US2107897A - Fan - Google Patents

Description

Feb. 8, 1 938. K. n. MCMAHAN FAN Filed July 24, 1935 2 Sheets-Sheet l Plow Cub/'c' w m 900 we InventoP: 1 Kenton D.MCM,Q|"1OT1, L-J

Flow- Cubic feet per' min.

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Feb. 8,1938. K. D. MQMAHAN FAN Filed July 24, 1935 2 Sheets-Sheet 2 way 200 4W I000 I200 14 I609 MW Flow Cub/c fet per min.

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Patented Feb. s, 1938 ran Kenton n. Mommas; Schenectady, Np!" usignor to General Electric poration of New York Company, a cor- Application July :4, 1m. Serial No. 32.850

'1 Claims.

My. invention relates to fans and more particularly to fans of the centrifugal type enclosed in a scroll or housing.

The object of my invention'is to provide an improved fan adapted to move a large quantity of air against a high back pressure.

What I consider to be novel and my invention will be better understood by reference to the following speciilcation and appended claims when considered'in connection with the accompanying drawings in which Fig. 1 is a side view broken away in part of my improved fan; Fig. '2 is a sectional view along the lines 2-2 of Fig. 1; Figs.

- 3 and 4 are curves illustrating the performance of the fanwith diflerent widths of scrolls; Fig. 5 is a side view of a double inlet fan; Fig. 6 is a sectional view of the fanin Fig. 5 along the line 6-6, and Figs. 7 and 8 are curves illustrating the performance of the double inlet fan with diflerg ent widths of scroll. v

Referring to Figs. 1 and 2 of the drawings, I0

indicates a scroll or housing in which a centrifugal fan II is mounted. Tan H consists of a number of curved blades 12 mounted parallel to 5 the axis of rotation of the fan and spaced uni-' formly about the axis. Blades I! are secured at one end to a disc II and at the other end to a ring l4. Disc i3-is connected to a hub Iii. Huh. I! is secured on the end of a shaft l6 which is adapted 30' to be connected to a source of power such asan electric motor or the like for driving the fan.

Scroll Ill is provided with an inlet opening H in which is mounted a curved orifice I8. An outlet I8 is provided for the housing ill from which the air passes from the scroll it. In operation the-centrifugal fans serve to convert a velocity head of air to a pressure head so that a certain volume of air may be passed through-a duct, machine or other apparatus against apres- 40 sure inherent'in the systemthrough which the air is passed.- In the application-ofa centrifugal fan to a particular system the back pressure against which the fan must operate is determined by -thecharacteristics of the system. The 5 amount of -,air which it is desired to pass through the. system is also determined, by-characteristics of.-=the system.

Referring tothe curves of Fig.4, it will be seen the-tithe flow produced-by a particular fande- 59 pendsupon the static-pressure against which it is operated. In mostsystems-the pressure of the system :will vary slightly and in other systems in which for example a damper is located the pressure against which the fan isoperating may,

. vary quite widely. Therefore, it is nece s y to select a fan which will operate on the portion of the curve past the knee. This means that, for example, referring to the solid 'line curve of Fig.

4. the fan of which this curve was made should be operated in a system in which the back pres- 5 sure is less than 1.1 inches of water so that upon any slight variation in pressure there will not be a greatvariation in flow. It would not be satisfactory to operate this .fan against a much higher back pressure because then the fan would be operating on the portion of the curve which is substantially horizontal. when operating on this portion of the curve the fan would tend to be unstable because a slight variation in the pressure would cause a verywide variation in flow. 15 Of course if the fan is to operate in a system in which the pressure varies widely. it would be necessary to operate the fan very' much further down on the steep slope of this curve. I have found that with agiven length of blade and diameter fan the operating characteristics of the fan may be altered by varying the width of the scroll. I have found that if this width is too small the flow which can, be producedby the fan I against a given pressure is reduced and if thisv scroll is too wide then the maximum pressure;

