Abstract:
A conventional machine package faces problems concerned with costs, weight, etc. and also a problem of antinomy that an attempt to enhance the noise reducing performance increases the airflow resistance and then deteriorates the cooling performance. Providing a sound absorbing structure with the heat radiation performance ensured within a practical range and then with considerably improved noise reducing effect achieves a low-noise machine package with a downsized casing and a reduced cooling fan power. There is provided a sound absorbing structure having a plurality of polyester fiber sound absorbing cylinders each formed into a circular-cylindrical shape, formed of a base material of polyester fiber whose surface is combined and covered with polymer nonwoven fabric of polyester fiber or the like, and arranged at a support member in at least either of a suction port and an exhaust port in such a manner that long axes of the sound absorbing cylinders intersect substantially perpendicularly with a flow direction of air flowing through the suction port or the exhaust port. This makes it possible to reduce noise while controlling the airflow resistance minimum, thus achieving downsizing of a cooling fan and reduction of a cooling fan power.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese application serial No. 2007-21594 filed on Jan. 31, 2007, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a machine package having a sound absorbing structure for reducing noise radiated from an opening provided for cooling down heat generated from a machine, such as an industrial machine, having a suction port and an exhaust port. 
     Most typically used as a conventional sound absorbing structure for an opening are: a lined duct using a porous material such as glass wool, a sprit type, a cell-type, etc., a basic form of each of which is a duct lined with a sound absorbing member. 
     The duct lined with a sound absorbing member faces a decrease in the amount of sound reduction in a high sound area where the wavelength of sound is smaller than the diameter or short side of a cross section thereof since a sound wave travels in a beam-like form. Often used to prevent this defect as much as possible are: a cell type as a parallel type of thin straight paths formed by dividing the duct cross section in a grid form by a sound absorbing member; and a splitter type sound absorbing duct dividing the flow path in parallel by a tabular sound absorbing member. 
     However, even with these types, the amount of sound reduction is controlled by sound absorbing properties of the sound absorbing member and the length of the duct subjected to sound absorbing processing. Thus, to provide effect for high sounds and further increase the sound absorption coefficient in a low sound area by providing the split type, the cell type, or the like, it is required to increase the thickness of the sound absorbing member, thus resulting in an increase in the fluid resistance. The conventional sound absorbing structure of a sound absorbing duct type requires space for noise in a band of 500 to 2 kHz which finds the widest applications, and thus faces problems concerned with costs, weight, etc. and also a problem of antinomy that an attempt to enhance the noise reduction performance increases the airflow resistance and then deteriorates the cooling performance. 
     Additionally, it is also possible to achieve noise reduction by installing a louver or forming the duct into a maze shape, although it suffers from the same problems as described above. 
     As their solution, Japanese Patent Application Laid-Open Publication No. H9-126666 describes a sound reducing assembly having substantially circular-cylindrical sound absorbing members formed of a sound absorbing material and also arranged in at least two rows across an air inlet. 
     Japanese Patent Application Laid-Open Publication No. 2000-87725 describes an acoustic damping material formed with a sound absorbing member and a acoustic reflection member provided on one side of this sound absorbing member and having a cross-sectionally concave-shaped reflection surface so that sound transmitted through and incident on the sound absorbing member is absorbed while being reflected by the reflection surface to elongate the sound absorbing distance in the sound absorbing member and then emitted to the side where sound S has arrived. 
     Japanese Patent Application Laid-Open Publication No. H9-26177 describes an air duct having a sound absorbing function that, by fitting a sound absorbing member using ion exchange fiber to a gas flow path, utilizes sound absorbing effect and gas pollutant removing operation to purify gas. Japanese Patent Application Laid-Open Publication No. 2002-266756 describes a sound absorber which has, inserted in a rectangular-cylindrical casing, a circular-cylindrical sound absorbing element having a pipe of an inorganic fiber whose front and rear surfaces are coated with an anti-scattering material of breathable inorganic fiber, organic fiber, glass cloth, nonwoven fabric, or the like. 
