Furnace Efficiency With Microporous Insulation

Mark Burton

July 1, 2004

We all know what a furnace is and what it does. It takes an energy source in the form of a combustible fuel, or electricity, and converts it into usable heat inside an insulated outer casing. The economics of this process are simple. An efficient insulation allows the fuel source to be used more efficiently, which lowers the costs of operating the process. Selecting the appropriate insulation also makes fine temperature control much easier and helps the furnace manufacturer to achieve the lightest, most compact design. Careful selection of insulation can mean tremendous cost savings.

Microporous insulation achieves its low thermal conductivity by resisting all forms of heat transfer. Heat transfer through materials takes place by a number of different mechanisms or modes; an efficient and effective insulator must address each of these modes in order to achieve the lowest possible thermal conductivity. The most important modes of heat transfer in insulation materials are gaseous conduction, solid conduction and radiation.

Insulations usually contain significant volumes of air in voids. The air in the voids conducts heat by collisions between molecules, allowing transfer of energy from fast-moving "hot" molecules to slow-moving "cold" molecules.

In microporous insulation, the void volume is 90 percent of the total volume of the material, but because the voids are so small, the collisions between gas molecules are eliminated. Effectively, each air molecule is trapped in a box unable to interact with its neighbors. Air under these conditions has a far lower thermal conductivity than free air. This is known as the microporous effect, and is the primary reason why a microporous material has a thermal conductivity lower than that of still air.

Solid conduction of heat is more significant than gaseous conduction, and occurs when atoms in a material are heated and increase their vibrational energy. Interactions with their neighbors pass the energy along chemical bonds from atom to atom through the structure leading to a transfer of energy away from the heat source.

Microporous insulation minimizes solid conduction in several ways. The most important is the low density of the material, with a high ratio of gas to solid. Furthermore, microprous insulation is largely composed of amorphous particles with a low intrinsic thermal conductivity compared to most solids. Finally, the particles are very small and randomly packed, which results in long heat paths through the material along which solid conduction can take place. Heat flux by conduction is inversely proportional to the distance along which the heat has to travel; so long heat paths reduce heat transfer.

As temperatures increase into the hundreds of degrees, the above modes of heat transfer become less important and most heat transfer takes place by direct infrared radiation. It is essential that high-temperature insulation is dimensionally stable and not subject to shrinkage or any other movement at very high operating temperatures.

Microporous insulation is formulated using controlled materials to withstand high temperatures without damage. It can also be used as a backup insulation behind other refractory materials.

The characteristics of mircroporous insulation make it a good choice in furnace design for a wide diversity of applications, from small laboratory furnaces to the very large process furnaces used in manufacturing.

An example of a large-process furnace application is a roller hearth reheat and treatment furnace built by Wellman Furnaces (United Kingdom) for use in an automotive application in the United States. The furnace is 85 feet long and gas-fired.

Inside the furnace, the expensive rollers, which are able to withstand the high operating temperature, are an important design cost consideration. The use of microporous insulation ensured the minimum wall thickness for optimum efficiency. This in turn allowed shorter, less expensive rollers to be used.

Another common application for microporous products is in glass furnaces for bottle manufacture. Because of the constant need to minimize temperature gradients in the molten glass, tailored kits of insulation components are supplied for forehearths, feeder bowls, feeder bowl covers, orifice rings as well as for the general insulation of walls and recuperators for optimized efficiency.

The quality of fit is critical, particularly at corners and penetrations where gaps in the insulation can result in serious heat leakage paths, which reduce operating efficiency and create hot spots on the outer casing.

Good design should not be about using the cheapest components and materials. The most cost-effective solutions include a well-engineered design, quality components and a microporous insulation that provides optimal performance for the specific application.