GB2145157A

GB2145157A - Axial flow fans

Info

Publication number: GB2145157A
Application number: GB08418298A
Authority: GB
Inventors: Rainer Blees
Original assignee: Kuehnle Kopp and Kausch AG
Current assignee: Howden Turbo GmbH
Priority date: 1983-08-16
Filing date: 1984-07-18
Publication date: 1985-03-20
Also published as: GB8418298D0; GB2145157B; DE3329563A1; IN162214B; DE3329563C2

Abstract

The fan comprises a cylindrical first housing part 1,1', a conical second housing part 2', a cylindrical third housing part 3' within which is a rotatable impeller 5, and a guide ring 7 adjacent the impeller to define an annular bypass channel through which, under partial load operation, controlled return of air or gas separations, forming in the region of the impeller can pass to prevent unstable operation. The conical guide ring 7 is fastened to the conical second housing part 2' so that the edge of the ring 7 terminates in the vicinity of the entry edge of the blades of the impeller 5. <IMAGE>

Description

SPECIFICATION Axial flow fan This invention concerns an axial flow fan comprising a cylindrical first housing part, a conically narrowing second housing part linking thereto, as well as a cylindrical third housing part linking thereto and within which a rotatable impeller of the fan is disposed, there being arranged at the entry side of the impeller, a guide ring externally of which is an annular bypass channel through which, under partial-load operation of the fan, controlled return of separations forming in the impeller region can take place.

Axial flow fans of large dimensions are nowadays frequently used for aeration or ventilation, and deaeration, of mines, motor tunnels, manufacturing plants, large combustion chambers and the like.

Such axial fans usually have an impeller which is driven by a motor, preferably an electric motor, and which is arranged behind a ring of non-rotating preguide vanes. With a view to achieving good operating efficiency, it is necessaryforthe diameter of the ring of pre-guide vanes to be made approximately 10 to 20% larger than the diameter of the rotating impeller, which leads to the fact that the flow medium conducted past the pre-guide vanes is accelerated prior to reaching the impeller. Such a dimensioning of the pre-guide vane ring with respect to the impeller then leads necessarily to the fact that the housing of the axial fan has two cylindrical housing parts of different diameters, which parts are connected together by a conical housing part.This interpolated conical housing part can be considered as a "nozzle" in which preacceleration of the medium to be accelerated by the rotating impeller comes about.

In such axial-flow fans, the wish often exists for the throughput of air or gas to be able to be varied through a wide range of regulation, according to the operating condition of the plant to be aerated or deaerated. Since rotational-speed regulation of the motor driving the impeller is, in most cases; technically extremely expensive or complicated, regulation of the amount of air or gas displaced by the axial fan is usually achieved by angular adjustment of the pre-guide vanes arranged at the entry side of the rotating impeller. With the aid of a "spin regulator" formed in this way, the amount of air or gas conveyed by the axial fan can be influenced over a wider range of regulation.

In the case of such axial fans having variable throughput, however, under partial load, there arise unstable operating conditions which are caused by air or gas separations in the region of the impeller vanes (see VGB Kraftwerkstechnik, 57th volume, No.

3, March 1977, page 160). In this respect, there occurs, along the edges of the individual vanes of the impeller, a break-away flow which can lead to vibration of the impeller vanes and thus to vibration breakages or failures of individual vanes. In addition to this, the efficiency of the axial fan is disadvantageously influenced, so that for this reason, too, such operating instability is not desirable.

Axial fans are already known (see, for example, illustration 7 of the above-mentioned publication) having, interposed between the cylindrical housing part serving for the reception of the guide vane ring and the conical housing linking thereto, a cylindrical ring which has externally thereof an annular return channel through which, under partial-load operation, the turbulences generated along the periphery of the rotating impeller are caught and are conducted in a controlled manner rearwards in the direction of the guide vane ring, so that extensive characteristic stabilisation is achieved.

From practical experience, it has become apparent that with provision of such a characteristic stabilisation device, axial fans can be operated stably over a very wide range of throughput regulation under the most varied pressures. What has proved to be disadvantageous, however, is alone the fact that subsequent incorporation of such a characteristic stabilisation device is, in practice, not feasible because subsequent insertion of the cylindrical ring, provided with a cylindrical return channel or duct, into the housing of the axial fan necessitates corresponding variations to the foundations or basic structure of the fan; at the same time the shaft carrying the rotatable impeller has to be lengthened, and then, in order to avoid critical rotational speeds, changes have to be made in relation to the shaft bearings and like components.Subsequent conversion for characteristic stabilisation proves, however, in many cases to be advantageous, but because the condition of such a characteristic stabilisation device necessarily represents an additional technical expenditure, it should be undertaken only in those cases where the respective operating conditions of the axial fan make this necessary. The necessity of such a measure can, however, as a rule, be established only after the relevant axial fan has already been erected 'in situ' and an appropriate number of test runs have been carried out.