     The conventional duct lined with a sound absorbing member or, as its application, the cell-type and the splitter-type have many problems in practical aspects such as sound reducing performance, space, weight, costs, etc., since an attempt to increase the amount of sound reduction for a band of 500 to 2 kHz in highest need of sound reduction requires narrowing down the duct length, the thickness of the lined sound absorbing member, and the opening, which results in an increase in the airflow resistance. 
     The configuration described in Japanese Patent Application Laid-Open Publications No. H9-126666 and 2000-87725 has the cylindrical sound absorbing member arranged in such a manner as to intersect with airflow, and thus provides the effect of reducing the airflow resistance, but did not give sufficient consideration to sound absorbing properties concerning the material of the sound absorbing member with respect to the sound absorbing effect. 
     Further, the configuration described in Japanese Patent Application Laid-Open Publications No. H9-26177 and 2002-266756 has the sound absorbing member arranged in parallel to airflow and thus has the same problem as the aforementioned cell-type and splitter-type have, and further does not give sufficient consideration to sound absorbing properties concerning the material of the sound absorbing member with respect to the sound absorbing effect. 
     SUMMARY OF THE INVENTION 
     To address the problem described above, a low-noise package according to one aspect of the present invention includes a sound absorbing structure having a plurality of polyester fiber sound absorbing cylinders formed into a circular-cylindrical shape and arranged at a support member in at least either of a suction port and an exhaust port in such a manner that long axes of the sound absorbing cylinders intersect substantially perpendicularly with a flow direction of air flowing through the suction port or the exhaust port. 
     The polyester fiber sound absorbing cylinder may be a sound absorbing body formed of a base material of polyester fiber whose surface is circular-cylindrically wound and combined with polymer nonwoven fabric. 
     A structure may be provided which has a solid shaft or a hollow shaft penetrated through a circular-cylindrical center of the polyester fiber sound absorbing cylinder. 
     On the polymer nonwoven fabric, a metallic or resin-based network structure or perforated structure may be provided. 
     The support member may be a polyester fiber sound absorbing member. 
     The polyester fiber sound absorbing member may be provided as a sound absorbing structure formed of a base material of polyester fiber whose surface is combined with polymer nonwoven fabric. 
     The base material may be glass wool or flexible urethane foam. 
     The sound-absorbing structure may be in a freely detachable cassette form. 
     The support member may also be provided at a region other than both ends of the polyester fiber sound absorbing cylinders. 
     A plurality of semicircular notches may be provided in the support member to provide a sound absorbing structure in which both ends of the polyester fiber sound absorbing cylinders can be fitted in the notches, and the support member and the both ends of the polyester fiber sound absorbing cylinders may be laid alternately to form an array. 
     According to the present invention, noise reduction can be achieved while reducing the airflow resistance, thus making it possible to minimize a decrease in the amount of cooled air and improve the package heat radiation performance. Moreover, since enough heat radiation performance can be provided, a cooling fan can be downsized, which makes it possible to reduce noise generated from the cooling fan, reduce the fan power, and make the sound-absorbing structure even smaller, thus permitting achieving downsizing of the package. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, objects and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings wherein: 
         FIG. 1A  is a side view of a sound absorbing structure according to a first embodiment; 
         FIG. 1B  is a sectional view taken along a line X-X in  FIG. 1A ; 
         FIG. 2  is a sectional view showing the structure of a sound absorbing cylinder according to the first embodiment along one diameter thereof; 
         FIG. 3  is a bird&#39;s-eye view showing a sound absorbing structure according to a second embodiment; 
         FIG. 4  is a bird&#39;s-eye view of a low-noise package having fixed therein the sound absorbing structure according to the first embodiment or the second embodiment; 
         FIG. 5  is a bird&#39;s-eye view of a low-noise package having the sound absorbing structure according to the first embodiment or the second embodiment in a cassette form; 
         FIG. 6  is a sectional view of an experimental device for checking sound absorbing effect provided by the sound absorbing structure according to the first embodiment; 
         FIG. 7  is a comparative diagram indicating the sound absorbing effect provided by the sound absorbing structure according to the first embodiment; 
         FIG. 8  is a comparative diagram indicating sound absorbing properties in a case where polyester nonwoven fabric is combined with the surface of a base material of polyester fiber; 
         FIG. 9  is a comparative diagram indicating sound absorbing effect provided by the sound absorbing structure according to the second embodiment; 
         FIG. 10  is a sectional view of an experimental device for checking sound absorbing effect provided by a conventional structure; 
         FIG. 11  is a comparative diagram indicating the sound absorbing effect provided by the conventional structure; 
         FIG. 12  is a configuration diagram showing the structure of the low-noise package provided with the sound absorbing structure according to the first embodiment or the second embodiment; 
         FIG. 13  is a comparative diagram comparing sound absorbing effect between the low-noise package of the second embodiment and a conventional package on actual machines; and 
         FIG. 14  is a bird&#39;s-eye view showing a sound absorbing cylinder support structure formed with a laminated support member in the sound absorbing structure according to the first embodiment or the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the embodiments of the present invention will be described, with reference to the accompanying drawings. 
