GB2114675A - Centrifugal fan impeller - Google Patents

Abstract

An impeller comprises a front disc and a rear disc 2, wherebetween blades 3 are secured. The impeller is provided with means for varying the profile of at least a portion of the blades, which means are arranged symmetrically with respect to the impeller axis. Each of the means is a flexible member 4 mounted on a corresponding blade 3 and bent to provide a loop 4a. With rotation the blade diameter is increased from D to D' and the exit angle from ??? to ???', which in turn changes the delivery and pressure developed by the fan. Varying the length of the flexible members 4 and/or the points of their fastening on the blade 3 provides a means for controlling the centrifugal fan performance. <IMAGE>

Description

SPECIFICATION Centrifugal fan impeller The present invention relates to the art of fan engineering and is specifically concerned with centrifugal fans, in particular with impellers thereof.

The invention may be employed to the best advantage for ventilation of mining industry enterprises. It may also prove useful in engineering metallurgy, chemical industry, and other industries where a transfer of gas flows is needed.

The chief problem which faces designers in developing powerful fan plants for the main ventilation of mines and pits lies in ensuring an adequate ventilation of underground workings whose extent, aerodynamic resistance, volume of gas, and temperature change continuously.

Inasmuch as fans of such plants consume up to 20% of the entire electric power spent for mining useful minerals, the problem of cutting down the power spent by a fan at every ventilating conditions required for a mine is of major importance.

It should be pointed out that the problem of selecting a fan for given operating conditions involves certain difficulties. The selection is based on predicted depth of occurrence of useful minerals, extent, aerodynamic resistance, temperature, and gas volume of underground workings. In the course of development of mining works, however, all the values of the above characteristics often turn out to exceed the predicted ones, and the fan plant becomes incapable of meeting the production requirements and restrains a further development of mining, in particular preventing to proceed to working of deeper horizons.

The rate of air supply to a mine can be increased in two ways, namely either by reducing the aerodynamic resistance of the ventilation network at a constant fan performance characteristic or by increasing the pressure produced by the fan at a constant aerodynamic resistance.

The aerodynamic resistance of a mine ventilation network can be reduced only by increasing the cross-sectional area of mine workings, but this calls for rather considerable costs, and therefore the only reasonable way for increasing the rate of air supply to a mine is to raise the pressure produced by a fan.

The pressure developed by centrifugal fans is at present controlled mainly by swirling the gas flow at the inlet to the impeller, which is accomplished with the aid of conventional guide vanes, and also by decreasing the impeller rotation speed.

These methods provide for the control only downwards from the extreme performance characteristic of a fan, i.e. allow make it possible to obtain a set of performance characteristics disposed below the extreme one and thereby to reduce, when required, the fan delivery and pressure in operation to a given ventilation network (for a better understanding, refer to the well-known delivery vs. pressure curve presented, e.g., in the book "Centrobezhnye ventilyatory" (Centrifugal fans). Ed. by T. S. Solomakhova.

Moscow, "Mashinostroenie", 1975, p. 256, Fig.

178). It should be noted that some, a rather small, rise in the pressure can be obtained by swirling the gas flow by said guide vanes in a definite direction, namely counter to that of impeller rotation, but this technique involves a disproportionately high rise in the power drawn by the fan, a sharp drop in the efficiency, and is therefore unsuitable.

Another possible way of providing the required airflow rate and pressure in many cases, especially for mining enterprises, consists in construction of a more powerful fan plant with a larger diameter of the impeller instead of the previously operating fan plant, but this calls for considerable costs as well as for a time during which the mine would not be supplied with the required amount of ain it follows from the above that the problem of ventilation of mine workings in a development of mining enterprises should be solved by providing a means for controlling a fan upwards from its extreme performance characteristic.

