Abstract:
An acoustic baffle for reducing noise of a centrifugal fan includes a base for mounting with a fan outlet and a projection extending from the length of the base at a back side of the base and curving away from a top surface of the base. The projection continuously tapers along at least one side from an area proximate the base to an apex. The apex aligns with a fan tangency point, and the apex or a trough of the projection aligns with a midpoint of the outlet, when the acoustic baffle is installed in the outlet. The acoustic baffle effects a gradual variation in radial and tangential airflow at the blower outlet, to reduce fan blade passage tone.

Description:
FIELD 
     This invention relates to fan blades within evaporator blowers, and to acoustic performance of evaporators in vapor cycle cooling systems. 
     BACKGROUND 
     Centrifugal fans are inherently noisy machines, due to the design and airflow interaction of the fan wheel and blower outlet. Air is drawn in at an inlet by a motor-driven rotating impeller. The impeller includes a number of passages arranged in a spiral. Air accelerates through these passages and emerges at an outlet. A cut-off area between the impeller housing and the outlet causes a sudden change of radial and tangential airflow at the outlet. The change in airflow, which is proportional to the blower speed, causes a pressure pulse that results in noise generation. 
     Conventional efforts to reduce noise generated by centrifugal fans include insulating the fan housing and ducts, both upstream and downstream. Alternately, sound reducing equipment may be installed at the fan inlet or at the fan discharge. For example, U.S. Pat. No. 3,191,851 to Wood describes a two-part system including a square sheet of metal that extends towards and slightly over a small portion of the fan, plus a perforated fairing to decrease size of the fan outlet. 
     U.S. Pat. No. 5,340,275 to Eisinger discloses a rotating cutoff device that is attached within a fan casing. Resonating chambers in the cutoff device are meant to absorb sound. U.S. Pat. No. 6,463,230 to Wargo describes a noise reduction device for smoothing airflow transition at a pinch point of a fan. Wargo focuses on reducing air stagnation at the point where the fan scroll is tangent to the scroll case. The noise reduction device has an airfoil cross section shape, and extends linearly over the fan opening. U.S. Pat. No. 6,575,696 to Lyons et al. combines a sound attenuating cavity, formed as part of the blower housing, with an angled cut off for disrupting pressure fluctuation near the intersection of the exhaust section and the fan scroll. 
     In another example, U.S. Pat. No. 5,536,140 to Wagner et al. discloses a furnace blower with a flat plate that is inserted parallel to a blower exhaust port. Notches cut in a specified pattern vary the quantity of airflow and reduce pulsing tones. U.S. Pat. No. 5,584,653 to Frank et al. discloses a device for reducing noise in a side channel fan, which appears to include notches or spikes cut into fan outlets and pointing into the intake/discharge, to reduce noise. 
     U.S. Pat. No. 3,034,702 to Larsson et al. is not concerned with noise suppression, but rather is directed towards a fan having great axial length and dual air inlets, one at each end. Larsson relies upon a series of baffles to provide uniform flow throughout the entire cross-section of the fan discharge opening. 
     U.S. Pat. No. 6,935,835 to Della Mora discloses various anti-noise stabilizers for centrifugal fans. In particular, Della Mora seeks to homogenize airflow and reduce vortices, in order to reduce noise and improve efficiency of the centrifugal fan. The stabilizers extend for the width of the discharge opening and include dual appendages that face the inlet cone of the fan, one on either side of the discharge opening. U.S. Pat. No. 6,039,532 to McConnell also discloses a device at a fan discharge opening. In particular, McConnell places a baffle in the outlet of a squirrel cage fan. The baffle either tapers continuously from one side of the fan outlet to the other side of the outlet, or is a rectangular insert with a plurality of holes that increase in size from one end to the other end of the baffle. 
     U.S. Pat. No. 3,687,360 also provides a noise suppressing baffle in a discharge duct. Prew&#39;s triangular baffle is inserted within the duct, proximate a chamber housing rotating blades (i.e., a centrifuge chamber). The baffle changes the effective shape of the opening between the duct and the chamber to a trapezoid, and further provides a gradual increase in cross-sectional area of the duct. This change in cross-section decreases velocity of material being discharged into the duct, in order to reduce tendency of the material to build up on walls of the duct. 
