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Thursday 13 July 2017

Louvre ventilation design: why the 50% Free Area rule-of-thumb doesn’t work


A common error when designing a Performance Louvre system, is the use of Free Area as the metric from which the required louvre area is calculated. This method can lead to an overestimation of the required louvre area or worse: an under-estimation of the area required.

Under-estimation will result in an inadequate ventilation area, leading to overheating and ultimately, premature failure of mechanical plant.

Free Area is the measured area, deducting the area of the louvre components from the total area of the assembly. This is quoted as a percentage of the total louvre area. Typically, 50% is commonly adopted as the standard to follow.

This form of rough evaluation came about as a result of planning consents where façades are typically required to feature louvres, but with no consideration of airflows. This is because, at the planning stage, the mechanical services requirements of the building (louvre areas) are unknown. By simply taking an air change rate for the type of building usage, a typical open area can be determined. And by assuming louvres restrict airflows, this area is doubled to achieve the required louver area required.

However, this percentage type of evaluation takes no account of whether the design uses single or multi-banked louvres – or, of profile, thickness, depth, spacing, airflow efficiency, etc.  Typical single bank and triple bank louvre designs can have a similar percentage free area, but the airflow efficiency (the ease at which air can pass from one side of the louvre to the other) can be vastly different.

The only way louvres can be sized accurately, is to establish the louvre co-efficient of airflow (Ci/d), which in turn can only be determined by wind tunnel testing at varying velocities.  Adding this value into known formula, the resulting pressure drop is determined and the louvre sized correctly.

A full evaluation of Performance Louvre requirements should always be carried out by a specialist with expertise in this field of design and the 50% Free Area rule-of-thumb should always be avoided. Please contact us if you need help with louvre sizing or product selection.

By David Mowatt of Lareine Engineering from original article by Paul Paffett, INNIVATE Pte Ltd”

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