April 13, 2015
The term refractory refers to a substance that is hard to fuse, while insulation refers to a substance with a high thermal resistance. Both are used as inner linings of furnaces, kilns, ovens, etc.. The reliability of the refractory is dependent upon the successful combination of materials, proper design, and installation of the refractory.
Experts agree that refractory designed and installed to save energy also saves money at a rate that is essential for efficient plant operation. Refractory that is properly selected and installed will always last longer, help minimize the amount of shutdowns required, and lead to savings in annual fuel cost.
Despite the advantages of some refractories over others, it is important to select the right refractory for the specific application. The selection of refractories aims to maximize the performance of the furnace, kiln or boiler.
Furnace manufacturers or users should take into account the following factors in the selection of a refractory:
Identify the type of your furnace and the type of fuel used. Different type of furnace has different applications and refractory challenges.
The chemical corrosion caused by slag attack is the most common cause for the failure of refractories. Slag resistance depends on chemical reactions that may occur between the slag and the refractory. The physical and chemical properties of the slag and refractory are very important for the stability and wear resistance of the refractory lining during service.
Selecting materials should always be based on properties and specifications suitable for a specific application. Refractories can be used for a wide variety of different applications. Using refractory materials which are not right for the specific application can be dangerous. The refractory materials may fail, cracking, exploding, or developing other problems during use which could pose a threat to safety in addition to fouling a project.
An operating temperature above the service limit for a particular refractory can weaken or melt the refractory. Operating temperatures determine extent of slagging and refractory quality required. Operating temperature can vary from very low to very high (approximately 900°F to 2900°F). The refractory materials are therefore needed to withstand temperatures over and above these temperatures.
Mechanical abrasion and impact of refractories is a key issue for many industrial furnace applications where particle-loaded gas jets are expected during the process. Those applications require a good resistance to mechanical abrasion and impact. A refractory with good resistance to abrasion should be mechanically strong and well bonded.
A furnace must be designed to carry its own structural load at high temperatures. If it is unable to hold its own weight, it may collapse at high temperatures.
The life time of a refractory brick in a furnace lining can be reduced by cracking and spalling due to the stresses due to temperature gradient in the structures and temperature fluctuations. Ordinarily one face or end of a brick is subjected to much higher temperatures than the opposite face or end of the brick and a temperature gradient exists in the brick. With radical changes in temperature at the hot end, fluctuations in the gradient cause stresses which are frequently relieved by cracking the brick.
The chemical compatibility to the furnace environment is also very important for consideration. These are acidic refractory, basic refractory and neutral refractory. Acidic refractories react with basic chemicals and are used for acidic composition of slag. Basic refractories react with acidic materials and used for basic slag condition. Neutral refractories may be used for all types of slag conditions.
Furnaces used for melting, heat treatment, heat regeneration or for any other purpose demand minimum heat loss and maximum heat conservation so as to minimize heat losses for maximum heat efficiencies and minimum fuel consumption as well as high production as a result of maintaining high working temperatures. Refractories with low thermal conductivity is desirable for fuel conservation.
Further, the cost and longevity of the refractories are to be considered before selection of the refractories.
No single refractory material can meet all these requirements. So, the selection of refractories for any particular application is made with a view to achieve the best performance.
April 26, 2010
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