Enclosure Cooling: Things to know about Airflow

Looking at datasheets to obtain the correct airflow for a filter fan unit is not always plain sailing. Many manufactures are coy in how this information is displayed. Here we explain what the figures mean and how best to determine the correct airflow at the correct frequency.

Maximum Airflow and Combined Airflow
Within the datasheet for a filter fan our attention is easily drawn to the maximum airflow figure. This is figure is just that, the maximum air the filter fan can deliver and one the manufacture is proud to highlight. However in reality, the filter fan is used with either an inlet or outlet to allow hot air out or cool air into the enclosure. Therefore, we are adding resistance to the airflow and consequently the actual airflow through a cabinet will be reduced. The combined airflow figure is either displayed in a graphical form (similar to below) or as a published figure and often for the same size of filter unit.

For example, the graph shows Axair’s filter fan FPF12KU230BE-100 has a maximum airflow figure of 57m³/h. When used with the same size outlet FPF12KUG-100 (yellow line) the combined airflow is reduced to 41m³/h. This is the actual airflow we can expect to get through the cabinet. If you are calculating your airflow figure on the maximum then clearly this is going to be problematic for the equipment you are trying to protect. If we use a larger outlet filter such as the FPF13KUG-100 (green line) which offers a greater area then the figure improves to 47m³/h as we have less resistance to the airflow within the cabinet.

Frequency
Let’s also be mindful of the fact that airflow figures are shown for operation at both 50Hz (blue line) and at 60hz (purple line). The 60Hz figure is always the greater and some manufacturers and distributors find this too tempting not to highlight as the maximum airflow figure a fan can produce. Whilst it is a true fact that the 60Hz figure is indeed the maximum available, the reality for the UK is that it is the 50Hz figure that is of most importance.

Filter Media and Ingress Protection
One final point to be mindful of is the filter media within the filter fan unit. Whilst filters improve the ingress protection of the enclosure, they can dramatically effect airflow by adding resistance. Usually, the higher the IP rating, the lower the airflow. Replacing standard filter media with a denser version, say IP55, will mean airflows are reduced but this is necessary where a high degree of protection is needed. The benefits often outweigh the reduced airflow but it is a factor to consider.