Abstract:
A damper comprising a duct defining an air flow path, opposing damper blades pivotally mounted within the duct, each damper blade having an end that is pivotally mounted in cantilevered fashion in the duct so that an opposing end is free to move within the duct, wherein each damper blade is planar in shape, cooperating flow straightening members disposed centrally in the air flow path, an air flow sensor for generating a signal cooperatively disposed between the flow straightening members, and an actuator for pivoting each damper blade within the duct in response to the signal from the air flow sensor to vary the damper blade position and thereby maintain a desired velocity of air exiting the damper.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a stack damper, and more particularly, to a stack damper which automatically maintains a downstream air flow velocity by adjusting a damper blade position in response to an air flow signal. 
       BACKGROUND OF THE INVENTION 
       [0002]    Suitable air flow velocity is a widely accepted means of maintaining entrainment of air borne contaminants. To this end various environmental rules require that air exiting from an exhaust stack or duct be discharged into the air at a predetermined height (or some calculated distance from intake ducts) and a predetermined velocity to ensure proper diffusion. 
         [0003]    One problem presented by this regulatory regime is the large constant volume of air which must be discharged to maintain proper velocity. 
         [0004]    Compounding the problem is the volume of air which is required to be discharged varies during the day. For instance, the volume of discharged air could be reduced dramatically during off hours. On the other hand, regulations often require the velocity of discharged air to remain the same throughout the day. This often requires that ambient air be pumped into the stack to ensure constant discharge velocity. This in turn substantially increases the associated energy costs required to run the facility, even though the percentage of entrained contaminants may be very small. 
         [0005]    It would be advantageous if the energy costs associated with the maintenance of a high velocity discharge could be reduced by reducing the amount of ambient air required to maintain the desired discharge velocity. 
         [0006]    Representative of the art is U.S. Pat. No. 6,071,188 (2000) which discloses an exhaust system has an air exhaust duct having a charge opening and a discharge opening and defining an air flow path. A damper is mounted in the duct for maintaining a constant air velocity for air exiting the discharge opening. The damper has opposing damper blades pivotally mounted within the duct. Each damper blade has an end that is pivotally mounted in cantilevered fashion within the duct so that an opposing end is free to move within the duct. Each damper blade is parabolic in shape and minimizes the air turbulence over the damper blade and reduces vibration. Each damper blade can be pivoted in response to a change in air volume discharged through the discharge opening to maintain a desired velocity of air through the discharge opening. 
         [0007]    What is needed is a stack damper which automatically maintains a downstream air flow velocity by adjusting a damper blade position in response to an air flow signal. The present invention meets this need. 
       SUMMARY OF THE INVENTION 
       [0008]    The primary aspect of the invention is to provide a stack damper which automatically maintains a downstream air flow velocity by adjusting a damper blade position in response to an air flow signal. 
         [0009]    Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings. 
         [0010]    The invention comprises a damper comprising a duct defining an air flow path, opposing damper blades pivotally mounted within the duct, each damper blade having an end that is pivotally mounted in cantilevered fashion in the duct so that an opposing end is free to move within the duct, wherein each damper blade is planar in shape, cooperating flow straightening members disposed centrally in the air flow path, an air flow sensor for generating a signal cooperatively disposed between the flow straightening members, and an actuator for pivoting each damper blade within the duct in response to the signal from the air flow sensor to vary the damper blade position and thereby maintain a desired velocity of air exiting the damper. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with a description, serve to explain the principles of the invention. 
           [0012]      FIG. 1  is a plan view of the damper. 
           [0013]      FIG. 2  is a perspective view of the damper. 
           [0014]      FIG. 3  is a side view of the damper. 
           [0015]      FIG. 4  is a side view of the damper. 
           [0016]      FIG. 5  is cross sectional view  5 - 5  from  FIG. 4 . 
           [0017]      FIG. 6  is a cross sectional view of the air flow measurement device. 
           [0018]      FIG. 7  is a chart showing average air velocity as a function of distance from the stack damper. 
           [0019]      FIG. 8  is a chart showing average air velocity as a function of distance from the stack damper. 
           [0020]      FIG. 9  is a chart showing average air velocity as a function of distance from the stack damper. 
           [0021]      FIG. 10  is a schematic representation of an example system. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]      FIG. 1  is a plan view of the damper. Damper  100  comprises a duct defining an air flow path and generally having sides  10 ,  11 ,  12 , and  13  arranged in any form suitable for installation in air handling system ductwork. Sides  10 ,  11 ,  12 ,  13  are connected together in a substantially air tight manner to avoid air leakage. Mounting member  14  is attached to side  10 . An end of air piston  20  is connected to mounting member  14 . Sides  10 ,  11 ,  12 ,  13 ,  14  and skirt  15  and flange  16  preferably comprise galvanized metal, but may also comprise any other material suitable for HVAC service. Portion  18  is disposed between sides  10 ,  11 ,  12 , and  13  and skirt  15 . 
         [0023]    Damper blade  50  is attached in a cantilever fashion to damper shaft  30 . Damper blade  51  is attached in a cantilever fashion to damper shaft  40 . Vane  52  is fixedly connected between sides  10  and  13 . Vane  52  is fixedly connected between sides  10  and  13 . Vanes  52  and  53  are substantially parallel. Damper blades  50  and  51  are substantially parallel. Damper blades  50 ,  51  and vanes  52  and  53  are substantially flat (planar). 
         [0024]    Damper blades  50  and  51  move in unison and express substantially the same position with respect to the air flow through the damper. 
