Patent Abstract:
An apparatus and method for attenuating the sound generated by a fan powered terminal unit in an HVAC (heating, ventilating, and air conditioning) system. The apparatus utilizes internal geometry to minimize noise due to air disturbances and aerodynamic effects within the apparatus.

Full Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional application no. 60/895,152, filed Mar. 16, 2007, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to an integrated fan powered silencing terminal unit for HVAC (heating, ventilating, and air conditioning) systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    Commercial HVAC systems have contained “Fan Powered Terminal Units” (“FPTUs”) for the purpose of providing an outlet for commercial ventilation systems into the rooms of a building or other structure equipped with an HVAC system. A FPTU typically consists of the following components: 1) centrifugal fan, 2) motor, 3) insulated casing, and 4) air inlet (with or without damper). 
         [0004]    In commercial HVAC installations, a “silencer” (or “attenuator”) is often attached to the inlet or outlet of an FPTU in order to attenuate the sound produced by the high-velocity air entering the FPTU. Such silencers have typically comprised an air duct (typically from three to five feet in length) that is lined internally with insulation to attenuate the noise produced by the air flowing through the FPTU. Such internal insulation is also known as a “baffle” and is usually held in place by perforated sheet metal. The perforations in the metal allow the air traveling through the silencer to interact with the insulation material contained inside the baffle. The silencer is attached to the inlet or the outlet of the FPTU and acts to attenuate the noise that is produced by the FPTU. This attenuation is achieved due to the conversion of acoustic energy into heat energy as the air molecules inside the silencer create friction when they collide with the lined insulation. 
         [0005]    The noise generated by an FPTU can be separated into two components: 1) noise due to the air disturbance created in the immediate vicinity of the rotating fan blades and 2) aerodynamic noise due to the fan-induced air flow that has variable pressure regions within the fan discharge velocity profile and the air flow interaction with geometry changes in the air stream. The insulation contained in silencers minimizes both sources of noise created by the FPTU. 
         [0006]    The noise generated by a given FPTU can vary widely depending on how it is utilized in a particular HVAC system and on the configuration of the HVAC system. Similarly, the acoustic performance of a given silencer can also vary widely depending upon the configuration of the HVAC system in which it is installed. Such unpredictability of the noise that will be generated by an FPTU and the attenuation achieved by a silencer is known as the “system effect” of the HVAC system in which the FPTU and silencer are installed. For instance, the manner in which the distribution ductwork is organized in a given building installation can affect the turbulence and air pressures created inside the ductwork. This, in turn, can affect the noise level generated by an FPTU and the acoustic performance achieved by a silencer attached thereto. 
         [0007]    The unpredictability produced by such system effects creates uncertainty when HVAC installers are selecting FPTUs and silencers for installation in a building. Manufacturers of traditional FPTUs and silencers typically test their products under artificial laboratory conditions and produce specifications as to the noise generated by their FPTUs and the noise attenuation achieved by their silencers. However, these specifications do not take into account the system effects produced by installing their products in an actual HVAC system. Thus, HVAC installers generally have only marginally reliable product specifications on which they can rely and often must utilize trial-and-error methods to choose the appropriate combination of FPTUs and silencers that will meet their needs in a particular HVAC installation. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention (a fan powered silencing terminal unit “FPSTU”) involves an apparatus and method for attenuating the sound generated by a fan powered terminal unit in a predictable and consistent manner. A further object of the invention is the integration of an FPTU and a silencer into a single unit. Another object of the invention is to attenuate sound to a greater degree than is possible with a combination of prior art FPTUs or silencers of a given size. 
         [0009]    Embodiments of the invention can minimize the noise generated by the variable pressure regions within the FPSTU unit by closely coupling the noise-attenuating, insulation-lined silencing portion of the unit to the housing of the centrifugal fan inside the unit. Such close-coupling minimizes the turbulence created by the centrifugal fan and thus minimizes the associated noise. 
         [0010]    Embodiments of the invention also minimize noise within the FPSTU by creating a constant, uniform cross-sectional profile of the air traveling through the unit. This uniform cross-sectional profile minimizes the turbulence created when air exiting a typical FPTU enters a silencer with a larger (or smaller) cross-sectional area. The decreased turbulence in the airflow of the invention, in turn, helps minimize the noise generated by the FPSTU. 
