Comparison of typical physical and chemical properties of materials used for furnaces lining in fireplace inserts.
Ceramic furnace lining creates the body protecting layer, is an decorative element, upgrades the quality of wood combustion. The choice of the elements that work in contact with fire decides of their reliability and has considerable influence on the performance and visual aspects of the combustion. The protective layer structure, the way of installation, split lines and dilatations are also very important. Lining doesn’t have to and even shouldn’t be single-layer one. The basic, traditionally used materials are chamotte plates and shapes made of insulated concrete – mainly on the basis of vermiculite. Both groups of materials vary from each other fundamentally in values, but they have common – protective function.
Aside from these materials Unirol company proposes to offer high quality refractory lining REFRABOX which outstanding feature is high durability. The elements of the furnace lining wores out usually because of the cracks which are not always the result of different kinds of mechanical and heat shocks. If an excessive stress didn’t occur but the material is being destroyed, usually it happens due to corrosive changes. Both chamotte and vermiculite are very porous and contain large amount of undesirable contaminants that can cause corrosion (they are made of low-processed fossil raw materials). This causes additional stress inside which together with thermal shock may exceed mechanical endurance of the materials. These disadvantages are, however, compensated to some extent by the relatively low price and better availability.
Comparison of typical physical and chemical properties of materials used for furnaces lining in fireplace inserts
One of the main problems here is the thermal stress. Because of the relatively low thermal conductivity of chamotte, on the cross-section of plates are fairly large differences in temperature and as a result, material expands very unevenly. There is also a problem of uneven plate heating over the entire surface what again is a result of low conductivity. Differences may reach up to several hundred degrees. Inflexible and nonplastic material – ceramics – in these condidions must respond by the net of cracks. Initially invisible microcracks cumulate and become bigger, embracing the whole cross section.
In vermiculite plates, which are quite flexible, the stresses in its interior can be reduced by deformation phenomenon. However, there is the additional problem of shrinkage cracks. This material when heated for the first time, shrinks (up to 1,5%) and the scratches can appear on the heated side. Besides it has very high extensibility (close to steel) which with low conductivity makes it more likely to crack despite the relatively flexible structure. Cracks related to the expansion appear first on the cooler side of the material and than during cooling, they diverge towards inner surface. Small density of the material cause that the cracks are being filled with loose grains what prevents from converging the cracks towards after the plates cools down. Unico company don’t apply vermiculite as a lining in the own fireplace inserts. The material of REFRABOX plates is free of most of disadvantages which traditional ones has. Apart from multiply smaller share of harmful contaminants it contains a lot more compounds that increase fire resistance, mainly aluminum oxide Al2O3, which is here the predominant component (more than 80%). It has a lower porosity, several times greater mechanical endurance, several tens of percent higher unit weight (density). Approximately twice higher thermal conductivity of the material used, what also has a significant impact on reducing the thermal stress due to smaller temperature differences (more rapid diffusion of heat) in different areas of the element. High density and conductivity also causes a significant increase in the heat capacity of the ceramic lining of the furnace, and thus an amount of accumulated energy.