How to Select a Heat Pipe by zhp16666


									How to Select a Heat Pipe?
Heat pipes are being used very often in particular applications w hen conventional cooling methods and heat s ink des igns
are not suitable. Once the need for heat pipe arises, the most appropriate heat pipe needs to be selected. Of ten selecting an
appropr iate heat pipe is not an easy task, and the follow ing needs to be considered.

     1.   Investigate and deter mine the follow ing operational parameters for the heat pipe application:
               a. Heat load and geometry of the heat source.
               b. Possible heat sink location, the distance and orientation of the heat sink r elative to the heat source.
               c. Temperature profile of the heat source, heat s ink and ambient
               d. Environmental condition (such as ex istence of corrosive gas)

     2.   Select the heat pipe material, w ick structure, and w orking fluid. (consult w ith an Shagal engineer or or iginal heat
          pipe manufacturer to select the most appr opriate heat pipe)
               a. Deter mine the heat pipe w orking fluid appropriate for your application
               b. Select heat pipe material compatible to the hea t pipe w orking fluid
               c. Select heat pipe w ick structure for the heat pipe operating orientation
               d. Dec ide on the pr otective heat pipe coating

     3.   Deter mine the heat pipe length, s ize of the heat pipe, and the shape of the heat pipe appr opriate for your
          application (consult w ith Shagal engineer)

Figure 1 gives the heat pipe performance w ith heat pipe diameters ranging from 3 to 22.23mm.

What materials can be used to construct a heat pipe?
A particular heat pipe w orking 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 betw een the fluid and the heat
pipe vessel. Corrosion w ill 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 temper ature range from -70 to +60°C and is compatible
w ith aluminum, nickel and stainless steel heat pipe vessel materials.

                                      Table 1. Typical Operating Characteristics of Heat Pipes
Temperature Range (                                   Heat Pipe Vessel         Measured axial(8) heat flux (   Measured surface(8) heat flux
                       Heat Pipe Working Fluid
        °C)                                               Material                     kW/cm2)                          ( W/cm2)
     -200 to -80            Liquid Nitrogen             Stainless Steel              0.067 @ -163°C                   1.01 @ -163°C
                                                      Nickel, Aluminum,
     -70 to +60             Liquid Ammonia                                                0.295                            2.95
                                                       Stainless Steel
                                                   Copper, Nickel, Stainless
    -45 to +120                Methanol                                              0.45 @ 100°C(x)                  75.5 @ 100°C
     +5 to +230                 Water                   Copper, Nickel                0.67 @ 200°C                     146@ 170°C
                      Mercury * +0.02% Magnesium
   +190 to +550                                         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                Silv er*           Tantalum +5% Tungsten                    4.1                            413
(8)Varies w ith temperature
(x)Using threaded artery w ick
*Tested at Los Alamos Scientific Laboratory
*Measur ed value based on reaching the sonic limit of mercury in the heat pipe
Reference of "Heat Transfer", 5th Edition, JP Holman, Mc Graw -Hill

The liquid am m onia heat pipe has been w idely used in space and only alum inum heat pipe vessels are used due to
lightw eight. Water heat pipes, w ith a temperature range from 5 to 230°C, are most effective for electronics cooling
applications and copper heat pipe vessels are compatible w ith w ater. Heat pipes are not functional w hen the temperature of
the heat pipe is low er than the freezing point of the heat pipe w orking fluid. Freezing and thaw ing of heat pipes is a des ign
issue, w hich may destroy the sealed joint of a heat pipe w hen place vertically. Pr oper 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 follow s;

     1.   Sonic limit - The r ate that vapor travels from heat pipe evaporator to condenser
     2.   Entrainment limit - Fr iction betw een heat pipe w orking fluid and vapor that travel in opposite directions
     3.   Capillary limit - The rate at w hich the heat pipe w orking fluid travels from heat pipe condenser to evaporator
          through the w ick
     4.   Boiling limit - The rate at w hich the heat pipe w orking fluid vaporizes from the added heat

What is the common heat pipe wick structure?
There are four common heat pipe w ick structures used in commercially produced heat pipes; groove, w ire mesh, pow der
metal and fiber/spring. Each heat pipe w ick structure has its advantages and disadvantages. There is no perfect heat pipe
w ic k. Refer to Figure. 2 for a brief glance of actual test perfor mance of four commercially pr oduced heat pipes. Ev ery heat
pipe w ick structure has its ow n capillary limit. The groove heat pipe has the low est capillary limit among the four, but w orks
best under gravity assisted conditions w here the condenser is located above the evaporator.

  Figure 2. The Actual Test Results of Heat Pipe w ith Different Wick Structure at Hor izontal and Vertical ( Gravity Assisted)

The rate of vapor traveling from the heat pipe evaporator to the condenser is governed by the difference in vapor pressure
betw een 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 w ill 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 dir ect function for both the sonic limit and
entrainment heat pipe limit.

Figure 3 compares the heat transport of heat pipes w ith different diameters. Also, the operational temperature of a heat p ipe
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 w orking 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 w ith 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 tw ice the w attage as a meter long heat pipe.

                       Figure 3. The Perfor mance of Various Groove Wic k Copper Water Heat Pipes

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