LIGHTWEIGHT INSULATION MATERIALS HT
Heat-insulating refractories are used for refractory lining and insulation to concentrate heat energy and reduce its flow, thereby reducing energy losses. They utilize the phenomenon of heat radiation shielding, low conductivity and low heat capacity resulting from high porosity. Porosityof lightweight insulation materials reaches 90%. Higher porosity allows for better thermal insulation but, on the other hand, leads to lower mechanical strength and increased gas permeability. Total porosity is not the only determinant of material’s heat conductivity. Very important are the size, shape and distribution of pores, as well as the material’s chemical composition.
High temperature thermal insulation
Thermal insulator is a substance, material or profiled shield with low thermal conductivity of ca. 0.1 W/ (m • K). On the other hand, in high temperature technology, also materials with conductivity over 1 W/ (m • K) are classified as insulators, due to rapid decrease in mechanical strength and concurrent increase in heat conduction along with the increase in temperature, and the need to use materials with a stronger structure and higher density. A special group are refractory insulators, also called HT insulators, which can stand fire or generally, temperatures above 500°C. ASTM C71 standard defines refractory and heat resistant materials as those with thermal resistance above 1000°F, i.e. 538°C.
Heat exchange mechanisms
Depending on the environment and the insulator’s operating temperature, but also on its structure and chemical composition, certain heat exchange mechanisms may vary. In industrial equipment, heat can be transferred in one of the three ways:
- Conduction, which involves transfer of energy through random motion of particles and their collisions, and of free electrons.
- Convection due to movement of masses of liquid or gas. Convection may be natural (free), generated by difference in density due to temperature difference, and forced by external factors such as pump, fan, etc.
- Radiation, transfer of energy by electromagnetic radiation, from every body warmer than absolute zero. Radiation does not require a medium, it can also occur in vacuum.
Which of these mechanisms will predominate, depends not only on the type of insulation material, but first of all on the temperature. At low temperatures prevails heat conduction through vibration of insulator particles and free electrons. This factor roughly linearly depends on temperature. At middle-range temperatures convection takes the lead and intensifies with temperature rise, which is particularly important in materials with large pores or high gas permeability. At high temperatures, which is the most important case, the most significant factor is radiation. Stefan-Boltzmann’s law:
States that the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature. Therefore, a 2x increase in temperature results in a16x increase in energy emission. Since the temperature in this formula is in Kelvin’s absolute scale, this corresponds to a temperature rise from ca. 300°C to 1000°C. Because of this, refractory thermal insulation materials have very non-linear thermal conductivity characteristics with a strong tendency to grow at high temperatures.
Key parameters of HT insulation
The parameters of a good insulating material that determine its insulating performance and durability, are:
- thermal conductivity, defined as “k” or “λ” coefficient
- surface emissivity coefficient, denoted by letter “ε”
- resistance to thermal shocks related to material’s strength, extensibility and elasticity
- thermal capacity or inertia dependent on insulator’s density and specific heat.
Mechanical strength is also important. Although thermal insulation materials’ operating conditions are static, their good strength reduces installation costs and often contributes to prolonged service life. In most cases, in order to achieve better insulating effects, density is reduced and porosity increased of the materials, while retaining their good mechanical strength. Also, pores’ size reduction and finer distribution improve the material’s insulating performance. A good example of perfect insulator is a quartz-based airgel, porous at the molecular level.
One of the most important parameters considered in the selection of insulation material for high temperature applications is its resistance to thermal shocks. Naturally, this resistance increases with the material’s increase in strength, but it’s also significantly influenced by reduction of CTE linear thermal expansion coefficient and Young’s modulus E – longitudinal elasticity coefficient. Such are features of fibrous materials and materials made of needle-like crystals such as aluminium silicate based mullite and calcium silicate based xonotlite. Refractory materials based on ceramic fibers arein fact completely resistant to thermal shocks in a wide range of temperature changes.
Insulating materials are made porous in many technological processes, such as foaming (expansion) and gas bubbling, evaporation or sublimation of liquids, firing of solids, and formation of fibrous structures and application of natural or synthetic additives. In kilns and industrial installations with low mechanical loads, where there are no strong corrosive factors, lightweight thermal insulation materials have almost completely supplanted the traditional heavy-duty refractory bricks.
Economic optimization of thermal insulation
A very important factor in the selection of thermal insulation materials is economic optimization. Of course, the thermal insulation material quality, which directly affects its service lifetime and the energy savings associated with its lower thermal transmittance, should be taken into account. Then the optimum insulation thickness should be determined. In this case, the costs of insulation and of energy losses throughout its assumed service life must be considered and the overall cost calculated.
In solutions, where the thickness is not determined by other factors, such as specific insulation jacket temperature, structural or ecological considerations, the minimum overall cost is the ruling factor in determining the optimum thickness, a.k.a. economic insulation thickness, as shown in Fig. 2.
PM porous mullite blocks
Porous mullite blocks are refractories with low and medium porosity and application temperatures above 1200°C. ASTM C 155-70 and DIN EN 1094 standards define the temperature at which the material shrinkage can not exceed 2%, the maximum apparent density and the minimum mechanical strength the material must meet under the given temperature load. PM blocks are made of raw materials with a content of Al2O3 and SiO2, and sometimes also CaO. Aluminum oxide is provided in raw materials such as clay, kaolin, chamotte, sillimanite, andalusite, cyanide, mullite, alumina and hydrated alumina.
The additive firing process is the best known and most commonly used in the manufacture of refractory lightweight blocks and bricks. Used as additives are fine sawdust, petroleum coke, ground wax, foamed polystyrene balls and highly processed by-products of the pulp and paper industry are used. It is important that while these additives are burning out, as little as possible of ash – a solid that can have a negative effect at high temperatures – is left in the material.
