Rooftop twin-flue chimneys for a Dutch food plant

The challenge

A food plant in the Netherlands needed to rebuild the exhaust of its baking and frying lines. Twelve process fans discharge warm, greasy off-gas at up to 250 °C, around the clock, through stacks on two existing roofs. The old chimneys had to come down, and the new ones had to discharge high above the building while the discharge became slightly quieter than before.

Everything had to stand on the existing roofs. That meant a new steel structure to carry the chimneys, silencers and ducts, with the loads spread so the building could take them, and an erection plan that worked from the ground with cranes. The old chimneys had 2 mm walls where 3 mm was needed, so only a few sections could be reused.

Our approach & engineering

Axces designed the system as two rooftop groups: four chimneys on the high roof discharging at +35 m, and four on the low roof discharging at +30 m. Each group stands on its own bolted, hot-dip galvanised steel frame, and each fan line runs through its own silencing before it reaches a chimney.

Twin-flue chimneys

Five of the eight chimneys are Ø900 twin-flue units: a 3 mm stainless partition plate runs the full length of the chimney, so two fan lines discharge through one shell with separate connections. Each has a condensate plate with drain and an inspection hatch. The other three are Ø630 single-flue chimneys. All flues are AISI 304/304L, pickled and passivated inside and out, and the chimneys are CE marked to EN 13084-7.

Guyed, and tied together at 31 m

The chimneys are guyed rather than free-standing. On the high roof a centring structure at 31 m links the four chimneys: it bears on the inner two, centres the outer two and carries six galvanised guy cables with tensioners. The low roof has twelve guy points. Old chimney sections with sound walls were reused only as top sections on the low roof, fitted with stiffening rings.

Steel frame, silencers and access

The high-roof base frame is built from IPE 450 and lighter sections in S235JR, hot-dip galvanised and bolted with HV 10.9 bolts, and was sized with an external structural engineer, who also confirmed the building could carry it. On the high roof, eight cylindrical silencers feed six horizontal splitter silencers of 1,150 × 1,150 × 2,000 mm whose splitters lift out from a grated walkway above them. The low roof has four cylindrical silencers. Galvanised railings, a short stair and a cable barrier along the roof edge give safe access.

Eight chimneys on two roofs, outlets at +35 m and +30 m
Five twin-flue chimneys with a full-length stainless partition plate
Guyed design with a centring structure at 31 m on the high roof
Bolted, hot-dip galvanised frames, pre-assembled on the ground and lifted onto the roof
Splitter silencers serviceable from a grated walkway

Scope of supply

5 × Ø900 twin-flue chimneys and 3 × Ø630 single-flue chimneys in AISI 304/304L
Centring structure, guy cables and tensioners
Hot-dip galvanised rooftop steel frames with operating platform, walkway, railings and stair
12 × cylindrical silencers and 6 × splitter silencers, with galvanised spiral ducts and bends
Dismantling of the old stacks, roof penetrations, and erection with cranes

Results

The new system was delivered and erected at the end of September 2013. The frames and chimney modules were pre-assembled on the ground beside the building, and the four-chimney cluster for the high roof was lifted into place as one module.