Tecnnical Field

The present invention relates to axial or mixed flow blowers or suction fans and more particularly to such Dlowe and fans which exhibit improved stability of operation. Bac grounά Art

It is Known that conventional fans suffer from tne problem of stalling when the gas flow past the fan blades breaks away from tne surface of the olades and/or tne nousin surrounding tne fan. The fans and Dlowers according to tn present invention are designed to significantly reαuce the liability of a stall condition occurring over a wide range- o possible operating conditions.

Disclosure of the Invention

The present invention consists in an axial or mixed flo blower or suction fan comprising an elongate tuoular casing within which is disposed an impeller comprising a huD rotataole about tne longitudinal axis of the casing and a plurality of blades extending radially therefrom, tne casing comprising a tubular inlet portion which terminates in a tubular intermediate portion of larger cross sectional dimensions tnan the inlet portion whicn in turn terminaces i a tubular outlet portion which is no larger in its cross sectional dimensions than the inlet portion, tne impeller being so positioned that the leading edge of the tip of each blade is positioned within tne intermediate portion wnile th trailing edge thereof is positioned witnin the outlet portio

The casing of tne olower or fan ^' is preferaoly circular in cross sectional shape. The inlet portion is preferaoly frusto-conical or cylindrical while the intermediate and outlet portions are preferably cylindrical. The diameter of the outlet portion snoulo not be greater than tne minimum diameter of the inlet portion and is preferably slightly smaller than that minimum diameter. As is conventional in such fans the diameter of the outlet portion should be only slightly larger than the diameter of tne circle swept oy tne

rotating blades of the impeller.

The diameter of the intermediate portion is larger than ^' the diameter, or minimum diameter, of the inlet and outlet portions. It is preferred that the diameter of the enlarged intermediate portion be from 1.1 to 1.5 times the diameter o the impeller and most preferably between 1.2 and 1.4 times that diameter.

There are preferably provided a plurality of fixed vane within the intermediate portion which vanes are preferbly disposed radially of the hub and axially of the casing. It is particularly preferred that these vanes narrow in a downstream direction.

The commencement and termination of the intermediate portion are preferably both abrupt with the intermediate portion being joined to the inlet or outlet portion by an annular wall portion which lies in a plane at right angles t the lorigitudinal axis of the- fan or blower casing. While it is not essential that the intermediate portion terminates abruptly in a downstream direction, it is essential that thi termination takes plac _r in an axial direction, between the leading and trailing edges of the impeller blades.

The arrangement according to this invention may be used with equal advantage in fixed or variable pitch fans and blowers.

In certain cases it is desirable to provide in the inle portion a plurality of radially directed guide vanes which are each rotatable about its own longitudinal axis. These guide vanes preferably extend between the wall of the inlet portion and a cylindrical stator disposed coaxially with the casing. These guide vanes are preferably used in axial flow fans and blowers in which the blades of the impeller are not themselves rotatable about their own longitudinal axes. These variable pitch guide vanes may be positioned at angles relative to the- longitudinal axis of the fan of from + 45 to - 90°. The angle is considered to be positive if the

vanes cause the incoming gas stream to be moved so as to increase the angle through which the direction of movement o the gas stream is changed by the impeller; it is considered negative when that angle is reduced. A similar convention i used when considering the fixed vanes in the intermediate portion. In fans and blowers where the impeller blades are of variable pitch such guide vanes are not generally require

The minimum diameter of the inlet portion is preferably slightly larger than the impeller diameter. The most preferred minimum diameter of the inlet portion is from 1.01 to 1.1 times the impeller diameter. The stability of the fa for any given minimum diameter of the inlet portion increase as the. axial distance between the leading edge of the impeller and the terminal edge of the inlet portions increases. This increased stability is however associated with a slight loss of efficiency.

Best Mode of Carrying Out the Invention Hereinafter given by way of example only are details of a variety of embodiments of the invention described with reference to the accompanying drawings in which •:-

Fig. 1 is a diagrammatic longitudinal sectional view through one embodiment of a mixed flow fan according to this invention.

