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Rectangular Bellows
Rectangular metal bellows are ideal for ducting systems where space constraints and directional flexibility are key. Common in HVAC, flue gas and exhaust systems, they absorb vibration and thermal expansion in tight configurations. Usually either made from metals or fabrics, these expansion joints offer high resistance and reliable performance in demanding applications where specific geometrical configurations are present.
What Are Rectangular Bellows?
A rectangular bellows is a flexible expansion joint designed specifically for rectangular or square ducting:
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Rectangular expansion joints;
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Duct expansion joints;
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Flue expansion joints;
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Rectangular duct bellows;
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Fabric bellows;
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Fabric expansion joints.
Rectangular bellows are typically used to manage:
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Thermal growth in ducting and flue runs;
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Vibration from fans, turbines, engines or exhaust systems;
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Misalignment between duct sections;
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System movement caused by temperature changes and structural deflection.
Because ducting systems are often large, hot and subject to movement in more than one plane, rectangular bellows are frequently specified as engineered components rather than 'standard parts.'
Where Rectangular Bellows Are Used And Why
Rectangular bellows are installed wherever non‑circular ducting must accommodate thermal movement, vibration or misalignment.
Power Generation & CHP:
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Flue gas ducting;
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Boiler connections;
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Air handling and exhaust runs.
Rectangular expansion joints are standard in power plants because flue gas ducts are large, hot, and often rectangular. They absorb thermal growth and vibration from boilers, turbines, and ID/FD fans.
Marine & Offshore:
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Exhaust ducting;
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Engine‑connected ventilation.
Marine exhaust systems often use rectangular sections due to space constraints, making rectangular bellows essential for vibration isolation and thermal expansion.
Industrial Process Plants:
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Hot air ducts;
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Fume extraction systems;
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Ventilation and process gas handling;
Process plants frequently use fabric or metal rectangular bellows to manage movement in large ductwork carrying heated air, fumes, or corrosive gases.
Energy‑from‑Waste & Thermal Systems:
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High‑temperature ductwork;
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Flue gas connections.
Waste‑to‑energy plants operate at high temperatures and require custom‑ engineered rectangular expansion joints to handle thermal cycling and structural movement.
HVAC & Building Services:
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Large rectangular ducts requiring flexibility;
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Fabric expansion joints for low‑pressure systems.
In HVAC systems, rectangular fabric bellows are common where vibration isolation or thermal movement must be managed in large air‑handling ducts.
If your requirement is for pipework rather than ducting, see Metal Piping, and if you are looking for information on more typical bellows, see Metal Bellows or Metal Expansion Joints.
Rectangular Bellows Types (Fabric Vs. Metallic)
Fabric Rectangular Bellows
Often selected where:
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Low-pressure duty and high-temperature gas flows are present;
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Large duct sizes, transitions or non-standard geometries are involved;
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Significant multi-directional movement must be absorbed (axial, lateral, angular);
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Light weight, high flexibility and low reaction forces are priorities.
Fabric bellows are typically built using multi-layered composite construction, combining materials such as PTFE, elastomers, insulation layers, and high-temperature fabrics. They are installed using frames, clamping bars or bolted flanges, depending on duct design and temperature class.
Metal Rectangular Bellows
Often selected where:
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Higher mechanical integrity or pressure capability is required;
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The installation requires robust metallic end frames or welded interfaces;
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System conditions demand more rigid engineering, such as defined movement limits or higher stability.
Metallic rectangular joints may incorporate:
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Flexible metallic elements (corrugated stainless steel, nickel alloys or other metals);
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Engineered hardware such as tie rods, hinges, gimbals, liners, covers or flow sleeves;
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Stability controls to prevent squirm or buckling in large rectangular geometries.
Depending on the application, metallic rectangular joints may incorporate flexible metallic elements and engineered hardware, with careful attention to stability and movement direction.
In most systems, the correct choice between fabric and metallic rectangular bellows is determined by:
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Pressure class;
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Gas temperature and chemistry;
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Required movement (axial, lateral, angular);
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Available envelope and installation constraints;
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Maintenance expectations and lifecycle requirements.
Key Design Considerations For Rectangular Bellows
Rectangular ducting behaves very differently from round pipework because flat panels, corners and large aspect ratios introduce unique stresses and movement patterns. As a result, rectangular bellows design is strongly influenced by:
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Duct dimensions - width, height, aspect ratio and any transitions or offsets. These directly affect bellows stability and corner loading.
