Compressor equipped with an acoustic outlet piece

Abstract: Outlet piece for a compressor, which outlet piece contains a damping body (3) applied symmetrically around a passage (2), characterized in that this passage has a constant diameter and in that the damping body (3) has a perimeter which gradually increases according to the direction of flow (V), so that this damping body (3) has a thickness which increases according to the direction of flow (V).

Description: The present invention relates to a compressor equipped with an acoustic outlet piece connected to the compressor outlet, which outlet piece contains a damping body applied symmetrically around a passage. It is known that in the outlet volume of compressors, and in particular of compressors of the displacement type, including screw-type compressors, considerable compression pulses occur. For various reasons, it is advisable for these compression pulses to be damped. A first reason is that the vibrations in internal components, such as temperature and pressure sensors, and in the conduits of the cooler which is usually used, must be restricted so as to prevent them from being damaged. A second reason is that undamped compression pulses cause impermissible sound emissions, both of the compressor itself and of the conduits and appliances which are connected to the outlet of the compressor. Until now, these compression pulses were damped by means of an acoustic outlet piece which is carried out as a quarter wavelength resonator. These known outlet pieces of the resonator type are dimensioned such that they reflect sound waves in the direction of the source, in such a way that damping occurs. As the outlet sound of a screw-type compressor is characterized by powerful, pure tones, a very good deadening of sound can thus be obtained by exactly adjusting the geometry of such an outlet piece to the quarter wave length of the above-mentioned pure tones. Outlet pieces of the resonator type mainly deaden sound which travels as longitudinal waves. In order to obtain an effective deadening, the dimensions of the outlet piece must be precisely adjusted to the compressor. Hence, every different application requires an outlet piece with different dimensions. Outlet pieces of the resonator type exclusively provide for a deadening of sound in the shape of longitudinal, flat waves. Moreover, with a correctly adjusted outlet piece of the resonator type, an accumulation of pulse energy may arise, which results in that structural adjustments have to be made to the compressor and/or the resonator for certain applications so as to exclude strong vibrations which could be generated due to this accumulated pulse energy. For a smooth service, the outlet pieces of the resonator type must be provided with sudden changes in the diameter, so that relatively large housings are often required. Mechanical reinforcements of these housings may be necessary in order to prevent a too high sound emission of these housings. The invention aims a compressor equipped with an outlet piece which does not show the above-mentioned disadvantages. In particular, it aims a screw-type compressor equipped with an outlet piece, which does not exclude other types of compressors, however. Also, it aims a compressor equipped with an outlet piece which allows for a deadening in a very large frequency range, in particular in the entire frequency range in which the compression pulses of a screw-type compressor normally occur, i.e. the range between 250 and 6,000 Hz. Moreover, the invention aims a compressor equipped with an outlet piece in which such materials are preferably used that the outlet piece can easily resist the high temperatures which may arise on the outlet of a screw-type compressor, which as is known may rise to 250°C. Naturally, the outlet piece should be able to resist the pressure felt, which as is known may rise to 10.5 bar in a screw-type compressor. To this end, the invention consists of a compressor equipped with an outlet piece consisting of a damping body applied symmetrically around a passage, whereby this passage has a constant diameter and this damping body has a perimeter (circular or polygonal) which gradually increases according to the direction of flow and thus has a thickness which increases according to the direction of flow. Thanks to the gradually increasing perimeter, it is excluded that standing waves are generated between the outlet of the compressor and the inlet of the outlet piece. EP-A-0.037.559 describes a compressor equipped with dampening means consisting in damping synthetic foam material surrounding the air filter which mounted in the air inlet of the compressor and is built-in in the compressor body, and in synthetic foam material covering a portion of the outer surface of the housing. US-A-3.602.333 describes a sound absorber to be mounted on a conduit for a fluid under pressure, there where said fluid is drawn in or released. This sound absorber contains a damping body which is erected around a conical passage which widens in the direction of flow. Said damping body is a metal plate in the shape of a conical surface which is covered with a layer of damping material of invariable thickness on the side of the passage. In the passage can also be erected a second damping body which consists of a metal plate in the shape of the surface of a truncate cone, which plate is covered on both sides with damping material. This known sound absorber is designed for damping turbulent flows and provides for damping material wherever there is turbulence. This sound absorber is not suitable