GB1589307A - Centrifugal fans - Google Patents

Description

(54) CENTRIFUGAL FANS (71) We, AB SVENSKA FLAKT FABRIKEN, a Company organised under the laws of Sweden, of Sickla Alle 1, S-131 00 Nacka, Sweden, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to centrifugal fans for handling air or gases.

In industrial manufacture of centrifugal fans for different purposes, the fans are usually classified into three categories, depending on the pressure increase across the fan when the gas is passing through the impeller, whereby its dyanmic as well as its static pressure is increased. Consequently, there are low pressure fans, medium pressure fans and high pressure fans in which the pressure increase is generally 3500 Pa, 7000 Pa and 12000 Pa, respectively. For the total range of pressure increase the manufacturer provides within his standard range a number of different fans having different capacities to be able to supply equipment for different purposes and demands and for different operational conditions.In order to standardize the manufacture of impellers and other components the dimensions of the impellers have been set in certain steps which are suitable for capacities covering the whole range of operation for the fan range. If these impellers, having stepwise increasing dimensions, are coupled for direct drive by induction motors, the rotational speed of the impeller will be dependent on the number of poles of the motor and the actual slip. The operational range of the fan can be graphically illustrated by means of curves in which the pressure increase is plotted as a function of the gas flow at certain rotational speeds. Thus, capacity curves of the function APt = f(q) are obtained but these curves will appear as rather scattered in the graph.This implies that by such an arrangement it will not be economical to provide a range of fans with a sufficiently great selection of performances to provide solutions for various operating conditions. In principle it would be possible, of course, to achieve a sufficiently varied range of fans by allowing the dimensions of the impellers to differ from each other with sufficiently small steps that the intervals between the corresponding curves APt = f(q) would be as small as desired, but such an arrangement would be quite unacceptable from manufacturing and economical points of view. Instead, in the solution hitherto chosen the speed of rotation and hence the fan capacity is varied in order to achieve the desired difference in performance of the available fans.The rotational speed of the impeller is most simply controlled by means of a belt transmission between the motor and impeller shafts since transmission pulleys and belts are standardized products which are commercially available in a great selection. Even if the arrangement with belt drive has involved considerably higher costs it has, however, hitherto been the best and cheapest method to provide the desired range of fans to meet the customer's different demands.

As a comparison it may be mentioned that the cost for a belt driven fan is between 40 and 50%higher than for a directly driven fan.

The object of the present invention is to allow the steps in the dimensions of the impellers to be reasonably large, which is desirable from the manufacturing, standardizing and economical points of view, while also eliminating the disadvantages of having a belt drive transmission.

According to the present invention, the performance of a centrifugal fan is modified by trimming the blades of the fan impeller to reduce the diameter of the circle through the radially-outer blade tips. When this method of adjusting the fan performance is adopted, all the fans in the range may have their impellers coupled for direct drive by means of induction motors operating at the asynchronous rotational speeds characteristic for each type of motor and working condition, and in this way a desired selection of fans in the range can be achieved by means of dimensional changes both of the entire impeller and of the tip diameter of the blades. Hence the capacity curves within the range of pressure increase can be brought as close together as is desired.At the same time, all the advantages of known systems are maintained as regards the possibilities of stockkeeping a standardized collection of impellers with certain dimensions. By allowing elimination of the belt drive transmission the disadvantages connected therewith can be eliminated, while the costs for the manufacture of the standard range of fans can be substantially lowered.

The fans to which the invention is applicable usually have impellers which each have a circular central opening for receiving an inlet flow in an axial direction, with the diameter of the central opening being between 25% and 80% of the outer diameter of the fan impeller.

The invention may be carried into practice in various ways, but certain specific embodiments will now be described by way of example, with reference to the accompanying drawings, of which: Figure 1 is a cross sectional view of a fan impeller; Figure 2 is a view of a fan impeller having the same outer diameter as the impeller shown in Figure 1, and mounted inside a housing; Figures 3a, 3b, and 3c are diagrammatic views, showing three generally similar fans, which differ primarily in their outer diameters; and Figure 4 is a graph showing a family of pressure flow characteristic curves for the fans shown in Figures 3a, 3b and 3c.

The fan impeller shown in Figure 1 comprises a front disc 1 and a rear disc 2 between which blades 3 are arranged. The front disc 1 carries a hub 4 which is intended for direct coupling to the shaft of an electric induction motor, not shown in the drawing. The rear disc 2 is provided with an axial inlet having a diameter d, for the gas, which after passing the blades 3 is expclled in a radial direction thereby undergoing a total pressure increase APt. As can be seen in the drawing the fan impeller of Figure 1 has an outer diameter D1 which is equal to the diameter D of the circle through the radially-outer tips of the blades 3.

The fan impeller shown in Figure 2 is arranged inside a housing 5 having a funnel shaped inlet sleeve 6 in front of the inlet. This impeller differs from the impeller of Figure 1 in that the diameter D of the circle through the tips of the blades 3 is smaller than the outer diameter D1 of the fan impeller. The remaining parametegs of the impeller, primarily its inlet diameter dl and its outer diameter D1, are the same as in Figure 1.

Thus, the ratio * is the same for the two impellers shown in Figures 1 and 2, while the ratio D s is approximately 1 in Figure 1 but is greater than 1 in Figure 2.

