Abstract:
A fume hood exhaust stack system ( 10 ) and method utilize a variable speed fan ( 24 ) and an exhaust stack ( 28 ) having an adjustable cross-sectional area. Toxic exhaust from one or more fume hoods ( 12 ) is conveyed through a header ( 16 ) to the fan. The fan forces the exhaust through the exhaust stack, and the exhaust is then discharged into the atmosphere at a sufficient velocity and momentum to ensure that the exhaust reaches an environmentally sound altitude. A variable speed drive ( 36 ), programmable controller ( 34 ), flow signals ( 26 ), and static pressure and total pressure signals ( 20 ) are utilized to modulate the speed of the fan and the area of the exhaust stack to maintain the desired fan inlet pressure and exhaust velocity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a nonprovisional application claiming priority from U.S. provisional application No. 60/201,226, filed in the United States Patent and Trademark Office on or about May 1, 2000. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to exhaust systems and methods, and more particularly to an energy efficient and environmentally sound advanced stack system and method for exhausting toxic air from fume hoods. 
     Laboratories and other facilities typically contain fume hoods in which chemical processes produce toxic fumes. These facilities necessarily contain fume exhaust stack systems that exhaust this toxic air from the building and send the toxic air through a stack to a prescribed minimum altitude such that fresh air contamination and environmental pollution is reduced. To satisfy environmental safety standards, the fume exhaust stack system must provide a minimum velocity and momentum to the toxic exhaust exiting the stack to ensure that the toxic exhaust reaches a minimum altitude substantially higher than the outlet of the stack. Due to architecture, structural, and economic limitations, however, the stack is often required to be as short as possible. 
     A typical fume hood exhaust stack system (depicted in  FIG. 1 ) consists of a stack, a constant-speed fan, a make-up air damper, fume hoods, a static pressure sensor, and a controller. In these conventional fume exhaust stack systems, the total toxic air flow rate and volume from the fume hoods are significantly below the optimal design values because the majority of the fume hoods are typically in a standby mode in which they exhaust little or no toxic air into the exhaust header. These system utilize a controller that modulates a make-up air damper to maintain a static pressure set point at the exhaust header. When the toxic air flow rate decreases to a value less than the design value, the controller opens the make-up air damper to maintain the desired static pressure set point; conversely, when the toxic air flow rate increases, the controller closes the make-up air damper to maintain the set point. Since the static pressure sensor is often located far from the mixing junction of the make-up air conduit and exhaust header discharge conduit, the static pressure at the inlet of the fan is significantly higher under partial exhaust air flow conditions than under full exhaust air flow conditions. 
     Under partial exhaust conditions—when the make-up air damper is open or partially open—the air flow rate through the fan is higher than the design value due to the higher static pressure at the inlet of the fan. The fan power consumption under these partial-exhaust high static pressure conditions is often up to 30% greater than the fan power consumed under full exhaust conditions; fan power consumption increases as exhaust air flow rate decreases. Fan power consumption increases as exhaust air flow decreases. The exhaust fans generally operate for 8,760 hours annually, while the fume hoods generally operate less than two hours per day. These existing systems therefore consume excess power and overload the fan motor when the exhausted toxic air flow from the fume hoods is less than the design value. 
     There is therefore a need for an energy efficient fume hood exhaust stack system and method that reduces fan power consumption while ensuring that the toxic exhaust discharged from the stack exits the stack with a relatively constant velocity and momentum sufficient to ensure that the toxic exhaust reaches an environmentally sound elevation. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an advanced fume exhaust stack system and method that consumes less power than conventional systems and methods. 
     It is also an object of the present invention to provide an advanced fume exhaust stack system and method that ensures that toxic exhaust discharged from the stack has a relatively constant velocity and momentum sufficient to ensure that the toxic exhaust reaches an environmentally sound elevation. 
     It is a further object of the present invention to provide an advanced fume exhaust stack system utilizing a variable-speed motor driven fan and an adjustable area stack outlet. 
     It is also an object of the present invention to utilize a programmable controller and variable speed drive to maintain the exhaust pressure at the inlet of the fan at a relatively constant desired level. 
     It is another object of the present invention to provide an advanced fume exhaust stack system that utilizes a programmable controller to modulate the outlet area of the exhaust stack to substantially maintain the total pressure at a constant total pressure set point, thereby ensuring that the toxic exhaust discharged from the stack has a relatively constant velocity and momentum sufficient to ensure that the toxic exhaust reaches an environmentally sound elevation. 
     It is yet another object of the present invention to provide an advanced fume exhaust stack system that utilizes a programmable controller to modulate the outlet area of the exhaust stack to substantially maintain the exhaust flow rate at a constant-design flow rate, thereby ensuring that the toxic exhaust discharged from the stack has a relatively constant velocity and momentum sufficient to ensure that the toxic exhaust reaches an environmentally sound elevation. 