The pressure developed by a fan at a constant impeller rotation speed is known to depend mainly on the impeller diameter and the exit angle of its blades. The most expedient method for the control upwards from the extreme performance characteristic consists therefore in simultaneously varying, namely increasing, the two parameters of the impeller. The only economically feasible way to attain this is to vary the profile of the blades.

Known in the art are impellers provided with blade profile variation means in the form of turnable ailerons coupled with the blades (refer to "Centrobezhnye ventilyatory" (Centrifugal fans). Ed. by T. S. Solomakhova, p. 235, Fig.

1 62). Such an impeller generally comprises a front disk and a rear disk with blades secured therebetween to transfer a gas flow. Every aileron is mounted, in particular, on a turnable spindle pivoted in said disks so that the aileron forms a continuation (exit portion) of the corresponding blade. This makes it possible to vary the exit angle and thereby the configuration of the blade by turning the aileron through various angles with respect to the blade. Setting the ailerons to positive angles (i.e. turning them in the impeller rotation direction) increases the blade exit angle and the impeller diameter, and hence raises the pressure upwards from the fan's extreme performance characteristic.

Drawbacks of controlling with the aid of ailerons are a sudden change ("break") in the working surface of the blade profile with the aileron turned as well as a vortex zone at the aileron rear side at the gas flow exit, stemming from a separation of flow from the rear surface of blades. This phenomenon causes the efficiency to drop in the controlling: a pressure increase, for example, by a factor of 1.3-1.4 is accompanied by a 810% drop in the efficiency.

Moreover, the practice has shown impellers with ailerons to offer a poor dependability in operation, accounted for by the action on an aileron of considerable centrifugal forces (up to 10,000 N per a kilogram of aileron mass), which sometimes break the aileron off from the impeller; this brings about a great unbalance of the latter and thereby results in serious failures.

Attaining, on the one hand, the possibility of a rapid turn on the aileron and, on the other hand, the required strength of its fastening to impeller disk is under alternating loads a hard-to-solve problem.

Higher technical and economic characteristics are offered by a centrifugal fan impeller comprising a front disk and a rear disk, disposed in a coaxial relation, wherebetween blades for transfer of a gas flow are secured, and also having means for varying the profile of the blades, which means are stiff cover pieces secured on the working surfaces of the blades at the gas flow exit end and having the configuration of trihedral prisms (USSR Inventor's Certificate No. 706546, Int. C12. FO4D 29/28). The number of said cover pieces corresponds to at least a part of the number of blades, and the cover pieces are arranged symmetrically with respect to the impeller axis.

Such a construction provides for increasing the impeller diameter by means of prism angles protruding beyond the blades with simultaneously increasing the exit angle of the blades.

This construction does not affect the configuration of the rear surface of a blade, and hence diminishes the vorticity in separation of flow from the blade profile surface, with the result that such a fan features a higher efficiency than that of fans with ailerons.

Further, impellers with prismatic cover pieces are more dependable in operation than impellers with ailerons, which is due to a stiff coupling between the cover pieces and the blades.

At the same time, however, such impellers as well have considerable drawbacks which impede their practical application. First of all, a sudden change of the working surface of the blade at the cover piece causes a substantial (68%) drop in the efficiency when the pressure is increased 1.3-1.4 times.

Next, even when manufactured from a relatively light material, the cover pieces nevertheless give rise to considerable additional centrifugal forces and thereby substantially increase the load on the impeller, which shortens the fan service life. Also, a breakaway of a cover piece may result in a serious failure due to an impeller unbalance caused by the breakaway.

In all the above-discussed impeller constructions the geometric parameters of ailerons or cover pieces are specified for attaining the rated operating conditions of a fan. When the operating conditions vary, the geometric parameters of the impeller remain the same, while the kinematic parameters of the flow change, and-the resulting mismatch therebetween sharply cuts down the fan efficiency.

The principal object of the present invention is to provide a centrifugal fan impeller wherein increasing the delivery and pressure upwards from the extreme fan performance characteristic is attained without a significant drop in the efficiency.

