This article was originally published on February 4, 2015.

Selecting products made with the right manufacturing process makes the difference.

Engineering design and the lining materials chosen are key factors in controlling the efficiency and energy usage of equipment used in iron and steel applications. As a result, it is critical that industrial designers understand the advantages and disadvantages of the materials they choose. For example, it is especially important to select insulating firebricks (IFBs) that minimize energy losses. Recent studies conducted on IFBs produced using the three most common manufacturing methods – cast, slinger and extrusion – show that the cast process offers the lowest thermal conductivity and provides the greatest energy savings.

IFB Manufacturing Techniques Vary Widely in Ability to Control Energy Losses

The versatile IFB is used in numerous iron and steel applications, including: blast furnaces, ductwork in direct-reduction processes and reheat furnaces, backup insulation in coke ovens, and in tundishes and ladles. They are also used extensively to form the sidewalls, roofs and hearths of a wide variety of heat-treatment, annealing and galvanizing lines. Figure 1 shows their use in a coke oven stack (top) and in a tunnel kiln (bottom).

IFBs are manufactured using a variety of techniques, the most common of which are casting, slinger and extrusion. The cast process uses gypsum plaster as a rapid-setting medium for a high water-content clay mix containing additional burnout additives. The slinger process is a form of low-pressure extrusion of a wet clay mix containing high levels of burnout additives. It includes an additional processing step in which the semi-extruded material gets “slung” onto a continuous belt to generate additional porosity before drying and firing. The extrusion process forces a damp-clay mixture containing burnout additives through an extrusion nozzle, where the extruded material is subsequently cut into bricks, dried and fired.

The brick chemistries and microstructures produced can differ widely among these methods, leading to a extensive variety of thermal conductivities within products of the same temperature rating. This variation, in turn, has an effect on the ability of different IFB types to control energy loss.

Comparing Manufacturing Methods

To understand the effect of the three main IFB manufacturing methods on thermal conductivity and energy-loss behavior, researchers conducted a study to quantify the differences in energy usage that can be achieved within Class 23 and Class 26 IFBs.

Figure 2 shows the thermal conductivity of the IFBs tested, a critical property since IFBs are primarily used for their insulating abilities. In each class of IFB, cast brick has the lowest thermal conductivity, followed by the slinger-produced brick, with the extruded brick displaying the highest conductivity.

Researchers designed two identical electrically heated laboratory muffle kilns (Fig. 3) and conducted energy-usage studies comparing the IFB bricks. They lined the first kiln with Class-23 cast IFBs, and this formed the benchmark since they had the lowest thermal conductivity in the class. Test results are shown in Table 1.

Figure 4 is a thermograph of the kilns during the 1000°C (1832°F) firing test. The cast-IFB-lined kiln is on the left. It shows how much heat is wasted through the body of the kiln lined with the higher thermal conductivity IFB and how the surface temperature of the kiln becomes overheated. This shows both the effect of wasting energy costs and health and safety issues caused by hazardous working temperatures.

Significantly less energy was needed to run the test kiln through a 1000°C firing cycle with the cast IFB compared to the extruded IFB (37% less for Class 23 and 38.5% less for Class 26). These energy-usage differences are due to the differing thermal conductivities of the IFBs. In materials of similar chemistry, thermal conductivity is controlled by the structure of the material. The different manufacturing methods of the IFBs studied produce materials with inherently different macrostructures and microstructures, and it is these that control the thermal behavior of the products. For example, Figure 5 illustrates the differing macrostructures of the Class-23 IFBs studied. The texture of the IFBs is finest for the cast product and coarsest for the extruded product.

Energy-Saving Calculations

After conducting laboratory tests demonstrating the potential to minimize energy usage through appropriate selection of IFB for an installation lining, researchers then ran heat-flow calculations to understand the effect on actual industrial installations. Calculations were done to assess energy running costs in strategic locations of two annealing applications that use IFBs as the lining material: a catenary strip-annealing furnace and a cast-iron part-annealing furnace.

The modeling was performed using the most common real-life IFB lining arrangements, where walls are normally built up using standard brick sizes, while roofs are constructed from special pre-assembled roof blocks. Figure 6 shows the results. The top graphs show heat-flow calculations for the catenary strip-annealing furnace wall using cast IFB (left) and extrusion IFB (right). The bottom shows heat-flow calculations for the cast-iron part-annealing furnace roof using cast IFB (left) and extrusion IFB (right).

Table 2 shows the significant differences that can be achieved for casting temperatures when using different IFB types. Using cast IFB produces much lower casting temperatures than extruded IFB. The lower surface temperature obtained using the cast IFB also produces a more comfortable working environment for operators and minimizes the risk of burns due to operators coming into contact with the surface of the installation.

For the catenary strip-annealing furnace wall, the heat-flow calculations show that the lining with the extrusion IFB requires 271 W/m² more energy to maintain the 1200°C (2192°F) operating temperature than the lining with the cast IFB due to the lower thermal conductivity of the cast compared to the extruded IFB. The difference in energy consumption between the two simulated furnace walls equates to a savings of 42,450 m³ of natural gas per year using the cast IFB compared to the extrusion IFB. Assuming a gas price of $0.192/m³,
this equates to an annual savings of $8,150/year for this wall section only. Assuming a furnace-wall lining life of 10 years, the total savings over the life of the kiln lining would be $81,500.

The savings on the complete structure would be significantly larger. A 127 m² working area in the wall of the catenary strip-annealing furnace would need approximately 7,200 standard-sized IFBs. Although the cast-IFB price is a little higher, the example shows an initial payback period of just over three months. There would be continuous cost savings due to the lower energy requirements for the rest of the 10-year service life.

For the cast-iron part-annealing furnace roof, the lining with the extrusion IFB requires 434 W/m² more energy to maintain the 930°C (1706°F) operating temperature than the lining with the cast IFB. For the 46.5 m² working area, the difference in energy consumption between the two simulated furnace roofs equates to a savings of 17,615 m³ of natural gas per year using the cast IFB compared to the extrusion IFB, which equates to an annual savings of $3,382 for just this small roof section. Assuming a furnace-roof lining life of 10 years, the total savings over the life of the kiln lining would be $33,820. The savings on the complete structure would be significantly higher. The 46.5 m² roof area would need approximately 2,600 standard-sized IFBs, so the payback for using cast IFB is less than three months.

Conclusion

By monitoring energy usage in laboratory kilns lined with IFBs manufactured by different processes and by modeling the effects on heat flow of using these same IFBs in two important iron and steel applications, research has demonstrated that IFBs manufactured by the cast process offer the lowest thermal conductivity available today at application temperatures and provide the greatest energy savings.

For more information:  Contact Wendy Evans, marketing communications, Morgan Advanced Materials Thermal Ceramics, 2102 Old Savannah Road, Augusta, Georgia 30906; tel: 706-796-4200; e-mail: wendy.evans@morganplc.com; web: www.morganthermalceramics.com