     SUMMARY 
     In an embodiment, an acoustic baffle for reducing noise of a centrifugal fan includes a base for mounting with a fan outlet. A projection extends from the length of the base at a back side of the base, and curves away from a top surface of the base. The projection continuously tapers from the base to an apex that aligns with a center line of the base. When the acoustic baffle is installed in the outlet, the projection extends over the fan wheel and tapers from left and right sides of the outlet to a fan tangency point at a midpoint of the outlet. 
     In an embodiment, an acoustic baffle for reducing noise of a centrifugal fan includes a base for mounting with a fan outlet. A projection extends from the length of the base at a back side of the base and curves away from a top surface of the base. The projection includes opposing left and right sides that are parallel to or aligned with left and right sides of the base, and an internal cutout forming a trough. A center point of the trough aligns with a center line of the base. Ends of the left and right sides opposite the base form left and right apices of the internal cutout. Opposing inner sides of the projection defining the cutout continually taper from the trough to the apices. When the acoustic baffle is installed in the outlet, the projection extends over the fan wheel and the left and right sides of the projection continually widen from left and right fan tangency points to the trough. 
     In an embodiment, an acoustic baffle for reducing noise of a centrifugal fan includes a base for mounting with a fan outlet. A projection extends from the length of the base at a back side of the base and curves away from a top surface of the base. The projection continuously tapers along at least one side, from an area proximate the base to an apex. The apex aligns with a fan tangency point, and the apex or a trough of the projection aligns with a midpoint of the outlet, when the acoustic baffle is installed in the outlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top, perspective view of an acoustic baffle having a linear, spike shape, according to an embodiment. 
         FIG. 2  is a top perspective view of an acoustic baffle shaped as a non-linear spike, according to an embodiment. 
         FIG. 3  is a top perspective view of an acoustic baffle shaped having a linear, vee shape, according to an embodiment. 
         FIG. 4  is a top perspective view of an acoustic baffle shaped as a non-linear vee, according to an embodiment. 
         FIG. 5  is a front view of a centrifugal fan with the baffle of  FIG. 2  installed proximate the blower outlet, according to an embodiment. 
         FIG. 6  is a perspective view of the fan and installed baffle of  FIG. 2 . 
         FIG. 7  is a cross-sectional view of a prior art centrifugal fan. 
         FIG. 8  is a cross-sectional view of the fan of  FIG. 7  showing an installed acoustic baffle that lacks a fan case extension, according to an embodiment. 
         FIG. 9  is a cross-sectional view of the fan and baffle of  FIG. 8  with a fan case extension, according to an embodiment. 
         FIG. 10  is a graph showing exemplary reduction of fan blade passage tones by the baffles of  FIGS. 1-4 . 
         FIG. 11  is a graph similar to that of  FIG. 10 , but illustrating level of tone reduction by the baffles of  FIGS. 1-4  from a baseline level. 
         FIG. 12  is an exemplary bar graph comparing maximum fan blade passage tone levels achieved with the baffles of  FIGS. 1-4  with a baseline level. 
         FIG. 13  is a graph comparing static pressure with evaporator flow rate, and illustrating performance of the baffles of  FIGS. 2-4  as compared to baseline and distribution duct flow. 
         FIG. 14  is a partial view of the graph of  FIG. 13 , further illustrating impact of the baffles of  FIGS. 2-4  on evaporator flow rate. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an acoustic baffle  100  having a base  102  for attaching with the outlet of a blower or fan (hereinafter, fans and blowers are referred to collectively as “a fan” or “the fan”). A spike-shaped extension  104  extends into the fan discharge or blast area and partially over a fan wheel or impeller of the fan, when baffle  100  is secured in the outlet. At least a back side  106  of spike extension  104  (alternately, most or all of spike extension  104 ) is curved or bent to conform to exterior geometry of the impeller. A fin  108  extends from a front surface  110  of spike extension  104  (and optionally, from base  102  as well) and tapers from base  102  to an apex  112  of extension  104 . Fin  108  may be formed with spike extension  104  and/or base  102  (for example, where baffle  100  is molded from plastic or other flowable material), or extension  104  may be formed as a separate part and attached with spike extension  104  and/or base  102 . Spike extension  104  and fin  108  effect a gradual change in airflow from the impeller to the outlet, in contrast to the sudden change in radial and tangential airflow typical at the outlet of a centrifugal fan. 
