The present invention relates to a method and an apparatus for stabiliz¬ ing the pressure of a centrifugal fan which comprises an impeller mounted in a fan casing, said impeller being shielded by a shroud ring on the inlet side, said shroud ring extending to the mouth of an inlet cone of the fan with a clearance between shroud ring and inlet cone.

Figures 1 and 2 illustrate a centrifugal fan of usual type and figure 3 a detail of figure 2. Such a fan comprises a bladed impeller 1 which is shielded at the side directed towards the cone-shaped inlet section 3 by a shroud ring 2. The ring 2 extends to the inlet cone and covers its mouth while leaving an annular clearance J therebetween, see figure 3.

The impeller is rotating in a "spiral" type casing 4, which means that the distance "d" between the impeller outlet and the casing is continuously increasing from the cut-off edge 6 of the casing to the discharge section 7, following the rotation of the impeller. The casing is a diffusor which concentrates the flow towards the discharge section 7.

A part of the flow is recirculating between the impeller shroud ring 2 and the sideplate 5 of the casing 4, see figure 2. This region will be called recirculation region. As the pressure level in this area is higher than in the inlet section, a recirculating flow enters into the impeller through the annular clearance J. The amount of recirculating flow plays a role for the apperance of the pressure-flow characteristic curve of the fan, and the dimension J is a significant parameter in the fan design.

Although the casing is not of a symmetric revolution shape it is gen¬ erally considered that this fact does not alter the flow inside the impeller itself; in the normal operating flow area of the fan, the flow is considered to be identical in the different passages between the blades. This assumption is usually adopted in flow calculation to be limited to just one passage between two blades. Moreover, the recirculating flow into the annular clear¬ ance J is normally not considered in the aerial calculation. As a result the flow in the different passages of the impeller and the recirculating flow are assumed to be independant of the position of the passage between two blades relatively to the cut-off edge 6 of the casing.

Experimentally it has been observed, however, that the flow patterns in the recirculating region between the impeller shroud 2 and sideplate 5 of the casing 4 are different at different angle positions around the inlet cone 3.

A more precise study preceding the present invention of the real flow in the fan impellers has indicated that the recirculating flow in a passage between two blades of the impeller is depending on the instantaneous position of this passage relatively to the cut-off edge 6 of the casing, even with a constant clearance J.

^• The purpose of the present invention is to propose a method and provide an apparatus for directing the recirculating flow towards the discharge section of the fan.

This purpose is obtained by a method and an apparatus of the kind defined in the introductory portion and having the characterizing features of claims 1 and 2 respectively, thus by eliminating the revolution symmetri in the region between the shroud ring and the side plate of the fan casing on the inlet side to influence the recirculating flow existing in this region.

By the method and apparatus according to the invention a modification of the pressure-flow characteristic is obtained, resulting in a more uniform curve over a larger range of flows. Thus a higher pressure and an improved efficiency is obtained in the most interesting operating range. At lower flow rates improved stability of the recirculating flow is also obtained.

According to advantages embodiments of the apparatus according to the invention the stabilizer means comprise one or more stabilizer fins attached to the outer surface of the inlet cone or the inner surface of the casing on the inlet side to influence the recirculating flow existing in this region.

By using stabilizer fins at different locations it is possible to influence different zones of the pressure-flow characteristic to adapt the fan for a given use. Thus it is possible to increase the maximum pressure of the fan or increase the pressure for lower flow rates to avoid unstable operating conditions like surge.

The stabilizer fins can also be of different size and shape. According to another embodiment of the apparatus according to the invention the stabilizer fin is located in the recirculation region. The stabilizer fin located in the recirculation region of the fan then controls the recirculation flow rate in the passages between blades of the impeller.

According to yet another advantages embodiment of the apparatus accord¬ ing to the invention the stabilizer fin is so located that there is a dis¬ tance between its extremity and the cut-off edge of the casing at the dis¬ charge section. At lower flow rates the stabilizer fin will control the recirculating flow and its stability and therefore there must be a free interspace between the extremity of the fin and the cut-off edge of the

casing and this interspace must be large enough to ensure a possible recir¬ culating flow at the reduced flow rate.

An embodiment chosen as an example of the apparatus according to the invention will now be described in connection with figures 4-6.

On the drawings figure 1 and 2 shows the general construction of a centrifugal fan in two different views, figure 3 shows a detail of figure 2, figure 4 and 5 shows a centrifugal fan with the apparatus according to the invention, and figure 6 shows the pressure-flow characteristic of a centrifu¬ gal fan illustrating the effect of the invention.

The fan shown in figure 4 and 5 is of the general type described above in connection with figures 1 and 2. In the recirculation region of the fan a stabilizer fin 8 is attached to the outer surface of the inlet cone 3. By the installation of this stabilizer fin the revolution symmetri in the region between the shroud ring 2 and the side plate 5 of the fan casing 4 is elimi¬ nated and the recirculating flow in this region, passing on the stabilizer fin, is directed towards the discharge section 7 above the cut-off edge 6 of the casing 4.

The stabilizer fin shown in figures 4 and 5 is formed in one piece attached to the inlet cone 3.

The stabilizer means can however be formed of two or more parts with e.g. one part attached to the inlet cone and the other to the casing 4.

Figure 6 shows the pressure flow characteristic of a centrifugal fan, chosen as an example, with a rather large ratio of impeller width W to impeller diameter D of more than 0,15. The plain curve applies to a centri¬ fugal fan without the apparatus according to the invention and this curve exhibits a drop in the pressure for flow rates of about 2,00 - 3,50m ³ /sec. In the same diagram the efficiency of the fan is shown and a corresponding drop in the efficiency is observed. Further the pressure curve shows variations or instabilities for lower flow rates.

By introducing the apparatus according to the invention to such a centrifugal fan the drop of the pressure is essentialy eliminated with a corresponding increase of the efficiency as a result, c.f., the dotted curve in figure 6. It can further be observed that the introduction of the appa¬ ratus according to the invention essentialy eliminates the variations in the pressure for lower flow rates.

To sum up, by the invention a smoother pressure curve is obtained over a broader range of flow rates and an improved efficiency is obtained in a region of flow rates which often is the most important one for practical purposes.

The invention has been described above in connection with a single inlet, however, it can be used on a double inlet fan as well .