Showing posts with label Insulating Refractories. Show all posts
Showing posts with label Insulating Refractories. Show all posts

September 19, 2010

Types of Insulating Refractory Bricks and Castables - their Manufacturing and Installation

No high temperature operation can go without heat management especially, in this ‘endless era’ of rising energy costs. The solution is of course, refractories and typically speaking, Insulating Refractories. The reason - it allows a furnace to reach temperature faster than without it, at the same time protects the unit’s surrounding environment from excessive heat and saves energy costs; add value to the customer’s product. 
In one of our earlier articles Insulating Refractories (Part–I), we reviewed the functions of insulating refractories, some of the fundamental technologies of high-temperature refractory insulation, mechanisms of heat transfer in industrial processes, rate of heat flow (heat loss), considerations of Thermal Conductivity in a refractory material, and how to calculate Heat Loss or Heat Transport and Thickness of Refractory Lining for a given furnace conditions etc. This article presents and discusses on the types or few qualities of insulating refractories, their manufacturing procedure including the various raw materials used, and also on the installation of insulating refractory bricks and castables.
There are several types of insulating refractories, including insulating fire brick (IFB), insulating castables, insulating pumpables, granular insulation, and ceramic fibre insulation [How effective are Insulating Refractory (Ceramic) Fibers?]. The insulating bricks may be classified mainly into two categories, one being used for the low temperatures, below 1000OC (CFI) and the other (HFI) for any temperature above 1000OC, depending on the raw material used in their manufacturing. Ceramic fibres of various compositions with corresponding application temperatures form another category of insulation.
To be a good insulating refractory brick they must have the following properties:
1. Low thermal conductivity.   
2. Mechanically strong enough to bear the load of the structure.
4. High porosity.
5. Low permeability.
6. Withstand the heat of at which they are used.
7. Must not shrink or react chemically with the material with which they are in contact during use.
It is well known fact that vacuum is the best insulator. Next to this comes the motionless air. But can we create vacuum in a brick for having insulating properties? For obvious reasons, the answer is no. This property is introduced in a brick by including a large number of air spaces in its body. The air spaces inside the brick prevent the heat from being conducted but the solid particles of which the brick is made conduct the heat. So, in order to have required insulation property in a brick a balance has to be struck between the proportion of its solid particles and air spaces. The thermal conductivity is lower if the volume of air space is larger. Importantly, the thermal conductivity of a brick does not so much depend on the size of pores as on the uniformity of size and even distribution of these pores. Hence, uniformly small sized pores distributed evenly in the whole body of the insulating brick are preferred. The brick should have enough pore space at the same time cellular in texture. This cellularity for manufacturing insulating refractory bricks can be introduced by one of the following ways:
(a) By addition of a combustible substance in the composition (mixture) of brick e.g. saw dust, paper fiber, coal dust, rice husk ash, styrophore etc. During firing this burns out leaving behind a porous structure.
(b) By using minerals which expand and open up on heating e.g. raw kyanite, some china clays.
(c) By addition of a volatile compound in the composition (mixture) of brick e.g. naphthalene.
(d) By using substances which by themselves have open texture e.g. insulating brick grog, vermiculite, ex-foliated mica, raw diatomite etc.
(e) By chemical bloating. This is generally done by using aluminium (Al) powder in combination with NaOH solution.
(f)  By aeration.
(g) By putting foaming agents in the mixture of the brick.
Amongst all these the first method is more common and easier for producing cellularity. The manufacturing of insulating refractory bricks and other insulating materials require a different approach. The low temperature insulating bricks are manufactured using granules of vermiculite, ex-foliated mica, and raw diatomite. While using any of the above raw materials, a good percentage of combustible or carbonaceous grains is used in the batch composition, which burns out during firing, leaving voids inside the texture of the brick. The high temperature insulating bricks are produced from mixtures of grains of calcined clay, raw kyanite with combustible material in the batch. Raw kyanite expands on heating by 15-18 per cent. This fact has made raw kyanite an excellent material for making insulating refractory bricks. When the bricks are fired, the kyanite expands and the bricks become porous. The addition of saw dust or any other combustible is helpful in the sense that on burning, the saw dust leaves open spaces and when kyanite expands, the expansion is borne well by these spaces and the structure is not disturbed [One complete 'Production Recipe' is given at the end of this article for manufacturing insulating refractory brick of two different compositions and properties].
