Centrifugal ventilator fan

A centrifugal ventilator fan having a base plate with an axis of rotation, an inlet ring having an opening therethrough and a plurality of generally flat blades, each having first and second side edges, a leading edge and a trailing edge is provided. Each blade is connected to the base plate along the first side edge, and is connected to the inlet ring along the second side edge. The blades are radially spaced about the axis of rotation and are backwardly inclined with respect to a direction of rotation of the fan. A portion of each blade adjacent to the leading edge has a radius of curvature such that the blade portion adjacent to the leading edge extends generally inwardly towards the axis of rotation to reduce noise generated by the ventilator fan.

FIELD OF THE INVENTION 
The present invention relates to ventilator fan devices, and more 
particularly, to centrifugal ventilator exhaust fans. 
BACKGROUND OF THE INVENTION 
Exhaust ventilators with centrifugal fan blades are generally known in the 
art. Typically, these fans operate at low static pressure, with air 
entering axially through an inlet and being discharge radially through 
openings in the ventilator housing. One problem with the known ventilator 
fan designs is that the fans often produce a high noise level based on the 
turbulence created by turning the air flow from the axial inlet to the 
radial direction for discharge. While certain tradeoffs based on the speed 
of the fan, the fan size and the required volumetric air flow are 
expected, there has been a constant need to reduce the noise produced by 
such ventilator fans without significantly sacrificing fan performance. 
In one known ventilator fan design, the blades are designed with a tapered 
leading edge with the surfaces of each blade being formed partially by 
truncated cones. The leading edges of the blades are curved forward in the 
direction of rotation to catch the air flow, and the trailing edges of the 
blades are curved in a direction opposite to the direction of rotation of 
the fan to allow the air to flow off the ends of the blades. In another 
known blower wheel design, the blades are formed from sheet metal and each 
also includes a curved inner tip on the leading edge which catches the air 
flow, and is therefore curved in the direction of rotation. 
U.S. Pat. No. 5,336,050 which is assigned to the assignee of the present 
invention, and is incorporated by reference as if fully set forth, also 
discloses a ventilator fan having arcuate blades formed from sheet metal 
which are backwardly inclined with respect to the direction of rotation of 
the fan. 
The present invention is a result of observation of the limitations of the 
known fans and efforts to provide a centrifugal ventilator fan which 
generates reduced noise in comparison to the known designs while still 
providing a high volumetric air flow. 
SUMMARY OF THE INVENTION 
Briefly stated, the present invention provides a centrifugal ventilator fan 
having a base plate with an axis of rotation, an inlet ring having an 
opening therethrough and a plurality of generally flat blades. Each blade 
has first and second side edges, a leading edge and a trailing edge. Each 
blade is connected to the base plate along at least a portion of the first 
side edge, and is connected to the inlet ring along at least a portion of 
the second side edge. The blades are radially spaced about the axis of 
rotation and are backwardly inclined with respect to a direction of 
rotation of the fan. A portion of each blade adjacent to the leading edge 
has a radius of curvature such that the blade portion adjacent to the 
leading edge extends generally inwardly towards the axis of rotation to 
reduce noise generated by the ventilator fan. 
In another aspect, the present invention provides a centrifugal ventilator 
fan having a base plate with an axis of rotation, an inlet ring having an 
opening therethrough, and a plurality of generally flat blades. Each blade 
has first and second side edges, a leading edge and a trailing edge. Each 
blade is connected to the base plate along at least a portion of the first 
side edge, and is connected to the inlet ring along at least a portion of 
the second side edge. The blades are radially spaced about the axis of 
rotation and backwardly inclined with respect to a direction of rotation 
of the fan. A portion of each blade adjacent to the leading edge has a 
radius of curvature such that the blade portion adjacent to the leading 
edge extends generally inwardly towards the axis of rotation. Each blade 
has a chord length between the leading and trailing edges defined as 
C.sub.L, a thickness defined as t, and a radius of curvature of the 
leading edge defined as R, which satisfies the equations: 
EQU H=(R/24)(R/R.sub.1).sup.2 (12-(R/R.sub.1).sup.2) 
EQU 2&lt;C.sub.L /H&lt;.infin. 