     At least one sidewall  114  provides an attachment point for bolting or otherwise fastening baffle  100  in the fan outlet. Base  102  may include a terminal lip  116  for extending over a bottom edge or end of the fan outlet, to facilitate positioning of baffle  100  with the outlet. Although not shown, base  102 , sidewall  114  and/or lip  116  may form openings for hardware to secure baffle  100  in place. An optional joiner  117  may be included to reinforce or stiffen the junction of sidewall  114  with base  102  and spike extension  104 . 
     A fan case extension  118  may be included on a bottom surface  119  of base  102 , for filling a gap between the fan impeller and the fan scroll cut off/blower case. Fan case extension  118  may include a longitudinal ridge  120  for fitting with the fan scroll cut off, to facilitate proper positioning of baffle  100  within the blower outlet. Fan case extension  118  tapers from bottom surface  119  to an end  121 , for example forming a roughly triangular shape, although shape of fan case extension  118  may vary depending on geometry of a gap to be filled. 
     In one aspect, a back side  122  of fan case extension  118  continues the curvature of back side  106  of spike extension  104 . In another aspect, back side  122  essentially forms an obtuse angle with back side  106 . When baffle  100  is secured with a fan outlet, fan case extension  118  fills in gaps that could otherwise remain between baffle  100  and the fan scroll cut off, thus enhancing acoustic performance of baffle  100 . A front side  123  of fan case extension  118  is curved or otherwise shaped for fitting with a blower case proximate the cut off, as shown in  FIG. 9  (described below). 
     It will be appreciated that geometry of back side  106  and back side  122 , as well as length and width of baffle  100  and dimensions of fin  108  may vary depending upon dimensions of the fan to be outfitted with baffle  100 . It will also be appreciated that geometry of fan case extension  118  may vary depending upon dimensions of the fan to be outfitted with baffle  100 . For example, an angle between back side  106  and back side  122  may be determined based upon dimensions of an existing fan case, such that apex  112  is a minimal distance from the fan scroll without interfering with the fan scroll during service or use. Base  102  may also include a cutout  124 , dimensions and placement of which may also vary to accommodate preexisting features of the fan outlet. 
     Left and right sides  128  and  130  of spike extension  104  may taper from base  102  to apex  112  in a linear manner, as shown in  FIG. 1 , or sides  128  and  130  may feature a non-linear taper from base  102  to apex  112 , as shown with respect to baffle  150 ,  FIG. 2 . 
       FIG. 2  shows an acoustic baffle  150 , which is similar to baffle  100 . Where baffle  100  has linearly tapering sides  128  and  130 , baffle  150  includes non-linearly tapering left and right sides  132 ,  134 . That is, sides  132  and  134  taper from base  102  to apex  112  in a non-linear manner. Identical features of baffles  100  and  150  are noted using the same reference numbers. 
       FIGS. 3 and 4  show acoustic baffles  200  and  250 , respectively. Baffles  200  and  250  share multiple identical features, which are denoted with the same reference numbers from one drawing to the other. Baffles  200  and  250  each have a base  202 , which is similar to base  102 , described above. A v-shaped (“vee” shaped) extension  204  extends from base  202  and shaped to conform to exterior geometry of a fan impeller when baffle  200 / 250  is secured in the fan outlet. In particular, at least a back side  206  of vee extension  204  is curved or bent to conform to exterior curvature of the impeller. Left and right fins  208 ,  209  extend from left and right sides  228  and  230  of vee extension  204 , forming sidewalls of extension  204 . Hereafter, fins  208  and  209  may be referred to as sidewalls  208  and  209 . 