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Movement requirements - axial, lateral, angular and combined or universal movements, plus the directionality of movement across each duct face. Rectangular bellows often see uneven movement due to duct geometry.
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Temperature profile - including thermal gradients across the duct section. Uneven heating can cause differential expansion, twisting or racking.
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Pressure class - even low‑pressure ducting can impose significant loads on large rectangular surfaces, requiring reinforcement or liners.
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Flow conditions - velocity, turbulence, particulate loading or corrosive media may require liners, flow sleeves or abrasion‑resistant materials.
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Corner and frame design - critical for avoiding stress concentration, squirm or premature fatigue. Corners are the highest‑stress regions in rectangular bellows.
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Installation access - including clamping clearance, bolting patterns, weld access and maintenance reach. Poor access often drives design changes.
Field Failure Insights
In real installations, rectangular bellows failures are most commonly linked to:
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Misalignment between duct sections;
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Unaccounted movement direction (especially lateral movement across wide duct faces);
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Insufficient allowance for duct thermal growth;
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Incorrect restraint conditions (tie rods, anchors, guides not behaving as assumed);
Getting these correct upfront is key.
Typical Applications
Rectangular bellows are often specified to solve problems like:
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Hot ducting thermal growth causing cracking, distortion or excessive loads at supports;
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Fan or turbine vibration transferring into duct sections and causing fatigue damage;
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Misalignment between ducting runs due to settlement, thermal bowing or structural tolerance;
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Large rectangular transitions where rigid joints would not tolerate movement safely.
If you’re dealing with an urgent failure or shutdown, a rapid-response approach may be appropriate (see FastTrack Service).
Options & Features (What We Tailor)
Rectangular bellows can be supplied with features matched to your project requirements, such as:
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Frames and connection interfaces: flanged frames, angle frames, or bespoke mating details;
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Corner configurations engineered to suit movement and duct geometry;
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Protective features where required (covers or shrouds for external protection or to reduce damage risk);
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Hardware and clamping arrangements designed for reliable fit-up and maintainability;
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Materials and extra finishes chosen to suit external environment (indoor, outdoor, marine, offshore exposure).
If your system involves particularly harsh external exposure, also review Any Environment Bellows for environmental design considerations that may influence joint protection and hardware choices.
How To Request A Quote (What We Need)
To quote for rectangular bellows and expansion joints, please provide:
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Duct size (width and height) and any transitions or offsets (drawings preferred);
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Available envelope for the bellows and frame or hardware;
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Operating and maximum temperature (and whether there is thermal cycling);
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Operating and/or design pressure (even if low);
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Media description (e.g., flue gas, hot air, exhaust) and particulate content if relevant;
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Required movement in each direction (axial, angular, lateral) and expected cycles;
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Connection details (frame type, flange drilling, mating duct details);
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Any required inspection or testing, documentation, or project standards.
Send drawings or specs and we’ll confirm feasibility, recommend the best construction approach (fabric versus metallic) and provide a quotation.
FAQs About 'Rectangular Bellows'
Are rectangular bellows the same as rectangular expansion joints? Yes. These terms are used interchangeably for rectangular duct expansion joints in flue gas, HVAC and industrial ducting systems. Both refer to flexible components designed to absorb movement, vibration and thermal growth in non-circular ductwork.
When should I use fabric rectangular bellows instead of a metallic joint? Fabric rectangular bellows are typically chosen for large ducting or flue systems, especially where: •Pressure is low •Gas temperature is high •Movement is significant and multi-directional •Low reaction forces and high flexibility are required Metallic rectangular joints are preferred when the system requires higher mechanical integrity, welded interfaces or more rigid engineered control of movement.
Can rectangular bellows handle multi-directional movement? Can rectangular bellows handle multi directional movement? Yes. Rectangular bellows can accommodate axial, lateral and angular movement, but the movement directionality must be defined clearly. Rectangular geometries behave differently from round bellows, so correct engineering and installation depend on knowing how each duct face will move.
Do you support outdoor, marine and offshore ducting environments? Yes. External environments influence material selection, hardware and protective features - especially for offshore and coastal exposure.
What is the fastest way to get a quotation? Send a duct drawing (dimensions, geometry, transitions) plus temperature, pressure, movement requirements (axial, lateral, angular) and any documentation, testing or specification expectations.
Read more FAQs here
