Abstract:
A fume exhaust stack system that includes an exhaust stack, at least one fume hood, a plurality of stack pieces, a retractable expander, a sensor, and a controller. The exhaust stack is coupled to the at least one fume hood and is adapted to emit exhaust conveyed by the at least one fume hood. The retractable expander is positioned at the exhaust stack, and is moveable between a first position in which the retractable expander is extended and a second position in which the retractable expander is retracted relative to the first position. Movement of the retractable expander between the first and second positions is operable to adjust a cross-sectional area of the exhaust stack. Based on an output signal from the sensor, the controller outputs a control signal to move the retractable expander into one of the first and second positions.

Description:
RELATED APPLICATION  
       [0001]     This application claims priority to U.S. Provisional Patent Application Ser. No. 60/701,594, filed on Jul. 22, 2005, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD  
       [0002]     Embodiments of the invention relate generally to exhaust systems and methods, and particularly to systems and methods to improve efficiency of central exhaust systems.  
       BACKGROUND  
       [0003]     Various types of facilities, such as research buildings, industry production facilities, medical buildings, manufacturing assemblies, and laboratories, often use exhaust systems equipped with fume hoods in order to process toxic fumes. Generally, an exhaust system includes a fan by which to draw fumes into the exhaust system, and a stack system by which to emit fumes into the atmosphere at predetermined altitudes.  
         [0004]     Differential safety requirements generally dictate the altitudes at which fumes are to be exhausted. To reach those altitudes, exhaust systems must emit fumes at predetermined velocities and pressures. For example, a make-up damper linked to the exhaust system can be opened to maintain a constant static pressure either at the fume hoods or at an inlet of the fan, when the fan is run at a constant speed. In such cases, however, the fan continues to consume the designed power regardless of the level of exhaust. In some cases, the fan can consume 50 percent more power than actually required. In addition, when airflow is low, excessive negative static pressure can result, which leads to noise problems and control stability problems with respect to operation of the fume hoods.  
       SUMMARY  
       [0005]     Embodiments of the invention provide an energy-efficient exhaust system that can be installed as a new exhaust system or can be retrofitted to existing exhaust systems.  
         [0006]     In one embodiment, the invention provides a fume exhaust stack system that includes an exhaust stack, at least one fume hood, a plurality of stack pieces, a retractable expander, a sensor, and a controller. The exhaust stack is coupled to the at least one fume hood, and is adapted to emit exhaust conveyed by the at least one fume hood. The retractable expander is positioned at the exhaust stack and is moveable between a first position in which the retractable expander is extended and a second position in which the retractable expander is retracted relative to the first position. Movement of the retractable expander between the first and second positions is operable to adjust a cross-sectional area of the exhaust stack. The sensor is positioned near the exhaust stack and outputs a signal indicative of an exhaust condition. The controller is coupled to the sensor, receives the output signal from the sensor, and, based on the output signal, outputs a control signal to move the retractable expander into one of the first and second positions.  
         [0007]     In another embodiment, the invention provides a method of controlling exhaust emissions from an exhaust stack having a plurality of stack pieces, wherein a retractable expander is positioned at the plurality of stack pieces. The method includes sensing an exhaust condition at the exhaust stack, comparing the condition with a threshold, retracting the retractable expander to reduce a cross-sectional area of the exhaust stack when the condition is below the threshold, and extending the retractable expander to enlarge the cross-sectional area of the exhaust stack when the condition is above the threshold.  
         [0008]     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a schematic of a fume exhaust stack system.  
         [0010]      FIG. 2  is a perspective view of a variable diameter stack that can be used with the fume exhaust stack system of  FIG. 1 .  
         [0011]      FIG. 3  is a top view of the variable diameter stack of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0012]     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.  
         [0013]     Embodiments of the invention provide fume exhaust stack systems that include a variable diameter stack. Embodiments herein can control an exit velocity of exhaust by adjusting a diameter of the stack when the exhaust airflow rate changes. Additionally, embodiments herein can maintain a constant static pressure or a required static pressure at different locations of a fume exhaust stack system. In some embodiments, a retractable expander is employed to adjust the diameter. In other embodiments, a fume exhaust stack system includes a fan that is controlled by a controller and a variable frequency drive. By modulating the speed of the fan, the fume exhaust stack system can minimize power consumption.  