Acid insulation bricks can be made similarly with crushed quartzite, fireclay and saw dust in batch. The use of combustible material may be eliminated by adopting the foaming technique during forming the shapes. In rice growing countries, the rice husk ash is a cheap and important insulating raw material suitable for use at a fairly high temperature 1500OC. A small percentage of plastic clay as bond is used in both low temperature and high temperature insulating bricks. The firings of insulated bricks are carried out at a temperature depending on the raw materials used as well as on the temperature of their application. The firing temperature should be preferably higher than the temperature of their application. Naphthalene is also used produced cellularity. It is mixed with fireclay and insulating grog in powder form and pressed into bricks. On firing naphthalene volatilizes leaving a cellular mass. Sometimes aluminium powder is used with NaOH to produce chemical bloating or froth. Chemicals like ammonium sulphate, ammonium chloride, ammonium nitrate, calcium phosphate, phosphoric acid, and sulphuric acid are also used for manufacturing insulating refractories. But these are generally used in manufacturing basic insulating refractories like magnesite. Sulphuric acid acts in green state. CO2 is expelled leaving the body of the brick porous. Other substances like ammonium chloride decompose on heating. The Cl2 mixed with water vapour acts with magnesite and CO2 is expelled.
Some Drawbacks of Insulating Fire Bricks (IFB)                         
Generally saw dust is used in the batch composition for manufacturing of insulating fire bricks which gives porous structure to the brick after firing. Although porosity decreases thermal conductivity and density of the brick, it also degrades the mechanical strength of the brick as compared to a dense refractory firebrick. The porosity also makes IFB more susceptible to chemical attack by gases, fumes, slags etc. The porosity in IFB or any other insulating refractories creates a large amount of free surface area. Since chemical attack starts at surfaces, porosity leads to poor chemical resistance as compared to dense refractories of similar compositions. Liquids such as slags, molten glass etc. at high temperatures can penetrate porous bricks easily, making insulating fire bricks unsuitable for direct contact with such liquids or gases.
The poor strength of IFB due to their high porosity can pose structural design problems. In addition, insulating fire bricks often suffer from thermal spalling problems, particularly in an environment of rapidly changing temperature. Since these bricks are good insulators, a substantial temperature gradient will occur between the hot and the cold face of each brick. The hot face will expand more than the cold face. The thermal gradient thus, gives rise to a mechanical stress in the body of the brick. Since, insulating fire bricks are not very strong, the surface can be spalled off by these stresses, especially if the temperature changes frequently.
Installation               
The procedure of installation of insulating fire bricks is same as dense brick. “Techniques of Installation of Fiber Refractory Linings” and “Techniques of Adding Insulation over the Existing Refractory Linings” have been discussed in a separate post. Insulating fire bricks are used as the hot-face refractory materials in ceramic kilns and many heat-treating furnaces. They can not be used on the hot-face when severe temperature or operating conditions exist. But insulating fire bricks are often used backup insulation in such circumstances. When used as backup insulation, it is important that the interface temperature between the working face of the furnace and the backup insulating brick is known so that the proper grade of these insulating fire brick (IFB) can be selected. Similarly, insulating castable refractories are monolithic refractory mixes into which a large amount of porosity has been introduced. The method of manufacturing is much the same as described above for insulating bricks. One way is to put saw dust or any other combustible material in the aggregate to make this porous when it is fired. Then the aggregate is crushed and sized and mixed with more conventional bonding chemicals to prepare the castable. Another approach is to put foaming agents (as mentioned above) in the mix, which are activated when water is added for installation. By this porosity is introduced in the matrix instead of the aggregate. Installation of insulating castable refractories is almost the same as for a dense refractory castable but again with due attention to the mechanical weakness of these castables in design of the system. Insulating refractory castables may be poured into an intervening space deliberately left between the steel shell and a free-standing wall of working refractory bricks. This technique ensures an excellent fit between any irregularities in the brickwork and irregularities that are bound to occur in the surrounding shell. Some poured-in backup insulation is simply made of loose, porous, granular fired refractories. This, of course, has no mechanical strength at all. Its use tends to be limited small furnaces whose brickwork is entirely self-supporting and to a number of other similar situations in relatively small vessels. Typically mineral fiber or other low-duty refractory materials used as backup insulation generally degrade over time, allowing heat to channel through. Insulating pumpables are refractory materials which provide quick and easy refractory lining repair. Common insulating pumpable applications include – re-insulating hot-spot in utility boilers, industrial furnaces and kilns, sealing around burner blocks and flues, and placement between fiber modules that have shrunk excessively. And more recent addition to these is the Insulating Foams that are cast with different cellular configurations.
Manufacturing and Composition Recipe
Materials
Volume  %
Firing Temp / ST (Shrinkage %)
Tentative Properties
China Clay (Ball Mill Fines)
Saw Dust (Fines)
Raw Kyanite (Fines)
45
32
23

