CHAPTER 10: THERMAL RESISTANCE OF HOT WATER PIPES AND INSULATION AS IT APPLIES TO SANS 10400 XA
For cylindrical objects, the heat loss is not the simple straight-through heat flow found with the flat surface material, but rather a radial heat loss. This is due to the surface areas of the inner and outer radiuses being substantively different. For cylindrical surfaces or insulation, the following equation should be used to calculate the “R” value:
R=
ln
( ) De Di
where De = External diameter, Di = Internal diameter, k =
2πkl Thermal conductivity, l = length. In the instance of the SANS 10400 XA one needs a unit value to get to the correct insulation thickness and can therefore express l as 1 and leave it out of the equation for simplicity sake. The correct radial thermal resistance
equation to use would subsequently be:
R=
ln
( ) De Di
2πkl The differences in results are worlds apart and cannot be compared. What one does need to remember is that, as the diameter of the pipe increases, the difference between the surface areas decrease. This would cause the R – value to decrease with the same thickness of insulation, as demonstrated when using the correct formula above. Using the 1/k formula would incorrectly show a constant value of R for increasing diameters. The table below gives you an indication of the differences in R - values when using the two differing equations.
Table 1
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SUSTAINABLE ENERGY RESOURCE HANDBOOK
EE 10 THERMAL RESISTANCE OF HOT WATER PIPES.indd 162
2013/12/06 12:39 PM