Calculate Surface Coefficients to Save Money
Most contractors and consultants have never heard of the “surface coefficient of heat transfer” but understanding this value can send insulation thicknesses tumbling and save money.
By raising the surface temperature of insulation above the dew-point temperature we can stop condensation from forming. Increasing the insulation thickness on a cold pipe has the effect of 'keeping the cold in' and giving you a warmer surface. But this isn't the only influencing factor in determining what thickness will prevent condensation.
Transferring heat into the air
When it comes to calculating the surface temperature of insulation it actually turns out that there is a more important factor than either the thickness of the insulation applied or the thermal conductivity. The surface coefficient of heat transfer.
A measure of how efficiently heat can transfer from the surface of insulation into the air surrounding it, the surface coefficient of heat transfer is a complex value that combines the radiative and convective components of heat transfer.
Heat transfers through an insulation material (as it does through any solid) primarily through conduction and the thermal conductivity is an important value for insulation. When the heat reaches the surface of the insulation and it must suddenly interface with free moving air it can no longer transfer easily by conduction. Radiative heat transfer and convective heat transfer become far more important and it is these two components that combine to create the surface coefficient of heat transfer.
Surface temperatures are determined by the surface coefficient
Take the following example:
- Ambient temperature: 25°C
- Pipe temperature: 0°C
- Maximum relative humidity: 75%
If the outer surface coefficient of heat transfer were 9 W/(m²·K) then the insulation thickness needed to prevent condensation would be 13 mm.
A slight reduction in the surface coefficient of heat transfer to 5 W/(m²·K) would result in an increase in thickness to 19 mm.
This is a dramatic increase but not an entirely unexpected one. Stopping condensation is all about controlling the surface temperature of insulation and the surface temperature is intimately connected to the surface coefficient of heat transfer of the insulation.
What makes the surface coefficient?
You might be wondering where the surface coefficients of 9 W/(m²·K) and 5 W/(m²·K) used in the above example come from. The truth is that 9 and 5 are simply representative values - the surface coefficient can actually vary quite significantly.
The surface coefficient of heat transfer is a combination of both radiative heat transfer and convective heat transfer. Neither of these factors is simple or constant and both are influenced by the temperature of the surface and the surrounding air.
Radiative heat transfer
Radiative heat transfer is dominated by the radiative emittance of a surface. Emittance increases as the temperature difference between a surface and the surrounding air becomes more significant but the main determining factor of the radiative emittance is actually the surface emissivity.
Surface emissivity is a dimensionless value between 0 and 1 where 0 is a perfectly reflective surface and 1 is a perfect “black body”. The “colour” of a surface influences the emissivity but so too do other factors, including how “reflective” the surface is. At the moment there is no standard test for measuring surface emissivity of materials but a list of values for commonly encountered materials can be found below:
|Paint (including white)||0.9|
Convective heat transfer
The convective contribution of heat transfer is more difficult to calculate. Like radiative heat transfer, convective heat transfer occurs at a faster rate as the temperature difference between the surface temperature and the ambient temperature becomes greater.
Other factors influence the rate of convective heat transfer too. Most obviously larger surfaces have a greater potential for heat transfer and the rate of air flow over the insulation surface also directly affects the convective heat transfer.
Calculating the heat transfer coefficient
Estimating the heat transfer by convection is complicated by the unpredictable nature of laminar and turbulent flows but, fortunately, the international standard EN ISO 12241 offers equations based on empirical results that can be used to calculate surface coefficients accurate enough for use.
The Kaimann Thermal Calculator automatically calculates the surface coefficient using the equations detailed in EN ISO 12241. Accessing the advanced calculation options of the thermal calculator makes it possible to experiment with changing variables like the surface emissivity and the air flow speed. Making just a few calculations can bring home just how much these impact not just the surface coefficient but also the insulation thickness needed for condensation control.