Thermal conductivity can be defined as the rate at which heat is transferred by conduction through a unit cross-section area of a material, when a temperature gradient exists perpendicular to the area. Thermal conductivity denoted by k, λ, or κ.
Thermal conductivity is dependent upon insulation materials. Every insulation material has a fixed thermal conductivity at mean temperature.
Mean Temperature is average of hot face (Maintain) temperature & cold face (Minimum Ambient) temperature. i.e.
Mean Temperature = (Hot face (Maintain) temperature + Cold face (Minimum Ambient) temperature)/2
For example:
Insulation Material : Rockwool/Glass wool
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| Table-1 |
Insulation Material : Calcium Silicate
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| Table-2 |
Insulation Material : Polyisocyanurate (PIR)/ Polyurethane (PUR)
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| Table-3 |
Now we have a question, how to calculate thermal conductivity at any mean temperature?
Solution:
k = (Difference between two mean temp of conductivity / Difference between two mean temp) X (First mean temp - calculated mean temp) + First conductivity at first mean temp
Now we understand with a example:
Hot face temp is 75 Deg C, Cold face temp is 0 Deg C & Insulation material is Rockwool. Calculate the thermal conductivity?
Solution
1st we calculate mean temperature:
Mean Temp. = (75+0)/2 = 37.5 Deg C
i.e. we calculated thermal conductivity at 37.5 Deg C mean temperature.
Now we calculate thermal conductivity
We putting some valve from Table-1
k = (0.043-0.052)/50 X (50-37.5) + 0.043
k = -0.00018 X 12.5 + 0.043
k = 0.04075W/M Deg C
Reference:
EIL Insulation Specification.
https://www.sciencedirect.com/topics/materials-science/thermal-conductivity
