NO751949L - - Google Patents

Description

Gas flow muffler

The history of noise control has been a development of step-by-step technological improvement with simultaneous progressive recognition of the physiological, psychological and sociological necessity of limiting the noise levels to which people are exposed. The scientific principles involved are taken and combined from fields such as acoustics, pneumatics and hydraulics. In the same way as in mechanics, known physical means are combined in new ways with dimensions and other construction criteria that are discovered to give increasingly better results.

In the special field relating to the treatment and control of gas drums, especially pulsating or pressure- and/or velocity-disturbed flow, it is necessary to tackle complicated problems relating to the elimination or at least high attenuation of sound frequency systems as well as equalization and end- ring off the flow in other ways to avoid the generation of noise directly on discharge into the atmosphere or indirectly by causing noise that creates vibrations in mechanical equipment. At the same time, the formation of secondary noise in the silencer itself or other treatment devices must be avoided.

It is an object of the present invention to provide a silencer for controlling the existing or potential noise in a gas stream and which is able to absorb noise upstream and smooth out irregular flow to avoid the formation of noise downstream, while at the same time avoiding that secondary noise is formed in the silencer itself. The device is able to process flows with highly varying pressure and velocity characteristics, including shock waves.

A further object of the invention is to provide such a silencer which has a long and reliable lifetime. Another

purpose is to provide such a multi-stage silencer, to which one or more stages can be added to provide more complete sound attenuation as the noise pollution standards become stricter. Another purpose is to provide such a silencer which has a very high ratio between performance and size. - Fig. 1 shows a side view of a typical silencer according to the invention. Fig. 2 is a longitudinal section of the silencer in fig. 1. Fig. 3 shows on an enlarged scale the construction and the effect of the shock-absorbing section of the unit. Fig. 3A and 3B show details of the perforation patterns for the inner mantles and of the diffusers in full size, respectively. Figs. 4 and 5 show detailed sections of alternative forms of vibration-isolating connections between adjacent sections of the silencer, the construction in fig. 1 and 2 is the one shown in fig. 4.

Fig. 6 is a disassembled representation of an "additional unit".

Fig. 7 is a side view of a modified silencer.

The silencer shown in fig. 1 to 4 is a two-section unit comprising cylindrical shells 1 and 2 with the inlet end closed by a cover 3. A first stage diffuser tube 4 is welded to the cover, and a standard connecting flange 5 is welded to the projecting end of the diffuser tube. The cylindrical walls of the rudder are perforated, and the monster is shown in full size in fig. 3B. An aperture 6 with an opening 7, whose function will be described in the following, is shown lying against the surface of the flange 5.

The abutting ends of the casings 1 and 2 are equipped with flanges 8 and 9. A partition wall 10 with a central opening 11 and a peripheral flange 12 is mounted at the transition between the sound dem-

the first and second stages of the pair by means of the peripheral flange, j Elastomeric spacers 13 are inserted between flange 12 and' flanges 8 and 9 for the connection is tightened by means of bolts 14. The purpose of the elastomeric spacers is to prevent the transmission of vibration between the jackets bound together by the compound.

Inner cylindrical casings 15 and 16 are mounted by means of rings 17 at a distance from the casings 1 and 2. The inner casings are, as shown, perforated to allow access from the interior of the silencer to the material packed in the frames between the inner casings and the muffler shrouds. The properties and function of these materials shall be described in more detail below. The inner mantle perforation sample is shown in full scale in fig. 3A.

A second stage diffuser rudder 18 is mounted in the opening 11 of the partition 10. This rudder is, as shown, perforated in the same way as the first stage diffuser rudder.

A perforated inner cover 19 is preferably mounted at a distance from the cover 3 and the space which is thereby filled with sound-absorbing material, such as a mat of copper or steel fibres. In addition, the casing 1 is surrounded by the silencer's first section. with two layers of casing vibration-damping material, such as lead or asbestos sheet 21. A cover made of thin sheet is also provided.

Although the gas flow silencer according to the invention can be connected in a line, i.e. that its inlet is connected to a supply channel for gas and that its outlet is also connected to a channel, the device shown in the drawing is constructed to form the end of the gas flow channel. The inner and outer casing at the outlet end 22 of the muffler is open to the atmosphere.

The gas drum, whose properties can vary from relatively steady flow to violently pulsating flow, is fed into the first stage diffuser tube 4. Since the downstream end of the tube is closed, the gases are forced to flow radially outward through

rudder perforations. In the course of each pulse, the gases are directed in a large number of individual jets towards the inner casing 15.

