ATS engineers designed a thermal solution for a telecom board, using ATS patented maxiFLOW heat sinks, which met the customer’s thermal requirements. (Advanced Thermal Solutions, Inc.)
Engineers at Advanced Thermal Solutions, Inc. (ATS) were brought into a project to assist a client with cooling a PCB that was going to be installed in telecommunications data center. The board currently had heat sinks embedded with heat pipes covering the two hottest components but the client wanted a more reliable and cost-effective solution.
ATS engineers used the company’s patented maxiFLOW™ heat sinks to replace the heat pipes and through analytical and CFD modeling determined that by switching to maxiFLOW™ the junction temperature and case temperature would be below the maximum allowed.
• Challenge: The client had a new PCB over which air could flow from either direction and two of the highest power dissipating components were on opposite sides.
• Chips/Components: WinPath 3 and Vector Processor
• Analysis: Analytical modeling and CFD simulations determined the junction temperature with air going from left-to-right and right-to-left and ensured it would be lower than the maximum allowable (100°C for one component and 105°C for the other).
• Test Data: With air flowing from left-to-right, CFD simulation determined that the junction temperatures would be 89.3°C and 101.4°C – below the maximum temperatures of 100°C and 105°C. With air flowing from right-to-left, the junction temperature of the most power-dissipating component was 100°C, which was right at the maximum, and the second was at 87°C, which was below it.
• Solution: The original heat sinks embedded with heat pipes were switched for maxiFLOW™ heat sinks, with their placement offset slightly to create a linear airflow, and the same levels of thermal performance were achieved.
ATS engineers changed the embedded heat sinks for maxiFLOW™ heat sinks and received the same thermal performance with a more reliable and cost-effective solution. (Advanced Thermal Solutions, Inc.)
• Net Result: The client received the required level of cooling in the PCB, regardless of the direction of air flow, and with a more reliable and cost-effective solution than had been previously been in use.
For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or [email protected].
Heat Pipes have been called Heat Superconductors! In this engineering article we’ll talk about what a heat pipe, how they are made, compare them with heat sinks, and talk about performance in various thermal management applications.
Figure 1 Schematic View of a Heat Pipe [1]
Heat pipes are transport mechanisms that can carry heat fluxes ranging from 10 W/cm2 to 20 KW/cm2 at a very fast speed. Essentially, they can be considered as heat super conductors. Heat pipes can be used either as a means to transport heat from one location to another, or as a means to isothermalize the temperature distribution.
The first heat pipe was tested at Los Alamos National Laboratory in 1963. Since then, heat pipes have been used in such diverse applications as laptop computers, spacecraft, plastic injection molders, medical devices, and lighting systems. The operation of a heat pipe is described in Figure 1.
Sections of A Heat Pipe
A heat pipe has three sections: the evaporator, the adiabatic, and the condenser. The interior of the pipe is covered with a wick, and the pipe is partially filled with a liquid such as water. When the evaporator section (Le) is exposed to a heat source, the liquid inside vaporizes and the pressure in that section increases. The increased pressure causes the vapor to flow at a fast speed toward the condenser section of the heat pipe (Lc). The vapor in the condenser section loses heat to the integral heat sink and is converted back to liquid by the transfer of the latent heat of vaporization to the condenser. The liquid is then pumped back to the evaporator through the wick capillary action. The middle section of the heat pipe (La), the adiabatic portion, has a very small temperature difference.
Figure 2 Pressure Drop Distribution in a Heat Pipe [1]
Figure 2 shows the pressure drop distribution inside a heat pipe. In order for the capillary force to drive the vapor, the capillary pressure of the wick should exceed the pressure difference between the vapor and the liquid at the evaporator. The graph also shows that if the heat pipe is operated against the force of gravity, the liquid undergoes a larger pressure drop. The result is less pumping of the wick with reduced heat transfer. The amount of heat transfer decrease depends on the particular heat pipe.
Structure of a heat pipe
Metallic pipe: The metal can be aluminum, copper or stainless steel. It must be compatible with the working fluid to prevent chemical reactions, such as oxidation.
Working fluid: Several types of fluids have been used to date. These include methane, water, ammonia, and sodium. Choice of fluid also depends on the operating temperature range.
Wick: The wick structure comes in different shapes and materials. Figure 3 shows the profiles of common wick types: axial groove, fine fiber, screen mesh, and sintering. Each wick has its own characteristics. For example, the axial groove has good conductivity, poor flow against gravity, and low thermal resistance.
Conversely, a sintering wick has excellent flow in the opposite direction of gravity, but has high thermal resistance.
Figure 3 Different Wick Structures – From top to bottom: Sintered powder, fine fiber, wrapped screen, axial groove
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]
Types of heat pipes
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
Commonly Used Flat Heat Pipes
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.
Thermal Spreading Resistances for Different Materials
Importance of an Heat Pipe
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.
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? Reach us by visitingATS Heat Pipe Page or email us at [email protected] 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
