CENTRIFUGAL FAN

Abstract: The centrifugal fan with single or double intake is provided with a fan impeller of the closed type which is formed of a rotating bladed wheel (10) having at least a covering element (13). The blades (11) of the fan impeller (16) are flat and are backwardly inclined with respect to the direction of rotation of the fan impeller and the covering element (13) comprises along the direction of flow a frustoconical surface portion (13A), a first curved surface portion (13B) and a second curved surface portion (13C), wherein the radius of curvature of the first curved surface portion is smaller than the radius of curvature of the second curved surface portion.

Description: CENTRIFUGAL FAN DESCRIPTION This invention generally relates to the ventilation apparatus field and, more particularly, an improved centrifugal fan. TECHNICAL FIELD As known, centrifugal fans generally comprise the following essential elements: a casing with a spirally shaped duct, the so called volute, which is provided with an outlet at its end, an inlet or eye at the center of volute, a fan impeller arranged downstream the volute eye and connected to a hub which is keyed on a driving shaft, a baffle or a diffuser arranged downstream the fan impeller. BACKGROUND ART In centrifugal fans the air or other process gaseous fluid, is drawn in by the fan impeller through the inlet, parallel to the rotation axis and is delivered by the fan impeller in a direction perpendicular to the rotation axis. The air stream delivered by the fan impeller is diverted by the baffle or further processed by the diffuser before entering the volute which collects and directs the air stream towards the outlet. Centrifugal fans can be of the type with single or double intake, i.e. they can draw in air from one side only of the fan impeller or from the two opposite sides thereof. The fan impeller is formed of a bladed rotating wheel consisting of a disc to which the blades are secured at their base. In the case of a fan with single intake the blades are arranged only on one side of the disc, whereas in the case of a fan with double intake the blades are arranged on both the sides of the disc. The blades can be arranged in radial direction or they can be forwardly or backwardly inclined with respect to the rotation direction of the fan impeller. The blades have often a streamlined profile, but in the prior art also flat blades, i.e. non-streamlined blades are known. The impeller of centrifugal fans can be of the closed or open type according to whether a covering element, so called impeller covering, consisting in a circular frame fastened to the tip of the blades is provided or not. Fan impellers provided with inclined blades are generally of the closed type in order to increase the bending strength of the blades. Several approaches have been adopted in the design of centrifugal fans in order to improve their operational characteristics and efficiency. These approaches consist for example in providing the inlet and the outlet with well jointed convergent and divergent ducts, respectively and, downstream the fan impeller, a diffuser formed of fixed blades suitably shaped and directed for guiding the air stream exiting the fan impeller so as to reduce flow losses. For similar purposes, fan impellers are provided with a great number of blades and these sometimes have a curved guide portion at their leading edge. DISCLOSURE OF INVENTION The object of the present invention is to improve the efficiency of centrifugal fans by employing an optimal combination of parameters such as the number, the shape and the arrangement of the blades of the fan impeller and the shape of its covering element. More particularly, this object is attained by providing a centrifugal fan with single or double intake having an impeller of closed type formed of a rotating bladed wheel provided with at least a covering element, characterised in that the blades of the fan impeller are flat and are backwardly inclined with respect to the rotation direction of the fan impeller and in that the covering element comprises along the flow direction a frustoconical surface portion, a first curved surface portion and a second curved surface portion, wherein the radius of curvature of the first curved surface portion is smaller than the radius of curvature of the second curved surface portion. BRIEF DESCRIPTION OF DRAWINGS The present invention will be now described in more detail with reference to the accompanying drawings, wherein: Figure 1 is a front elevation view of the centrifugal fan impeller of the invention; Figures 2 and 3 are a front elevation view and a side elevation view, respectively, of a blade of the impeller of Figure 1 ; Figure 4 is a partial side view of the covering element of the impeller of Figure 1 ; Figures 5 and 6 are a front elevation view and a side elevation, respectively, of the inlet arranged upstream the impeller of Figure 1 ; Figures 7 and 8 are a front elevation view and a side elevation view of the baffle arranged downstream the impeller of Figure 1 ; and Figure 9 is a diagram illustrating the performance curves of the fan of the invention. BEST MODE FOR CARRYING OUT THE INVENTION Referring to Figure 1 of the drawings, there is shown the impeller according to the present invention, generally designated by 10. The impeller is adapted to be used in connection with a centrifugal fan with single or double intake and is formed of a closed type rotating bladed wheel. In the case of a centrifugal fan with single intake, the impeller 10 draws in air or other process gaseous fluid from one side only and comprises therefore a single row of blades connected on one side to a disk 12 and on the other side to a ring 13 serving as a covering element. Instead, in the case of a centrifugal fan with double intake, the impeller 10 draws in air from the opposite sides and comprises therefore a pair of blade rows 11 , one on each side of the impeller. The blades of each row are connected on one side to the disc 12 and on the other side to a covering ring 13. In both the above cases the impeller disc 12 is provided with a flanged hub for connection to a drive shaft. The covering ring 13 of the impeller is made of sheet steel and, as shown in Figure 4, it comprises along the direction of air flow represented in the drawing by the arrow F a frustoconical surface portion 13A, a first curved surface portion 13B and a second curved surface portion 13C. According to the invention, the radius of curvature of the first curved surface portion 13B is smaller than the radius of curvature of the second curved surface portion 13C. The impeller blades are made of sheet steel and are welded at their base and tip to the disc 12 and the covering ring 13, respectively. Their number may range from nine to thirteen and is preferably equal to eleven. As shown in Figures 1-3, the impeller blades are flat, i.e. their profile is not streamlined, and they are backwardly inclined with respect to the direction of rotation, thereby forming an angle α with the radial direction which ranges from 40 to 45 degrees, and is preferably equal to 42.88 degrees. As mentioned above, the impeller 10 is adapted for use in connection with a centrifugal fan. Such fans comprise in a manner known per se a volute for collecting the air stream delivered by the impeller and directing it towards the outlet. The centrifugal fan is furthermore provided with an inlet as shown in Figures 5 and 6 of the drawings. The inlet is formed of an annular flange 14 made of sheet steel or machined and arranged at the center of the volute. The flange 14 comprises in the direction of air flow represented in the drawing by the arrow F a converging surface portion 14A, a substantially cylindrical surface portion 14B and a diverging surface portion 14C which are well jointed together. The fan further comprises a baffle 15 shown in Figures 7 and 8 of the drawings. The baffle is made of sheet steel and is arranged parallel to the plane of the outlet which in this case has a square shape. The baffle is defined in its upper portion by two surface portions 16 which are curved along the direction of air flow and have an inclination of 70 degrees with respect to the longitudinal central plane of the fan, thereby generating a V- shape baffle which improves the aerodynamical interference with the pulsating air stream at the periphery of the impeller. Preferably, the height of the baffle, at the highest points on the sides, is equal to 38% of the length of the side of the square shaped outlet. The baffle is also provided with a stiffening rib 17 arranged substantially parallel to its base. Test carried out on a fan having the above features have shown an increase in the efficiency of the fan and a reduction of its noise level. The results of these tests are shown in Figure 9. The tests refer to two fans, the first according to the teachings of the present invention and the second of a type known in the art. The impeller of the fans had a nominal diameter of 900 mm, a nominal speed of 500 rpm and the air had a nominal density of 1.20 kg/m 3 In Figure 9 on the abscissa axis the rate of flow Q measured in m 3 /h, and on the ordinate axis the static pressure PS measured in Pa are indicated. On the same ordinate axis there are represented the mechanical power PM measured in W, the fan efficiency E in %, the noise level S in dB. In the diagram the curves H, U, V and W represent the static pressure, the noise level, the efficiency and the mechanical power, respectively, of the first fan as a function of the rate of flow Q, whereas the curves H', IT, V and W represent the static pressure, the noise level, the efficiency and the mechanical power, respectively, of the second fan as a function of the rate of flow Q. By comparing the curves the improvements obtained with the first fan according to the invention are readily apparent.

