N. Oya, Y. Kawajiri, H. Kitajima, K. Matsumoto, K. Sugiura, Y. Imae, K. Imae
Osaka Prefectural College of Technology,
Keywords: heat insulation, silica fiber, glass fiber, aluminum foil, graphite, aerogel, combustion test, FEM analysis
Summary:There are urgent needs for fire-proof heat insulations that are particularly used for protection of electric cables in critical power plants. Those insulations must provide sufficient coverages to enormously long cables and even have reduced size and weight to facilitate the installation procedure. Once a fire incident occurred at a site, such insulations must shut and prevent the fire from spreading for a duration of at least 60 minutes until the rescue from the fire station arrives. IKK Inc. Japan has already launched a project to invent a high-performance and light-weight heat insulation which has a laminated structure including silica-fiber woven fabrics, silica-fiber nonwoven mats, aluminum foils, and aluminized glass-fiber woven fabrics. The proposed heat insulation has the heat resistant silica-fiber woven fabrics at the upper and lower outermost layers. Between those outermost layers, four silica-fiber nonwoven mats are placed in parallel and each of them is covered by two aluminum foils and two aluminized glass-fiber woven fabrics. The silica-fiber nonwoven mat has a unique characteristic to exhibit a condensation reaction where water is generated and evaporated under heated environments. In addition, the aluminum layers provide heat reflection to hinder the thermal conduction along the thickness of heat insulation. The thermal insulation performance obtained by the proposed heat insulation with the total thickness of 25 mm is fairly good; exposing its surface to gas-burner flame over 1000°C for a duration of 60 minutes, the temperature at the other surface can be kept under 80°C. Owing to the soft texture of silica-fiber nonwoven mat, the heat insulation provides a flexible handling which enables the installation even in a confined site. Following the background, the authors aim to have further improved thermal insulation performance and even more downsized structure of the silica-based heat insulation. In order to achieve the goals, expanded graphite sheets and aerogel papers are newly introduced into appropriate layer positions; those positions are examined and optimized by performing combustion experiments with a gas-burner and a steady heat conduction study based on 3D-FEM analysis. The graphite sheet has an anisotropic layer structure which enhances in-plane thermal conduction. In contrast, the aerogel paper contains the aerogel constituent of up to 30% which contributes to extremely high thermal insulation and low mass density. Testing and analyzing various laminated structures including the graphite and the aerogel candidates, it is revealed that the graphite layer should be placed in heated layers near the fire and the aerogel paper should be positioned apart from the fire replacing the silica-fiber nonwoven mats there. In fact, the thermography measurement and the FEM analysis indicate that the graphite layer positioned near the fire can disperse the heat throughout the insulation surface and transfer it effectively to the whole area of silica-fiber nonwoven mats to generate more condensation reaction. The suppressed heat is further insulated by the aerogel paper, and the output temperature finally obtained is lower than 70°C. The total weight of the heat insulation can be reduced by 19% using the aerogel paper.