GB2237323A

GB2237323A - Fan silencer apparatus

Info

Publication number: GB2237323A
Application number: GB8922577A
Authority: GB
Inventors: Bruce Nigel Smith
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1989-10-06
Filing date: 1989-10-06
Publication date: 1991-05-01
Also published as: GB8922577D0

Abstract

Silencer apparatus for silencing fluid flow, for example upstream or downstream of a fan, includes a casing 11 in which a plurality of chambers 13, 14, etc of different volumes are formed. A Helmholtz resonator to suppress frequencies from a fan are formed by making A hole in one wall of each chamber is provided so that the chambers act as Helmholtz resonators. The holes may be of different sizes in order to suppress different frequencies. The silencer apparatus could be applied to hydraulic circuits. <IMAGE>

Description

FAN SILENCER APPARATUS This invention relates to fan silencer apparatus arranged to be disposed in use connected to a fan.

For large fans used in industrial applications there is always a problem arising from noise. It is necessary for environmental protection reasons to be able to eliminate or at least reduce the more obtrusive frequencies generated by the fan in the hearing range.

Industrial fans are used in many applications where large quantities of air have to be moved. For example, they are used as ventilating fans in food and manufacturing industries where a temperature has to be controlled, in air conditioning apparatus and# for providing fresh air to remote locations, such as, for example, to ventilation shafts in mines Industrial fans are normally mounted in a housing and, depending on the application, the housing has attached to it lengths of ducting usually on the downstream side although there may also be lengths on the upstream side.

Noise generated by fans can be created by the actual movement of the fan impeller, by vibration of the fan and by turbulence caused by the fan. Some of the noise can be reduced by appropriate design of the fan blades and tip profiles and by controlling the operating speed of the fan. However in spite of this a large amount of noise can still be generated.

The normal audible frequency from a fan is in the range of 100 to 8000 Hertz. Most fans in industrial application will generate noise in the lower range and a typical range of noise generation from an industrial fan is 100 to 4000 Hertz.

Currently noise reduction is achieved by use of sound absorptive materials, such as mineral wool, inserted within the fan ducting, either as a wall lining or as panels placed within the air flow, ie splitters. These provide a noise reduction over a wide range of frequencies. However in industrial applications such as the coal mining industry, 'his type of noise suppression is prone to contamination from dirt and after a time its performance suffers. Once the mineral wool has become contaminated it has to be replaced.

There have been attempts at reducing the noise of fans by using a tuned resonator. The resonator is efficient only at its resonance frequency and provides little reduction away from this frequency.

It is an object of the present invention to provide a fan silencer apparatus based on resonators which substantially reduces noise generated by a fan over a wide range of frequencies.

According to the present invention, fan silencer apparatus arranged to be disposed in use connected to a fan comprises a housing having a plurality of chambers formed therein, at least some of the chambers being of a different size to the others and each having the hole formed in one wall so that each chamber acts as a separate Helmholtz resonator.

The housing is conveniently constructed as a plurality of rectangular compartments although other configurations could be used. The compartments are closed on all sides except where the hole is formed. It is convenient to have all the holes formed on a common side of the housing.

Preferably the housing includes means for incorporation of the housing in the ducting of a fan. The housing may be incorporated into the wall of the ducting or may be placed in the ducting in a position such that it is in the direct flow of air from the fan.

The housing would normally be placed on the downstream side of the fan although in certain constructions it may be useful to place it on the upstream side or on both sides of the fan.

A plurality of housings having different compartments which resonate at different frequencies may be provided in a single installation. Some of the compartments may be of the same dimensions in order to maximise the suppression at separate frequencies.

In order that the invention may be readily understood, one explanation of the opera Lion o: the apparatus in an example thereof will now be made with reference to the four figures of the accompanying drawings. In the drawings figure 1 illustrates in schematic terms a simple resonator, figure 2 explains graphically how the resonator works, figure 3 is an example of a silencer in accordance with the present invention and figure 4 illustrates the effect of the silencer in graphical terms.

Referring first to figures 1 and 2 of the drawings, these figures are used to illustrate the principles adopted into the invention. The Helmholtz resonator is itself known and is illustrated as a cross-section in figure 1 of a rectangular housing having walls 1 of which one wall has a hole 2 formed in its side.

The hole is normally circular for manufacturing purposes but it is not restricted to a circular form, since it is the area of the hole which is important relative to the volume of the silencer.

Depending on the size and area of the hole and the resonator volume, the frequency which is absorbed by the resonator varies.

In figure 2 to which reference is now made, three plots are shown for holes of 2 iwn, 2.5 mm and 3 mm diameter. It will be seen from figure 2 that the maximum absorption coefficient of frequency for a given volume is shown by curve 3 for a 2 mm hole at a frequency of just over 400 Hz. The curve 4 shows that with a 2.5 mm diameter hole, the frequency suppression has moved up the range between 500 and 600 Hz but the absorption coefficient has reduced to about .9.