where R.sub.1 =R+t/2, and C.sub.L /H is the radius coefficient, for a flow 
coefficient of between approximately 0.1 to 0.5 where the flow coefficient 
is defined by the equation: 
EQU .phi.=Q/AU 
where Q is a volumetric flow rate for air moved by the fan and is between 
150 and 3000 cubic feet per minute, A is a peripheral area of the fan, and 
U is a peripheral velocity of the fan. The radiused portion of each blade 
reduces noise generated by the ventilator fan.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Certain terminology is used in the following description for convenience 
only and is not limiting. The words "right," "left," "lower" and "upper" 
designate directions in the drawings to which reference is made. The words 
"inwardly" and "outwardly" refer to directions toward and away from, 
respectively, the geometric center of the centrifugal ventilator fan and 
designated parts thereof. The terminology includes the words above 
specifically mentioned, derivatives thereof and words of similar import. 
Referring to the drawings, wherein like numerals indicate like elements 
throughout, there is shown in FIG. 1-6 centrifugal ventilator fan 10 in 
accordance with a first preferred embodiment of the invention. As shown in 
FIG. 1, the centrifugal ventilator fan 10 comprises a housing 12, a motor 
14 located in the housing 12, and a fan blade assembly 18. Air, 
represented by flow arrow 16, is drawn axially into the centrifugal 
ventilator fan 10 and is forced radially outwardly by the fan blade 
assembly 18 through an opening in the housing 12. 
As shown in FIGS. 1-3, the fan blade assembly 18 of the centrifugal 
ventilator fan 10 includes a base plate 20 having an axis of rotation 22. 
The fan blade assembly 18 also includes an inlet ring 24 having a central 
opening 26 extending therethrough. Preferably, an annular, generally 
inwardly extending flange 28 is formed on the inlet ring 24 around the 
opening 26. A plurality of generally flat blades 30 are mounted between 
the base plate 20 and the inlet ring 24. 
In the first preferred embodiment 10, the drive shaft of the motor 12 is 
connected directly to the base plate 20 to rotate the fan blade assembly 
18, and the housing 12 is adapted for roof top mounting, such that exhaust 
air 16 can be drawn upwardly from an enclosed space by the fan 10 and 
forced radially outwardly through openings in the housing 12. However, it 
will be recognized by those skilled in the art that the fan 10 can be 
adapted for other types of mounting arrangements, or may be used to force 
air through a duct system, if desired. Additionally, the fan blade 
assembly 18 may be driven by other means, such as a belt or gear drive, in 
order to allow the motor 14 to be mounted separately from the housing 12 
and to provide different turning speeds for the fan blade assembly 18 
through the use of step-up or step-down pulleys, gears or other motion 
connecting devices (not shown), if desired. 
Referring now to FIGS. 2-6, the fan blades 30 each have first and second 
side edges 30a, 30b, a leading edge 32 facing the axis 22 and a trailing 
edge 34 facing the housing 12. Each blade 30 is connected to the base 
plate 20 along a portion of the first side edge 30a, and is connected to 
the inlet ring 24 along a portion of the second side edge 30b. The blades 
30 are generally equally radially spaced from each other about the axis of 
rotation 22 and are backwardly inclined with respect to a direction of 
rotation of the fan, represented by arrow 40. In the preferred embodiment, 
there are ten to twelve blades 30, and more preferably, twelve blades 30, 
as shown. However, it will be understood by those skilled in the art from 
the present disclosure that the number of blades can be varied, and there 
may be more or less blades depending on factors such as the volume of air 
flow required and others. 
As shown in FIGS. 2, 4 and 5, in the first embodiment of the centrifugal 
ventilator fan 10, each blade 30 includes an upper flange 42 and a lower 
flange 44 which are used to attach the fan blades 30 to the inlet ring 42 
and the base plate 20 respectively. Preferably, the blades 30 are attached 
with mechanical fasteners, such as rivets, screws, bolts or the like. 
However, it will be recognized by those skilled in the art from the 
present disclosure that other attachment means, such as adhesive or other 
bonding or welding could be used, if desired. Additionally, the flanges 
42, 44 could be omitted depending upon the assembly procedure and method 
and the fixtures used to position the blades 30 prior to attaching them in 
position by a method such as welding. 