       FIG. 12  is a longitudinal sectional view showing the schematic structure of an air compressor unit to which low-noise packages of the embodiments are applied. In  FIG. 12 , the air compressor unit  1  is fixed on a base  2   a  in a casing  2  forming a contour or framework of the air compressor unit  1 . The air compressor unit  1  includes: a well-known motor  3  of the type that is fixed to the support member  2   b  supported by support poles  14  in the casing  2 ; an outer peripheral driving type scroll compressor  4  that is fixed to the support member  2   b  in the same manner and that generates compressed air; a cooling fan  5  that attracts external air into the casing  2  to air-cool the motor  3  and the outer peripheral driving type scroll compressor  4 ; a heat exchanger  6  that cools down the compressed air from the outer peripheral driving type scroll compressor  4  to an adequate temperature; and a dryer  7  that dehumidifies the compressed air from the heat exchanger  6  to an adequate humidity. 
     The outer peripheral driving type scroll compressor  4  includes a V pulley  8 . In conjunction with rotational driving of the motor  3 , to the outer peripheral driving type scroll compressor  4 , a rotative power is transmitted via a V pulley  9  provided on a side (right side in  FIG. 12 ) of one of motor rotation axes  3   a  of the motor  3  and a V belt  10  mounted on these V pulleys  8  and  9 . 
     The cooling fan  5  has a rotation axis thereof coupled to a side (left side in  FIG. 12 ) of the other of the motor rotation axis  3   a , and is driven in conjunction with driving of the motor  3 . Then driving of this cooling fan  5 , as shown by an arrow A in  FIG. 12 , flows external air into the casing  2  from a suction port  11 A that has arranged therein sound absorbing cylinders  40  to be described later, and exhausts it via the cooling fan  5  and a duct  12  from an exhaust port  13 A that has arranged therein sound absorbing cylinders  40  to be described later. 
     Consequently, the motor  3 , the outer peripheral driving type scroll compressor  4 , etc. in the casing  2  are cooled down with the external air. Moreover, simultaneously therewith, external air from the suction port  11 A is discharged to the heat exchanger  6  provided in the duct  12  via the cooling fan  5  and then exhausted from the exhaust port  13 A. Consequently, the heat exchanger  6  cools down the compressed air from the outer peripheral driving type scroll compressor  4  down to an adequate temperature. 
     The dryer  7  includes a compressor, a condenser, a capillary tube, and an evaporator, and thereby dehumidifies the compressed air from the heat exchanger  6  to an adequate humidity. Moreover, at this point, since the dryer  7  is provided with a fan  7 C that air-cools the condenser and the evaporator, the air is exhausted from an exhaust port  13 B as shown by arrow C in  FIG. 12 . 
       FIG. 4  is a bird&#39;s-eye view of the structure of the air compressor unit  1  shown in this embodiment as viewed diagonally from above the right front side. In the air compressor unit  1 , the outer peripheral driving type scroll compressor  4 , the cooling fan  5 , and the motor  3  serve as main sources of vibration and noise. In this embodiment, a sound-absorbing structure is formed which has a plurality of air-absorbing cylinders  40  arranged at the suction port  11 A and the exhaust port  13 A in parallel to the surfaces of these ports, that is, in a manner such that the longer axes of the sound absorbing cylinders  40  intersect substantially perpendicularly with the air flow direction. 
