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Business sectors that use energy-intensive processing equipment, such as the ceramic industry, are actively reviewing the efficiency of the refractory and insulating systems in these units. While many traditional high temperature kiln linings offer long life, these refractory products also act as heat sinks, absorbing BTUs to heat the lining itself. Moreover, they generally have a high thermal conductivity that leads to excessive heat loss through the lining (see Figure 1).
The energy wasted by inefficient heat management systems is fueling the change to more efficient kiln lining materials, such as insulating firebrick (IFB), ceramic fiber modules, non-ceramic fiber blankets and modules, and microporous insulation.
Insulating FirebrickLightweight insulating firebrick (IFB) offer good thermal efficiency, low heat loss and structural properties that minimize the need for support steel. Although IFB have been around for a long time, they have been largely overlooked by users, many of whom transitioned from dense firebrick or castables directly to ceramic fiber modules.
IFB are available in a range of temperature grades up to 3200?F (see Table 1). Lower-temperature grades, rated for use to 2000, 2300 and 2500?F, have densities of 30 to 42 lbs/ft3 (480 to 670 kg/m3). Thermal conductivity ranges from 1.6-2.0 BTU in/hr ?F ft2 (0.23 to 0.29 W/m K) at a mean temperature of 2000?F make these materials approximately three to six times more thermally efficient than similar temperature-rated dense castables or firebrick.
High-temperature-grade IFB, for use at 2600, 2800, and 3000?F, have densities ranging from 50-80 lb/ft3 (800 to 1280 kg/m3). They offer a thermal conductivity in the range of 2.4-3.8 BTU in/hr ?F ft2 at 2000?F, making them four to eight times more thermally efficient than dense castables or firebrick in the same temperature range. Some of the newer IFB products have a structure more like the lower temperature IFB, with thermal conductivity 25% below conventional 2600?F IFB.
For very high temperature usage up to 3200?F, bubble alumina IFB are often the choice. They are especially well suited for cyclic kilns for firing high alumina or technical ceramic materials, where their unique chemistry resists spalling on the typical high temperature IFB surface.
Both the high and low temperature IFB are durable products that withstand a variety of kiln conditions. They are good choices for lining ductwork, stacks or other applications where gas velocity is an issue. IFB with a special anorthite chemistry (Ca-Al2-Si) withstand chemical attack very well with excellent resistance to alkalis, such as sodium. These IFB also have excellent hot load resistance and perform very well where there are mechanical requirements in the kiln design.
Ceramic Fiber ModulesLightweight fiber modules offer low thermal conductivity and less heat storage than IFB or castables. They are available in a variety of ceramic fiber compositions, attachment anchoring systems and construction designs for lining industrial furnaces. Lighter than IFB, module densities range from 8-15 lb/ft3 (128-240 kg/m3). Thermal conductivity ranges from 2.25-2.75 BTU in/hr ?F ft2 (0.32-0.4 W/m K) at a mean temperature of 2000?F (1095?C).
Ceramic fiber modules offer greater thermal efficiency than IFB and insulating castables, particularly in cyclic operations, and are much easier to install. However, they are not structurally supporting and are somewhat more vulnerable to chemical attack and velocity, as the surface areas of the modules may react with corrosive chemicals causing fiber deterioration.
Ceramic fiber modules are ideal for ceramic processing equipment operating up to 2350?F (900 to 1290?C). Traditional whiteware kilns operating near the top of this temperature range transitioned from high temperature board/blanket designs to module systems over the last 15 years for installation ease and cost reasons.
For higher temperature use, alumina-silica-zirconia composition fiber modules can withstand operating temperatures to 2450?F (1340?C). New modules, which contain a chromia addition in the benign trivalent state, can withstand continuous temperatures as high as 2500?F (1370?C). The chrome-containing modules offer lower thermal shrinkage at higher temperatures than other alumina-silica based modules. For certain ceramic applications up to 2800?F (1500?C), high alumina mullite fiber modules (72% Al2O3) are the product of choice for fiber type linings.
Besides use as kiln lining materials, ceramic fiber modules have found widespread use in low mass kiln cars. Both modules and insulation log material are commonly used in conjunction with hollow refractory tubes and lightweight setter plates, replacing the heavy brick and castable found in old-style kiln cars. These materials harden on firing and provide a rigid monolithic block of fiber insulation. Due to the lightweight nature of the fiber modules and log material, cars that cycle in tunnel kilns experience high thermal storage savings, typically 30% or greater, due to their low mass to heat up.
Non-Ceramic Fiber Blankets and ModulesNon-ceramic fiber blankets and modules are typically made of a special calcia-silica-magnesia chemistry. Compared to traditional high temperature insulation, these products have been formulated to have low biopersistence, or reduced capability to persist in the body. In the early 1990s, the first bio-soluble fibrous insulation material, Superwool® 607™, was introduced to the market. This amorphous wool material contains no organic binders that cause outgassing or undergo changes from atmospheric conditions during their life cycles. In addition, its low density and thermal conductivity, which approximates that of traditional ceramic fiber insulation, reduces energy consumption.
Maximum continuous use temperatures of Superwool 607 and the new Superwool 607 MAX™ insulation product are 1822?F (1000?C) and 2200?F (1200?C) respectively. Both materials exhibit very low shrinkage at their use limit (<2%) and are available in a wide variety of product forms, including blankets, modules, boards, flexible paper and mastics.
Microporous InsulationMicroporous silica material is the most thermally efficient insulation on the market today, with a thermal conductivity less than even still air. The thin, lightweight products, available in board or stitched panel forms, are a highly effective backup to IFB, castables or ceramic fiber. Because of its low density, microporous insulation material is particularly useful in areas in which thickness or weight is an issue. Just 1⁄4- to 1⁄2-in. (6 to 13 mm) thick microporous silica material as a cold face lining will result in a significant reduction in heat loss. In fact, 1 in. (25 mm) of microporous silica insulation delivers the same thermal efficiency as 3 to 4 in. of standard ceramic fiber blanket or board.
Microporous insulation also offers high compression strength, low shrinkage at high temperatures and good resistance to vibration. It is a good back-up insulation material where space is at a premium, such as near flues and burner blocks, and was used in the recent tunnel kiln reline at the Thermal Ceramics facility (featured in a Ceramic Industry February 2001 article).
Partnering for SuccessAnyone who is looking at upgrading their insulating and refractory systems should consider all the options before proceeding. In some cases, a combination of materials may work best.
Using parameters such as hot face temperature, target cold face temperature and energy consumption, most suppliers will work with you to design a customized system that will cut heat loss and lower fuel costs.