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Патент USA US2107897

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Feb. 8, 1.938. -
K.‘ n. MCMAHAN
2,107,897
FAN
Filed July 24, 1935
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InventoP:
1
Kenton D.MCM,Q|"1OT1,
bL-J
His Atto?neg.
Feb. 8,1938.
K. D‘. MQMAHAN
2,107,897 ‘
FAN
Filed July’ 24, 1935
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2 Sheets-Sheet 2
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2,107,897
Patented Feb. s, 1938
UNITED STATES PATENT OFFICE
3.107.897
ran
.
Kenton n. Mommas; Schenectady, Np!" ‘u
signor to General Electric Company, a cor
poration of New York
Application July :4, 1m. Serial No. 32.850
'1 Claims. (cl. 280-127)
select a fan which will operate on the portion of
My. invention relates to fans and more particu
larly to fans of the centrifugal type enclosed in the curve. past the knee. This means that, for
a scroll or housing.
.
'
example, referring to the solid 'line curve of Fig.
The object of my invention'is to provide an im
5 ' proved 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 fol
lowing speciilcation and appended claims when
10 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 sec
tional view along the lines 2-2 of Fig. 1; Figs.
- 3 and 4 are curves illustrating the performance
15 of the fan°with di?erent widths of scrolls; Fig. 5
is a side view of a double inlet fan; Fig. 6 is a
sectional view of the fan‘in Fig. 5 along the line
6-6, and Figs. 7 and 8 are curves illustrating the
performance of the double inlet fan with di?er
g0 ent widths of scroll.
v -
Referring to Figs. 1 and 2 of the drawings, I0
‘ indicates a scroll or housing in which a centrifu
gal fan II is mounted. Tan H consists of a
number of curved blades 12 mounted parallel to
25 the axis of rotation of the fan and spaced uni-'
formly about the axis. Blades I! are secured at
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 sat
isfactory to operate this .fan against a much
higher back pressure because then the fan would 10
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 pres
sure would cause a very- wide variation in ?ow. 15
Of course if the fan is to operate in a system in
which the pressure varies widely. it would be nec
essary to operate the _fan very' much further
down on the steep slope of this. curve. vI have
found that with a- given length of blade and 20
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;
one end to a disc II and at the other end to a ring a against
is, reduced.
whichInthe
Fig.fan
2, will
I have
operate
indicated
satisfactorily;
the ef
l4. Disc i3-is connected to a hub Iii. Huh. I! is
secured on the end of a shaft l6 which is adapted fective length of blade by the, letter b, the clear
30' to be connected to a source of power such as- an ance within the scroll at each end of the blade 30
electric motor or the like for driving the fan. by the letter 0, and the overall width of the scroll
‘ Scroll Ill is provided with an inlet ‘opening H by the iletter 's. I have °found that when the
in which is mounted a curved orifice I8. An ratio of the clearance c at each side of the blade
outlet I8 is provided for the housing ‘ill from to the effective blade length b is .3, the perform
ance of the fanis the most desirable. In other. 5
which the air passes from the scroll it. In op
eration the-centrifugal fans serve to convert. a words for a most satisfactory ‘operation the scroll
velocity head of air to a pressure head so that a width s should be approximately 1.6 times the
certain volume of air may be passed through-a effective blade length b... The difference in per
formance of the‘ fan with a lesser and greater
duct, machine or other apparatus against apres
40 sure inherent'in the systemthrough which the clearance ,is indicated in Figs. 3 and 4. The ‘0
‘ air is passed.~ In the application-ofa centrifugal curves of Figs. 3 and 4 were made for a fan‘hav
‘fan to a ' particular system the back pressure ing an effective blade length b of 3% inches and
against which the fan‘ must operate is determined adiameter of 71/2 inches. The dotted line curve
by ~the- characteristics of the system. ‘The
5 amount of -,air which it is desired to pass through
the. system is also determined, by-characteristics
‘
ofv 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.
inchgand the ratio of the clearance .c to the
Referring to‘the curves of Fig.4, it will be seen -' blade length -.is- .234. The solid line curves of
of.~=the
system.