1200OC / 2hrs
(1%)
Al2O3 = 40%   
Fe2O3 = 2%
Service Temp = 1400OC (max)
BD = 1.1 gm / cc
PCE = 32 ½
Apparent Porosity = 58%
CCS = 40 kg/cm2
Thermal Conductivity at 600OC H/F =         0.45 K.Cal/m/hrOC  

China Clay (Ball Mill Fines)
Saw Dust (Fines)
Insulating Grog (0 - 3mm)
55
35
10
1220OC / 2hrs
(1%)
Al2O3 = 30%   
Fe2O3 = 2%
Service Temp = 1300OC (max)
BD = 0.8 gm / cc
PCE = 30
Apparent Porosity = 70%
CCS = 15 kg/cm2
Thermal Conductivity at 600OC H/F =         0.35 K.Cal/m/hrOC  

  
production Process       
(a) Saw dust (containing 30% moisture max.) is screened through a Rotary screen (2mm). (b) Dry mixture is made. Materials are added by Volume per cent as per composition (e.g., here it is 8 boxes China Clay + 5 boxes Saw Dust + 1 box Insulating Grog = Total 14 boxes. If we calculate the same by weight then it comes about - China Clay 64%, Saw Dust 28% depending on its Moisture%, Insulating Grog 8%). (c) This dry mix is Pug Milled adding only water & kept in a Bunker under a plastic cover to avoid rapid drying. (d) Showering of water is done over this mix for 10-12 days. For accountability starting & last date of showering should be marked on the respective Bunker wall. (e) After 10-12 days of showering the same mixture is remixed in a Muller Mixer after adding some organic bond. After this final mixing, the Mixture is taken for moulding into clots as per the required size (provision in mould size should be kept for firing-shrinkage & final cutting). (f) After floor drying, clots are fired in a batch type kiln at about 1200-1220OC / 2hrs or as mentioned above. (g) Fired clots after cutting & little bit finishing are ready for packing and despatch.                     

May 23, 2010

Installation of Refractory Fiber (Ceramic Fibers) Kiln and Furnace Linings

Considerable technology has been developed in the installation of refractory (ceramic) fiber kiln and furnace linings. Techniques for lining new equipment as well as for addition of refractory fiber insulation to existing dense brick, refractory castable, or IFB (Insulating Fire Brick) linings have been devised. The major manufacturers of fiber and their distributors can provide application booklets and advice.
Replacing Dense or Hard Linings with Fiber in existing Furnace or Kilns 
Although refractory fibers are easier to install than other refractories, let me tell you there is still considerable technology involved in it. Proper design and installation are absolutely critical. As you go through this article you will find how the technology is completely different from that required for installing other refractories. Fascination with the advantage of refractory fiber linings has prompted some users to consider replacing “hard or dense” linings with fiber in existing furnaces and kilns. Such a change-over requires expert guidance and should not be undertaken lightly. While it can be successful, many failures have been recorded due to technical misapplication of fiber materials or due to lack of consideration of the consequences. Obviously, changes in temperature profiles will result, all the way from the hot face to furnace or kiln shell. Major changes in the distribution and storage of heat in the vessel also result, including radical changes in start-up and cool-down time but also changes affecting steady-state processing. Even the fuel consumption characteristics of a given furnace can be strongly affected by replacement of a brick and castable lining by ceramic fiber. Pitfalls also await attempts to augment an existing “hard” refractory lining by the addition of fiber, either inside or outside. Most such concepts are well-founded; but they must also be well-analyzed, well-designed, and well-executed. When they are the rewards can be impressive.
But before we go into the techniques for installation of fiber refractory linings, let us first consider few more things involving Heat Flow and Energy Saving calculations here.
Heat-loss Rate, Heat-up, Firing, Cool-down, and Benefits (energy savings)
We mentioned in our earlier article How effective are Insulating Refractory (Ceramic) Fibers, that one of the features of a fiber refractory is that it sores or retains very little heat. This means that a furnace or kiln can be brought up to operating temperature very quickly and economically, and likewise cools down again very rapidly, if its working lining is of fiber. For ‘periodic’ and other cyclically operated kilns, both the heat energy saved and the time saved during heat-up and cool-down of each cycle is money in the bank. Let us explore just how great this energy saving can be. Take for example, a 4 inch thick lining of a 6 pcf (lbs/ft3) fiber blanket, working at a hot-face temperature of 2600OF and with its cold-face at 400OF. The mean temperature for the lining is then 1500OF. Looking at the adjacent figure showing Typical Thermal Conductivities for Refractory Fiber Blanket Materials we read off an average k of 1.6 BTU.in./(ft2.OF.h). First we need to use this k to find an equivalent thickness of, say ‘x’, a dense firebrick lining that this fiber lining might replace. Equivalent could mean, having the same heat-loss rate in the same application. The heat-loss rate for this fiber lining is, per ft3 of area (i.e., A = 1):
             





