GB2139696A - Absorptive exhaust gas silencer - Google Patents

Abstract

A duct lined with acoustically absorbent material (2) through which exhaust gases flow to atmosphere has means for maintaining and replenishing a distinct layer of protective gas between the exhaust gases and the absorbent material. A screen (7) of acoustically transparent material such as expanded metal in conjuction with an inlet for ambient air or air under pressure may provide the protective gas layer. <IMAGE>

Description

SPECIFICATION Absorptive silencer The present invention relates to silencers for reducing the noise emitted by the working gases emitted from a fluid powered machine such as an internal combustion engine or upon otherwise venting of a high pressure gaseous medium into the atmosphere.

In particular the invention is concerned with absorptive silencers. The gases thus emitted or vented into the atmosphere are referred to hereinafter as exhaust gases, and the noise emitted thereby into the atmosphere is referred to as exhaust noise.

The principles of exhaust noise reduction are a known art. Exhaust gases may be passed through a series of alternate expansions and contractions in an exhaust duct, thereby reducing the fluctuating pressures present in those exhaust gases by destructive interference of pressure waves. A device operating in this manner is commonly referred to as a "reactive silencer".

As an alternative, or as an additionai technique, exhaust gases may be passed into a device containing an acoustically absorbent material, in which the main exhaust flow passes across the surface of the absorbent material. The pressure pulsations present in those exhaust gases are thus reduced by converting the energy present in the pressure pulsations into heat generated by friction between the exhaust gases and the absorbent material. A device operating in this manner is commonly referred to as an "absorptive silen cer The acoustically absorbent material is normally a fibrous or porous material, and it is a recognised problem of the absorptive silencer that the porous or fibrous material within the device may become damaged or removed by the forces applied to it by the exhaust gases passing across the porous or fibrous material.

The effectiveness of the device is thereby reduced. It is a further known problem of the absorptive silencer that the temperature of the exhaust gases passing through the device may cause damage to the porous or fibrous materal within the device. It is a third known problem of the absorptive silencer that products of incomplete combustion, such as carbon, present in the exhaust gases may become deposited on the surfaces of the absorbent material within the device, thereby reducing its effectiveness. This latter known problem is often particularly acute when an absorptive silencer is employed to reduce exhaust noise associated with an engine operating on the Diesel cycle.

It is a yet further known problem of the absorptive silencer when employed to reduce noise from high pressure air exhausts that moisture present in the exhaust air may be caused to condense or freeze within the device due to the rapid fall in temperature of the exhaust air associated with its expansion from the high pressure exhaust. Condensed or frozen moisture reduces the effectiveness of the fibrous or porous material within the device.

The present imvention seeks to provide an absorptive silencer formed in such a way as to avoid or mtigate at least in part the aforementioned known problems.

According to the invention there is provided an absorptive silencer comprising an inlet for exhaust gases, an outlet for exhaust gases, a layer of acoustically absorbent material for reducing exhaust noise, and means for maintaining and replenishing a protective gas layer distinct from the exhaust gases between the exhaust gases and the acoustically absorbent material.

The means for maintaining and replenishing the protective gas layer preferably includes a screen of acoustically transparent material, and expanded metal is eminently suitable for this purpose. Nevertheless, in some instances, especially where the flow path of the exhaust gases past the acoustically absorbent layer is short, the need for a screen may be avoided.

The important consideration is that exhaust gases should be inhibited by the presence of the protective layer from reaching any part of the absorbent layer. Normally the protective gas layer is continuously replenished by maintaining it as a continuous flow, although an arrangement in which the flow is intermittent might be envisaged. In all cases, the objective is to ensure that the quantity of harmful matter contained in the exhaust gases reaching the acoustically absorbent material is minimsed. The protective gas layer serves to protect the fibrous or porous material from the forces, temperature, moisture or particulate contamination associated with the exhaust gases entering the silencer.

Preferably there is provided a protective gas layer inlet, and means for continuously flowing the protective gas layer therethrough and between the exhaust gases and the acoustically absorbent material.

Conveniently, the exhaust gas inlet, the protective gas layer inlet and the acoustically absorbent material layer are arranged about a common axis, so that in use the protective gas layer forms a substantially annular flow around the exhaust gases.

Conveniently, the protective gas layer inlet communicates with means for supplying gas at a pressure slightly above the surrounding ambient pressure. This could conveniently be the cooling system of a machine which in use produces the said exhaust gases.