A not less important object of the invention is to provide an impeller which cuts down the electric power consumption by a fan plant under conditions of control.

A further object of the invention is to provide an impeller which is designing new fans makes it possible to diminish their overall dimensions and the amount of material required for their manufacture.

Stiil further object of the invention is to provide an impeller which reduces both costs and time in modernization of existing fan plants.

Other objects of the invention include the provision of an impeller offering a high dependability, repairability, and convenience in service.

In accordance with the invention, there is provided a centrifugal fan impeller comprising a front disk and a rear disk, disposed in a coaxial relation, wherebetween blades for transfer of a gas flow are secured, and also having means for varying the profile of the blades, the number of said means corresponding to at least a part of the number of the blades, said means being disposed symmetrically with respect to the axis of said impeller, and each of the means being a flexible flat member mounted on a corresponding blade and bent in the plane of the profile of the blade so that said member has in the cross-section the configuration of a loop.

Tests conducted by the present inventors have shown that under the action of the three forces (the force of aerodynamic action of the gas flow, the centrifugal force due to the mass of the member itself, and the force of its tension), the member mounted on a blade assumes a fairly well streamlined loop-shaped configuration to form a new profile of the blade and change both the exit angle of the blade and the diameter of the impeller, and hence the delivery and pressure developed by the fan are changed as well. A smooth conjugation of the flexible member with the blade surface in the course of operation yields a substantially separation-free flow of gas over the latter, which results in a high efficiency of the fan. A change in the fan operating conditions, brought about, e.g., by a change in the aerodynamic resistance of the ventilation network, upsets the equilibrium between the three above-mentioned forces, and the flexible member assumes a new configuration which best corresponds to the new conditions. Owing to such a self-adjustment of the flexible member for every operating conditions, the pressure loss in controlling turns out to be the minimum. This greatly cuts down the electric power drawn by the fan.

One of possible modifications of the proposed impeller is such one wherein the flexible member is rigidly secured by its both ends on the surface of a blade so that the length of the portion of this surface, facing the member, the portion being defined by the points of fastening of the ends of said member, is less than the length of the latter, measured between its ends in a straightened state. Despite its apparent simplicity, such a modification provides for a required control of fan operating conditions both upwards and downwards from the extreme performance characteristic.

When a pressure control only upwards from the extreme performance characteristic is needed, then a modification is useful wherein at least one end of the flexible member is secured on the rear side of the blade at the gas flow exit portion.

Such an arrangement yields a smooth transition from the blade profile to the profile defined by the flexible member and thereby excludes an origination of a vorticity zone on the blade rear side at the gas flow exit.

When the pressure is to be controlled only downwards from the extreme characteristic, then a modification is advisable wherein one end of the flexible member is secured on the rear side of the blade at the gas flow entry portion, while the other end is secured on the working side of the blade.

Such a construction provides the best conditions for gas flow entry onto the blade along with a high dependability of flexible member fastening because of friction forces between the latter and the blade surface adjoining this place.

The most simply accomplished is a modification of the proposed impeller, wherein both ends of the flexible member are secured on the blade surface in one point A modification of the impeller is also possible, wherein the flexible member completely encompasses the blade in the plane of the profile thereof, the flexible member ends are fixed to each other, and the length of the member, measured between its ends in a straightened state, exceeds the length of the blade profile contour.

This modification is preferable when fastening the flexible member ends to the blade proper is undesirable for some reasons.

The best efficiency of controlling is attained when the thickness of the flexible member meets the condition: 6 P1 -=(0. 1-0.8)-, D P2 where: 6=thickness of said flexible member; D=impeller diameter measured at blade ends; P1 and density of the gas transferred by the fan and of the flexible member material respectively.