The space 23 which encloses the diffuser tube is filled with a durable material, such as metal shavings or copper scrap, which provides many small spaces in connection with each other. Although it contributes to the absorption of sound vibrations in the gases from upstream sources, this space's main function is to receive and dampen the shock of the rays from the diffuser tube 4. In other words, although sound absorption is a desirable property of material contained in the space 23, this property is partly sacrificed in order to achieve resistance to destruction by the gases which flow at high speed through the perforations of the inner mantle 15 and into the small voids of the room. The space 24, which is located downstream of the diffuser, but which is nevertheless part of the silencer's front shock section, is preferably filled with a sound-absorbing material of the hold-bar type. Sound energy is absorbed by the material in the spaces 23 and 24 and also by the sound-absorbing material 20 which is also connected to the space in the silencer's first stage.

The treatment of the flow in the silencer's first section provides a partial but significant equalization of the gas volume after placement of the shock and sound absorbing parts.

The muffler's second stage is designed to further dampen pulsations and absorb sound energy still present in the gas drum as it flows from the first stage into the second stage diffuser vane 18. Although the treatment in the second stage works in the same way as the treatment in the first stage of the muffler , the emphasis here is changed from shock absorption to sound absorption with continued smoothing of the flow in preparation for outflow into the atmosphere with minimal noise-creating noise. The sound energy is absorbed by a material that is better sound absorbing than the material contained in the first section, such as perforated felt or glass fiber contained in the space 25. Noise, whose sources are located upstream of the silencer and noise that can form in the gas system inside the silencer, is absorbed to an acceptable level before it reaches the muffler outlet.

The gas flow silencer must be designed so that it not only absorbs as much as possible noise energy that reaches it from upstream sources and smooths the flow to avoid the formation of j

of noise at or after the muffler exits, but also to avoid the formation of noise inside the muffler itself. Formation of low-frequency current noise that tends to occur

when a stream with a relatively high speed flows into a large volume, such as e.g. when a current from an inlet channel flows into a relatively large silencer chamber, is avoided in the silencer described here. The inlet current is divided into a large number of jets 26 (fig. 3), and the shock-absorbing material is arranged somewhat beyond the cores of the jets, as shown at 27 on an enlarged scale in fig. 3. The preferred distance between the inlet pipe 4 and the perforated inner jacket 15 is from four to twenty times the diameter of the perforations in the diffuser pipe. With this design, damage to the shock-absorbing material is avoided due to the high core speed of the jets, and the gases are received and processed without causing low-frequency noise. This distance can be somewhat smaller or somewhat larger, with a consequent loss of life for the shock-absorbing material or acoustic performance of the silencer, but the stated dimensions give optimal effect.

After treatment of the gas drum in the first shock stage of the silencer, the gases flow into the second stage diffuser tube 18 and radially from there through the perforations therein. The perforations in the second stage diffuser rudder are preferably somewhat smaller, and the rudder is somewhat longer compared to the corresponding dimensions for the first stage diffuser. This results in more and smaller rays. Although the second stage diffuser rudder provides further smoothing of the flow, the treatment in the second stage is primarily acoustic,

i.e. absorption of sound waves propagating in the gas chamber. Room 25 is packed with an effective sound-absorbing material, such as hair felt or fiberglass. As shown in fig. 1, the exposed surface of the sound-absorbing material is extended well past the closed downstream end of the diffuser pipe to provide continued absorption of sound energy as the stream is collected for discharge into the atmosphere at a relatively low and steady velocity through the open downstream end of the muffler. The silencer must necessarily be designed for. to be able to process a predetermined maximum of flowing gases. Although a valve can be arranged in the upstream channel that leads to the sound

the damper to limit the flow, it is likely that only then noise will form downstream of the valve. This can be avoided by using a shutter 6 wood. the inlet to the first stage diffuser vane. The total open surface formed by the perforations in the diffuser tube is proportional to the size of the aperture opening 7. If the perforations in the second stage diffuser tube are somewhat smaller than those in the first diffuser tube, which is preferable, the total open area for flow will through the perforations in the second rudder be approximately the same as for the first rudder to give optimal results because the flow into the second diffuser rudder is distinctly smoother.

For a two-stage silencer, it is usually only necessary to provide means for preventing the emission of noise into the atmosphere from the casing for the first stage or shock stage. This is done by surrounding the mantle 1 with vibration dampening

layer as described above. Transfer of vibrations in the first stage casing 1 to the second stage casing 2 is largely avoided by inserting elastomeric material in the connection between them as described above, and the relatively smoother flow of gas into and through the second stage usually causes no problematic vibrations in the casing.