         [0014]      FIG. 1  is a schematic of a fume exhaust stack system  100  having a variable diameter stack (“VDS”)  104  on top of an outlet duct  108 . The fume exhaust stack system  100  includes one or more fume hoods (not shown) coupled to an inlet duct  112 . The fume exhaust stack system  100  also includes a first sensor  116  located at the inlet duct  112 . The fume exhaust stack system  100  uses a fan  120  to draw exhaust from the fume hood(s) to the inlet duct  112  in a direction indicated by arrows  124 . In the embodiment shown, a variable frequency drive (“VFD”)  128  drives the fan  120  and controls a speed of the fan  120 . The fume exhaust stack system  100  also includes a second sensor  132  located below the VDS  104  in the outlet duct  108 . The fan  120  continues to convey the exhaust to the outlet duct  104  in a direction indicated by arrows  136 .  
         [0015]     The sensors  116 ,  132  monitor, sense, measure, or determine one or more conditions of the fume exhaust stack system  100 . For example, the sensors  116 ,  132  sense conditions indicative of a static pressure at the respective inlet duct  112  and outlet duct  108 . Sensed conditions can then be converted into calibrated signals that are indicative of the static pressures of the fume exhaust stack system  100 . The sensors  116 ,  132  can be equipped with calibration circuitry and/or microprocessors that internally convert the static pressures to a calibrated form. Alternatively, the sensed conditions can be converted into calibrated signals by other external processes or devices in a manner known in the art. In the embodiment shown, the sensor  116  measures a static pressure near the inlet duct  112 , and the sensor  132  measures a total static pressure near the outlet duct  108 . Although only one sensor is shown at the inlet duct  112  and the outlet duct  108 , respectively, the fume exhaust stack system  100  can include additional sensors.  
         [0016]     In some embodiments, the fume exhaust stack system  100  includes multiple fume hoods. In such embodiments, the fume hoods can be connected to the inlet duct  112  via ductwork, and the first pressure sensor  116  can be mounted near the fume hood that is farthest from the fan  120 .  
         [0017]     A controller  140  receives the sensed conditions from the sensors  116 ,  132 , processes the conditions, and adjusts the VFD  128  and an actuator  144 . For example, the controller  140  compares each of the sensed conditions with a corresponding condition set point stored in a memory (not shown) of the fume exhaust stack system  100 , or in the controller  140 . Once the controller  140  has compared each of the sensed conditions with the corresponding condition set point, the controller  140  adjusts the speed of the fan  120 . For example, in the embodiment shown, when the static pressure determined at the first sensor  116  near the inlet duct  112  is greater than the corresponding static pressure set point at the inlet duct  112 , the controller  140  speeds up the fan  120  via the VFD  128 . Conversely, when the static pressure determined at the first sensor  116  near the inlet duct  112  is less than the corresponding static pressure set point at the inlet duct  112 , the controller  140  slows down the fan  120  via the VFD  128 .  
         [0018]     Furthermore, when the second condition determined at the second sensor  132  is greater than the corresponding set point, the controller  140  sends a signal to the actuator  144  to enlarge a cross-sectional area or a diameter of the VDS  104 . Conversely, when the second condition determined at the second sensor  132  is less than the corresponding set point, the controller  140  sends a signal to the actuator  144  to reduce a cross-sectional area or a diameter of VDS  104 .  
         [0019]      FIG. 2  is a perspective view of an exemplary variable diameter stack system  200  that can be used to implement the VDS  104  of  FIG. 1 , wherein like numerals are used to refer to like parts. The VDS  104  includes two or more stack pieces  204  resulting from vertically cutting a portion of the outlet duct  108 . In some embodiments, the stack pieces  204  are of equal height. Each of the stack pieces  204  is joined to another of the stack pieces  204  at a joint  208  with a seal  212 . Although three stack pieces  204  are shown in  FIG. 2 , the VDS  104  can include more stack pieces  204 . When joined together with the seals  212 , the stack pieces  204  form a substantially cylindrical VDS  104  with a cross-sectional area and a stack diameter that are substantially similar to those of the outlet duct  108 . In some embodiments, the length (L) of each of the stack pieces  204  or the vertical cut is expressed in EQN. (1).  
             L   =       1   -     α         2   ⁢           ⁢   sin   ⁢           ⁢   β               (   1   )               
 In EQN. (1), α is a ratio between a minimum desired exhaust airflow rate and a maximum desired exhaust airflow rate, and β is a maximum allowable bend angle of a stack piece  204  that depends on materials used for the stack pieces  204 . In some embodiments, the maximum allowable bend angle of the stack piece  204  is less than about 15°. In some embodiments, the length L is generally less than two times the variable stack diameter. 
 
         [0020]     In the embodiment shown, the seals  212  are made of corrosion-resistant rubber. However, the seals  212  can be made of other corrosion-resistant materials. The mechanical properties of the seals  212 , aided by the static pressure at the outlet duct  108 , substantially prevent air leakage at the joints  208 .  
