Note: Descriptions are shown in the official language in which they were submitted.
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AIR ~ANDLING STR~CT~RE FOR FAN INLET AND OUTLET
This invention relates to air duct apparatus for use
in conjunction with air supply fan units, particularly
such units designed for buildings or other large
structures. This application is a division of Canadian
patent application serial No. 2,093,534 filed April 5,
1993.
It is well known to provide an air supply system for
a building, which system includes a main air supply duct,
branch supply ducts and a fan unit. Often an air
conditioning unit will form part of this system in order
to cool the air that is being forced through the ducts.
A problem often encountered with such systems is that the
fan unit can produce a substantial noise and this noise
i5 can be carried through the ductwork and thereby be heard
by persons in the building or structure. Not only is
this a problem downstream of the fan unit, but it can
also be a problem, at least in the immediate vicinity of
the fan unit, on the upstream side since sound can travel
out through the passageways that feed air to the fan
unit. The noise created by the large fans in these
systems is a particular problem in those buildings which
must or should be kept reasonably quite, for example in
hospitals and other buildings where the occupants are
sleeping on a regular basis.
In addition to providing some noise attenuation, an
air duct structure located downstream from a fan unit
often is required to deliver the airflow from the fan to
one or more air filters or perhaps to an air conditioning
unit. In such cases it can be important for the air
stream provided at the outlet of the duct structure to be
relatively uniform across the width and height of the
outlet. In this way, the amount of air flowing through
each filter or each section of the filter, would be
approximately the same.
In the construction of the duct structure located
immediately downstream from a fan unit, it can be
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advantageous if the size of the air flow passageway is
gradually increased from the inlet to the outlet of the
duct structure. By increasing the size of the passageway
in this manner, the air flowing through the passageway is
allowed to expand gradually, thus permitting the velocity
energy of the air to be recovered. As a result, the
static pressure of the airflow is thereby increased. A
gradual expansion of the size of the passageway is
important in order to obtain maximum regain of air
velocity pressure. By constructing the outlet duct
structure in this manner, one could use a smaller size of
fan motor to supply the same size of building than would
otherwise be the case.
Another requirement of the duct structure located
downstream from an air supply fan unit, is the frequent
need to convert the airflow passageway from one having a
round cross section at the outlet of the fan unit to one
having a rectangular cross section. It will be
appreciated that a rectangular air supply duct generally
provides a more efficient use of the space available in
a building for such ducts. Accordingly, it is often a
requirement in a building that the air supply ducts and
particularly the main ducts be substantially rectangular
or square. The distance available to a duct designer or
an air duct supplier for making this transition from a
round cross-section to a rectangular one will vary from
one job site to the next but, at least for some building
sites, the transition distance can be quite short.
U.S. patent No. 4,418,788 issued December 6, 1983 to
Mitco Corporation describes a combined branch take-off
and silencer unit for an air distribution system. This
combined apparatus has two series-coupled sections, the
first being a static pressure regain section and the
second section having a main airflow passageway extending
along its centre axis and branch ducts which connect
smoothly with the main passageway. The structure is
constructed with internal walls made of perforated metal
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sheets which overlays fibreglass packing provided for
sound absorption. The main duct in this apparatus has a
circular cross-section.
U.S. patent No. 4,295,416 issued October 20, 1981 to
Mitco Corporation describes a building air distribution
system with a mixing plenum for receiving and mixing
outside and return air. There is an input flow
concentrator and integral silencer disposed within the
plenum. The output port of this unit is connected to a
fan unit which drives the air to the main duct of the
building. The concentrator/silencer has inner and outer
sections which are axially symmetrical about a vertical
axis. It has an input port which extends symmetrically
about this axis and a circular output port at the top.
The inner and outer sections are lined with acoustically
absorbing material.
U.S. patent No. 4,986,170 dated January 22, 1991
issued to the present applicant describes a branch take-
off airflow device which can be used immediately
downstream of a fan unit. In the take-off section of the
unit, the take-off passageways are rectangular in
transverse cross-section whereas the main airflow
passageway extending axially through the unit has a
circular cross-section. In this main passageway there is
an elongate airflow defining member which has a round,
transverse cross-section with a maximum diameter equal to
the diameter of the hub of the adjacent fan.
British patent No. 1,423,986 in the name of Alan
Dodson et al, published February 4, 1976, describes a
silencer duct designed for use in a roof opening where an
extractor fan is located above the opening. Opposite
sidewalls of the duct are lined with sound absorbing
material such as glass fibre slabs. Additional silencing
is provided in the form of flow-splitter baffles which
are flat and parallel to each other. This duct unit has
a rectangular cross-section. The baffles themselves
contain sound absorbing material.
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The present invention provides improved air duct
structure for both the inlet and the outlet sides of an
air supply fan unit for a building or other large
structure. Both the inlet and the outlet apparatus are
provided with improved sound attenuating capabilities.