Fig. 2 is a pressure/volume diagram for the mixed flow fan of Fig. 1.

Fig. 3 is a diagrammatic longitudinal sectional view through another embodiment of a mixed flow fan according to this invention.

Fig. 4 is a pressure/volume diagram for the mixed flow fan of Fig. 3.

Fig. 5 is a diagrammatic longitudinal sectional view through a further embodiment of a mixed flow fan according to this invention.

Fig. 6 is a diagrammatic longitudinal sectional view through a still further embodiment of a mixed flow fan

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according to this invention.

Fig. 7 is a pressure/volume diagram for the mixed flow fans of Figs. 5 and 6.

In describing the various embodiments of this invention the same numeral will be used to describe the same integers in the different embodiments of the invention.

As is seen in Figs. 1, 3, 5 and 6 the fan 10 comprises casing 11 or circular cross sectional shape which surrounds and is coaxial with an impeller 12 which comprises a hub 13 and a plurality of radially directed blades 14. The cylindrical casing 11 is divided into three principal parts. A frusto-conical inlet part 15 terminates in a cylindrical imtermediate part 16 of enlarged diameter which in turn terminates in a cylindrical outlet part 17. The intermediat part 16 terminates and the outlet part commences intermediat the leading and trailing edge of the impeller blades 14.

Upstream of the impeller 12, and coaxial therewith is-a centre tube 18. A plurality of radially directed guide vane 19, each mounted on a spindle 20, -are positioned about the - centre tube 18. Each vane 19 extends radially outwardly to the inside surface of the inlet port 15 of the casing 11 and is rotatable about its spindle 20 such that the angle of the vane to the longitudinal axis of the fan may be varied.

The intermediate part 16 may include an array of fixed vanes 21 which may be axially of the fan as in Figs 3 and 5 or at an angle to that axis as in Fig. 1.

In the arrangement shown in Fig. 1 the minimum diameter of the inlet part 15 is equal to the diameter of the outlet part 17, i.e. approximately equal to the diameter of the impeller 12.

.Tne intermediate portion includes a number of fixed vanes 21 each of which extends at 45 to the axis of the casing and is radially directed relative to the impeller.

The inlet portion 15 is extended rearwardly as cylindrical wall 22 which continues the minimum diameter of

inlet portion 15 and is connected to the radially inner edge of the vanes 21.

Figure 2 is a pressure/volume diagram for the fan shown in Fig. 1. Each solid line represents the measured P/V relationship for the angular setting of moveable guide vanes indicated. The dotted lines represent contours of equal efficiency with the point of maximum efficiency marked with cross. It can be seen from Fig. 2 that the fan of Fig. 1 showed a smooth P/V curve for all vane angles tested showing that the fan was not stalling at .any point throughout its full operating ranges.

Figure 3 shows another embodiment of the invention in which the intermediate portion includes a number of fixed vanes which extend axially of the casing and radially relative to the impeller. The inlet portion 15 is extended with the frusto conical wall 22 defining the minimum diamete ' of the inlet portion.

In this arragnement an aperture was left in the fixed vanes adjacent the leading edge of the intermediate portion to increase interaction of circulating flow between each pai of fixed stabilising vanes.

In can be seen from Fig. 4 that the above embodiment is free from stall characteristics.

Figures 5 and 6 show further embodiments of the invention.

Figure 5 shows an arrangement in which axially and radially disposed vanes 21 are provided in the intermediate portion. Figure 6 shows a very similar arrangement but without the vanes. In each of these situations the inlet portion stops abruptly at the start of the intermediate portion.

Figure 7 compares the P/V performance of the fans of Fig. 5 and 6 with the Fig. 5 fan results shown in dashed lines and the Fig. 6 fan in solid lines. It can be seen that while the removal of the fixed guide vanes has not induced

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stalling in the fan, it has substantially reduced pressures at low volumes.