© Photo | Fotolia © Robert Daly

“Air-guiding system” guar­an­tees low noise output

Air inlet grille for axial and centrifugal fans


In appli­ca­tions relating to refrig­er­a­tion, air condi­tioning and venti­la­tion tech­nology, users frequently observe that the selected fan installed in the device does not run as quietly as expected. Such fans often generate a level of oper­ating noise consid­er­ably higher than that stated in the product docu­men­ta­tion. The reason for this is simple: the values stated in the docu­men­ta­tion are based on repro­ducible measure­ments under stan­dard condi­tions (without any disrup­tions). However, the appli­ca­tions them­selves are subject to inflow disrup­tions of varying inten­sity that are caused by the instal­la­tion condi­tions in the respec­tive device. These lead to addi­tional noise gener­a­tion. Help is now at hand thanks to an air inlet grille that essen­tially has a straight­ening effect, consid­er­ably reducing unde­sired disrup­tions and effec­tively reducing the level of noise gener­a­tion.

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Figure 1: Typical inflow in customer device with turbu­lence

Heat pumps and air condi­tioners are installed in different ways, with varying dimen­sions and designs. For example, differ­ences occur in the posi­tioning of the open­ings required for venti­la­tion, in the heat exchanger surface and in the packing density. The air flow that enters the fan is there­fore influ­enced by the other compo­nents such as the heat exchanger, in addi­tion to being influ­enced by the instal­la­tion situ­a­tion. Depending on the appli­ca­tion, the inflow to the fan is subse­quently highly uneven and contains non-stationary compo­nents. For example, the housing walls of rectan­gular heat exchangers exhibit back­flow regions with corre­sponding circu­la­tion – that is, air turbu­lence (figure 1). These in turn are then directed towards the loca­tions with the least amount of clear­ance between the fan and the housing wall. It is here that the turbu­lence from both sides is combined, causing ‘vortexes’ that generate high levels of turbu­lence. This causes major fluc­tu­a­tions in the pres­sure and speed at the front edge of the blade, which can some­times lead to dras­ti­cally increased levels of addi­tional noise, partic­u­larly in the low-frequency range. This gener­ates broad­band noise and narrow­band, tonal frequency compo­nents, also referred to as tonal noise.

The tonal noise consists of the blade-passing noise and its harmonics. The formula for this is f=n*z*k. The frequency of the blade-passing noise f is calcu­lated from the product of the speed of the fan n and the number of blades z. The harmonics of the blade-passing noise are the multiple of this, desig­nated as k. Accord­ingly, an axial fan with five blades and 1,200 rpm (20 rps), for example, would result in a blade-passing noise at a frequency of 100 Hz. We are all familiar with the unpleasant ‘humming’ noises that are typi­cally gener­ated as a result.

Putting an end to ‘humming’ noises

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Figure 2: The air inlet grille dras­ti­cally reduces the noise-gener­ating disrup­tions to the inflow and is equally effec­tive with both axial and centrifugal fans. It has virtu­ally no effect on power input and air perfor­mance

The addi­tional noises gener­ated by heat exchangers and air condi­tioners are not only unde­sired, they are intol­er­able. This is partic­u­larly the case in resi­den­tial envi­ron­ments, although elim­i­nating these noises is far from easy. It is not possible to compen­sate for disrup­tions to the inflow by opti­mising the fan. Providing addi­tional insu­la­tion for the housing also brings little success in prac­tice, as corre­sponding insu­la­tion panels are typi­cally only effec­tive as of higher frequency levels. The fan specialist ebm-papst Mulfingen there­fore took a different approach: if you improve the inflow of air to the fan, this reduces the turbu­lence and there­fore also reduces the unpleasant low-frequency noises that are caused by this.

With this in mind, the Mulfingen engi­neers devel­oped the special Flow­Grid air inlet grille which has a straight­ening effect on the inflowing air (figure 2). This dras­ti­cally reduces the noise-gener­ating disrup­tions to the inflow and is equally effec­tive with both axial and centrifugal fans. In the case of a condenser fitted with an axial fan, for example, the deploy­ment of the air inlet grille reduces the noise level by 3.9 dB(A) and the tonal noise by 16 dB. With regard to a low profile air condi­tioning device (diam­eter 250 mm), the Flow­Grid reduces the noise level by 2.5 dB(A) and reduces the tonal noise by 9 dB. Figure 3 shows the actual results of a noise measure­ment performed on an example condenser.