Claims: CLAIMS 1 ) A centrifugal fan with single or double intake having a fan impeller of the closed type formed of a rotating bladed wheel provided with at least a covering element, characterised in that the blades of the fan impeller are flat and are backwardly inclined with respect to the direction of rotation of the fan impeller and in that the covering element comprises along the direction of flow a frustoconical surface portion, a first curved surface portion and a second curved surface portion, wherein the radius of curvature of* the first curved surface portion is smaller than the radius of curvature of the second curved surface portion. 10 2) A centrifugal fan according to claim 1 , characterised in that the angle of inclination of the impeller blades with respect to the radial direction ranges from 40 to 45 degrees and is preferably equal to 42.88 degrees. - | 5 3) A centrifugal fan according to claim 1 , characterised in that the number of the impeller blades is equal to eleven. 4) A centrifugal fan according to claim 1 , characterised in that it is provided with an inlet arranged upstream the impeller and secured to the fan casing 20 and is formed of a flange comprising along the direction of flow a converging surface portion, a cylindrical surface portion and a diverging surface portion which are well jointed together. 5) A centrifugal fan according to claim 1 , characterised in that it comprises 25 a baffle arranged downstream the impeller which is formed of a surface parallel to the plane of the outlet and is defined in the upper portion by two surface portions which are curved along the direction of flow and have an angle of inclination of 70 degrees with respect to the longitudinal center plane of the fan, thereby generating a V-shape baffle which is adapted to 30 improve the aerodynamical interference with the pulsating air stream at the periphery of the impeller. 6) A centrifugal fan according to claim 5, characterised in that the height of the baffle, at the highest points on the sides, is equal to 38% of the length of the side of the square shaped outlet. 7) A centrifugal fan according to claim 4, characterised in that the converging portion has an angle of inclination with respect to the flange ranging from 27 and 35 degrees according to the size of the fan. 8) A centrifugal fan according to the above claims, characterised in that the dimensions of the centrifugal fan vary with an homothetic relationship according the size of the fan.

Inventors: BRIVIO DARIO, CANALI UMBERTO, BRIVIO, DARIO, CANALI, UMBERTO
Applicants: BRIVIO DARIO, CANALI UMBERTO, NICOTRA IND S P A, BRIVIO, DARIO, CANALI, UMBERTO, NICOTRA INDUSTRIALE S.P.A.
Application year: 2001
Main classification: F04D29
IPC codes: F04D29