for a closed circuit under pressure, such as in the case of a compressor, and it is not optimized or designed to sufficiently suppress the reflection of waves to the inlet of the damper, which is necessary for the strong tonal frequency components of a compressor. BE-A-361.081 describes various embodiments of sound absorbers which are designed for combustion engines, however. They contain a damping body which is provided symmetrically around a passage, but none of these embodiments has a passage with an invariable diameter as well as a damping body whose perimeter and thickness increase in the direction of flow. None of these embodiments excludes sound reflections on the inlet of the damper and none of these embodiments is suitable to be mounted in a closed circuit with a compressor. Preferably, the perimeter gradually increases over the entire length of the pulse damper according to the invention, in other words as of the inlet of the passage up to the outlet. The perimeter of the inlet is preferably equal to or almost equal to the perimeter of the passage. If required, the gradually widening part can be followed by a part with an equal perimeter so as to further increase the damping. The damping body is hereby preferably conically shaped and has a circular cross-section for every cross-section. The passage is preferably rectilinear and has an invariable diameter over the entire length, so that the compressed gas flowing through it experiences a minimum of flow resistance. The outside of this damping body preferably consists of a plate-shaped jacket. The space of the outlet piece surrounding the passage is preferably mainly filled with a filling of sound-absorbing material. This filling is made of a porous material, for which use is made according to one of the preferred embodiments of a fibrous mass or fibrous material. The above-mentioned passage can be surrounded by a preferably round, tubular wall of porous and sound-transparent material. This tubular wall makes sure that the material of the above-mentioned filling does not end up in the passage and also makes sure that the passage is limited by a smooth or rather smooth wall, so that no turbulence can arise in the flow of compressed gas coming through it. Around the above-mentioned tube wall can also be provided an air-permeable protective layer, for example in the shape of a thimble, so as to prevent that fine material particles, fibres and such of the filling go through the porous tube wall and end up in the flow of air. According to a variant, use is made of a filling made of a material with a solid, porous structure, such as a ceramic foam or a highly porous, sintered powder of stainless steel. This variant offers the advantage that no particles, such as for example fibres, can come loose from the filling. Also, from a constructional point of view, this offers the advantage that the-above-mentioned separate tube wall and protective layer can be omitted, as they are no longer necessary. The filling may in this case be formed of a homogenous structure which extends from the passage to the outer wall of the outlet piece. In order to better explain the characteristics of the invention, the following preferred embodiments are described as an example only without being limitative in any way, with reference to the accompanying drawings, in which: figure 1 shows an outlet piece of a compressor according to the invention as a section; figure 2 shows a variant according to figure 1. The outlet piece 1 according to the invention mainly consists of a damping body 3 provided around a passage 2. The passage 2 is hereby connected with its inlet 4 to the outlet 5 of a compressor, and with its outlet 6 to an inlet 7 of an after-cooler which is part of the compressor, or to the inlet of a consumer device. According to the invention, the damping body 3 has a diameter which gradually increases according to the direction of flow V and this body thus has a thickness which gradually increases according to said direction of flow V, preferably as of the inlet 4 to the outlet 6. The damping body 3 is hereby tapered or cone-shaped. Preferably, near the inlet 4, the diameter of this damping body 3 is equal or almost equal to the diameter of the passage 2. The conicity is preferably characterized by an angle A of 10 to 25°. The passage 2 is rectilinear and has an invariable diameter around which the damping body is symmetrically provided. The outside of the damping body 3 consists of a plate-shaped jacket 8, for example made of metal, in particular steel. The space between the passage 2 and the jacket 8 is mainly filled with a filling 9 of sound-absorbing material which preferably consists of a fibre mass, for which one can choose for example between rock wool, glass wool, ceramic wool and steel wool made of stainless steel. In order to prevent the filling 9 from entering the passage 2, said passage 2 is surrounded by a tube 10 made of porous and sound-transparent material which can be made among others of a material selected from the following series: filtering material with fine pores formed of woven wire mesh; sintered, non-woven wire mesh, preferably made of stainless steel; and sintered and reinforced non-woven wire mesh, preferably made of stainless steel. The filling 9 preferably occupies the entire space between the tube 10 and the jacket 8. Around the above-mentioned tube 10 can be provided an air-permeable protective layer 11 which prevents that material particles of the filling 9, such as fibres, go through the porous tube 10 and end up in the passage 2, whereby this protective