Sound attenuator for low frequencies, in particular for air ducts in paper mills

Abstract: The invention concerns a sound attenuator for low frequencies for air-condiboning ducts, in particular for intake-air and/or exhaust-air ducts in paper mills. The sound attenuator comprises a frame (20), whose flow duct substantially coincides with the air-conditioning duct and in which frame (20) there is at least one plane attenuator member (30) parallel to the flow direction and consisting of one or several attenuator units operating by the Helmholtz and/or plate resonator principle. The attenuator member or members (30) are fitted in the frame (20) singly or as a group of several attenuator members to be replaced for cleaning and/or servicing of the sound attenuator and/or of the attenuator members.

Description: The invention concerns a sound attenuator for low frequencies for air-conditioning ducts, in particular for intake-air and/or exhaust-air ducts in paper mills, comprising a frame, whose flow duct substantially coincides with the air-conditioning duct and in which frame there is at least one plane attenuator member parallel to the flow direction and consisting of one or several attenuator units operating by the Helmholtz and/or plate resonator principle. Ever stricter requirements are imposed on suppression of noise in the environment. One important source of noise consists of the intake and exhaust air pipes for ventilation in connection with various industrial plants and other large buildings, through which pipes especially the noise of blowers is spread into the environment. The blowers are usually chosen on the basis of the quantity of air produced by them, and attention is frequently not paid to the noise produced by them. The noise produced by the blowers has quite a wide spectrum, which also imposes particular requirements on the noise suppression. In view of suppression of noise, paper mills are particularly demanding, because the ventilation of the paper machine hall and in particular the elimination of moisture from the drying section of the paper machine require large quantities of air. It is a particular problem in the air-conditioning of a paper machine, e.g. in respect of the exhaust air from the wire part, that the exhaust air contains a large amount of moisture and a certain extent of fibres and various paper fillers, such as kaolin clay. Said materials tend to block the sound attenuators. The prior-art sound attenuators in general, and the sound attenuators used in paper machine ventilation in particular, have been difficult to maintain, for they are often difficult to cleanse, and the replacement of their attenuator members is often difficult. This may have had the consequence that sound attenuators which have been designed and dimensioned efficient in themselves operate unsatisfactorily, because they are "blocked" as a result of impurities. Since the noise produced by blowers has quite a wide spectrum, in the intake and exhaust air ducts connected to the blowers it is frequently necessary to use both absorptive and reactive sound attenuators. Absorptive sound attenuators operate primarily at higher frequencies; the maximum of their attenuation is at a frequency of about 1000 Hz, whereas reactive sound attenuators, which are mainly based on various resonator constructions, operate most efficiently at low frequencies, and their maximum attenuation is, as a rule, tuned in a range of about 100 to 200 Hz. For sound attenuation at low frequencies, there are various principles, whose applications have been used and are used in sound attenuators, as is well known. One well-known application in sound attenuation at low frequencies is the plate resonator, i.e. membrane attenuator, wherein the oscillation of the plate converts the sound energy to thermal energy. In a plate resonator, in which the plate or membrane that operates as the mass is placed at a certain distance from a rigid wall and closes the air space, which operates as a spring, behind it. In a plate resonator, the maximal attenuation occurs at a resonance frequency which depends on the mass of the plate and on the distance between the rigid wall and the plate that operates as the mass. A second well-known solution for sound attenuation at low frequencies is the Helmholtz resonator, in which the resonator consists of an air space which communicates with the "outer air" through an opening. An air plug present in the opening forms the mass that resonates on support of the spring force formed by the air enclosed in the hollow space. The resonance frequency of a Helmholtz resonator depends on the area of the opening, on the volume of the air space, and on the length of the air plug formed in the opening. In a Helmholtz resonator, the frequency range and the extent of attenuation can be regulated by changing the dimensions of the chamber