The design work not only includes standard mechanical ventilation processes, but also the following processes for natural ventilation:

Single-zone model / multi-zone model

The zone model is based on the abstraction of a building as a flow network consisting of zones and flow elements. The zones represent a volume in the building that can be described in terms of pressure, temperature and foreign matter concentration.

These zones and nodes are linked by flow elements which represent the airflow rate based on pressure difference. Flow elements can be doors, air intake louvres and gravity roof ventilators, or the components in a simple ventilation system such as ducts, grilles and connectors.

It is also possible to take the airflow pressure characteristics of fans into account. Air pressure coefficients combine the pressure at the external nodes with the current airspeed. Not only the effects of wind, but also the level of air exchange due to chimney effects (as a result of differences in the density of the air in the building and external air) can be calculated. The requirements of mass conservation in each zone are used to solve non-linear equations for each timeframe.

This dimensioning process enables,

  • the calculation of any number of vents,
  • the variation of all relevant parameters for these vents, such as installation height, geometric or aerodynamic surface, pressure loss coefficient (or drag coefficient) and local wind pressure coefficient,
  • the incorporation of values for outside air temperature, wind direction and wind speed,
  • an assessment of the effects of mechanical ventilation or air extraction on natural ventilation.

Generally, precise data can be obtained in the dimensioning process on air temperature in the work zone, exhaust air temperature, mass air flows and air velocity, as well as the neutral plane, depending on the values of the above parameters.

CFD - Computational Fluid Dynamics

CFD software enables the computer simulation of flow patterns and the associated physical phenomena, such as heat and mass transfer in industrial ventilation components and systems.

The applications for CFD extend from modelling laminar and turbulent internal and external air flows, though the calculation of temperatures, including heat exchange and radiation, to the acquisition of data on of mixing processes, mass transfer and the expansion of concentration fields, such as smoke spread and fire simulation.

In order to use CFD models, it is initially necessary to prepare a three-dimensional geometry of the building, including all details that are relevant for air flows. The resulting volume is then divided up into a finite number of small reference units (cells). The number of cells required depends on the size and complexity of the building (and can vary up to more than 10 million cells).

The final step is to solve balance equations in respect of momentum, energy, mass and, where applicable, turbulence and gas type for each individual cell. Since the conditions in each of the cells are influenced by the neighbouring cells, the ensuing equation system is iterative.

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