Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DAMPENER FOR A FLUID ACCELERATOR SUCH AS A PNEUMATIC BLOWER
FIELD
The present disclosure is directed to apparatus used for movement of fluids
such as
gases, and more particularly, an apparatus to eliminate, reduce or prevent the
formation or
transmission of sonic or oscillatory waves from the apparatus to the
environment or between
parts of the apparatus.
BACKGROUND
There are numerous types of apparatus that generate streams of accelerated
fluid.
One such apparatus is a pneumatic blower. In simple terms, a pneumatic blower
takes in air
at one or more intakes (inputs) and blows air out at one or more discharges
(outputs),
typically with a high output velocity. Pneumatic blowers have hundreds of
applications. Some
of those applications entail moving gases, such as air, and nothing else.
Other applications
may involve using gases as a vehicle for moving solids, such as sand, wood
chips or grain.
Accelerating gases by apparatus such as a pneumatic blower can result in the
generation of noise. Some of the noise is attributable to the sounds made by
the accelerated
gases that are being drawn into the intake or expelled from the discharge of
the pneumatic
blower.
Noise is, by its nature, generally undesirable. In some circumstances, noise
is not
merely a nuisance, but noise becomes a hazard or a safety concern. The hazard
can be in
the form of potential damage to people's hearing, for example, or disruptive
vibrations, or
masking sounds (such as alarms or other communications) that are important to
hear.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of an assembled dampener.
FIG. 2 is an exploded view of the dampener of FIG. 1.
FIG. 3 is a cross-section of an illustrative reflective head, with attached
fins and
discharge element.
FIG. 4 is a plan view of an illustrative fin.
FIG. 5 is a perspective view of a cross-section of a dampener depicting fluid
flow and
streamlines.
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DETAILED DESCRIPTION
The present disclosure describes apparatus for eliminating, or reducing, or
preventing
the formation of, or preventing the transmission of sonic or oscillatory waves
(generally,
"noise") that may otherwise be generated by fluid-accelerating apparatus, such
as a
pneumatic blower. For purposes of the discussion that follows, such noise
reduction may be
referred to as "silencing," even if it does not eliminate all of the noise, or
"dampening."
Pneumatic blowers may be of many variant configurations, having different
shapes,
different sizes, and different apparatus to accelerate a gas. For purposes of
simplicity, it will
be assumed that the gas being accelerated is air, although the concepts
described here are
not limited to air.. The accelerated air may be air only, or it may be air
mixed with other
materials (such as solids or liquids or particulates), with the air serving as
the vehicle for
accelerating and moving the materials.
For purposes of describing the context of the concepts and various features
and
potential advantages thereof, it will be assumed that the fluid-accelerating
apparatus is a
pneumatic blower, and that the pneumatic blower is a rotary lobe roots-type
blower, useful in
applications such as mills for blowing wood chips. Such blower systems may be
medium
pressure, medium volume systems. Rotary lobe roots-type blowers tend to
discharge
accelerated air in pulses, rather than a continuous stream of air at a steady
rate, and the
pulses may generate pulsation noise. The noise can be significant enough to be
a safety
concern. The concepts described herein may be useful with such a blower
system, and may
have been subjected to experimentation with such a blower system. The concepts
are not
necessarily limited to application to such a blower system, however. The
concepts may be
applied to other kinds of blowers or fluid-accelerating apparatus, such as
high frequency
screw compressors, high pressure high volume applications like air
compressors, and low
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pressure, high volume applications such as fans. Some such systems may
generate
significant noise other than pulsation noise. The concepts may further be
applied to systems
that are not conventionally thought of as blowers, such as exhaust systems for
diesel engines
for generators or vehicles, or other systems that generate noise when
discharging an
accelerated fluid.
For simplicity, however, the concepts will be discussed in context with a
pneumatic
blower.
Pneumatic blowers generally include one or more intakes. The intakes are the
sites
where the air is drawn into the pneumatic blower. Typically, a pneumatic
blower creates
internally a pressure that is lower than the ambient air pressure, such that
ambient air at a
higher pressure tends to move through the intake into the blower with its
lower internal
pressure. The drawn air moves in rapidly and may be accelerated further by the
blower.