that operates as the air space and the size of the opening. When the volume of the air space becomes larger, the resonance frequency is shifted towards lower frequencies, and when the area of the opening is made smaller, the resonance frequency is shifted towards lower frequencies. In respect of the prior art closely related to the present invention, reference is made to the US Patent 4,787,473, in which a solution is suggested for attenuator members in which there are Helmholtz resonators at both sides. In the version described in said US patent, attention has, however, not been paid to a possibility of cleansing or to ease of maintenance of the attenuator units, nor to designing the attenuator so that it is suitable for different purposes of use. One important property of sound attenuators connected to the input or output ducts of blowers is, besides the attenuation of noise, the loss of pressure produced by them. The object of the present invention is to provide novel solutions for the problems discussed above. It is a particular object of the invention to provide such a sound attenuator for low frequencies as is in particular suitable for air-conditioning of paper machines and as can be serviced and cleansed quickly and in which, when necessary, the attenuator members can be replaced, but whose construction is, nevertheless, relatively simple. It is a further object of the invention to provide such an easily serviceable sound attenuator for low frequencies as operates under demanding conditions, such as in wet, contaminating, hot, and/or dry environments. In view of achieving the objectives stated above, the sound attenuator for low frequencies in accordance with the invention is characterized in that the attenuator member or members are fitted in the frame singly or as a group of several attenuator members to be replaced for cleaning and/or servicing of the sound attenuator and/or of the attenuator members. The sound attenuator in accordance with the invention is highly efficient at low frequencies, and the pressure losses occurring in same are very low, approximately equal to those occurring in a duct portion of the same length. When a sound attenuator in accordance with the invention is employed, the low frequencies can be attenuated efficiently, in which case the overall length of the attenuators can also be made considerably shorter, in particular so if a sound attenuator in accordance with the invention is used in combination with an absorptive attenuator, in a suitable ratio depending on the particular situation. Further, the cleansing of a sound attenuator in accordance with the invention is easy, and it does not require any other maintenance operations. When a solution in accordance with the invention is employed, the attenuators can be made as circular and rectangular attenuator members of standard sizes. In the following, the invention will be described in more detail with reference to the figures in the accompanying drawing, wherein Figure 1 is a schematic, axonometric view of a circular sound attenuator for low frequencies in accordance with the invention, which can be opened and cleansed on the site, Figure 2 is a schematic, axonometric view of an angular sound attenuator for low frequencies in accordance with the invention, which can be opened and cleansed on the site, Figure 3 is a schematic, axonometric view of a sound attenuator for low frequencies in accordance with the invention when applied to an absorptive attenuator, Figure 4 is a schematic sectional view of a sound attenuator for low frequencies in accordance with the invention when applied to an absorptive attenuator, Figures 5A to 5C are schematic illustrations of a fixed sound attenuator for low frequencies in accordance with the invention wherein a Helmholtz sound attenuator is applied, Figure 6 is a schematic illustration of an exemplifying embodiment of a sound attenuator for low frequencies in accordance with the invention which can be suspended on a wall, e.g., in a room for motor cooling fans, Figures 7A to 7D are schematic illustrations of an exemplifying embodiment of a sound attenuator for low frequencies in accordance with the invention which can be cleansed after the attenuator has been removed from the system of ducts. The embodiment of the invention shown in Fig. 1 comprises a sound attenuator for low frequencies which can be opened and cleansed on the site and which has low pressure losses. The frame 20 forms a flow duct, which substantially coincides with the air-conditioning duct when the sound attenuator is placed in its position in the air-conditioning duct. The direction of air flow is denoted with the arrow A, and the plane attenuator member 30 placed inside the frame 20 is parallel to the flow A. The sectional shape of the frame 20 of the sound attenuator is circular in the direction perpendicular to