Alternatively, the protective gas layer inlet may communicate with the surounding atmosphere at ambient pressure, the protective gas layer being drawn therethrough by jet pumping action of the exhaust gases.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which Figure 1 is a sectional elevation of one form of absorptive silencer in accordance with the invention.

Figure 2 is a sectional plan view on the line AA of the absorptive silencer shown in Figure 1.

Figure 3 is a sectional elevation of an alternative design of absorptive silencer in accordance with the invention.

Figure 4 is a sectional plan view on the line BB of the absorptive silencer shown in Figure 3.

Referring first to Figures 1 and 2, the absorptive silencer shown therein comprises an inlet duct 1 for exhaust gases, a tube 2 of acoustically absorbent fibrous or porous material, an outer cover 3,a mesh protective covering tube 4, a mounting surface 5, a top retaining plate 6, a tubular mesh gas separator screen 7 and a plurality of flexible links 8.

The tube 2 of fibrous or porous material is retained at its outer diameter by the outer cover 3 and at its inner diameter by the mesh tube 4. The cover 3 and the tube 4 are anached to the mounting plate 5 which, together with the top retaining plate 6, provides axial location for the tube 2. The outer cover 3 and the top retaining plate 6 are rigidly attached to each other by welding.

The mounting plate 5 would, in a typical application, forms part of a system of covers over the internal combustion engine, high pressure air source or other device to be silenced. Exhaust gases from the device to be silenced are caused to pass into the inlet duct 1, which is positioned concentric with the tube 2 and entering a short distance into the tube, through an opening provided in the mounting plate 5. The relative dimensions of this opening, the tube 4 and the inlet duct 1 are such that an annular gap exists between the inlet duct 1 and the mounting plate 5 and the tube 4.

Typically, the inlet duct 1 may be mounted on the internal combustion engine or high pressure air source, and is permitted a small amount of radial movement to allow for vibrations transmtted to it. One end of the mesh screen 7 is loosely fitted over the end of the inlet duct 1, and at the other end it is held in a position concentric with the inlet duct 1 by a plurality of flexible links 8 which link the mesh screen 7 to the mesh tube 4 close to the top retaining plate 6.

The operation of the device is as follows.

Exhaust gases or exhaust air emerge from the inlet duct 1 at high speed, and pass through the interior of the mesh screen 7, exiting from the device through the top retaining plate 6.

Air at approximately ambient temperature flows through the annular gap between mounting plate 5 and inlet duct 1 the flow being induced by separate supply of air at low pressure fed beneath plate 5. This air supply can be maintained by the small differential pressure existing across the mounting plate 5 in cases where this mounting plate forms part of a system of covers over an internal combustion engine (or high pressure air source) provided with a forced cooling system. The aforementioned air flow forms a protective gas layer, protecting the porous or fibrous tube 2 from the effects of heat, moisture and/or contamination present in the exhaust gases.

The mesh screen 7 is designed to assist in minimising mixing of the exhaust gases and the protective gas layer within the device. The mesh screen 7 and mesh tube 4 are acoustically transparent, that is to say they are designed to permit pressure waves associated with exhaust noise to pass through them and into the porous or fibrous tube 2. The sound energy within the exhaust gas can thus be attentuated by the material of the tube 2, whilst the protective gas layer minimises contact of the tube 2 by the exhaust gas. The exhaust noise is thus reduced as the exhaust gases pass through the device, but harmful contact between the exhaust gases and the acoustically absorbent material is avoided or minimised.The mesh screen 7 and possibly also the mesh tube 4 are conveniently formed of expanded metal, the screen 7 being so arranged that the elements of the expanded metal thereof are angled inwardly in the direction of flow of the exhaust gas. Outward flow of exhaust gas through the mesh, so as to mix with the protective gas layer, is thus minimised.

Turn to Figures 3 and 4 which show an alternative embodiment of the invention, the silencing device shown therein comprises an inlet duct 9 for exhaust gases or exhaust air on which are mounted a plurality of fins 10, a tubular outer cover 11, and a bellmouth 1 2.

A tube 1 5 of acoustically absorbent material, eg of a fibrous or porous nature is located inside the tubular cover 11 and retained in place by a pair of annular end walls 13, 14 and a mesh tube 1 6. One end of a mesh gas mixing screen 1 7 is located loosely over the end of the inlet duct 9 and the other end is held in a position concentric with the inlet duct 9 by a plurality of flexible links 18 which join the mixing tube 1 7 to the mesh screen 16.

