Heat Pipe Assemblies Design Guidelines
For the best performance, the application should have gravity working with the system; that is, the evaporator section (heated) should be lower, with respect to gravity, than the condenser (cooling) section. In other orientations where gravity is not aiding the condensed liquid return, the overall performance will be degraded. Performance degradation depends on a number of factors including wick structure, length and working fluid of the heat pipe along with heat flux of the application. Careful design can minimize the performance loss and allow an accurate prediction performance.
Temperature Limits
Most pipes use water and methanol/alcohol as the working fluids. Depending on the wick structure, pipes will operate in environments with as low as -40癈. Upper temperature limits depend on the fluid, but 60癈 to 80癈 is the average limit.
Heat Removal
Heat can be removed from the condenser using air cooling in combination with either extrusion, bonded-fin heat sinks, or flat-fin stock. Enclosing the condenser in a cooling jacket allows liquid cooling.
Reliability
Heat pipes have no moving parts and have demonstrated life of over 20 yrs. The largest contributor to heat pipe reliability comes from control of the manufacturing process. The seal of the pipe, purity of the materials used in the wick structure and cleanliness of the internal chamber have measurable effect on the long term performance of a heat pipe. Any leakage will eventually render the pipe inoperable. Contamination of the internal chamber and wick structure will contribute to the formation of non condensable gas (NCG) that will degrade performance over time. Well developed processes and rigorous testing are required to ensure reliable heat pipes.
Forming or Shaping
Heat pipes are easily bent or flattened to accommodate the needs of the heat sink design. Forming heat pipes may affect the power handling capability as the bends and flattening will cause a change in fluid movement inside the pipe. Therefore design rules that take into consideration heat pipe configurations and the effect on thermal performance ensure the desired solution performance.
Effects of Length and Pipe Diameter
The vapor pressure differential between the condenser end and the evaporator end controls the rate at which the vapor travels from one end to the other. Diameter and length of the heat pipe also affect the speed at which the vapor moves and must be considered when designing with heat pipes. The larger the diameter, the more cross sectional area available to allow vapor to move from the evaporator to the condenser. This allows for greater power carrying capacity. Conversely, length when in opposition to gravity has a negative effect on heat transport as the rate at which the working fluid returns from the condenser end to the evaporator end is controlled by the capillary limit of the wick which is an inverse function of the length of the pipe. Therefore, shorter heat pipes carry more power than longer pipes when used in application not assisted by gravity.
Wick Structures
Heat pipe inner walls can be lined with a variety of wick structures. The four most common wicks are:
- a) groove
- b) wire mesh
- c) sintered powder metal
- d) fiber/spring