the flow direction A, and the frame 20 is tubular. As the material of the frame 20, it is possible to use, e.g., aluminium, steel, etc. Standard flanges (not shown) can be attached to the frame 20. Attenuator members 30 are placed in the frame 20, which members may operate either by the Helmholtz principle or by the membrane principle and be either one-sided or two-sided. The attenuator members 30 are placed in the frame 20 so that their plane faces are substantially parallel to the direction A of the air flow. In the embodiment shown in Fig. 1, Helmholtz attenuators are used. One to three members can be placed in the frame 20, depending on the size of the system of ducts. In the exemplifying embodiment shown in the figure, two members are used. For the material of the members 30, the same materials are suitable as for the material of the frame 20. An attenuator member 30 comprises a number of attenuator units 33, which consist of a closed volume 31 and of an opening 32. The size of the opening 32 and the chamber size of the closed volume 31 are determined by the attenuator size and by the frequency to be attenuated. The attenuator members 30 are supported on the frame 20 by their top and bottom edges by means of horizontal rails 25. The attenuator members 30 can be removed from the frame 20 along the rails 25. To facilitate the pulling-out, the attenuator members 30 are provided with handles 29. The removable attenuator members 30 permit washing of the attenuator members 30 without necessity to remove the attenuator itself from the system of ducts. The members 30 can be replaced quickly by equivalent spare parts, and the contaminated attenuator members 30 can be washed later at a suitable time. If plate resonator attenuators are used in the attenuator member 30, as the membrane material, it is possible to use, e.g., a thin metal sheet, plastic, teflon, or an equivalent material. No opening 32 is made into the membrane material. The membrane material is fixed to the face of the attenuator member, e.g., by means of thin rails and screws (not shown) or by welding. The membrane in the attenuator member may also be made of thin metal sheet which is provided with openings 32 and onto whose face a thin plastic film or any other synthetic film is glued as protection. The exemplifying embodiment of the invention shown in Fig. 2 comprises an attenuator whose frame 40 is shaped as a rectangular prism. The frame 40 is placed in an air-conditioning duct so that its flow duct substantially coincides with the air-conditioning duct. The frame 40 has a rectangular section in the direction perpendicular to the flow direction A. The attenuator comprises two attenuator members 30 composed of Helmholtz or plate attenuators 33. At its outer edges, the attenuator may also comprise two attenuator members 50 operating by the Helmholtz or plate resonator principle. The attenuator members 30,50 are plane, and their plane faces are substantially parallel to the air flow direction A. The attenuator members 30,50 are supported on horizontal rails 25 provided on the frame 40 by their top and bottom edges, along which rails the attenuator members 30,50 can be pulled by the handles 29 out of the frame 40. In the exemplifying embodiment shown in the figure, the attenuator members 30 placed in the middle are two-sided, and the attenuator members 50 placed at the outer sides are one-sided. The attenuator members placed at the outer sides may also be equal to those in the middle, or they may also be omitted. Attenuator members can also be constructed inside the air intake openings of the system of ventilation ducts. The size and shape of the frame 20;40 are adapted to the size and shape of the system of ducts. If necessary, some of the attenuator members in an attenuator can also be constructed so that they operate by the principle of absorptive attenuation, i.e. so that they comprise, e.g., a perforated plate in whose interior there is foam plastic or wool. In the exemplifying embodiment shown in Fig. 3, the sound attenuator comprises a box part 60 shaped as a rectangular prism, to whose frame construction opposite fixed vertical walls 61 and 62 have been attached. The other vertical walls consist of openable door pairs 63a,63b and 64a,64b provided with hinges 68. Through the box part 60, an inner tube 69 of circular section passes, which restricts the air duct and whose wall plate is provided with through perforations 69a and forms the frame 70. The inner tube 69 is fixed by means of its end walls 65a,65b. Around the perforated 69a inner tube 69 in the box part 60, a ring 71 of absorption material is fitted, which is, for example, of mineral wool or foam plastic of permanent shape. In the frame 70 of the sound attenuator, there are two attenuator members 30 and 80. The attenuator members 30,80 are plane and parallel to the flow A. The