Pneumatic blowers also generally include one or more discharges that expel the
accelerated
air. Air moving through the intakes and the discharges can create noise.
Structures that reduce the noise may adversely affect the flow of the air. In
other
words, some noise reduction structures may reduce the speed of the airflow, or
may require
the pneumatic blower to draw more energy to draw air into an intake or expel
air from a
discharge at a desired speed, or both.
The concepts described below are directed to silencing noise at an intake or a
discharge, or both, without substantially compromising air flow speed or
significantly
increasing energy consumption. Once again, for simplicity, the concepts will
be discussed in
the context of air accelerated from a discharge.
When air is accelerated by a pneumatic blower, the accelerated air exits the
blower
through a discharge in a stream. The noise generated at the discharge is not
localized in the
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stream but radiates outward in all directions.
FIG. 1 is an isometric view of a dampener 100 illustrative of the concepts
described
herein. The dampener 100 is shown in FIG. 1 in an assembled state, and later
drawings will
illustrate some of the internal components of the dampener 100. The dampener
100 is in an
elongated shape with most of its length being substantially uniform circular
cross-sections.
The dampener 100 includes a first end 102 and second end 104. For purposes of
explanation, it will be assumed that the first end 102 is physically coupled
to a discharge of a
pneumatic blower (not shown), such that air expelled by the discharge moves
into the first
end 102 of the dampener 100. The air moves out of the dampener 100 at the
second end
104, and may thereafter move into the ambient atmosphere or into additional
apparatus (not
shown). Although the air may be discharged linearly from the dampener 100, any
other type
of discharge, such as radial discharge, is also possible.
The dampener 100 includes a first head 106 (or first head ring 106) near the
first end
102, which physically couples the dampener 100 to the discharge or manifold of
the blower.
The first head ring 106 may be physically coupled to the blower in any
fashion, including
bolted or welded. In FIG. 1, the first head ring 106 includes holes 108, that
may
accommodate bolts or other fasteners to hold the first head ring 106 to the
blower. Ordinary,
the first head ring 106 would be physically coupled to the blower in a very
secure fashion,
though not necessarily a permanent fashion. The first head ring 106 may
typically be
attached to the blower like a collar, such that virtually all of the air
exiting from the blower
discharge is directed into the dampener 100, with no leaks.
In the case of a dampener 100 at the intake of a blower, the first head ring
106 would
be physically coupled to the blower. Thus, for a dampener 100 at the intake,
air would move
into the second end 104 and out of the first end 102 and into the blower; but
for a dampener
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100 at the discharge, air would move out of the blower into the first end 102
and out of the
second end 104.
A second head 110 is near the second end 104. The second head 110 may include
structures such as holes 112 that may accommodate bolts or other fasteners to
hold the
second head 110 to another piece of equipment, such as a tube.
Near the first end 102 is a spool structure 114, which will be shown in more
detail in
FIG. 2. The components of the spool structure 114 may be welded together for
strength and
durability, and so the spool structure may be described in some embodiments as
a spool
weldment 114. The first head 106 is a part of the spool structure 114.
An outer cylinder 116 surrounds the dampener 100 and encloses internal
components
to be described below.
The rigid or most physically durable parts of the dampener 100 may be
constructed
from any number of materials or combinations of materials, including but not
limited to metals,
plastics, and ceramics. For purposes of simplicity, however, the rigid or
physically durable
parts of the dampener 100 may be constructed from steel. As will be mentioned
below, some
components of the dampener 100 are not rigid and may be made from more
flexible
materials.
FIG. 2 shows an exploded view of the dampener 100 shown in FIG. 1.
