AIR AND VENTILATION POLLUTION TION 2 LOUVRES
Increasing the size of the louvre face will enable the same airflow to be achieved at lower velocity
When louvres are being specified, it is important to consider a range of issues that could affect the overall performance of the installation. Chris Marney offers some practical advice
G
etting air into or out of a building without letting water through the hole is a simple concept but it does have its complications.
Critically, you need to address the unavoidable trade-off between the louvre’s ability to let air through the grilles, and its ability to keep water out. With louvre design, if you reduce resistance to airflow, you are also likely to be reducing resistance to water penetration. Ambiguous performance claims and an increasing emphasis on energy reduction mean that specifying weather louvres as ‘50% free area’ no longer represents an adequate design brief. As with any product, knowing the performance requirement for a weather louvre is crucial. However, getting from a notional requirement to a practical product specification capable of delivering that performance can prove problematic. And when you come to product selection, how can you be sure the louvre will perform as promised?
Knowing your requirement Airflow: The volume of air required by your system and the size of the opening through which the air is introduced are key factors affecting louvre selection and the subsequent fan power requirement. The system designer should quantify the
system’s required volume of air in m3/s. This should then be expressed as a velocity onto the
52 CIBSE Journal March 2012
louvre, taking into account the area of the louvre face. For example, a volume requirement of 2 cu m/s through a louvre with a face area of 1 sq m equates to a face velocity of 2 m/s. The same requirement through a louvre with double the face area will equate to half the face velocity. The test standard for weather louvres (EN 13030:2001) provides a means of rating the louvre’s air resistance, based on the pressure drop across the louvre. This is expressed as a class based on the coefficient of discharge between 1 and 4, with 1 representing the lowest air resistance. A typical, reasonably water-resistant louvre will achieve a rating of 3. Designers selecting a unit with a higher class should be aware of potentially higher, long-term fan power costs. Water penetration: For some applications, water penetration is not an issue – perhaps ductwork behind the louvre has drains that can handle the ingress. For others, however, penetration of water is a major consideration and getting it wrong can mean major disruption and cost to rectify the problem. Defining this water rejection requirement
can be difficult. The test standard helps by banding water rejection effectiveness, based on the percentage volume of water rejected by the louvre when subjected to simulated wind and rain. The closest you are likely to get to a useful definition of your water penetration requirement is in considering which of these bands best matches your need.
As well as how critical water rejection is to
your application, you also need to consider the ultimate positioning of the louvre, both in terms of geographical climate and whether its position will be sheltered or exposed.
Finding the balance Having considered your ideal requirement for airflow and water rejection, you now need to combine the two in order to begin product selection. The first point to appreciate is that the ‘perfect’ louvre does not exist and that, for most systems, you should expect at least a small amount of water penetration. The second point is that there is a direct relationship between face velocity and water rejection capability for any given louvre. This is emphasised in the standard test for weather louvres, which measures water penetration at eight different face velocities ranging from zero to 3.5m/s. Broadly speaking this data indicates whether the louvre will meet the water rejection requirement at your known face velocity. It follows that improved water rejection, without sacrificing freedom of airflow, requires a more highly engineered, and usually higher-priced, product.
Size matters Increasing the size of the louvre face will enable the same air volume to be achieved at lower velocity. This not only reduces the fan power requirement but also improves the water resistance of the louvre, since the faster you suck air through the louvre, the more challenged the louvre is in keeping droplets out. Of course, this relies on having the space (and design foresight) for this to be an option.
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