ABSORPTIVE SOUND ATTENUATOR, IN PARTICULAR FOR AIR DUCTS IN PAPER MILLS

Abstract: Absorptive sound attenuator, which is intended above all for intake and/or exhaust air ducts at paper mills. The sound attenuator comprises a box (10), in whose interior there is an inner tube (19), which is provided with through perforations (19a) and which forms the air duct (20), and, around said inner tube, material (21,22) that absorbs sound. The inner tube (19) defines the air duct (20) in its interior, the sound being absorbed in the area of said air duct (20). In view of cleaning the sound attenuator and the absorption material (21,22,26) in same and in view of facilitating the replacement of the latter, the sound attenuator is provided with a box (10) which has openable outer walls (13a, 13b, 14a, 14b, 13A,13B). When these walls are open, the absorption material (21,22,26) can be removed and/or replaced from outside the inner tube (19).

Description: Absorptive sound attenuator, in particular for air ducts in paper mills The invention concerns an absorptive sound attenuator for air-conditioning ducts, according to the precharacterising clause of claim 1 and disclosed in FR-A-85 3 854 or FR-A-2 386 767. 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 pity 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 solid materials tend to block the perforated plates in an absorptive sound attenuator, and the condensing moisture wets the absorption materials and deteriorates their properties of sound absorption. The prior-art sound attenuators in general, and the sound attenuators used in paper machine ventilation in particular, have been difficult to maintain, for their absorption material is frequently susceptible of wetting, and the absorption material is difficult to cleanse and to replace. This may have had the consequence that absorptive sound attenuators which have been designed and dimensioned efficient in themselves operate unsatisfactorily, because they are "blocked" as a result of water and/or 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...200 Hz. 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 an absorptive sound attenuator in particular suitable for air-conditioning of paper machines as can be serviced and cleansed quickly and easily and in which, when necessary, the absorption material can be replaced, but whose construction is, nevertheless, relatively simple. It is a further object of the invention to provide such an easily serviceable absorptive sound attenuator as operates under demanding conditions, such as in wet, contaminating, hot, and/or dry environments. A further object of the invention is to provide a sound attenuator that is also capable of operation when the absorption material is wet. In view of achieving the objectives states above and those that will come out later, the invention has the characterising features of claim 1. A preferred embodiment of the invention comprises a box part shaped as a rectangular prism, through which the perforated inner tube passes. The space between the box part and the inner tube is filled with pieces of absorption material. The plane side walls of the box part, which are parallel to the air flow, have been arranged as doors, through which the pieces of absorption material as well as absorption plates or membranes, if any, that may be provided in the air duct of the inner tube can be replaced from outside the box part. The absorption material can be washed and replaced after it has been removed from the interior of the box part. In some cases, the absorption material and the various perforated plates can also be washed when they are in their operating positions. The perforated wall plates of the perforated plate in the inner tube in the sound attenuator and of possible absorption plates, if any, protect the absorption material placed inside said plates. Said perforation is susceptible of being blocked, in particular so in sound attenuators at paper mills because of the fibres and fillers, but by means of the invention it is relatively easy to cleanse said perforations so that the attenuation capacity of the sound attenuator remains good. In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accompanying drawings, the invention being in no way strictly confined to the details of said embodiments. Figure 1 is a schematic and axonometric view of a sound attenuator in accordance with the invention and of the openability of its construction. Figure 2 is a more detailed side view of an embodiment of the sound attenuator in accordance with the invention partly in section. Figure 3 shows the same as Fig. 2, seen in a contrary direction (axial direction of the flow duct). Figure 4 shows a detail D4 in Fig. 2. Figure 5 is a schematic and axonometric view of a second principle of realization of the invention. Figure 6 is an axial view of a third principle of realization of the invention. To start with, mainly with reference to Fig. 1, the main principles of the construction and the operation of the absorptive sound attenuator in accordance with the invention will be described. According to Fig. 1, the absorptive sound attenuator comprises a box part 10 shaped as a rectangular prism, to whose frame construction opposite, stationary vertical walls 11 and 12 have been fixed. The other vertical walls consist of openable pairs of doors 13a,13b and 14a,14b provided with hinges 18. Through the box part, an inner tube 19 of circular section passes, which tube defines the air duct 20 and whose wall plate is provided with through perforations 19a. The inner tube 19 is fixed to the box part by the intermediate of its end walls 15a, 15b. According to Figs. 2 and 3, the end walls 15a, 15b are provided with pipe connectors 16a, 16b, which are provided with flanges 17a, 17b, by whose means the sound attenuator is fixed to the inlet or outlet air pipe (not shown) of the air system so that the inner tube 19 communicates with the air pipes without