There is a possibility of applying the innovative high-temperature-resistant furnace lining with increased durability. The lining that is totally resistant to fire, combustion products and mechanical damage. The system includes a set of vertical panels and ceramic deflector supporting the combustion process.
The protective refractory lining system of the combustion chamber.
The invention of the REFRABOX system is a result of research conducted by the Unirol company. We were looking for a solution that could provide long-term use of the fireplace inserts without troublesome repairs and replacement of the plates forming refractory lining of the furnace. Modern knowledge of the high temperature resistant linings gives the opportunities of better (than it does in the general old practice) choice of material and construction method. During the reasearch we mainly focused on the damage causes.
Here we can distinguish two basic mechanisms . The first one is a result of mechanical and thermal stresses. In the plate material, the forces appears, the cause of which lies in the thermal expansion. Almost every material subject to this regularity. With few exceptions, each one expands when the tempterature increase. This process is not destructive if heating is being performed slowly and there is no temperature difference in various locations of the element. It’s hard to achieve such conditions. It is very unique. However in such “perfect” conditions the heat transport wouldn’t exist what could be a problem, because the transport of energy is the one of the basic principles of the lining structure. In real conditions the material isn’t evenly heated so there is always some kind of heat transport which can occur only due to temperature differences. The effect is that the individual layers expand differently. Ceramics will not respond to us on this with elastic deformation, but just a gradual loosening of the structure and microcracks occurence. Then after microcracks overlap, we can see a scratch. It means that the higher temperature and conductivity the material will have and lower expansion coefficient, the better will retain. It happens so, because the optimal level of these properties provide a lower level of thermal stress. There will be smaller temperature differences in cross-section, and with a lower expansion, “good response” of the element will increase. A perfect example can be a heat-resistant glass . Its heat resistance lies precisely in the very low (virtually zero) expansion – here stress do not appear due to lack of source. Materials with high expansion tolerance and good conductivity wont be resistant to thermal shock. The answer to the fundamental question – Does the ceramic product has to crack? – is surprisingly simple. If inside there are stresses exceeding the mechanical strength – surely the damage will appear. Designing the system REFRABOX we had to find out what to do to minimize the risk of such a situations.
By crack we mean a visible lack of material continuity causing its gradual fragmentation. The material “tries” in this way to change its shape, because elastic deformation of this range is not possible. Ceramics is the material of very high so called modulus of elasticity and large difference between the compressive and tensile strength, what means that any attempt to deformation causes big stress significantly exceeding the durability. The cooler layers are stretched, and this is such stress, (in contrast to the compression) that no mineral material can resist, even in a small range. But if we replace big crack with hundreds or thousands of micro-cracks, which size and shape are predicted in advance, and the path of propagation is limited, the structure of the material will be sufficiently “flexible” to lift the load without visible damage while still retaining high density.
The basic material used in REFRABOX system in its structure contains a network of pores in the form of randomly oriented small canals that has circular cross section and perfectly smooth surface. They close the way of micro-cracks that are unable to connect. The canals also make easier to discharge the accumulated moisture from the material, which could also cause damage (cracking of the material) during intense heating. High in comparison to traditional materials, mechanical strength of Refrabox system, which doesn’t fall (even grow significantly) with temperature also plays an important part (see chart).
The cracks are not always the result of different kinds of mechanical and heat shocks. If an excessive stress didn’t occur but the material was destroyed, usually it happens due to corrosive changes, which are the second basic destructive mechanism. 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. The simplest example of the corrosion can be the reactions of iron oxide contained in chamotte (visible rust spots) which are accompanied by significant changes in the volume of the inclusions – even more than 100%
These changes follows due to the very low thermal stability of this type compounds and as a result of deposition of soot in the pores of the material, that cause disintegration of carbon dioxide into oxide, which presence in turn causes the release of metallic iron (a process which is well known and used in steel industry) undergoing later re-oxidation. Such process occurs during every use of the fireplace. This mechanism has enough power to destroy the chamotte plate and yet it’s not the only one. To make matters worse, metallic iron is a strong catalyst for production of soot. The material used in the system REFRABOX is free of significant iron contamination compounds and other harmful ingredients, such as manganese, alkali and it contains very little calcium compounds in harmless and very particulate form. With the elimination of contamination, one of the main problems is the thermal stress. As mentioned, 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 uneven. 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. 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.
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.
In layer on the side of the steel body the fireproof insulator is applied. It slows the heat flow in a direction perpendicular to the plate surface. Energy then has a possibility of faster propagation in parallel directions. The heat capacity is there also increased because with more balanced heating, the average temperature and accumulated amount of heat is higher than it would due to the very fact of increased density of the material. The kind of material of plates insulation we use allows them to be “smart” during the combustion. It helps to keep high temperature in the furnace, support thermal purification of the plates and allows to transfer the heat energy to the outside. Unico uses a different type of insulation in water and convective inserts. In the first case it is a refractory non-woven fabric that has heat resistant of 1260oC, which in the initial burning duration, even at low temperature, inhibits the heat flow – the conductivity is very low. The panels and the whole hearth heat up faster. As the temperature rises, after stabilization of combustion, conductivity of the additional layer decreases fast and the heat is being transferred more intensively than it does in case of traditional chamotte plates (almost twice higher conductivity), not to mention about vermiculite.This also helps to achieve a lower amount of pollutants in the exhaust gases which level is increased always during the ignition phase.
Summing up, the REFRABOX system at low temperatures behave (when it comes to heat conductivity) similar to the chamotte plates but at high temperatures can provide much more heat through the body of the insert. In the water insert series Top Eco, the insulating layer is made of dense insulating material – calcium silicate. When the temperature grows, its properties changes completely different than it is in the above-described non-woven fabrics (see chart).
Ceramic deflector, being the last element of the system uses a large heat capacity of the material and its catalytic properties (lowering the gas ignition temperature). It stabilize temperature of gases leaving the combustion chamber and helps to sustain the burnout which is a result of the recirculation. It increases the temperature in the chamber significantly and helps to keep the glass of the insert clean. Due to the fact that it is not being cooled from the outside, this part of the lining has a much higher temperature than the others. In the case of temporary disturbance in the combustion process, (caused for example by adding some fresh wood) enables the ignition of volatile fuel flowing around.