For comparison, a 2600OF - rated firebrick is chosen, whose k might be 8.0
Now we can determine the thickness (x), here for this dense firebrick -
   



(Refer to our earlier article Insulating Refractories (Part - I) where we have rearranged the Heat-flow Calculations and discussed in detail how to calculate Heat Loss or Heat Transport and Thickness of Refractory Lining etc.)          
So, a 20 inch thick wall of firebrick is equivalent in heat loss to a 4 inch thick fiber blanket, during the steady-state party of operating cycle. But no one in his right mind would erect a 20 inch thick brick wall for 2600OF duty in a chemically “clean” kiln or furnace. Let us build the wall instead, of 9 inch “straights” in the alternating header and stretcher courses. It will then be only 9 inch thick, and its heat-loss rate at steady-state will be (20/9) or 2.2 times that of the 4 inch thick ceramic fiber blanket. We will just swallow that disadvantage, and now set about to compare the heat wastage in start-up (heat-up) and shutdown (cool-down) for a 9 inch thick brick wall versus a 4 inch thick fiber blanket.
The density of a low-duty refractory firebrick is about 128 lb/ft3. Now, every square foot of area of a 9 inch thick wall has a volume of 0.75 ft3. So every square foot of this firebrick wall weighs (0.75) (128) or 96 lb. By contrast, every square foot of 4 inch fiber blanket has a volume of 0.33 ft3, and from its density of 6 lb/ft3, we get the weight of a square foot of this fiber blanket, (0.33) 6 or 2 lb.
Recall that we started here with a mean temperature of the working fiber refractory of 1500OF. That will be about the same for the brick wall as well. A rule of thumb for oxide refractories is that their heat capacity is relatively constant at roughly 0.25 BTU per lb per OF. So the heat we have to store in these refractories is the weight times the heat capacity times the rise in their mean temperature. The first time we go for heat-up these, from say 100OF to 1500OF mean, we have to store the following in every square foot of lining:
(a) In 9 in. thick brick, (96 lb) (0.25) (1400OF) = 33600 BTU
(b) In 4 in. thick brick, (2 lb) (0.25) (1400OF) = 700 BTU              
Cycling will give somewhat smaller numbers in both cases, because cool-down would not be all the way to room temperature. If cycling is between mean-temperatures of 500OF and 1500OF, every heat-up would take 24000 BTU for brick but only 500 BTU for fiber, per square foot of lining. Since refractory linings of kilns or furnaces can easily measure in the thousands of square feet, the difference could be large.
Let us see the consequences using these numbers in another case, for example, in a shuttle kiln of the form of a cube, 14 ft. on each side. The total refractory lining area is 6 (14)2 or 1200 ft2. Suppose this kiln is firing ceramic wares, requiring 6 hours at steady-state and (for brick lining) 6 hours for heat-up, coo-down, loading and unloading. In a 24-h day, two loads could be fired. The wasted heat at steady-state would be 12 h times (2.2) (880) BTU/ft2.h times 1200 ft2 or 27.9 million BTU per day. And the wasted heat stored in two heat-ups of the refractory brick lining would be 2 times 24000 BTU/ ft2 times 1200 ft2 or 57.69 million BTU per day. This loss of stored heat is over twice the loss due to heat flow out through the walls at steady state.
By contrast, with the fiber lining on the same daily schedule the wasted heat at steady state would be (12 h)(880 BTU/ ft2.h)(1200 ft2) or 12.7 million BTU per day. And the wasted heat in two heat-ups of this refractory lining would be (2)(500 BTU/ft2)(1200 ft2) or only 1.2 million BTU per day. So the comparison of the wasted heat per day is as follows:
                                                9 in. Brick           4 in. Fiber                      Fiber saving
   Firing (steady state)                 27.9         –         12.7           =          15.2 million BTU
   Cycling (heat-up)                     57.6         –          1.2            =          56.4 million BTU  

