Pressure losses in an air-intake plenum in relation to fire-safety systems: comparing variants to guarantee the regulatory flow rates.
The major restructuring of office buildings at La Défense entails the reconfiguration of a plenum acting as an air collector, which feeds numerous air-handling units (AHUs). These supply air to the fire-safety systems.
The EOLIOS studies made it possible to compare the different pressure-loss levels across the variants devised by the architects.
The essentials. CFD pressure-loss study of the AF1 air-intake plenum (office restructuring at La Défense): full 3D modelling from on-site surveys, grilles sub-modelled from the manufacturer's datasheets, comparison of the layout variants devised by the architects and validation of the configurations compliant with fire-safety requirements.
As part of the restructuring of office buildings at La Défense, the AF1 plenum acts as an air collector: it distributes air to the units feeding the smoke-control and pressurisation devices. A controlled pressure loss in this plenum is essential so that the safety fans deliver the regulatory flow rates in the event of fire.
The AF1 plenum is one of the project's main air intakes; its geometry and the layout of its grilles directly determine the pressure loss seen by the smoke-control fans.
Technical volume acting as a collector: it draws in fresh air through its intake grilles and distributes it to the air-handling units. In a smoke-control system, it provides the make-up air compensating the extractions.
The entire plenum was modelled from on-site surveys. The air-intake grilles were “sub-modelled” from the manufacturer's datasheets, in order to reproduce their pressure loss faithfully without needlessly increasing the mesh.
This approach makes it possible to simulate precisely the air velocities and pressure fields inside the collector, and to locate the turbulence and restriction zones responsible for the pressure losses.
Pressure drop experienced by the flow due to friction and singularities (bends, grilles, constrictions). It determines the flow rate actually delivered by the fans; see our technical paper on pressure loss and hydraulic resistance.
Simplified representation of a component (here the air-intake grilles) through its manufacturer characteristics rather than its detailed geometry: the calculation remains faithful while keeping the mesh size under control.
Several layout variants devised by the architects were simulated and compared on the pressure-loss criterion. The studies made it possible to rule out the configurations that degrade the plenum's performance.
The selected configurations ensure the required fire-safety flow rates are maintained, while remaining compliant with regulatory requirements. CFD asserts itself here as a decision-support tool to reconcile architectural constraints with the performance of safety systems.
Key point. Compliance rests on maintaining the air-intake flow rates in a fire situation: any layout variant modifying the plenum's geometry or its grilles must be re-simulated before validation.
Role of the plenum, pressure-loss criterion, sub-modelling of the grilles and validity of the variants: the answers to the questions this type of study raises.
It is a collector that distributes fresh air to the air-handling units providing the air intake of the fire-safety systems. Its geometry and the layout of its grilles directly determine the flow rates actually available. See also our study of the pressure losses in the CNIT plenum.
Every bend, grille or obstacle adds resistance to the flow. If the total pressure loss exceeds the capacity of the fans, the air-intake flow rates required in a fire situation are no longer reached: the variants are therefore compared on this criterion.
The entire plenum was rebuilt in 3D from on-site surveys. The air-intake grilles were sub-modelled from the manufacturer's datasheets, which makes it possible to simulate air velocities and pressure fields precisely.
Each layout variant was simulated under identical conditions, then the pressure-loss levels were compared. The configurations degrading the plenum's performance were ruled out.
No: any new variant modifying the plenum's geometry or the layout of its grilles must be re-simulated to check that the fire-safety flow rates remain guaranteed.
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CFD pressure-loss study of the AF1 air-intake plenum (office restructuring at La Défense): full 3D modelling from surveys, grilles sub-modelled from manufacturer datasheets, and comparison of architectural variants to rule out penalising configurations and guarantee the compliance of the fire-safety systems.
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