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Showing posts with label Selection of Refractories. Show all posts
Showing posts with label Selection of Refractories. Show all posts

Criteria for Furnace or Kiln Design and Selection of Refractories

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The primary function of any refractory material is to withstand high temperature in a hostile environment. However, in actual application a refractory lining is required to perform many other functions depending upon the place of use and prevailing service conditions which in turn help us to decide what should be the outline or design of furnace, its parts followed by the selection of proper refractories. 

First let us see the various general requirements which a refractory material is to fulfill :
1. Ability to withstand high temperature.
2. Ability to withstand temperature fluctuation.
3. Ability to withstand the actions of processing materials and product of combustion.
4. Ability to withstand load under high temperature.
5. Ability to withstand impact and abrasion of solid, liquid and dust laden gases moving with high speed.
6. The refractory material should be volume stable.
7. It should not contaminate the finished product.
8. The refractory material should have low co-efficient of thermal expansion.
9. It should not conduct much heat.
For a proper design of any refractory lining system it is essential that the complete information of furnace or kiln type and prevailing service conditions are available.
The most important operational data required for the selection of refractories are as follows:
Furnace / Kiln Type :  For which industry the furnace or the kiln is to be used.
Process : Details of process to be adopted. Will the refractory material come in direct contact with slag, metal, dust, fluxing agent, gas or flame? Which part of the furnace or kiln will be subjected to the destructive actions of the above elements, etc.
Fuel  :  Type of fuel to be used for generation of heat energy. How the furnace will be heated.
Operation :  How the furnace (kiln) will be operated: continuous or intermittent. What is the extent of temperature fluctuation and over what period of time. To what extent the refractories will be exposed to thermal shock.
Operation - Temperature :  What will be the highest temperature to which refractories will be exposed. What will be the peaks.
Limiting - Temperature:  What are the maximum and minimum temperatures of the furnace or kiln design components e.g. steel shell temperature etc.
Heat Loss  :  What heat loss will take place? Is the heat to be conducted through refractories or retained within the furnace?
Surrounding Conditions   :  What are the surrounding conditions such as heat flux calculations, influence of any adjacent plant or component, maximum and minimum ambient temperatures, wind speed, radiation co-efficient etc.
Furnace Atmosphere :  Is it neutral, oxidizing, reducing or changing?
Furnace Pressure :  What operation pressure is expected? Is the furnace part under suction or under positive pressure.
In actual situation the refractories may have to work under some or all of the above conditions. They may act simultaneously and demand suitable refractories to withstand the destructive forces. No single refractory material can satisfy the entire requirement. Hence, a compromise is made and the most demanding requirements are first met at the cost of other lesser requirements. For example, in a hot air or gas carrying system the thermal conductivity would be the vital criteria. Therefore from every saving point of view insulating properties of the refractory material becomes more important than other properties for design considerations.
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3 Things You Must Do To Get Better Performance Of Refractories

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Nothing counts like the ‘performance’. Getting or giving a better performance is one thing which everyone tries to do. Reasons obvious! For getting better performance of refractories from an installation one must take care of the following three simple but very important things:
1. Proper Selection of Refractories.
2. Proper Installation-Application.
3. Proper Operation Practice.

Selection of Refractories
Though there are specific refractories for different applications, operation practices lead to certain criteria on which, depends the life of refractories. As such these need to be properly considered. Customers should disclose the actual operating practices and conditions so that some important properties, required to such conditions can be taken care during the selection and manufacturing stage of refractories. Among various physical, chemical, thermo-chemical and thermo-mechanical properties of Refractories, there are a few properties which contribute markedly to its performance. These are called ‘Key Properties’. For ensuring better performance and quality these key properties should be tested. 

Application - Installation of Refractories
Depending on the method of application or installation there has to be a set of guidelines in respect to laying of refractory bricks, their dimensions, selection of mortars, expansion joints and many other minute but very important things. So from case to case basis the supplier of refractories should specify this properly and also ensure that the methods are actually being followed. Transportation, handling, timely arrival of refractory bricks, mortars, skill of masonry work, proper equipment for application e.g. mixer machine for castable, vibrator for installation, forma etc. are very important. Maintaining proper expansion gaps, correct dimension of bricks and monolithics, fixing anchors etc. all are very important to achieve better life of refractories. Once the installation of refractories is over, the initial heating of the lining before starting the actual operation is of prime importance. Customers should demand the initial heating schedule from the refractory supplier.

Operational Practices
Proper operation is not only important for getting right quality output but also, help in getting the optimum life of refractory lining, less downtime, maximum availability of the furnace and thus, the benefit of lower cost of refractories per tone of finished product. Customers must be aware of the reasons which can damage the refractories arising because of improper operations. During the training of the furnace operators, apart from the method of the furnace operation etc. they must be given some knowledge regarding the proper usage and importance of refractories also.