CENTRIFUGAL FAN OR PUMP

Abstract: Apparatus for damping low frequency sound waves such as infrasound in centrifugal fansor pump apparatus. The apparatus is composed of one or more solely sound wave mediating channels (5, 10, 11) which connect the fans etc. housing (1) and conventional flow channel (3) or channels. The damping of the sound waves from the fan etc. is achieved through interference at the coupling locations between sound waves in the flow channels (3) and the solely sound wave transmitting channels (5, 10, 11). Because the extra channels (5, 10, 11) are solely sound wave transmitting suppression of disturbances in the flow and consequent instabilities with decreased efficiency and increased noise is achieved. The extra channels (5, 10, 11) can be flow channels of conventional type, but with one or more membranes (6, 7) or other sound wave transmitting organ mounted in the channels (5, 10, 11). Those membranes (6, 7) can advantageously be mounted in the openings of the extra channels (5, 10, 11) in order to be fitted to the conventional flow volumes form.

Description: Centrifugal fan or pump The present invention concerns a centrifugal fan or pump. It is known that fans and pumps of this type generate low frequency noise where the frequency has a certain connection with the rotation rate of the fan- or pumpwheel. The'noise is caused by uneveness in the wheels ' rotation symmetry, etc, arid is a long known environmental problem in both inter¬ nal and external environments. Because of the very low frequency of the noise, usually between 5 and 50 Hz, it cannot be shielded off or absor¬ bed with known techniques in a practical way. An attempt has been made to solve that problem according to the intro- ductory part of patent claim 1. Here two or more flow transmitting out¬ let channels, have been connected to the fans etc. housing. These have been connected to a common outlet channel. Then inteference has been obtained either through connection of the channels to the housing at different points or through construction of the channels with different lengths. In the first case there is good possibility of damping sound wave in a . narrow frequency interval, but simultaneously allowing noise in other frequency intervals because of generation of instabilities in the flow. In addition the fan≤ etc. efficiency is significantly reduced, for which reason the fan etc. must be dimensioned for increased effect, which means hat the intended reduction of the noise level can even be replaced by an increase. In .the second case there occurs in principle the same problem as in the first case but possibly not quite so pronounced, when the channels can come from a common outlet from the housing. With a solution of this type a very stable rotation rate for the fan etc. is required,unaffected by variations in the load, which is impossible to obtain without expen¬ sive control apparatus. Further, this solution is' very bulky as the channel lengths are determined by the wavelength. In the frequency in- terval 5 - 50 Hz that means that fan.channels of about 35.m are required. For liquid pumps that length is increased significantly because of the higher speed of sound. According to the present invention the problem is solved in a way w is given in the characterization part of patent claim 1. By making the extra channel or channels solely sound wave transmitt the instabilities which occur in previous constructions are avoided The faπ-5 etc. efficiency can therefore be maintained. This means th solving the problemsby meansof different channel lengths and conseq problem is unnecessary. An appropriate form of the invention is described in detail below r ferring to the attached drawings. Fig. 1 shows a schematic section of a centrifugal fan with channel system according to one construction. Fig. 2 and 3 show two different constructions of the channel attach in somewhat larger scale. Fig. 4 show a schematic side view of an alternative construction in ded for damping of noise in the entrance channel. The housing of the centrifugal fan, which is indicated by 1, is of ventional type. The outlet 2 of the housing is connected to an outl channel 3 . The fan wheel 4, seen from the inlet side, is mounted for rotation the fan housing 1 in the usual manner, and appropriately driven fro the opposite side of the fan from the inlet. In the example shown an extra channel 5 is connected to the housing essentially diametrically opposed to the fan outlet 2 . This channe is connected to the outlet channel 3 at its opposite end from the housing. The distance from the housing 1 to this connection is esse ially equal for the two channels3 and 5 . Sound waves which are generated in the area of the fan wheel 4 thus reach the connections of channels 3 and 5 simultaneously but with opposite phase relativ to each other. In this way these sound waves can be extinguished in a well-known manner. The essentially new in the present invention is that only the conven¬ tional outlet channel 3 is flow carrying while the extra channel 5 is constructed so as to transmit solely sound waves. For that purpose a membrane 6 is mounted between the extra channel 5 and the housing 1;. , in the first instance at this connection. This mem¬ brane is shaped to conform to the interior form of the housing 1 so that the least possible disturbance of the flow can be achieved as is best shown in Fig. 2 and 3 . The membrane 6 is appropriately made of an elastic such as rubber,plastic or something similar. The material and characteristics can be chosen to conform to the type of flow and pressure as well as to the sound frequency. Corresponding factors are considered when the invention is applied to a centrifugal pump. A corresponding membrane 7 can advantageously be attached to the connection between the two channels 3 and 5 . It is then located such that the evenest possible surface is obtained between the channel 3 and the connection position. From Fig.3 it is further clear that the channel 5 can have smaller cross sectional area than the channel 3 by constructing the connection piece of the channel 5 with an impedance match 8 . In this case such an im¬ pedance match must be made .at each end of the channel 5 . In the example of Fig. 4 the noise reduction steps have been concen- trated on the fan inlet. To the -inlet channel, disignated by 9, an extra channel 10 has therefore been connected, which is shown with full ... " linies. This channel corresponds to the previous channel 5 and is there¬ fore equipped with membranes corresponding to the membranes 6 and 7 at its connections. Alternatively an extra channel 11 can be connected between the fan outlet 3 and inlet 9 channels, which is shown by point-dashed linies. Also in this construction membranes are mounted at the connecting ends of the channel 11. Even if the membranes discribed above provide the most advantageous 95 solution it is also possible according to the invention to impede fl in the extra channel or channels " by inserting in this pistons with limited mobility and spring loaded against the fan pressure. In cert application , a fluid or a liquid loch can give a corresponding effect That which is discribed in connection with centrifugal fan can be 100 applied with appropriate modification to centrifugal liquid pumps. P