In one version of the apparatus, there is provided a
resonator mechanism for reducing the narrow band peak
noise generated by the fan blade passages, which
mechanism includes a hollow resonator chamber extending
around or located adjacent to the inlet or the outlet
that is connected to the fan unit. Furthermore, in an
improved version of the outlet duct structure, there are
provided at least first and second series of splitters
with the splitters of each series being spaced apart to
form smaller air passageways and mounted side-by-side in
a row. The second series is positioned downstream in the
airflow passageway relative to the first and is staggered
with respect to the first series. In addition to
improved sound attenuation, these splitters promote the
regain of air velocity pressure in the unit.
According to one aspect of the invention, a sound
attenuating duct unit for placement at an outlet of an
air supply fan unit for a building or other large
structure includes a housing having exterior sides and
internal walls surrounding a substantially curved, main
airflow passageway, a circular air inlet for said
passageway in one of said exterior sides for arrangement
next to an outlet of said fan unit, and a rectangular air
outlet for said passageway in another exterior side of
said housin-g, said side extends at a substantial angle to
said one exterior side, said internal walls providing a
gradual transition in the transverse cross-section of the
main airflow passageway from circular to rectangular,
wherein sound absorbing material is arranged between said
internal walls and said exterior sides of the housing and
surrounds said airflow passageway.
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According to another aspect of the invention, a
sound attenuating duct unit suitable for placement at an
outlet of an air supply fan unit comprises an exterior
housing having exterior walls forming outer surfaces of
said housing, an air inlet with a circular outer
perimeter lying in a first plane, and a rectangular air
outlet in a second plane arranged at a substantial angle
to said first plane, said air inlet being adapted for
connection to the fan unit in order to receive airflow
from the fan unit, interior walls arranged in said
housing and defining a main airflow passageway which is
substantially curved in its lengthwise direction and
which extends lengthwise from said air inlet to said air
outlet, said interior walls providing a gradual
transition in the transverse cross-section of said
airflow passageway from circular to rectangular, sound
absorbing material arranged between said interior walls
and said exterior walls and covering sides of said
airflow passageway and splitter apparatus rigidly mounted
in said airflow passageway and extending transversely
from one side of said passageway to an opposite side
thereof, said splitter apparatus dividing said main
airflow passageway into smaller passageways.
According to a further aspect of the invention, a
sound attenuating duct unit suitable for placement
adjacent an air supply fan unit for a building or other
large structure comprises an exterior housing having top,
bottom and end walls forming outer surfaces of said
housing, an annular opening in one end wall of said
30 ~ housing for arrangement next to one end of said fan unit,
said annular opening having a central axis extending
perpendicular to said one end wall, two rectangular
openings located on opposite sides of said housing, said
opposite sides extending at a substantial angle to said
one end wall and interior walls arranged in said housing,
connected to said top, bottom and end walls, and defining
airflow passageways which are substantially curved in an
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axial plane extending through said central axis, said
annular opening and rectangular openings being connected
by said airflow passageways so that the airflow through
said annular opening also flows through said rectangular
openings, wherein said interior walls contain sound
absorbing material which surrounds said airflow
passageways.
Preferably the two rectangular air outlets are
substantially larger than the circular air inlet.
In the preferred embodiment of the air duct outlet
apparatus, there is a central airflow defining member
rigidly mounted in the housing in the airflow passageway.
This member extends to the inlet adapted for connection
to the fan unit and creates an airflow passageway that is
annular at the inlet. There can be a resonator chamber
located at one end of this airflow defining member.
Further features and advantages will become apparent
from the following detailed description taken in
conjunction with the accompanying drawings which
illustrate preferred embodiments of the invention.
In the drawings,
Figure 1 is plan view of a typical equipment room in
a building wherein air duct silencing apparatus has been
installed;
Figure 2 is a perspective view showing vertical
sides and the top of both an air duct inlet structure and
an air duct outlet structure constructed in accordance
with the invention and in approximate relationship;
Figure 3 is another perspective view showing the
outlet ends of the air duct inlet structure and the air
duct outlet structure of Figure 2 in which the top panel
of the outlet structure has been exploded and in which
the outlet structure is broken away for purposes of
illustration;
Figure 4 is a side elevational view, partly in
cross-section, taken in the direction of the arrow 4
shown in Figure 2 showing the air duct inlet structure
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(in the lower half, a central interior wall has been
broken away to reveal an inner air passage and a cone
member);
Figure 5a is one half of a composite section of the
air duct inlet structure taken along the line Va-Va of
Figure 4;
Figure 5b is the other half of the composite section
of the air duct inlet structure taken along the line Vb-
Vb of Figure 4 showing the flat floor of the upper
section and in chain dot lines the outline of the
passageway above the plane of the section;
Figure 6 is a plan view of an air duct outlet
structure with one half of the view in cross-section
along the line VI-VI of Figure 3;
Figure 7 is a detail view of the transverse cross-
section of a splitter used in the air duct outlet
structure of Figure 6;
Figure 8 is a detail view, with sections removed, of
the splitter of Figure 7, which view shows an inner
horizontal plate support;
Figure 9 is a graph or chart plotting flow
resistivity versus duct height, which design chart can be
used to select the flow resistivity for the sound
absorbing material; and
Figure 10 is a graph plotting sound power (d~3)
against the octave band (Hz) and showing the results of
tests conducted with the inlet silencer and outlet
silencer described herein.