attenuator members 30 and 80 operate either by the Helmholtz principle or by the plate resonator principle. Also, the attenuator member 30 may operate by the Helmholtz or plate resonator principle and the attenuator member 80 by the absorption principle. By means of this combination, both high-frequency and low-frequency sound oscillations can be attenuated by means of the same attenuator. The attenuator members 30,80 are supported on horizontal rails 25 fitted in the frame 70 by their top and bottom edges, along which rails 25 the attenuator members can be pulled out of the frame 20 by the handles 29 either completely or partly. The space inside the box construction 60 that is placed outside the absorption ring 71 can be filled, e.g., with absorption pieces 72, which are arranged and dimensioned in such a way that they can be removed for cleansing and/or replacement through the opposite door pairs 63a,63b and 64a,64b fitted on the box 60. Fig. 4 shows an exemplifying embodiment in which Helmholtz/membrane attenuator members 30;50 are employed in an absorptive attenuator. The ring that is made of an absorption material and placed inside the frame 70 is denoted with the reference numeral 71. The absorption material may be wool, foam plastic, or some other, equivalent material. This absorption material is not replaceable and, being a construction of lower cost, is suitable for clean applications, for example as a sound attenuator for exhaust-air ducts in a machine hall. The thickness of the absorption material is, e.g., 100 to 200 mm. The attenuator also includes plane attenuator members 30;50, which are parallel to the flow direction, which can be pulled out, and which operate either by the Helmholtz principle or by the membrane principle, depending on the requirements of the particular application. The number of members used in a pipe is 1 to 3, depending on the diameter of the pipe in the system of ducts. Figs. 5A to 5C show an attenuator that has a fixed construction and operates by the Helmholtz principle and whose frame 30 consists of a material layer 91 of solid plate placed on the outside face and of a material layer 92 provided with openings 32 and placed on the inside face. As the material of the frame 90 of the attenuator, aluminium, aluminium zinc, steel, or stainless steel can be used. The thickness of the inner layer 92 may be, e.g., 1 mm, and the thickness of the outer layer 91, e.g., 2 to 3 mm. The attenuator is composed of attenuator units 33, in which the outer wall of the closed volume 31 consists of the outer wall 91 of the frame 90 and, in a corresponding way, the inner wall consists of the inner wall 92 of the frame 90, said inner wall 92 and outer wall 91 being placed at a distance from one another. The horizontal walls 93 of the frame 90, which are placed at a distance from one another and extend from the inner wall 92 to the outer wall 91, form the bottom and top walls of the closed volume 31, and the vertical walls 94 formed in the frame 90, which are placed at a distance from one another and extend from the inner wall 92 to the outer walls 91, form the side walls of the closed volume 31. Thus, each attenuator unit 33 comprises a closed volume 31, into which an opening 32 passes which has been formed into the inner wall 92 of the frame 90. In the solution of attenuator shown in Fig. 6, the attenuator is constructed in a framework 100, which consists of horizontal walls 101 and vertical walls 102, which form a compartment 103, into which an attenuator member unit, for example a unit 33 operating by the Helmholtz principle, is fitted. Into the compartment 103, it is also possible to fit a plate resonator, i.e. an attenuator unit operating by the principle of membrane attenuation. The attenuator units 33 can be attached to the framework 100, e.g., by means of rapid connectors (not shown in the figure), in which case the attenuators can be opened quickly, for example, for cleansing and/or replacement. The embodiment shown in the figure is well suitable for use, e.g., as suspended on a wall, for attenuation of low frequencies, e.g. in motor cooling rooms, on a heat recovery plane, and at apparatuses placed on the ceiling. In the exemplifying embodiment shown in Figs. 7A to 7D, plane attenuator members 30;50 parallel to the flow direction A are placed in a circular frame 110 in octagonal shape, and the attenuator members 30;50 can be removed vertically. Such removal can, however, not be carried out on the site, but the whole attenuator must first be removed from the system of ducts. The attenuator members 30;50 are placed in the frame 110, e.g., in octagonal shape. As is shown in Fig. 7D, which is an enlarged detail from Fig. 7C, the triangular space 87 that remains between the attenuator members 30;50 can be filled, e.g., with foam plastic. Fig. 7B shows an attenuator member 30, which