Achieving High Value in Kiln Insulation and Furnace Refractories

The importance of correct material selection

In all of these areas, poor product selection or incorrect matching of the material to the application has the potential to result in inefficient or ineffective processing, wasted energy and process time, and possibly even reputational damage if a crucial order deadline is missed due to an unresolved processing issue. Above all, unnecessary downtime while failed materials, such as insulation systems, are replaced, cannot be countenanced. Put simply, best practice in material selection, system design and installation will go a long way towards achieving optimum production efficiency and system uptime based on the amount of cycles that can be undertaken before these systems need replacing

It is inevitable that products will at some stage need to be replaced and, perhaps for that reason, the focus among many individuals with responsibility for specifying or purchasing kiln insulation and furnace refractories has traditionally been on achieving the lowest unit cost. However, as with so many things, the best price does not necessarily mean best value and the focus should always be on finding the best solution rather than the cheapest product which will almost inevitably require more frequent replacement. With the cost differential between, for example, a lower-grade insulation system and a premium product paling into insignificance compared with the cost of downtime while these systems are replaced, it is in an area where even a modest time investment or investigation of total life cycle costs can pay major dividends.

Achieving the best insulation system

Insulation systems are integral to ensuring processing temperatures are not just high enough but consistent throughout the kiln or furnace and help to ensure uniformity of parts. Minimising the loss of heat through the sides and top of the kiln or furnace is not just key for processing, though – it also contributes towards operator safety as external surfaces are not so hot, while energy costs are reduced too as more heat is retained inside. While natural gas costs are relatively low currently, it cannot be assumed that this will always be the case, and many companies are in any case taking commendable steps towards reducing their carbon footprint on a voluntary basis, irrespective of any legislative compulsion to do so.

Insulation systems need not just to offer minimal heat transfer. There may also be a requirement for thermal shock resistance, in applications where rapid heating or cooling are employed. Resistance to corrosion is also often a key attribute depending on the application as are physical strength and flexibility.

Depending on the process, a choice can be made between high temperature insulation wools such as refractory ceramic fibers (Kaowool^® or Cerafiber^®), alkaline earth silicate fibers (Superwool^®, and polycrystalline fibres or combinations of high temperature fibers with insulating firebricks or special duty castables [Kao-Tab, Kao-Tuff^®]). Whatever solution is considered, specifiers and purchasers should take steps to satisfy themselves about the quality and consistency of the materials used, and the integrity of the manufacturing process of these products. Inconsistency in either of these areas will likely result in substandard performance, with more frequent replacement also necessary. Rather than simply buying on price, therefore, investigations should first be made to ascertain how suppliers’ processes are benchmarked between different facilities, as well as establishing the existence of quality plans and what use is being made of comparative testing methods. If the manufacture of the lining material can be proven to be consistent, then its performance in key areas such as heat transfer can be accurately predicted and relied upon – vital where accurate temperature control is needed.

For processors seeking to reduce downtime even further, the use of a lining material suitable for a higher temperature than the actual process will employ is more and more frequently being considered. The latest polycrystalline fibres, for example, are manufactured by a chemical process rather than the melt and attenuation process more commonly used for other lining materials. However, the chemical and thermal stability of polycrystalline systems mean they can, for example, offer extended service life – up to 10 years is not uncommon – compared with refractory ceramic fibres, while also delivering better performance in areas such as heat transfer.

Meeting the unique demands on kiln furniture

The story is similar when it comes to kiln furniture, which is subject to intense demands on its performance including the effects of rapid heating and cooling, making materials choice and design absolutely critical.

There are many systems available in various materials which are suitable for lower temperature processing. However, demands from within the industry for kiln furniture led Morgan to develop a new high-performance nitrite-bonded silicon carbide material suitable which can withstand temperatures of up to 1,500ºC (2,732ºF).

The material, known as Halsic^®-N™, combines the properties of proven materials such as silicon carbide (SiC) and silicon nitride (Si₃N₄) in a microstructure which delivers strength, excellent refractory properties and resistance to oxidation and thermal shock.

Standard designs include setter plates, beams and supports, while components can also be produced to individual customer specifications.

Thermocouple protection tubes made from Halsic-N™ are ideal for the melting of non-ferrous metals such as aluminium and magnesium, where its strong metal-repelling characteristics help ensure long service life and optimised performance.

With flexural strength of 160MPa, Halsic^®-N™ can even be coated with a specially formulated coating for the firing of porcelain or technical ceramics.

The shape of things to come?

When it comes to fired refractory shapes, the need to employ best practice is equally pronounced. While it is recognised that these products are generally used only once and so need to withstand a single firing, they are generally subject to rapid heating and cooling and so need to be strongly resistant to thermal shock. More and more frequently they are used with expensive cast metals and superalloys, an area where material loss as a result of a failed consumable is not acceptable due to the high material costs involved.

Once again, material quality is paramount. Even under high-temperature firing, it is crucial that oxidisation of consumable products is kept to an absolute minimum to prevent contamination of the materials being processed. This means purity of consumable products is vital and so another area where rigorous investigation of suppliers’ raw materials sourcing and manufacturing processes represents time well spent.

The best options for inert and vacuum furnaces

As detailed above for ceramic furnace consumables, high temperature inert and vacuum furnaces pose similar challenges for purchasers and designers.  In these ultra-high temperature furnaces. which are capable of reaching temperatures of 3000ºC (5,432ºF), designing the proper insulation solution of carbon or graphite felt, and carbon or graphite rigid board, is incredibly important, as the costs of these heat treating processes play an increasing role in the expansion of high-end technologies such as solar and semiconductor products.  There are a variety of standard materials available designed specifically for these applications. Many of these materials can also be enhanced in a number of ways to provide the process or furnace designer with additional options.  For instance, felt products can be specifically designed to provide enhanced insulation performance or longer life. The rigid board products offered by Morgan include a standard material as well as a low thermal conductivity material, RGB-LTC, for energy improvements across many applications.  In addition, for use in harsher environments, specially developed coatings can be applied to improve life or reduce friability. Purity of the insulation is also vital for product performance.  Many levels of purity can be supplied, offering the designer even more options.  Whatever the application or need, providing a specific solution to solve a specific problem is key in providing the best cost of ownership for the purchasing company. 

Product specification and system design are not areas where processors should feel they are on their own. The leading manufacturers of these systems are keen to be more than just suppliers and to work in a strategic and collaborative way with customers, gaining a deep understanding of their individual application requirements, then assisting with system design and recommending and supplying the most appropriate consumables, whether these be standard products or need to be developed on a bespoke basis.

They will even partner with installers to help ensure best design practices and that issues such as poor sealing around doors, for example – an issue which can rapidly negate the investment made in a premium product – are avoided. A good installer will ensure the very best is obtained from the product selected.

Above all, consumables should not be treated as commodities – they are vital tools without which effective processing cannot take place.

For further information visit www.morganthermalceramics.com/kiln-furnace