Near the first end 102 is the spool structure 114. The spool structure
includes the first
head 106 and a mid head 118, which resemble the flanges of a spool, with a
drum 120
between the flanges. Although the first head 106, mid head 118, and drum 120
may be
constructed as separate components, they are typically physically coupled to
form a a spool
structure 114 that behaves as a single unitary element. The center of the drum
120 may be
hollow. The drum 120 may have a circular cross-section, though this is not
essential. The
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walls of the drum 120 may be constructed of a mesh-like material. The mesh-
like material
may resemble a screen or a sieve, or may be embodied as a solid material with
perforations,
for example. Perforations, if used, need not be in any pattern, and may be
randomly
distributed; the perforations may be any openings in the solid material of any
number, shape,
or size. Surrounding the drum 120, and disposed between the first head 106 and
the mid
head 118 and encased by the cylinder 116, is an absorptive material 122. A
small section of
the absorptive material 122 is shown in FIG. 2; in practice, the drum 120
would ordinarily be
fully surrounded by a torus of absorptive material 122.
Absorptive material 122 is absorptive of sound. Absorptive material 122 may be
composed of packed polyester, for example. Other types of materials, such as
fiberglass
and/or ceramics'and/or other polymers and/or combinations thereof may also be
used; but
polyester has been found by experimentation to work well and has a low risk of
breaking
down. High density polyester is optimal for sound absorption and cohesion.
Cohesion of the
absorptive material is important, as any fractured fiberglass or ceramic
material, for instance,
can damage the blower, pump, fan or other impeller, and if downstream, can
contaminate a
conveyed material.
In a variation, the drum 120 may be formed in cooperation with the absorptive
material
122. The mesh-like material may be reduced or eliminated such that the walls
of the drum
120 through which air passes would be formed, perhaps in part, by the
absorptive material
122.
In a further variation, there may be no drum, but rather an absorbent material
may be
formed against the interior surface of the dampener in order to defin a
longitudinal hole
through which gasses, usually air, may pass. The interior wall of the
absorbent material itself
may form a retaining barrier.
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In operation, most air passing through the drum moves through the center of
the drum
120. The air has variations in air pressure or compression, however, which
generate or
convey sound or noise. The variations in compression enable some air to
transmit sound
through the mesh of the drum 120 into the absorptive material 122. Though air
does not flow
through the absorptive material 122, the variations in compression of the air
may be conveyed
to the absorptive material 122. The noise is less easily transmitted though
the absorptive
material 122 than it is through air, and so the absorptive material 122 acts
as a dampener to
reduce or eliminate some of the noise.
The spool structure 114 with absorptive material 122, encased by the cylinder
116,
forms a first dampening section of the dampener 100.
The center of the drum 120 may be thought of as defining an axis, along which
the air
traveling through the drum 120 generally follows. Air exiting from the drum
120 of the spool
structure 114 flows toward a second dampening structure, which will now be
described.
The air exiting from the drum 120 encounters a reflective head 124, mounted to
one or
more fins 126. The reflective head 124 is near the second end 104, that is,
nearer to the
second end 104 than to the first end 102. The fins 126 are nearer the 138 end
104 than is the
reflective head 124.
The reflective head 124 is disposed coincident with the axis of the drum 120.
The
reflective head 124, though generally not strictly planar, may define a plane
that is orthogonal
to the axis of the drum 120. For example, an outer edge of the reflective head
124 may be
thought of as defining a plane, and the axis defined by the drum 120 would be
at or very close
to a right angle where the axis intersected that plane.
The fins 126 may be planar, and the planes of the fins 126 may be parallel to
or
coincident with the axis defined by the drum 120. In other words, the planes
of the fins 126
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are generally orthogonal to the plane defined by the reflective head 124.
Optimally, the fins
126 are disposed radially about the axis of the drum 120, and are attached at
one end to the
reflective head 124 and at the other end to the second end 104.
In the embodiment shown in FIG. 2, six fins 126 are used, although not all of
the six
fins 126 can be seen. The fins 126 are arrayed with approximately sixty
degrees of
separation between adjacent fins. The reflective head 124 and the fins 126 are
physically
coupled to one another in a fixed arrangement. The reflective head 124 and the
fins 126 may
be formed from a single piece of material, or may be joined or physically
coupled together by
a fastening technique such as welding.
Surrounding the reflective head 124 and fins 126 is a liner 128. Neither the
reflective
head 124 nor the fins 126 are physically coupled to the liner 128, except
through intermediate
elements as will be discussed below. As shown in FIG. 2, the liner 128 is
almost a cylinder,
but for a gap 130. The liner 128 has a diameter smaller than the diameter of
the cylinder 116.