The three fan impellers A, B and C shown in Figures 3a, 3b and 3c are of low pressure type, medium pressure type and high pressure type, respectively, intended for different capacitires. In order to cover the range of pressure increase of interest, the outer diameters D1,, D," and D,"' of the impellers are in a geometric progression, so that D1" = 1.25 D,', and Dot"' = 1.25 Dl" As shown in the drawings, the diameters D', D" and D11,,, just as in Figure 1. Thus, three basic forms of impeller are provided.

For fans which use these basic forms of impeller, in which, generally expressed, D 1, with the impellers being driven by induction motors at the asynchronous rotational speed for these motors, capacity curves will be obtained as shown in Figure 4 by the curves A, B and C which are drawn as continuous lines. As Figure 4 shows, these curves A, B and C are fairly widely spaced.

As mentioned in the introduction to the specification, it is desired, however, to have smaller intervals between the curves in order to realize the desired great number of different fan performances, and therefore the capacity has up till now been varied by varying the rotational speed for each impeller size by means of belt transmissions. With the fan impellers shown in the drawings, however, the impeller is always driven directly by the induction motor shaft, and the fan capacity is instead changed by varying the blade tip diameter D of the fan impeller. The broken lines in Figures 3a, 3b and 3c illustrate how the blade tip diameter can be reduced to a value D', Dr or D"', to obtain the desired fan capacity. In the drawings, three possible reduced diameters are indicated for each impeller; these reduced diameters are achieved by trimming the radially-outer ends of the fan blades 3, without trimming the peripheries of the side discs 1 and 2. In that way, intermediate capacity curves are obtained, which are shown in broken line in the graph of Figure 4. Thus, the whole family of curves will represent the desired assortment of fan performances. Since the impellers shown in Figures 3a, 3b and 3c can each exhibit one of four different blade diameters, four capacity curves are obtained for each impeller, namely the capacity curve of the basic form of impeller and three intermediate capacity curves between the corresponding basic impeller curve and the basic curve for the next smaller impeller.

It is desirable that an impeller whose blade tip diameter has been reduced to the minimum for that basic impeller should give rise to a capacity curve above the capacity curve for the basic form of the next smaller impeller, and this will in its turn have the consequence that the ratio between the outer diameter D1 of the impeller and the reduced blade tip diameter D must not be greater than 1.25, since this is the ratio between the outer diameter of a given impeller and the outer diameter of the next smallest impeller.

Thus, for each basic form of impeller, with given outer dimensions, impellers can be obtained for several fans having different capacities, by altering only the blade tip diameter. Since the blades usually are made of soft material it will be possible to adjust the blade tip diameter to individual demands on fan capacity simply by shearing the blades of a suitable impeller, chosen from the range of basic forms of impeller.

WHAT WE CLAIM IS: 1. A method of modifying the performance of a centrifugal fan, comprising trimming the blades of the fan impeller to reduce the diameter of the circle through the radially-outer blade tips.

2. A method as claimed in Claim 1, in which the fan impeller has a circular central opening for receiving an inlet flow in an axial direction, the diameter of the central opening being between 25 % and 80% of the overall diameter of the fan impeller.

3. A method as claimed in Claim 1 or Claim 2, in which the trimming operation reduces the blade tip diameter of the fan impeller to not less than 80% of the overall diameter of the impeller.

4. A method as claimed in Claim 1 or Claim 2 or Claim 3, in which the fan impeller comprises two side discs connected by the fan blades, and the trimming operation involves trimming the radially-outer ends of the fan blades, without trimming the peripheries of the side discs.

5. A method of modifying the performance of a centrifugal fan, the method being substantially as herein described, with reference to the accompanying drawings.

6. A method as claimed in any of the preceding claims, when applied to a fan whose impeller is directly coupled to the shaft of an induction motor.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. tip diameter has been reduced to the minimum for that basic impeller should give rise to a capacity curve above the capacity curve for the basic form of the next smaller impeller, and this will in its turn have the consequence that the ratio between the outer diameter D1 of the impeller and the reduced blade tip diameter D must not be greater than 1.25, since this is the ratio between the outer diameter of a given impeller and the outer diameter of the next smallest impeller. Thus, for each basic form of impeller, with given outer dimensions, impellers can be obtained for several fans having different capacities, by altering only the blade tip diameter. Since the blades usually are made of soft material it will be possible to adjust the blade tip diameter to individual demands on fan capacity simply by shearing the blades of a suitable impeller, chosen from the range of basic forms of impeller. WHAT WE CLAIM IS:

1. A method of modifying the performance of a centrifugal fan, comprising trimming the blades of the fan impeller to reduce the diameter of the circle through the radially-outer blade tips.

2. A method as claimed in Claim 1, in which the fan impeller has a circular central opening for receiving an inlet flow in an axial direction, the diameter of the central opening being between 25 % and 80% of the overall diameter of the fan impeller.

3. A method as claimed in Claim 1 or Claim 2, in which the trimming operation reduces the blade tip diameter of the fan impeller to not less than 80% of the overall diameter of the impeller.

4. A method as claimed in Claim 1 or Claim 2 or Claim 3, in which the fan impeller comprises two side discs connected by the fan blades, and the trimming operation involves trimming the radially-outer ends of the fan blades, without trimming the peripheries of the side discs.

5. A method of modifying the performance of a centrifugal fan, the method being substantially as herein described, with reference to the accompanying drawings.

6. A method as claimed in any of the preceding claims, when applied to a fan whose impeller is directly coupled to the shaft of an induction motor.