• fire safety,
  • smoke control

Ventilation opening areas explained - Part 1

Part 1/3: Equivalent area

The (required) ventilation area in buildings is a broad one and involves numerous definitions that are not fully transparent to the users. This series of blog posts aims to clarify the definitions and demonstrate the calculations on worked examples.

Approved Document F of the Building regulations gives guidance for means of ventilation for common building situations, including car parks. The guidance refers to “equivalent area” for openings instead of “free area” because it is argued that “the physical size of the aperture may not accurately reflect the airflow performance”.

Equivalent area definition according to Approved Document F is:

  • A measure of the aerodynamic performance of a ventilator. It is the area of a sharp-edged orifice which air would pass at the same volume flow rate, under an identical applied pressure difference, as the opening under consideration.

In addition to the “equivalent area” there are also other terminologies that are being used in the ventilation design process such as aerodynamic area, geometric area, gross area, physical free area, visual free area, net open area, discharge coefficient, k-factor, zeta etc. We believe the use of all these terms without sufficient clarification introduces ambiguity and eventually mistakes in designs. With this series of blog post we aim to shed some light on this topic.

Coefficient of discharge - Cd

The discharge coefficient is defined as “the ratio of actual flow rate […] to the theoretical flow rate […]“ according to EN 12101-2. Based on the assumption that the actual flow cannot be larger than the theoretical flow rate, the Cd value can be between 0 and 1. In essence, the coefficient is a reduction factor due to the resistance of the opening.

Geometric area (gross area)

The geometric area is defined by EN 12101-2 as “Area of the opening through a ventilator, measured in the plane defined by the surface of the construction works, where it contracts the structure of the ventilator. No reduction will be made for controls, louvers or other obstructions”.

It is also called as gross area.

Aerodynamic free area - Aa

Aerodynamic free area equals the geometric area (Av, gross area) multiplied by the coefficient of discharge.

Equivalent area (effective area or gross area)

Having established the definition of the equivalent area, it can be formulized as below by incorporating the coefficient of discharge and aerodynamic area. Equivalent area is only applicable in relation with the compared orifice. Equivalent area is equal to gross area when Cd of the opening is equal to Cd of square edged orifice.




To see the worked example, continue to the Part 3 of the blog.

Free area

Free area can be expressed in two ways:

-Physical free area (net open area)

-Visual free area


K-factor, or minor loss coefficient, is a dimensionless value that expresses the aerodynamic resistance of the grille/duct or other impediments to the airflow. It can be derived from coefficient of discharge. Further information is given in Part 2 in this blog series.

K-factor, ξ :

Why “free area” is not a well-defined quantity

Going back to the definition, the reason why the free area and air performance are not correlated is the pressure loss through the opening. For example, the pipes in the figure below have identical inlet free areas however due to their geometry, they create different resistances for the airflow. While the flow in (a) can pass through the pipe without any restriction or direction change, pipe (b) forces the flow to go through a less straightforward path. This yields to higher resistance in (b) and therefore to reach the same amount of flow rate a higher pressure difference is needed in (b) compared to (a). This is a simple demonstration as to why only “free area” might not provide sufficient information while designing a ventilation system.

In the next part of this blog the k-factor and its relation to coefficient of discharge by deriving with equations are explained.