Experiments have demonstrated that when the condition is met, then even a 60-% pressure rise provided by the invention is accompanied by not more than a 4-% drop in the efficiency, which yields a substantial saving in electric power at all fan operating conditions as against the prior art impeller. Provided S Pt - < 0.1- D P2 the aerodynamic forces acting upon the flexible members exceed the centrifugal forces, with the result that the effect of each such member upon the gas flow declines and a relatively small rise in the fan pressure is obtained.Provided a P1 - > 0.8- D P2 the centrifugal forces predominate over the aerodynamic ones and cause the flexible member to take a radial (with respect to the impeller axis) position, which brings about a sudden change in the blade profile, gas flow separation from the blade, and drop in the efficiency.

The exact nature of the invention will be more clear from the consideration of embodiments thereof, described below, with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal sectional view of a fan impeller constructed according to the present invention (arrows in this Figure, as well as in subsequent Figures, indicate the direction of gas flow movement through the impeller); Fig. 2 is an enlarged sectional view taken along the line Il-Il in Fig. 1 and showing a blade of the impeller for a modification wherein the ends of a flexible member are secured on the blade surface at two points; Fig. 3 is a view similar to Fig. 2, but for a modification wherein at least one end of the flexible member is secured on the rear surface of a blade at a gas flow exit portion.

Fig. 4 is a view similar to Fig. 2, but for a modification wherein one end of the flexible member is secured on the rear side of a blade at the gas flow entry portion, and the other end, on the working side of the blade; Fig. 5 is a view similar to Fig. 2, but for a modification wherein both ends of the flexible member are secured on the blade surface at one point; and Fig. 6 is a view similar to Fig. 2, but for a modification wherein the flexible member completely encompasses the blade in the plane of profile of the latter.

The proposed centrifugal fan impeller comprises (Fig. 1) a front (with respect to gas flow movement direction) disk 1 and a rear disk 2, wherebetween blades 3 serving for the gas flow transfer are secured. The front disk 1 has an inlet opening 1 a, and the rear disk 2, a hub 2a for mounting on a fan shaft (not shown).

The impeller has also means for varying the profile of the blades, which means are flexible members 4 with a thickness S (Fig. 2). The number of flexible members 4 corresponds to the whole, or to a part of the number of the blades 3, and the members 4 are mounted on the blades 3 symmetrically with respect to the impeller axis.

Each said member 4 is bent in the plane of the profile of the blade 3 so as to form in the crosssection a loop 4a, as is clearly seen in Fig. 2.

The flexible member 4 may be fastened to the blade 3 in various manners. In particular, as shown in Fig. 2, the flexible element 4 is secured by its both ends 4b and 4c on the surface 3a of the blade 3 so that the length of the portion of the surface, facing the member 4, the portion being defined by the points of fastening of the ends 4b and 4c (in Fig. 2 this portion is conventionally denoted by a dotted line), is less than the length of said member 4, measured between its ends in a straightened state (i.e. the state of a blank before its bending into a loop).

Each blade 3 is interposed between the disks 1 and 2 so that its surface 3a is a working side, and the surface 3b, a rear side with respect to the gas flow (with the impeller rotating in the direction indicated by an arrow in Fig. 2).

A centrifugal fan with the proposed impeller operates as follows (see Figs. 1 and 2). When a drive (not shown) is turned on, the impeller starts rotating in the direction indicated by the arrow in Fig. 2. This causes a gas flow, in particular an air flow, to enter the impeller through the opening 1 a in the front disk 1 , to move further towards the blades 3 as shown by arrows in Fig. 1, and then to pass between the blades 3 and to emerge to the outside. Flowing over the blades 3, the air flow acts upon each flexible member 4, which is also acted upon by the centrifugal force arising due to the mass of the member and by the force of tension of the material thereof.

The effect of the three forces causes the flexible member 4 (see Fig. 2) to stretch and to assume a well streamlined configuration, and hence to change the initial profile of the blade 3.

This changes the impeller diameter from D to D' and the exit angle of the profile of the blade 3 to P to p', which in turn changes the delivery and pressure developed by the fan. Varying the length of the flexible members 4 and/or the points of their fastening on the blade 3 provides a means for controlling the centrifugal fan performance both downwards and upwards from its extreme characteristic.

