High temperature insulation wool

High-temperature insulation wool is a manmade material from alumino silicate glass which is used for thermal insulation in High Temperature applications and processes >700°C.

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History

  Man has used fire for melting and heat treatment of metals for thousands of years. To ensure safe working with the fire, for melting and working metals (bronze, iron), special refractory materials were needed very early on, as these made the handling of liquid or hot metals possible. To meet the needs of the wide-ranging applications, a large number of shaped, dense materials (refractory bricks, chamotte), shaped heat-insulating materials (lightweight refractory bricks) and unshaped refractory materials (heavy- and lightweight ramming mixes) have been developed, which are used for special high-temperature applications. For decades, however, other manmade materials have been used for thermal insulation, glass and rock wool being used in the low-temperature range (around 200°C to maximum 500°C).

In the 1960s aluminium-silicate-based “ceramic fibres” were launched on the market in Europe, and thanks to their high temperature-resistance and good technical properties (i.e. good thermal shock resistance and low thermal conductivity), they quickly became the reference for industrial high-temperature insulation. The term “ceramic” fibre is ambiguous as also other materials are subsumed under the term. For this reason, and because other new materials have been developed for the high-temperature range, the nomenclature of high-temperature insulation wools was redefined in Germany at the end of the 1990s. (VDI 3469)

In response to various energy crises (1970s to date) and the constantly rising prices for oil and gas, energy-saving high-temperature insulation wool was used increasingly commonly, wherever technically possible. Thermal insulation with high-temperature insulation wool enabled a more lightweight construction of industrial furnaces and other technical equipment (heating systems, automobiles), resulting in many economic and ecological benefits. Consequences are smaller wall thicknesses and considerably lower lining masses.

Comparison of the mass for the different wall linings

Heavyweight lining: 1500 to 3500kg/m³,
Lightweight lining:  500 to 1000kg/m³,
Lining with HTIW:    160 to  300kg/m³.

High-temperature insulating wool (HTIW)

High-temperature insulation wool is an accumulation of fibres of different lengths and diameters, produced synthetically from mineral raw materials. The group of the high-temperature insulating wools includes amorphous alkaline earth silicate wool (AES) and aluminium silicate wool (ASW) as well as polycrystalline wools (PCW) (VDI 3469; DIN-EN 1094) with a classification temperatures >1000°C. Besides the differences in the chemical composition, manmade fibres have parallel edges in contrast to natural fibres.

Alkaline-earth-silicate wool (AES wools)

Also known as “high-temperature glass wools (HTGW)”, AES wools consist of amorphous fibres, which are produced by melting a combination of CaO-, MgO-, SiO₂ and ZrO₂ (see also VDI 3469, Parts 1 and 5). Products made from AES-wool are generally used at application temperatures <900°C and in continuously operating equipment and domestic appliances.

Aluminium silicate wool (ASW)

Aluminium silicate wool, also known as “refractory ceramic fibre” (RCF), are amorphous fibres produced by melting a combination of Al₂O₃ and SiO₂, usually in a weight ratio 50:50 (see also VDI 3469 Parts 1 and 5, as well as TRGS 521). Products made of aluminium silicate wool are generally used at application temperatures >900°C and in intermittently operating equipment and critical application conditions (see Technical Rules TRGS 619).

Polycrystalline wools (PCW)

Polycrystalline wool consists of fibres containing > 70 wt.% Al₂O₃; they are produced by a "sol-gel method" from aqueous spinning solutions. The water-soluble green fibres obtained as a precursor are crystallized by means of heat treatment (see also VDI 3469 Parts 1 and 5). Polycrystalline wools are generally used at application temperatures >1300°C and in critical chemical and physical application conditions, also at lower temperatures.

Definitions

Classification temperature

The classification temperature is defined as the temperature at which a linear shrinkage of 4% is not exceeded after 24h heat treatment in the electrically heated laboratory oven and in a neutral atmosphere. Depending on the type of product, the value may not exceed the following limits: 2% for boards and shaped products, 4% for mats and papers. The classification temperature is specified in 50°C steps (starting at 850°C and up to 1600°C). The classification temperature does not mean that the product can be used continuously at this temperature. In the field, the continuous application temperature of amorphous HTIWs (AES and ASW) is typically 100°C to 150°C below the classification temperature. Products made of polycrystalline wool can generally be used up to classification temperature.

Wools

Wool is the term used to describe an ordered accumulation of fibres of varying length and diameter. “High-temperature insulation wools” fulfil this definition and are therefore covered by the term “wool”.

Amorphous AES and aluminium silicate wool (ASW) are produced by melting the raw materials in a melting pot by means of electrical resistance melting. The jet of melt discharged from the pot is accelerated in a blowing or spinning process and pulled into fibres with different length/diameter ratios.

Continuous fibres/textile glass fibres (VDI 3469 Part 1)

These fibres are produced by means of the continuous filament process with defined nozzle diameters, all fibres having the technical defined and required diameter. During handling, only fibres of the given diameter but different length are released.

High-temperature insulation wools are by definition not fibres, therefore the term “ceramic fibres” is not correct.

Health hazards

Dust

Some dusts if inhaled over a longer period may cause lung disease. Fibrous dust can be released during handling and working with manmade inorganic mineral and high-temperature insulation wool. Fibres with a length exceeding 5 µm, a diameter below 3µm and a length to diameter ratio that is larger than 3:1 are regarded as respirable. Consequently, regulations on handling and use must be observed that are drawn up and published by the Federal Ministry of Labour and Social Affairs|Federal Ministry of Labour and the associations of the producers of high-temperature insulation wool. Detailed specifications are currently in preparation and will be added.

References

VDI Guideline: VDI 3469 Part 1 - Overview VDI Guideline: VDI 3469 Part 5 – High-Temperature Insulation Wool