Highly effi­cient centrifugal compact fan for house­hold appli­ances

Forced air circu­la­tion can increase the effi­ciency of the heat exchanger and with this, it is possible to use a special compact heat exchanger, which would not func­tion with natural convec­tion alone. In addi­tion, the temper­a­ture distri­b­u­tion in the device can be adjusted by targeted distri­b­u­tion of cold air, creating the perfect storage condi­tions in terms of temper­a­ture and humidity. The forced air circu­la­tion inside the device also prevents air mois­ture from condensing on the equip­ment parts and stored items.

Best possible level of device effi­ciency at minimum power consump­tion

To achieve the best possible level of device effi­ciency and there­fore to minimise power consump­tion, every compo­nent must be opti­mised and partic­ular atten­tion must be paid to how the indi­vidual compo­nents interact with one another. In fans that are located inside the device to be cooled, energy is not just used to drive the fan; the motor heat gener­ated by the fan must also be included in the overall thermal balance. The third crucial design para­meter for house­hold appli­ances is the noise emis­sion. This must be kept as low as possible, as noise is an impor­tant quality factor.

Design spec­i­fi­ca­tions for the fan can be derived from these basic condi­tions. In the instance described, the require­ments led to a new centrifugal compact fan being devel­oped, which, with an input capacity of around one watt, achieves a maximum effi­ciency of 22 %. The aero­dy­namic compo­nents have been espe­cially designed and perfected for their appli­ca­tion. The impeller and worm gear unit have been designed in unison so that an ideal inter­ac­tion between the indi­vidual compo­nents can be guar­an­teed as early as the design phase.

A low circum­fer­en­tial speed for the impeller was very impor­tant in this process, as it results in low noise emis­sions. in line with the aero­dy­namic and aeroa­coustic config­u­ra­tion, a motor that produced partic­u­larly low levels of struc­ture-borne noise has been designed and adapted to the required speed and torque.

Figure 1: Completed fan and CDF model. The direc­tion of rota­tion in the model is the oppo­site of the completed fan and the minimum distance between the impeller and housing has been increased in favour of noise emis­sions. There are pockets at the fan inlet for balancing weights, which are used to guar­antee that the fan runs with low vibra­tions.

The aero­dy­namic compo­nents have been entirely designed with modern, three-dimen­sional compu­ta­tional fluid dynamics (CFD). These were used in the refrig­er­a­tion unit in a much earlier state of devel­op­ment when defining the require­ments for the fan. in the course of the devel­op­ment, the reac­tion of the fan to the noise behav­iour was also eval­u­ated by means of simu­la­tions. for this, the incom­press­ible Navier stokes equa­tions involving fric­tion are solved for the given fan geom­etry. The geom­etry, i.e. the design of the blades, current cross-section and housing geom­etry, are modi­fied until the current moves through the fan with as little noise as possible and with as little loss as possible. More details of the design process and the descrip­tion of the exper­i­mental devices used can be found in Schmitz, et. al., Design and Test of a small High-Perfor­mance Diag­onal fan, Proc. iGTi 2011, Vancouver.

Rapid-Proto­typing

Figure 2: Aero­dy­namic power data of the fan unit

In the next state of devel­op­ment, the compo­nents of the fan are built in a rapid proto­typing proce­dure, the motor is inte­grated and the entire unit is eval­u­ated with regard to air perfor­mance and acoustics. The oper­ating point of the fan is within the range 30-40 m³/h. In this range, the pres­sure build-up in the fan corre­sponds to the system resis­tance of 15-25 Pa. The fan achieves an overall effi­ciency of around 20 %. This means that one fifth of the elec­trical energy is converted into current energy. for a fan of this size and perfor­mance level, this is a very good figure, partic­u­larly when one considers the compro­mises that have to be made.

To achieve a higher level of effi­ciency, the gap between the rotating and static compo­nents must be kept as small as possible. However, if the gap is too small, there is a danger of the refrig­er­ating unit freezing, which is why a minimum gap size must be adhered to. The smaller the distance between the casing tab and the impeller, the more audible the aeroa­coustic inter­ac­tions of both compo­nents. These contra­dic­tory require­ments mean that compro­mises have to be made and the compro­mises needed for good acoustics were deter­mined in exper­i­ments using proto­types.