Acoustics – Sound-modifying means – Muffler – fluid conducting type
Reexamination Certificate
2001-04-17
2003-03-11
Hsieh, Shih-Yung (Department: 2837)
Acoustics
Sound-modifying means
Muffler, fluid conducting type
C181S224000, C181S247000, C181S248000, C181S249000, C181S250000
Reexamination Certificate
active
06530452
ABSTRACT:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The object of the present invention is a reactive silencer, specified in the preamble of the independent claim presented below, for industrial supply air and exhaust air channels or comparable applications, especially in paper mills.
In different types of industrial plants, especially in paper mills, fans and vacuum pumps constitute a considerable noise source from which the noise spreads through air channels or the like into the environment. Fans are generally selected on the basis of the amount of air required and the pressure loss of the system, and it is thus not often possible to pay sufficient attention to the noise they produce. Therefore, the noise has to be attenuated by means of silencers fitted in the air channels. In large plants, lowering the noise level below increasingly stringent requirements requires larger and larger silencers or ever greater numbers of silencers, that is, considerable investments. This means that the silencers also take considerably lot of space, which is not always available, especially in older plants.
The noise produced by the fans covers a wide spectrum. However, different types of silencers function best only within a specific spectral area. The conventionally used absorptive silencers, in which the sound energy is absorbed and converted into heat in a porous material, function best at higher frequencies, their maximum attenuation being at a frequency of about 1000 Hz. Low noise at a frequency below 200 Hz is mostly left unattenuated by an absorptive silencer of any reasonable size.
To attenuate lower frequencies, it is known to use so-called reactive silencers, in which sound attenuation is achieved by means of the specific geometrical shape of the device. A typical reactive silencer, the so-called tube resonator, comprises a tubular chamber larger than an air channel, into which is arranged a partition wall across the direction of flow and a narrow flow pipe through the partition wall.
The sound-attenuating effect of the tube resonator is based on the fact that when an air current flows to the resonator, it first meets with a sudden expansion and thereafter with a considerable contraction, whereby the resonator reflects a part of the sound energy back towards the sound source. The length of the tube resonator chamber determines the frequency of its maximum attenuation; the longer the chamber, the lower the frequency. The ratio of the cross-sectional area of the chamber to the cross-sectional area of the flow channel passing through the partition wall for its part determines the level of attenuation.
The flow pipe passing through the partition wall in a tube resonator is often provided with an extension part provided with perforations, which part extends from the end of the pipe proper to the supply or discharge opening of the resonator. The perforated pipe extensions reduce pressure loss in the resonator. Metso Paper Inc.'s American patent U.S. Pat. No. 5,285,026 discloses a tube resonator of the above type, which in addition has the special feature that the partition wall is fitted in an oblique position in order to avoid so-called zero attenuation frequency.
From the perspective of noise prevention, particularly demanding sites are paper mills in which, for example, the ventilation of the paper machine room, the removal of moisture from the dryer section of the paper machine, and the creation of an underpressure require discharging of large amounts of air by means of fans or vacuum pumps. In this case it is a question of both large single amounts of air and numerous smaller amounts of air.
It has been found that the tube resonators described above function efficiently in the smaller size categories. In larger size categories, for example, when their diameters exceed 630 mm, some of the sound waves pass through the resonator unattenuated. In paper mills, air exhaust channels may have diameters of up to 2 meters. The sound attenuation problem thus arising has, where possible, been solved by dividing the air current between several smaller channels, in each of which is installed its own silencer. However, dividing the air current between several channels and using separate silencers in each channel gives rise to considerable additional costs, and is often impossible to implement due to the lack of space.
The aim of the present invention is to bring about an improvement to the problems described above.
The aim is especially to achieve a reactive silencer suitable for use in large exhaust air and supply air channels.
The aim is also to achieve a reactive silencer suitable for use in conjunction with several smaller exhaust air or supply air channels.
SUMMARY OF THE INVENTION
In order to achieve the above aims, the reactive silencer according to the invention, which is comprised of a sound attenuator chamber fitted with a partition wall and a flow pipe or the like passing through the partition wall, is characterised by what is presented in the characterising part of the independent claim presented below.
A typical reactive sound attenuator chamber according to the invention, which is intended for industrial air channels or similar applications, thus comprises
a partition wall which divides the sound attenuator chamber into a first and second chamber part,
a feed opening in the first chamber part,
a discharge opening in the second chamber part, and
two or more flow channels or pipes which are fitted in the partition wall in order to connect the air spaces of the first and second chamber parts, and the cross-sectional area A
1
of which pipes or channels is substantially smaller than the cross-sectional area A
2
of the sound attenuator chamber proper.
Preferably, the total cross-sectional area &ugr;A
1
of the flow channels is less than one fifth of the cross-sectional area of the sound attenuator chamber, that is, &Sgr;A
1
<⅕*A
2
.
According to the first preferred embodiment of the invention, two or more feed openings and two or more discharge openings are fitted in the sound attenuator chamber. The sound attenuator chamber in this case preferably has one feed opening and one discharge opening per each flow channel fitted in the partition wall. The feed openings and the discharge openings are preferably fitted in pairs, concentrically opposite each other. Each flow pipe or channel is preferably fitted concentrically between one pair of feed and discharge openings.
The partition wall is fitted in the sound attenuator chamber preferably so that the partition wall divides the chamber into a first chamber part and a second chamber part in such a way that the length l
1
of the first chamber part is less or greater than the length l
2
of the second chamber part. Typically l
1
=½*l
2
or l
1
=2*l
2
.
In special cases, the sound attenuator chamber can be divided in the direction of flow, by means of several consecutive partition walls, into several consecutive parts depending on the attenuation requirement and the frequency range to be attenuated.
The flow pipe is fitted in the partition wall preferably in such a way that the length l
3
of its pipe section projecting into the first chamber part equals half the length l
1
of the first chamber part in the direction of flow. Similarly, the length l
4
of the flow pipe section projecting into the second chamber part equals half the length l
2
of the second chamber part in the direction of flow.
The diameter of the flow pipe fitted in the partition wall is preferably equal in size to the diameter of the feed opening and/or discharge opening. A perforated pipe extension can then be fitted between the end of each flow pipe and the feed opening and discharge opening of the chamber, in order to reduce pressure loss.
Most typically, the silencer according to the invention is formed of an elongated box-like structure which is divided by means of a longitudinal-partition wall into two elongated chamber parts. The partition wall is p
Heikkilä Tauno
Jokinen Juhani
Nissilä Keijo
Pettersson Henrik
Hsieh Shih-Yung
Lathrop & Clark LLP
Metso Paper Inc.
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