An alternative form of vibration isolating connection is shown in fig. 5. In this arrangement, the ends of the sheaths 1a and 2a overlap and are attached to each other with an elastomer spacer 13a in between as a vibration barrier.

Although it is generally more desirable, both from a cost and performance point of view, to construct the silencer with cylindrical casings, these can alternatively be conical as shown in fig. 7. The construction considerations described in connection with the silencer shown in fig. 1 to 4 also apply to the unit shown in fig. 7, which differ largely only in that the mantles are conical instead of cylindrical.

Developments in recent years have required increasingly better sound attenuation to reduce noise pollution. What can be accepted j and tolerated today is not sufficient to fashion tomorrow-

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today's more stringent standards. With this development for the city, and the likelihood that it will continue in the future, the invention aims at possible future extensions by adding one or more processing steps to the originally installed silencer which may comprise only two steps. This concept is shown in fig. 6 where the original two-stage unit is equipped with a connecting flange 28. An additional stage 29 comprising a jacket 30 with a lining of sound-absorbing material 31, diffuser pipes 32 and connecting elements is shown in position for connection to the outlet end of the original two-stage unit. If it is. If desired at a later stage, one or more further stages may be inserted between the original unit and the end stage 29.

The silencer shown in fig. 6 illustrates a further alternative design feature. To further dampen the sound of the gas drum, a sound absorbing element 33, which is contained between an inner solid tube 34 and an outer perforated tube 35, is arranged as an extension of the diffuser tube 32. Extended sound absorption is achieved by allowing the stream to pass through the annular passage formed by the surface of the sound-absorbing element.

The muffler according to the invention is very flexible in use. While attenuation of relatively violent flow is its forte, it is also effective at pressure reduction, such as sound attenuation of pressurized steam escaping into the atmosphere.

Claims (10)

1. A multi-stage gas flow silencer with circular jackets for recirculating gas, where each section has a central inlet opening and where the inner surfaces of at least the side walls of the jackets are covered with a layer of sound-absorbing material held in position by perforated inner jackets, characterized in that a diffuser tube is mon- ter in each inlet opening and extending downstream from there mn in, but not through, associated processing section, that each rudder is open at the upstream end, is closed at the downstream end and has perforated side walls, that the distance from the perforated side walls to the perforated inner jackets surrounding the same is larger than four times the diameter of the perforations in the side walls, that the sound-absorbing material of the layer surrounding the diffuser in the treatment section at the inlet end of the silencer consists of a durable material, such as metal shavings, and that the layer of sound-absorbing material in subsequent treatment sections comprises material with high sound absorption ability, such as perforated hair felt or fiberglass. 2. Silencer according to claim 1, characterized in that the distance from the perforated side walls to the perforated inner mantles surrounding the same is from four to twenty times the diameter of the perforations in the side walls. 3. Muffler according to claim 1, characterized in that it includes a layer of vibration-damping material that surrounds the jacket or jackets that define the treatment section at the inlet end of the silencer in order to minimize the emission of noise from the jackets to the atmosphere. 4. Silencer according to claim 1, characterized in that the sheaths that define adjacent sections are interconnected with compliant material that is inserted between and separates the sheaths in order to minimize transmission of vibration from the sheaths in one section to the sheaths in the adjacent section. 5. Silencer according to claim 4, characterized in that the connections that connect adjacent treatment sections comprise flanges mounted on the respective casings, a layer of compliant material arranged between the flanges and fasteners that pull and hold the flanges together with said layer inserted in between. 6. Silencer according to claim 4, characterized in that the connections that connect adjacent treat- .ling sections comprise telescoping end parts of the sheaths arranged at a distance from each other and a layer of yielding ^ material which fills the space between the telescoping end parts, and means for joining the telescoping end parts with the yielding material inserted in between. 7. Muffler according to claim 1, characterized in that the outlet end of the last treatment section of the muffler is equipped with means for connecting a further treatment section, whereby the degree of treatment of the gas drum can be increased at any time after the original installation and use of the muffler . 8. Silencer according to claim 1, characterized in that it includes a downstream extension of a diffuser tube, the extension having a layer of sound-absorbing material exposed to the gas drum and defining an annular passage for the gas drum together with the inner jacket. 9. Muffler according to claim 8, characterized in that the extension of the diffuser tube is the muffler's last processing section. 10. Silencer according to claim 1, characterized in that the mantles are conical and that the smallest end is the inlet end of the silencer and the largest end is open to the atmosphere.