     Accordingly, the present invention provides for an energy efficient and environmentally sound advanced fume hood exhaust stack system and method that maintains a relatively constant static pressure at the inlet of the fan and Maintains a relatively constant desired velocity and momentum of the exhaust by modulating the diameter of the exhaust stack outlet. The system and method utilize a stack with an adjustable outlet, a fan, a variable speed drive, a flow sensor or total pressure sensor, a static pressure sensor, and a controller. No make-up air is utilized. The variable speed drive receives signals from the controller and modulates the fan speed to maintain a desired static pressure set point. When the measured static pressure is less than the static pressure set point, the variable speed drive reduces the speed of the fan to increase the static pressure to the set point. When the measured static pressure is greater than the static pressure set point, the variable speed drive increases the speed of the fan to reduce the static pressure to the set point. The advanced stack system is generally controlled and operated in three modes to maintain an exhaust flow from the stack having a desired constant velocity and momentum; the controller may be programmed with a variety of algorithms, equations, and set points, including static pressure set points, total pressure set points, stack outlet diameter set points, and exhaust flow rate set points. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, which form a part of the specification and are to be read in conjunction therewith, and in which like reference numerals are used to indicate like parts in the various views: 
         FIG. 1  is a schematic of a conventional fume exhaust stack system having a constant speed fan, make-up air damper, and fixed area stack. 
         FIG. 2  is a schematic of an advanced fume exhaust stack system having a flow sensor and a controller which modulates the outlet area of the stack based on the measured exhaust air flow. 
         FIG. 3  is a schematic of an advanced fume exhaust stack system having a total pressure sensor, flow sensor, and a controller which modulates the outlet area of the stack to maintain a total pressure set point. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in greater detail, and initially to  FIG. 2 , an advanced fume hood exhaust stack system is designated generally by the numeral  10 . One or more fume hoods  12  collect and discharge toxic exhaust through individual fume hood exhaust conduits  14  and into a common exhaust header  16 . Individual fume hood exhaust dampers  18  may be positioned in the individual fume hood exhaust conduits  14 , as depicted in  FIG. 2 , to enable a particular fume hood  12  to be isolated from the system. 
     A static pressure sensor and transmitter  20  is located at exhaust header  16  and measures the static pressure of the exhaust within the header  16 . The pressure sensor and transmitter  20  is adapted to transmit a signal proportional to the static pressure of the exhaust within the header  16 . The proportional transmitter signal may be a pulse signal, a 4-20 mA signal, or other electrical or digital signal commonly employed by and well known to those skilled in the art. 
     A header discharge conduit  22  conveys exhaust from the header  16  to the inlet of a fan  24 . The fan  24  is generally motor driven. As seen in  FIG. 2 , a flow sensor and transmitter  26  is located at header discharge conduit  22 , upstream of the fan  24 , and measures the flow rate of the exhaust flowing from the header  16  to the inlet of the fan  24 . The flow sensor and transmitter  26  is adapted to transmit a signal proportional to the flow rate of the exhaust flowing from the header  16  to the inlet of the fan  24 . The proportional transmitter signal may be a pulse signal, a 4-20 mA signal, or other electrical or digital signal commonly employed by and well known to those skilled in the art. 
     Exhaust is conveyed from the exhaust header  16 , through the header discharge conduit  22  and motor-driven fan  24 , into and through exhaust stack  28 , and into the atmosphere. Exhaust stack  28  has an inlet  30  and an outlet  32 . The outlet  32  of exhaust stack  28  has an adjustable area; the area may be increased or decreased by varying the diameter of the outlet  32  or otherwise modulating the area through which exhaust exits the stack  28  and is ejected into the atmosphere. The area of outlet  32  is modulated by a controller  34 . The controller  34  is typically a programmable logic controller (PLC) or other programmable controller of the type commonly used by and well known to those skilled in the art. The controller  34  receives and processes a signal from the static pressure sensor and transmitter  20  proportional to the static pressure of the header  16 . The controller  34  also receives and processes a signal from the flow sensor and transmitter  26  proportional to the rate of exhaust flow from the header  16  to the fan  24 . The controller may be programmed with a variety of desired set points, including various static pressure set points, total pressure set points, stack outlet diameter set points, and design exhaust flow rates. The controller  34  is adapted to transmit a signal to variable speed drive  36  which, in turn, is adapted to transmit a signal to the electric motor of motor-driven fan  24  to modulate the speed of fan  24 . It will be understood that variable speed drive  36  may be a variable frequency drive or other electrical or electromechanical drive (e.g. an eddy current drive or viscous drive) commonly used and well known to those skilled in the art. 
     In operation, and in the configuration depicted in FIG.  2  and described above, the controller  34  is programmed with a desired static pressure set point, a design flow rate, and a maximum design diameter of stack outlet  32  for the design flow rate. The controller modulates the diameter of outlet  32  based on the flow rate measured by flow sensor and transmitter  26 . The set point of the diameter of outlet  32  is calculated from the following equation and is based on the measured flow rate:
 