     Fins/sidewalls  208  and  209  taper in height from base  202  to opposing left and right apices  212  and  213  of vee extension  204 . Sidewalls  208  and  209  may be formed with vee extension  204 , for example where baffle  200 / 250  is molded from plastic or other flowable material), or sidewalls  208  and  209  may be formed as separate parts and attached with vee extension  204  and/or base  202 . The junction of sidewall  208  or  209  with base  202  and a respective sidewall  214  of base  202  may be reinforced or stiffened with an additional joiner  217 . In one aspect, sidewall  214  and sidewall  208  or  209  form a continuous sidewall, for example where baffle  200 / 250  is formed as a unitary piece. Joiner(s)  217  may be added if stiffening or reinforcement is desired. Like spike extension  104  and fin  108  ( FIGS. 1 and 2 ), vee extension  204  and sidewalls  208  and  209  effect a gradual change in airflow from the impeller to the outlet. 
     Sidewall(s)  214  extend from base  202  and provide an attachment point for bolting or otherwise fastening baffle  200 / 250  in the fan outlet. Base  202  may also include a terminal lip  216  for extending over a bottom edge or end of the fan outlet, to facilitate positioning of baffle  200  with the outlet. Although not shown, base  202 , sidewall  214 , one or both of sidewalls  208  and  209  and/or lip  216  may form openings for hardware to secure baffle  200 / 250  in place. 
     A fan case extension  218  extends from a bottom surface  219  of base  202 , for filling a gap between the fan impeller and the fan scroll cut off/blower case, when baffle  200 / 250  is installed in a centrifugal fan. Fan case extension  218  may include a longitudinal ridge  220  for fitting with the fan scroll cut off, to facilitate positioning of baffle  100  within the blower outlet. Fan case extension  218  tapers from bottom surface  219  to an end  221 , for example forming a roughly triangular shape, although shape of fan case extension  218  may vary depending on geometry of a gap to be filled. 
     In one aspect, a back side  222  of fan case extension  218  continues curvature of back side  206  of vee extension  204 . In another aspect, back side  222  essentially forms an obtuse angle with back side  206 . When baffle  200 / 250  is secured with a fan outlet, fan case extension  218  fills a gap that could otherwise remain between baffle  200 / 250  and the fan scroll cut off, thus enhancing acoustic performance. A front side  223  of fan case extension  218  is curved or otherwise shaped for fitting with a blower case proximate the cut off (see, e.g., baffle  150  in housing  314 ,  FIG. 9 ). 
     It will be appreciated that geometry of back side  206  and back side  222 , as well as length and width of baffle  200 / 250  and dimensions of sidewalls  208  and  209  may vary depending upon dimensions of the fan to be outfitted with baffle  200 / 250 . It will also be appreciated that geometry of fan case extension  218  may vary depending upon dimensions of the fan to be outfitted with baffle  200 / 250 . For example, an angle between back side  206  and back side  222  may be determined based upon dimensions of an existing fan case, such that left and right apices  212 ,  213  are a minimal distance from the fan scroll without interfering with the fan scroll during service or use. Base  202  may also include a cutout  224 , dimensions and placement of which may also vary to accommodate preexisting features of the fan outlet. 
     Vee extension  204  of baffle  200  ( FIG. 3 ) has inner, left and right sides  232  and  234  that taper from apices  212  and  213  (respectively) to a trough  236  in a linear manner. Vee extension  204  may alternately feature a non-linear taper of its opposing internal sides. Baffle  250 ,  FIG. 4  includes inner left and right sides  236  and  238 , which taper from apices  212  and  213  to base  202  in a non-linear manner. 
     Baffles  100 ,  150 ,  200  and  250  may be made of any material or materials that are compatible with the fan to be outfitted. In one aspect, baffles  100 - 250  are made of plastic, such as a thermoformed plastic. Fan case extensions  118 ,  218  may be integral to baffles  100 ,  150  and  200 ,  250 , respectively, or fan case extensions  118 ,  218  may be formed of the same or another material and attached with their respective acoustic baffles. 