against which the fan will operate satisfactorily; a is, reduced. In Fig. 2, I have indicated the effective length of blade by the, letter b, the clearance within the scroll at each end of the blade by the letter 0, and the overall width of the scroll by the letter 's. I have found that when the ratio of the clearance c at each side of the blade to the effective blade length b is .3, the performance of the fanis the most desirable. In other. words for a most satisfactory operation the scroll width s should be approximately 1.6 times the effective blade length b. The difference in performance of the fan with a lesser and greater clearance ,is indicated in Figs. 3 and 4. The 0 curves of Figs. 3 and 4 were made for a fan having an effective blade length b of 3% inches and adiameter of 7 inches. The dotted line curve of Figs. 3 and 4 represent the performance of the fan with a scroll width 3 of 5% inches in which case the .clearance c on each side is- V. inch and the ratio of the clearance .c to the blade length -.is- .234. The solid line curves of Figs. 3 and .4 represent the performance of the fan with a scroll width s of. 6 inches in which the clearance c each side of the fan is 1% inches and so the ratio of the clearance c to the effective blade length b is .3. The dash and dot. line curvesof Figs. 3 and 4 arev the performance curves for the fan with a scroll width so! 8% inches in which the clearances c are 1% inches or the ratio of clearance c to the effective blade length I) is .368. From reference to Fig. 3 it may be seen that the efiiciency of the fan with the wider scroll is substantially the same as the efficiency of the fan with the intermediate scroll width. However, the efiiciency curvefor the fan with the smallest clearance or smallest scroll width falls off more rapidly from the peak value. By reference to Fig. 4 it may be .seen that the curve of performance of the fan with the intermediate width of the three scroll widths for which the curves were taken shows a higher flow throughout the operating range of the fan than the corresponding curve for the 'fan with a smaller scroll width. Also, the fan with the intermediate width operates satisfactorily against va higher back pressure than the fan with the greater scroll width. Taking a specific example for a flow of 500 cubic feet per minute the fans are operating near their maximum efiiciency but the pressure against which the intermediate scroll width will operate is approximately ..1 inch of water higher than either of the other two fans with the lesser or greater scroll width. However, the fan with the greater scroll width cannot satisfactorily operate at that point because a slight variation in the back pressure would cause a wide variation in the flow produced by the fan. Similarly for a flow of 600 cubic feet per minute the fan with the intermediate width would operate satisfactorily against a back pressure greater than .9 inch of water, whereas the fan with the lesser width would not produce so great a flow against'this back pressure or would f'only produce this flow against a back pressure of .8 inch of water. The fan with the greater width would still ,be operating on an unsatisfactory portion of the curve. For a flow of- 700 cubic feet per minute the fan with the wider scroll would operate the most satisfactorily, but the difference between it and the fan with the intermediate width is not very great. However, it is obvious that the fan with the greater width does not operate satisfactorily over as wide a range as the fan with the intermediate width. At this point the fan with the lesser width cannot produce the flow against a back pressure more than two thirds of the pressure against which the other two fans will operate satisfactorily. It is to be noted that the difference in the performance of this fan is relatively great when it is considered that the scroll width has been changed'from the one with the most satisfactory operation by only 1 inch in total width or A inch 'in clearance on each side of the fan. Thus it may be seen that when the ratio of the clearance c to the effective blade length I) is reduced from .3 to .234 or increased from .3 to .368 the operation of the fan is greatly altered. However, in general practice this clearance has been much less than thatrepresented by the dotted curves of Figs. 3 and 4. It is an almost universal practice to provide a clearance which is approximately .1 of the blade length. This means that the overall scroll width is generally made about 1.2 times the blade length whereas the curves of Figs. 3 and 4 represent a variation from approximately 1.5 times the blade length to 1.7 times the blade length. Thus by comparison" with present practice-any one of the curves of Figs. 3-

and 4 represents an improvement in performance of the fan \over previous practice. Therefore, when other factors afi'ect the dimensions of the scroll such as the available space, it may be found satisfactory to use a scroll width which varies between 1.5 and 1.7 times the blade length. However, for the most satisfactory performance the scroll width should be approximately 1.6 times the effective blade length.

In Fig. 1 it may be seen that a cut-off 20 is provided with an inclined edge 2| at an angle a: to a radial line extending from the axis of the fan. Thecut-oif 20 serves to prevent eddies at this point in the outlet opening I9. The inclined edge 2| also serves to reduce the noise of the fan by compensating in part for deviations in the inclination of the blade l2. If a sharp edge were provided at this point, the variations in the position of the blade would cause one jet of air to pass on one side of the edge and another jet of air from the following blade to pass on the other side of the edge. This produces a pulsation which is proportional to the rate of'passage of the blades l2 by this point. These pulsations sometimes cause a vibration of the blades when their frequency approaches the natural period of vibration of the blades and thereby produce considerable noise. This inclined edge therefore serves to compensate for slight deviations in the inclination of the blades and so eliminates these pulsations and the resultant noise. I have found that an angle at of about 15 at which the edge'2l is inclined is satisfactory, however, it is not critical but must not be so great as to causetoo large an obstruction in the passage of air from the opening l9.

Referring to Figs. 5 and 6, a double inlet centrifugal fan is shown. This type of fan with a double inlet is generally provided where the space requirements limit the diameter of fan or where other considerations such as the flow required would otherwise make it necessary to use two single inlet fans for which one double inlet fan may be substituted.

Referring to Figs. 5 and 6, 22 indicates a scroll .in which are located two centrifugal fans 23 and 24. Fans 23 and 24 are provided respectively with curved blades 25 and 26 mounted parallel and spaced from the central axis of the fan. Fan blades 25 and 26 are secured respectively to -discs 21 and 28 which are mounted on a hub 29.

