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Fabric Expansion Joints Glossary

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General Terms

Fabric Expansion Joint: An assembly that utilizes a fabric belt element to allow for the movement of ductwork, as opposed to a metal expansion joint which allows for the movement by means of convoluted metal bellows.

Cold Shell: A duct that is internally insulated with refractory material.

Hot Shell: A duct that is externally insulated.

Mating Flange: A structural angle or channel that is welded to the end of the ducting facilitate bolting or welding the expansion joint in to the system.

Outboard Flange: A design where both sides of the belt attachment flange are accessible from the outside of the expansion joint.

Inboard Flange: A design where the inside of the belt attachment flange is not accessible from outside the expansion joint.

Inside Replaceable: The expansion joint design that allows the belt to be installed and replaced from inside the duct.

Fabric Over Metal: A design that allows for a fabric expansion joint to be installed over a metal expansion joint.

Flutter: The phenomenon that can occur in the fabric material due to turbulence of the gas flow. Flutter can occur in locations where pressure fluctuations occurs such as near dampers and fans. Intense flexing of the fabric could lead to premature failure, thus the use of liners, fillers, or heavier fabric is recommended.

Heat Seal Splice: For Fluoroplastic material, a process of splicing or repairing the fabric material with means of a special iron and a film of PTFE sheet.

Vulcanize: A process of heating and adding sulfur to an elastomeric material. Vulcanizing is required in the manufacturing and splicing of elastomers.

Dimensional Terms

Breach: The opening in the duct where the expansion joint is installed.

Face to Face: See Span.

ID: Inside Duct Dimension.

OD: Outside Duct Dimension.

Standoff: The distance of the fabric belt from the gas stream. The proper standoff allows belt cooling in high temperature applications and prevents the belt from being pulled into the gas stream in negative pressure.

Setback: See Standoff.

Fabric Expansion Joint Basic Component Terms

Fabric Belt: The flexible non-porous element of a fabric expansion joint. The fabric belt must be able to withstand the thermal, pressure and chemical conditions. Modern fabrics accomplish this with a chemically inert barrier bonded to high strength substrate. For high temperature applications, a layer of insulation is incorporated into the belt.

Belt Attachment Flange: The face of the expansion joint where the belt is clamped using a back-up bar. The flange can be turned out (see Outboard Flange) or turned in (see Inboard Flange). Either way, many designs allow for the belt attachment flange to be offset from the duct (see Standoff).

Gasket: A non-porous deformable material that is installed between the belt attachment flange and the fabric belt. The gasket allows a gas tight seal when the back-up bar clamping action is applied.

Insulation Tape: A thin layer of insulating material, usually woven fiberglass, that is installed between the belt attachment flange and the fabric belt. In certain cases, insulation tape is also used between the fabric belt and the back-up bar. The purpose of the tape is to prevent the conduction of high temperatures to the fabric material where it is clamped.

Accumulation Barrier: In applications where flash is present in the gas stream, a barrier is installed in the cavity of the expansion joint. The barrier is constructed of low density insulation wrapped in fiberglass cloth. The accumulation barrier helps prevent the particulate from building up in the cavity along the bottom of a joint in horizontal ducting.

Insulation Pillow: In applications where the gas stream temperature is greater than the fabric belt alone can withstand, an insulation pillow can be installed, usually by pinning to telescoping liners. An insulation pillow is constructed of high density material wrapped in cloth with wire mesh reinforcement.

Radius Corner: The belt radius at the corner of rectangular expansion joints. The radius corner helps prevent sharp creases that may shorten belt life.

Back-Up Bars: Metal bars with fastener holes used for clamping along the edges of the fabric gas seal to the belt attachment flange.

Clamp Bars: See Back-Up bars.

Liner: The liner is metal plate or plates that are designed to protect the fabric belt from the gas stream while at the same time allowing the expansion joint movement. The liner can be bolted, welded, floating, or integral to the standoff frame. A single liner is connected to the upstream side of the expansion joint. In some cases, a second "telescoping" liner is used on the downstream side.

Baffle: See Liner.


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