So, that shows how the energy saving effected by using the fiber instead of a dense brick lining is much more important in heat-up part of the kiln operating cycle than it is in the firing or working part. But the time saved in heat-up and cool-down can be very important too. In this case we might be able to cut the non-productive time from 6 h down to 2 h and thus fire three loads per day. That would be a 50% increase in the productivity of this kiln. The bottom line strongly favours the use of fiber linings where the processing environment permits.
Now, how are these fiber refractory linings installed?
Techniques of Installation of Fiber Refractory Linings
There are three basic installation techniques:
(1) Layer;
(2) Edge-stacked;
(3) Newer modular concepts.
Layer or “wallpaper” construction involves applying a number of layers of material by impaling them over special metallic or ceramic anchors. This has been the most commonly used method of construction. It allows lower temperature and/or lower density back-up insulation materials to be used as cold-face layers. Such materials are less expensive than the denser, higher temperature materials that must be used at the hot face. The construction technique is basically simple, but it is very important to use the proper anchor materials, to have the proper anchor density and positioning, and to make certain the joints in the various do not line up. Although the materials for a fiber lining may be more expensive than conventional IFB construction, installation labour cost is usually considerably lower. “The two main deficiencies of the layer approach are problems with the anchor system and shrinkage of the hot face layer.” Some anchor materials are listed in the following table:
                    TEMPERATURE USE LIMITS FOR FURNACE LINING
                  ANCHORING MATERIAL
Anchor Material
Use Limit (OF)
Type 304 stainless steel (SS)
Types 309 & 310 stainless steel (SS)
Inconel 601 (Trade name of Int’l. Nickel Co.)
RA 330 (Trade name of Rolled Alloys Co.)
Ceramic anchors
1500
1850
2250
2050
2600

Metallic anchors can not be used above 2250OF, and ceramic anchors are prone to thermal shock in many applications, such as in forge furnaces. At elevated temperatures, the shrinkage of the hot-face layer can cause joints to open up and can even result in tearing of the hot-face layer. Tearing will often occur at the anchors, resulting in loss of support for the layer. This is particularly troublesome in a roof or crown of a furnace or kiln. The flue in a furnace crown is also a problem in layer construction in high-temperature furnaces. If support anchors are brought in close to the flue opening, the metallic portion of the stud system is close to the flue and can fail due to exposure to too high a temperature. If the anchor is moved away from the flue opening, the hot-face refractory layer does not receive proper support and can sag.
The second construction approach is the edge - stacked blanket approach. In this technique, strips of fiber blanket are stacked up so that their edges are exposed as the hot-face. The strips can be anchored to the shell with hidden anchors, so that there are no exposed anchors at the hot - face. The layers are normally compressed to help compensate for shrinkage. The layer edges are more resistant to high velocity gases, which is an advantage over layer construction. However, the same high temperature material must normally be used through the entire lining thickness. This increases material costs. At elevated temperatures, the joints can open, leading to failure. This is more likely to occur if insufficient compression is used. Also, the thermal conductivity is measurably higher (30% or more) in the edge - grain configuration as compared to the layer configuration. This results in a thermally less efficient lining. 
Installation of Refractory Fiber Kiln or Furnace Linings: Modular Blanket Furnace Lining Module image
Fig: Modular Blanket Furnace Lining Module
A number of modular techniques have also been developed. These are designed to provide a very rapid installation, which decreases installation cost and furnace down-time for relining. They also provide a hot - face with no exposed hardware. The earlier modular approaches used edge - stacked blanket, usually in 12 by 12 inches modules. Another modular concept for fiber installation uses a vacuum - formed fiber “box” filled with blanket. One of the most successful concept or installation technique is an “accordion - folded” blanket as shown in the adjacent figure. The attachment hardware is near the cold-face, and the module is mechanically fastened to the kiln shell. Each module is held under lateral compression by bands and cardboard. The modules are installed in parquet fashion, and the bands and cardboard are then removed. The compression is thus released, and this compensates for shrinkage at elevated temperatures. This concept of installation extends the upper use temperature of fiber installations where it previously had been largely unsuccessful, such as in forge furnaces. However, as is the case for all refractories, proper installation is critical for a successful kiln or furnace lining. The suppliers of fiber blanket and modules also provide detailed instruction technical advice for their installation, including the selection and placement of attachment hardware.