Claims: Patent claims 1. Centrifugal fan or pump, in which extinguishment of disturbing sound waves is intended to be achieved through interference by including, in addition to conventional inlet and outlet channels (3), one or more extra channels (5,10,11) which directly or indirectly exit from the fanέ etc. housing.(1) and which are constructed to transmit sound waves generated in the housing which are intended to interfere with the di¬ rectly radiated sound waves from the conventional channels (3) via the housing volume, c h a r a c t e r i z e d b y t h a t the one or the extra channels (5,10,11) is solely sound wave transmitting. 2. Fan or pump according to claim 1, c h a r a c t e r i z e d b y t h a t a flow inhibiting but sound wave transmitting organ is mounted in or in conjunction with the extra channel (5,10,11) or each of the extra channels (5,10,11). 3. Fan or pump according to claim 2, c h a r a c t e r i z e d b y t h a t said organ is one or more membranes (6,7) in series. 4. Fan or pump according to claim 3, c h a r a c t e r i z e d b y t h a t the membrane (6,7) or membranes are mounted essentially attach¬ ed to the inner form of.the housing (1) or to the inner form of the inlet or outlet (1,3) respectively. 5. Fan or pump according to claim 2, c h a r a c t e r i z e d b y t h a t the said organ consists of a piston or corresponding construc¬ tion. 6. Fan or pump according to claim 2, c h a r a c t e r i z e d b y t h a t the said organ consists of a fluid in a liquid lock. 7. Fan or pump according to the above claims, c h a r a c t e r i z e d b y t h a t the total connection area of the extra channels (5,10,11) is essentially the same or larger than the area of one of the fan channels (3,9). MPI 8. Fan or pump according to the above claims and including only one ex channel (5) c h a r a c t e r i z e d b y t h a t that channel i connected to the housing (1) essentially diametrically opposed to flow outlet (2). 9. Fan or pump according to claims 1-7 and including two or more extra channels (5) c h a r a c t e r i z e d b y t h a t these channel are connected to the housing (1) with essentially even angular dist bution around it. 10. Fan or pump according to claim 9, c h a r a c t e r i z e d b y t h a t the extra channels (5) are connected to each other. 11. Fan or pump according to claim 1-9, c h a r a c t e r i z e d b y t h a t the one or .more extra channels (5,10,11) are connected to inlet and outlet channels (2,9) in a collection volume. 12. Fan. or pump according to claims 1-9, c h a r a c t e r i z e d b t h a t the one or more extra channels (5,10,11) and the inlet or let channels (2,9) are exposed to the same area in which the distu bance is to be reduced.

Inventors: HELLSTROEM P
Applicants: HELLSTROEM P
Application year: 1980
Main classification: F04D29
IPC codes: F04D29