Role of Emissivity in Thermal Insulation
While thermal conductivity is an important property for gauging the thermal performance of any material, an equally significant consideration is its surface emissivity. Despite being distinct, these two properties are often confused.
Thermal conductivity, refers to a material's ability to conduct heat. It describes how efficiently heat travels through a substance via conduction.
Emissivity, on the other hand, is a measure of how well a material emits thermal radiation. It indicates the ability of a surface to emit heat energy in the form of electromagnetic radiation.
While thermal conductivity relates to the ability of a material to conduct heat internally, emissivity relates to how effectively a surface can release heat to its surroundings through radiation.
Therefore, while both properties play roles in heat transfer processes, they describe different aspects of heat exchange and are not directly dependent on each other.
In condensation control, as we all know, basic goal is to keep any surface temperature above the dewpoint temperature, dewpoint temperature is when RH (Relative Humidity) becomes 100% and condensation begins to happen. If all surface temperatures are above the dewpoint temperature in a room, ambient temperature will not get the opportunity to interact with any surface and cool down below dewpoint simply because there is not temperature below dewpoint to interact with.
For example when you put a glass of cold beverage on the table,
and we have the following environment in the room:
1. Suppose the ambient temperature of the room is 35degree C
2. Suppose that the total amount of moisture in the room currently is 5gm/kg of air.
3. Suppose that the total capacity of air to contain moisture at 35degree C is 10gms/kg of air. (Remember hot air can contain more moisture than cold air)
At this point with the available data above we can calculate the current RH which is available moisture in the air divided by the capacity of the air to contain moisture at current temperature. Current RH would be 5gm/kg of air (supposition number 2) / 10gms/kg of air (supposition number 3) Current RH is 50%
Now,
4. Suppose the surface temperature of the glass holding cold beverage is 6degree C.
(Remember that hot air travels towards cold temperatures, (second law of thermodynamics))
Ambient air in our example of 35degree C will travel towards the cold surface of the glass, the surrounding air will interact with glass surface temperature of 6degree C and start to cool down.
5. Suppose that the surrounding air around the glass cools down to 20degree C.
6. Suppose that at 20degree C the capacity of air to contain moisture is 5kg/kg of air instead of 10kg/kg of air at 35degree (Remember again that hot air can contain more moisture than cold air)
As per our 2nd supposition we currently have 5gms of moisture/kg of air in the room. We will have a new RH for the temperature around the glass which will be 5gms moisture available in the room divided by 5gms moisture/kg of air capacity at 20degree C.
5gms/kg of air / 5gms/kg of air will make the RH around the glass to be 100%, In this case the dewpoint temperature is 20degree C where the RH becomes 100% and you will begin to see droplets of moisture over the glass surface.
So, if we want to avoid condensation in this case we need to maintain the temperature at the surface of the glass to be above 20degree C. This we normally do by using thermal insulation. Thermal insulation helps to maintain surface temperature above dewpoint and we get an added advantage of keeping the beverage cold for longer. (which is desirable in most cases)
This highlights the importance of surface emissivity in selecting insulation materials. Higher emissivity surfaces cool down quicker than low emissivity surfaces. Therefore, when choosing insulation materials, we must consider surface emissivity. Insulation materials with high emissivity require thicker insulation to keep surface temperature above dewpoint, preventing condensation.
To understand this, consider a typical room. Every room more or less has uniform ambient temperature because of thermal equilibrium. Typical room has a number of different surfaces made of different materials. Table surface is wood, laptop surface is metal, picture frame is glass and water bottle is plastic etc. if you touch these different surfaces with your hand you will discover that each surface has a different temperature. At same ambient temperature of the room surfaces are at different temperatures.
This is due to the different emissivity of these surfaces.
In HVAC systems, ducts and pipes usually have significant surface areas, making insulation surface emissivity crucial. For a typical project with 10,000m2 ducting that's one hector of ground space. Surface with high emissivity can significantly impact the cost of HVAC applications.
In conclusion, understanding the role of emissivity alongside thermal conductivity is essential for effective thermal insulation solutions. By considering both properties, we can optimize insulation performance, prevent condensation, and enhance energy efficiency in various applications.
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