-
~
'
the-tithe ?ow produced-by a particular fan- de
59 pendsupon the static-pressure against which it is
“operated. Inv most- systems-the pressure of the
Figs. 3 and .4 represent the performance of the
' system :will vary slightly and in other systems
and so the ratio of the clearance c to the effective
blade length b is .3. The dash and dot. line
in which for example a .damper is located the
pressure against which the fan isoperating may,
. vary quite widely. Therefore, it is necessary to
fan with a scroll width s‘ of. 6 inches in which
the clearance c each side of the fan is 1% inches
curves§of Figs. 3 and 4 arev thev performance .
curves for the fan with a scroll width so! 8%
2
12,107,897
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 e?iciency of vthe fan with the
wider scroll is substantially the same as the effi
ciency of the fan with the intermediate scroll
width. However, the e?iciency ‘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 inter
mediate width of the three scroll widths for
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 in
5
clined edge 2| also serves to reduce the noise of 10
the fan by compensating in part for deviations
in the inclination of the‘ blade l2. If a sharp
which the curves were taken shows a higher flow , edge were provided at this point, the variations
throughout the operating‘ range of the fan than in the position of the blade would cause one jet
of air to pass on one side of the edge and another 15
15 the corresponding curve for the 'fan with a small
er scroll width. Also, the fan with the interme
diate width operates satisfactorily against va
higher back pressure than the fan with the great
er scroll width. Taking a speci?c example for
jet of air from the following blade to pass on
the other side of the edge. This produces a pul-_
sation which is proportional to the rate of'pas
sage of the blades l2 by this point. vThese pulsa
20 a ?ow of 500 cubic feet per minute the fans are
tions sometimes cause a vibration of the blades
operating near their maximum e?iciency 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. How
ever, the fan with the greater scroll width can
when their frequency approaches the natural pe
riod of vibration of the blades and thereby pro
duce considerable noise. This inclined edge
therefore serves to compensate‘ for slight devia
tions in the inclination of the blades and so elim
inates these pulsations and the resultant noise.
not satisfactorily operate at thatwpoint because
I have found that an angle at of about 15° at
a slight variation in the back pressure would
cause a wide variation in the flow produced by
30 the fan. Similarly for a flow of 600 cubic feet
per minute the fan with the intermediate width
would operate satisfactorily against a back pres
sure greater than .9 inch of water, whereas the
fan with the lesser width would not produce so
35 great a flow against'this back pressure or would '_
f'only produce this ?ow against a back pressure
"40
of .8 inch of water. The fan with the greater
width would still ,be operating on an unsatisfac
tory portion of the curve. For a ?ow 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
45 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 ?ow against, a back pressure more
than two thirds of the pressure against which
50
the other two fans will operate satisfactorily.
It is to be noted that the difference in the per
formance 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 1A
inch 'in clearance on each side of the fan. Thus
it may be seen that when the ratio of the clear
ance c to the effective blade length I) is reduced
from .3 to .234 or increased from .3 to .368 the
which the edge'2l is inclined is satisfactory, how
ever, it is not critical but must not be so great as
to causetoo large an obstruction in the passage 30
of air from the opening l9.
Referring to Figs. 5 and 6, a double inlet cen
trifugal fan is shown. This type of fan with a
double inlet is generally provided where the
space requirements limit the diameter of fan or 35
where other considerations such as the flow re
quired 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 40
.in which are located two centrifugal fans 23
and 24. Fans 23 and 24 are provided respective
ly 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 con
nected to a source of power such as an electric
motortfor 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 50
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 cut
off 20 with its inclined edge 2l_ of the single in
let fan illustrated in Figs. 1 and 2.
In single inlet fans the fan diameter is usually
about twice the blade length and in double in
let fans the blade length is approximately equal
60 operation of the fan is greatly altered. ' However,
in general practice this clearance has been much to the fan diameter. I have found that by divid 60
less than thatrepresented by the dotted curves ing the blades of a double inlet fan in half and
of Figs. 3 and 4. It is an almost universal prac- ' spacing them as though two single inlet fans
tice to provide a clearance which is approximate
were being used the performance of the fan can
ly .1 of the blade length. This means that the be made substantially the same as though two
65
overall scroll width is generally made about 1.2 single inlet fans of ' the same diameter were 65
times the blade length whereas the curves of used. The lengths of blades 25 and 26 are in-.
