When specifying insulating materials for use as back-up lining in kilns and furnaces designers and specifiers must ensure their system delivers optimum long-term performance. Dave Barrington reports
Selecting the most effective insulation material for furnace applications is a key consideration and one that can deliver a major performance advantage and measurable commercial return. When the impact of thermal conductivity on long-term cost and energy efficiency is taken into account, the benefits of an effective insulation material soon become apparent. A range of materials is available for the market, yet it can be difficult to understand the benefits of one material when compared to another. However, whatever insulation material is chosen, its key attribute should be low thermal conductivity, which will enable it to restrict the flow of heat from the furnace to the external environment.
Heat loss from a high temperature source such as a furnace is dominated by infra-red radiation. This is blocked by the fibres contained in a fibrous insulation material. The larger the number of fibres, the more effective the insulation will be. A superior insulation material will therefore have the best possible fibre index and contain a minimal number of ‘shot’ (unfibreised globular glass fibre) particles. Some materials on the market feature high shot content and coarse fibres, neither of which are beneficial for blocking high temperature thermal radiation.
When specifying insulating materials for use as back-up lining in kilns and furnaces, which have castable or brick forming the hot face, designers and specifiers must look beyond the initial purchase cost of the insulating materials to ensure their system will deliver optimum long-term performance and return on investment. The main options for these applications are typically calcium cilicate or low biopersistent fibre-based boards. Calcium Silicate has been commercially available for more than 50 years, its high compressive strength makes it well suited to kiln car bases.
The compressive strength of Calcium Silicate might seem like a key benefit, and while it can endure heavy loads, its lack of flexibility does mean the material can be prone to cracking when put under certain strains that are difficult for the material to withstand. Fibre-based boards derive their strength from the interlinking of fibres during manufacture. The more fibres that are available to link together, then the greater the strength and durability of the board. The advantages to a board with high fibre count include easy installation and handling, excellent strength and resistance to cracking.
Testing thermal conductivity
Low biopersistent fibre-based boards were introduced to the market in the mid 1990s. The latest versions combine high-specification low biopersistent fibres, fillers and organic binders. These boards are engineered to maximise the content of insulating low biopersistent fibres by reducing the size and amount of ‘shot’, and so deliver significantly reduced thermal conductivity offering enhanced energy-saving properties.
Recent tests carried out at the most common operating temperatures for furnace back-up board – between 600ºC and 800ºC – revealed that in the key area of thermal conductivity, the latest low biopersistent fibre-based board outperformed Calcium Silicate by an average of 20 per cent at 600ºC and 15 per cent at 800ºC. While Calcium Silicate typically costs less than low biopersistent fibre-based board, the wasted heat and associated energy costs more than outweigh the lower initial purchase cost.
The physical properties of the two materials should also be evaluated by specifiers. Calcium Silicate is brittle, making it prone to chipping, crumbling and breakage during transportation, handling and stacking. These issues are made worse during machining and installation. Calcium Silicate also creates considerably greater levels of dust than low biopersistent fibre-based board when chopped, shaped or handled, which potentially exposes operatives to the inhalation of a particulate. Dealing with this requires the use of appropriate respiratory protective equipment (RPE), which adds to the cost. [Page Break]
A further key issue with these products is that of water absorption, as one installation option is to apply a castable material directly onto the back-up lining material. While low biopersistent fibre based board products are treated to be water repellent (hydrophobic), Calcium Silicate boards are highly water absorbent. This can result in the castable becoming dry and not curing correctly. It can also result in water becoming trapped in the back-up lining, leading to possible material damage. Therefore, in addition to accelerating heat loss and requiring more energy, the Calcium Silicate board will physically deteriorate and compromise the effectiveness of the system, resulting in a shorter product lifespan and potentially unsafe working conditions. Leading low biopersistent fibre-based boards do not require a water vapour barrier.
Demand for energy around the world is steadily increasing year-on-year, and looks set to continue to rise in years to come. There are numerous reasons for companies to prioritise the energy efficiency of their operations, including surcharges on energy bills, tax credits for energy-saving initiatives and increased capital allowances for resourceful machinery investments. However, one of the quickest and most cost-effective ways of achieving savings is by fitting low biopersistent fibre-based boards in furnaces.
Although Calcium Silicate is one option, low biopersistent fibre-based boards almost always offer the most energy efficient solution. For example, a typical industrial application might be a preheating cyclone for cement manufacture. In recent testing, Morgan Thermal Ceramics compared its Superwool Plus Blok product against a leading brand of Calcium Silicate board, covering a surface of 380m2 with 80mm back-up insulation. Results showed that although Calcium Silicate typically has a lower initial purchase price, the difference in cost will be repaid over the course of just seven weeks, due to the superior energy efficiency of Superwool Plus Blok fibre modules. When the energy-saving difference between the two materials is considered over a 12-month period, the difference in cost is paid back more than seven times. Furthermore, Superwool Plus Blok fibre modules reduced CO2 emissions by 190 tons a year compared with the leading Calcium Silicate board.
When these performance advantages are considered, alongside other factors such as ease of handling, on-site fabrication, operator satisfaction and durability, it is clear that specifiers are likely to reach a very different decision on material choice than specifying on initial purchase cost alone.
Dave Barrington is general manager, Morgan Thermal Ceramics UK, Wirral, Merseyside, UK. www.morganthermalceramics.com