An Introduction to. Heat Pipes

Transcription

1 An Introduction to Heat Pipes

2 Contents 1. What is a heat pipe?... Page What is a heat pipe made out of?... Page 3 3. How does a heat pipe work?... Page 4 4. Advantages of using heat pipes... Page Reliability of heat pipes Page 9 6. Factors that need to be considered in heat pipe design... Page Complimentary Engineering Support and Design Assistance.. Page 16

3 1. What is a Heat Pipe? Today's modern electronics generate heat that can cause damage. Effective thermal management is a very important factor to consider. With its ability to transfer and dissipate heat, the heat pipe plays a crucial role in cooling many modern sensitive electronic systems. Page 1

4 1. What is a Heat Pipe? ( Continued ) The use of heat pipes for thermal management is a proven and widely applied technology. The modern concept of a capillary driven heat pipe was first invented by General Motors in N.A.S.A. later adapted and further developed this concept. Heat pipes have become commonplace in some of today s most advanced electronics systems. Computers, pipes along the Trans-Alaska Pipeline, nuclear reactors, heat-sensitive electronics aboard satellites and the International Space Station, all rely on heat pipes to manage thermal output effectively. Page 2

5 2. What is a heat pipe made out of? A heat pipe is a metal tube, sealed under partial vacuum, with an inner wick lining (capillary material) and a small amount of fluid. There are a variety of fluids and wicks used in heat pipes, but the principle is the same: a fluid evaporates into a gas that travels to the cooler end of the pipe, where it condenses back into a fluid and returns via the wick to the hot end. The selection of fluid and wick capillary system is dependent on several factors, which we will later discuss. So how does this system work? Page 3

6 3. How does a heat pipe work? As heat is applied to the heat pipe s surface, this causes the evaporator region to heat the fluid inside and change it into a vapor. This phase change from fluid to vapor creates pressure. As pressure increases, vapor will naturally flow into the cooler section. Heat is released as the vapor condenses back into a fluid. The fluid will then flow back into the warm region, where the cycle will repeat (as long as there is heat applied). Note: A radiator or heatsink is usually needed on the condenser end to dissipate the heat to ambient air and hence condense vapor back to fluid. Page 4

7 4. Advantages of using heat pipes 1. Very high thermal conductivity Heat pipes are considered a type of thermal superconductor. They possess an extra ordinary heat transfer capacity and rate. The effective thermal conductivity of a heat pipe is up to 90 times greater than the solid copper for the same size. Lighter in weight when compared to solid copper of the same size. Page 5

8 4. Advantages of using heat pipes (Continued) 2. Flexibility in spatial location The most versatile feature of using heat pipes is the wide variety of geometries that can be constructed to take advantage of the available space around the electronics to be cooled. This is very useful in applications where there is strict space limitation near the heat source or lack of adequate air flow. Page 6

9 4. Advantages of using heat pipes (Continued) 3. Improve heat sink efficiency A heat pipe will improve heat sink efficiency by carrying heat to underutilized areas. This is useful when the fins are tall and heat has trouble reaching the top of the fins. Heat pipes can be used to effectively carry heat from the base of the heatsink to the underutilized sections of the fins. Page 7

10 4. Advantages of using heat pipes (Continued) 4. Improve heat spreading at the heat sink base You can improve the heat spreading across the heat sink base by embedding the heat pipes within the base of the heat sink. This is useful in cases where there is a small concentrated heat source versus a large heat sink base. Page 8

11 Operational life 5. Reliability of heat pipes No moving parts or corrosive materials inside heat pipes The working fluid and wick structures permanently sealed in a copper vessel. No mechanical or chemical degradation over time that has been reported by Radian customers. Typical operational life is ~20 years MTBF. Page 9

12 6. Factors that need to be considered in heat pipe design. A) Heat transport limitation of the heat pipe B) Wick structure of the heat pipe C) Length and diameter of the heat pipe D) Bending and flattening of the heat pipe E) Working fluid F) Care for usage and design guidelines Page 10

13 A) Heat pipe design Heat Transport Limit Capillary Limit Maximum heat that can be transported by the heat pipe before Vapor pressure (from Hot to cold regions) and gravitational force exceed Fluid capillary forces (from cold to hot region), i.e., fluid does not return fast enough or does not get back to hot region. Dry-out occurs in the evaporator region when capillary limit is reached Heat pipes no longer function properly if dry-out occurs. Possible solutions: modify heat pipe wick structure design, increase heat pipe diameter, add more heat pipes or reduce input power. Page 11

14 B) Heat pipe design Wick Structure Grooved Wire Mesh Sintered Lowest cost Lowest performance Does not work well against gravity Most commonly used Good performance Works well against gravity Highest cost Highest performance Works best against gravity Page 12

15 C) Heat Pipe Design Length & Diameter Length The capillary pumping pressure is the only driving force to circulate the working fluid inside the heat pipe. Higher chances of dry-out in a longer heap-pipe as the vapor has a greater distance to travel to the condenser section. Therefore, a bigger diameter heat pipe may be required for longer distances. Diameter Heat pipes with larger cross sectional areas (i.e. larger diameter of the heat pipe) allow more vapor to be transported from the evaporator region to the condenser region. Heat pipes with larger cross sectional areas have higher heat transport capacity. Page 13

16 D) Heat Pipe Design Bending, Flattening, & Working Fluid Bending and flattening Flattening or bending of a heat pipe will reduce the heat transport capacity. Factors affecting heat transport limits: flattened thickness, number of bends, and the angle of each bend. Working fluid Working fluid chosen must be able to operate within the heat pipe s operating temperature range. Generally, as the operating temperature range of the working fluid increases, the heat transport capability increases. The working fluid must be compatible with the wick and container material. Water is the most common working fluid in electronics cooling. Page 14

17 E) Heat Pipe Design - Care for usage Remove heat pipes before subjecting them to temperature above intended applications (e.g, pcb through reflow oven). Recommended Maximum temperature is 160 C. Restrain power within the heat transportation limit of the heat pipe. Parameters Power (per heat pipe)* Length* Diameter* Life (MTBF)* Ambient Temperature Value Up to 80 W Up to 300 mm 3~8 mm 20 Years 0-85 C * Please note that these are the suggested operation conditions for typical applications copper heat pipes with water as the working fluid. They are not necessarily the maximum capabilities. Please contact us if you have any questions. Page 15

18 F) Complimentary Engineering Support and Design Assistance Featuring our experienced team of thermal engineers with several years of experience in designing thermal solutions for various applications Full board level thermal analysis using CFD (Computational Fluid Dynamics) software Optimization of existing heatsink designs to improve performance Verify heatsink performance using actual testing in thermal lab Thermal lab equipped with wind tunnel and anemometers to test airflow velocity and temperatures throughout system Please contact us if you have any questions. Page 16