In particular, for a control only upwards from the extreme performance characteristics, a modification (Fig. 3) may be employed wherein at least one end of the flexible member 4, in this case the end 4c thereof, is secured on the rear side 3b of the blade 3 at the gas flow exit portion (in the drawing, at the extreme right-hand portion of the blade). Its other end 4b may in this case be secured at any point of the blade 3, such as on its working side 3a as shown in Fig. 3. Such an arrangement yields a smooth merging of the profile of the blade 3 into the profile of the flexible member 4, which provides for a separation-free air flow over the rear side 3b of the blade 3 and hence for a high efficiency of the fan.

When the control only downwards from the extreme performance characteristic is needed, it is advisable to resort to the modification illustrated in Fig. 4. Here one end 4b of the flexible member 4 is secured on the rear side 3b of the blade 3 at the gas flow entry portion (in the drawing, at the extreme left-hand portion of the blade 3), and the other end 4c on the working side 3a of the blade 3. In operation, a stable gas flow separation region (indicated by a closed line "m") develops downstream of the loop 4a of the flexible member 4 and at a relatively small length of the loop 4a is localized on the blade 3 by the action of the Coriolis force.The flexible member 4 jointly with the gas flow line "n" which separates the through flow and the flow separation region form as if a new configuration of the blade 3 with the exit angle smaller than that in the initial blade with the result that the fan pressure is reduced. It is well known that localized gas flow separation regions cause no significant rise in the aerodynamic resistance, and therefore the drop in efficiency with this control method is insignificant.

Also possible is a modification shown in Fig. 5, wherein both ends of the flexible member 4 are secured on the surface of the blade 3 in one point 4d. Such a fastening of the flexible member 4, as compared with that in the previously described modifications, is simpler, but nevertheless does not impair the technical and economic characteristics of a fan with the proposed impeller.

If fastening of the ends of the flexible member 4 to the blade 3 proper is undesirable because of, e.g., design features of the blade, then it is effective to resort to the modification shown in Fig. 6, wherein the flexible member 4 completely encompasses the blade 3 in the plane of the profile thereof, the ends of the flexible member 4 are fixed to each other, and the length of the member 4, measured between its end in a straightened state, exceeds the length of the contour of the profile of the blade 3.

The best result in embodying the present invention are attained when the thickness â of the flexible member 4 meets the condition: S P1 -=(0. 1-0.8)-, D P2 where P1 and P2 are respectively the density of the gas transferred by the fan and the density of the flexible member material. In this relation, the numerical values of a and D should be in the same units (this holds also for P1 and P2) It should be emphasized that the increase in the fan efficiency allows to considerably cut down the electric power consumption.Thus, for a centrifugal fan with a 3,200-mm diameter impeller and a 1,250-kW drive, the use of the flexible member 4 yields a gain of about 46% in the efficiency over the ailerons or cover pieces (because in this case increasing the pressure, e.g., by a factor of 1.4 entails a not more than 24-% drop in the efficiency), which allows to save, on the average 440-660 thousand kilowatt-hours of electric power a year.

Owing to a relative lightness of the flexible members; their use, in contrast to prior art constructions, causes no appreciable loading of the impeller by additional centrifugal forces. For the same reason, even an accidental breakaway of the flexible member 4 from the blade 3 brings about no unbalance of the impeller. It will be apparent that these features enhance the dependability and durability of a fan.

It should also be pointed out that the application of the above described embodiments in modification of existing fan plans of mines and pits makes it possible to considerably increase the rate of air supply to underground workings and thereby to improve the atmospheric conditions at the miner's work places, creating thus prerequisites for increasing the mining of useful minerals.

