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
Tunnel kiln roof prior to back-up lining installation.
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
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
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
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
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 firstname.lastname@example.org; or visit www.unifrax.com.