It is accordingly the task of the present invention to provide an axial fan having an adjustable throughput in which unstable operation under partial load is largely avoided, the arrangement being such that subsequent conversion of an already existing or projected installation is readily feasible with a view to a characteristic stabilisation.

In accordance with the invention this is achieved by providing an axial fan comprising a cylindrical first housing part, a conically narrowing second housing part linking thereto, as well as a cylindrical third housing part linking thereto and within which a rotatable impeller of the fan is disposed, there being, arranged at the entry side of the impeller, a guide ring externally of which is an annular bypass channel through which, under partial-load operation of the fan, controlled return of separations forming in the impeller region can take place, characterised in that the guide ring is a conical ring which, whilst maintaining the desired width of the annular bypass channel, is fastened to the conical second housing part in such a way that the front edge of the conical ring terminates in the vicinity of the entry edge of the rotor blade(s) of the impeller.

Advantageous further developments of the invention are defined by the sub-claims appearing at the conclusion of this specification.

The invention will be described further, by way of example, with reference to the accompanying drawing, in which: Fig. 1 shows a fragmentary schematic partial sectional side view showing a known form of axial fan without characteristic stabilisation, but also illustrating how it may be adapted for characteristic stabilisation in accordance with the invention; and Fig. 2 is a view comparable with Fig. 1 but showing the fan having been provided, by conversion, with a characteristic stabilisation device in accordance with the invention.

The axial fan of known construction shown in Fig.

1 comprises housing parts 1,2 and 3 which are connected together using appropriate bolts. The first housing part 1 serves to accommodate a regulator such as a spin or twist regulator 4 in the form of a ring or crown of stationary pre-guide vanes which are angularly adjustable for the purpose of adjusting the delivery or throughput of the axial fan.

The second housing part 2, fastened to the first housing part 1, is of conical configuration so that a corresponding cross-sectional reduction of the interior space of the axial fan is achieved. The conicity of this second housing part 2 lies in the region between 15 and 20 , so that by appropriate choice of the longitudinal dimensions of this second housing part 2 the result can be achieved that a diameter reduction of 10 to 20% relative to the cylindrical housing part 1 carrying the spin regulator 4 is achieved. This conical housing part 2 accordingly acts as a "nozzle", within which the medium delivered by the spin regulator 4 is, subjected to predetermined acceleration with a view to optimising the efficiency of the axial fan.

Fastened to the conical housing part 2 is, in turn, a cylindrical housing part 3 which has a reduced diameter as compared with the first housing part 1.

Mounted rotatably inside this housing part 3 is an impeller 5 which is driven at a constant speed by way of a shaft (not shown) of a motor (also not shown), for example an electric motor.

If, now, such an axial fan is mounted at its place of use, within the framework of test runs to be carried out, it may emerge that, for reasons of impreciseiy known operating conditions, upon partial load the fan passes into an operating condition in which unstable operation occurs. Should this be the case, then within the scope of the present invention conversion of the axial fan shown in Fig. 1 is readily possible, in order to avoid such unstable operating conditions under partial load.

As shown in Fig. 2, this conversion requires the provision of an additional housing ring 1', whereby the housing part 1 serving for the reception of the regulator 4 is lengthened rearwardly. The third housing part 3, serving to accommodate the rotatable impeller 5, must furthermore be appropriately shortened, so that a shortened cylindrical housing part 3' is present.In this connection, the shortening of the third housing part 3 Is undertaken In such a way that a maximum of 25% of the width (or axial length) of the Impeller 5 projects out of the shortened housing part 3 Interposed between the housing nng 1 and the shortened housing part 3 is, finally, a conical housing part 2 of which the conicity amounts to about 30' and is thus greater than the conicity of the original housing part 2 of Fig. 1 Fastened to this conical housing part 2', using radially extending supporting plates 6, is a conical ring 7 which serves for the characteristic stabilisation and which has the same conicity or obliquity as the conical housing part 2'.The inside diameter of the conical ring 7 at its narrower end is dimensioned so as to be slightly larger than the overall outside diameter of the impeller 5, so that exchange of the impeller 5 by axial extraction using its shaft (not shown) is possible, without the conical housing part 2', with the conical ring 7 fastened thereto, having to be removed.The conical ring 7 is arranged so that the rearward edge at its narrow end is in the immediate vicinity of the front outside edge of the impeller 5, whereby the result is achieved that separations, occurring in the case of specific operating conditions in the rotor blade region of the impeller 5, are conducted as indicated by the depicted black arrows through the annular chamber formed between the conical ring 7 and the conical housing part 2', in orderthen to be fed anew to the main flow delivered by the regulator 4.

The measures provided for within the scope of the present invention, such as shortening of the cylindrical housing part 3 and enlargement of the conicity of the housing part 2, contribute essentially to the positioning of the annular space, serving for the characteristic stabilisation, between the conical housing part 2' and the conical ring 7 as close as possible to the gas or airflow separations which would otherwise lead to characteristic instabilities.