       FIGS. 5 and 6  show a centrifugal fan  300  with baffle  150  (with a non-linear spike extension  104 , as shown in  FIG. 2 ) installed in an outlet  302 .  FIGS. 5 and 6  are best viewed together with the following description. 
     Base  102  of baffle  150  is sized to span a width w 0  of the outlet, for example fitting over or with a cut off of fan  300  (shown in  FIGS. 7-9 ) via features  118 - 120 . Extension  104  extends over and conforms to curvature of an impeller  304  of fan  300  (at least along back side  106 ). When baffle  150  is in place, extension  104  tapers over impeller  304  from opposing sides  306  and  308  of outlet  302  to a midpoint  310  of outlet  302  (i.e., a point halfway between sides  306  and  308 , shown marked as a half point of width w 0 ). Apex  112  overlies (but does not touch) impeller  304  proximate a fan tangency point  312  (see  FIGS. 8 and 9 ). Fin  108  of extension  104  tapers from base  102 , proximate the fan cut off, to apex  112  proximate tangency point  312 . Thus, baffle  150  smoothes changes in both radial and tangential airflow at outlet  302  to reduce fan noise (known as the fan blade passage tone). 
       FIGS. 7-9  are cross-sectional views of a fan scroll/housing  314 , taken along line A-A (see  FIG. 6 ).  FIG. 7  shows outlet  302  without an acoustic baffle.  FIG. 8  shows outlet  302  fitted with baffle  150 , with fan case extension  118  removed for purposes of viewing a gap at the fan scroll cut off.  FIG. 9  shows outlet  302  fitted with baffle  150  and showing fan case extension  118 . It will be appreciated that although baffle  150  is shown and described with respect to fan  300 /housing  314 , baffles  100 ,  200  or  250  may also fit with fan outlet  302  to provide noise reduction as described herein. 
       FIGS. 7-9  are best viewed together with the following description. Note the relatively large gap between impeller  304  and fan scroll cut off  318  in  FIG. 7 , whereas, in  FIG. 8 , the gap is reduced by baffle  150 . Baffle  150  extends out over impeller  304  to fan tangency point  312  and gradually varies the flow area of outlet  302  after tangency point  312  (for example, via tapering left and right sides  128  and  130 , and via tapering fin  108 ). However, in  FIG. 8 , a reduced gap  316  between baffle  150  and a fan scroll cut off  318  remains unfilled. 
     In  FIG. 9 , baffle  150  includes fan case extension  118 , which fills gap  316 . Baffle  150  and fan case extension  118  together encase impeller  304 . In laboratory tests, filling gap  316  improved acoustic performance of baffle  150  by up to about 50%. As shown, fan case extension  118  is somewhat triangular in cross section; however, shape of fan case extension  118 / 218  may vary according to a gap to be filled. 
     Fan blade passage tone (objectionable fan noise) is dependent upon the quantity of fan blades in the fan impeller, and the speed of the fan. The fan blade passage frequency, which generates the objectionable noise, can be calculated as follows: 
                       Frequency   Fan     ⁡     (   Hz   )       =       RPM   Fan     60             Eq   .           ⁢   1                   Frequency   FanBladePassage     ⁡     (   Hz   )       =       Frequency   Fan     ×   FanBladeQuantity             Eq   .           ⁢   2               
Once the fan blade passage frequency is known, it may be isolated during acoustic surveys of the fan, and overall effectiveness of an acoustic baffle may be measured.
 
       FIGS. 10-14  are graphs showing experimental results obtained in testing acoustic baffles  100  and  200 . Turning first to  FIG. 10 , graph  1000  plots evaporator fan blade passage tone (dB) against fan blade passage frequency (Hz). Line  1002  shows baseline fan blade passage tone of a fan without an acoustic baffle, at frequencies from about 900 Hz to about 2,550 Hz. Line  1004  illustrates fan blade passage tone of a fan outfitted with baffle  200  or  250  at these same frequencies. Line  1006  illustrates fan blade passage tone of a fan outfitted with linear tapered baffle  100 , again at frequencies between about 900 Hz and about 2,550 Hz. Line  1008  shows, at these frequencies, fan blade passage tone of a fan outfitted with non-linear tapered baffle  150 . 
