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Watch out when selecting fans!

When selecting a fan, it is often more useful to look at the static pres­sure instead of the total pres­sure. Our Danish colleague Torben Lintrup Kirk­holt explains why this is the case and talks about why manu­fac­turer details based on overall pres­sure should be treated with caution.

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.

Torben Lintrup Kirk­holt is Managing Director of ebm-papst Denmark (Photo: ebm-papst)

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.

When comparing an axial fan – here an AxiEco Perform – with a centrifugal fan – here a RadiPac Pres­sure – a uniform basis is crucial. (Photo: ebm-papst)

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.

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  • Manoochehr Darvish on said:

    I do not agree with the state­ment that the static pres­sure is more suit­able. It might be correct when comparing axial and radial fans, but not every­where else. For example, when a radial fan is ducted, the dynamic pres­sure at the outlet of the fan is not a loss, and in this case it is more mean­ingful to work with the total pres­sure. Using the static pres­sure in this case leads to an over­sized fan. If the radial fan is not ducted, i. e. open outlet, then the static pres­sure is more suit­able. In other words, selecting between the total and the static pres­sure depends on the boundary condi­tions under which the fan should operate.

    • mag-Team on said:

      Thank you for your comment, below please find the answer from our author Torben:

      The amount of dynamic pres­sure that can be recov­ered in a duct system depends on various factors, including the design of the duct system, the flow condi­tions, and the pres­ence of any devices or compo­nents within the system.

      In an ideal situ­a­tion, with a well-designed duct system and smooth, stream­lined ducts, it is possible to recover a signif­i­cant portion of the dynamic pres­sure. This can be achieved through careful consid­er­a­tion of factors such as duct size, shape, and routing, as well as the use of devices like diffusers or expan­sion cham­bers. By mini­mizing flow restric­tions, avoiding sharp bends or sudden changes in direc­tion, and employing appro­priate flow control mech­a­nisms, it is possible to reduce the losses in dynamic pres­sure.

      However, it’s impor­tant to note that no duct system can achieve 100% recovery of dynamic pres­sure. There will always be some level of pres­sure loss due to factors such as fric­tional losses along the duct walls, turbu­lence, and the conver­sion of dynamic pres­sure into other forms of energy (e.g., heat). The actual amount of dynamic pres­sure that can be recov­ered in a specific duct system will vary depending on the afore­men­tioned factors and the specific appli­ca­tion.

      To maxi­mize dynamic pres­sure recovery, engi­neers and designers often employ compu­ta­tional fluid dynamics (CFD) simu­la­tions or conduct exper­i­mental tests to opti­mize the duct system’s config­u­ra­tion. These tech­niques help iden­tify areas of high pres­sure losses and enable the imple­men­ta­tion of design improve­ments to enhance overall effi­ciency.

      Deter­mining the average percentage of dynamic pres­sure recovery in a duct system is chal­lenging, as it depends on numerous factors specific to the system and its appli­ca­tion. Recovery effi­cien­cies can vary signif­i­cantly depending on the design, oper­ating condi­tions, and the intended purpose of the duct system.

      In prac­tice, the percentage of dynamic pres­sure recovery can range from a few percent to over 90%, but it is impor­tant to note that achieving extremely high recovery effi­cien­cies is rare. In most cases, a recovery effi­ciency of around 10-20% or lower is consid­ered typical for duct systems without special­ized compo­nents or devices.

      To achieve higher recovery effi­cien­cies, engi­neers employ various tech­niques and compo­nents such as diffusers, expan­sion cham­bers, and flow control devices. These elements help to mini­mize pres­sure losses, opti­mize airflow, and increase the poten­tial for pres­sure recovery. However, the actual recovery percentage will still depend on the specific design choices and trade-offs made during the system’s devel­op­ment.

      It’s worth mentioning that the term “recovery” is often used in different contexts, such as heat recovery or energy recovery, where the focus is on capturing and utilizing waste heat or energy rather than specif­i­cally targeting dynamic pres­sure. Thus, the recovery percentage can vary signif­i­cantly depending on the partic­ular objec­tive and the nature of the system being consid­ered. Recov­ering dynamic pres­sure means changing it to static pres­sure or vice versa. These changes are always involving losses.

      To sum up the static pres­sure still accounts for the major para­meter when comparing fans. For example, an axial fan can be said to have 20% higher effi­ciency than the radial fan because the dynamic pres­sure consti­tutes that extra part. Now if these 20% are recov­ered with an effi­ciency of 10% the extra effi­ciency of the axial fan when consid­ering static pres­sure incl. the recov­ered dynamic pres­sure will be only 2%.

      As the radial fan also has a dynamic compo­nent, however smaller than the axial fan we will also see a recovery in this system so the effi­ciency of the radial fan incl. the recov­ered dynamic pres­sure will also be higher – in real life if we include dynamic pres­sure we bring in a compo­nent of eval­u­ating effi­ciency that is volatile between systems and should be avoided.

      If you want to have an example you can use our axial fan with and without the AxiTop: The AxiTop does exactly what I described – change the dynamic into static and increase effi­ciency – you can also see that it takes a great deal of engi­neering and compo­nents to achieve some­thing here – this is not often done in the market.

      Best regards, Torben

  • Petteri Sippola on said:

    As far I under­stand, fan total pres­sure should always be the correct base­line for fan selec­tion and compar­ison. If the actual instal­la­tion is not ideal, both the static pres­sure and total pres­sure are reduced due to instal­la­tion losses, making direct compar­isons to stated pres­sures doubtful.

    The static pres­sure rise measured on site (and in lab) is affected by the flow velocity at inlet and outlet sides basing on Bernoulli’s prin­ciple. If the inlet and outlet areas are not equal, the differ­ence in respec­tive dynamic pres­sures should be taken into account when comparing the measured pres­sured to the stated total or static fan pres­sure.

    It is prob­lem­atic that fan manu­fac­tures often do not clearly indi­cate how the static or total fan pres­sures they state are defined, and what is the refer­ence instal­la­tion type for each fan (ducted or not). More­over, there is constant confu­sion between measured static pres­sure rise and “Fan static pres­sure” which is defined in ISO 5801 as “differ­ence between *static* pres­sure at the fan outlet and the *total* pres­sure at the fan inlet”, which can not be measured directly.

    In any case, it is crucial that the stated total pres­sure is the actual pres­sure achieved in given setup, and not (wrongly) defined as a sum of the measured static pres­sure rise + the calcu­lated dynamic pres­sure at fan outlet.

    Best Regards, Petteri