Water Vapour Permeability and µ Factor
This section is concerned with the transmission of moisture within thermal insulation materials.
The efficiency of thermal insulation materials such as Class O Armaflex to prevent condensation and minimise heat gain over a time span measured in years is dependent upon the initial thermal conductivity and the effective performance of the vapour barrier. The purpose of the vapour barrier is to minimise the ingress of water vapour into the insulation material so that any change in the thermal conductivity value over a long period is very small. The insulation material will continue to prevent condensation and minimise heat gain.
Water has a much higher thermal conductivity, ƒl = 0.56 W / (m.K) at 0 °C, compared with materials normally used for thermal insulation, ƒl = 0.035 W / (m.K). Thus a small amount of water within an insulation material will significantly increase the overall thermal conductivity. In the long term this may lead to increased energy losses and surface condensation.
When insulation is used to minimise the transfer of heat into cold pipes which are below ambient temperature, i.e. refrigeration, chilled water and air conditioning systems, there is a continuous pressure to force water vapour through the material due to the difference in the partial pressure of water vapour on the two sides of the insulation material. The partial pressure of water vapour depends on the temperature and relative humidity of the air and when these values are known the partial pressures may be calculated or obtained from tables.
Using standard equations of physics the partial pressure difference across the insulating material may be determined and then related to the amount of water vapour within the insulation material.
As the water vapour is forced through the material it will reach a point where the surface temperature is below the dewpoint and condensation will occur.
Consider the following example:
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| Ambient temperature | 20 °C |
| Line temperature | 6 °C |
| Relative humidity | 70 % |
| Pipe diameter | 35 mm OD |
| Class O Armaflex | 19 mm |
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For the thermal insulation material:
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| Thermal conductivity (l) | 0.035 W / (m.K) at 0 °C |
| Emissivity | > 0.9 |
| µ Factor | > 5,000 |
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The partial pressures for water vapour will be:
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| On the outside of the insulation, Ambient conditions | 1637 Pa |
| Between pipe and insulation (6 °C and 100 % RH) | 936 Pa |
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There is, therefore, a continuous net pressure of 701 Pa acting to push water vapour into the insulation.
From the partial pressure difference and permeability the total diffusion resistance may be calculated and then the average moisture content of the material. The moisture coefficient for Armaflex has been determined at 4.3% giving a relation for the increase of thermal conductivity with moisture content determined experimentally as:
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| lt = lo +(0.035 × n × vt) |
Where
lt = thermal conductivity of Armaflex after t years.
lo = initial conductivity of dry Armaflex.
n = moisture coefficient of thermal conductivity.
vt = volume fraction of moisture after t years. |
Using these equations we may determine for Armaflex the change in thermal conductivity over time and hence calculate any increasing energy losses.
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Time | years | 0 | 5 | 10 |
l value | W / (m × K) | .035 | .037 | 0.39 |
Increase | % | 0 | 5 | 11 |
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translating the increase in thermal conductivity values into heat gain, and therefore increased energy loss, we obtain:
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| Energy loss in W/m | µ > 5,000 | µ > 1,000 |
| On start up | 3.7 | 3.7 |
| After 5 years | 5.5 % | 21 % |
| After 10 years | 11 % | 43 % |
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Thus it can be seen that the long term effect of using low µ factor insulation material is to increase considerably the energy losses from the system. |
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