Components like dampers, expansion joints, insulation, lagging, LPA screens, access doors, gaskets, instrumentation, and platforms/ladders are meticulously designed to interface with the ductwork, each serving a specific purpose within the system. 

1. Introduction

The list of components typically supplied with industrial ductwork is integral to the overall design and function of the ductwork system. These components are often designed in conjunction with the ductwork itself and have a significant impact on its overall design. Each component serves a specific function within the ductwork system and interfaces with the ductwork in various ways. The detailed description of their functions and interactions with the ductwork enhances the understanding of their role in the system.

2. Dampers

Dampers play a crucial role in duct systems by providing plant operators with the means to control or redirect flow within the ductwork. Additionally, isolation dampers are employed to isolate specific sections of ductwork for various operational requirements. There are three primary types of dampers commonly used in ducts:

  • Guillotine
  • Louver
  • Butterfly

Each damper comes with its own frame, seamlessly integrating with the duct to become an integral part of the system. The placement of dampers influences the design and transient pressure forces acting on the duct and its supporting structure.

Photo 1 : Damper

Guillotine dampers are often chosen for isolation purposes due to their generally superior leak-tightness compared to multi-blade louver dampers. In cases where absolute isolation is necessary, double-blade guillotine dampers may be employed. However, it’s important to note that guillotine dampers can be heavy and may require substantial space when the blade is outside the duct.

To ensure complete isolation, pressurized-seal air frames are installed around dampers. This design prevents any leaks from occurring outside the flue-gas stream, maintaining a controlled environment within the ductwork system.

3. Expansion Joints

In a ductwork system, expansion joints are incorporated to partition the duct into sections, effectively controlling thermal expansion and accommodating differential movements, especially in high-temperature environments. These joints are strategically placed at the inlets and outlets of major equipment like fans, boilers, economizers, SCR units, air heaters, and scrubbers.

Photo 2 :Expansion Joint

Expansion joints come in two primary types: non-metallic expansion joints and metal expansion joints. Non-metallic expansion joints are typically constructed from fabric or rubber-like materials, while metal expansion joints are made from light-gauge metal. It’s crucial to size expansion joints to accommodate both axial movement and rotation of the duct. Some joints, particularly non-metallic ones, may permit a limited amount of shear movement, while others, usually metal joints, can be designed to transfer shear forces without allowing shear movement. This versatile design ensures that expansion joints effectively address the specific movement requirements of the ductwork system. Also it maintains its structural integrity under varying conditions.

4. Insulation

Insulation serves a dual purpose in duct systems, either applied internally or externally. Its primary functions include retaining heat within the duct and providing personnel protection from external heat. Older ducts were often insulated internally using materials like masonry or mortar. However, contemporary ducts typically utilize external insulation, employing spun mineral fiber or fiberglass.

Photo 3: Duct Insulation

Inadequate insulation can lead to issues such as condensation forming on the inside of the duct at cold spots, resulting in significant corrosion of the steel. Additionally, insufficient insulation on hot-side ducts may lead to thermal shock. This may cause cracks at or near cold spots. Proper insulation is critical for maintaining temperature control, preventing condensation-related corrosion, and safeguarding against thermal shock-induced damage.

5. Lagging

The term for the fiberglass, aluminium, or stainless steel sheets that cover ductwork insulation is “lagging.” Lagging serves several essential functions;

  • keeping the insulation securely in place
  • protecting it from damage caused by weather or personnel
  • preventing water from coming into contact with both the insulation and the duct. 

Photo 4: Duct Lagging

In essence, lagging acts as a protective covering for the insulation, ensuring its effectiveness and longevity in various environmental conditions.

6. LPA (Large Particle Ash) Screen

In certain systems, a screen is employed to filter out larger particles of ash that have the potential to clog equipment, such as the catalyst in Selective Catalytic Reduction (SCR) reactors. These screens are usually positioned to discharge the Low-Pressure Ash (LPA) into the hoppers of the boiler outlet ductwork. The primary purpose of these screens is to prevent the entry of larger ash particles into sensitive equipment, ensuring the smooth and efficient operation of the system.

7. Access Doors

Access doors are typically installed in duct walls, facilitating entry for maintenance during periods when the boiler is out of service. Their design is carefully considered to balance the need for easy entry without compromising the structural integrity of the duct wall. These doors must be capable of withstanding the same pressures and wind loads as the duct wall itself. Hinges, latches, or bolts used for the doors are sized to handle these loads, ensuring the overall stability of the system.

To maintain an airtight environment, access doors incorporate a gasket seal around the entire perimeter. This sealing mechanism is crucial for preventing air leaks and maintaining the efficiency of the ductwork system.

Additionally, access doors are designed with proper insulation and lagging. This feature is essential to prevent;

  • the formation of cold spots 
  • to safeguard the doors against corrosion 
  • to ensure their long-term durability in varying environmental conditions.

8. Gaskets 

The selection of an airtight gasket for sealing is a critical consideration, taking these factors into account;

  • Design pressure
  • Temperature
  • Flow chemistry. 

In certain instances, gaskets are also applied at connection points, such as where a duct section links to equipment, expansion joints, and dampers. This comprehensive approach to using gaskets helps maintain the desired airtight conditions throughout the ductwork system.

9. Instrumentation

Flanged pipe pieces are commonly integrated into duct roofs or walls at different points to facilitate the insertion of air or gas measurement equipment into the stream. Certain instrumentation is permanently affixed to these ports, while others are capped to allow for the insertion of temporary or portable instruments as needed. The placement of these ports is determined by;

  • regulatory requirements,
  • the configuration of the duct,
  • test data,
  • operational requirements,
  • recommendations from instrument manufacturers, 
  • guidance from major equipment manufacturers. 

This multifaceted approach ensures that the positioning of these ports aligns with both industry standards and the specific operational and testing needs of the system.

10. Platforms and Ladders

In certain scenarios, access platforms and ladders are directly supported off ductwork, and if this approach is adopted, careful attention must be given to the structural design. The structural considerations should include accounting for the differential thermal expansion between the hotter duct and the cooler ladder or platform support steel. It’s crucial to address this factor when connecting the ladder or support steel to the duct. This will ensure the stability and integrity of the overall structure, especially when temperature differentials may impact the materials involved.

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