operates, in the exemplifying embodiment shown in the figure, by the Helmholtz principle, i.e. the attenuator units 33 are provided with an opening 32 and with a closed volume 31. The attenuator member may also be an attenuator member 50 operating by the plate resonator principle, in which case it has no holes and it comprises a membrane/plate and an air space. The sound attenuators in accordance with the invention are suitable for use in different applications of sound attenuation. In particular, all sound attenuators with circular shape are suitable for use in exhaust-air duct systems together with circular absorptive attenuators. In such a case, the overall length of the attenuators can be made shorter. Suitable materials for the frame of a sound attenuator include metallic materials, such as steel, or different plastic materials, e.g. PTFE ("teflon"), PP (polypropylene), or some other, equivalent plastic material. In plate resonator attenuators, suitable materials for the membrane are, e.g., the above steel materials (thickness, e.g., 0.2 mm), aluminium, fluorine plastic PTFE ("teflon"), polypropylene PP, or some other plastic material. Further, as the membrane, it is possible to use a thin perforated metal plate which is coated with an adhesive plastic film or some other synthetic film. In Helmholtz attenuators, to prevent access of contaminations into the chamber, it is possible to use a thin "superlon" plastic layer on the faces of the attenuator members. A thin plastic material, such as polypropylene or teflon, can also be glued onto the face of the plate. In such a case, the attenuator operates by the membrane principle. The plastic material must be thin but, yet, thick enough to tolerate washing. In tests carried out with an attenuator in accordance with the invention, attenuations of up to 30 decibels have been achieved at low frequencies. In said test, a membrane-based attenuator was concerned in which the membrane was a 0.3 mm thick teflon film with rib fastening.

Claims: Sound attenuator for low frequencies for air-conditioning ducts, in particular for intake-air and/or exhaust-air ducts in paper mills, comprising a frame (20,40,70), whose flow duct substantially coincides with the air-conditioning duct and in which frame (20,40,70,110) there is at least one plane attenuator member (30,50) parallel to the flow direction and consisting of one or several attenuator units operating by the Helmholtz and/or plate resonator principle, characterized in that the attenuator member or members (30,50) are fitted in the frame (20,40,70,110) singly or as a group of several attenuator members to be replaced for cleaning and/or servicing of the sound attenuator and/or of the attenuator members. Sound attenuator as claimed in claim 1, characterized in that the attenuator members (30,50) are fitted in the frame (20,40,70,110) so that their plane faces are substantially parallel to the direction (A) of the air flow in the air duct. Sound attenuator as claimed in claim 1 or 2, characterized in that the frame (20,40,70) is fitted with horizontal rails (25), on whose support the attenuator members (30,50) are supported by their top and bottom edges and along which the attenuator members (30,50) are fitted to be removed from the frame (20,40,70) either completely or at least partly. Sound attenuator as claimed in any of the claims 1 to 3, characterized in that the frame (20,70,110) of the sound attenuator is of circular section in the direction perpendicular to the flow direction (A). Sound attenuator as claimed in any of the claims 1 to 3, characterized in that the frame (40) of the sound attenuator is of substantially rectangular section in the direction perpendicular to the flow direction (A). Sound attenuator as claimed in claim 1 or 2, characterized in that the sound attenuator members (30,50) are fitted in the frame (110) in the shape of a polygon so that the principal shape of the polygon consisting of the attenuator members coincides with the sectional shape of the frame (110) in the direction perpendicular to the flow direction (A). Sound attenuator as claimed in any of the preceding claims, characterized in that at least one attenuator member (30,50) is an attenuator member (80) based on absorptive sound attenuation. Sound attenuator as claimed in any of the preceding claims, comprising a framework (100), into which compartments (103) have been formed by means of vertical (102) and horizontal (101) walls, one attenuator unit (33) being fitted into each of said compartments (103), characterized in that the attenuator units (33) are attached to the framework (100) by means of rapid connectors to permit quicker replacement of the attenuator units (33).

Inventors: PETTERSSON HENRIK, PETTERSSON, HENRIK
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1992
Main classification: F16L55
IPC codes: F24F13, F16L55