         [0025]      FIG. 1  illustrates the damper closed position. In the “closed” position the discharge area of the damper is not fully taken to zero air flow (0 CFM). Instead, the discharge area is in the range of approximately 40% to approximately 60% of the fully open discharge area. This has the effect of maintaining a suitable air flow velocity (FPM) for a suitable distance downstream of the damper during periods of reduced air flow rate (CFM). Of course, the position of damper blades  50  and  51  may be adjusted in the range of movement between fully open and the “closed” position depending upon the air flow requirements at the time. 
         [0026]    Air flow measuring sensor  60  is disposed between vanes  52  and  53 . Sensor  60  senses air flow through the damper. Vanes  52  and  53  act as air flow straightening members to improve the measurement accuracy of sensor  60  by reducing turbulence near the sensor. Vanes  52 ,  53  are substantially parallel to the air flow direction. Vanes  52  and  53  also serve to reduce turbulence and vibration in the duct caused by damper blades  50 ,  51  when they are in the partially closed or closed position. 
         [0027]      FIG. 2  is a perspective view of the damper. Vanes  52  and  53  are not moveable in the preferred embodiment, but in an alternate embodiment some movement may be desirable to adjust air flow characteristics. 
         [0028]      FIG. 3  is a side view of the damper. Damper  100  comprises sides  10 ,  11 ,  12 , and  13  arranged in any form suitable for installation in air handling system ductwork. Sides  10 ,  11 ,  12 ,  13  are connected together in a substantially air tight manner to avoid air leakage. Mounting member  14  is attached to side  10 . An end of air piston  20  is connected to mounting member  14 . Sides  10 ,  11 ,  12 ,  13 ,  14  and skirt  15  and flange  16  preferably comprise galvanized metal, but may also comprise any other material suitable for HVAC service. 
         [0029]    Skirt  15  and flange  16  are attached to an end  17  of the damper in an air tight manner to avoid air leakage. Flange  16  receives fasteners such as bolts to attach the damper to ductwork (not shown). 
         [0030]    Linkage arm  21  is connected to a damper shaft  30 . Linkage arm  22  is connected between damper shaft arm  23  and damper shaft arm  24 . Damper shaft arm  24  is connected to damper shaft  40 . 
         [0031]    Actuator  20  moves the damper blades  50 ,  51 . Actuator  20  may comprise any suitable device known in the art, including an electric motor or air piston. In this embodiment, air piston  20  is connected to a pressurized air source (not shown) and controller  603 , see  FIG. 10 . As air piston  20  extends and retracts it actuates linkage  21  and thereby damper shaft  30  and damper shaft  40 . 
         [0032]      FIG. 4  is a side view of the damper. Flange  16  mates up to adjacent system ductwork. 
         [0033]      FIG. 5  is cross sectional view  5 - 5  from  FIG. 4 . Damper blade  50  and  51  are shown in the “Closed” position. Air flow measuring device  60  is disposed between vanes  52  and  53 . 
         [0034]      FIG. 6  is a cross sectional view of the air flow measurement device. Device  60  comprises a static pressure measuring chamber  61  and a total pressure measuring chamber  62 . Relative air flow is as shown in the figure. Air flow is a function of the differential pressure between chambers  61  and  62 . Instruments known in the art (not shown) are connected to each chamber to measure the differential pressure, for example, the Honeywell model P7640A differential pressure sensor, see  601  in  FIG. 10 . 
         [0035]      FIG. 7  is a chart showing average air velocity as a function of distance from the stack damper. With the damper at full open, the average velocity at a distance of 20 feet from the damper discharge is approximately 1180 FPM. The measured air flow is approximately 8866 CFM through a representative stack damper. 
         [0036]      FIG. 8  is a chart showing average air velocity as a function of distance from the stack damper. With the damper at full open but at 60% of the air flow as in  FIG. 7  (˜5319 CFM), the average velocity at a distance of 20 feet from the damper discharge is approximately 890 FPM. This represents approximately 80% of the velocity for the full open configuration in  FIG. 7 , but at only 60% of the air flow for  FIG. 7 . 
         [0037]      FIG. 9  is a chart showing average air velocity as a function of distance from the stack damper. With the damper at 50% open and at 60% of the air flow as in  FIG. 7  (˜5319 CFM), the average velocity at a distance of 20 feet from the damper discharge is approximately 865 FPM. This represents approximately 79% of the velocity for the full open/full flow configuration in  FIG. 7 . 
         [0038]      FIGS. 7 ,  8  and  9  illustrate the capability of the stack damper to maintain appropriate air flow velocities in a stack system at reduced damper openings and air flow rates. For example, the stack damper would be used to maintain air flow velocities during off hour system operation such as during evening hours in an office building. 
         [0039]      FIG. 10  is a schematic representation of an example system. Device  60  is connected to the differential air pressure sensor  601  by tubing  61 ,  62 . Sensor  601  is electrically connected to a programmable controller  602  as required by the controller instructions (known in the art). Programmable controller  602  is connected to a motor or piston air pressure controller  603  (as described by the controller  602  and  603  instructions known in the art). Controller  603  is connected to piston  20  by pneumatic lines  27 ,  28  as shown in  FIG. 3 . Table  604  describes some of the information that can be obtained from the programmable controller  602  for other system uses. Controller  603  is operatively connected to the sensor  60  and the actuator  20  for automatically controlling pivotal movement of each damper blade  50 ,  51  in response to the measured change in the air flow so as to maintain a substantially constant discharge air velocity from the damper. 
         [0040]    Although a form of the invention has been described herein, it will be obvious to those skilled in the art that variations may be made in the construction and relation of parts without departing from the spirit and scope of the invention described herein.