Adding Insulation over the Existing Refractory Linings 
Often the existing refractory lining of heat processing equipment is in good condition but is inefficient from an energy standpoint. Much of the equipment currently in use was designed and built when energy was inexpensive. Insulation and energy conservation were not considered particularly important. This of course, is no longer true, and adding insulation to existing lining is receiving much attention as opposed to removing and replacing the old refractory.
There are only two places where insulation can be added to existing refractory linings. It can be added at the cold - face or it can be added at the hot - face. Adding insulation at the cold-face can be very effective in decreasing heat flow, which is desirable. But this results in a marked increase in the mean temperature to which the existing refractory lining is exposed. Drastic increases in the cold-face temperature of the original lining can occur which can result in actual failure of the working lining, in accelerated deterioration. The magnitude of the temperature increase will be greater when the original refractory lining has high thermal conductivity and when a considerable thickness of insulation is added.
It is very important never to place insulation over the existing structural steel or steel shell. Serious buckling or loss of structural integrity can result. Before adding any refractory insulation to the cold - face of an existing furnace or kiln, very careful two-layer heat - flow calculations must be performed (heat-flow calculations for  two-layered refractory lining and their thickness has been discussed a separate post) to determine what the new temperature profile will be after and to decide whether this is safe. For example, several inches of ceramic blanket insulation added to the basic brick in the crown of a glass tank regenerator may increase the cold face temperature of the brick from 400 - 500OF to over 2000OF. Many basic brick compositions lose structural rigidity at temperatures above 2000OF, and the crown might start a steady, disastrous slumping resulting into a total failure. Also, adding insulation to the cold face of a hard brick periodic kiln can often increase the heat storage more than the heat-loss is decreased. The result is an increase in fuel consumption, not a decrease. Careful heat capacity calculations such as we illustrated in the beginning of the present discussion must be performed, making use of the new temperature profiles as well.
Adding insulation to the hot face of an existing kiln or furnace lining is usually more difficult to accomplish. The main problem is usually finding an adequate method of attachment. One technique is to drill holes in the existing refractory, mortar-in appropriate anchors, impale layers of fiber blanket on to the anchors, and attach anchor washers. All of the advantages and problems of layer linings apply. Another technique is to mortar-on modules made of edge-stacked blanket. This system actually works surprisingly well. The existing refractory must have reasonable structural integrity, and the surface should be clean and not glassy. Very thick vacuum-formed fiber blocks have been sawed into appropriate veneering modules, which can also be mortared on to a refractory surface. These materials are denser but lack the flexibility of blanket and thus, do not conform to surface irregularities as easily. They offer better insulation and greater resistance to mechanical abuse. In either case, a high quality air-setting mortar which has high water retention must be used. Insulating materials can quickly “dewater” mortars with poor water retention.
Adding fiber refractory insulation at the hot face lowers the exposure temperature of the original refractory and can significantly extend its service life. However, the insulating material must be able to withstand the operating conditions in the process involved. Often the available materials can not do this, or there is insufficient room to install insulation, or no adequate installation technique can be devised. The use of a plastic (i.e. trowelled) or gunned monolithic refractory might well be considered in such case.
There is a very considerable and specialized technology involved in using fiber refractory materials. Since energy costs are likely to increase continually, interest and use of these materials seem likely to increase as well. But you should by now appreciate that fibers are just one available form of insulating refractories. They are clearly superior in some applications, inappropriate or impractical to install in others. The wise user employs both calculations and the “track record” of experience to make his choice.       
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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)
1239
201
220
23400
4603
1546
424
175
182
98
22
5.75
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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March 18, 2010