At the same time, with a view to permitting simple exchangeability of the impeller 5 the conical ring 7 is dimensioned in such a way that the impeller 5 can be drawn through the same. Whilst maintaining the existing axial conditions, such an arrangement of the conical ring 7, which serves to provide characteristic stabilisation, necessarily involves modification of the housing consisting of the housing parts 2 and 3.

Since the characteristic stabilisation is achieved substantially with the aid of the conical ring 7 and the annular space formed between the ring 7 and the conical housing part 2', in Fig. 1 two further possibilities for the mounting of the conical ring 7 are shown, in which no external modification of the housing, consisting of the housing parts 2 and 3, have to be undertaken.

The first possibility consists in that the inside diameter of the conical ring is selected, as indicated by the broken lines 8 in Fig. 1,so as to be smaller than the outside diameter of the impeller 5. In this case, it is specifically accepted that the relevant conical ring 2 has to be removed first before exchange of the impeller 5 can be undertaken by removal in the direction of the regulator 4. In order, nevertheless, to provide for simple mountability or assembly, in this case the fastening of the conical ring is advantageously effected with the aid of a row of bolts which are provided with appropriate spacer elements. Since.In the embodiment illustrated by the broken lines 8, the rearward edge of the conical ring terminates inside the diameter region of the front outer edge of the impeller 5, in the relevant case the gas or air separations occurring, when operating under partial load, at the rotor blades are still better intercepted, so that this embodiment has, relative to the desired characteristic stabilisation, improved stabilisation properties in comparison with the embodiment illustrated in Fig. 2. The conical ring indicated by the broken lines 8 should, of course, be dimensioned in such a way that its diameter at the narrow end is not less than 95% of the outside diameter of the impeller 5.

The second possibility for the mounting of the conical ring 7, serving for the characteristic stabilisation, consists in that the ring is fastened whilst maintaining an appropriately large diameter as exemplified by the dot-dash lines 9 of Fig. 1, offset relatively far forwards, inside the conical housing part 2, so that in a manner similar to the embodiment of Fig. 2, withdrawal of the impeller 5 without prior removai of the conical ring 7 is possible. In most cases, such an arrangement of the conical ring 7 yields adequate characteristic stabilisation, because during operation adequate suction occurs within the annular space between the conical ring 7 and the conical housing part 2, so that even the separations occurring at a distance at the front outer edge of the impeller 5 are sucked in a satisfactory way into the relevant annular space.

It will be evident that the conical ring 7 provided in the arrangement of the present invention can be provided in axial fans in which characteristic stabilisation is to be provided for right from the very start. In these cases, however, the possibility exists, in the embodiment illustrated in Fig. 2, of the cylindrical housing part 1 and the housing ring 1' being designed in one piece.

Claims

1.An axial fan comprising a cylindrical first housing part, a conically narrowing second housing part linking thereto, as well as a cylindrical third housing part linking thereto and within which a rotatable impeller of the fan is disposed, there being, arranged at the entry side of the impeller, a guide ring externally of which is an annular bypass channel through which, under partial-load operation of the fan, controlled return of separations forming in the impeller region can take place, characterised in that the guide ring is a conical ring which, whilst maintaining the desired width of the annular bypass channel, is fastened to the conical second housing part in such a way that the front edge of the conical ring terminates in the vicinity of the entry edge of the rotor blade(s) ofthe impeller

2.An axial fan as claimed in claim 1, characterised in that the conical ring is so dimensioned that its inside diameter at the narrow end is equal to or greater than the outside diameter of the impeller.

3. An axial fan as claimed in claim 1 or 2, characterised in that the rotatable impeller is arranged, relative to the three housing parts, in such a way that it projects up to a maximum of 25% of its width into the conical second housing part.

4. An axial fan as claimed in claim 1, 2 or 3, characterised in that the conical ring is fastened to the conical second housing part by means of radially extending supporting plates which are distributed uniformly around the periphery.

5. An axial fan as claimed in any preceding claim, characterised in that, whilst maintaining a predetermined distance between an adjustable regulator and the impeller, the cylindrical first housing part is so lengthened to beyond the adjustable regulator that, with a diameter ratio of the cylindrical first and third housing parts in the range from 1.1 to 1, to 1.2 to 1, an enlarged obliquity of the conical housing part carrying the conical ring is obtained.

6. An axial fan as claimed in claim 5, characterised in that the second conical housing part, carrying the conical ring, has an obliquity of about 30".

7. An axial fan substantially as hereinbefore described with reference to and as illustrated in Fig.

2 of the accompanying drawing.

8. An axial fan as claimed in claim 1 and substantially as illustrated and described as modifications of the fan of Fig. 1 of the accompanying drawing.