Showing posts with label Basics. Show all posts
Showing posts with label Basics. Show all posts

January 19, 2011

Refractory Bricks – Shapes and Sizes

There is no limit to which a refractory brick can have different shapes and sizes to suit a particular design or lining requirement. Production of refractory shapes depends upon the design and size of furnaces or other service constructions where such refractories are to be used. Numerous shapes and sizes of refractory bricks are produced to meet the specific lining requirements of straight walled, cylindrical, arched, dome and other special types of construction work besides the shapes like wedges, keys, sleeves, nozzles, burner blocks, tiles etc. For example, only an iron blast furnace requires refractory bricks of so many types, shapes and sizes. However, most of the lining works are made of certain standard refractory shapes and sizes which are always available in the market. Special refractory shapes are only produced to meet the specific lining requirements of each furnace or refractory structures like some typical coke oven shapes, stopper heads, arch tiles etc. The shapes which are universally accepted and used are listed below (see figure):
Refractory Bricks - Shapes and Sizes image
Fig: Refractory Shapes
Standard Refractory Shapes
1. Straight (Rectangular)                
2. Side Arch
3. End Arch
4. Wedge
5. Key
6. Flat Circle
7. Combined Arch and Wedge
8. Circle
9. Splits
10. Dome Brick
11. Skew (End / Side)
12. Bullnose or Jamb Brick
13. Soap or Closer
Refractory Bricks shapes and sizes image
Fig: Refractory Brick Shapes / Sizes 
Refractory Ladle Well Block image
Fig: Ladle Well Block
Refractory Lining | Steel Technology - Stopper Sleeve image
Fig: Stopper Sleeve
Stopper Pin image
Fig: Stopper Pin
Refractory Stopper Head image
Fig: Stopper Head
(For a short definition of each shape see the ‘Glossaries > Refractories’ in the menu navigation bar above)
Standardization and rationalization of refractory shapes becomes important as a host of refractory shapes needed for lining even a single part of a production unit involving very complex and complicated designs necessitates use of numerous kinds of opening and aperture details besides inter-chamber dimensions et. As such one can not imagine preparation of moulds either by mechanical pressing or by hand / pneumatic moulding to suit production of large number of shapes with different design details without involving a very high cost of production. So, it is always preferable to design a lining with commonly used refractory shapes as they facilitate easy availability, reduce cost (moulds / liners are generally available with manufacturer), and easy to repair.  
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April 24, 2010

How effective are Insulating Refractory (Ceramic) Fibers?

With the advancement of technology and involvement of very high temperatures by various industries as well as continuously increasing energy costs, there has been always a demand for new and more dependable insulating materials. Finally, there has been one of the most exciting developments in the field of high temperature insulation as ceramic fibers. These are a family of insulating refractory products based on refractory or ceramic fibers. Such products are very light and highly porous resulting in an excellent insulating efficiency with decrease in the material consumption of insulators by 40 to 60 percent. Thus use of such materials reduces overall weight of the structure, reduces fuel consumption and increases the productivity. These materials have seen rapid sales growth in recent years because of their excellent insulating properties, light weight, and ease of installation. Most refractory fiber materials are basically high temperature fiberglass materials. They have alumina-silica compositions made from pure alumina and silica or from kaolin clay. There are also chemically made alumina (Al2O3) fibers which are useful for high temperatures but which are quite expensive. Zirconia fibers (generally glass bonded zircon) have also found considerable market acceptance for service up to 3300OF or even a little higher. [Insulating refractories in general, their types, raw materials used for their manufacturing, method of heat - flow through such refractories and its calculations, what should be the thickness of insulating refractory linings etc. have been discussed in detail in posts Insulating Refractories (Part - I) and Insulating Refractories (Part - II)].   
Refractory fiber products can take on a variety of forms.
Bulk fiber can be used for packing or stuffing. The fiber can be collected into a mat and wetted with an organic binder. When this binder is cured it yields a felt. Available in flexible rolls in densities of 3, 4, 6, and 8 pcf (lb/ft3) or in sheets passed to densities as high as 24 pcf, these felts have served a wide variety of purposes. Another development has been the production of binder-free blankets. Often these have the fibers mechanically interlocked by a “needling” process which substantially increases mechanical strength without the using any organic binder. Mechanical strength at high operating temperatures is thus preserved, since any organic binder burns out during initial heat-up. Refractory fibers can also be vacuum formed to give rigid board and shapes, such as combustion chambers. A tremendous variety of products have thus resulted. Just to mention a high technology application, the insulating tiles on the re-entry surfaces of the Space Shuttle are of this type. Formulated of ceramic fibers and with a special ceramic bond, those tiles are capable of withstanding extremely high surface temperatures and temperature gradients without failure, while protecting the vehicle substructures by virtue of their very low thermal conductivity.

