Selecting Heat Pipe

How to Select a Heat Pipe?

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.

  1. Investigate and determine the following operational parameters for the heat pipe application:
    1. Heat load and geometry of the heat source.
    2. Possible heat sink location, the distance and orientation of the heat sink relative to the heat source.
    3. Temperature profile of the heat source, heat sink and ambient.
    4. Environmental condition (such as existence of corrosive gas).
  2. Select the heat pipe material, wick structure, and working fluid. (consult with an Enertron engineer or original heat pipe manufacturer to select the most appropriate heat pipe)
    1. Determine the heat pipe working fluid appropriate for your application
    2. Select heat pipe material compatible to the heat pipe working fluid
    3. Select heat pipe wick structure for the heat pipe operating orientation
    4. Decide on the protective heat pipe coating
  3. Determine the heat pipe length, size of the heat pipe, and the shape of the heat pipe appropriate for your application (consult with Enertron engineer)

What materials can be used to construct a heat pipe?

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 +60°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%
Magnesium +0.001%
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
1 Varies with temperature
2 Using threaded artery wick
*Tested at Los Alamos Scientific Laboratory
*Measured value based on reaching the sonic limit of mercury in the heat pipe
Reference of "Heat Transfer", 5th Edition, JP Holman, McGraw-Hill

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.

What are the four heat transport limitations of a heat pipe?

The four heat transport heat pipe limitations can be simplified as follows;

  1. Sonic limit - The rate that vapor travels from heat pipe evaporator to condenser
  2. Entrainment limit - Friction between heat pipe working fluid and vapor that travel in opposite directions
  3. Capillary limit - The rate at which the heat pipe working fluid travels from heat pipe condenser to evaporator through the wick
  4. Boiling limit - The rate at which the heat pipe working fluid vaporizes from the added heat

What is the common heat pipe wick structure?

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.

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