GB1563439A - Centrifugal fan with sound-absorbing means - Google Patents

Abstract

The noise of the radial-flow fan (5) is attenuated by a sound-absorbing insert (3). In order to achieve attenuation even at low frequencies, the thickness of the sound-absorbing insert (3) deviates from the average thickness. The internal interface (2) over more than one tenth of the rotor perimeter of said insert has the approximate shape of a helix. <IMAGE>

Description

(54) CENTRIFUGAL FAN WITH SOUND-ABSORBING MEANS (711 We, FÜTÖBER ÉPÜLET- GEPESZETI TERMEKEKET GYART6 VALLALAT, of Budapest XIV, Gvadanyi u.67, Hungary, a body corporate organized under the laws of Hungary, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to centrifugal fans having housings accommodating soundabsorbing material for absorbing a significant portion of the noise that is inevitably generated during the operation of the fan.

Solutions are well-known, in which the noise of centrifugal fans is reduced by absorptive noise dampers connected downstream of the pressure (delivery) pipe and/or the suction (inlet) pipe. The space requirements of such dampers often present an almost insoluble problem for the designer: the use of such dampers involves an increase of flow resistance which can only be overcome by an undesirable increase in pressure and an attendant increase in the noise of the fan.

Centrifugal fans are also known. in which sound-absorbing material of uniform thickness is arranged in the housing in the manner of a lining on the inside surface of a casing defining the fluid flow path. The sound absorbing material arranged in a uniform thickness along the inner surface of the casing provides a damping of 8-15 dB in the range of the higher regions of the audio frequency range (above 800 Hz). In the lower regions of the audio frequency (AF) range the value of damping is, however, only 2-4 dB. The application of a uniform and thicker layer could improve these values but only at the price of narrowing the flow cross-section, thus leading to disadvantageous flow conditions, or of a significant increase of the external dimensions of the casing and of the fan itself.

It is a well-known fact that the damping of low AF tones range represents a rather difficult task. The problem is aggravated by the fact that a centrifugal fan with a spiral casing functions as an internally excited Helmholtz-resonator, the natural frequency f0 of which is given by:

where:: a=the velocity of propagation of sound in m/s Lsz, L,,=lengths of the suction and delivery pipes respectively (m) A,,, Any=cross-sectional area of the suction and delivery pipes, respectively (m2) V=volume of the casing (m3) For a fan of average dimensions, with an impeller diameter of 570 mm, a casing volume of V=0.35 m3, Lsz=0.12 m, Ass=0.25 m2, Lay=0. 12 m, Any=O.l3 m2, a=330 m/s, fro~150 Hz For the upper harmonics, standing waves are formed provided that the wave-length A < 2 lmaX, where 1 max indicates the greatest linear dimension of the spiral casing.In this example lax=1.4 m, consequently l < 2.8 m (f > 120 Hz).

Both the lower resonance frequency, and the upper harmonics can be found in the spectra of centrifugal fans. The fact that they are independent of the frequency of excitation (state of operation, impeller r.p.m.) confirms the presence of spaceresonance.

It would seem expedient to extend the use of damping of uniform layer thickness for absorbing lower AF noise: A construction is known, where the housing of the fan is parallelepipedal or the side-plates functioning as load-bearinp structures are rectangular. In such constructions there is unutilized space between the enveloping parallelpiped and the spiral casing.

The present invention seeks to utilize this enveloping space by occupying it with sound-absorbing material. In this way it becomes possible to extend the damping effect of sound absorbing inserts, hitherto effective only above about 700 Hz, to below 700 Hz.

According to the present invention there is provided a centrifugal fan which is as claimed in the accompanying claims.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, wherein: Figure 1 illustrates a casing and soundabsorbing insert configurations of the preferred fan, the effective cross-section for flow remaining unchanged, as measured along the axes indicated by chain lines, and Figures 2 and 3 on the one hand, and Figure 4 on the other hand illustrate practical embodiments of the invention.

As shown in Figure 1, the enveloping parallelepiped fan housing 1 supports an internal casing 2.1 -defining the outer boundary of a fluid flow path la. Soundabsorbing material 3 is disposed between the housing 1 and the casing 2.1. The casing consists of six planar walls one of which is supported directly by the housing 1 so that no sound-absorbing material is disposed outwardly of this one wall.

The six walls approximate in shape to a spiral and this small deviation of the casing shape from a spiral does not give rise to important drawbacks from the point of view of streaming efficiency, but allows the sound-absorbing material 3 to be of geometrically simple shapes having straight sides, i.e. being polygonal prism shape. This construction results in improvements of acoustical efficiency and reduction of fabrication costs.

In Figures 2 and 3 there is shown a centrifugal fan, Figure 2 being a perspective view partially broken away and Figure 3 being a cross-section. The housing 1 of the fan is formed by sides 1.1, holders 1.3 and cover plates 1.2; these structural elements being rigid enough to support a rotatably mounted impeller or rotor 5, a driving motor 6 and to provide mountings 7 for the fan.

The casing 2.1 is expediently a solid, sound-permeable plate or sheet, e.g. a perforated sheet, which is covered by a foil or fabric (synthetic fabric or glass fibre) 2.2 in order to promote smooth flow.

