Replies

1. 
   Jamie_Buxton | Mar 28, 2003 01:48am | #1

   Oh c'mon!  You don't get to post a question like that without telling us what you're doing.  Thermal conductivity indeed!

1. 
   cherryjohn | Mar 28, 2003 04:39am | #2

   now come on.......would Gen.Tommy Franks tell Sad Saddy his plans?

2. 
   Clay | Mar 28, 2003 06:09am | #3

   Wood is a poor conductor of heat in any species.  Perhaps you should consider an alternative material?

1. 
   cherryjohn | Mar 28, 2003 02:15pm | #5

   Estetics in this application dictate the use of wood.  Not only that, if I were to use an alternate material it would still need to be something that can be nailed to.  Plastics can be nailed but are as bad as wood for thermal conductivity

3. 
   Jamie_Buxton | Mar 28, 2003 09:03am | #4

   I found a table containing the thermal conductivity of a half dozen domestic woods, along with the density.  There's a strong correlation between density and higher thermal conductivity.   I'd look for the densest, oiliest wood I could find -- lignum vitae, for instance.  I think I've read that it was used for deck planks on submarines in the mid-twentieth century, for two reasons.  First, it is so dense that it sinks in water.  Second, it is highly rot-resistant even in that extremely wet application.

1. 
   cherryjohn | Mar 28, 2003 02:21pm | #6

   In this table of densities that you refer to.....is there anything superior to white oak for density and rot resistance?  Something that is also easy to get and relatively inexpensive?  Lignun Vitae ( iron wood) has the rot resistance and the density but oh the price!  This project has a price ceiling to it and I have to consider the inclusion of distribution in the overall price.  The only way I know to make my profit margins and have a good profit for distribution is to keep the price down as much as possible so if I can find something that works relatively well and it is priced right, the right marketing will seel the project.

1. 
   Jamie_Buxton | Mar 28, 2003 06:09pm | #7

   The following table is from the CRC Handbook of Chemistry and Physics.   From the point of view of a woodworker, it omits lots of interesting woods.   The best that can be said about it is that denser woods conduct heat better.

    Wood

    Density

    Thermal Conductivity

    Balsa

    8.8

    .38

    Cypress

    29

    .67

    White pine

    32

    .78

    Mahogany

    34

    .90

    Virginia pine

    34

    .98

    Oak

    38

    1.02

    Maple

    44

    1.10

    

    

    

    (Glass)

    

    6

   The units of Density are pounds per cubic foot.   The units of Thermal Conductivity are BTU per hour per square foot, with a temperature gradient of 1 degree farenheit per inch of thickness.

   Bruce Hoadley, in his book Understanding Wood, has a table showing the decay resistance of many woods.   In the "exceptionally high decay resistance" column, he has: black locust, red mulberry, and osage-orange.  White oaks are in the next category (resistant).  Red oaks and maple are in the "slightly resistant" group.

1. 
   Froed | Mar 28, 2003 07:31pm | #8

   Jamie,

   I looked at your units for conductivity, and I thought you must have missed something, so I looked it up.  Sure enough, that's what the CRC handbook says.  That's a very strange way to give conductivity, and confusing, since the temperature gradient doesn't change the conductivity of the wood.  I wonder why they did it that way.  Goofy.

   The higher number means it conducts heat better, so I guess the units don't matter much here, though.  But I am curious about what CherryJohn is planning, and is he at least gonna tell us after he's done with it?

1. 
   jjf | Mar 28, 2003 08:39pm | #9

   Conductivity is a property of the material but it is dependent on the Temperature gradient,  The greater the difference of the temperature from one side as compared to the other side, the greater the heat transfer.  The thinner the material the more heat transfer also.

   Black Locust and osage-orange would be very good for rot resistance.

2. 
   Froed | Mar 28, 2003 09:23pm | #10

   Yes, conductivity is a material property.  As a material property, it is dependent on the temperature of the material, not the temperature gradient.  Remember that conductivity and heat transfer are not the same thing.

   Heat flux through conduction can be determined from q = kA(Thot-Tcold)/t, where k is the material conductivity, A is the surface area of the plate, wall, whatever, (Thot-Tcold) is the temperature gradient thru the material, and t is the thickness.

   You can increase the heat transfer rate by making (Thot-Tcold) and/or A bigger, or by making t smaller (reducing the thickness of the material).  The only practical way to change k is to change the material itself.  If the temperature gradient is large enough, the conductivity can change thru the material, but this is because of the temperature at a given point in the material, not the temperature gradient.  For an application with wood in it, any change in conductivity would be negligible, since I can't imagine the hot-to-cold temperature swing would be that severe.

3. 
   Jamie_Buxton | Mar 28, 2003 09:32pm | #11

   Froed --

     Yes, the CRC's language is a little strange.   I think the temperature gradient stuff is not really part of the definition of the conductivity, but rather a description of the measurement conditions.  But, as you say, for this discussion it doesn't really matter.  The two salient points are: the higher the wood density, the better the heat transfer; and wood transfers lots less heat than many other solid materials.

        Jamie

4. 
   Jamie_Buxton | Mar 28, 2003 09:39pm | #12

   Cherryjohn --

      As all the posters have observed, wood is a bad conductor of heat.   Could you use wood veneer over some high-conductivity material, like metal?  For instance, the thermal conductivity of aluminum is over 50 times greater than that of wood.

            Jamie