         [0011]    Embodiments of the invention minimize high-frequency noise due to the internal angled or curved geometry of the FPSTU. Such geometry obstructs any direct line-of-sight pathway out of the unit that would otherwise allow high-frequency noise to escape without much attenuation. Traditional silencers lack any such internal geometry and instead allow high-frequency noise to exit the silencer without contacting the baffles of the silencer. Therefore, the high-frequency noise in a traditional silencer can escape without much attenuation. 
         [0012]    Further objects, features, and advantages will become apparent upon consideration of the following detailed description of the invention when taken in conjunction with the drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  is a side elevation view of a centrifugal fan and the velocity and pressure profile of the air leaving the centrifugal fan in a prior art FPTU. 
           [0014]      FIG. 2A  is a top cut away view of a prior art FPTU coupled to a prior art silencer with vertical baffles. 
           [0015]      FIG. 2B  is a side cross-sectional view of a prior art FPTU coupled to a prior art silencer with horizontal baffles. 
           [0016]      FIG. 3A  is a top cut away view of a prior art FPTU coupled to a prior art silencer. 
           [0017]      FIG. 3B  is a side cross-sectional view of  FIG. 3A . 
           [0018]      FIG. 3C  is an end view along line  3 C of  FIG. 3B . 
           [0019]      FIG. 3D  is a cross-sectional view along line  3 D of  FIG. 3B . 
           [0020]      FIG. 4A  is a top cut away view of an embodiment of an FPSTU in accordance with the invention. 
           [0021]      FIG. 4B  is a side cross-sectional view of  FIG. 4A . 
           [0022]      FIG. 4C  is an end view along line  4 C of  FIG. 4B . 
           [0023]      FIG. 4D  is a cross-sectional view along line  4 D of  FIG. 4B . 
           [0024]      FIG. 4E  is a magnified cross-sectional view of inset  4 E of  FIG. 4B . 
           [0025]      FIG. 5A  is a top cut away view of an embodiment of an FPSTU in accordance with the invention. 
           [0026]      FIG. 5B  is a side cross-sectional view of  FIG. 5A . 
           [0027]      FIG. 5C  is an end view along line  5 C of  FIG. 5B . 
           [0028]      FIG. 5D  is a cross-sectional view along line  5 D of  FIG. 5B . 
           [0029]      FIG. 5E  is a magnified cross-sectional view of inset  5 E of  FIG. 5B . 
           [0030]      FIG. 6A  is a top cut away view of an embodiment of an FPSTU in accordance with the invention. 
           [0031]      FIG. 6B  is a side cross-sectional view of  FIG. 6A . 
           [0032]      FIG. 6C  is an end view along line  6 C of  FIG. 6B . 
           [0033]      FIG. 6D  is a cross-sectional view along line  6 D of  FIG. 6B . 
           [0034]      FIG. 6E  is a magnified cross-sectional view of inset  6 E of  FIG. 6B . 
           [0035]      FIG. 7A  is a top cut away view of an embodiment of an FPSTU in accordance with the invention. 
           [0036]      FIG. 7B  is a side cross-sectional view of  FIG. 7A . 
           [0037]      FIG. 7C  is an end view along line  7 C of  FIG. 7B . 
           [0038]      FIG. 7D  is a cross-sectional view along line  7 D of  FIG. 7B . 
           [0039]      FIG. 7E  is a magnified cross-sectional view of inset  7 E of  FIG. 7B . 
       
    
    
     DETAILED DESCRIPTION  
       [0040]      FIG. 1  is an illustration of the velocity and pressure profile of a centrifugal fan  101  in a typical prior art FPTU  100 . The centrifugal fan  101  is enclosed in a housing  103  and blows air out into a discharge duct  102  or attached silencer. The housing  103  of the fan  101  has a cutoff plate  104  on the lower edge of the housing  103 . The cutoff plate  104  creates a low pressure area  105  immediately behind the cutoff plate  104 . The high-velocity air exiting the fan  101  exhibits a non-uniform bulge  106  of high pressure. As the air travels down the discharge duct  102 , the bulge of high pressure will gradually even out as illustrated in  107 ,  108 ,  109 , and  110 . The turbulence generated as the high pressure bulge gradually evens out will create noise in the FPTU  100 . 