Figure 1 illustrates a typical equipment room
constructed to house the air supply equipment for a
building or other large structure. Outlined in dashed
lines are the walls 10 and 12 of this room 14. Located
at one end of the room and also indicated in dashed lines
are three inlets 16 which supply outside air to the room
and to the air supply equipment. Centrally located in
the room and preferably accessible for removal or repairs
is an air supply fan unit 18 which drives the air from a
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combined air duct inlet apparatus and silencer 20 to a
combined air duct outlet apparatus and silencer 22. It
will be understood that the air duct apparatus 20
incorporates at least one aspect of the present
invention. The fan 18 itself can be of standard
construction and the unit 18 ~_ se does not form part of
the present invention.
In the arrangement shown, the outlet apparatus 22
supplies air to a bank of or series of air filters 24
through which the air flows to a rectangular plenum 26
shown in dashed lines and possibly to several smaller,
rectangular supply ducts 28 to 30. Alternatively, the
outlet apparatus 22 may supply air directly to a large
rectangular supply duct.
It will be understood that incoming air enters the
duct inlet apparatus 20 from opposite vertical sides 32
and 34 and accordingly these sides should be spaced an
adequate distance from the walls of the room, for example
four to five feet. It will also be understood that the
standard fan unit 18 has a circular air inlet at the end
36 of the unit and a circular air outlet at its
downstream end 38. Accordingly, the outlet for the air
duct apparatus 20 and the inlet for the air duct outlet
apparatus 22 are also circular and preferably of
corresponding size.
Referring now to Figures 2 and 3 of the drawings,
the duct inlet apparatus 20 includes an exterior housing
40 having two principal air inlets 42 and 44 located at
sides 32 and 34 respectively, that is on opposite
vertical sides. This unit also has a single annular air
outlet 46 located at one end of the housing and adapted
for connection to the fan unit for air flow to the
latter. The air inlets 42 and 44 and the outlet 46 are
connected by~ an airflow passageways 48 defined by
interior walls 50, 52 and 54, which passageways curve
about 90 degrees from the inlets to the outlets. At
least sections of these walls are preferably made of
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perforated sheet metal to provide sound attenuation.
Preferably the air passageway extending from each inlet
is divided into four quadrants as illustrated but with
larger units more than four segments for the inlet on
each side can be constructed. The upper and lower
quadrants are separated by a horizontal divider 56 which
extends from a front wall 58 to rear wall 60. The left
and right quadrants are separated by the aforementioned
interior wall 52 which is shaped like one half of a
funnel in the passageway. It thus has a curved section
62 which extends to a semi-cylindrical section 64. The
interior wall 50 is a vertical wall that is curved in
plan view. Its leading edge 66 is located at the front
wall 58 while its rear edge 68 is located near the outlet
46 as shown in Figures 5a and 5b.
With respect to interior wall 54, it forms an
annulus at 70 which is semi-circular in cross-section.
The purpose of this annulus is to help smooth the flow of
air into the fan unit and to help avoid a direct line of
sight from the inlet of the fan unit through the
passageway 48. Because the sound is unable to pass
directly from the front of the fan to the interior of the
room 14, the amount of noise is reduced.
The duct inlet apparatus is also provided with a
central airflow defining member in the form of conical
plate 72, which plate is rigidly mounted in the housing
in the airflow passageway 48. The wide end of this
member is located at the outlet 46. With this conical
plate, which is also made of perforated metal and
contains sound absorbing material, and the internal walls
50 and 54, the two airflow passageways 48 join and form
an' annular passage at the outlet 46 (see Figure 3).
Thus, the shape and size of the combined passageway at
this outlet corresponds to the shape and size of the
inlet (not shown) of fan unit 18.
In order that the duct inlet apparatus 20 will also
act as a silencer, the housing contains sound absorbing
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material, which material is indicated generally at 76.