         [0021]      FIG. 2  also shows a retractable expander assembly  216  that can be actuated by the actuator  144 . In the embodiment shown, the retractable expander assembly  216  includes multiple spring arms  220  positioned within the VDS  104 . In some embodiments, the spring arms  220  are spaced apart radially in an equiangular manner. Each of the spring arms  220  includes a spring  228  and an expander rod or extendable arm  232  contained in a guide track or housing  236 . One end of the extendable arm  232  is supported by the spring  228  in the housing  236 , and the other end of the extendable arm  232  protrudes from the housing  236  and is mounted on a stack piece  204  at a midpoint  233  between the joints  208  of the stack pieces  204 . In the embodiment shown, the housing  236  is about 60 percent of the length of the spring arm  220 , and the extendable arm  232  is about 40 percent of the length of the spring arm  220 . In other embodiments, other dimensional ratios can be used. The distance between the top edge of the VDS  104  and the midpoint  233  can be determined based on EQN. (2).  
             Y   =         (         1   -     α       2     -   Z     )     /   sin     ⁢           ⁢   β             (   2   )               
 In EQN. (2), Y is a distance from the top of the VDS  104  to the midpoint  233 , and Z is a ratio between a length of the extendable arm  232  and the stack diameter. When extended, the spring arms  220  push the stack pieces  204  radially outwardly, thereby enlarging the diameter of the VDS  104 . When retracted, the spring arms  220  move radially inwardly, thereby reducing the diameter of the VDS  104 . 
 
         [0022]     In the embodiment shown, the retractable expander  216  also includes a chain  240  guided by a plurality of guide rings  244  positioned external to the VDS  104 . In other implementations, other types of devices can be employed in lieu of a chain, such as cable, wire, rope, other devices including links, or compressive devices (e.g., a vise positioned external to the stack pieces  204 ). As shown, each of the stack pieces  204  has one guide ring  244 . The guide rings  244  are generally mounted on an exterior center of the stack piece  204 . In the embodiments shown, the maximum distance between a guide ring  244  and the midpoint of the joints  208  is less than 10 percent of the stack diameter. In other embodiments, a stack piece  204  may have multiple guide rings  244 . During operation of the fume exhaust stack system  200 , the chain  240  is generally wrapped around the VDS  104 . When the actuator  144  extends the chain  240 , the stack pieces are released tangentially, thereby enlarging the cross-sectional area. Conversely, when the actuator  144  retracts the chain  240 , the stack pieces  204  are compressed tangentially, thereby reducing the cross-sectional area. In some embodiments, the actuator  144  uses linear or rotational motion to extend or to retract the chain  240 .  
         [0023]      FIG. 3  is a top view of the VDS  104  of  FIG. 2 . View A of  FIG. 3  shows that each of the stack pieces  204  is joined to another of the stack pieces  204  at the joint  208  with a seal  212 . In addition, the spring arms  220  have a length of R, and the extendable arms  232  have a length of b. The stack diameter is therefore 2R.  
         [0024]     As described earlier, when the chain  240  is retracted, the extendable arms  232  are compressed against the springs  228 . The extendable arms  232  are therefore pushed by the stack pieces  104  inwardly. As a result, the VDS  104  is compressed radially inwardly in a direction indicated by arrow  288 , while the stack pieces  204  are compressed tangentially in a direction indicated by arrow  290 . Thus, R and b are reduced, thereby reducing the diameter and the cross-sectional area of the VDS  104 .  
         [0025]     When the chain  240  is extended, pressure is exerted on the extendable arms  232 , resulting in lessened pressure against the springs  228 . As such, the extendable arms  232  push against the stack pieces  204  until the extendable arms  232  protrude from the housing  236  by a value of b, and the stack pieces  204  are released tangentially in a direction indicated by arrow  294 . As a result, the VDS  104  can expand radially outwardly in a direction indicated by arrow  292 . Thus, R and b are increased, thereby enlarging the diameter and the cross-sectional area of the VDS  104 .  
         [0026]     Accordingly, when the condition determined at the second sensor  132  of  FIG. 1  is greater than a predetermined condition set point, the chain  240  can be extended, thus enlarging the cross-sectional area of the VDS  104  in order to cause the condition to decrease and converge with the set point. Conversely, when the condition determined at the second sensor  132  of  FIG. 1  is less than the predetermined condition set point, the link  240  can be retracted, thus reducing the cross-sectional area of the VDS  104  in order to cause the condition to increase and converge with the set point.  
         [0027]     Other embodiments of the invention may be implemented. For instance, in addition to, or in lieu of, a chain placed external to the stack pieces  204 , an actuator may be positioned in or near the housing  236  to electromechanically expand and retract the extendable arms  232 . It is to be appreciated that such components may need to have corrosion-resistant properties.  
         [0028]     Various features of the invention are set forth in the following claims.