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Figure 3: The air inlet grille achieves a signif­i­cant reduc­tion in the sound pres­sure level and consid­er­ably weakens tonal noise. The figure shows the actual results of a noise measure­ment performed on a condenser

The use of the air inlet grille reduces the sound pres­sure level and consid­er­ably weakens the tonal noise. There­fore, there is signif­i­cantly less need for insu­la­tion and noise protec­tion. The air inlet grille is made of injec­tion-moulded plastic and satis­fies various fire protec­tion classes up to UL94-5VA. You can use screws to quickly and easily secure the Flow­Grid to axial and centrifugal fans, and it is possible to imple­ment any neces­sary appli­ca­tion-specific adjust­ments while securing the Flow­Grid. Depending on the design of the end device, it is even possible to retrofit the Flow­Grid, for example in the course of any upcoming service work. In the case of a size 800 axial fan, the air inlet grille requires only around an addi­tional 15 cm of instal­la­tion space in the axial direc­tion, which is typi­cally avail­able in the appli­ca­tions.

A further option for acoustic improve­ment

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Figure 4: The diffuser reduces the noise gener­a­tion in the medium-frequency range, and can be combined with the air inlet grille

Purchasers in the field of venti­la­tion, refrig­er­a­tion and air condi­tioning tech­nology can take addi­tional action regarding noise reduc­tion: further opti­mi­sa­tions can be achieved with the help of another passive compo­nent. If users take a fan and combine the air inlet grille described above with an “AxiTop” diffuser on the discharge side, this both increases energy effi­ciency and reduces noise emis­sions still further – above all in the medium-frequency range. This means that the Flow­Grid reduces the deep-frequency range, while the AxiTop is respon­sible for a further reduc­tion in the medium-frequency range. It is easy to grasp how this works: in fans with free outflow, the exit losses that result from the oper­ating prin­ciple are frequently under­es­ti­mated as an energy consumer. The diffuser can help minimise these losses. It oper­ates rather like a reverse nozzle and signif­i­cantly reduces the exit losses thanks to its pres­sure-increasing effect (figure 4). Effi­ciency is increased and the oper­ating noise is simul­ta­ne­ously reduced.

Ideal oper­ating condi­tions for fans

The acoustic improve­ment is primarily of interest when fans are in oper­a­tion in envi­ron­ments where noise is a crit­ical factor and the user addi­tion­ally combines the diffuser with the air inlet grille (figure 5), for example when using a tested condenser. In this case, the outside air is drawn through a heat exchanger.The condenser is fitted with an axial fan that has a diam­eter of 800 mm and runs a diffuser fitted on the pres­sure side. The noise level can be reduced by an addi­tional 3 dB(A) with the help of the air inlet grille.

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Figure 5: The combined use of the AxiTop and Flow­Grid resulted in a 5.8 dB(A) reduc­tion in noise emis­sions from the tested condenser, primarily in the low-frequency range

Thanks to the combined use of the AxiTop and Flow­Grid, the noise level is reduced by 5.8 dB(A) and the tonal noise is reduced by 20 dB – any persons present will find this to be very pleasant. Like the air inlet grille, the diffuser is also made from light, resis­tant plastic and is easy to mount and retrofit. As it is only 250 mm high it is gener­ally not neces­sary to change the design of the appli­ca­tion, even for the purpose of retro­fitting. Purchasers will soon be able to benefit from the advan­tages of noise reduc­tion in the low-frequency range: Flow­Grid models for size 500 and size 630 axial fans, which are the typical sizes for use in heat pumps, will be avail­able in the first quarter of 2014. Versions for size 710 and size 800 centrifugal fans, as used in larger air condi­tioners, will also be entering series produc­tion soon; addi­tional versions will follow. The AxiTop has already entered series produc­tion.

Using passive compo­nents – the diffuser and the air inlet grille – the Mulfingen special­ists have once again succeeded in contin­uing the devel­op­ment of fan tech­nology and setting new stan­dards. The opti­mi­sa­tion of inflow and outflow provides ideal oper­ating condi­tions for fans, enabling energy-effi­cient oper­a­tion that is as quiet as possible.

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