Typical Thermal Conductivities for Refractory Fiber Blanket Materials graphics
Refractory Fiber products have unique properties.
In many respects they have revolutionized insulating refractory lining technology. Refractory Fiber products have exceptionally low thermal conductivity values, as can be seen in the adjacent figure (graph) given for typical refractory fiber blanket products. Note that the higher density materials have lower k values. Most of the heat transfer occurring in fiber products is by radiation. Higher density fiber products have more fibers in the same volume and thus block radiation more effectively. Solid conduction is minimal, since an 8 pcf fiber blanket contains 95% air. Air conduction is also important, however. Note that the k values increase rapidly as the temperature increases. This too, is the result of the major role that radiation plays in energy transport in refractory fiber materials. The low density of refractory fiber means that very lightweight insulation systems are possible. Furnace or kiln linings can be exceptionally light. This also results in very low heat storage, which is very important in cyclical operation. It allows rapid heat-up and cool-down and is a major factor in energy conservation with these materials. Insulating refractory fiber linings also greatly reduces the mechanical load on supporting structures, so that these can be made lighter and less expensive. The resilience of fiber materials makes thermal shock practically impossible. Extraordinarily rapid temperature changes have no effect on refractory fibers or their mats. Various types of felts based on ceramic fibers and available in rolls have proved to be useful as their use promote speedy laying with minimum joints. They also guarantee a unique advantage of lining surfaces bearing complicated contours.   
TABLE: Thermal Comparison of Refractory Fiber Lining with IFB and Fireclay
Brick Linings for Furnace Operating at 1800OF
Wall Construction
Heat Loss (BTU/ft2/hr)
Heat Storage (BTU/ft2)
Cold Face (OF)
Lining Weight (lbs/ft2)
9 in. fireclay brick
9 in. 2000OF IFB
6 in. refractory fiber    (3 in. 8 pcf blanket, 3 in. mineral wool back-up)
Like all refractories, fiber materials do have some limitations.
The chief limitation is shrinkage at high temperatures. A high quality ceramic fiber blanket rated for continuous use at 2400OF will have 5% shrinkage after 24 hr exposure at 2400OF. Shrinkage will not continue past this level in normal operating conditions, but this shrinkage must be carefully considered in designing a furnace lining. The mechanical strength of ceramic fibers is understandably poor. Even the rigid vacuum formed products are not really structural materials. Proper support must be given to all refractory fiber products. Since these are for most part glass fiber materials, they may sag at high temperature due to softening of fibers if improperly supported. Devitrification also occurs, causing a loss of resilience. Since their first introduction to the market, refractory (ceramic) fiber products have been considerably improved in many of these respects. Their manufacturers are happy to call attention to those improvements; but in every case it is wise to pay close attention to the properties of fiber materials and to the technical design and installation advice given by their prior users. A limitation that is always present is that fiber insulating materials are handy repositories of dusts, fogs, and combustible fumes; not to mention for process liquids like slags and metals. These materials are definitely not indicated for service in such severe environments. They are used with great success, on the other hand, in metal treating furnaces, ceramic kilns, and numerous other periodic operations whose atmosphere do not negate their revolutionary thermal and lightweight qualities. Fiber mats also continue to be used in expansion joints and door seals, and in tunnel kilns and other exposed - brick structures as either original or retrofit layers on the outside or cold-face surface.
Refractory fiber materials tend to be more expensive than conventional refractories, although that differential has shrunk or disappeared as fiber prices have held more or less steady. Installation labour savings and energy savings have made refractory fiber the most economical material in a very wide variety of ‘clean’ applications. It is the combination of low heat loss and low heat storage that make fiber so attractive.
Our next post is on the subject: Installation of Refractory Fiber Kiln and Furnace Linings.  