A not unimportant advantage is the simplicity of its accomplishment, which calls for no considerable time, labour input, amount of materials, nor for a high skill of workment. The experience gained in modernizing large mine fan plants, has shown, for example, that mounting and dismounting of the flexible members on a 3,200-mm diameter eight-blade impeller takes as little as 1.5-2 hours, while the cost of the modernization amounts to from 0.4% (without replacing the electric motor) to 10% (when replacing the motor by a more powerful one) of the initial cost of the plant or, which is the same, respectively from 0.2% to 5% of the cost of a more powerful plant capable of the same performance as is the modernized one. Said advantages enhance the repairability and simplify the maintenance of a fan.

The use of the above described embodiments in developing new fans designed for the same delivery and pressure range as are fans without the flexible members makes it possible to considerably reduce the impeller peripheral speed (blade tip speed), diminish the overall dimensions, cut down the material content, and upgrade the dependability of a fan plant as a whole as compared with the last mentioned fans.

For example, for operation to the same ventilation network, a fan with an impeller diameter of 2,500 mm, a peripheral speed of 94 m/s, and a mass of 1 5 t, developing a pressure of 2,800 Pa, may be successfully employed instead of a fan with a 3,200-mm impeller diameter, 120m/s peripheral speed, and 25-t mass, developing a rated pressure of 4,500 Pa. In the course of service of the former fan, the pressure developed by it can be, if required, raised to a desired value (in particular, to the same 4,500 Pa) by appropriately selecting the above-described flexible members and mounting them on the fan impeller. Thus, the overall dimensions of the fan are reduced by 22%, and its mass, by 40%.Such a modernization of an existing fan plant is most effective in cases when a fan during a large part of its service life runs at conditions below the rated ones, which occurs, for example, in constructing an enterprise and bringing its output to the design value.

Presented below, for the purpose of illustration, are specific examples of testing fans with the proposed impeller.

Example 1 Flexible members of a synthetic material were installed on an operating mine fan plant according to the modification shown in Fig. 3 of the accompanying drawings. The impeller diameter was of D=3,200 mm, the impeller outlet width, of 640 mm. The fan drive was rated at 1,250 kW.

The flexible member length in a straightened state was of 600 mm; the width, of 630 mm; and the thickness, of d=0.44 mm. The density of transferred air was of p,=l .2 kg/m3, and of the flexible member material, ofp2=1,060 kg/m3 at P1 (-=0.121-).

D P2 The test results demonstrated that the use of the flexible members raised the pressure developed by the fan from 4,500 Pa to 6,700 Pa (i.e. by 48%), while the efficiency dropped insignificantly (from 84% to 81%). During 7 months of test the plant ran normally, without an appreciable wear of the flexible members Example 2 Flexible members were installed on a fan plant similar that of, and in the same manner as in Example 1. The length of the flexible member in a straightened state was of 500 mm; the width, of 620 mm; and the thickness, of 2.4 mm. The density of the transferred air was of Pi=1 .2 kg/m3, and of the flexible member material, of p2=l ,230 kg/m3 (5 P1 (-=0.77-).

D P2 After mounting the flexible members, the pressure developed by the fan rose from 4,300 Pa to 6,800 Pa (by 57%), while the efficiency drop was small (from 83% to 79.5%). During 4 months of the test the plant ran normally, without an appreciable wear of the flexible members.

Example 3 Flexible members were installed on an operating mine fan plant according to the modification shown in Fig. 4 of the accompanying drawings. The impeller diameter was of 2,500 mm; the impeller outlet width, of 500 mm. The fan drive was rated at 630 kW. The length of the flexible member in a straightened state was of 550 mm; the width, of 500 mm; and the thickness, of 0.9 mm. The density of transferred air was of p,=l .2 kg/m3, and of the flexible member material, of P2=1 ,060 kg/m3 P1 (-=0.32-).

