Furnace lining refits and repairs in the metals processing, chemical processing and ceramics manufacturing industries typically have been done using anchored and bonded ceramic fiber module systems. An alternative method is to use a new, gunnable foam/fiber insulation developed by Unifrax Corp. The Foamfrax monolithic insulation system was developed to repair and/or upgrade existing furnace linings or to install new linings in a shorter time than that required for conventional blanket or module linings. The system offers fast, easy installation and good lining performance and overall cost savings for existing fiber linings, furnaces linings patches and refits and linings over hard refractory.

Existing refractory surface of the batch forge furnace has chipped and broken away due to thermal cycling and repeated repair.

Material characteristics

The three-component system consists of specially conditioned bulk ceramic or soluble fibers, an inorganic binder (colloidal silica) and an organic foaming binder (primarily consisting of polyvinyl alcohol). The bulk fiber material and binders are combined in the installation process in a patented mixing mechanism creating a homogeneous foam/ fiber mixture. The organic portion of the binder is sacrificial and burns out during the firing process. After exposure to operating temperatures, the inorganic binder adds strength and integrity to the furnace lining.

The system differs from other sprayable fiber techniques in that the fibers are completely coated with the foaming binder solution the mixing chamber, encapsulating the fibers within the foam matrix. By comparison, other sprayable methods feed dry fiber and binder in separate streams, which are partially combined after the product leaves the installation spray nozzle.

System characteristics include:

• Speed and ease of installation
  
• Low rebound during installation
  
• Low thermal conductivity
  
• Low thermal shrinkage
  
• Low heat storage
  
• Excellent thermal shock resistance
  
• Good chemical resistance

Case study: Forge furnace lining repair

Applying the new system over a partially deteriorated furnace lining can extend the furnace lining life and improve furnace efficiency. One of the advantages of the system is that the end user realizes energy savings immediately after the product is installed and the furnace is put back into service.

The system was evaluated in a field trail at National Forge Co.'s Irvine, Pa. plant by applying it to the existing hard refractory surface of a batch forge furnace, which has an operating temperature of 2300F (1260C). Due to thermal cycling, regular maintenance was required to preserve the integrity of the existing rammed plastic refractory lining. The trial consisted of applying a 3-in. (76-mm) thick, 8 PCF Foamfrax Grade II fiber veneer over the existing hard refractory surface of the furnace. The nominal composition of the material is 30% Al2O3, 54% SiO2 and 16% ZrO2, with an average fiber diameter of 1 - 2 micron. To test the performance of the repair lining in this demanding application, the insulation was initially applied only to the 340-ft2 (31.5 m2) furnace roof surface. Installation speed

Prior to the repair installation, the hard refractory surface was cleaned of all loose material to provide a stable substrate for the application. Dilute refractory cement was then applied to the refractory surface as a surface preparation step. The temperature of the refractory surface must be lower than 200F (90C) before applying the cement coating. The cement improves the adhesion of the insulation to the refractory and controls the rate at which the moisture is transferred from the insulation veneer to the refractory substrate.

To maintain an even thickness throughout the installation, the veneer is installed in sections with a perimeter of insulation gunned in place for each section and then the interior is filled in. The binder system is dyed blue so the installer can visually check that the proper fiber-to-binder ratio is maintained during installation.

Trained contractors using patented installation machinery install the insulation quickly and easily. The fibers and binders are combined within the mixing chamber to create a homogeneous foam/ fiber mixture, which is propelled through a feed hose and nozzle and gunned onto the target surface.

Insulation can be installed at rates in excess of 1000 board feet per hour, which, depending on the application, can be two to six times faster than rates for traditional installation techniques. Compared with repair of existing furnace linings that would otherwise require complete removal prior to a traditional installation, the time savings is even greater. In addition, unlike traditional installation techniques that often can require five or more installers, the new technology requires only a three-person installation crew. Reduced airborne-fiber exposure

The insulation system is specially designed to control airborne fiber levels. Unlike sprayable fiber methods that feed dry fiber and binder in separate streams that are partially combined after the product leaves the installation nozzle, the fiber in the new system is completely coated within the foaming binder solution while in the mixing chamber. This significantly reduces the potential for airborne fiber release into the surrounding environment.

Measurable airborne fiber levels within the system can vary according to the specific application. Average real-time exposures to airborne fiber levels with this technology are similar to or less than those experienced during conventional blanket or module installations. Also, because the system is installed much faster than conventional methods, the duration of exposure is significantly decreased.

In addition to the advantages of the mixing chamber and proprietary binder system, a dust collection hood located on top of the fiber feed hopper further controls the release of airborne fiber. A vacuum drawn on the fiber hopper hood keeps the bulk fiber chamber under negative pressure during operation. Limit switches installed on the binder-mixing chamber also prevent discharge of fiber prior to complete mixing with the foam binder.

Trial results

The lining upgrade on the furnace roof of batch forge furnace was completed in less than four hours and the furnace was immediately put back into service. The installation remained intact after a three-month service trial period at an operating temperature 2300F (1260C). Following the successful roof installation, the sidewalls and backwall of the furnace were also veneered with the insulation system. According to the company, a variety of other refractory patch materials tried on the furnaces had very little success. The new insulation system by comparison has maintained its integrity and provided decreased heat storage and heat loss, resulting in immediate savings. Also, better temperature uniformity within the furnace results in more uniformly heated parts. Therefore, product quality improved as a result of easier forgeability.

Conclusions

The primary benefit of the new insulation system is that the end user realizes energy savings immediately after the product is installed and the furnace is put back into service. When installed as a lining upgrade or lining over existing refractory, the foam/ fiber system has demonstrated a substantial improvement in thermal performance, which translates into energy cost savings. The typical payback period in terms of energy savings for an installation with this technology is less than six months.