     Here, the sound-absorbing structure will be described in more detail.  FIG. 1A  is a side view showing the side of the sound-absorbing structure, and  FIG. 1B  is a sectional view taken along a line X-X in  FIG. 1A . Intervals W 1  and W 2  between the sound absorbing cylinders  40  are determined in view of the flow resistance so that they are in a range of 50% to 150% of a diameter D of the sound absorbing cylinder  40 . Moreover, as described above, the sound-absorbing structure is formed which has a plurality of sound absorbing cylinders  40  arranged in a manner such that the longer axes L of the sound absorbing cylinders  40  intersect substantially perpendicularly with the air flow direction M, as shown in the figure. Providing this structure achieves a structure capable of effectively reducing noise from the suction port  11 A and the exhaust port  13 A without increasing the flow resistance to cooling air A and B. In this embodiment, the sound absorbing cylinders  40  are arrayed in zigzag alignment, although they may be arrayed in alignment other than the zigzag alignment. 
     Next, the structure of the sound absorbing cylinder  40  will be described.  FIG. 2  is a sectional view showing the structure of the sound absorbing cylinder  40  along one diameter thereof. As shown in  FIG. 2 , the sound absorbing cylinder  40  is structured of a base material  40   a  of polyester fiber formed into a circular-cylindrical shape whose surface is wound, combined, and covered with polymer nonwoven fabric  40   b  of polyester fiber or the like. For example, the surface of a base material of polyester fiber, having a thickness of 30 mm and a bulk density of 44 kg/m 3 , is heat-sealed and combined with polyester nonwoven fiber by powder-like hot melt to thereby form a sound absorbing cylinder. 
     To check the effect of this embodiment, under the condition that a speaker S is placed in an experimental box B as shown in  FIG. 6  and pink noise is generated, sound pressure levels for ⅓ Oct. Band central frequency under the presence and absence of a sound-absorbing structure formed of sound absorbing cylinders A are measured with a microphone M for comparison.  FIG. 7  shows results of this comparison. CASE  1  refers to a case where the sound-absorbing structure is completely absent. CASE  2  refers to a case where sound absorbing cylinders each formed of a base material of polyester fiber (35 kg/m 3 ) whose surface is combined with polyester fiber nonwoven fabric are installed. CASE  3  refers to a case where sound-absorbing cylinders each formed of only a base material of polyester fiber (35 kg/m 3 ) whose surface is not combined with polyester fiber nonwoven fabric are installed. It can be understood that even in CASE  3 , as compared to CASE  1 , noise is more reduced in a wide band of 500 to 4 kHz centered at 1.25 kHz, and that noise is even more considerably reduced in CASE  2 . 
     This is attributable to an improvement in sound-absorbing properties as a result of combining the surface of the base material of polyester fiber with the polyester nonwoven fabric. The ground for this is shown in  FIG. 8 .  FIG. 8  is a diagram making a comparison between a sound absorbing cylinder (∘ marks in the figure) formed of a base material only and a sound absorbing cylinder (● marks in the figure) formed of a base material (of polyester fiber having a thickness of 30 mm and a density of 44 kg/m s ) whose surface is heat-sealed with and combined with polyester nonwoven fabric by powder-like hot melt, where a horizontal axis represents the frequency and the vertical axis represents the normal incidence sound absorption coefficient. As is obvious from this figure, as compared to the sound absorbing cylinder formed of a base material only, combining the surface of the base material with the nonwoven fabric by using the heat-sealing powder is proved to dramatically improve the sound absorbing properties. 