The sound absorbing material extends to and is covered by
the internal walls 50, 52 and 54. In preferred
embodiments of both the duct inlet apparatus 20 and the
outlet apparatus 22 there are at least two types of sound
absorbing material used. The first type is the
relatively thin layer, for example, one half inch, of
fibreglass insulation which has a cloth backing. A
suitable form of this insulation indicated at 78 in
Figures 5a and 5b is Knauf Ductliner-M. This material has
zero erosion of the fibreglass insulation at air
~elocities up to 6,000 feet per minute. Because of this
zero erosion characteristic it is placed directly against
the back of the perforated metal plate which forms the
interior walls of the duct/silencer with the cloth
backing lying against the perforated sheet metal. Behind
the material 78 is placed standard low density acoustical
filler 80 which is used to fill the rem~-n~er of the
cavity between the internal walls and the exterior walls
of the housing. For example, this standard fibreglass
acoustical filler can be purchased in the form of bats
that are 3 inches thick and when placed in the
duct/silencer it is compressed to some extent (for
example from 3 inches to 2 inches in thickness) in order
that it will completely fill the space and have good
sound absorbing capabilities.
In a preferred embodiment of the apparatus 20, only
a portion of the internal wall 52 is made of perforated
metal sheet. In fact, all of the side of wall 52 that
faces the internal wall 50 and the conical plate 72 is
made of imperforate galvanized metal sheet (for example
16 gauge). The imperforate sheet metal is indicated at
82. Only the curved portion of internal wall 52 which
faces the internal wall 54 is constructed of perforated
metal sheet, typically 22 gauge. This perforated sheet
is indicated at 84 in Figure 3. The reason for the use
of the two different sheet materials is that the
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perforated sheet is only used where there is room for
sound absorbing material to be placed behind the metal
sheet.
It will also be appreciated by those skilled in this
art that the apparatus 20 could also be used as a duct
outlet apparatus/silencer for placement immediately
downstream of the fan unit, if desired. Such a use would
provide enhanced sound attenuation as well as uniform air
delivery to the two outlets of the duct unit.
10Reference will now be made to the main components
and structure of the duct outlet apparatus/silencer 22
which is connected to the outlet side of the fan unit 18.
The duct apparatus 22 includes an exterior housing 90
with sidewalls 92, a front end wall 94 containing an air
15inlet 96 and a rectangular air outlet 98. The inlet 96
and the outlet 98 are connected by a main airflow
passageway 100 defined by interior walls 102 of the
housing (see Figure 6).
The duct apparatus 22 contains a central airflow
defining member 104 which is rigidly mounted in the
housing in the passageway 100. This conical member 100
tapers and extends from the region of the inlet 96 to a
centrally located splitter 106 described further
hereinafter. Thus, between the member 104 and the
interior wall 102, the passageway 100 is substantially
annular. The member 104 is filled with sound absorbing
material in the manner described above in connection with
the inlet apparatus 20. This sound absorbing material
also fills the space behind interior walls 102 and
surrounds the passageway 100. In the outlet duct
apparatus 22 the main passageway 100 is shown as
substantially straight although the passageway increases
in transverse cross-section from the inlet to the outlet.
However, it will be appreciated that an outlet duct
apparatus constructed in accordance with the invention
can be made with a curved main passageway that, for
example, curves about 90 degrees from the air inlet to
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the air outlet. In this case the outlet of the unit
would be at a side of the housing rather than at the end
thereof which is opposite the end wall 94. The air inlet
96 corresponds substantially in size and shape to the
outlet (not shown) of the fan unit 18.
The outlet apparatus 22 has a top sidewall 108 and
a bottom sidewall 110. Between these two walls or panels
extend at least first and second series of air stream
splitters 112 and 114 with the splitters of each series
being spaced apart to form smaller air passageways 116.
The splitters of each series are mounted side-by-side in
a row as shown in Figures 3 and 6 with the second series
comprising the splitters 114 positioned downstream in the
airflow passageway 100 relative to the first series
comprising the splitters 112. Also, the splitters 114
are staggered relative to the first series transverse to
the direction of air flow in the passageway. In this way
there is no direct line of sight from the inlet 96 to the
outlet 98, thus preventing sound waves from travelling
directly from the inlet to the outlet. This is due in
part to having the width of the splitters correspond
closely to the width of the passageways 116 between the
splitters of the other series.
The construction of each splitter will now be
discussed in detail with references to Figures 3, 6, 7
and 8. Each splitter 112 and 114 contains sound
absorbing material 76. Again, this material can comprise
the two types of fibreglass material described above in
connection with inlet apparatus 20. Each splitter is a
straight elongate member which extends vertically
substantially the entire height of the outlet duct
apparatus 22. Each splitter is formed with perforated
sheet metal 120 which covers the sound attenuating or
sound absorbing material 76 contained in the splitter.
Preferably the fibreglass insulation in the nose area 122
is packed to a higher density to improve the sound
attenuating characteristics of the splitter. In the
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illustrated preferred embodiment the nose area is packed
with acoustical filler to a density of 1.6 lbs per cu.ft.
while the r~m~;n~er of the splitter is packed with the
same filler to a lower density of only 1.2 lbs per cubic
foot. The nose section 122 including the rounded nose
124 which forms the upstream end is made of imperforate
metal. The nose 24 is preferably a length of metal
tubing 126 (for example, 2 inch outer diameter tubing).
