Patent Publication Number: US-6217281-B1

Title: Low-noise fan-filter unit

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a low-noise fan-filter unit used in a clean room, and particularly relates to a fan-filter unit applicable in a clean room or hospital for providing filtered airflow at pre-defined cleanliness and flow quality. The unit performs at a very low noise level. 
     BACKGROUND OF THE INVENTION 
     The fan-filter unit of the invention is an air filtration device incorporating a centrifugal fan and a filter, and provides purified air to the space of a room. Fan-filter units have been widely used in the industry, medical and research facilities. They are especially popular for their characteristics of space-saving, energy-saving and easy installation. However, existing fan-filter units often generate high levels of noise in operation and may psychologically or mentally annoy the personnel within the room where precise and delicate manufacturing processes are being handled. Therefore, there is a need for a clean room fan-filter unit to operate and generate noise at a low level. 
     A fan-filter unit usually incorporates a centrifugal fan. The airflow drawn into the unit is guided by interior air passageways to the filter in the unit. After passing through the filter, the air is propelled into the designated environment, such as a clean room. The construction of the fan-filter unit makes less space in the axial direction of the fan than in the radial direction. The limited space increases the difficulty of incorporating noise reduction constructions into the fan-filter unit. For example, in a conventional fan-filter unit, for the airflow from the inlet to the outlet, the contact area of the airflow to some sound-absorbing materials is so limited that it performs low effectiveness. 
     People have been trying to solve the serious problem of the high noise levels of conventional fan-filter units. An exemplary prior art for reducing the noise of a clean room fan-filter unit is disclosed in U.S. Pat. No. 5,803,721. It provides a fan-filter unit having a perforated plate situated within the airflow path in the fan-filter unit housing such that the perforations diffuse the airflow before it passes through a filter and exits the housing. The uniform airflow provided by the perforated plate produces a reduced noise level. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is therefore to provide a low-noise fan-filter unit and to reduce the noise influence to the environment. 
     To achieve the aforesaid objective, a fan-filter unit according to the present invention includes a housing having at least an air inlet and a coaxial air outlet; a centrifugal fan installed in the housing for drawing air from the air inlet into the housing and propelling it out of the air outlet; a filter attached with or inserted into the housing for removing impurities from the air; and noise reduction means installed along the air passageway between the fan and the filter. The housing includes a front wall, a back wall, a right wall, a left wall and an upper wall. The front, back, right and left walls are in the direction parallel to the axis of the fan. The air inlet is on the upper wall. The centrifugal fan is installed in the interior of the housing and positioned under the air inlet. The noise reduction means are installed in the housing along the air passageway between the fan and the filter. 
     The noise reduction means include at least a tortuous air passageway formed by a flow-guiding plate and a noise-reduction plate, and sound-absorbing materials furnished on walls of the air passageway. The flow-guiding plate is positioned under the fan and extends in the radial direction of the fan. The noise-reduction plate, which can be in one piece or two separate pieces, attaches to the left and right walls of the housing, and extends to the front and the back walls. The position of the noise-reduction plate in the axial direction of the fan is between the flow-guiding plate and the filter. Therefore, the tortuous air passageway is formed among the flow-guiding plate, the noise-reduction plate and the filter. The flow-guiding plate first guides the discharged airflow moving in the radial direction of the fan. The airflow turns into direction parallel to the axial direction of the fan after reaching the walls of the housing. Then, the airflow is further guided by the noise-reduction plate extending inward in the general direction parallel to the radial direction of the fan. And finally, the airflow passes through the opening of the noise-reduction plate, diffuses into the cavity under the noise-reduction plate and exhausts through the filter into a clean environment. Hence, the airflow turns several times in the tortuous air passageway before passing through the filter. The tortuous and extended air passageway, together with the sound-absorbing materials furnished along the air passageway, increases the contact area of the airflow to the sound-absorbing materials, and enhances noise reduction. 
     An exemplary embodiment of the sound-absorbing materials in the noise reduction means according to the present invention is foam pads furnished on the surface of the air passageway. The foam pads have a wavy or wedged-like surface to absorb noise in the air passageway. 
     Another exemplary embodiment of the sound-absorbing materials in the noise reduction means according to the present invention is perforated plates mounted along the surface of the air passageway and separated by a clearance with aforementioned plates and walls. Acoustic waves penetrating into the clearance would reflect therein and dissipate into heat, and, thereby, the noise is reduced. Meanwhile, the surfaces of the perforated plates are preferably smooth so that friction losses can be minimized. 