     On the other hand, under the condition that, as shown in  FIG. 10 , in the experimental box B described above, a conventional structure having glass wools G of 32 kg/m 3  machined into a size of 60 mm×160 mm and arranged at intervals of 40 mm is provided, a speaker S is placed, and pink noise is generated, sound pressure levels for ⅓ Oct. Band central frequency under the presence and absence of the sound absorbing structure formed of glass wools G is measured with a microphone M, the results of which are shown in  FIG. 11 . As shown in  FIG. 11 , in CASE  4  referring to the conventional structure, as compared to CASE  1  where the sound absorbing structure is completely absent, sound absorbing effect is observed, but with much more unfavorable results than those of this embodiment especially in a high frequency band. Thus, this embodiment provides a structure that is not only more excellent in the sound reduction performance but also more advantageous in the flow resistance. 
     As described above, since not only the base material of polyester fiber is provided, but also the polymer nonwoven fabric of polyester fiber or the like is combined with the surface of the base material, the sound absorbing performance dramatically improves, thus providing great sound absorbing effect. Moreover, the shape is circular-cylindrical, which facilitates air circulation and, also due to a short passage, the airflow resistance considerably improves. This solves a problem of antinomy between the sound absorbing effect and the airflow resistance which a conventional air absorbing duct faces. 
     Since the sound absorbing cylinder  40  is structured of the base material  40   a  of polyester fiber formed into a circular-cylindrical shape whose surface is covered with the polymer nonwoven fabric  40   b  of polyester fiber or the like, the sound absorbing cylinder  40  may be inferior in strength, thus probably failing to maintain its shape when an external force acts thereon. Thus, the sound absorbing cylinder  40  may be structured such that, as a core material of the sound absorbing cylinder  40 , a solid or hollow shaft for reinforcing fitting penetrates therethrough. 
     Moreover, to protect the surface of the sound absorbing cylinder  40 , a metallic or resin-based network structure or perforated structure may be provided on the polymer nonwoven fabric  40   b  on the surface of the sound absorbing cylinder  40 . 
     Instead of the base material  40   a  of polyester fiber, a base material of glass wool or flexible urethane foam also fulfills the same function. 
     Further, as shown in  FIG. 1 , to insert the sound absorbing cylinders  40  in support members  31  and  32  to form an array of the sound absorbing cylinders  40 , one ends of the sound absorbing cylinders  40  first need to be inserted in the support member  31  and then the other ends thereof need to be inserted in holes of the support member  32 . If the number of sound absorbing cylinders  40  forming the array is small, it is possible in some way to insert the other ends of the sound absorbing cylinders  40  in the support member  32 . However, as the number of sound absorbing cylinders  40  increases, it may become more difficult to insert the sound absorbing cylinders  40  in the holes of the support member  32 . 
     Thus, as shown in  FIG. 14 , an array of the sound absorbing cylinders  40  and layered support members  45  as members fixing the sound absorbing cylinders  40  may form a sound-absorbing structure. At portions of this layered support member  45  where the sound absorbing cylinders  40  are to be fitted, semicircular notches are provided. In the plurality of semicircular notches of the layered support member  45 , the sound absorbing cylinders  40  are respectively fitted. Thereafter, a different layered support member  45  is fitted in such a manner as to sandwich the fitted sound absorbing cylinders  40 . By repeating this, the array is formed. Forming the array in this manner can solve the difficulties in fitting the sound absorbing cylinders  40  due to an increase in the number of sound absorbing cylinders  40 , thus considerably improving the operability. 
     As a method of fitting the sound absorbing cylinders  40  to the air compressor unit  1 , as shown in  FIG. 4 , they may be fixed directly to the suction port  11 A and the exhaust port  13 A. For easier maintenance, as shown in  FIG. 5 , members, like cassettes  43  and  44 , including a combination of sound absorbing cylinders  40 , may be provided in a freely detachable cassette form. Providing the cassette structure has the advantage that it can be easily fitted as a module for noise reduction. 
     Next, the second embodiment of the present invention will be described. In this embodiment, in addition to sound absorbing cylinders each formed of a base material of polyester fiber whose surface is combined with polyester-fiber-based nonwoven fabric, a support member supporting this sound absorbing cylinder is also structured to have sound absorbing effect. Specifically, as shown in  FIG. 3 , a polyester fiber sound absorbing member formed of a base material of polyester fiber whose surface is combined with polymer nonwoven fabric of polyester fiber or the like is provided with holes for supporting the sound absorbing cylinders and provided at the both ends of a package opening so that the sound absorbing cylinders are inserted therein. This structure achieves overall noise reduction. Other structure of an air compressor unit  1  to which a low-noise package of this embodiment is applied is the same as that of  FIG. 12  and thus its description will be omitted here. 