Referring again to FIGS. 1-6, a portion 36 of each blade 30 adjacent to the 
leading edge 32 has a radius of curvature such that the blade portion 36 
adjacent to the leading edge 32 extends generally inwardly toward the axis 
of rotation 22 to reduce the noise generated by the ventilator fan 10. As 
indicated in FIG. 6, the radius of curvature of the leading edge 32 is 
indicated as R. Each blade 30 has a chord length, indicated as C.sub.L, 
between the leading and trailing edges 32, 34. The chord length C.sub.L of 
each blade 30 preferably decreases from the base plate 20 toward the inlet 
ring 24. Each blade has a thickness, indicated as t, and a height of the 
leading edge 32 from the generally flat portion of the blade 30, indicated 
as H. A radius coefficient C.sub.L /H for each blade 30 satisfies the 
equations: 
EQU H=(R/24)(R/R.sub.1).sup.2 (12-(R/R.sub.1).sup.2 (Equation 1) 
EQU 2&lt;C.sub.L /H&lt;.infin. (Equation 2) 
where R.sub.1 =R+t/2. 
In the first preferred embodiment 10, the radius of curvature R and chord 
length C.sub.L are defined such that the radius coefficient C.sub.L /H is 
between approximately 5 and 50, and more preferably between 5 and 15. The 
optimum fan performance was achieved with a radius coefficient C.sub.L of 
between 8 and 11. This provides a reduced noise level by the fan 10 due to 
the radius of curvature on the blade portion 36 adjacent to the leading 
edge 32, with an increase in air flow in comparison to a flat blade. 
In the first preferred embodiment, the fan 10 has a flow coefficient of 
between approximately 0.1 to 0.5, where the flow coefficient is defined by 
the equation: 
EQU .phi.=Q/AU (Equation 3) 
Q is a volumetric flow rate for air moved by the fan 10, and is preferably 
between 150 and 3000 cubic feet per minute, A is a peripheral area of the 
fan 10 which is calculated by multiplying the fan circumference by the 
blade height, and U is a peripheral velocity of the fan 10. However, 
improved performance and reduced noise levels are also realized at higher 
and lower flow rates through the use of blades 30 having a radiused 
portion 36 adjacent to the leading edge 32. 
Preferably, in the first preferred embodiment 10, the leading edge 32 of 
each blade 30 includes a radiused relief 32a adjacent to the opening 26 in 
the inlet ring 24, as shown in FIGS. 1 and 3. However, it will be 
recognized by those skilled in the art from the present disclosure that 
the radiused relief 32a may be omitted, if desired. 
Preferably, the base plate 20, the inlet ring 24 and the blades 30 are made 
of aluminum or an aluminum alloy, with the base plate 20 being 
approximately 0.060-0.080 inches thick and the blades 30 and inlet ring 24 
being made from approximately 0.040-0.0625 inches thick material. However, 
it will be recognized by those skilled in the art from the present 
disclosure that the material and thickness of the base plate 20, inlet 
ring 24 and blades 30 could be varied depending upon the size of the 
centrifugal ventilator fan 10. For example, the base plate 20, inlet ring 
24 and blades 30 could be made of any type of sheet metal polymeric 
material, composite or the like, depending upon the particular 
application, and the thickness of the material could be varied 
accordingly. 
Three fans in accordance with the first preferred embodiment 10 were tested 
for performance along with a similarly configured fourth fan with flat 
blades. The blades 30 for all of the fans were approximately four inches 
tall with a maximum chord length of 2.88 inches at the first edge 30a and 
a radius relief 32a of 1.0 inches on the leading edge 32 adjacent to the 
inlet ring 24. The chord length C.sub.L decreased from the first side edge 
30a toward the second side edge 30b based on a constant leading edge taper 
of about 9.75.degree.. The radius of curvature R for the blade portion 36 
adjacent to the leading edge 32 for the blades 30 of each of the four fans 
which were tested were 0.75 in., 0.313 in., 0.13 in., and 0.0 in. The 
height H was determined according to Equation 1 above based on each radius 
of curvature R and a thickness t of 0.050. The blades 30 were mounted to a 
base plate 20 having a diameter of 9.875 inches and the blade assemblies 
18 were rotated at 1550 rpm. 
Two graphs are shown in FIGS. 9 and 10 which show data obtained as a result 
of testing the fans described above. As can be seen from FIG. 9, the noise 
level decreased as the radius of curvature R increased from 0 to 0.8, from 
a maximum of approximately 7.57 sones for the flat blades to a minimum of 
approximately 6.7 sones. A sone is defined as the loudness of a sound with 
a frequency of 1,000 Hz and a sound pressure of 0.02 microbars (40 dB), 
and generally a noise level of 7.1 sones or less is considered to be 
acceptable. Sones were calculated by the method defined in ANSI S 
3.4-1980. 
