- THE MAGAZINE
Microporous insulation has been used successfully in Europe for years in a variety of ceramic, aluminum and steel applications, and it is also being used in an increasing number of similar applications in North America. Because of the insulation's low thermal conductivity, it offers significant energy and space savings compared to many traditional refractory insulation materials.
Recently, a new microporous insulation* composed of inorganic oxides (primarily fumed silica) has been introduced into the North American market. It offers lower thermal conductivity than some existing forms of microporous insulation, enabling users to save more space, reduce more weight and significantly reduce energy consumption. The new insulation also provides a high level of thermal stability, consistent operating temperatures and easy fabrication. In fact, although the price of the new insulation tends to be higher than ceramic and glass fibers or mineral wools, the savings in energy, space and/or weight can often justify the added cost.
With the new microporous insulation, manufacturers can maximize their operating efficiencies.
The Microporous PrincipleMicroporous insulation significantly reduces the passage of heat energy through the insulation material by minimizing the three modes of heat transfer: convection, conduction and radiation (see Figure 1).
Convection is minimized due to the microscopic voids that the ceramic particles and fibers form. These voids are smaller than the path necessary for air molecules to travel effectively and, as a result, do not allow air molecules to carry heat easily through the material.
Conduction is minimized by limited particle-to-particle contact of the various components that comprise the microporous formulation. Although the particles are close enough together to form voids that minimize air convection, they are also far enough away from one another to minimize the solid conduction of heat energy.
Finally, radiation is minimized through the use of specially selected particles called "opacifiers." These particles are designed to reflect, refract and re-radiate radiation energy and prevent it from traveling easily or freely through the microporous material.
Enhanced PerformanceThe new microporous insulation is composed of inorganic oxides (primarily fumed silica) and features 10-15% lower thermal conductivity than many of the existing microporous materials currently sold in the North American market. This extremely low thermal conductivity is primarily due to the use of improved opacifiers that block heat transfer due to radiation. The insulation's low thermal conductivity can be used to increase the capacity of ladles, kilns, industrial ovens and commercial appliances while maintaining or improving thermal performance. It can also reduce weight (which, in turn, can reduce structural requirements for furnaces and other high-temperature vessels), save energy and reduce operating costs by reducing heat loss compared to conventional refractory linings. The insulation is designed to withstand continuous operating temperatures up to 1832 degrees F. Additionally, due to its superior insulating characteristics, processes can be easier to regulate and control, which often results in a more consistent final product.
The microporous insulation is available in several off-the-shelf forms, including a rigid board in thicknesses up to 50 mm (2 in.); a thin, flexible "flexliner" in thicknesses up to 10 mm (0.4 in.); and a vacuum insulation panel designed for lower-temperature insulation in appliances, packaging and other cold storage applications. All forms are completely encapsulated to eliminate the penetration of moisture. Additionally, both the board and flexliner forms can be easily cut to size and installed with commonly available tools using a high-temperature adhesive or mechanical anchoring system. (The vacuum insulation panels must be fabricated by the supplier.)
Ceramic ApplicationsIn the ceramic industry, the new microporous insulation can offer benefits in both tunnel and roller kilns.
Tunnel Kilns. The board form of the insulation is being used as backup insulation in specific high-temperature zones of ceramic tunnel kilns, which are used to fire tableware, sanitaryware, brick or ceramic tile. Due to the extremely low thermal conductivity of the new insulation, the energy required per ton of manufactured product is greatly reduced. Other advantages include a reduction in cold face temperature from 160 to 140 degrees F, a reduction in heat loss of 30%, an improvement in product quality, lower operating costs and easy on-site fabrication.
Roller Kilns. The board form of the new product can also be used as backup insulation in roller kilns. Advantages in this application include an average improvement in heat loss of approximately 25% versus conventional insulation materials; an increase in the usable volume of the kiln; improved thermal consistency in the kiln, which results in improved product quality; and easy on-site fabrication.
Aluminum ApplicationsSeveral applications in the aluminum industry can benefit from the performance advantages of the new insulation, including aluminum melting furnaces, aluminum launders and over-the-road transfer ladles.
Aluminum Melting Furnaces. These furnaces must maintain an optimal temperature distribution throughout the melt to keep a homogenous liquid, which is imperative to achieving a quality end product. Refractory insulation systems are crucial to ensuring proper temperature control throughout the aluminum melter. In these applications, the board or flexliner form of the new insulation material can be used as backup insulation to reduce heat loss, improve product quality and reduce operating costs. In most cases, the insulation can provide a reduction in insulation thickness of 50% compared to ceramic fiber boards, as well as a 30% heat loss reduction. The new insulation can also provide a 26 degrees F reduction in the cold face temperature compared to a similar thickness of ceramic fiber board, which leads to improved product quality and an extended furnace life.
Aluminum Launders. When moving molten aluminum from the melters to the casting operation, aluminum processors incorporate the use of launders to distribute the molten aluminum. Insulation is used to maintain the temperature of the molten material at approximately 1,400 degrees F (750 degrees C) to avoid "freezing" the molten aluminum and to maintain a consistent flow throughout the distribution system. Insulation materials are also used to minimize heat loss in the launders, which can reduce the requirement for superheating the aluminum. The flexliner form of the new microporous insulation can typically be used in this application, as long as the required insulation thickness is between 0.125 and 0.39 in. (If the required thickness is between 0.125 and 2 in., the board form can be used.) Advantages include a reduction in insulation thickness of 1.5 in. compared to conventional insulation, such as calcium silicate board; a reduction in cold face temperature of 43% compared to a similar thickness of calcium silicate board; and a heat loss reduction of up to 67%, which provides improved product quality and extended launder life.
Over-the-Road Transfer Ladles. Aluminum manufacturers often transfer molten metal between plant sites using "over-the-road transfer ladles." In this application, the new insulation provides a significant improvement in thermal conductivity--for example, just 2 in. of the board form of the new insulation can provide the same heat loss and cold face benefits as 7 in. of mineral wool board. This provides increased ladle capacity and reduced overall operating costs. Other benefits include the ability to maintain the current lining profile, minimize heat loss and reduce energy consumption.
Steel ApplicationsIn the steel industry, tundish insulation and steel ladles are two potential application areas for the new insulation.
Tundish Insulation. During the continuous casting of steel, tundishes are used as a reservoir to hold the molten steel and maintain a continuous flow of material to the caster. These tundishes are insulated with refractories that are backed up by insulation materials to minimize heat loss and cold face temperatures and allow for extended tundish operation. The flexliner form of the new microporous insulation can be used as backup insulation in these applications to reduce heat loss, improve product quality and reduce operating costs. The insulation thickness can be 1.1 in. less than ceramic fiber board with the same level of performance, which increases the usable volume of the tundish. The insulation also reduces the cold face temperature by 76 degrees F compared to similar thicknesses of ceramic fiber board; lowers heat loss by 40%, which results in improved product quality and reduced operating costs; and improves temperature homogeneity.
Steel Ladles. Because steel ladles are required to hold molten steel at temperatures of approximately 3,000 degrees F for extended periods of time, efficient refractory insulation systems are needed to improve productivity and lower production costs. The insulation must also be as thin as possible to increase ladle volumes without compromising thermal performance. The flexliner form of the new microporous insulation has been successfully used in this application. Advantages include an increase in the usable volume of the ladle by up to 10%, which results in reduced operating costs; lower cold face temperatures, which improves plant safety; improved energy efficiency of the unit by up to 50%; and easy on-site fabrication. In some cases, a castable or gunning mix can be placed over the insulation to further increase performance.