SPECIAL REPORT/REFRACTORIES: Back it Up

February 1, 2007
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Back-up insulation plays a very important role in the thermal performance of refractory lined kilns that are used to fire ceramic products. Commonly used back-up insulation is typically lower in density relative to the hot face refractory, therefore serving to decrease the overall density of the refractory lining and lowering both the heat loss and heat storage of the refractory system.

The amount of thermal benefit gained from an efficient refractory back-up system is a direct function of the number of days the kiln is in operation. In general, ceramic kilns are continuous operations (tunnel kilns) with hard refractory hot face or working linings. These types of kilns are excellent candidates for an engineered back-up insulation to improve kiln efficiency.

Tunnel kiln roof prior to back-up lining installation.

Traditional Methods

Traditional methods of refractory back-up insulation usually involve cutting and fitting low-temperature boards or blanket systems in place. These materials must be cut to fit around the required anchoring system for the hot face refractory materials, or around other structural members of the kiln. The cutting of the back-up material often results in areas of heat loss at the seams left at board joints or anchor penetrations.

Due to the cutting and material handling involved, installing back-up material in this manner can also be very labor intensive and expensive. Many low-temperature rated back-up materials contain a significant amount of organic binder that provides green handling strength. Over time and exposure to the temperatures inherent to the firing process, the organic component of these products burns out, and the remaining back-up material may be susceptible to potential settling and degradation. This can result in decreased kiln efficiency or problems with hot spots on the steel casing.

Tunnel kiln roof after back-up lining installation.

A Foam Alternative

Recently, FS Sperry Co. of Memphis, Tenn., installed a back-up lining in a tunnel kiln roof for a brick manufacturer using insulation that is a combination of ceramic fiber and refractory cement.* This material, which sets to an installed density of 25 pcf, is basically inorganic and is rated for a continuous use limit of 1800°F, providing a significant safety margin for most back-up applications. It is installed using a foam-based gunning process that serves to minimize airborne fiber exposure during installation.

The three-component system includes specially conditioned bulk ceramic or soluble fibers, an inorganic binder, and an organic foaming binder. The installation process combines the bulk fiber material with the binders in a patented mixing mechanism to create a homogeneous foam/fiber mixture. The installation machinery propels this mixture through a feed hose and nozzle, and the material is then gunned onto the target surface.

The application system for the foam/ fiber back-up insulation is specially designed to control airborne fiber levels. Unlike "sprayable fiber" technologies that feed dry fiber and binder in separate streams that are partially combined once the product leaves the installation nozzle, the fiber in this system is completely coated within the foaming binder solution while in the mixing chamber, significantly reducing the potential for airborne fiber release into the surrounding environment.

During the brick manufacturer's installation, FS Sperry found that the system installed up to three times faster than board type products and eliminated many material handling issues associated with difficult-to-access areas. This particular installation was a new unit, but for a typical existing unit, the benefits include a shorter downtime, which results in a significant cost savings when compared to traditional methods.

The main performance benefit of the system is that it is installed as a monolith, and the reduced heat loss through the refractory lining is very uniform. This resulted in more uniform firing within the brick manufacturer's tunnel kiln, as well as the subsequent improvement in product quality and consistency. Also, because the product has less than 1% organic content, settling or degradation over time and temperature exposure is minimized.

*Foamfrax® RG Insulation, produced by Unifrax Corp.

Choosing a Refractory Back-up System

When considering refractory systems, the effect of using back-up material and the resulting change in interface temperatures throughout the lining cross-section need to be carefully considered. Heat flow calculations must be done to ensure that the existing refractory anchoring system will adequately withstand the expected interface temperatures of the lining system. Choosing to upgrade a tunnel kiln with an efficient back-up lining system can provide cost savings and product quality improvements for many years.

For additional information on back-up insulation systems, contact Unifrax Corp. at 2531 Whirlpool St., Niagara Falls, NY 14305; (716) 278-3800; fax (716) 278-3900; e-mail info@unifrax.com; or visit www.unifrax.com.

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