layer can be formed of a textile layer or a material layer made of a non-woven fibre material, preferably glass fibre. In order to prevent material of the filling 9 from coming loose of the outlet piece 1, the widest end can be closed off by means of an end plate 12, so that the filling 9 is entirely contained by the jacket 8, the tube 10, the protective layer 11 respectively, and the end plate 12. The jacket 8, as well as the end plate 12, and possible extra sealing parts 13, which can actually be part of the jacket 8, make sure that the whole is air-tight in order to exclude pressure losses. The working of the outlet piece is as follows. The air of the compressor is carried from the inlet 4 to the outlet 5 through the passage 2. As soon as the air enters the passage, the compression pulses are damped in the filling 9 by means of absorption. Thanks to the gradually increasing diameter, this absorptive damping further increases, so that the required damping is obtained in a large frequency range. The filling 9 hereby makes sure that the compression pulses are absorbed such that the waves which are reflected as a result of the sudden diameter decrease at the end of the outlet piece acquire a negligible amplitude. This filling 9 can be made of a flexible or rigid porous material. Due to the incoming compression pulses, the oscillating air particles experience resistance as they go through the pores of this porous material. This leads to dissipation of energy, due to viscous effects as a result of shearing forces in the boundary layer flow at the edges of the pores on the one hand, and due to heat loss over the boundary layer on the other hand. This viscous/thermal process of energy dissipation, caused by variations in sound pressure and variations in the speed of the sound particles in this porous medium result in absorptive damping. Figure 2 shows a variant in which, instead of using the tube 10, the protective layer 11 and the filling 9, use is made of a homogenous filling 14 consisting of a solidly cohering porous material. By this is meant a material which is not made up of separate particles, such as for example fibres, and of which, as a consequence, no material particles can come loose. For the filling 14, use can be made of a ceramic foam or a highly porous, sintered powder of stainless steel. The present invention is by no means limited to the embodiments described as an example and represented in the figures; on the contrary, such an outlet piece can be made in various forms and dimensions while still remaining within the scope of the invention.

Claims: Compressor equipped with an acoustic outlet piece connected to the compressor outlet (5), which outlet piece (1) contains a damping body (3) applied symmetrically around a passage (2), whereby this passage has a constant diameter and the damping body (3) has a perimeter which gradually increases according to the direction of flow (V), so that this damping body (3) has a thickness which increases according to the direction of flow (V). Compressor equipped with an outlet piece according to claim 1, characterized in that the damping body (3) is cone-shaped. Compressor equipped with an outlet piece according to any of the preceding claims, characterized in that the space of the outlet piece (1) which surrounds the passage (2) is mainly filled with a filling (9-14) of sound-absorbing material. Compressor equipped with an outlet piece according to claim 3, characterized in that the filling (9) consists of a fibre mass. Compressor equipped with an outlet piece according to claim 3 or 4, characterized in that the sound-absorbing material of the filling (9) is made of a material of the following series: rock wool, glass wool, ceramic wool, basalt wool and steel wool made of stainless steel. Compressor equipped with an outlet piece according to any of the preceding claims, characterized in that the passage (2) is surrounded by a tubular wall (10) made of porous and sound-transparent material. Compressor equipped with an outlet piece according to claim 6, characterized in that the porous material of the above-mentioned tubular wall (10) is selected from the following series: filtering material with fine pores formed of woven wire mesh; sintered, non-woven wire mesh, preferably made of stainless steel; and sintered and reinforced non-woven wire mesh, preferably made of stainless steel. Compressor equipped with an outlet piece according to claim 7, characterized in that an air-permeable protective layer (11) is provided around the above-mentioned tubular wall (10). Compressor equipped with an outlet piece according to claim 8, characterized in that the protective layer (11) consists of a textile layer or a material layer made of a non-woven fibre material, preferably glass fibre. Compressor equipped with an outlet piece according to claim 3, characterized in that use is made of a filling (14) which is formed of a material with a solid, porous structure. Compressor equipped with an outlet piece according to claim 10, characterized in that the material of the filling (14) is selected from a ceramic foam, a metallic foam, a highly porous, sintered powder of stainless steel or a sintered ceramic powder. Compressor equipped with an outlet piece according to any of the preceding claims, characterized in that the damping body (3) is limited on the outside by a plate-shaped jacket (8-12-13).

Inventors: PAUWELS RAPHAEL, PAUWELS, RAPHAEL
Applicants: ATLAS COPCO AIRPOWER NV, ATLAS COPCO AIRPOWER N.V.
Application year: 1996
Main classification: F04B39
IPC codes: F04B39, F01N1, F04C29