In one preferred embodiment, the total depth of the
splitter from the nose 124 to tail end 128 is 45 inches
while the depth of the splitter 112 is 25 inches. In
this version, the splitter 114 has the maximum width of
12 inches while the corresponding splitter 112 has a
maximum width of 8 inches. Also, as shown in Figure 6,
the nose portion of each splitter 112 is semi-circular in
cross-section and is more rounded than the nose area of
each splitter 114. The nose area 129 can be made from
imperforate 18 gauge galvanized sheet metal that is
welded to the perforated metal forming the sides of each
splitter 112. The use of imperforate metal in the nose
region has distinct advantages in that it reduces air
friction at the region of impact of the air flow with the
splitter and it helps maintain airflow speed through the
duct unit. Optionally one can provide an internal
partition wall 131 that separates the nose area from the
rest of the splitter. This plate extends the entire
height of the splitter and acts to separate the two
densities of filler material.
The number of splitters in each row and their
geometry can vary based on the desired length, width,
height and sound absorption capacity of the duct
apparatus 22. Also, if the main airflow passageway bends
from inlet to outlet, the splitters can also bend or
curve in their transverse horizontal cross-section to
match the curve of the passageway.
Figure 8 illustrates how each splitter 112, 114 can
be provided with one or more intermediate, horizontal
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support plates 130 which are welded to the exterior metal
sheets by means of flanges 132. Each support 130 can,
for example, be made of 18 gauge imperforate metal sheet.
In addition to providing added strength, the support
plates 130 help to support the sound absorbing material
and prevent it from settling unduly. Figure 8 also
illustrates the use of imperforate top and bottom plates
134 and 136 which are used to connect the splitter to the
top and bottom walls of the housing.
As shown in Figure 7, the preferred splitter 114 has
three sections moving in the direction of airflow through
the duct unit. These include a short nose section 140, a
larger central section 141 with flat opposing sidewalls,
and a tapering tail section 142. This provides the
splitter with a streamlined exterior that will not slow
down the flow of air an undesirable amount. Preferably
the sidewalls 144 diverge slightly in the direction of
airflow.
It will be appreciated that the aforementioned
internal walls 102 provide a gradual transition in the
transverse cross-section of the main airflow passageway
100 from circular to rectangular, it being noted that the
air inlet 96 has a circular periphery while the air
outlet 98 is rectangular. This gradual transition takes
place over a relatively short distance indicated by the
letter D in Figure 6 relative to the total front to back
~;men.~ion of the outlet apparatus 22. For example, in
one preferred version of the apparatus 22, the distance
D is 2 feet whereas the total distance from end wall 94
to the outlet 98 is 7 feet. Accordingly, in the region
of the air passageway where the splitters 112 and 114 are
mounted, the passageway has a rectangular cross-section.
The transverse cross-section of the passageway 100
gradually increases from the air inlet 96 to the air
outlet 98 as shown, whereby the air velocity pressure of
air flowing through the passageway is recovered. The
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rectangular air outlet 98 is substantially larger than
the circular air inlet.
In addition to the function of sound attenuation,
another function of the splitters 112 and 114 is to
divide the airflow in the main passageway evenly across
the width thereof. For this reason the splitters in each
series are substantially evenly spaced apart as shown in
Figure 6 SO as to create the smaller air passageways 116
between them, which are substantially equal in
transverse width (as well as in height). Small outer
passageways 150 have a width about one half the width of
passageways 116 between the splitters 114. It will be
understood that by having the splitters so arranged that
they split the stream of air evenly at each series of
splitters, one will achieve a substantially uniform air
stream at the outlet 98 where the air is combined again
into a single air stream. In this way the air stream
will strike the air filters 24 evenly, thus causing the
filters to operate with maximum efficiency and to have a
longer operating life before cleaning or replacement.
Also, a gradual expansion of the air flow in the duct
apparatus 22 (as permitted by the splitters) results in
maximum static pressure regain. The outlet duct
apparatus 22 has the basic advantages of saving both
space and energy, the space being gained by having the
transition from circular to rectangular cross-section
incorporated into the body of the silencer.
Preferably in the region of outlet 98 there are
additional flat splitters 152. These can be made of
flat, imperforate sheet metal~connected at the top and
the bottom to the housing (typically by welding).