D P2 The test results demonstrated that the use of the flexible members fastened according to the modification shown in Fig. 4 made possible a control downwards from the extreme performance characteristic of the fan: a decrease in the developed pressure from 4,000 Pa to 3,500 Pa (by 13%) was attained at an insignificant drop in the efficiency (from 85% to 83%). During 6 months of the test the plant functioned normally, without an appreciable wear of the members.

Example 4 A model of the fan described in Examples 1 and 2 was tested under laboratory conditions.

Flexible elements were installed according to the modification shown in Fig. 3 of the accompanying drawings on an impeller whose diameter was of 500 mm, and the outlet width, of 100 mm. The model plant drive was rated at 2 kW. The length of the flexible member in a straightened state was of 90 mm; the width, of 96 mm; and the thickness, of 0.04 mm. The density of transferred air was ofp1=1.2 kg/m3, and of the flexible member material, of p2=1,200 kg/m3. Hence the a - P1 ratio was of 0.08 D P2 i.e. below the recommended one.

A rustling noise in fan operation and pulsations of the pressure developed by the fan were noted sometimes in the course of the test; this impaired to a certain degree the stability of fan operation.

The developed pressure increased to a lesser extent than in Examples 1 and 2 (by 15%), while the drop in the efficiency was greater (by 6%), which as due to a disproportionate rise in the power.

Example 5 Flexible members were installed on the same model and in the same manner as in Example 4.

The length of the flexible member in a straightened state was of 90 mm; the width, of 96 mm; and the thickness, of 0.54 mm. The density of the transferred air was of p1=l .2 kg/m3, and of the flexible member material, of p2=l ,060 kg/m3. Hence, the (5 P1 - ratio was of 0.95 -, D P2 i.e. exceeded the recommended one.

A rise in the noise level with an appearance of howlings tones in the noise was noted in the course of the fan test. Along with a considerable rise in the pressure (by 53%), attained by the use of the flexible elements, a significant drop in the efficiency (by 10%) took place.

While only some particular embodiments of the present invention have been presented above, it will be quite clear that the invention is not limited to the disclosed embodiments thereof and that various modifications apparent to those skilled in the art may be introduced therein without departing from the spirit and scope of the invention as defined in the following claims.

Claims (8)

Claims

1. A centrifugal fan impeller comprising a front disk and a rear disk, disposed in a coaxial relation, wherebetween blades for transfer of a gas flow are secured, and also having means for varying the profile of the blades, the number of said means corresponding to at least a part of the number of blades, said means being disposed symmetrically with respect to the axis of said impeller, and each of the means being a flexible flat member mounted on a corresponding blade and bent in the plane of the profile of the blade so that said member has in the cross-section the configuration of a loop.

2. An impeller in accordance with claim 1, wherein the flexible member is secured by its both ends on the surface of the blade so that the length of the portion of the surface, facing the member, the portion being defined by the points of fastening of the ends of the member, is less than the length of the latter, measured between its ends in a straightened state.

3. An impeller in accordance with claim 2, wherein at least one end of the flexible member is secured on the rear side of the blade at the gas flow exit portion.

4. An impeller in accordance with claim 2, wherein one end of the flexible member is secured on the rear side of the blade at the gas flow entry portion, and the other end, on the working side thereof.

5. An impeller in accordance with claim 2, wherein both ends of the flexible member are secured on the surface of the blade in one point.

6. An impeller in accordance with claim 1, wherein the flexible member completely encompasses the blade in the plane of the profile thereof, the ends of the member are fixed to each other, and the length of the member, measured between its ends in a straightened state, exceeds the length of the contour of the profile of the blade.

7. An impeller in accordance with any of claims 1 to 6, wherein the thickness of the flexible member meets the condition: (5 Pi -=(0.l1-0.8)-, D P2 where #=thickness of said member; D=diameter of the impeller, measured at blade ends; Pi and p2=density of the gas transferred by the fan and of the material of the flexible member respectively.

8. A centrifugal fan impeller substantially as hereinbefore described with reference to, and as illustrated in the accompanying drawings.