Referring to FIG. 10, the flow rate in cubic feet per minute increased from 
approximately 706 cubic feet per minute (cfm) for the flat blade to a 
maximum of approximately 732 cfm for the blades having a radius of 
curvature R of 0.3 inches and then decreased as the radius of curvature R 
of the blades increased. While optimum performance was obtained with 
blades having a radius of curvature R at the leading edge 32 of 
approximately 0.3 inches with an acceptable noise level of approximately 7 
sones, reduced noise levels can be achieved through a trade off in fan 
performance. Based on the acceptability of a noise level of under 7.1, an 
optimum configuration for the radius of curvature R of the blades would be 
a radius of approximately 0.3 inches, with further reductions in noise 
level down to 6.75 sones being possible while achieving an equivalent air 
flow to the flat bladed fan. 
Based on the above data and additional testing, the best fan performance 
based on volumetric air flow with acceptable noise levels of 7.1 sones or 
less is achieved when the radius coefficient C.sub.L /H is between 5 and 
15, and more preferably, between approximately 8 and 11. 
Referring now to FIG. 7, a blade 130 for a second embodiment of a 
centrifugal ventilator fan is shown. The second embodiment of the 
centrifugal ventilator fan is identical to the first embodiment 10, except 
for the blade configuration. The blade 130 is similar to the blade 30 of 
the first embodiment, and like elements have been identified with the same 
reference numerals. 
The blade 130 is generally flat, and includes the portion 36 adjacent to 
the leading edge having a radius of curvature R. The blade 130 also 
includes a portion 138 adjacent to the trailing edge 34 which includes a 
radius of curvature R2. The curved blade portion 138 adjacent to the 
trailing edge 34 extends generally outwardly, in an opposite direction 
from the curved blade portion 36 adjacent to the leading edge 32. 
Based on experimental testing, the second embodiment of the fan with blades 
130 having the curved portion 138 adjacent to the trailing edge 34 further 
reduces noise level. Two centrifugal ventilator fans having a base plate 
with a diameter of 11.0 in. with the radius of curvature of R at the 
leading edge of 0.75 inches were tested at 1550 rpm. One fan also included 
a curved portion 138 adjacent to the trailing edge 34 having a radius R2 
of 0.75 in. with a height at the trailing edge of -0.057 in. The sound 
level produced by the centrifugal ventilator fan with the curved portion 
138 adjacent to the trailing edge 34 was 6.6 sones for an air flow of 860 
cfm. The fan having a blade without the curved portion 138 adjacent to the 
trailing edge 34 produced a noise level of 8.8 sones for an air flow of 
864 cfm. This is a significant improvement in noise level with only a 
nominal difference in volumetric air flow. 
Referring now to FIG. 8, a third embodiment of the centrifugal ventilator 
fan 210 is shown. The third embodiment of the ventilator fan 210 is 
similar to the first embodiment 10 and similar reference numerals with the 
prefix 2 have been used to identify like elements. The differences between 
the third embodiment of the centrifugal ventilator fan 210 and the first 
embodiment 10 are described in detail below. 
As shown in FIG. 8, the inlet ring 224 is curved and the second side edge 
230b of each blade 230 is curved to match the curvature of the inlet ring 
224. Additionally, the radius relief on the leading edge 232 has been 
omitted. Preferably, the blades 230 are assembled to the curved inlet ring 
with welds. Positioning holes may be provided in the inlet ring 224 and 
tabs may be located on the second side edge 230b of the blade 230 in order 
to properly align the inlet ring 224, in a similar manner to that 
described in U.S. Pat. No. 5,336,050, which is incorporated herein by 
reference as if fully set forth. The portion 236 of each blade 230 
adjacent to the leading edge 232 similarly includes a radius of curvature 
with the leading edge 232 extending generally inwardly toward the axis of 
rotation 222 to reduce noise generated by the ventilator fan 210. Again, 
optimal fan performance was achieved when the radius 420 coefficient 
C.sub.L h was between 5 and 15 and more preferably when the radius 
coefficient was between 8 and 11. 
It will be appreciated by those skilled in the art that changes could be 
made to the embodiments described above without departing from the broad 
inventive concept thereof. It is understood, therefore, that this 
invention is not limited to the particular embodiments disclosed, but it 
is intended to cover modifications within the spirit and scope of the 
present invention as defined by the appended claims.