CA2265860A1 - Fan rotor - Google Patents

Abstract

A fan comprises fan blades (24) extending from a hub (18) provided with a deflector ring (22) at the air delivery end of the rotational axis.

Description

CA 02265860 l999-03- 10W0 98l19070 PCT/GB97/02940FAN ROTORFan units are often manufactured and sold to stockists without the exactperformance requirements of the eventual user being known. In such cases itis normal that when sold, the fan manufacturer ensures that as far as possible,the included motor has sufficient capacity so as to cover the fans operationover the entire published fan characteristic.Generally axial fan units exhibit a "stall" or abrupt change in the slopeof their performance characteristic when they reach their maximum attainablepressure value. Operation beyond (i.e. to the left) of this ’stall point’ is notrecommended because of the unstable nature of the airflow/pressurecharacteristic, its associated increase in sound level; and its effect on themechanical integrity of the fan unit. However, the resulting powercharacteristic to the left of the ’stall point’ is generally of declining value withany increase at very low flow (i.e. towards the ’shut off’, or zero flowcondition) back to the peak value attainable when operating in its stablecharacteristic.The attained peak pressure of an axial fan is very much dependent onits tip speed (i.e. proportional to its rotation speed) and to that of the total tipchord or total width of the blades in the tip region.One kind of fan with a large tip chord is the so called scimitar bladedfan. Another is the sickle bladed fan. The greater chord results in higherpressure generation being achieved at the same tip speed.As these types are particularly useful for their extra pressure generationcapability over conventional axial impellers then it is important that theresulting power characteristic takes on a sensible and acceptable contour. Ithas been found from test work that very useful pressure developments canSUBSTITUTE SHEET (RULE 26)*rBW0 98/ 19070CA 02265860 l999-03- 102occur on these impellers with wide tip chord when placed within a ducted fanarrangement although hitherto at least scirnitar fans have usually been ductless,i.e. they have been made as partition fans or table fans. The inventors havediscovered that these large tip chord fans when used in ducted configurationsexhibit a rapid increase in power when run under low flow conditions whichis well in excess of the "normal" peak power experienced towards the higherflow conditions; such that powers can be as much as 150% of the normal peakpower.According to the invention fan blades extend from a hub provided witha deflector ring at the air delivery end of the rotational axis.It has been found that by the introduction of the hub ‘deflector ring’that this low flow power surge can be controlled with only marginal reductionof the achieved increased performance. This then can result in the normalmotor sizing associated with the normal ’peak power’ being safely made.The presence of the hub deflector ring has been found not to seriouslyeffect the throughput of the fan unit from any extra ’blockage effect’. Whatit does do is stop an increasing flow reversal back through the hub region ofthe blade, which increases as the main fan throughput is reduced.The natural centrifugal flow components become more dominant as themain flow reduces this gives a useful increase in pressure attainment to theimpeller, be it at the expense of power due to recirculation.The hub deflector plate in the fan which can be of a flat, inclined orspecially profiled section, controls the power to the designers requirement.Thus the hub deflector ring may help a cross-section which broadens,SUBSTITUTE SHEET (RULE 26)PCT/GB97I02940U:[QU130CA 02265860 l999-03- 10‘fl3preferably increasingly, towards the air delivery end of the rotational axis.The invention is not solely applicable to axial flow fans. It has been foundexperimentally to be useful in mixed flow fans. These have generally radiallyextending impeller blades like an axial fan. but the fan is designed so as toproduce a flow characteristic including generally axial intake but with deliveryfrom the fan including a centrifugal or any rate radially directed component.This may result from shaping of the hub and/or of the duct in which the bladesrotate, or by using a shroud which is effectively equal to the duct section inwhich the blades rotate but which is rotatable with the blades. Additionally oralternatively the effect may be caused by the shape and/or location of theblades. In particular. the blades may have axes which lie in planes generallyradial of the rotational axis. but which axes are inclined in that plane so as to becontained on the surface of an imaginary cone instead of lying in a plane normalto the axis of rotation.It has been found that with mixed flow fans occurrences happen similar to theeffect described above in relation to scimitar sickle bladed axial fans, especiallywhen the mixed flow fan has a spherical hub and spherical shroud designed toaccommodate at least adjustable pitch blades, that is to say ones which arerotatable about their individual axis to any of a range of positions to givedifferent fan characteristic.It has been found beneficial when designing such impellers for installationsrequiring either radial or axial airflow discharge that the tip shroud be relativelyshort in the rearward direction so as to allow the main airflow to radiallyexpand off the protruding blade tips when the installation allows. That is to say,the shroud has an axial length which is less than the developed axial length ofthe tip chord.AMENDED SHEET30CA 02265860 l999-03- 10-1This flow benefit. when the mixed flow fan is installed in a roof unit. is alsoapparent when the impeller is run in a ducted configuration. However. in thelatter the same increasing power phenomena at reduced flow as experienced inthe axial fans is present. This can be overcome by the use of a flat, tapered orcontoured hub deflector ring at the rear of the blade root. with little or no effecton the main flow achievement.The root of each fan blade may extend substantially to the outer edge of thedeflector ring. The fan blade may include an extension adjacent to the hub inthe air delivery direction. The extension may be triangular.A typical application of the invention is now more particularly described withreference to the accompanying drawing wherein the sole figure is a sectionalelevation view of a mixed flow fan. In the drawing the fan motor may beprovided at the location 10 upstream of the fan, that is to say at the inlet end oralternatively at the location 12 which is at the downstream or the delivery end.The arrangement includes a fixed duct 14 providing the inlet and a portionwhich is substantially a continuation of the fixed duct but is a rotating shroud16.The blades are fixed to a hub structure 18 including a part spherical portion 20and an outwardly flared hub deflector ring portion 22. The one curve may fairsmoothly into the other as shown. The shroud 16 may also be part sphericaland the centre of curvature of the part spherical area of the shroud and that ofthe hub 20 may be common.A plurality of blades 24 are provided fixed at each end to the hub and shroudrespectively. The inner and outer ends are shaped to make a close fit with theshroud and with the hub.AMENDED SHEETCA 02265860 l999-03- 10WO 98/19070 PCT/GB97/029405In the drawing, the blades are seen to conform closely to the hubincluding the portions of the blades extending over the hub deflector ring. Inthe drawing it appears that the blades do not fit closely against the shroud butthis is due to the limitation of a two dimensional drawing and to the twist ineach blade where the comers 30 and 32 are located at different circumferentialpositions in relation to the diametric section of the drawing. In practice aclose fit is achieved at the shroud.The blades may be adjustably mounted so that they can be turned aboutaxis A. The portion of the blade end lying against a part spherical surfacemay have an appropriately shaped end edge which will always lie against acomplementary part-spherical surface and hence can be made a close fit.However, in the case of the heel portion of the blade which lies against thehub deflector ring, this is not true, and in this area the blade has to be shapedso that it makes a perfect fit against the ring at an extreme position of twistand a progressively increasing less perfect fit i.e. clearance as the blade settingangle is decreased.It will be noted that the shroud 16 extends over only a portion of theblade width so that the airflow from inlet to outlet can have a substantial radialdirection at the outlet, which is particularly useful in the case of roof unitswhich draw air axially from within the building or ducting and deliver itradially around the perimeter into a roof cowl and thence to atmosphere.It will be noted that the blades have a somewhat triangular extension36 fitting against the hub deflector ring and it is particularly important that thearea 38 bounded by the broken line 40 joining the comer 30 with acorresponding comer of the extension 36 is not filled in. This shaping permitsthe driven air to escape from the fan without being caught up in a recirculatorySUBSTITUTE SHEET (RULE 26)CA 02265860 l999-03- 10WO 98/19070 PCT/GB97/029406flow leading to undesirable power absorption which would negate the purposeof the hub deflector ring.SUBSTITUTE SHEET (RULE 25)

