ABSTRACT
Properties of insulation are imparted to a material by reducing heat transfer through all three modes-conduction, convention, and radiation. Reduction of conductive heat transfer is achieved by selecting a material of low thermal conductivity. Because ceramic materials are generally nonconducting at room temperature, they are usually selected. Gases have even less conductivity, but they can conduct heat by convection. To reduce this mode of heat transfer, the thickness of gas enclosures must also be reduced. In large spaces, this is achieved by separating the space into thin compartments, which reduces the circulation of the gas. Air is the most commonly used gas for insulation purposes. The radiation mode of heat transfer is reduced by radiation shields that are in the form of thin sheets. They function by reecting the heat back to the source. One shield can reect back 50% of the heat it absorbs. Two shields in parallel can reect back 67%, and three can reect back 75%. Thus, as the number of shields increases, the less heat is absorbed.