     For an embodiment of the present invention, in a sectional view taken along a plane parallel to the axis of the fan, the height of the tortuous air passageway should be the same as or close the height at the outlet of the fan, but the opening in or between the noise-reduction plate should be at least twice of the width of the aforementioned height, so that the pressure drop in the air passageway can be minimized. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The above objectives and advantages of the present invention will become more apparent from the following detailed descriptions of preferred embodiments with reference to the attached drawings which are given by way in which: of illustration only, and thus are not limitative of the present invention, and 
     FIG. 1 is a sectional view of the first embodiment of the present invention showing the construction of a fan-filter unit incorporating a housing, a fan, a filter and noise reduction means; 
     FIG. 2A is a sectional view (A—A) of FIG. 1, showing first exemplary embodiment of the flow-guiding plate; 
     FIG. 2B is a sectional view (A—A) of FIG. 1., showing another exemplary embodiment of the flow-guiding plate; 
     FIG. 3 is a sectional view of the second embodiment of the present invention based on FIG. 1; 
     FIG. 4 is a sectional view of the third embodiment of the present invention further based on FIG. 1; 
     FIG. 5 is a sectional view of the fourth embodiment of the present invention, showing an example of using perforated plates as sound-absorbing materials; 
     FIG. 6 is a sectional view of the fifth embodiment of the present invention based on FIG. 5; 
     FIG. 7 is a sectional view of the sixth embodiment of the present invention further based on FIG. 5; 
     FIG. 8 is a sectional view of the seventh embodiment of the present invention, showing another exemplary embodiment of the noise-reduction plate; and 
     FIG. 9 is a sectional view of the eighth embodiment of the present invention, showing further another exemplary embodiment of the noise-reduction plate. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The fan-filter unit according to the present invention substantially includes a rectangular housing, a centrifugal fan installed inside the housing for drawing air into the housing and propelling it out, a filter for removing impurities from the air, and noise reduction means for reducing noise. The noise reduction means further includes several parting plates installed in the interior of the housing and positioned between the fan and the filter. The parting plates incorporate with the housing to form at least a tortuous air passageway which U-turns the airflow at least two times. The tortuous and extended air passageway increases the contact area of the airflow with the sound-absorbing materials, and, thereby, enhances the noise reduction. 
     Referring to FIG. 1, a fan-filter unit according to the present invention includes a rectangular hollow housing  10  which has at least an air inlet  11  and a coaxial air outlet  12 . A centrifugal fan  20  with blades  22  is installed in the housing  10  and draws air passing through the air inlet  11  into the housing  10 , forces the air through a filter  30  and propels it out of the air outlet  12  to a clean environment (such as a clean room). The housing  10  includes a front wall  13   a,  a back wall  13   b,  a left wall  14   a,  a right wall  14   b  and an upper wall  15 . The front, back, left and right walls are in the direction parallel to the axis of the fan  20 . The air inlet  11  is on the upper wall  15 . The centrifugal fan  20  is attached inside the housing  10  and positioned under the air inlet  11 . The filter  30  is attached with or inserted in the housing  10 . And, the noise reduction means are installed inside the housing  10  creating the air passageway between the fan  20  and the filter  30 . A bell mouth  21  can be formed as the air inlet  11  for guiding the air flowing into the housing  10 . The filter  30  can be generally made of any filtering media or an air-penetrating meshy material. To enhance the filtration performance, specific filters such as HEPA, ULPA or electrostatic filters, can be used. 
     The noise reduction means include at least a tortuous air passageway  70  formed by the flow-guiding plate  40 , the noise-reduction plate unit  50  having first plate portion  501 , second plate portion  502 , and sound-absorbing materials  80 ,  81  furnished on walls of the air passageway  70 . 
     To achieve the noise reduction function, the noise reduction means can be realized as described in the following embodiments. 
     First embodiment 
     As shown in FIG. 2A, the flow-guiding plate  40  is positioned under the fan  20 , extended in the housing  10  parallel to the radial direction of the fan  20  and between the front and the back walls  13   a,    13   b.  The noise-reduction plate unit  50  having first plate portion  501  and second plate portion  502  respective attached to the left and right walls  14   a,    14   b  of the housing  10 , extended to the front and the back walls  13   a,    13   b  and in parallel with the radial direction of the fan  20 . The noise-reduction plate unit  50  has an opening between the first plate portion  501  and second plate portion  502 . The position of the noise-reduction plate unit  50  in the axial direction of the fan  20  is between the flow-guiding plate  40  and the filter  30 . Therefore, the tortuous air passageway  70  are formed by the flow-guiding and noise-reduction plate unit  40 ,  50 , incorporating the front, back, left and right walls  13   a,    13   b,    14   a,  and  14   b  of the housing  10 . 
     In the other exemplary embodiment of the flow-guiding plate  40   a  is illustrated in FIG. 2B, wherein the flow-guiding plate  40   a  doesn&#39;t contacts any walls of the housing  10 , so the air may flow through the flow-guiding plate  40   a  in a direction radial of the fan  20 . 
     Where the first plate portion  501  and second plate portion  502  are attached t 0  the left and right walls  14   a,    14   b,  smooth turning of the airflow, by using rounded corner  51  is preferred. 
     The sound-absorbing materials  80 , shown in FIG. 1, in the noise reduction means according to the present invention are foam pads furnished on the surfaces of the air passageway  70 , i.e. on the lower surface of the flow-guiding plate  40 , and on the upper surfaces of the noise-reduction plate unit  50 . The sound-absorbing materials  80  preferably have wavy or wedge-like surfaces  81  for better sound-absorbing performance. 