The inlet duct 9, outer cover 11, bell mouth 12, annular end walls 13, 14, acoustically absorbent tube 15, -and mesh tubes 1 6 and 1 7 are arranged in a concentric fashion, with the end of the inlet duct 9 projecting inwardly slightly past the bellmouth 12 and into the end of the porous or fibrous tube 1 5 as shown in Figure 3. The internal diameter of the bellmouth 1 2 and mesh tube 1 6 are such that an annular gap exists between these items and the inlet duct 9 and the mesh screen 17.

In use of the device, exhaust gases emerge from the inlet duct 9 at high speed, and pass through the interior of the mesh screen 17, exiting from the device through the retaining ring 14. In this embodiment, the mesh screen 1 7 is designed to allow a proportion of the exhaust gas to pass outwardly through the mesh. A controlled mixing is thus effected between the exhaust gases and the air present in the annular gap between the screen 1 7 and the mesh tube 16. A jet pumping action is thus caused to take place within the device, and an annular flow of air at substantially ambient temperature and pressure is thus in duced through bellmouth 12.To this end, the mesh tube 1 7 is conveniently formed of ex panded metal, the elements of the expanded metal being angled relative to the direction of flow of the exhaust gas, to the extent necessary to induce the desired limited degree of mixing with the air present in the annular gap.

This annular air flow forms the protective gas layer, protecting the porous or fibrous tube 1 5 from the effects of heat, moisture and/or contaminant present in the exhaust gases. The mesh screen 1 7 and mesh tube 1 6 are acoustically transparent that is to say they are designed to per it pressure waves associated with exhaust noise to pass through them and into the porous or fibrous tube 15, thereby reducing exhaust noise as the exhaust gases pass through the device.

It is a feature of the device shown in Figure 3 and 4 that it requires no additional supply of air (other than ambient atmospheric air) for satisfactory operation, and may be mounted directly onto an exhaust system, independent of any system of covers associated with the internal combustion engine or high pressure air source silenced by the device.

Absorption silencers in accordance with the invention find particularly useful application in all cases where the noise emitted by exhaust gases is to be reduced, and direct contact between the exhaust gases and the acoustically absorbent material in the silencer would be harmful. Examples of their useful application are thus for silencing diesel engines, in which the dirt and soot contained in the exhaust gases would tend to be deposited on the surfaces of the absorbent material, reducing its effectiveness; air compressors in which periodic venting of pressurised air to atmosphere may be necessary, when water or ice condensing from the exhaust gas may clog the absorbent material; and pressure vessels from which there are regularly ejected pressurised gases which are corrosive towards the absorbent material, or which contain entrained liquids which would soak the absorbent material. High velocity exhaust gases can cause physical damage to, or removal of the absorbent material, and the separation effected by the protective gas layer can be highly beneficial from this viewpoint also.

Claims (11)

1. An absorptive silencer comprising an inlet for exhaust gases, an outlet for exhaust gases, a layer of acoustically absorbent material for reducing exhaust noise, and means for maintaining and replenishing a protective gas layer distinct from the exhaust gases, between the exhaust gases and the acoustically absorbent material.

2. An absorptive silencer according to claim 1 wherein the means for maintaining and replenishing the protective gas layer includes a screen of acoustically transparent material.

3. A absorptive silencer according to claim 2 wherein the acoustically transparent material is expanded metal.

4. An absorptive silencer according to any one preceding claim wherein there is provided a protective covering of an acoustically transparent material which covers the acoustically absorbent material.

5. A absorptive silencer according to claim 4 wherein the protective covering is of expanded metal.

6. An absorptive silencer according to any one preceding claim comprising a protective gas layer inlet, and means for continuously flowing the protective gas layer therethrough and between the exhaust gases and the acoustically absorbent material.

7. An absorptive silencer according to claim 6 wherein the exhaust gas inlet, the protective gas layer inlet, and the acoustically absorbent material layer are arranged about a common axis so that in use the protective gas layer forms a substantially annular flow around the exhaust gases.

8. A absorptive silencer according to claim 6 or claim 7 wherein the protective gas layer inlet communicates with plans for supplying gas at a pressure slightly above the surrounding ambient pressure.

9. An absorptive silencer according to claim 8 wherein the means for supplying gas is a cooling system of a machine which in use produces the said exhaust gases.

10. An absorptive silencer according to claim 6 or claim 7 wherein the protective gas layer inlet communicates with the surrounding atmosphere at ambient pressure, the protective gas layer being drawn therethrough by jet pumping action of the exhaust gases.

11. An absorptive silencer according to claim 1 and substantially as hereinbefore described.

1 2. An absorptive silencer substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.