Often the filter fan for an electrical panel is chosen based on the size and airflow, however, a deeper understanding is required to determine how the filter fan unit will actually behave in a real application. In this article, we will address the definitions of airflow and static pressure, then examine the relationship between them and the importance of the working point.
Airflow and static pressure
Let’s start with the definitions:
- Airflow is the volume of air produced by the fan over time and is measured in cubic meters per hour (m³/h) in metric units, or cubic feet per minute (CFM) in imperial units. For example, if you have a 5m x 5m x 5m enclosure and a fan that generates 125 m³/h of airflow, approximately the fan will take 1 hour to take the hot air from the enclosure (even if the experience teaches us that it’s not always that simple!).
- Static pressure is the amount of force exerted by the air on the walls of an enclosure (pressure) and is measured in Pascal (Pa) or in inches of water (inH2O), in simple words how far the airflow will be able to arrive.
How to choose the filter group: example
This graph shows the performance of a filter fan unit through airflow and static pressure values.
It is important to know that even if maximum values for airflow and static pressure are specified, the filter fan will never be able to provide the two maximum values at the same time.
In this second graph, however, the inverse correlation between the airflow and the static pressure is shown.
As the airflow increases, the static pressure decreases and vice versa.
The three points on the graph represent the possible and hypothetical scenarios in which the filter fan unit could work, in this case at 60Hz (red curve), but the same is true for the frequency of 50Hz (blue curve).
In order to better understand the three scenarios, it is necessary to imagine an enclosure on which we mount a filter group, referring to the graph above with the three points designated 1, 2, and 3.
For point 1 we have to imagine an enclosure completely open on one end. There is nothing obstructing the movement of air from the fan and all the airflow is expelled from the other end. In this case, we will have the maximum airflow (unimpeded) and the static pressure at zero.
For point 2 we have to imagine a closed enclosure except for a small exhaust hole, or air outlet, at the other end. The size of this exhaust hole is smaller than that of the filter assembly mounted in the intake, which impedes the flow of air. The constant accumulation of air inside the enclosure, not being able to escape, increases the static pressure inside it. In this case, the airflow is limited by the increase in static pressure and will be lower than its maximum value.
For point 3 we have to imagine a completely closed enclosure. In this case, the flow of air entering the enclosure will increase the static pressure as there is no room for the air to flow away. Once the static pressure specification is exceeded, even if the filter fan continues to operate, the high static pressure will no longer allow air to enter. In this case, the maximum static pressure has been reached and the volume of the airflow drops to zero.
In real applications, cases 1 and 3 are not realistic. In a practical example of ventilation of an electrical cabinet, most of the filter fan units approach example 2. However, to produce the graph, a method similar to the examples described above is used, using an air chamber.
The importance of the aeraulic impedance (or pressure drop)
The actual airflow and static pressure are determined by the aeraulic impedance. Aeraulic impedance is defined as resistance to the passage of airflow which could be in the form of electronic components, walls, or anything that impedes the airflow, in our specific case from the air outlet filter.
For once, let’s leave the mathematical formulas aside and see the effect of this aeraulic impedance on the graph of a filter fan.
Let’s go back to the graph initially studied in which we found the performance curves of the FF15A230UF filter fan shown in blue (50Hz) and in red (60Hz) and add the filter impedance curves shown in green (for an FF15U filter with the same filter fan size) and dashed orange (for an FF20U filter that is larger than the filter fan unit).
The actual airflow and the static pressure that we will find in the cabinet on which the filter fan unit and filter are mounted are determined at the point where the impedance curve (green) intersects the performance curve (blue or red).
Choose a filter fan unit: the working point
For a correct design of the electrical cabinet ventilation, it is therefore important to keep in mind that the correct value to be taken into consideration is not the maximum flow rate in free air, but rather the “working point” of the filter fan unit with an output filter.
In addition to the correct selection of the filter fan, the positioning of the components within the electrical cabinet and their density must also be taken into account.
The number of components within a cabinet determines the “installation density”.
With fewer components (low installation density), there is more room for air to pass through and the filter assembly can produce high airflow.
With more components (high installation density), there are more obstacles in the path of the airflow. In this case, the high induced static pressure drastically reduces the airflow, bringing it well below its working point.
Sometimes it is enough to take small precautions to avoid large losses in airflow, one of these for example is to avoid mounting the filter fan unit right in front of a large component (for example a transformer, an inverter, etc.), or, in the absence of specific programs for calculating the air flows, imagine what the airflow could be that is created inside the panel by optimizing the position of the components and/or of the filter fan unit (and of the filter), uniforming the airflow and trying to avoid hot spots.
The Fandis range of filter fan units
The maximum airflow, static pressure, and duty point values that you will find in the technical data sheets of our products refer to an empty electrical cabinet. Take this into consideration when choosing the most suitable model for your electrical cabinet with high-density installation.
To find out more, visit our website fandis.com and discover our products for electrical panels, or consult our articles on filter fan units. To receive information, contact us or leave a comment on this article. One of our technicians will answer you as soon as possible.