with the heat source.

The length of the heat pipe greatly affects its heat transport capacity, as well as the length of the evaporator section (the area of the pipe in contact with the heat source) and the condenser section (the area of the pipe in contact with the heat dissipation method).

Orientation with respect to the direction of gravity also affects the thermal capacity of the heat pipe.

The method of contact between the heat pipe, the heat source, and other components of the thermal module is also important. The heat pipe contact with the heat source and heat diffuser has a certain level of thermal contact resistance. This can be lessened with thermal greases or thermal pads for less permanent bonds, and thermal epoxy or solder for bonds that require greater mechanical strength and permanence.

Press-fitting the heat pipe into a fixture and/or machining the heat pipe surface to a desired flatness value requires a thicker heat pipe wall to withstand the mechanical stresses involved in those procedures.

Another critical design decision is the type of heat pipe wick that is employed. Each of Baknor’s products uses a certain type of wick structure. The links below briefly explain each wick structure, and their various strengths and weaknesses.

While heat pipes are effective heat conductors that can be used in various thermal situations, not every heat pipe is suitable for all applications. The following is a summary of what will need to be considered when designing with heat pipes.

  • 1. Heat load, or heat to be transported
  • 2. Operating temperature
  • 3. Pipe material
  • 4. Working fluid
  • 5. Wick structure
  • 6. Length and diameter of the heat pipe
  • 7. Contact length at the evaporating zone
  • 8. Contact length at the compensating zone
  • 9. Orientation
  • 10. Effects of bending and flattening of the heat pipe

Wick Structures In Standard Heat Pipes

Sintered Wick

The sintering process used to create this type of heat pipe wick is the same as that used in the creation of metallic filters and certain machine components.


Mesh/Fiber Wick

Mesh or fiber wicks use a weave of either metallic or non-metallic material wrapped and pressed or otherwise bonded against the heat pipe wall.


Grooved Wick

Grooves are broached or extruded in the heat pipe wall, creating channels for the liquid phase to flow from the condenser section to the evaporator section of the heat pipe.


Composite Wick

Creating a composite wick from a variety of wick types can maximize performance by using the unique benefits of each individual wick structure to overcome the weaknesses of a single wick type.


Thermal Testing And Prototypes

Once the design inputs are decided, a heat pipe design is either created by Baknor or supplied by the customer. This is done through both engineering drawings and CAD-generated 3-D models. The 3D models are used to evaluate machining and manufacturing tolerances, and to program various CNC machines with finalized models.

Once the design has its tolerances cleared for production, a set of prototypes is constructed and tested for their thermal performance. These thermal tests are the final validation before readying the design for mass production, and can be carried out either at Baknor’s facilities or at the customer’s desired testing facilities. If the thermal performance is not as desired, Baknor will quickly and efficiently correct defects and refine the manufacturing process in order to create a final product that exceeds customer expectations.

Below is a table that shows the thermal performance of some straight sintered-wick heat pipes with no bends and at horizontal orientations to gravity at a variety of dimensions. This is only a set of reference data; please contact Baknor for a heat pipe design that is suited to your needs.


Use Our Design Services or Send Us Your CAD Drawings

Start the conversation with our thermal design solution professionals, by using our thermal design check list, to analyze and solve your thermal challenges. Baknor’s experience and knowledge gained over the years in many different applications enables us to help you find the right innovative cooling solutions for your application.