 D=D   o ( Q/Q   o )
 
Where Q o  is the design flow rate, Q is the flow rate measured by flow sensor and transmitter  26 , and D o  is the maximum design diameter of outlet  32 .
 
     As the variable speed drive  36  increases the speed of fan  24 , the flow rate of exhaust from header  16  to stack  28  increases and the static pressure at header  16  and the inlet of fan  24  decreases toward a desired static pressure set point. As the variable speed drive  36  decreases the speed of fan  24 , the flow rate of exhaust from header  16  to stack  28  decreases and the static pressure at header  16  increases toward the desired static pressure set point. In this manner, the static pressure at header  16  is substantially maintained at the programmed static pressure set point. 
     To maintain a relatively constant desired exhaust velocity and momentum at the outlet  32  of stack, the diameter of stack outlet  32  is modulated by the controller  34  in accordance with the above programmed equation. If the measured flow rate Q exceeds the design flow rate Q o , the controller  34  reduces the diameter D of stack outlet  32 , thereby reducing the flow rate Q measured by flow sensor and transmitter  26 . If the measured flow rate Q is less than the design flow rate Q o , the controller  34  increases the diameter D of stack outlet  32 , thereby increasing the flow rate Q measured by flow sensor and transmitter  26 . In this manner, the flow rate Q is continually modulated toward the programmed design flow rate Q o  to provide a relatively constant and sufficient exhaust velocity and momentum as the exhaust exits the stack  28  through outlet  32 . 
     Referring now to  FIG. 3 , another embodiment of the advanced fume hood exhaust stack system  10  is depicted. In this configuration, as in the configuration depicted in  FIG. 2 , one or more fume hoods  12  discharge exhaust through conduits  14  and into an exhaust header  16 . Individual fume hood exhaust dampers  18  may be used to isolate a particular fume hood  12  from the system. Exhaust from header  16  flows through header discharge conduit  22 , through motor-driven fan  24 , and through exhaust stack  28 . Stack  28  has an inlet  30  and an adjustable area outlet  32 . The area of outlet  32  is modulated increased or decreased—by a programmable controller  34 . 
     Again referring to  FIG. 3 , a static pressure sensor and transmitter  20  measures the static pressure at the header  16  and transmits a signal proportional to the static pressure of the exhaust within the header  16  to the programmable controller  34 . A header discharge conduit  22  conveys exhaust from the header  16  to the inlet of the motor-driven fan  24 . A flow sensor and transmitter  26  is located at header discharge conduit  22 , upstream of the fan  24 , and measures the flow rate of the exhaust flowing from the header  16  to the inlet of the fan  24 . The flow sensor and transmitter  26  is adapted to transmit to the controller  34  a signal proportional to the flow rate of the exhaust flowing from the header  16  to the inlet of the fan  24 . In addition to the static pressure sensor and transmitter  20  and the flow sensor and transmitter  26 , the system depicted in  FIG. 3  includes a total pressure sensor and transmitter  38 . Total pressure sensor and transmitter  38  measure the total pressure of the exhaust within header discharge conduit  22  downstream of fan  24  and upstream of exhaust stack  28 , as seen in  FIG. 3 , and transmits a signal proportional to the total pressure to the controller  34 . The proportional pressure signal, flow rate signal, and total pressure signal transmitted to the controller  34  may be pulse signals, 4-20 mA signals, or other electrical or digital signals commonly employed by and well known to those skilled in the art. The controller is adapted to transmit a signal to the variable speed drive  36  which, in turn, is adapted to transmit a signal to the electric motor of motor-driven fan  24  to modulate the speed of fan  24 . 
     In one mode of operation, the system depicted in  FIG. 3  utilizes the programmable controller  34  to modulate a total pressure set point based upon the diameter of the outlet  32  of exhaust stack  28 . In this mode of operation, the controller requires signals from the static pressure sensor and transmitter  20  and the total pressure sensor and transmitter  38 ; input from the flow sensor and transmitter  26  is not required. The set point of the total pressure is determined by the following equations: 
         P   set     =       P   0     ⁢           (       D   0     D     )     2     ⁡     [     1   +       f   0     ⁢     L     D   0       ⁢       D   0     D       +         k   e     ⁡     (     D     D   0       )       2       ]         1   +       f   0     ⁢     L     D   0                   
         P   0     =       (     1   +       f   0     ⁢     L     D   0           )     ⁢       V   0   2       2   ⁢   g       ⁢   ρ         
  f   o   =f ( x   0 )
 
