Showing posts with label Fiber Blanket. Show all posts
Showing posts with label Fiber Blanket. Show all posts

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 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:
                  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

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)
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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