         [0041]      FIGS. 2A and 2B  are illustrations of the close-coupling of a prior art FPTU  201  with a prior art silencer  202 . Such silencers typically have vertical baffles  203   a  or horizontal baffles  203   b  (with respect to the FPTU  201 ) in order to attenuate the sound produced by the FPTU  201 . Prior art silencers  202  typically have a wider cross-sectional area than a corresponding FPTU  201 , creating a wide area  204  inside the silencer  202 . This wide area  204  creates a space where turbulence can develop in the silencer  202 , thus unnecessarily increasing the noise level in the silencer  202 . In addition, prior art FPTUs  201  contain the cutoff plate  205  described previously, which also increases the noise generated by the FPTU  201  due to the non-uniform bulge of high pressure exiting the FPTU  201 . The cross-sectional area of the blower outlet  210  of prior art FPTUs  201  is typically larger than the cross-sectional area of the air pathway  206  of prior art silencers  202 . Therefore a “nose”  209  is created where the air exiting the blower outlet  210  collides into the baffles  203   a ,  203   b  inside the silencer  202 . This causes added turbulence and increased noise. 
         [0042]    Prior art FPTUs  201  and silencers  202  also have a direct line-of-sight pathway  206  from the centrifugal fan  207  of the FPTU  201  to the discharge outlet  208  of the silencer  202 . As a consequence of such a direct line-of-sight pathway  206 , high-frequency sounds can travel relatively unobstructed through the silencer  202 . This is because the shorter wavelengths of high-frequency sound waves produce less displacement of the air molecules and hence those air molecules are less likely to collide with the baffles  203   a ,  203   b  inside the silencer  202 . This “beaming” effect of high-frequency sounds thus reduces the effectiveness of prior art silencers  202  in reducing high-frequency noise. 
         [0043]      FIGS. 3A-3D  are depictions of a prior art FPTU  301  closely-coupled to a prior art silencer  304  with only a half-baffle design. That is, the silencer  304  contains a baffle  306  on only a single internal wall. This half-baffle silencer  304  still contains a nose  302  which leads to increased turbulence and noise. The nose  302  is caused because the cross-sectional air pathway  305  of the silencer  304  is narrower than the cross-sectional area of the blower outlet  303  of the FPTU  301 . 
         [0044]      FIG. 3C  depicts an end view of the silencer  304  and the perforated metal casing  353  that encloses the insulating material  354  of the baffle  306 .  FIG. 3C  also shows the casing  351  of the silencer  304  and the casing  352  of the FPTU  301 . 
         [0045]      FIG. 3D  depicts a cross-sectional view of the insulating material  354  that comprises the baffle  306  of the silencer  304 .  FIG. 3D  also shows the casing  351  of the silencer  304  and the casing  352  of the FPTU  301 . 
         [0046]      FIGS. 4A-4E  depict an embodiment of an FPSTU  401  in accordance with the invention. FPSTU  401  contains a silencer inlet extension  402  which connects the top edge  403  of the baffle  409  contained in the silencing portion  404  of the FPSTU  401  directly to the cutoff plate  405  of the centrifugal fan  406  housed in the FPSTU  401 . The silencer inlet extension  402  eliminates the low-pressure area  105  caused by the cutoff plate  104  in prior art FPTUs ( FIG. 1 ). Therefore, the air exiting the centrifugal fan  406  does not contain a non-uniform bulge of high pressure as it travels down the air pathway  407  of the silencing portion  404  of the FPSTU  401 . 
         [0047]    In addition, the cross-sectional area of the blower outlet  408  substantially equals the cross-sectional area of the air pathway  407  of the silencing portion  404  of the FPSTU  401 . Therefore, the FPSTU  401  contains no nose, unlike the nose  209 ,  302  present in prior art silencers  202 ,  304  ( FIGS. 2B ,  3 B). 
         [0048]      FIG. 4C  depicts an end view of the FPSTU  401  and the perforated metal casing  453  that encloses the insulating material  454  of the baffle  409 .  FIG. 4C  also shows the casing  451  of the silencing portion  404  of the FPSTU  401  and the casing  452  of the plenum portion of the FPSTU  401 . 
