Looking at the thermal insulation of a boiler and just how thin I could make an insulator I started looking at Multi-Layer Insulation or MLI’s. The effective thermal conductivity of an MLI is in the order of 10-5 Wm-1K-1 as given by Sutheesh and Chollackal [ref 1].
The thermal insulation I achieved for the vertical boiler has an effective thermal conductivity of 0.06 Wm-1K-1. Rather a poor design in comparison to an MLI. It would be great to reduce the thermal conductivity by two or three orders of magnitude.
MLI’s were designed for the cryogenic and space industry and consist of multiple layers of aluminized mylar and glass fabric.
The radiation heat from the outer space strikes on the first reflective layer where a part is reflected back to the environment, and the remaining radiation energy heats up the first layer of spacer.
P M Sutheesh and Alex Chollackal, “Thermal performance of multilayer insulation: A review“, IOP Conference Series: Materials Science and Engineering
As the temperature of the layer increases, solid conduction, gas conduction and radiation takes place through the spacer material to the next foil. Thus the second foil temperature will increase. The second foil reflects some radiation back to the first foil and the remaining energy transfers to the third foil. This process continues up to the bottom layer.
As per the application this insulation stops heat getting to the Cold Boundary where the cryogenic gas/liquid is held. One challenge of this material is in the spacer. The spacer needs to be a poor conductor. Hence any bridging needs to be via a tortuous path, thus extending the length and reducing conducted heat.
Comparison of Insulators
It is interesting to compare the MLI to other insulators that we are familiar with in everyday life.
| Material | Thermal Conductivity [Wm-1K-1] |
| Multi-Layer Insulator | 0.00001 |
| Monolithic Aerogel | 0.018 |
| Extruded Polystyrene Foam | 0.028 – 0.031 |
| Polyurethane | 0.035 |
| Expanded Polystyrene Foam | 0.04 |
| Rockwool | 0.04 |
The challenge is that these still tend to be quite thick overall in the range 10 to 30mm. Using the physics of this approach in a material that has a total thickness of around 1.5mm is interesting and most likely not possible without some exotic materials.
References
- P M Sutheesh and Alex Chollackal, “Thermal performance of multilayer insulation: A review“, IOP Conference Series: Materials Science and Engineering
- Özlüsoylu, Ismail & Istek, Abdullah. (2019). “The Effect of Hybrid Resin Usage on Thermal Conductivity in Ecological Insulation Panel Production”