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Increasing fan effi­ciency

The Euro­pean Union’s Ecode­sign Direc­tive (ErP Direc­tive), is the basis for assessing the effi­ciency of various compo­nents and devices. For example, the minimum effi­ciency of fans is spec­i­fied in the fan regu­la­tion, Regu­la­tion (EU) No. 327/2011, which also considers fans that are built into refrig­er­a­tion, air condi­tioning and venti­la­tion equip­ment. Such equip­ment can only be granted a CE marking if its fans satisfy the fan regulation’s require­ments.

The effi­ciency require­ments for fans have been set out since 2013. They were raised in 2015, and there are plans to raise them again in the near future. The effects of the regu­la­tion have become impos­sible to over­look as energy-effi­cient EC fans are strength­ening their posi­tion on the market, reducing CO2 emis­sions and bene­fiting the envi­ron­ment. In addi­tion to energy-effi­cient motors, aero­dy­namic improve­ments are primarily respon­sible for the high effi­ciency levels of the modern fans.

Proper eval­u­a­tion of a fan’s effi­ciency means testing the fan as it will later be used. Other­wise it – and any equip­ment it is installed in – may not bear the CE marking. There are good reasons for this. Any slight modi­fi­ca­tion, for example in the design of nozzles or support struts, affects the fan’s effi­ciency and thus its ErP confor­mity.

Support struts cause unavoid­able blockage of the air flow at the intake or outlet, resulting in effi­ciency decreases of a few crucial percentage points depending on the oper­ating point (Fig. 1a). Nozzle geom­etry has similar effects, even when the air gap remains unchanged (Fig. 1b). The effi­ciency also decreases by a few percentage points when an axial fan is oper­ated in a short nozzle instead of a full nozzle. The posi­tion within the nozzle is also crucial; an axial shift of a few millime­ters can result in effi­ciency losses (Fig. 1c).

Those wanting to be on the safe side with regard to the fan regu­la­tion should there­fore select solu­tions whose energy effi­ciency has been deter­mined in exactly the condi­tions under which those solu­tions are ulti­mately to be used (Fig. 2 and text box). Other­wise the users must prove ErP confor­mity them­selves. Motor and fan specialist ebm-papst has a wide range of fans which, thanks to Green­Tech EC tech­nology and opti­mized aero­dy­namics, not only satisfy but substan­tially exceed the require­ments of the ErP Direc­tive.

Thanks to Green­Tech EC tech­nology and opti­mized aero­dy­namics ebm-papst fans exceed the require­ments of the ErP Direc­tive.

If one wishes to opti­mize a fan’s effi­ciency, one must examine the aero­dy­namic loss mech­a­nisms with an eye to reducing them. In axial fans, losses typi­cally arise at the gap between the rotating impeller and the stationary fan housing. Possi­bil­i­ties for improve­ment are rapidly exhausted since the gap will always have a certain size for manu­fac­turing reasons. In addi­tion, there are turbu­lence losses that arise due to differ­ences in flow speed and losses at the outlet.

In centrifugal fans, gap losses arise because some of the air is trans­ported in a circle through the nozzle gap. There are also pres­sure losses at the blades, which do not receive optimum air flow in all areas. For both fan types, the greatest poten­tial for improve­ment is in the outlet, where high outlet speeds lead to losses as the speed and thus the dynamic pres­sure go unused.

Guide vanes and diffusers

Figure 2: Fully assem­bled AxiBlade fan with fan housing, guard grill and guide vanes.

Much can be achieved by the use of guide vanes or a diffuser on axial fans. Since the rota­tion of an axial fan’s impeller imparts angular momentum to the expelled air, attaching stationary guide vanes results in a static pres­sure increase as the kinetic energy conveyed by the angular momentum is converted to static pres­sure. Diffusers exter­nally mounted at the outlet side offer a means of mini­mizing outlet pres­sure losses in larger axial fans. In prac­tice, use of the AxiTop diffuser from ebm-papst enables lower energy consump­tion and greater freedom for engi­neers and users. For example, the diffuser’s config­u­ra­tion can be opti­mized for various prop­er­ties depending on the appli­ca­tion. Higher fan output with the same energy input and lower energy consump­tion for the same air perfor­mance are equally possible. Acoustic perfor­mance can also be improved consid­er­ably with the diffuser.