     The structure for supporting the absorbing cylinders  40  is achieved in the following manner. As shown in  FIG. 3 , in the sound absorbing members  41  and  42  arranged at the both ends of the sound absorbing cylinders  40 , holes  41   c  and  42   c  for supporting the sound absorbing cylinders  40  are provided, and then the both ends of the sound absorbing cylinders  40  are respectively inserted in the holes  41   c  and  42   c  of the sound absorbing members  41  and  42 . The sound absorbing members  41  and  42  are respectively formed of base materials  41   a  and  42   a  of polyester fiber whose surfaces are respectively combined with polymer nonwoven fabric  41   b  and  42   b  of polyester fiber or the like. 
     Here, sound absorbing effect provided by the sound absorbing members  41  and  42  will be described, referring to  FIG. 9 . In  FIG. 9 , CASE  1  refers to a case where the sound-absorbing structure is completely absent. CASE  2  refers to a case where only sound absorbing cylinders each formed of a base material of polyester fiber (35 kg/m 3 ) whose surface is combined with polyester fiber nonwoven fabric are installed. CASE  3  refers to a case where sound absorbing cylinders each formed of a base material of polyester fiber (35 kg/m 3 ) whose surface is combined with polyester fiber nonwoven fabric are installed together with the aforementioned polyester fiber sound absorbing members (of 35 kg/m 3 , and 25 mm in thickness). As is obvious from  FIG. 9 , it is proved that providing this structure, with sound absorbing effect provided by the sound absorbing members  41  and  42  in addition to the sound reducing effect provided by the sound absorbing cylinders  40 , can achieve more effective sound reducing effect especially in the range of 630 Hz to 1 KHz than is achieved by the first embodiment described above. 
     Further, the amounts of sound reduction achieved by a conventional structure combining together a sound absorbing duct using flexible urethane foam for a suction port and an exhaust port and sound absorbing processing in the package and by the structure of this embodiment adopting the sound absorbing cylinders  40  and the sound absorbing members  41  and  42  are checked on actual machines, results of which are shown in  FIG. 13 . It is proved that the package to which this embodiment is applied provides greater sound reducing effect even on the actual machine than the conventional structure. Needless to say, the package can also keep down temperatures of the different parts in the package to the same degrees as are achieved by the conventional structure. 
     Moreover, instead of the base materials  41   a  and  42   a  of polyester fiber, base materials of glass wool or flexible urethane foam also fulfill the same function. 
     To more stably support the sound absorbing cylinders  40 , the sound absorbing cylinders  40  can be supported at a portion other than the both ends of the sound absorbing cylinders  40 . Further, needless to say, the noise reduction performance can also be enhanced by disposing a polyester fiber sound absorbing member on a surface other than the surfaces of the package supporting the sound absorbing cylinders. 
     Also in this embodiment, forming an array of sound absorbing cylinders  40  with sound absorbing members of a layered structure as described in the first embodiment can solve the difficulties in fitting due to an increase in the number of sound absorbing cylinders  40 , thus considerably improving the operability. 
     As a method of fitting the sound absorbing cylinders  40  to the air compressor unit  1  in this embodiment, as described in the first embodiment, the sound absorbing cylinders  40  may be fixed directly to the suction port  11 A and the exhaust port  13 A, or may be provided in a freely detachable cassette form for easier maintenance. Also in this embodiment, providing the cassette structure has the advantage that it can be easily fitted as a module for noise reduction. 
     The model experiments and the evaluation described above demonstrate excellent performance of a low-noise package of this embodiment that solves the antinomy between the heat radiation performance and the noise reduction performance. The noise reduction performance in particular, as compared to other methods, is excellent, providing great sound absorbing effect in a wider frequency band.