Propagation of vibration of the rotor 5 to the sheet surfaces is hindered by the use of so-called vibroisolators (dampers) 4.1, 4.2, 4.3 and 4.4 respectively disposed adiacent the fixings 7, rotor 5, the holders 1.3 and the driving motor 6.

Regulation of the direction of blowing can be altered by rotating the whole of the fan.

Figure 4 shows a fan with bilateral suction but otherwise this structure is essentially identical with the previously described structural arrangments.

WHAT WE CLAIM IS: 1. A centrifugal fan having a box-like (i.e.

substantially parallelpipedic) housing, the housing having therein substantially planar internal walls which define the outer boundary of a fluid flow path through the fan, the outer boundary being approximately spiral in shape, and an impeller rotatably mounted within the internal walls, the regions disposed between the box-like housing and the internal walls being of polygonal prism shape and being occupied with sound absorbing material also of polygonal prism shape.

2. A centrifugal fan according to claim 1, wherein part of the length of said boundary coincides with a portion of the box-like housing so that no sound absorbing material lies outside said part of the boundary.

3. A centrifugal fan according to claim 2, wherein the outer boundary is defined by six of said internal walls, one of which is supported directly by the housing so that no sound absorbing material lies outside said one wall.

4. A centrifugal fan according to any one of the preceding claims, wherein each internal wall is formed by a layer of foil or fabric backed, on the side of the wall remote from the flow path, by a perforated sheet.

5. A centrifugal fan according to claim 1, substantially as herein particularly described with reference to Figures 1 to 3, or as modified by Figure 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. between the enveloping parallelpiped and the spiral casing. The present invention seeks to utilize this enveloping space by occupying it with sound-absorbing material. In this way it becomes possible to extend the damping effect of sound absorbing inserts, hitherto effective only above about 700 Hz, to below 700 Hz. According to the present invention there is provided a centrifugal fan which is as claimed in the accompanying claims. Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, wherein: Figure 1 illustrates a casing and soundabsorbing insert configurations of the preferred fan, the effective cross-section for flow remaining unchanged, as measured along the axes indicated by chain lines, and Figures 2 and 3 on the one hand, and Figure 4 on the other hand illustrate practical embodiments of the invention. As shown in Figure 1, the enveloping parallelepiped fan housing 1 supports an internal casing 2.1 -defining the outer boundary of a fluid flow path la. Soundabsorbing material 3 is disposed between the housing 1 and the casing 2.1. The casing consists of six planar walls one of which is supported directly by the housing 1 so that no sound-absorbing material is disposed outwardly of this one wall. The six walls approximate in shape to a spiral and this small deviation of the casing shape from a spiral does not give rise to important drawbacks from the point of view of streaming efficiency, but allows the sound-absorbing material 3 to be of geometrically simple shapes having straight sides, i.e. being polygonal prism shape. This construction results in improvements of acoustical efficiency and reduction of fabrication costs. In Figures 2 and 3 there is shown a centrifugal fan, Figure 2 being a perspective view partially broken away and Figure 3 being a cross-section. The housing 1 of the fan is formed by sides 1.1, holders 1.3 and cover plates 1.2; these structural elements being rigid enough to support a rotatably mounted impeller or rotor 5, a driving motor 6 and to provide mountings 7 for the fan. The casing 2.1 is expediently a solid, sound-permeable plate or sheet, e.g. a perforated sheet, which is covered by a foil or fabric (synthetic fabric or glass fibre) 2.2 in order to promote smooth flow. Propagation of vibration of the rotor 5 to the sheet surfaces is hindered by the use of so-called vibroisolators (dampers) 4.1, 4.2, 4.3 and 4.4 respectively disposed adiacent the fixings 7, rotor 5, the holders 1.3 and the driving motor 6. Regulation of the direction of blowing can be altered by rotating the whole of the fan. Figure 4 shows a fan with bilateral suction but otherwise this structure is essentially identical with the previously described structural arrangments. WHAT WE CLAIM IS:

1. A centrifugal fan having a box-like (i.e.

substantially parallelpipedic) housing, the housing having therein substantially planar internal walls which define the outer boundary of a fluid flow path through the fan, the outer boundary being approximately spiral in shape, and an impeller rotatably mounted within the internal walls, the regions disposed between the box-like housing and the internal walls being of polygonal prism shape and being occupied with sound absorbing material also of polygonal prism shape.

2. A centrifugal fan according to claim 1, wherein part of the length of said boundary coincides with a portion of the box-like housing so that no sound absorbing material lies outside said part of the boundary.

3. A centrifugal fan according to claim 2, wherein the outer boundary is defined by six of said internal walls, one of which is supported directly by the housing so that no sound absorbing material lies outside said one wall.

4. A centrifugal fan according to any one of the preceding claims, wherein each internal wall is formed by a layer of foil or fabric backed, on the side of the wall remote from the flow path, by a perforated sheet.

5. A centrifugal fan according to claim 1, substantially as herein particularly described with reference to Figures 1 to 3, or as modified by Figure 4 of the accompanying drawings.