Claims (11)

1. An axial or mixed flow fan comprising fan blades extending from a hub provided with a deflector ring at the air delivery end of the rotational axis each fan blade including an extension adjacent the hub in the air delivery direction such that an area which is not filled in is bounded by the blade edge and a notional straight line joining the extension and a corner of the blade.

2. A fan as claimed in Claim 1, wherein the hub deflector ring in the fan is of a cross section which broadens towards the air delivery end of the rotationalaxis.

3. A fan as claimed in Claim 2, wherein the hub deflector ring is of a curved cross-section becoming increasingly broad towards the air delivery end of the rotational axis.

4. A fan as claimed in any of claims 1, 2 and 3 wherein the extension is triangular.

5. A fan as claimed in any preceding Claim, wherein the fan is a mixed flow fan and the fan blades have axes which lie in planes generally radial of the rotational axis, but which axes are inclined in that plane so as to be containedon the surface of an imaginary cone instead of lying in a plane normal to the axis of rotation.

6. A fan as claimed in any preceding claim, wherein the fan includes a shroud and the shroud has an axial length which is less than the developed axiallength of the blade tip chord.

7. A fan as claimed in any preceding claim wherein the blades have adjustable pitch.

8. A fan as claimed in Claim 7, wherein the mixed flow fan has a part spherical hub.

9. A fan as claimed in Claim 7 or 8 wherein the fan has a part spherical shroud.

10. A fan as claimed in any preceding claim wherein the extension extends the root of each fan blade substantially to the outer edge of the deflector ring.

11. A fan as claimed in any preceding claim wherein the fan is a large tip chord fan.