Ductwork is primarily circular or rectangular, with exceptions for transition configurations. The choice between circular and rectangular cross-sections depends on flow, cost, structural behaviour, and external constraints.
1. Basic Ductwork Configuration
The fundamental decision is whether the duct should be circular or rectangular. Factors affecting the ductwork configuration decision include flow, cost, structural considerations, available space, support means, connection to equipment, and fabrication, shipping, and erection limitations.
- Circular ducts with minimal stiffeners are cost-effective for smaller diameters.
- Larger ducts may make circular configurations less economical due to fabrication, shipping or erection limitations.
- Square cross sections are generally preferred for rectangular ducts for efficiency in flow and structure.
Key Cost Considerations:
- Effect on plate thickness with different stiffener spacings.
- Amount of welding for a given stiffener arrangement.
- Transportation mode, temporary bracing for shipping, erection and maximum shipping piece size.
- Methods for supporting ductwork.
- Effect on load magnitudes for ash, wind and snow.
- Relative cost of shop labour versus field labour.
2. Rectangular Ducts
Rectangular ducts are common in utility and large industrial boiler applications. Panels are fabricated from flat plates with rolled steel shapes for support. Reinforcements at corners and internal trusses maintain squareness. Rounded corners may reduce pressure drop but are complex to analyse and fabricate.
Photo 1: Rectangular and Circular Ducts
3. Circular Ducts
Circular ducts are efficient in resisting positive internal pressure loads, requiring fewer and smaller stiffeners. Advantages include:
- Efficient resistance to positive internal pressure.
- Fewer and smaller stiffeners.
- Smaller perimeter for a given cross-sectional area, reducing weight and cost.
- Better internal flow characteristics and lower pressure drop.
- Smaller wind loads and better drainage of rainwater and snow.
- Stiffeners for circular ducts are typically fabricated from plate or relatively small rolled shapes.
- Transitions to rectangular shapes can be challenging.
- Approximately 13% wider internally than a square duct.
- Designing access doors and providing access to instrument connections may be more challenging.