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Heat pipes are being used very often in applications when conventional cooling methods and heat sink designs are not suitable. Once the need for heat pipe arises, the most appropriate heat pipe needs to be selected. Often selecting an appropriate heat pipe is not an easy task, and the following needs to be considered.
A particular heat pipe working fluid can only be functional at certain temperature ranges. Also, the particular heat pipe working fluid needs a compatible vessel material to prevent corrosion or chemical reaction between the fluid and the heat pipe vessel. Corrosion will damage the heat pipe vessel and chemical reaction can produce a non-condensable gas.
Refer to Table 1. For example, the liquid ammonia heat pipe has a temperature range from -70 to +60Â°C and is compatible with aluminum, nickel and stainless steel heat pipe vessel materials.
Table 1. Typical Operating Characteristics of Heat Pipes
|Temperature Range (°C)||Heat Pipe Working Fluid||Heat Pipe Vessel Material||Measured axial 1 heat flux ( kW/cm2)||Measured surface1 heat flux ( W/cm2)|
|-200 to -80||Liquid Nitrogen||Stainless Steel||0.067 @ -163°C||1.01 @ -163°C|
|-70 to +60||Liquid Ammonia||Nickel, Aluminum, Stainless Steel||0.295||2.95|
|-45 to +120||Methanol||Copper, Nickel, Stainless Steel||0.45 @ 100°C 2||75.5 @ 100°C|
|+5 to +200||Water||Copper, Nickel||0.67 @ 200°C||146 @ 170°C|
|+190 to +550||Mercury* +0.02%
|Stainless Steel||25.1 @ 360°C*||181 @ 750°C|
|+400 to +800||Potassium*||Nickel, Stainless Steel||5.6 @ 750°C||181 @ 750°C|
|+500 to +900||Sodium*||Nickel, Stainless Steel||9.3 @ 850°C||224 @ 760°C|
|+900 to +1,500||Lithium*||Niobium +1% Zirconium||2.0 @ 1250°C||207 @ 1250°C|
|1,500 + 2,000||Silver*||Tantalum +5% Tungsten||4.1||413|
The liquid ammonia heat pipe has been widely used in space and only aluminum heat pipe vessels are used due to lightweight. Water heat pipes, with a temperature range from 5 to 200°C, are most effective for electronics cooling applications and copper heat pipe vessels are compatible with water. Heat pipes are not functional when the temperature of the heat pipe is lower than the freezing point of the heat pipe working fluid. Freezing and thawing of heat pipes is a design issue, which may destroy the sealed joint of a heat pipe when place vertically. Proper engineering and design can overcome this heat pipe limitation.
The four heat transport heat pipe limitations can be simplified as follows;
There are four common heat pipe wick structures used in commercially produced heat pipes; Groove, Wire mesh, Sintered powder metal and Fiber/spring. Each heat pipe wick structure has its advantages and disadvantages. There is no perfect heat pipe wick. Refer to Figure. 2 for a brief glance of actual test performance of four commercially produced heat pipes. Every heat pipe wick structure has its own capillary limit. The groove heat pipe has the lowest capillary limit among the four, but works best under gravity assisted conditions where the condenser is located above the evaporator.
Figure 2. The Actual Test Results of Heat Pipe with Different Wick Structure at Horizontal and Vertical (Gravity Assisted) Orientations.
The rate of vapor traveling from the heat pipe evaporator to the condenser is governed by the difference in vapor pressure between them. It is also affected by the diameter and the length of the heat pipe. In the large diameter heat pipe, the cross sectional area will allow higher vapor volume to be transported from the heat pipe evaporator to the condenser than in a small diameter heat pipe. The cross sectional area of a heat pipe is the direct function for both the sonic limit and entrainment heat pipe limit.
Figure 3 compares the heat transport of heat pipes with different diameters. Also, the operational temperature of a heat pipe affects the sonic limit. We can see, in Figure 3, the heat pipes transport more heat at higher operational temperatures.
The rate of heat pipe working fluid return from the condenser to the evaporator is governed by capillary limit and is the reciprocal function of the heat pipe length. A longer heat pipe transports less heat versus the same heat pipe with a shorter length. In Figure 3, the unit of the Y-axis is QmaxLeff (W-m) representing the amount of heat a heat pipe can carry per meter length. If the heat pipe is half a meter long, it can carry approximately twice the wattage as a meter long heat pipe.
Figure 3. The Performance of Various Groove Wick Copper Water Heat Pipes
As it can be seen, the selection of an appropriate heat pipe can be a complicated process. For any assistance in the heat pipe section process you can consult with an Enertron engineer.