     As shown, at 1500 Hz, a non-baffled fan produced a fan blade passage tone of about 100 dB. In contrast, a fan outfitted with baffle  200 / 250  produced about 89 dB of noise. A fan outfitted with baffle  100  produced about 83 dB fan blade passage tone, and a fan outfitted with baffle  150  produced about 81 dB. 
       FIG. 11  features a graph  1100  showing reduction of fan blade passage frequency from baseline  1002  ( FIG. 10 ). Line  1104  shows reduction by baffle  200 / 250 , line  1106  shows reduction by baffle  100 , and line  1108  shows reduction by baffle  150 . At 1500 Hz, baffle  200 / 250  reduced fan blade passage tone by about 11 dB. Baffle  100  reduced tone by about 17 dB, and baffle  150  reduced fan blade passage tone by about 19 dB. At about 2,100 Hz, baffles  100 / 150  achieved about a 1 dB reduction in fan blade passage tone, whereas baffles  200 / 250  reduced tone by about 11 dB. 
       FIG. 12  shows a bar graph  1200  illustrating maximum evaporator fan blade passage tone level over a fan speed sweep of 600-5,700 RPM. Over this range, the maximum baseline (baffle-free fan) passage tone level was 100 dB. Bar  1202  represents the baseline. At this tone, baffles  100  and  150 , represented by bars  1206  and  1208 , respectively, reduced noise by about 17 dB. Baffles  200  and  250 , represented by bar  1204  achieved about an 11 dB reduction. 
     Experimental results suggest that overall, “spike” style acoustic baffles such as baffles  100  and  150  have better noise reduction in the 1,200-1,700 Hz fan blade passage frequency, while “vee” style baffles  200  and  250  have better noise reduction in the 2,100-2,600 Hz fan blade passage frequency. 
     Inclusion of baffle  100 ,  150 ,  200  or  250  in the blower outlet of a centrifugal fan (i.e., outlet  302  of fan  300 ) results in minimal reduction of flow into the distribution duct (e.g., a duct attached at outlet  302 ). Impact on blower flow rate was calculated by measuring the static pressure at multiple flow rates for a baseline configuration, and with the acoustic baffles installed.  FIG. 13  shows a graph  1300  that plots static pressure (InH 2 O) against flow rate (ACFM). Line  1302  is a baseline depicting flow rate of a baffle free fan. Line  1304  shows flow rate of a fan outfitted with vee-style baffle  200  or  250 . Line  1308  illustrates flow rate of a fan outfitted with baffle  150 . Line  1309  illustrates flow within a distribution duct of the fan. Data collected from fans outfitted with acoustic baffles  150  or  200 / 250  (lines  1308  and  1304 ) was compared with the distribution duct performance (line  1309 ) and flow losses calculated.  FIG. 14  is a graph  1400  that shows losses using baffle  150  and baffle  200 / 250 . Line  1402  represents a 5,700 RPM baseline, while line  1404  represents a fan with baffle  200 / 250  and line  1408  represents the fan with baffle  150  installed. Line  1409  shows flow within the distribution duct. With baffle  150  installed, a 4.5 cfm loss was measured at 5,700 RPM. A 6.0 cfm loss in flow at 5,700 RPM was measured with baffle  200 / 250  in place. These losses amount to a 2.27% reduction in flow with baffle  150 , and a 3.02% reduction with baffle  200 / 250 . The measured losses minimally impact performance of the centrifugal fan, and are well outweighed by gains in acoustic performance (see  FIGS. 10-12 ). 
     Certain changes may be made in the above systems and methods without departing from the scope hereof. For example, features and use shown or described with respect to one of baffles  100 - 250  may be incorporated into or pertain to another of baffles  100 - 250 . Thus, it is intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are to cover generic and specific features described herein, and all statements of the scope of the invention which, as a matter of language, might be said to fall there between.