Kiln car construction - high temperature materials solutions
As companies drive for increased efficiency from their manufacturing chain, the materials speciﬁed as part of the process are under increased scrutiny. Ermanno Magni and Didier Finck from Morgan Advanced Materials discuss some of the available options.
Kiln cars are subjected to highly challenging operating conditions where the kiln may be either operating at a consistently high temperature for extended periods or be set to a high temperature at which it operates for a certain period before being allowed to cool. While red bricks, for example, are typically heated to 800–1,200°C, clay products may require an even higher temperature.
This issue has been exacerbated in recent years by the drive to extract maximum value from plant assets, meaning kilns and kiln cars are heated up more rapidly as cycle times are reduced. Depending on the size of the kiln, a process that previously took 24–48 hours may now be completed in less than half that time.
Speciﬁers must therefore seek products with excellent thermal shock resistance and thermal stability. Materials without these properties will physically deteriorate over time, thereby impacting their performance and resulting in more frequent replacement, with the associated downtime. It also goes without saying that the products used in kiln cars should not react chemically with the products being manufactured, not only because of the contamination and damage to the product but also because of the potential for noxious emissions. Similarly, where burners are used, the product must offer resistance to gas velocity and composition.
Ease of installation is also key. Products should be easy to cut and not give off potentially harmful particulates or fumes. Light weight and low bulk density will help to decrease energy use, but this should never be at the expense of physical strength. The product must be able to support the weight of the internal load, and that strength must not be affected by changes in temperature.
The key inﬂuencers on the speciﬁcation and performance of kiln car lining materials are the temperature at which the kiln will operate, the rate at which the kiln is required to rise or fall in temperature, the possibility of water or chemical ingress from the products being kilned, and the required charge weight of the kiln.
Lining products should ideally be light in weight, but should be sufficiently strong to support the mechanical stress of the kiln and its contents. Products should also ideally be simple and quick to cut and install and, for safety reasons, should create minimal levels of dust during the cutting and installation process.
Dense refractory products such as cordierite (magnesium aluminium silicate) and mullite (based on oxides of aluminium and silicon) are widely used in kiln car applications to create both the car surface and also the pilasters in kiln cars made from lighter materials, where extra support is needed for the load. While mullite has been used in Germany since the Middle Ages in the manufacture of the ﬁrst Hessian crucibles, cordierite was discovered in 1813. Both materials are typically around 2.4t/m3 in density, meaning they can take a long time to heat up, impacting on energy costs. On the other hand, they are very strong and do not require additional support, as well offering long life and being less prone to chemical attack.
Light refractory products such as insulated ﬁ re bricks (IFB), ﬁbre blankets, calcium silicate or light concretes are less dense (typically around 0.3t/m3), more cost-effective and more energy-efficient, but can be prone to thermal shock in short cycles and may also be subject to chemical attack. They generally cannot support heavy loads directly as these will cause physical deformation. In such applications, additional supports or pilasters made from cordierite or mullite will be needed to support the load. Light refractories can also be more prone to attack from chemicals – a particular issue when products containing signiﬁcant amounts of impurities are being heated, as these impurities are more likely to be aggressive at higher temperatures.
Calcium silicate products can be used successfully in a variety of applications where thermal insulation is required. Products with speciﬁc properties are available depending on the speciﬁc requirements of the application. 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.
The problem with calcium silicate board is that it is brittle, making it prone to chipping, crumbling and breakage during transportation, handling and stacking. It is also highly water absorbent, which can compromise performance. These issues are made worse during machining and installation. Calcium silicate board 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.
The latest low biopersistent ﬁbre-based boards are engineered to maximise the content of insulating ﬁbres by reducing the size and amount of shot. This delivers signiﬁcantly reduced thermal conductivity, offering enhanced energy-saving properties. Where greater ﬂexibility is required, a blanket product should be speciﬁed.
Irrespective of the material being used, the key issue to consider when designing kiln cars is the expansion that inevitably occurs, both dimensionally and in terms of volume, when the kiln car enters the furnace and is subjected to a sudden increase in heat. This expansion must be calculated and managed through the judicious placement of expansion joints. These must allow enough ﬂexibility for the heat expansion, but not so much that the kiln car becomes physically unstable. This issue is more and more important as cycle times are reduced to optimise productivity, with kiln cars heated up more rapidly and therefore subject to more rapid expansion.
In most instances, rather than requiring investment in new cars, existing lining and insulation products can be replaced with superior alternatives without the need for the cores of the kiln cars themselves to be changed. However, it should be noted that the full beneﬁt of these alternative products can generally only be realised through a redesign of the kiln.
For more information, contact Scott Bentley, email@example.com