US 2634048 A
Description (OCR text may contain errors)
April 7, 1953 J. E. M DONALD 2,634,048
4 FAN SYSTEM Filed Dec. 29, 1948 VOLUML INVENTOR 05i) 5. MtDana/d YWJYZ- ATTORN EY Patented Apr. 7, 1953 r i 1 I John1E. ll icljonaldi hlewtonpltdassgassignor a. Westinghouse Electric Corporation, East Pitts ere were erase-er I ApplicationiDe'cember 29, 1948, SerialNo.67,82
" fife classif er tac -1'29) T is invent on; relatcsi-toffan two or gnoreiians are connected in parallel to a common load.
orrnore fans in parallel for providing more ca pacitythan a single fan can provide, for handling a common load. Thus, it may be desired to con- 193M1 3 Systems, it is desirable toconnecttwo.
nect two fans in parallel to a common; outlet duct, with each fantohandle oneehalf the load.
However, it ispractically impossible to divide the loadin exactequality, since the resistance to the two fans will never. be exactly .thesame in a commercial installation. 1 1 is of no consequence the case of fansjsuch as thosehaving backwardly curved blades, which fans have pres sure-volume characteristic curves. with. steep Howevensome fans, such as the radial blade I and the forwardly curved blade types, have pressure volume characteristic curves with portions extending substantially parallel the volume ordinates or havingsdipsrSatisfactory and efiicient operation of such fansin parallel may re-.,
quire that'the fans operate at. points on such portions which may resultininstability.
If for any-reason, such as a momentary change of resistance, one of two such fans operating in p lallelnear instability, shou1d deliver less than; its, proportionate, share. of load, the second. -fan.
would; deliver more than, its proportionate share of load with resulting increased power consumption and possible overloading of the driving motor. Surging between fans may occur and there may even result a back flow through one fan and a very large flow through the other.
This invention enables fans having operating points on pressure-volume characteristic curves which extend substantially parallel the volume ordinates, or on dips in such curves, to be operated in parallel with stability. The outlets of the fans are connected through converging nozzles to a common exhaust duct, the nozzles of the fans terminating at the same point. The velocity pressures of the fans are increased and their static pressures are correspondingly reduced, resulting in the fans having pressurevolume characteristic curves with steep slopes through their operating points, thus providing stable operation.
An object of the invention is to increase the stability of fans connected in parallel to a common load.
1 immediate tension at erence t0 the drawing of which: L iate fans normally having pressure-volume .charac teristiccurves witlrr working portions extending: substantially parallel tov theyolumeordinates parallel, with'stability. i
Another object r; the inventionis toj (sea "f n n m ll z e e. pr s r o me 3 9 931.;
teristic curves with working portions having dius therein, in parallel, with,stability..
The invention will now; be described with re Fig. 1 :is a diagrammatic view illustr ,ing M conventional method of connect'ng tworians paralleltoacommonload;.
Fig. '2, is. achart showin the pressurewolum characteristic curves of eachpf two unstable ifansoperated i pa le a i us rated. b F 1 Fi ,3 is a;, a rammati. view l ll stratinetwe fans; operated. in parallelaccording :to this; in vention;..... t. .v
Fig. 14 isza chart showing the pressure volum V characteristic curves of each of the two fans asp! connected as, illustrated in Fig; 3, and. .11. L
Fig. .5 is a,diagrammaticviewillustrating three. unstable fans arranged, according to this inven-;; tiomfor stable operation. 1,;
Referring first to Fig. 1, the two unstable centrifugal fans l0 have theiroutlets connected to; dischargeintoa common outlet duct I I. ,Theirinlets i2 may beconnected to an upstream load,, or the loadmay be. a downstream one connected; 7
V. LQQI, T Tl} f SUP.
Referring. nowito, g-.,2,jthe ;curve A hows: he: total pressures of, each.1the; two fans of;Fig 2 i:ordifferent volumes. The ;c11.1've; B is, the static pressure. curve .forthe two fans. ,-The curve; 9;;
is the load resistance urve-forpne tan -carrying. the total load, and the curve D is the load resistance curve for each of the two fans, assuming the load to be shared equally. The operating point of the parallel connected fans is where the curve D crosses the static pressure curve B.