Insulating Refractories (Part - I)


Insulating refractories are thermal barriers that keep in the heat and save energy. Furnaces used for melting, heat treatment, heat regeneration or for any other purpose demand 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. As the cost of energy has increased, the role of insulating refractories has become more important. Not too long ago, energy costs were low and stable, while the costs of insulating materials and, particularly, installation labour were moving northwards. Those circumstances dictated the use of minimal insulation. The situation is quite different now. The use of considerable quantity of refractories is socially and economically justified. With today’s energy costs at such higher levels has come the development of a wide range of new insulating refractory materials and technology of high-temperature insulation which are capable to restrict the escape of heat even at a much elevated temperature. Instead of going direct into the discussion of insulating refractories, their types, raw materials, manufacturing, properties and applications etc., here we will first review some of the fundamental technology of high-temperature insulation.     
The function of insulating refractory is to reduce the rate of heat flow (heat loss). Although it is not possible to totally prevent the flow of heat energy when there exists a temperature differential between two points, but it can be retarded. There are three mechanisms of heat transfer that we must understand. These are conduction, convection, and radiation. We must consider all these three mechanisms when we study the overall conductivity of a given material.
Heat transfer by Conduction occurs via the transfer of energy from atom to atom (or molecule to molecule) in a material. Atoms vibrate faster in higher temperature as they possess more energy. This energy will be passed to the adjacent atoms having lower energy. Since atoms and solids are bonded to one another and are in close contact, conduction in solids is higher than in liquids. Metals, especially, have high rates of conduction because both the atoms and their electrons conduct the electrons much more rapidly. Liquids generally have lower conduction rates than solids because of their lack of regular structure and strong bonding. Gases have much lower rates of conduction since their molecules exist at much lower concentrations and are in relatively infrequent contact. So, within metals, dense ceramics, and dense refractories Conductivity is the main mechanism of heat transfer.   
Energy transfer by Convection relies on the mass movement of a fluid. The moving fluid may be either a liquid or a gas. Convection does occur horizontally; but it depends on the gravitational force of the earth. Again, in case of dense refractory bricks heat transfer through this process can not happen since there is no fluid for convection.
Radiation process of heat transfer does not require the presence of any material. Radiation occurs most readily through empty space. The sun radiates energy through space to earth. Similarly all hot bodies radiate heat, and if they are hot enough they also radiate visible light which we call as glow.
When one studies heat transfer mechanisms in industrial processes, all three modes of heat (or energy) transfer must be considered. In a high temperature furnace or kiln, for example, energy is transferred from the heat source i.e. a burner to the material being heated and to the surrounding furnace refractory walls by all the three processes. The amount of energy transferred by radiation increases dramatically as the temperature increases. It is the dominant heat transfer mechanism at high temperatures. The load and the refractories of the furnace wall absorb energy, get hot, and re-radiate energy. The moving gases within the carry heat with them and transfer it when they come in contact with cooler solid. A small amount of gas conduction occurs, and conduction is the main process of transferring energy or heat from the surface of the solid or liquid load to its own interior.
One of the prime roles of a refractory is to withstand the effects of heat usually in a hostile environment. That is why for the selection of refractory and its designing Thermal Conductivity is one property which one has to consider. Usually one would like to have a refractory with low thermal conductivity so that heat may be more effectively contained within a furnace or kiln. Sometimes, however refractories and materials having high thermal conductivity are desired. For example, a protective muffle in certain ceramic kilns is designed to prevent combustion gases from reaching the ceramic ware. It must transfer as much heat to the ware as possible, so conductive ceramic materials like silicon carbide are often used for muffles.       
Since insulation refractories find application in processes involving thermal energy, an understanding of thermal properties especially, thermal conductivity of these refractories is quite important. Thermal Conductivity of a refractory material, k, is a measure of the amount of heat that it will allow to pass under certain conditions. Thermal conductivity can be defined as the quantity of heat transmitted through a material in unit time, per unit temperature gradient along the direction of flow and unit cross sectional area. First, let us understand the material conditions affecting this thermal property of a refractory brick whether it is insulating or normal brick, and then the most common method used to measure (or calculate) the same. While there are many factors affecting the thermal conductivity of refractories, some of the most important are [Reference: J.E. Burke, Progress in Ceramic Science, Vol. 2, Ed., Pergamon Press, Chapter 4, 1962]: 
1. Temperature
2. Complexity of structure (crystal and microstructure)
3. Defects (impurities, solid-solution, and stoichiometry)
Temperature dependence of thermal conductivity for several materials graph
                  Fig: Temperature dependence of thermal conductivity for several materials
The temperature dependence of thermal conductivity of several materials is shown in the adjacent figure. In general, the thermal conductivity is expected to decrease with increasing temperature when the temperature exceeds the Debye temperature. The Debye temperature is a characteristic temperature for a given material and may be below or above room temperature. The structural features such as, anisotropic arrangement of ions, relative mass difference between anion and cation, pores, and grain boundaries etc. do affect thermal conductivity of a material. Spinel (MgAl2O4) for instance, has a thermal conductivity lower than that for either MgO or Al2O3. Another example is reducing the thermal conductivity of a solid by introducing porosity and this is the most common technique of manufacturing insulating refractories.
Fortunately for us, the thermal conductivity of a refractory material is ordinarily measured in such a way as to account for all of the heat transfer processes that happen to be operating in that material. We do not have to unscramble them or deal with tem separately, for most ordinary purposes. Once that property is known for each material in the vessel, some very sophisticated calculations can be performed to find out where the heat goes in a given operation. In the next following lines we will discuss only the simplest of these calculations. This will be enough to enable you or someone to select among various insulating refractories and also to measure what will be the refractory lining thickness.
Imagine a large flat slab or wall of refractory, whose hot face (hot side), is at some fixed temperature, Th. Its cold face (cold side) perhaps in contact with a steel shell, is at some lower temperature, Tc. We will call the thickness of the refractory X. Let us assume that the heat is supplied to the hot face at some fixed rate by process fluids, and that heat is removed from the cold face (may be by the steel shell and the air outside it) at exactly the same rate. Two things then follow: (a) heat flows through the refractory at exactly the same rate as well and (b) temperatures Th and Tc do not change with time. This is called Steady State situation. If we call some amount of heat H flows in time interval t then the rate of heat-flow Q would be H / t. If you think about it, you will understand that this rate of heat-flow or heat transport has to be proportional to the area of refractory wall, A, through which heat is flowing. One mathematical equation connects all of these things at once is:
Refractory Lining Technology

 where, k is the value of thermal conductivity.


