Today, modern centrifugal and axial fans are very effi­cient and quiet. However, once installed in an appli­ca­tion, their behavior will change if the inflow or outflow condi­tions are disturbed. In an air-condi­tioning unit, fans are ulti­mately a means to an end. Nega­tive effects are there­fore usually due to func­tion. Flaps and filters can change the air flow and the distance to walls and heat exchangers, as well as the use of guard grills, have an impact. Typical selec­tion programs (Fig. 1) can only take into account these effects to a limited extent, as they use the values deter­mined under labo­ra­tory condi­tions (Fig. 2).

Fig. 1: Selec­tion programs should be config­ured in such a way that they request as many para­me­ters as possible that are based on the instal­la­tion situ­a­tion. (Photo | ebm-papst)

These must ulti­mately be based on repro­ducible measure­ments under (undis­turbed) stan­dard condi­tions. In the appli­ca­tion, however – due to the instal­la­tion condi­tions in the respec­tive unit – the turbu­lence is more or less pronounced. This turbu­lence leads to noise gener­a­tion that is diffi­cult to calcu­late under real condi­tions. Once installed in the unit, the docu­mented values often do not match reality.

Fig. 2: The data sheet spec­i­fi­ca­tions are deter­mined on the test stand. These must be based on repro­ducible measure­ments under (undis­turbed) stan­dard condi­tions. (Photo | ebm-papst)

Figure 3 shows how energy consump­tion and noise change depending on the intake-side instal­la­tion situ­a­tion – depending on how much the housing hinders the flow, i.e. whether it is drawn in axially from the front, centrifu­gally from all sides, or on one side. In the worst case, this signif­i­cantly increases power consump­tion and noise level at the same oper­ating point.

To ensure safety, axial fans usually have to be fitted with guard grills. Here, it is impor­tant to adapt the geom­etry of the guard grill to the flow control in order to generate as few losses and noises as possible. Nowa­days, indi­vidual acoustic effects of guard grills have a much stronger effect than a few years ago. Modern fans in them­selves work so quietly that the effects caused by the guard grill can be heard more strongly. It is there­fore not useful to rely only on catalog values when it comes to power consump­tion and noise emis­sions. Selec­tion programs should there­fore be config­ured in such a way that they request as many para­me­ters as possible that are based on the instal­la­tion situ­a­tion.

Fig. 3: Energy consump­tion and noise change, depending on how the housing influ­ences the flow – i.e. whether it is drawn in axially from the front, centrifu­gally from all sides, or on one side. (Graphic | ebm-papst)

Taking the instal­la­tion situ­a­tion into account during devel­op­ment

Since inter­ac­tions between the fan and the appli­ca­tion can always occur, fan and motor specialist ebm-papst endeavors to take the subse­quent instal­la­tion situ­a­tion – wher­ever possible – into account when devel­oping a fan. Here, appli­ca­tion exper­tise gained over the course of decades is incor­po­rated, and the wide range of disci­plines involved in devel­op­ment work closely together, starting with high-perfor­mance simu­la­tion tools and test stands, right up to the psychoa­coustic testing labo­ra­tory.

Fig. 4: With the RadiPac fans, ebm-papst not only opti­mized the impeller, motor, and control elec­tronics in terms of energy effi­ciency and noise emis­sions, but also took into account the actual instal­la­tion situ­a­tion in AHUs. (Photo | ebm-papst)

The latter is becoming increasing impor­tant. This is because today an air-water heat pump set up outside can annoy the neigh­bor­hood with disturbing noise, even if it may meet the German “TA” noise regu­la­tion. To be on the safe side, fans should be opti­mized in accor­dance with psychoa­coustic criteria in addi­tion to the common eval­u­a­tion of the noise level.

If the subse­quent instal­la­tion situ­a­tion is taken into account during devel­op­ment, this signif­i­cantly improves the results, as shown in the example of RadiPac fans (Fig. 4). Here, ebm-papst not only opti­mized the impeller, motor, and control elec­tronics in terms of energy effi­ciency and noise emis­sions, but also took into account the actual instal­la­tion situ­a­tion in AHUs. The wide optimum effi­ciency range of centrifugal fans means that the fans operate at prac­ti­cally every oper­ating point with the lowest possible power consump­tion, and there are also no unpleasant surprises in terms of noise levels.

Flow­Grid improves noise level

Fig. 5: The Flow­Grid is suit­able for centrifugal fans (left) and axial fans (right) (Photo | ebm-papst)

There is a worst case for every manu­fac­turer of an AHU: When installed, the selected fan is too loud or not effi­cient enough. In the worst-case scenario, market launch can be delayed by this consid­er­ably. In precisely such cases, it may be worth­while to ask the special­ists. Passive compo­nents such as the Flow­Grid air-inlet grille (Fig. 5), which is suit­able for axial and centrifugal fans, can also mini­mize unwanted effects after instal­la­tion retroac­tively.

If the fans used are retro­fitted with this, this dras­ti­cally reduces noise-gener­ating turbu­lence in the inflow without reducing the air perfor­mance or power consump­tion (Fig. 7). Good results can be obtained here in noise-sensi­tive appli­ca­tions such as heat pumps, resi­den­tial venti­lation units and air puri­fiers for class­rooms.

Fig 6.: Addi­tional parts such as the Flow­Grid can also be used to reduce unpleasant effects after instal­la­tion. (Graphic | ebm-papst)

Devel­op­ment under real condi­tions

Anyone wanting to be on the safe side right from the start should involve the fan manu­fac­turer in an early devel­op­ment phase of their unit. In the case of large and small fans, it is equally worth­while to precisely analyze and eval­uate the flow situ­a­tion and find suit­able opti­miza­tion measures. As a compe­tent partner, the fan and motor special­ists at ebm-papst constantly set the stan­dard here when air-guiding units are concerned to prevent prob­lems in advance or to achieve the best possible noise and effi­ciency before product launch.