Best possible effi­ciency at actual oper­ating point

To ensure that the axial fans really work at the best possible effi­ciency level when installed and oper­ated at appli­ca­tion-specific oper­ating points, the AxiBlade series was devel­oped; its axial fans operate in a wide range of appli­ca­tions at an effi­ciency of up to 54% with respect to the static pres­sure increase. At the same time, the noise level can be reduced by up to 8 dB(A) compared with the stan­dard range by increasing the diam­eter while main­taining the same space require­ments as the industry stan­dard and using a modular design; depending on the required pres­sure ranges, the fans can be combined with guide vanes. Further features include impellers with profiled blade geom­etry and winglets to mini­mize noise. The impellers are specif­i­cally designed for the different motors with which they can be combined, which also increases the effi­ciency.

Figure 3: Diam­eter increase at constant dimen­sions reduces outlet losses: compar­ison of HyBlade (orange) with AxiBlade without (blue) and with guide vanes (green). (Graph | ebm-papst)

In this way, the axial fans can be opti­mally designed for a partic­ular appli­ca­tion, with an effi­ciency increase of up to 40% compared with the proven HyBlade series. The version without guide vanes is suit­able for low to middle pres­sure ranges – up to 200 Pa for the axial fan depicted in Fig. 3. In this case, the bene­fits of the guide vanes do not come into play. Even without them, the effi­ciency and oper­ating noise are much better than the current industry stan­dard. Guide vanes are essen­tial at high back pres­sures of up to 260 Pa in order to reach high effi­ciency.

Since the foot­print of these axial fans corre­sponds to the current industry stan­dard, virtu­ally no design changes to customer equip­ment are neces­sary. And the fans require lower spaces than the usual models. Their height is likely to play a minor role for most appli­ca­tions, but not for trans­port. Every centimeter counts when the fans have to be loaded onto pallets or into containers.

Figure 4: Centrifugal fan in scroll housing (green) for increased pres­sure gener­a­tion and effi­ciency compared with a centrifugal blower (orange). (Graph | ebm-papst)

A centrifugal fan with scroll housing

With a view to the increas­ingly strict require­ments regarding the effi­ciency of venti­la­tion equip­ment for resi­den­tial build­ings, ebm-papst has aero­dy­nam­i­cally opti­mized its proven RadiCal centrifugal fans.

To further reduce outlet losses, the fans can be combined with a scroll housing that has also been aero­dy­nam­i­cally opti­mized. The scroll housing has an outlet with a round cross section for direct attach­ment to the pipe fitting on the unit’s air outlet. This consid­er­ably reduces the usual turbu­lent flow losses. At the same time, the char­ac­ter­istic curve becomes very pres­sure-insen­si­tive, and the effi­ciency increases by up to 38% compared with centrifugal blowers of the same design (Fig. 4). In addi­tion, the noise level decreases by 3.5 dB(A). These fans are also avail­able in a volume-flow-controlled design with a vane anemometer for extremely precise control.

For users, this makes it easy to be on the safe side of the ErP Direc­tive without great effort for both centrifugal and axial fans. Aero­dy­namic opti­miza­tion improves effi­ciency and noise char­ac­ter­is­tics, even in extremely different installed config­u­ra­tions.

The EU has finally defined the fan

There were previ­ously many gray areas in the fan regu­la­tion. For example, it was long unclear what minimum config­u­ra­tion a fan subject to the regu­la­tion should have and how it has to be eval­u­ated in terms of energy. The prin­ci­ples for deter­mining effi­ciency were thus insuf­fi­cient for manu­fac­turers and market regu­la­tors.

The new EN 17166 stan­dard, expected to take effect at the begin­ning of 2020, will elim­i­nate the gray areas and estab­lish a reli­able foun­da­tion for ErP assess­ments. A fan always consists of “signif­i­cant elements” that contribute to the conver­sion of elec­trical or mechan­ical power into pres­sure and air flow. These include the rotor, i.e. the impeller, and the stator, which consists of the housing, inlet ring, fan housing, etc., and the motor.

Of funda­mental impor­tance is the require­ment that the fan always has to be eval­u­ated as it will later be used in order to rightly bear the CE marking, because the motor and the entire flow machine always deter­mine the effi­ciency.


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