steps. In Figs. 1 to 3, outside the inner tube 19 provided with perforations 19a in the box part 10, a ring 21 of absorption material is fitted, which is made, e.g., of mineral wool of permanent shape. The absorption ring 21 may be composed of strips which can be attached to their holders. The thickness S of the ring 21 is, e. g., S = 50 mm. According to Figs. 1 to 3, the inner tube 19 in the sound attenuator is provided with two absorption plates 25, which are defined by two opposite plane wall plates 27 provided with through perforations 27a. The inner space between the plates 27 is filled with an absorbing material layer 26, such as mineral wool or foam plastic. The absorption plates 25 can be detached best in the way coming out from Fig. 1, for which purpose the absorption plates 25 are mounted by their top and bottom edges in horizontal rails 24a and by one of their vertical edges in vertical rails 24b. One end 28 of the absorption plates 25 is provided with a handle 29, by whose means the absorption plates 25 can be pulled out for cleaning or equivalent. The space in the box 10 outside the absorption ring 21 is filled with absorption pieces 22, which are arranged and dimensioned in such a way that they can be removed for cleaning and/or replacement through the opposite pairs of doors 13a, 13b and 14a, 14b in the box. According to Fig. 1, absorption pieces 22 are placed in four vertical stacks, four absorption pieces in each stack one on top of the other. The round inner sides of the absorption pieces 22 are placed in tight contact with the outer face of the absorption ring 21. The other sides of the absorption pieces 22 are plane. In Fig. 1, one of the absorption pieces 22' is shown as partly removed from the box 10. After the pieces 22 have been removed, the absorption plate 25 can also be pulled out of the box 10 if necessary, as is shown in Fig. 1. The absorption plates 25 can be removed in one direction only, i.e. towards their handle 29. After the absorption pieces 22 have been removed from the box 10 and the absorption plates 25 been detached from their horizontal guides and removed out of the box 10, the absorption ring 21 can also be removed if necessary. Hereupon the various absorption parts and the holes 27a in the walls defining them can be cleaned, e. g., by means of water jets. Likewise, the holes 19a in the inner tube 19 in the sound attenuator can be cleaned by means of water jets. After cleaning, the components of the sound attenuator can be assembled into the box 10 in the reversed sequence while replacing any defective parts when necessary. In some cases, it is also possible to cleanse a sound attenuator without removing the absorption pieces. In such a case, the perforations 19a and 27a,27b are cleansed from inside. The absorption piece 22 is most appropriately made of such a material, preferably foam plastic, as does not become permanently wet, because water flows away from it. An example of a suitable material is foam plastic, which is not compressed, does not become wet, and has favourable sound attenuation properties. The axial length L of the box part, i.e. the height in vertical installation, is usually L = 1500...3000 mm in ventilation pipes at paper mills. The diameter D of the inner tube 19 is, as a rule, in the range of D = 500...2000 mm. The diameters of the holes 19a and 27a are, as a rule, in the range of 5... 10 mm, preferably about 8 mm. The sound attenuator is preferably of the vertical design shown in Figs. 1 and 2, but horizontal or diagonal designs are also possible. Fig. 5 shows a second variation of the invention, in which, instead of the sound attenuation plates 25, there is a cylindrical central absorption core 25A, which is coaxial with the perforated 19a inner tube 19 fitted in the box part. Between the inner tube 19 and the perforated 27b mantle tube 27B of the core 25A, an air duct 20 shaped as a cylindrical ring remains. Inside the core 25A, there is a piece 26B of absorption material, such as mineral wool or foam plastic. The core 25A is fixed to the box 10 and/or to the inner tube 19 by means arms (not shown). The box 10 of the sound attenuator as shown in Fig. 5 is provided with two fixed opposite walls 11 and 12 as well as with ends 15a and 15b and walls 13A and 13B, which are provided with openable hinged doors, as is shown in Fig. 6, which doors can be opened so that the opposite absorption pieces 22 can be removed. The core 25A can be removed in the direction of its longitudinal axis. Fig. 6 shows such an embodiment of the invention in which, inside the inner tube 19, there are absorption plates 25B fitted in X-shape, said plates being interconnected at the centre line of the duct 20 by their chamfered outer ends 23. Each of the absorption plates 25B is provided with an inner part 26A of absorption material and with perforated 27a walls 27. The absorption plates 25B can be removed in the direction of the arrows A in their guides 24. Owing to this direction A of removal, the pairs of doors 13a, 13b and 14a, 14b are provided with hinges 18a. The absorption pieces 22 can be removed in the direction of the arrows B after the pairs of doors 13a, 13b and 14a, 14b have been opened completely. A sound attenuator in accordance with the invention can also be accomplished without absorption plates 25,25A,25B fitted in the inner tube 19. In some applications, in particular when sound attenuation is desired at relatively low frequencies, the absorption plates can be substituted for by corresponding membrane plates that are capable of oscillating. The dimensioning of a sound attenuator in accordance with the invention, of course, depends on the quantity of air that passes through its duct 20 and on the level and frequency spectrum of the noise to be attenuated. Figs. 1 and 2 show a somewhat asymmetric construction (L₁ > L₂), by whose means the sound attenuation is intensified and the effective frequency range of the attenuator is widened.