     The airflow through the fan  20  outlet is first guided by the flow-guiding plate  40  to move in radial direction of the fan  20  in the housing  10 . Then, the airflow turns into the direction parallel to the axial direction of the fan  20 . Then, the airflow is further guided by the noise-reduction plate unit  50  in the direction generally parallel to the radial direction of the fan  20  (the airflow makes a first U-turn here). And finally, the airflow passes through the opening “d” formed in or between the noise-reduction plate  50 , diffuses into the cavity under the noise-reduction plate  50  (the airflow makes a second U-turn here) and exhausts through the filter  30 . 
     Second embodiment 
     FIG. 3 shows a second embodiment of the present invention in which the noise reduction means are modified by the embodiment of FIG.  1 . Two curvy extrusions  202  are formed on two edges of the flow-guiding plate  40 . The extrusions  202  contract the output ports  201  of the fan  20  to guide the output airflow flowing in parallel with the upper wall  15  of the housing  10  and to increase the velocity of the airflow. The edges of the extrusions  201  turn into the vertical direction which is parallel to the side walls  14   a,    14   b  of the housing  10  so as to guide the airflow turning into the direction parallel to the axis of the fan  20 . 
     Another modification is to provide sound-absorbing materials  80   a  on the lower surfaces of the noise-reduction plate unit  50 . When the airflow passing through the opening “d” between the noise-reduction plate unit  50 , a part of the airflow will move tinder the lower surfaces of the noise-reduction plate unit  50  before exiting through the filter  30 , therefore, a further reduction of the noise can be obtained. 
     Third embodiment 
     FIG. 4 shows a third embodiment of the present invention in which the noise reduction means was modified by the embodiment of FIG.  3 . The modification is to furnish each corner of the air passageway with a rounded corner. For example, in the corners of the upper wall  15  of the housing  10  to the left and right walls  14   a,    14   b,  curvy corners  16  are formed and furnished with sound-absorbing materials  80  so as to smooth the airflow when it turns in the air passageway and to further reduce the noise. 
     Further embodiments 
     FIGS. 5 to  7  illustrates respectively a fourth, a fifth and a sixth embodiment of the present invention in which the noise reduction means are similar to the embodiments of FIG. 1,  3  and  4  but different in the sound-absorbing materials  80 . The sound-absorbing materials used in these embodiments are perforated plates  90 , which are furnished with a plurality of penetrating holes  91 . The perforated plates  90  are mounted above the parting plates. There are clearances  92  formed between the perforated plates  90  and the parting plates  40 ,  50  of the air passageway  70 . A preferred manner is to form some supporting elements  93  on the edges of the perforated plates  90  to maintain a suitable clearance between the perforated plates  90  and the plates  40 ,  50 . The clearances are preferably less than a quarter of the wavelength of the acoustic wave so that the acoustic wave penetrating through the holes  91  into the clearances will reflect therein and dissipate into heat, and the noise is reduced. 
     Meanwhile, the surfaces of the perforated plates  90  are preferably smooth so that frictional loss of the airflow can be minimized due to the low friction of the airflow moving along the perforated plates  90  of the air passageway  70 . 
     Basically, in the aforesaid embodiments, the width “b” of the tortuous air passageway  70  is preferred of the same as the width “a” of the output port  201  of the fan  20 . While, since the fan  20  provides two radial airflow outputs to the air passageway  70  which combine into one before passing the filter  30 , the width of the air passageway  70  before the filter  30 , i.e. the opening “d” formed between the noise-reduction plate unit  50  should be at least twice of the width “a” of the output port  201  of the fan  20 , so that the pressure drop in the air passageway  70  can be minimized. 
     As for the arrangement of the noise-reduction plate unit  50 , it can be in parallel with the upper wall  15  of the housing  10  as shown in FIG. 1; or incline from the left and right walls  14   a,    14   b  toward the filter  30  as shown in FIG. 8; or even bend down from the left and right walls  14   a,    14   b  toward the filter  30  as shown in FIG. 9, so that a suitable airflow performance can be achieved. But substantially, the pressure drop of the air passageway should not be increased. So, the minimum width “c” of the air passageway between the flow-guiding plate  40  and the noise-reduction plate unit  50  should not less than the width “a” of the output port  201  of the fan  20 . 
     The housing  10  has to be made from sound retarding materials, and furnished with sound-absorbing materials on the surface thereof, so as to prevent noise transmission from the housing  10  to the exterior. 
     The advantage of the present invention is that the parting plates and the sound-absorbing materials make the fan-filter unit a lower noise device. In the lower noise fan-filter unit, the airflow makes at least two U-turns in the tortuous air passageway before passing through the filter. The tortuous and extended air passageway and the sound-absorbing materials furnished along the air passageway increase the contact area between the airflow and the sound-absorbing materials, and enhance the effect of noise reduction. 
     It is to be understood that the drawings are designed for purposes of illustration only, and are not intended as a definition of the limits and scope of the invention disclosed.