Hub 29 is mounted on a shaft 30 which is connected to a source of power such as an electric motor for driving the fan. Scroll 22 is provided with two inlet openings 3| and 32 in which are located curved orifices 33 and 34 respectively. A single outlet opening 35 is provided. A cut-off 36 is provided in the opening 35 having an in clined edge3'l. The cut-off 36 with its inclined edge 31 performs the same functions as the cutoff 20 with its inclined edge 2l of the single inlet fan illustrated in Figs. 1 and 2.

In single inlet fans the fan diameter is usually about twice the blade length and in double inlet fans the blade length is approximately equal to the fan diameter. I have found that by dividing the blades of a double inlet fan in half and spacing them as though two single inlet fans were being used the performance of the fan can be made substantially the same as though two single inlet fans of the same diameter were used. The lengths of blades 25 and 26 are in-.

dicated by letters in and b: respectively. The clearance on each side of In is indicated by the letter or. Similarly, the blade length of blades 26 is indicated by b: and theclearance on each side by oz. The total scroll-'-"-width is indicated by letter St. The ratio of or tofu and to b: is .3. In other words, the width of the scroll St equals 1.6 times b1+bz. To indicate the difference in the performance of the fan with the fan blades compared with a fan with the blades divided in two and spaced apart so that the total clearance is equal to twice the clearance of a single inlet fan, curves of Figs. 'Land 8 have been taken. The solid line curves of Figs. 7 and 8 are the performance curves for the fan as illustrated in Fig. 6'. The dash-dot curves of Figs. '1 and 8 represent the performance of a fan with the same clearance between the blades and the sides 01' the scroll but with the blades abutting each other without any clearance between them and so the total scroll width St is equal to 1.3 times bi+h.

From the curve of Fig. '7 it will be seen that the lesser scroll width is reached at a lower flow than for the curve of the greater scroll width. Also, the curves of Fig. 8 show that the flow of the fan with the lesser scroll width is lower against any given back pressure than the flow of the fan with the greater scroll width. By a comparison of the solid line curve of Fig. 8 with the solid line curve of Fig. 4, it will be seen that for any given back pressure the flow of the double inlet fan is practically twice the flow of the single inlet fan. This indicates that the double inlet fan of the same diameter with the clearance between the two halves of the fan being equal to twice the clearance between the fan and the scroll of the single inlet fan operates as though it were two single inlet fans or each half of the fan operates independently of the other half. Without this clearance there is interference in the flow produced by the two fans and so a reduction in the total flow produced.

From the foregoing it may be seen that an improved centrifugal fan has been provided by the provision of a clearance between the fan blades and scroll to produce the higher flow against a higher back pressure. In addition, means have been provided to reduce the noise of operation of fan blades mounted in parallel relation about a common axis, means to drive said fan blades, and a scroll having an inlet opening and an outlet opening, the width of said scroll being between 1.5 and 1.7 times the eilfectivelengtn of said fan blades.

3. A centrifugal fan comprising a plurality of fan blades mounted in parallel relation about a common axis, means to drive said fan blades, and

a scroll having an inlet opening and an outlet opening, a clearance being provided on each side of the fan blades within the scroll equal to .3 times the eflective length of said fan blades.

4. A centrifugal fan comprising a plurality of fan blades mounted in parallel relation about a common axis, means to drive said fan blades,

and a scroll having an inlet opening and an outlet opening, a clearance being provided on each side of the fan within the scroll between .24 and .35 times the effective length of said fan blades.

5. A centrifugal fan comprising a plurality oi.

fan blades mounted in parallel relation about a common axis, means to drive said fan blades;

and a scroll having an inlet opening and an outlet opening, the width of said scroll being approximately equal to 1.6 times the effective length of said blades, and a cut-off located in said outlet opening having an edge inclined at an angle of approximately 15 to a radial line intersecting the edge.

6. A centrifugal fan comprising a plurality of fan blades mounted in parallel relation about a common axis, means to drive said fan blades, and a scroll having two inlet openings and an outlet opening, the-width of said scroll being approximately equal to 1.6 times the effective length of said fan blades, and a cut-off located in said outlet opening having an edge inclined at an angle of approximately 15 to a radial line intersecting the edge. f

7. A centrifugal fan comprising a plurality of fan blades mounted in two sets about a common axis with a spacing between thetwo sets equal to .3 .times the combined length of the fan blades, means to drive said fan blades, and a scroll having two inlet openings and an outlet opening, clearances provided on each side of said fan blades being approximately equal to .15 times the'combined effective length of said fan blades, and a cut-oi! provided in said outlet opening having an edge inclined at an acute angle to a radial line intersecting the edge.

KENTON D. MGMAHAN.

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