A special type of high-temperature fabric expansion joint developed by U.S. Bellows, Inc. is the furnace sealing bag. The objective of the bag is to seal the air inlet conduits penetrations into a furnace and thus prevent heat loss. Because of thermal expansion of both the conduits and the furnace, the bag must be able to expand and contract during normal cycles of operation.
The bag consists of a tapered sleeve formed from layers of flexible, flame and heat-resistant impervious fabric, which is connected at its upper end to another sleeve in the furnace floor. The lower end of the bag is connected to the air inlet conduit (usually a pipe) so as to form an airtight seal. The connection to this pipe is made such that the fabric is collapsed when the furnace is cold and extended when the furnace is hot. When required, a tapered coil spring formed from suitable metal is installed around the conduit inside the bag. This prevents the fabric from collapsing inward during vacuum conditions inside the furnace. For most applications, band straps may be used to attach the bag at both ends.
This expansion joint provides a simple and cost-effective solution to a problem with potentially high energy losses. These bags function through the maintenance 'shut-down' cycles for large furnaces and can be replaced after several years of operation.