A 23.9 m stainless steel stack with a built-in fan silencer for a steel plant

The challenge

A process engineering contractor building an installation at a steel plant in Western Europe needed a stack for the exhaust of a large fan: up to 35,000 Nm³/h of flue gas at 211 °C, about 72,500 m³/h at operating conditions. The fan duct would reach the stack about 2.5 m above its base.

The fan’s noise had to be dealt with before the gas left the stack, and the plant’s own steel structure stood next to the stack position and could take lateral loads. The contractor also needed the stack’s support forces early, because the design of the foundations and the steelwork depended on them.

Our approach & engineering

A silencer inside the stack

Instead of a separate silencer in the fan duct, Axces built a tube-type silencer into the bottom of the stack. The gas passes through rows of perforated tubes welded into a tube plate. This type of silencer is short, keeps the back pressure low, and its housing is part of the stack itself.

The silencer section is Ø1,900 mm, wider than the Ø1,200 mm shaft above it, so the gas passes it at a lower velocity. A transition on top of the silencer carries the shaft. The fan duct enters the silencer section through a rectangular connection at 45°.

Guided by the plant’s steelwork

The 23.9 m shaft is guided by brackets on the plant’s steel structure at four levels, at 4.9, 8.9, 13.9 and 18.9 m, so only the top 5 m stands free. With that support the calculated critical wind speed for vortex shedding is 93 m/s, far above any design wind, so the stack does not need a vibration damper. Our article on industrial stack design loads explains why vortex shedding governs slender stacks.

316L with no corrosion allowance

Axces advised 3 mm AISI 316L for the whole stack, with no corrosion allowance, rather than carbon steel with a 2 mm allowance and a heat-resistant coating. The stainless shell needs no coating, and the stack is designed for a 25-year life.

• Tube-type silencer built into a Ø1,900 mm bottom section
• Ø1,200 mm shaft in 3 mm AISI 316L, 23.9 m high
• Guided at four levels by the plant’s steelwork; the top 5 m free
• No vortex-shedding damper needed
• Support forces sent to the contractor in October 2024 for the foundation and steel design

Scope of supply

• 1 × stack, 23.9 m, 3 mm AISI 316L: Ø1,200 mm shaft on a Ø1,900 mm bottom section, delivered in three flanged sections
• Tube-type silencer built into the bottom section, with a rectangular fan connection at 45°
• Guide brackets for four support levels, with bolts, nuts, washers and gaskets
• Measurement ports, earthing lugs and a nameplate
• Structural and dynamic calculation, including the support forces for the plant’s steelwork

Results

The stack was delivered to the steel plant in three sections in 2025, with its guide brackets and fixings on a separate pallet. Because the support forces were known in October 2024, the contractor could design the foundations and the steelwork around the stack before it was built.

• One stainless steel stack with an integrated fan silencer, on site in 2025
• Silencer and stack in one component: no separate silencer in the fan duct
• Guided by existing steelwork, so no separate support structure was needed
• 316L shell with no coating to maintain

Related reading

• Freestanding, guyed, or self-supporting: choosing an industrial stack configuration: when a stack is better stayed by a nearby structure
• Industrial stack design loads: wind, seismic, and vortex shedding explained: the load cases that govern a stack’s design
• Understanding dB(A) reduction targets: how to specify acoustic performance: how to set the noise requirement a silencer is designed to