Another advantageous feature which is found in the
outlet duct apparatus 22 is diffusing baffle means
rigidly mounted in the airflow passageway 100 to provide
more uniform air distribution at the air outlet 98. In
the illustrated embodiment, the diffusing baffle means
comprises a single baffle member 152 made of imperforate
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metal plate. In one preferred embodiment, the diffusing
baffle member is made of 16 gauge galvanized sheet metal
and has a length of about 2 feet, the same as the length
of the gradual transition from circular to rectangular in
the cross-section of the main airflow passageway. The
member 152 extends about a central axis of the airflow
passageway 100 and acts to reduce the angle of expansion
of air flowing through this passageway. The sheet metal
member is formed with multiple bends so that its
transverse cross-section goes from round at the inlet 96
to rectangular (see Figure 3). The member 152 also
increases the performance of the outlet duct apparatus 22
from the standpoint of velocity regain in the air flow.
The downstream end of baffle member 154 is arranged
to meet the nose 129 of the outer splitters 112,
preferably in the centre of this nose as shown in Figure
6. It will thus be appreciated that air entering the
inlet 96 at the point 160 is forced to flow on the
outside of the baffle member 154 and once it reaches the
outer splitter 112, is forced to flow on the outside
thereof.
Both the inlet duct apparatus 20 and the outlet duct
apparatus 22 are preferably provided with resonator means
for reducing the noise created by the operation of the
fan unit, particulary peak blade passage frequency noise.
In each duct unit, this resonator means comprises one or
two hollow resonator chambers located adjacent the one
inlet or outlet that is adapted for connection to the fan
unit. As shown in Figures 4, 5a and 5b, in the inlet
duct apparatus 20, there are two resonator chambers 170
and 172, each of which is provided with a number of holes
174, 176. The use of only one resonator chamber is also
possible. Each of these chambers is enclosed by chamber
walls including a peripheral wall which contains the
holes 174 and 176. The chamber 172 is annular extending
around the outside of the air passageway 48 while the
chamber 170 is a flat, circular chamber ha~ing a diameter
CA 02208l90 l997-06-l3
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equal to that of the wide end of the perforated plate
that forms conical member 72. Thus, the chamber 170 is
encircled by the air passageway. In each case, the
peripheral wall that contains the holes 174 and 176 faces
the airflow passageway. Also, as shown in Figure 4 and
5, the annular chamber 172 is defined by four walls
including inner and outer circumferential walls 178 and
180, radially extending sidewall 182, and the rear wall
60 of the housing. In a preferred embodiment, the chamber
walls are made of 16 gauge sheet metal and are
imperforate except for the aforementioned holes 174, 176.
In one preferred embodiment wherein the outside diameter
of the annular outlet is 55 inches, the annular chamber
172 had 23 holes each measuring one inch in diameter
spaced evenly about the circumference of the chamber. The
outside diameter of the chamber 172 was 61 inches and its
height was 3 inches. In this same embodiment, the
circular chamber 170 had a diameter of 28 inches, a width
of 2 5/8th inches and 23 holes of the same one inch size.
Two resonator chambers were used in the inlet duct unit
because the annulus area at the outlet was treated as two
annular areas with each being treated as a separate duct.
Thus the chamber 170 is provided for the inner annular
area while the chamber 172 is provided for the outer
annular area. The total volume of the two chambers and
the number of holes adds up to the required volume and
holes for a single duct of the same size.
Turning now to the resonator chamber of the outlet
duct apparatus 22, this chamber 184 is located at the
wide end of the conical air flow defining member 104. It
is a flat, circular resonator chamber similar to the
above described chamber 170. The chamber 184 is
surrounded by the annular airflow passageway and evenly
distributed about its circumference are a number of holés
186. In one preferred embodiment of the outlet duct
apparatus wherein the outer diameter of the annular
passageway at the inlet 96 was 4'7", the chamber 184 had
CA 02208190 1997-06-13
- 19
an outside diameter of 21 inches and a width of 5 1/8th
inches. In this embodiment there were 20 holes, each
having a diameter of 1 1/4 inch.
The resonators 170, 172 and 184 incorporated into
the air duct apparatus of the invention provide means for
changing the acoustic impedance of the air supply system.
These resonator chambers act as additional noise control
elements. The transmission loss of a resonator installed
in an air duct having a cross sectional area S1 is given
by the following formula:
LT~ = IQ io~,~[l+~ f~0-~O~ d~
'~h~ ~ res-)n~tor r~5lit~ c~ .dim~ ion',e~s) = ~lR~j~o~
-~ = reson~o~ r~actance !dime-s~ar.l~s~) = .S,c!27rfOY
re~ of rn~in ~c~. m~-
R.s--flo-~ resis~ Ce :n re~on;l;o. I~_bcs. m~s r;~yl~
~--volu~ne of ~s~n..tor. m~
'~D - to~a!~er!ure-~re~ m;'
,fO = re~-~ni~ce ~re~iuc~ ; H~
p ~ ~ensi~ of ga~ kg!~
c = spe~d of sound m~sec
S1 here is the size of the annular open area at the
outlet or inlet in the case of an annular airflow
passageway. The total aperture area A~ is obtained by
simply multiplying the number of small holes (174 or 176)
into the chamber by the area of each hole. Thus, the
selected size and number of holes is not critical but as
a practical matter, the holes should not be too small and
it is preferred that they be at least 1/2 inch in
diameter.