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April 19, 2010

Some Basic Guidelines for Laying Refractory Brickwork or Lining

Certain basic rules exist for the laying installation of refractory bricks or lining. They generally apply for all designs and construction parts of any furnace, pipe, chute, chimney, foundation, tank or any other vessel etc. Some of these rules have been summarized below:
=> Refractory bricks must always be laid horizontally unless the design of the plant requires inclined positions or inclinations as is the case for crowns or inclined planes.
=> The construction dimensions in the design and drawings must always be observed taking the indicated tolerances into consideration. The first refractory layer (course) must be installed with extreme care, aligned and checked before giving the “go ahead” for further brick laying (lining) work.
=> All joints must be filled with the prescribed joint material. Thickness of the joints must be observed taking the indicated tolerances into consideration.
=> All joints must be filled over the entire surfaces with the joint material. It is permissible to apply the mortar with a ‘Collar’ because there is the danger of hollow spaces forming in the joints.
=> If, due to the size tolerances of the bricks, the prescribed joint thickness can not be accomplished without obtaining ‘Naked Surfaces’, the person responsible for the refractory design will have to decide if thicker joints can be allowed. This is only permitted as a better solution cannot be found by sorting or changing the shapes. A grinding of the bricks should only be a possibility in exceptional cases.
=> Expansion joints should never contain any contamination, e.g. by insertion of joint templates or by gluing.
=> Refractory bricks which have been already laid can only be readjusted in the direction of the bed or vertical joint.   
=> Readjustment of brickwork already laid is not possible if the mortar has started to harden to a greater degree. Depending on the type of mortar used, there will possibly be only few minutes for readjustment once the bricks have been positioned. Sometimes, it may be necessary to remove bricks not placed correctly, clean them, and re-install them once again with fresh refractory mortar.
=> Refractory bricks with smaller spalls, hair cracks or slight inclusions may only be installed (laid) provided these irregularities are insignificant for the proper functioning of the construction part. This also applies to the rear side of the hot slide layer and for the brickwork behind. The criteria for the acceptance or rejection are indicated in the specifications or must be agreed upon mutually by the customer, manufacturer, and supplier before the start of lining or brick laying work.
=> Brickwork out of refractory materials must be designed in such a way that no hollow space forms. Dust and fly ash can penetrate hollow spaces. This results in uncontrolled pressure buildup which may destroy the refractory brickwork. Damages can also occur by roaming gases.
(To be continued)
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June 21, 2009

The Function of a Furnace and Kiln used in Ceramic and other Industries

There are so many definitions for Kiln and Furnace. Actually kilns are an integral part of the manufacture of all ceramics, which require heat treatment, often at high temperature. The distinction of a Kiln and a Furnace is often done on the basis of user industry than on the design of the device. Generally the term kiln is used when referring to high temperature treatment of non-metallic materials such as in the ceramic, cement (cement rotary kiln), lime (lime kiln) industries etc. When melting is involved the term furnace is used as in steel manufacture (Blast Furnace, Basic Oxygen Furnace, Ladle Furnace), glass industries (Glass Melting Tank Furnace) etc.

As a practical working definition, it has been proposed to restrict the term Furnace (or Kiln) to an industrial appliance, constructed to heat a material through a cycle involving temperatures exceeding 400OC. This temperature has been chosen in order to exclude a large number of industrial process in which steam is used as a medium of transferring heat. It is essential that the heat released in the space of the furnace should be so utilized that the maximum heat economy is effected. A good working furnace must therefore -

>> have very good control of temperature.

>> require a minimum amount of fuel or other energy sources and other auxiliary materials.

>> require minimum capital and maintenance costs.

It is felt that a greater use of thermodynamics in furnace design can lead to more accurate and closer way of thinking about such problems as those of preheat, utilization of waste heat, recirculation of the flue gases and different qualities of energy sources, etc.

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Criteria for Selection of Furnace (and Kiln)