The liner 128 fits inside the cylinder 116 and is physically coupled to the
cylinder 116; the
physical coupling of the liner 128 to the cylinder 116, however, need not be
rigid in all places.
In one embodiment, the liner 128 is welded to the cylinder 116 in a select
number of
attachment sites. FIG. 2 shows an illustrative welding spot attachment site
132 and shows
five similar attachment sites; the concept is not limited to that number of
attachment sites, nor
the disposition of the sites.
The less-than-fully-rigid attachment of the liner 128 to the cylinder 116, and
the
geometrical features of the liner 128 such as the gap 130, give some limited
freedom of the
liner 128 to move relative to the cylinder 116. This limited freedom of
movement may allow
the liner 128 to act as a dampening element. It has been discovered that a
less-than-fully-
rigid attachment of the liner 128 to the cylinder 116 is better at absorbing
sound than a more
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rigid attachment.
The air exiting from the drum 120 cannot pass through the reflective head 124,
and
must flow around the reflective head 124. Accordingly, the air stream exiting
from the drum
120 may change from a column of air to a tube of air (or to several columns of
air) as the air
flows around the reflective head 124. The cross-sectional circular area of the
center of the
drum 120 may be (but need not be) comparable to the cross-sectional area of
the ring-shaped
area around the reflective head 124. As a result, the air may change from a
columnar flow to
a tubular flow without major changes in speed or pressure, and laminar flow
may be largely
maintained, with slight loss. In another variant, the air may change from a
single columnar
flow to multiple columnar flows by fins 126.
Due to the natural viscosity of air, a boundary layer of air may be expected
to form next
to the reflective head 124. The boundary layer does not move (or does not move
very much)
with respect to the reflective head 124. In other words, the boundary layer
does not form part
of the airstream around the reflective head 124.
Experimentation has shown that noise resulting from the moving airstream does
not
necessarily follow the airstream. It has been discovered through
experimentation that, as the
airstream flows around the reflective head 124, some of the noise in the
airstream, in the form
of variations in air pressure and compression, is conveyed in a more-or-less
straight line
thorough the boundary layer and into the reflective head 124. The reflective
head 124 reflects
this noise back toward the spool structure 114.
FIG. 3 shows a cross-section of an illustrative reflective head 124. In FIGS.
2 and 3,
the reflective head 124 is depicted as a concave structure (that is, concave
in relation to, or
viewed from the point of view of, the first end 102) such as a spherical
reflector. The
reflective head 124 may be shaped to be any kind of reflector, such as a
concave elliptical
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reflector, a concave parabolic reflector, a corner reflector, a concave
conical reflector, a
concave frustro-conical reflector, or a combination of reflectors, or any
other shape of
(typically non-planar) reflector. Planar reflectors and convex reflectors have
been found to be
typically less effective than other shapes selected to reflect sound back in
the general
direction it came from.
Reflected noise may, to some extent, interfere constructively or destructively
with non-
reflected noise. The geometry of the dampener 100, such as the distance
between the mid
head 118 and the reflective head 124, may be selected to improve noise
cancellation by
destructive interference, though this is not essential to the concept.
Basically, noise is
reflected back to the spool structure 114, where the noise may be absorbed by
the absorptive
material 122. In other words, noise that did not get absorbed by the
absorptive material 122
on the first pass may reflected back for a second chance at absorption.
It has been discovered by experimentation that reflection of noise does not
result in
appreciable resistance to the airstream. It has further been discovered that
the change from
a columnar flow of air to a tubular flow (or to multiple columnar flows) adds
fairly little
resistance, so that the amount of additional energy expended to push the air
through the
dampener 100 is manageable and not a serious impediment. It has further been
discovered
through experimentation that the noise reduction achieved as described above
can also be
achieved if the air is moving in the opposite direction (as it may be when a
dampener 100 is
disposed at an intake for a pneumatic blower).
