Heat Pipe

A Two Phase Heat Transfer Device

DESIGN CONSIDERATIONS

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