To use this equation, we will adopt a set of English units that engineers in the fields of processing and refractories are familiar with. The unit of heat energy, the BTU (British thermal unit), is defined as the amount of heat that will raise the temperature of 1 pound of water by exactly 1OF. The unit of time will be hour (hr). We shall take units of area A in square feet (ft2), the thickness X in inches (in.) and temperature in OF. Clearly if the situation described by A, X, Th, and Tc is held fixed but different materials are studied, the rate of heat transport (Q or H/t) will be proportional to the k (thermal conductivity) of each material. Since k is a property of each material, we can get different values for the rate of heat transport by choosing different materials or mixtures of them. Thermal conductivities i.e. values of k for different materials are measured in the laboratory and published. We can use them in calculations with the above equation. Only we need to make sure that the units of k are (BTU.in)/(ft2.OF.hr).
In fact, k is numerically equal to the rate of heat transport when the slab area (here, area of the refractory or furnace wall) is exactly 1 ft2 and the temperature gradient is exactly 1OF/in. The table below lists some of the typical values of thermal conductivity (k) for different solid materials: some metals, some ordinary “working” refractories, some insulating and some highly conducting refractories. Given below are some examples of how to calculate Heat Loss or Heat Transport and Thickness of Refractory Lining:      
Suppose we have a furnace lined with Superduty refractory brick, and the total wall area of this furnace is 1350 ft2 and also suppose the refractory lining thickness is 12 inch. Say, the process we are conducting in this furnace keeps its hot-face temperature at 3000OF. With thermocouples we find that the cold-face is at a steady temperature 600OF. Then, what will be the rate of heat loss through all the walls of this furnace ?    
We find from the table given below that k for Superduty brick is 9.5. Then by putting all the given numbers into our heat transfer equation mentioned above we get the rate of heat flow (heat loss) Q as per -
Refractory Lining Technology

Refractory Lining Technology





It will be instructive to check here as how much less refractory it would take to match this heat loss keeping all the conditions same if we used, say, an insulating refractory firebrick whose thermal conductivity (k) value is 3.0, also taken from the table below. Suppose that this insulating brick can survive at 3000OF, to make the question reasonable. Here we will find out the required thickness of the insulating brick lining for which we first rearrange the heat transfer equation to be explicit in X so that we can solve it for the refractory thickness. Then by putting all the given numbers into the equation except 3.0 for k, we get -
Refractory Lining Technology
That is 3.8 inch of insulating firebrick has the same heat transfer resistance as 12 inch of conventional Superduty refractory firebrick ! We would be na├»ve to replace the one refractory by the other until we learn more; but the effectiveness of insulating refractories in containing heat is impressive. If we were to keep the refractory lining thickness at 12 in. for example, and solve our heat transfer equation with k = 3.0, we would find that the total rate of heat loss is only 810,000 BTU/hr., instead of 2,565,000 BTU/hr. Now imagine how much thousands of dollars we could save per month in fuel costs !     
However, on practical ground or real - life, calculations are never this simple for numerous reasons. For one thing, the value of thermal conductivity itself changes with temperature as the relative contributions of conduction, convection and radiation change. The second complication we will mention here is that in most cases the refractory lining of a furnace or kiln is done with several refractory layers of varying qualities:
1. A working face of refractory layer or, interior layer of refractory lining that is exposed to the process;
2. The refractory lining between the furnace or kiln shell and working lining, often referred to as the Safety Lining or Insulating Lining. Insulating linings are used to limit heat loss and to maintain the vessel (furnace) shell temperatures at reasonable levels.
Such refractory lining arrangements definitely complicate the heat transfer calculations. But even with the simple introduction about insulating refractories what we have given above, you can appreciate that a process operator can intelligently design a refractory lining that will endure its use temperature and chemistry, and at the same time meet the restrictions on refractory lining thickness or on heat loss that are specified for the situation.
In our next post Insulating Refractories (Part - II) we will look at the different types of insulating refractories and their manufacturing etc.                      
      Table :  Typical Thermal Conductivity Values
Refractories / Materials
k (BTU.in/ft2.OF.hr)
Metals (dense solid)
Copper
Aluminium
Gold
Silver
304 Stainless Steel
310 Stainless Steel
1020 Carbon Steel

2500
900 - 1500
2060
2900
113
96
360
Dense Refractories
Silica Brick
Superduty Brick
Periclase
High Alumina
Chrome - magnesite
Zirconia

13
9.5
20 - 50
10 - 40
14
5
Insulating Refractories
Insulating firebrick 2800
Insulating firebrick 2600
Insulating firebrick 2300
Ceramic Fiber Blanket 4 pcf (lb/ft3)
Ceramic Fiber Blanket 8 pcf (lb/ft3)
Vacuum formed board
Backup insulation

2.5 - 3.0
2.0 - 2.5
0.9 - 1.3
0.6 - 3.0
0.35 - 2.0
0.4 - 1.5
0.3 - 1.0
Conducting Refractories
Silicon Carbide
Baked Carbon
Graphite

100 - 200
300 - 800
500 - 1200