Fresh air through closed windows

Constantly improving insu­la­tion in build­ings is accom­pa­nied by increas­ingly refined solu­tions for room venti­lation. Today controlled venti­lation systems with heat recovery are used with increasing frequency. The used but warm air from the inte­rior is routed with a fan through a heat exchanger to the outside, while cool fresh air is brought in from the outside via this heat exchanger and warmed up. This mini­mizes heating losses and reduces the amount of required elec­trical energy.

Figure 1: Window frame with controlled venti­lation.

Active or controlled venti­lation systems can be designed as central­ized or decen­tral­ized systems. In the former, a central system distrib­utes the air via ducts throughout the house; in the latter, each room is supplied inde­pen­dently of the others. The devices used in decen­tral­ized systems are much smaller and can be inte­grated in a building’s existing struc­ture. A partic­u­larly elegant solu­tion is to inte­grate the fan, the heat exchanger and the control elec­tronics directly in the window frame (Figure 1). This was the approach taken by REHAU in the devel­op­ment of its GENEO INOVENT window fan in coop­er­a­tion with the fan special­ists at motor and fan manu­fac­turer ebm-papst.

Require­ments for fans and air duct design

Figure 2: Series fan with 65 mm impeller diam­eter.

Such a solu­tion imposes special require­ments on fans and air duct design. The conflicting aims of achieving the required air flow with compact dimen­sions and low noise emis­sions need to be recon­ciled. In the case described here, the fans from ebm-papst were specially designed to meet the condi­tions of REHAU’s GENEO window system. The avail­able space in the window frame made it neces­sary to design for two sepa­rate fan-heat exchanger systems for air intake and exhaust instead of a single system with a bulky coun­ter­flow heat exchanger. The fans were designed using modern 3D flow simu­la­tions, first for the fans alone, later together with the window frame compo­nents rele­vant to air flow.

Fan, heat exchanger and control elec­tronics are inte­grated in the window frame.

Special use for cylin­drical rotor fans

Figure 3: Effects of motor on sound power of fan.

For this task newly designed compact centrifugal fans from ebm-papst were chosen, so-called Sirocco fans, some­times also called squirrel-cage fans. These DC fans have elec­tron­i­cally commu­tated drives with elec­tronic reverse polarity protec­tion. The elec­tronics are inte­grated in the fan’s impeller hub to save space. The outstanding effi­ciency of the brush­less drive results in lower heat stress for the bear­ings, which increases the service life of the fan. The air flows in through a round inlet orifice, is radi­ally deflected inside the fan and leaves it through the tangen­tially posi­tioned outlet. Sirocco fans are char­ac­ter­ized by a high number of blades that are bent in the direc­tion of rota­tion. The blades transfer the rota­tional energy to the air flow while the static pres­sure increase takes place in the spiral. Without the spiral the air would simply be mixed. Due to its design prin­ciple, this fan type has highly turbu­lent air flow with lower aero­dy­namic effi­ciency compared to other centrifugal fan designs. On the other hand, the high number of blades is bene­fi­cial as it results in lower noise emis­sions with fewer disturbing tonal compo­nents. Today, fluid mechan­ical calcu­la­tions for centrifugal fans with forward-curved blades are possible with modern simu­la­tion methods. Using numer­ical flow simu­la­tions, the complex air flow struc­ture in the fans can be computed and the results are used to opti­mize the fan geom­etry with respect to the appli­ca­tion.

Three-step fan design

Figure 4: Geomet­rical adap­ta­tion of centrifugal fan to the avail­able space.

The fan design took place in three steps. In the first step, a low-vibra­tion elec­tron­i­cally commu­tated three-phase motor was inte­grated in a fan with similar dimen­sions (Figure 2). This drive ensures minimal gener­a­tion of the struc­ture-borne noise that is often consid­ered disturbing at low speeds. In the second step, the aero­dy­namic compo­nents (scroll housing), number of blades and blade angles) were designed using 3D flow simu­la­tions and related exper­i­ments. Figure 3 shows the fan’s overall sound power plotted against the back pres­sure of the flow chan­nels through the window frame.
With increasing air flow through the flow chan­nels, the back pres­sure increases, thereby requiring a higher fan speed. Conse­quently, the values on the x-axis are also a measure of the fanspeed: the higher the speed, the greater the air flow, the higher the back pres­sure and the higher the sound power level. The black curve shows the behavior of the series fan with a basic single-phase motor. The blue curve denotes a fan with the same dimen­sions and drive but with opti­mized aero­dy­namics and the red curve shows the acoustic behavior of the series fan when equipped with a three-phase motor.

Improve­ments to aero­dy­namics and motor

It can be seen that improve­ments in aero­dy­namics are just as effec­tive as choosing the right motor tech­nology. For pres­sures above 90 Pa, both effects can be approx­i­mately added without nega­tive inter­fer­ence, so both improve­ments were used in the new fan. The fan dimen­sions were further reduced and adapted to the geomet­rical require­ments of the appli­ca­tion (Figure 4).

Thanks to the easy window replace­ment the building suffers no nega­tive visual impact.

Further opti­miza­tion with CFD

Figure 5: Flow simu­la­tion of complete fan in window frame.

Since the fan cannot be oper­ated under optimum intake and outlet condi­tions, the actual condi­tions of instal­la­tion have to be included in further opti­miza­tion efforts. For this purpose, compu­ta­tional fluid dynamics (CFD) is a very effec­tive method. The rela­tively large effort involved in performing CFD calcu­la­tions enables the simu­la­tion of complex flow processes in a unique way (Figure 5). Effec­tive visu­al­iza­tion of the results helps in assessing modi­fi­ca­tions and improve­ments so the amount of exper­i­men­ta­tion can be reduced consid­er­ably. To inte­grate the system into the window frame, two fans with iden­tical aero­dy­namics and motors but different direc­tions of rota­tion are needed: a clock­wise fan for the intake and a coun­ter­clock­wise fan for the exhaust.

Figure 6: Air perfor­mance and sound power of centrifugal fan with point of oper­a­tion and aero­dy­namic resis­tance curve for window frame.

With respect to air perfor­mance and noise emis­sions (shown as sound power), the fan oper­ates close to its acoustic minimum. Regarding the scale of the red curve in Figure 6 the devi­a­tions are well within measure­ment toler­ances and product vari­a­tions.
Decen­tral­ized resi­den­tial venti­lation systems inte­grated in window frames provide the conve­nience of fresh, preheated air. Thanks to the easy window replace­ment without extra instal­la­tion effort, the building suffers no nega­tive visual impact. The window’s func­tions of noise suppres­sion and protec­tion against break-ins remain in effect. To fully achieve these bene­fits, ebm-papst devel­oped espe­cially effi­cient and quiet fans with a design opti­mized for the limited avail­able space in window frames.