Claims: Absorptive sound attenuator for air-conditioning ducts, in particular for intake and/or exhaust air ducts at paper mills, said sound attenuator comprising a box (10), in which interior there is an inner tube (19) provided with through perforations (19a) or equivalent and defining the air duct (20) in its interior, and sound absorption material (21,22) disposed around said inner tube (19), whereby the sound is absorbed in the area of said air duct (20), characterized in that, for the purpose of cleaning the sound attenuator and the absorption material (21,22,26) and for the purpose of facilitating the replacement of the latter said box (10) has outer side walls (11,12,13a,13b, 14a,14b, 13A,13B) of which two opposite walls (13a,13b, 14a,14b) have been arranged as openable doors, through which the absorption material (21,22,26) can be removed and replaced from outside the inner tube (19). Sound attenuator as claimed in claim 1, characterized in that said box (10) is of rectangular shape and the inner tube (19) of circular shape in the transverse plane perpendicular to the passage of the air flow, and that the planes of the outer walls of the box part (10), including the openable outer walls (13a, 13b; 14a, 14b) are parallel to the air flow. Sound attenuator as claimed in claim 1 or 2, characterized in that the axial direction of the inner tube (19) of the sound attenuator is vertical and that the box (10) of the sound attenuator is provided with vertical plane side walls (11,12, 13a,13b, 14a,14b). Sound attenuator as claimed in any of the claims 1 to 3, characterized in that around the perforated (19a) inner tube (19) in the sound attenuator there is a ring (21) of absorption material, which is made of mineral wool or of an equivalent absorption material, and that around said absorption ring (21) there are pieces (22) of absorption material, which can be removed through the openable outer doors (13a, 13b, 14a, 14b, 13A, 13B) of the box. Sound attenuator as claimed in any of the claims 1 to 4, characterized in that the absorption material that fills the box (10) in the sound attenuator consists of pieces (22) of absorption material, whose number is at least four separate pieces, which have a cylindrical inner face and plane outer faces, said plane faces being placed against the inner faces of the outer walls of the box (10). Sound attenuator as claimed in claim 4 or 5, characterized in that said absorption pieces (22) are made of unwetting and non-compressible plastic foam. Sound attenuator as claimed in any of the claims 1 to 6, characterized in that in the interior of the inner tube (19) in the sound attenuator, a plate or plates (25,25B) of absorption material are provided, whose plane is preferably parallel to the air flow, and that said plates (25;25B) of absorption material consist of perforated (27a) plate walls (27) and of layers (26;26A) of absorption material placed inside said plate walls. Sound attenuator as claimed in claim 7, characterized in that, for the purpose of their cleaning and/or replacement, said absorption plates (25;25B) can be shifted in their guides (24,24a) through the openable outer walls of the box (10) outwards from the box (10) and, correspondingly, back into their operating positions. Sound attenuator as claimed in any of the claims 1 to 6, characterized in that, in the interior of the inner tube (19) in the sound attenuator, there is an absorption core (25A), which is provided with a perforated (27b) outer wall (27B), which is preferably coaxial with the inner tube (19) in the sound attenuator, an inner part (26B) of sound-absorption material being placed in the interior of said absorption core (25A) (Fig. 5). Sound attenuator as claimed in any of the claims 1 to 9, characterized in that the box part (10) of the sound attenuator is shaped substantially as a rectangular prism and comprises four outer walls of equal lengths (L) in the axial direction and of substantially equal widths, of which outer walls two opposite walls are openable door walls, and that the box part (10) includes plane ends (15a,15b), to which pipe connectors (16a, 16b) have been fixed, which join the inner tube without steps and which are provided with flanges (17a, 17b) for attaching the sound attenuator to the air duct pipes so that the air duct, which preferably is circular, continues through the entire sound attenuator so that its outer wall has substantially no steps.

Inventors: LEMETYINEN, MARKKU, PETTERSSON, HENRIK, SJOESTRAND, ESKO, Sjöstrand, Esko
Applicants: VALMET PAPER MACHINERY INC.
Application year: 1991
Main classification: F16L55
IPC codes: F16L55