Figs. 3 and 4 represent a variation from approxi
dicated by letters in and b: respectively. The
mately 1.5 times the blade length to 1.7 times clearance on each side of In is indicated by the
the blade length. Thus by comparison" with letter or. _ Similarly, the blade length of blades
70 present practice-any one of the curves of Figs. 3
26 is indicated by b: and theclearance on each 70
and 4 represents an improvement in perform
side by oz. The total scroll-'~"-’width is indicated
ance of the fan \over previous practice. There
by letter St. The ratio of or tofu and 0: to b: is .3.
fore, when other factors a?'ect the dimensions of In other words, the width of the scroll St equals
the scroll such as the available space, it may be
1.6 times b1+bz. To indicate the difference in
found satisfactory to use a scroll width which the performance of the fan with the fan blades 75
.
2,107,097
formed as a single fan with the spacing at each
side of the blades the same as would be provided
for a single inlet fan of the same diameter as
. 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 per
3
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 out
let opening, the width of said scroll being be
tween 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
formance curves for the fan as illustrated in ' common axis, means to drive said fan blades, and
The dash-dot curves of Figs. '1 and 8 a scroll having an inlet opening and an outlet 10
-10 Fig. 6'.
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
15
20
25
30
35
45
50
opening, a clearance being provided on each side
of the fan blades within the scroll equal to .3
times the e?ective length of ‘said fan blades.
without any clearance between them and so the
4. A centrifugal fan comprising a plurality of
total scroll width St is equal to 1.3 times bi+h. fan blades mounted in parallel relation about a
From the curve of Fig. '7 it will be seen that the . common axis, means to drive said fan blades,
peak of the efiiciency curve for the fan with the and a scroll having an inlet opening and an‘
lesser scroll width is reached at a lower ?ow ‘ outlet opening, a clearance being provided on
than for the curve of the greater scroll width. each side of the fan within‘ the scroll between
Also, the curves of Fig. 8 show that the flow of .24 and .35 times the effective length of said fan 20
the fan’ with the lesser scroll width is lower blades.
against any given back pressure than the ?ow
5. A centrifugal fan comprising a plurality oi’.
of the fan with the greater scroll width. By a fan blades mounted in parallel relation about a
comparison of the solid line curve of Fig. 8 with ' common axis, means to drive said fan blades;
the solid line curve of Fig. 4, it will be seen that and a scroll having an inlet opening and an out
for any given back pressure the flow. of the let opening, the width of said scroll being ap
double inlet fan is practically twice the ?ow of proximately equal to 1.6 times the effective length
the single inlet fan. This indicates that the of said blades, and a cut-off located in said out
double inlet fan of the same diameter with the let opening having an edge inclined at an angle
clearance between the two halves of the fan of approximately 15° to a radial line intersect .30
being equal to twice the clearance between the ing the edge.
fan and the scroll of the single inlet fan operates
6. A centrifugal fan comprising a plurality of
as though it were two ‘single inlet fans or each fan blades mounted in parallel relation about a
half of the fan operates independently of the common axis, means to drive said fan blades, and
other half. Without this clearance there is in
a scroll having two inlet openings and an outlet
terference in the ?ow produced by the two fans opening, the-width of said scroll being approx
and so a reduction in the total flow produced.
imately equal to 1.6 times the effective length of
From the foregoing it may be seen that an im
said fan ‘blades, and a cut-off located in said
proved centrifugal fan has been provided by the outlet opening having an edge inclined at an
provision of a clearance between the fan blades angle of approximately 15° to a radial line inter
40
and scroll to produce the higher flow against a secting the edge. f
higher back pressure. In addition, means have
7. A centrifugal fan comprising a plurality of
been provided to reduce the noise of operation of fan blades mounted in two sets about a common
the fan.
axis with a spacing between thetwo sets equal
What I claim as new and desire to secure by to .3 .times the combined length of the fan
Letters Patent of the United States, is:
. blades, means to drive said fan blades, and a. 45
1. A centrifugal fan comprising a plurality of scroll having two inlet openings and an outlet
fan blades ‘mounted in parallel relation about a opening, clearances provided on each side of
common axis, means to drive said fan blades, said fan blades being approximately equal to .15
and a scroll having an inlet opening and an times the'combined effective length of said fan
outlet opening, the width of said scroll being ap
blades, and a cut-oi! provided in said outlet 50
proximately equal to 1.6 times the effective length opening having ‘an edge inclined at ‘an acute
of- said fan blades.
‘angle to a radial line intersecting the edge.
2. A centrifugal fan comprising a plurality of
KENTON D. MGMAHAN.
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