With a 3 inn diameter hole as indicated by curve 5, the peak of the frequency range suppressed has moved to above 600 Hz but the absorption coefficient is now only .6.

It will be noted that although there is a definite peak for the frequency absorption coefficient, each hole does have a spread of frequencies which it absorbs to a lower degree and thus if there are a number of resonators with different size holes, it will be appreciated that a wide spread of frequencies can be suppressed to a larger or greater extent. Thus it is possible to provide a series of resonators each of the same size and having different holes.

As an alternative to th sthe same principle can be applied to a number of different sizec resonators i.e., resonators having different volumes and each having a common size of hole. Similarly a combination of variations. in volume of the resonators and the holes can provide a ready means for spanning a wide spectrum of frequencies for silencing purposes.

Absorption coefficient is always related to a particular surface area and its value for the resonators will vary depending on how many resonators there are in that area. By varying the number of a particular type of resonator lying within this area, the absorption at a particular frequency can be optimised. However there are physical restraints on how many resonators can be fitted into a given area.

One example of a fan silencer element designed particularly for use in a ventilation fan for ventilating a coal mine is shown diagrammatically in figure 3. Here the silencer element 10 has a general outer casing 11 which is closed but is divided internally by walls 12 into a number of compartments. As can be seen from figure 3, the left hand compartment 13 is the largest of the compartments and with the adjacent compartment 14 spans the whole width of the casing 11. zfter this, subsequent compartments are subdivided across the width by walls 15 so as to provide three compartments across the width along the length of the casing. The spacing between the walls 12 varies from compartment to compartment so that progressively the compartments become smaller in size. The central compartments are larger in this example than the side compartments although the side compartments on each side of a central compartment are the same area.

In this example the facing plate and casing 11 have walls with a thickness of 0.2 cm on a backing plate of 0.6 cm. The inner depth of the cells is 5.0 cm and their cross sectional dimensions vary as indicated in figure 3. The dimensions given are in centimetres. The holes (not shown) are 3 mm diameter. The number of cells within the casing 11 and their cross sectional area in square centimetres is set out in the following table.

TABLE No. Area cm2 1 16.000 1 12.000 1 9.600 1 8.400 1 7.500 1 6.600 1 6.000 1 5.400 1 5.100 1 4.800 1 4.500 1 4.200 1 3.900 1 3.600 1 3.300 1 3.000 1 2.700 2 2.560 2 2.240 2 2.000 2 1.760 2 1.600 2 1.440 2 1.360 2 1.280 2 1.200 2 1.120 2 1.040 2 0.960 2 0.880 2 0.800 2 0.720 Owing to the large numcer of different cells, a wide range of frequencies is able to be suppressed by the silencer. If reference is now made to figure 4, where the absorption coefficient is shown plotted against frequency for the silencer of figure 3 with the peaks of the frequency suppression joined into one curve 20, it can be seen that the silencer is able to suppress effectively frequencies between a range of 400 and 1000 Hz to a better than 0.8 absorption coefficient. Clearly by design of different size silencer elements, other ranges can be covered. The ranges would need to be designed for each installation depending on the noise frequencies which are being generated and which have to be suppressed.

The silencer formed out of the element shown in figure 3, can be built into the wall of the ducting of a fan or it may be placed directly in the flow from the fan. Since in this example it is only 5.8 cm in external depth, it will not cause a great obstruction to flow and hence there will be no significant pressure drop in the fan output.

Although the invention has been described only in relation to an air fan, the principles can be adapted to deal with fluids other than air. The invention could also, for example, be applied to hydraulic circuits.

Claims

1. Fan silencer apparatus arranged to be disposed in use adjacent a fan comprising a housing having a plurality of chambers formed therein, at least some of the chambers being of a different size to the others and each having a hole formed in one wall thereof so that each chamber acts as a Helmholtz resonator.

2. Apparatus as claimed in cm aim 1 wherein some of the holes are of different dimensions in order to give a different resonator effect.

3. Apparatus as claimed in claim 1 or claim 2 wherein the chambers are of a rectangular construction.

4. Apparatus as claimed in any preceding claim wherein the holes are formed on a common side of the housing.

5. Apparatus as claimed in any preceding claim and including means on the housing for incorporating it into the ducting of a fan.

6. Apparatus as claimed in any one of claims 1 to 4 in which the housing has means for incorporating it into the wall of the ducting of a fan.

7. Apparatus as claimed in any one of claims 1 to 5 in which the housing is arranged to be placed in the direct flow of air to the fan.

8. Apparatus as claimed in claim 7 wherein the housing is placed upstream of the fan.

9. Apparatus as claimed in claim 7 wherein the housing is placed downstream of the fan.

10. Apparatus as claimed in any preceding claim and incorporating a plurality of housings.

11. Fan silencer apparatus substantially as hereinbefore described and with reference to figure 3 of the accompanying drawings.