PM blocks’ and mullite bricks’ characteristic features include:
- Low thermal conductivity, which provides good thermal insulation and allows for thinner brickwork structures.
- Low accumulation of heat. Because of ceramic materials’ high porosity and low specific heat, PM bricks accumulate a small amount of heat, resulting in significant energy savings in cyclically-operated furnaces.
- High purity. Low content of iron oxides and other impurities ensures a stable, reducing atmosphere in the furnace and mitigates the risk of interfering with the reaction.
- Precise dimensions. Owing to the blocks’ precise cutting, walls are erected faster, welds are thin and uniform, and stronger and more stable structures can be build.
HT ceramic fibre materials
Because of their low density, heat capacity and good insulation, and excellent mechanical properties, ceramic fibre products are very effective insulating materials up to as much 1600°C for zirconium oxide based polycrystalline fibres. With current ecological and energy costs, higher capital expenditure on this type of insulation depreciates very quickly.
With a few exceptions, high temperature ceramic fibres for thermal insulation applications are derived from Al2O3 – SiO2 system. Derivative of this system are also the fibres in which alumina Al2O3 is in part, up to ca 15%, replaced with zirconia ZrO2. Other types of fibre belong to the calcium silicate and calcium aluminate sets with CaO content ca. 20 to 40%, so-called “bio” fibres that are partially soluble and do not cause skin irritation.
Ceramic fibres are manufactured by two methods. Fibres are made of compressed air-blown stream of molten charge at temperature ca. 2000°C. They are up to 50 mm long, and their diameters are ca. 2-3 μm. This method mainly produces amorphous fibres with Al2O3 content 60% or less. They have a glassy structure and are designed for lower operating temperatures 1 000 to 1 260°C.
Better quality fibres are produced in vortex method whereby are formed by centrifugal force and rapid cooling. Fibre so produced can be 250 mm in length and about 3 – 5 μm in diameter.
In both methods, the fibres are terminated with small balls that break during cooling. Remaining in volume of material, they are non-fibrous particles.
The share of these particles (so-called shot) is about 40 to 60%. Mechanical separation during the process reduces it to 10%. An aluminosilicate based fiber can have up to 60% Al2O3 content, above this limit the surface tension is so high that the fibres are formed very short or not at all.
Technically more complicated is the manufacture of high quality polycrystalline fibres. So, their price is higher. Here aluminium salts the material’s base. To produce the fiber, organic polymers are added to the appropriate composition when blown or spinned. Silicic acid is also added as a crystalline growth stabilizer in the heat treatment process. Finally, the fully crystalline structure and pore removal are obtained by thermal treatment. Polycrystalline fibres contain mullite, corundum and their mixture, and have an unspecified length of 5 to 100 mm and a diameter of ca. 3 μm. These fibres are intended for higher temperatures ranging from 1200 to 1450°C.
Ceramic fibre’s classification temperature is determined according to DIN EN 1094 standard and refers to irreversible linear changes (permanent shrinkage). The shrinkage of a ceramic mat must not exceed 4% after 24 hours, or only 2% in case of plates. Fibre’s shrinkage and friability, which depend on its chemical composition, determine working conditions of ceramic fibre products. There are methods available to compensate the
shrinkage resulting from fibrous materials’ operation: pre-compression of mat or module linings, overlapping, fibrous pulp filling, reinforcing coatings.
CS calcium silicate based materials
Calcium silicate products are produced by hydrothermal treatment of finely ground raw materials: lime CaO and quartz sand SiO2 in water suspension with low solids and impurities. Although calcium silicate plates are less resistant to thermal shock than other refractory materials, they are easier to handle and install. These materials have no harmful health effects, are CE marked and approved for use in general construction.
Calcium silicate based materials are manufactured by filter pressing of plates or casting of blocks of high density materials. Mineralogical transformation into xonotlite 6CaO • 6SiO2 • H2O is carried out in an autoclave under pressure and at high temperature.
The average pore size of these materials is in the micrometer range, and their porosity is over 80%, which is good for their insulation properties and installation ease.
Because calcium silicate products have a crystalline matrix and do not require binding by the addition of organic binders, they are fully water resistant. Therefore, these materials do not rot and provide nutrients for fungi and bacteria, and after wetting and drying they fully recover their properties. CS insulation plates are most commonly used on the back side of lining in furnaces and installations with reduced atmosphere, where no diatomite profiles, vermiculite boards, nor mineral wool, should be used.
SiC microporous materials
Silicon carbide and pyrogenic silicon based insulating materials, with IR radiation shielding additives. In these materials, the insulating effect is achieved not only by greater pore diffusion and density reduction, but also by internal blocking of warm emission transfers, so-called “matted structure”. SiC microporous materials outperform the insulating performance of all standard refractory insulation and can only compete with aerogel and vacuum panel insulation.
Microporous plates have several times better insulating properties than the best ceramic fiber or calcium silicate plates, and this effect is even better with increasing temperatures. Therefore, despite the high cost of microporous materials’ manufacture, they have a much better economic effect in many applications. They are also indispensable for installations in sites with limited space or load capacity. Even in multilayer insulation solutions where a SiC microporous sheet accounts for one third of the total insulation thickness, the total thickness and weight may be reduced by more than 50%.
Microporous materials are new products on the high temperature insulation market and are still being intensely developed. Their characteristic feature is low mechanical strength which makes installation difficult and can cause faster wear, so intensive work is under way to further improve this material. The work concentrates on increasing the strength of the microporous material itself, and with addition of quartz or ceramic fibre reinforced composites. Undoubtedly today it is the most advanced materialin the industrial insulation technology, with the fastest growing market share.