The density of gas p is simply the density of the
gas or air that is flowing through the duct unit. It is
a preselected density based on the design parameters of
the system. The above-mentioned resonator chambers were
constructed to attenuate fan blade passage frequencies in
the 237 Hz range based on a fan unit with eight blades
operating at 1775 R.P.M.
-
CA 02208190 1997-06-13
- 20
Using this formula, o~e can obtain the necessary
information for calculating the details of a resonator
chamber useful in a particular air supply duct system.
These details include volume, throat diameter and
acoustic resistance. It will be appreciated that the
size and arrangement of the resonator chamber to be used
and the number of holes in the peripheral wall will vary
depending upon the frequency of the noise created by the
fan unit which is to be reduced.
In a preferred embodiment, the space between the
internal wall 102 and the external sidewall 92 of the
outlet duct apparatus 22 contains a number of partition
walls indicated at 190 which can be vertical walls
extending from top to bottom of the unit. The arrangement
and spacing of these walls can vary depending upon the
particular structural support required. The space
between these walls 190 is filled with the aforementioned
glass fibre insulation and the partitions 190 help to
support same. They also support the interior wall 102
which is made of relatively thin sheet metal.
In a preferred embodiment of outlet duct apparatus
22, the density of the sound absorbing material packed
between the interior walls and the exterior walls of the
housing is varied along the length of the air flow
passageway in order to increase sound attenuation by the
apparatus. One can obtain optimum performance in this
unit if the acoustic impedance of the silencer walls is
kept within a certain range of values. In particular,
the flow resistivity of the dissipative or sound
absorbing material should have a value given by the
following equation:
R = 6.6 (duct dimension)(design frequency)/(d) MKS rayls/m
In this equation, the letter R is the flow
resistivity, a factor that varies according to the
density of the sound absorbing material used. The letter
d is the thickness of the sound absorbing material at a
selected location along the length of the airflow
CA 02208l90 l997-06-l3
- 21
passageway. The duct ~imen~ion referred to is the width
or diameter of the airflow passageway at the selected
location and the design frequency is the frequency of the
sound which the duct apparatus is made to absorb or
attenuate. The ~;m~n~ion d is normally constrained to
yield 50% open area of the silencer. In other words, the
thickness of the sound absorbing material adjacent a
particular location along the duct should be at least 50~
of the immediately adjoining airflow passageway. In
order to obtain optimum performance, the flow resistivity
must be altered to suit the particular application and
required duct arrangement. For sound absorbing materials
commonly used in air duct silencers, the flow resistivity
is given by the following equation:
R = R(bulk density) 1'53 wherein K stands for a constant
that would depend on the particular material used.
It will be appreciated that the flow resistivity of
a given material can be increased by increasing the
packing density. The design chart shown in Figure 9 of
the drawings can be used to select the proper value of
flow resistivity. This procedure can be used to maximize
the silencer's performance at a specific frequency or to
provide a wide band of virtual constant attenuation.
In the particular preferred embodiment of an outlet
duct apparatus that is shown in Figure 6, the above
method for determining optimal flow resistivity of the
sound absorbing material was used and this procedure
resulted in the use of low density acoustical filler
having a density of 0.8 lbs per cubic foot in the
compartment 200 located closest to the inlet 96 and
extending between the end wall 94 and the first partition
wall 190. The acoustical filler in the rem~; nl ng,
smaller compartments, had a higher density of 1.2 lbs per
cubic foot. In other words, this higher density~was used
from the first of the partition walls 190 to the outlet
end of the unit. In this particular preferred embodiment
constructed by the applicant, the depth of the first
compartment containing the lower density filler was two
CA 02208190 1997-06-13
- 22
feet and the rem~; nt ng compartments had a total depth of
five feet. The width of the housing for this outlet duct
apparatus was twelve feet. The diameter of the inlet
opening of the unit was 4'7".
Figure 6 is drawn substantially to scale so that all
the ~;m~nsions of the various components and sections of
this unit can be seen from the drawing.
In this unit, as indicated earlier, the density of
the acoustic filler in the splitters is also varied. In
10 particular, in each of the splitters 112 and 114 of this
preferred embodiment, the density of the filler in the
nose area was 1.6 lbs per cubic foot while the density of
the filler in the remainder of the splitter was 1.2 lbs
per cubic foot.
It will be seen that in this particularly preferred
embodiment of the outlet duct apparatus, the density of
the sound absorbing material for the entire length of the
airflow passageway does not exceed 2 lbs per cubic foot.
This compares to conventional air flow silencers where
the density of the sound absorbing material is
substantially higher throughout the unit, typically in
the 3 lbs per cubic foot range.