The stability of the fans determined by the shapes of their static pressure characteristic curves. As illustrated by Fig. 2, the curve B is substantially horizontal on each side of its operating point where the load resistance curve D crosses it. If the resistance offered momentarily to one of the fans is larger than that offered to the other, it tends to drop its load and its operating point tends to shift towards the point of zero volume, while the other fan tends to increase its load, and to shift its operating point towards the point of maximum volume. This action is well known to those skilled in the art, and may result in unstable operation.
However, despite the disadvantage of fans having operating points on horizontally extending pressure-volume characteristic curves, there are some duties where they are preferred. A typical duty is where the fans blow air for combustion into a number of gas furnaces. Operating conditions may require air for one or more furnaces to be shut off, resulting in a decreased load upon the fans. Due to the lack of slope of their static pressure-volume characteristic curves, the drop in load will not cause an increase in pressure, so that the volume of air delivered to each active furnace remains constant. Stable operation of such unstable fans has been accomplished in the past by continuously monitoring their operation, and actuating dampers for adjusting the loads on each fan, or by the use of elaborate, pressure responsive, automatic controls.
According to this invention, the outlet of each fan terminates in a converging nozzle, the outlets of the nozzles of all of the fans terminating at the same point. Figs. 3 and 5 illustrate two embodiments of this invention. Referring first to Fig. 3, the fans I are connected through the venturi I4 to the outlet duct H. The splitter sheet I 5 extends from the junction point of the outlets of the two fans into the narrowest portion of the venturi, and provides the similar converging nozzles I6 in the venturi. The air velocities will increase and the static pressures will decrease at the center of the venturi, the drop in static pressure being proportioned to volume. Static pressures measured at the outlets of the nozzles 16 will provide a static pressure-volume characteristic curve B of Fig. 4 with a steep slope at its operating point where the load resistance curve D crosses it. This results in stable operation of the fans, since the steepness of the slope through the operating point indicates reserve pressure for stable operation.
Fig. 5 illustrates how more than two fans may be connected in parallel, according to this invention, for stable operation. The adjacent walls l8 of the outlet passages ll of adjacent fans '10, merge together and extend into the narrowest portion of the venturi l4, and form the constricted nozzles N5, the nozzles having the samecross section at their point of merger, which is at the narrowest point of the venturi.
The air after passing through the venturis of Figs. .3 and 5, expands into the outlet ducts II in which the total pressure is the same as the sum of the total pressures at the outlets of the individual fans, less the slight losses due to pressure drops in the venturis.
While embodiments of the invention have been described for the purpose of illustration, it should be understood that the invention is not limited to the exact apparatus and arrangement of apparatus illustrated, as modifications thereof may be suggested by those skilled in the art, without departure from the essence of the invention.
What is claimed is:
1. A fan system comprising a plurality of separate, independent fans having separate fan wheels and separate driving shafts for the wheels, each fan having a fixed casing with a gas outlet around its wheel, a plurality of relatively small converging nozzles, one for each fan, each nozzle having an inlet connected to the outlet of one of said fans, a relatively large outlet duct for connecting said fans to a common load, and means forming a single diverging passage connecting the outlets of said nozzles with said duct.
2. A fan system comprising a plurality of separate, independent fans having separate fan wheels and separate driving shafts for the wheels, each fan having a fixed casing with a gas outlet around its wheel, a plurality of relatively small, converging nozzles, one for each fan, each nozzle having an inlet connected to the outlet of one of said fans, said nozzles having outlets arranged in a common plane, a relatively large duct for connecting said fans to a common load, and means forming a single diverging passage connecting said outlets of said nozzles with said duct.
3. A fan system comprising a plurality of separate, independent fans having separate fan wheels and separate driving shafts for the wheels, each fan having a fixed casing with a gas outlet around its wheel, a relatively large duct for connecting said fan to a common load, means forming a Venturi passage connected to said duct, and means forming converging passages connected to the outlets of said fans and terminating in the narrowest portion of said Venturi passage.
JOHN E. MCDONALD.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 1,080,763 Kerr Dec. 9, 1913 1,210,030 Baumann Dec. 26, 1916 1,261,457 Stott Apr. .2, 1918