We’re concluding a two part series today on what a heat pipe is and how it works in thermal management. In part one we talked about what a heat pipe is and the anatomy of a heat pipe. Here in part 2 we’ll conclude with factors that can limit a heat pipes effectiveness, differences in the thermal performance of various heat pipe types, and the spreading resistance of different materials.
Table 1 shows experimental data for the operating temperature and heat transfer for three different types of heat pipes [1].
Table 1: Heat Pipes with Different Structures and Operating Conditions [1]
Certain factors can limit the maximum heat transfer rate from a heat pipe. These are classified as follows:
- Capillary Limit: Heat transfer is limited by the pumping action of the wick
- Sonic Limit: When the vapor reaches the speed of sound, further increase in the heat transfer rate can only be achieved when the evaporator temperature increases
- Boiling Limit: High heat fluxes can cause dry out.
- Entrainment Limit: High speed vapor can impede the return of the liquid to the condenser
A heat pipe has an effective thermal conductivity much larger than that of a very good metal conductor, such as copper. Figure 4 shows a copper-water heat pipe and a copper pipe dipped into an 80oC water bath. Both pipes were initially at 20oC temperature. The heat pipe temperature reaches the water temperature in about 25 seconds, while the copper rod reaches just 30oC after 200 seconds. However, in an actual application when a heat pipe is soldered or epoxied to the base of a heat sink, the effective thermal conductivity of the heat pipe may be drastically reduced due to the extra thermal resistances added by the bonding. A rule of thumb for the effective thermal conductivity of a heat pipe is 4000 W/mK.
Figure 4. Experiment Comparing Speed of Heat Transfer Between a Heat Pipe and a Copper Pipe [1].
Heat pipe manufacturers generally provide data sheets showing the relationship between the temperature difference and the heat input. Figure 5 shows the temperature difference between the two ends of a heat pipe as a function of power [2].
Figure 5. Temperature Difference Between the Evaporator and the Condenser in a Heat Pipe [2]
There are many heat pipe shapes in the market, but the most common are either round or flat. Round heat pipes can be used for transferring heat from one point to another. They can be applied in tightly spaced electronic components, such as in a laptop. Heat is transferred to a different location that provides enough space to use a proper heat sink or other cooling solution. Figure 6 shows some of the common round heat pipes available in the market.
Figure 6. Typical Round Heat Pipes in the Market.
Flat heat pipes (vapor chambers) work conceptually the same as round heat pipes. Figure 7 shows a flat pipe design, they can be used as heat spreaders. When the heat source is much smaller than the heat sink base, a flat heat pipe can be embedded in the base of the heat sink, or it can be attached to the base to spread the heat more uniformly on the base of the heat sink. Figure 8 shows some common flat heat pipes.
Figure 7. Conceptual Design Schematic of a Flat Heat Pipe
Figure 8. Commonly-used Flat Heat Pipes
Although a vapor chamber might be helpful in minimizing spreading resistance, it may not perform as well as a plate made from a very high conductor, such as diamond. A determining factor is the thickness of the base plate. Figure 9 shows the spreading resistance for 80 x 80 x 5 mm base plate of different materials with a 10 x 10 mm heat source. The vapor chamber has a spreading resistance that is better than copper, but worse than diamond. However the price of the diamond might not justify its application. Figure 9 also includes the spreading resistance from the ATS Forced Thermal Spreader (FTS), which is equal to that of diamond at a much lower cost. The FTS uses a combination of mini and micro channels to minimize the spreading resistance by circulating the liquid inside the spreader.
Figure 9. Thermal Spreading Resistances for
Different Materials.
Heat pipes have a very important role in the thermal management arena. With projected lifespans of 129,000-260,000 hours (as claimed by their manufacturers), they will continue to be an integral part of some new thermal systems. However, with such problems as dry out, acceleration, leakage, vapor lock and reliable performance in ETSI or NEBS types of environments, heat pipes should be tested prior to use and after unsatisfactory examination of other cooling methods have been explored.
That ends our two part series on heat pipes. Have you got a question on heat pipes or their application? How about an interest in bringing ATS’s team of experienced thermal engineers into one of your projects? You can reach us by visiting http://www.qats.com Purchase heat sinks through our Heat Sink eShop or email us at ats-hq@qats.com or give us a call at 781-769-2800
References:
1. Faghri, A. Heat Pipe Science and Technology Taylor & Francis, 1995.
2. Thermacore Internation, Inc., www.thermacore.com.
3. Xiong, D., Azar, K., Tavossoli, B., Experimental Study on a Hybrid