A test was conducted on behalf of the applicant
wherein an 84,000 CFM (cubic feet per minute) axial flow
flan unit was connected to an inlet silencer and an
outlet silencer constructed in accordance with the
invention. In this test, heat exchanger coils and
filters were installed on the inlet unit and filters on
the outlet unit. In order to provide some load to the
fan, additional filter media was installed. Under these
operating conditions, the pressure rise across the fan
was measured to be 1.5 inch water gauge with a nominal
delivery of 84,000 CFM. Sound level readings were taken
with a calibrated B & K 2204 sound level meter connected
to an octave filter set. Sound pressure levels were
converted to sound power levels using the standard method
of area corrections. Measurement locations were selected
CA 02208190 1997-06-13
around the entire unit. A microphone was placed four
inches from the unit under test, assuring that the
conversion from sound pressure to sound power could be
performed with errors of the order of 0.5 dB. The
results are summarized in the following Table 1.
TABLE 1
Measured Sound Power M&l Heat Transfer 84,000 CFM Unit
Fan Fan Inlet Outlet
Hz (Woods data) Casing Silencer Silencer
31.5 95 90 92
63 108 92 87 91
125 110 99 87 85
250 116 103 95 90
500 114 96 97 84
1,000112 90 94 84
2,000108 85 90 80
4,000102 83 90 72
8,00096 82 85 73
It is evident from these results that the fan casing
is the dominate radiator at low frequencies and that the
inlet silencer radiates most of the high fre~uency
energy. Some of the high frequency noise is generated by
airflow through small gaps. The inlet system in the test
was not sealed because of the need to disassemble it
after the test. These gaps and the panels would of
course be sealed when the unit tested is installed at an
actual operating site. The acoustic energy passing
through the silencer suffers additional attenuation as it
travels down the air ducts. Using typical performance
data from ducting and diffusers, one can expect NC35 in
a 4,000 cubic foot room with 30 air exchanges per hour or
NC28, if there are 6 air exchanges per hour. It will be
understood that this system, as tested, is constructed
for installation in an enclosure inside a mechanical room
CA 02208190 1997-06-13
- 24
of a building. The wall construction of a typical
enclosure normally has an STC rating of 25. Thus, the
sound transmitted from the unit into the mechanical room
will result in sound level equivalent to NC60.
Figure 10 is a graph which plots sound power against
octave bands. This graph is a plot of the test results
listed in the above Table 1.
It will be understood by one skilled in this art
that the type of duct structure shown in Figure 6 with
two series of splitters can also be used to construct an
inlet duct apparatus/silencer. If such an inlet
duct/silencer is constructed, it will be understood that
the splitters are modified so that they converge from the
air inlet of the air duct unit towards the fan and the
round nose of each splitter is arranged on the upstream
side in the air flow passageway, the pointed end being at
the downstream side. A diffusing baffle member is not
required in an inlet duct silencer of this type.
It will be further appreciated by those skilled in
the art that an outlet duct silencer similar to the inlet
duct silencer of Figures 2 and 3 could be constructed if
desired, that is in this type of outlet duct silencer the
air passageways would extend through a substantial curve,
for example, 90 degrees. There can be a single
passageway curving in one direction or two air flow
passageways curving in two opposite directions. The
splitters used in this outlet duct silencer can have a
circular quadrant shape.
As illustrated in Figures 2 and 3, in a preferred
embodiment the interior wall 52 is fitted with a
projecting extension member 192 which is wedge shaped as
shown. This can be made of imperforate 16 gauge sheet
metal and, in one embodiment, it has a horizontal length
of 18 inches. This extension can be located within
adjacent coil mounting frames which are part of air
conditioning units indicated at 194 and 196 in Figure 1.
CA 02208l90 l997-06-l3
- 25
The advantages of the applicant's improved duct
inlet apparatus and duct outlet apparatus will be
apparent from the above detailed description. Both have
very good sound attenuation characteristics for both high
frequency and low frequency sounds. The splitters or
dividers in both duct apparatus 20 and 22 also provide
for a uniform or even airflow within the airflow
passageway. In case of the duct inlet apparatus 20, the
use of both vertical and horizontal splitters or dividers
helps to assure that each section of the fan inlet gets
an equal amount of air. The outlet 46 of the apparatus
20 iS divided into equal areas by solid metal dividers.
The apparatus 20 provides a shallow bell arrangement with
a large turning radius for the air flow. The apparatus
20 has advantages over the use of a deep bell
construction which could cause pressure losses, flow
separation and unequal flow distribution. In some cases,
the use of a deep bell in this situation could even cause
the fan to stall.
It will be apparent to one skilled in the
construction of air supply units and systems that various
modifications and changes could be made to the above
described air supply duct apparatus without departing
from the spirit and scope of this invention.
Accordingly, all such modifications and changes as fall
within the scope of the appended claims are intended to
be part of this invention.
