The static pressure is a measure of the pressure that the fan can generate in the duct system. This is the pressure required to move air through the system. The total pressure comprises both the static pressure and the dynamic pressure generated by the air movement. As the dynamic pressure is not directly related to the power of the fan system, the total pressure can be misleading in the design process.
For example, the dynamic pressure with axial fans is generally higher than with centrifugal fans. If you base the system on the total pressure, you may conclude that axial fans have a higher output than centrifugal fans. However, this is not the case if the fans are connected to a duct system. In this case, when designing based on static pressure, the centrifugal fan is usually the best choice.
The static pressure also relates directly to specific energy consumption. That is why it is also the right parameter to use when choosing the fan that will give you optimum energy savings. This is because, once again, the axial fan seems to have a higher efficiency level than the centrifugal fan, measured by the total pressure. However, the centrifugal fan uses less electrical power to achieve the same or an even better result. To assess how much energy is actually consumed to achieve the desired result, it is therefore always best to use the static efficiency.
Total pressure can lead to inaccuracies
Measuring the static pressure is also a relatively simple and straightforward process that can be performed using a variety of instruments. However, the total pressure is more difficult to measure, as the flow rate is measured. In a fan or duct system, finding a measuring point that is not influenced by local speed differences and turbulence, for example, is difficult, leading to inaccuracies in the measurement.
It often takes a lot of experience to correctly interpret measurements containing the dynamic pressure. The situation is different in a measurement laboratory that evaluates fan data, but ultimately it comes down to how energy efficient the fan is where it is actually being used. The static pressure helps to assess this relationship more easily.
Many fan manufacturers indicate the performance of their products using static pressure, making it easier to compare different models and select the right fan for a particular system and application. Some manufacturers use the overall pressure to advertise their products so that they can indicate supposedly better values. For example, if an axial fan with an efficiency level of over 90 percent is advertised compared to a 60 percent centrifugal fan, this discrepancy may be based on different basic principles.
In this case, the efficiency of the axial fan relates only to the efficiency of the fan impeller based on the total pressure, compared to a complete centrifugal fan, including motor and control, based on static pressure. Comparisons are also made between two complete fan systems, with the axial based on the total pressure and centrifugal based on static pressure. The axial system is given here with a 20 percent higher efficiency level. The difference between the efficiency levels of the two fans is not objective, but rather a marketing method.
Some manufacturers use the overall pressure to advertise their products so that they can indicate supposedly better values.
Axial fans can operate at the same efficiency levels as centrifugal fans. However, it is clear that the laboratory data for axial fans is often difficult to reproduce in practice, as they are more sensitive to disturbances in the installation space than centrifugal fans. This can result, for example, in efficiency losses when converting existing ventilation systems to new energy-saving axial fans. Here, centrifugal fans are less sensitive and can reproduce the data ascertained in the laboratory more effectively, even in real-life operation.
Static pressure is the more suitable measure
In summary, although the total pressure is an important measure of flow dynamics, the static pressure is the more suitable measure for analyzing and comparing fan systems. It is more relevant and easier to measure for the performance of the fan system in a real-world scenario and is often used by manufacturers as a measure of fan performance.