 x   0   =ε/D   0 
 
 f=f ( x )
 
 x=ε/D 
 
 f ( x )=−53129 x   4 +6033.6 x   3 −233.99 x   2 +4.434 x+ 0.013
 
         k   e     =       0.9598   ⁢           ⁢       (     D     D   0       )     4       -     1.9541   ⁢           ⁢       (     D     D   0       )     2       +   0.9818         
 
     Where L is the length of the exhaust stack  28 , D 0  is the maximum diameter of the adjustable stack outlet  32 , V 0  is the design velocity of the exhaust at the outlet  32 , and ε is the roughness of the inner surface of the stack  28 . 
     In another mode of operation, the system depicted in  FIG. 3  utilizes the programmable controller  34  to modulate the diameter of the outlet  32  of exhaust stack  28  to maintain a desired total pressure set point. The total pressure set point is determined based upon the above equations. In this mode of operation, the controller  34  requires signals from the static pressure sensor and transmitter  20 , the total pressure sensor and transmitter  36 , and the flow sensor and transmitter  26 . 
     In operation, the controller  34  is programmed with a desired static pressure set point, a design flow rate, and a maximum design diameter of stack outlet  32 . To maintain the static pressure at header  16  at a substantially constant programmed set point, the controller  34  and variable speed drive  36  modulate the speed of motor-driven fan  34 . To decrease the static pressure at header  16  to a desired static pressure set point, variable speed drive  36  increases the speed of the fan  24  to increase the flow rate of the exhaust from header  16 . To increase the static pressure to a desired static pressure set point, variable speed drive  36  decreases the speed of fan  24  to decrease the flow rate of the exhaust from header  16 . In this manner, the speed of fan  24  is continually modulated to substantially maintain the static pressure at the desired programmed set point. 
     To maintain the velocity and momentum of the exhaust exiting stack outlet  32  at a substantially constant minimum level, the system depicted in  FIG. 3  utilizes total pressure sensor and transmitter  38  to measure the total pressure at the inlet  30  of stack  28  and transmit to controller  34  a signal proportional to the measured total pressure. If the measured total pressure is less than the programmed total pressure set point, minus a dead band value, the diameter of the stack outlet  32  is decreased by controller  34  to satisfy the set point. If the measured total pressure is greater than the programmed total pressure set point, plus a dead band value, the diameter of the stack outlet  32  is increased by controller  34  to satisfy the set point. The total pressure set point is generally updated at programmed time intervals (e.g. every three minutes). In this way, the total pressure is maintained at a minimum desired value, and the velocity and momentum of the exhaust passing through stack  28  are maintained at desired substantially constant values to ensure that the exhaust reaches the desired altitude upon exiting stack outlet  32 . 
     It will be seen from the foregoing that this invention is one well adapted to attain the ends and objects set forth above, and to attain other advantages which are obvious and inherent in the system and method. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and within the scope of the claims. It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, all matter shown in the accompanying drawings or described hereinabove is to be interpreted as illustrative and not limiting.