HeatPipe Basic Knowledge And Design Points

      Heat pipes are often used in current thermal  design, including our common laptops,PC and mobile phones. The following factors need to be considered in the design of heat pipe:

1. Heat load or heat to be transferred

2. Working temperature

3. Pipe Material

4. Working fluid

5. Capillary structure

6. Length and diameter of heat pipe

7. Contact length of evaporation zone

8. Contact length of compensation area

9. Gravity direction

10. Influence of heat pipe bending and flatness

What is capillary structure? How does it affect the performance of heat pipes

    There are  four common capillary Heat pipes structures, including grooves, wire mesh, sintered powder, metal and fiber. The capillary structure is lined on the inner wall of the heat pipe container and allows the liquid to flow from one end of the heat pipe to the other through capillary action. Each capillary structure has its advantages and disadvantages. There is no perfect capillary structure. Each capillary structure has its own limit.

Groove structure: The capillary limit is the lowest, but the effect is the best when the condenser is located above the evaporator

Wire mesh structure: With the most uniform cotton core, its working principle is in the gravity direction where the evaporator is located above the condenser.

Sintered powder structure: The effect is best in the direction of gravity. Because the sintered powder metal core is bonded to the pipe wall through metal, its heat conduction from the pipe wall to the core or vice versa is the best of the four common cores.

Metal and fiber structure: suitable for small radius bending heatpipe.

How do length and diameter affect the performance of heat pipe

    The steam pressure difference between condenser and evaporator determines the rate of steam propagation between condenser and evaporator. In addition, the diameter and length of heat pipe will affect the propagation velocity of steam, so it must be considered in the design of heat pipe. The larger cross-sectional area of the heat pipe (i.e. the larger diameter of the heat pipe) will allow a larger amount of steam to be transferred from the evaporator to the condenser. The cross-sectional area of the heat pipe is a direct function of the acoustic limit and entrainment limit of the heat pipe. However, the working temperature of the heat pipe will also affect the acoustic limit of the heat pipe. By comparing the heat transfer of heat pipes with different diameters. It can be seen that the heat pipe transmits more heat at higher operating temperature.

     The rate at which the working fluid returns from the condenser to the evaporator is controlled by the capillary limit and is a reciprocal function of the heat pipe length. Longer heat pipes transfer less heat than shorter heat pipes.

How does the direction of gravity affect the performance of heat pipes

    The structure with high capillary limit can overcome gravity and transfer more working fluid from the condenser to the evaporator. However, as mentioned earlier, the sintered powder metal core heat absorber with the highest capillary limit works best under gravity assisted conditions (the evaporator is above the condenser).

How does heat pipe bending affect performance

    If the bending radius is too small, the powder metal sintering or wire mesh layer will be damaged. Therefore, the elbow of the heat pipe may reduce the heat that can be transmitted. The experimental results show that if the bending radius is equal to or greater than 3 times, the bending will not affect the performance.

How does flattening thickness of heat pipe affect the performance

    If the heat pipe is flattened, the sound limit and entrainment limit will be reduced relative to the flattened thickness. Therefore, excessive flattening of the heat pipe will reduce the heat that can be transmitted and even completely block the passage of steam. The experimental results show that proper flattening will not affect the performance, but excessive flattening will affect the performance. If the thickness of the steam channel after flattening is greater than 2mm, the performance will not be reduced compared with the circular pipe.

How does the average operating temperature affect the performance

    The average working temperature of heat pipe will affect the performance . The higher the average temperature, the better the performance. This is due to the lower viscosity of the working fluid at higher temperatures, which allows more working fluid to flow from the evaporator to the oil core through the condenser. At higher temperatures, the working fluid can also become more volatile into a gaseous state.

Does heat pipe reliable

      The heat pipe has no moving parts and has high reliability. However, care must be taken in the design and manufacture of heat pipes. Two manufacturing factors will reduce the reliability of heat pipe: tightness and cleanliness. Any leakage in the heat pipe will eventually cause the heat pipe to fail. If the internal chamber is not thoroughly cleaned, when the heat pipe is heated, the residue will produce non condensable gas and reduce the performance of the pipe.