Free Area versus CD Value

Free Area versus CD Value

Free area and discharge loss coefficient (CD value) are measurements used to determine the amount of airflow through a louvre. The one often seen is free area, this is due to its simplicity. The free area is the smallest dimension measured between the two louvre blades, for example:

Free area = 52/100 x 100 = 52%

This would mean that for a 1m2 louvre with a free area of 52%, then the active area (the open area) would be 520mm2.  However, it is a very rudimental measurement and makes the assumption that the higher free area will allow for higher airflow. There is also an issue that some louvre manufacturers state that the pitch is the free area or ‘visual free area’. This makes it difficult for the customer to be sure that they have the correct information and compare correctly between different manufacturers.

A more accurate method of determining the airflow through a louvre is the Discharge Loss Coefficient (CD value).  This is the measurement of ease with which air will flow across a louvre.  Looking at the two tubes below, while both have the same free area the one on the left hand side would offer better airflow as there are no obstacles to block the air.

The same principle can be applied to louvres; the shape of the blade can have a big impact to the airflow. However, because it is a ratio it is sometimes hard to visualise what impact this would have on the louvre size. The engineer also needs to take into account what flow rate needs to be achieved and the maximum pressure drop. Sample calculations are shown based on a flow rate of 20m3/s and a maximum pressure drop of 20pa.

Louvre System

KW50Z

KW75Z

KW75S

KW100Z

KW75HPG

Free Area

52%

58%

50%

61%

47%

CD Value

0.274

0.273

0.254

0.277

0.311

Area of Louvre required

12.83m2

12.69m2

13.64m2

12.51m2

11.14m2

*CD values are based on the louvre with 4mm insect mesh.

These are just example calculations, in order to confirm if the louvre face area (i.e. physical area) is the correct size we would require the flowrate and the maximum pressure drop.

For more information and for assistance with airflow calculations, please contact Kingfisher’s Technical team on 01773 814 102 or technical@kingfisherlouvres.com.