Further noise dampening may occur before the air exits the dampener 100. After
flowing in a column from the drum 120, and flowing around the reflective head
124, the air
may be divided into columns once again. The fins 126 may break the airstream
into multiple
columnar flows.
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FIG. 4 is a plan view of an illustrative fin 126. The fin 126 is perforated.
Two kinds of
perforations are illustrated in FIG. 4. The fin 126 may have one or more holes
134. In FIG. 4,
there are three such holes 134 and they are circular. A molecule one side of a
fin 126 could
pass through a hole 134 and, be on the other side of the same fin. Another
kind of perforation
is a core cutout 136, which enables a molecule proximate to any fin to pass
through and be
proximate to any other fin. These perforations are merely for illustration.
The concept is not
restricted to any number or shape or arrangement of perforations.
Perforations, if used, need
not be in any pattern, and may be randomly distributed. The perforated fins
126 also may
assist in noise dampening.
As shown in FIG. 3, the fins 126 are physically coupled to a discharge element
138.
Illustrative attachment sites 140, where fins 126 are physically coupled to
the discharge
element 138, are shown in FIG. 3. The fins 126 may be physically coupled to a
binding ring
142, shown in FIG. 2. Attachment may be by welding, for example. The binding
ring 142
may include one or more perforations 144, which may have any of several sizes
and shapes
and distributions. The second head 110 may be a fixed component of the
discharge element
138, as shown in FIG. 2. The discharge element 138 may also include a flange
146 that may
be physically coupled to the cylinder 116. By physical connections such as
these, the
reflective head 124, which is physically connected to the fins 126, may be
held in a fixed
place within the dampener 100.
FIG. 5 is a perspective view of a cross-section of a dampener 100. FIG. 5
shows
illustrative streamlines 148, which are paths traced out by representative
fluid elements (such
as "particles" of air). The streamlines 148 through the dampener 100 are
roughly the same
regardless of which direction the air is moving.
As shown. in FIG. 5, about two-thirds of the length of the dampener 100
includes the
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spool structure 114 with the absorptive material 122, and about one-quarter of
the length of
the dampener 100 includes the reflective head 124, fins 126, and discharge
element 138.
These proportions are for purposes of illustration, and other proportions are
also possible. As
shown in FIG. 5, there is a gap or displacement 150 between the end of the
spool structure
114 and the reflective head 124. The gap 150 may be selected to obtain a
desired balance of
fluid flow and noise dampening. Disposing the reflective head 124 closer to
the spool
structure 144 may result in more noise reflection and thus more noise
reduction. Disposing
the reflective head 124 closer to the spool structure 144 may also result,
however, in more
constrictions in airflow and more energy needed to move air through the
dampener 100.
The disclosed apparatus may realize one or more potential advantages, many of
which
were discovered or verified by experimentation and some of which have been
mentioned
already.
The dampener 100 has been found by experimentation to significantly reduce the
noise
generated by some very noisy pieces of equipment. Further, the noise reduction
may
typically come at a cost (it generally takes more energy to move a fluid
through a dampener
than not to do so), but the cost is low in comparison to other known dampening
techniques.
Experimentation has indicated that the concepts described above are more
efficient in
dampening noise with less air resistance. Tests suggest that, in comparison to
other
dampeners, the apparatus disclosed here can cut the noise by half or more,
while maintaining
a comparable fluid flow.
The dampener 100 can be adapted to a number of different types of equipment by
adjustment of its dimensions. The dampener 100 may be sturdy and need not
include any
mechanical moving parts, making it durable.
The subject silencer or dampener 100 has been shown through acoustic modeling
and
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the construction of prototypes and test models to significantly reduce noise
without adversely
affecting air flow. The net effect of the subject silencer is to drastically
reduce the operating
noise of silenced machinery, while not adversely affecting airflow, therefore
allowing
machinery to run at lower HP requirements than with existing silencers, which
substantially
affect the airflow in order to reduce noise levels. This results in
significantly lower power use
in plants, mills or factories using the subject silencer.
The embodiments described above are intended to be examples only. Alterations,
modifications and variations can be effected to the particular embodiments
without departing
from the scope of the concept, which is defined by the claims appended hereto.
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