         [0049]      FIG. 4D  depicts a cross-sectional view of the insulating material  454  that comprises the baffle  409  of the silencing portion  404  of the FPSTU  401 .  FIG. 4D  also shows the casing  451  of the silencing portion  404  of the FPSTU  401  and the casing  452  of the plenum portion of the FPSTU  401 . 
         [0050]      FIGS. 5A-5E  illustrate an embodiment of the invention wherein the baffle  502  of the silencing portion  503  of the FPSTU  501  flares outward in a “tail”  504 . This tail  504  allows the expanding air that is traveling down the air pathway  505  to maintain a constant pressure. This is because the increased cross-sectional area of the tail portion  504  of the FPSTU  501  provides additional space for the expanding air to occupy, thus preventing a buildup of pressure within the FPSTU  501 . 
         [0051]      FIG. 5C  depicts an end view of the FPSTU  501  and the perforated metal casing  553  that encloses the insulating material  554  of the baffle  502 .  FIG. 5C  also shows the casing  551  of the silencing portion  503  of the FPSTU  501  and the casing  552  of the plenum portion of the FPSTU  501 . 
         [0052]      FIG. 5D  depicts a cross-sectional view of the insulating material  554  that comprises the baffle  502  of the silencing portion  503  of the FPSTU  501 .  FIG. 5D  also shows the casing  551  of the silencing portion  503  of the FPSTU  501  and the casing  552  of the plenum portion of the FPSTU  501 . 
         [0053]      FIGS. 6A-6E  illustrate an embodiment of the invention with a high-frequency splitter  602  placed in the air pathway  603  of the FPSTU  601 . The high-frequency splitter  602  scatters high-frequency sound waves that would otherwise pass relatively unobstructed through the air pathway  603  due to the “beaming” effect of high-frequency sound. The scattered high-frequency sound waves will therefore tend to impact the baffle  605  directly or bounce off the casing  604  and then into the baffle  605 , which will attenuate the sound. 
         [0054]      FIG. 6C  depicts an end view of the FPSTU  601  and the perforated metal casing  653  that encloses the insulating material  654  of the baffle  605 .  FIG. 6C  also shows an end view of the high-frequency splitter  602 .  FIG. 6C  also shows the casing  651  of the silencing portion of the FPSTU  601  and the casing  652  of the plenum portion of the FPSTU  601 . 
         [0055]      FIG. 6D  depicts a cross-sectional view of the insulating material  654  that comprises the baffle  605  of the silencing portion of the FPSTU  601 .  FIG. 6D  also shows the casing  651  of the silencing portion of the FPSTU  601  and the casing  652  of the plenum portion of the FPSTU  601 . 
         [0056]      FIGS. 7A-7E  depict an embodiment of the invention wherein the air pathway  702  of the FPSTU  701  is angled or curved, thus minimizing or eliminating the line-of-sight pathway from the centrifugal fan  703  to the discharge outlet of the FPSTU  701 . This elimination of the line-of-sight pathway will likewise minimize the high-frequency noise emitted by the centrifugal fan  703  and prevent high-frequency sound waves from traveling down the air pathway  702  unobstructed. The silencing portion of the FPSTU  701  can be up to five feet in length with an optimal length of three feet or less. 
         [0057]      FIG. 7C  depicts an end view of the FPSTU  701  and the perforated metal casing  753  that encloses the insulating material  754  of the angled top baffle  704 .  FIG. 7C  also shows the casing  751  of the silencing portion of the FPSTU  701  and the casing  752  of the plenum portion of the FPSTU  701 . 
         [0058]      FIG. 7D  depicts a cross-sectional view of the insulating material  754  that comprises the top and bottom baffles  704 ,  705  of the silencing portion of the FPSTU  701 .  FIG. 7D  also shows the casing  751  of the silencing portion of the FPSTU  701  and the casing  752  of the plenum portion of the FPSTU  701 . 
         [0059]    While this invention has been described with reference to the structures and processed disclosed, it is to be understood that variations and modifications can be affected within the spirit and scope of the invention as described herein and as described in the appended claims.

Technology Classification (CPC): 5