Abstract:
A multi-stack exhaust system and method including at least one fume hood, multiple exhaust stacks, each stack having a return duct with a return damper and a discharge duct with a discharge damper, a fan, a flow sensor, a first static pressure sensor for measuring the static pressure at the inlet duct, a second static pressure sensor for measuring the static pressure at the at least two exhaust stacks, and a controller.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 60/315,475, filed on Aug. 28, 2001. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     TECHNICAL FIELD 
     This invention relates generally to exhaust systems, and more particularly to a stack exhaust system designed to minimize energy consumption. 
     BACKGROUND OF THE INVENTION 
     The goal of stack exhaust systems is to exhaust toxic air from buildings to heights sufficient to avoid fresh air contamination and environmental pollution. Further considerations include prevention of condensed moisture in the stack and prevention of rain in the stack. Due to architectural and structural requirements, stack heights of stack exhaust systems are often required to be as short as possible. In order to satisfy environmental concerns and meet the architectural and structural requirements, stack exhaust systems are designed to exhaust air at sufficient velocities to create sufficient momentum to send the toxic air substantially higher than the stack and avoid contamination by the toxic air or recirculation of the toxic air. 
     FIG. 1 depicts a prior art stack exhaust system flow chart, which includes a stack  28 , a fan  24 , a make-up air damper  36 , fume hoods  12 , a static pressure sensor  20 , and a controller  34 . Because the fume hoods  12  are in standby mode most of the time, the total exhaust airflow from the fume hoods  12  is usually below the airflow for which the system is optimally designed. 
     The stack exhaust system of the prior art maintains the static pressure of the system by adjustment of the make-up air damper  36 . That is, in order to maintain a constant discharge velocity, the system most operate at a constant volume. Therefore, when the total exhaust airflow from the fume hoods  12  is less than the design airflow, the controller opens the make-up air damper  36  to maintain the static pressure of the system. 
     The static pressure is typically measured at a common exhaust header  16  different than the mixing-joint  85  of the make-up air and the exhaust air. For example, the static pressure sensor  20  may be located either in the exhaust system farthest from the main riser or in the main plenum, because adequate static pressure must be maintained in the ductwork farthest from the exhaust fan plenum. As a result, the static pressure at the inlet of the fan is much greater under partial-exhaust airflow with make-up air conditions, than under full-exhaust airflow without make-up air conditions, and, thus, the airflow through the fan is higher than the design airflow. Consequently, fan energy consumption is higher under conditions of partial-exhaust airflow than under design conditions of full-exhaust airflow. In addition, fan motor overload is common due to the higher-than-design airflow. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a multi-stack exhaust system and method. It is desirable for the system to include at least one fume hood adapted for intake of toxic exhaust into an inlet duct in fluid communication with the at least one fume hood. In addition, it is desirable for the system to include at least two exhaust stacks, each stack having a return duct with a return damper and a discharge duct with a discharge damper. The return duct returns air and/or toxic exhaust to the inlet duct and the discharge duct discharges toxic exhaust to the environment. In another desirable embodiment the system further includes a fan, adapted to convey the toxic exhaust from the inlet duct to the at least two exhaust stacks. The system also may include a flow sensor for measuring the flow of toxic exhaust in the inlet duct, wherein the discharge dampers are adjusted such that the total flow from the discharge ducts is approximately equal to the flow of toxic exhaust in the inlet duct. It is also desirable that a first static pressure sensor for measuring the static pressure at the inlet duct, wherein an inlet duct static pressure set point is maintained by adjustment of the return dampers. Finally, it is desirable that a second static pressure sensor for measuring the static pressure at the at least two exhaust stacks, wherein an outlet duct static pressure set point is maintained by adjustment of the speed of the exhaust fan. 
     A desirable method for exhausting toxic exhaust includes generating toxic exhaust in at least one fume hood and passing the exhaust from the at least one fume hood to an inlet duct in fluid communication with the at least one fume hood. It is also desirable that the method include conveying the toxic exhaust to at least two exhaust stacks by a fan, each stack having a return duct with a return damper and a discharge duct with a discharge damper, wherein the return duct returns air and/or toxic exhaust to the inlet duct and the discharge duct discharges toxic exhaust to the environment. The method may also include measuring the flow of toxic exhaust in the inlet duct, wherein the discharge dampers are adjusted such that the total flow from the discharge ducts is approximately equal to the flow of toxic exhaust in the inlet duct. Also, it is desirable that the method include measuring the static pressure at the inlet duct, wherein an inlet duct static pressure set point is maintained by adjustment of the return dampers and measuring the static pressure at the at least two exhaust stacks, wherein an outlet duct static pressure set point is maintained by adjustment of the speed of the exhaust fan. 
     Additional objects, advantages, and novel features of the invention will be set forth in the description that follows and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, which form a part of the specification and are to be read in conjunction therewith: 
     FIG. 1 is a flowchart of a prior art stack exhaust system; and 
     FIG. 2 is a flowchart of a multi-stack exhaust system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A multi-stack exhaust system generally embodying the principles of the invention is shown in FIG.  2 . The multi-stack exhaust system of the present invention maintains the toxic exhaust exit velocity or momentum from the stacks. Further, the fan energy consumption is minimized by adjusting the flow rate through the stacks and adjusting the number of stacks in operation. 
     At least one fume hood  12  collects and discharges 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 multiple exhaust stacks  28 , and into the atmosphere. 
     Each exhaust stack  28  has an adjustable discharge air damper  30  and an adjustable return air damper  32 . The discharge air dampers  30  are adjusted based upon the measured flow rate at the flow sensor and transmitter  26  and based upon the design flow of each stack. The discharge air dampers  30  are modulated by a controller  34 . Preferably, the discharge air dampers  30  are adjusted such that the number of discharge air dampers  30  in the open position are minimized. 
     The discharge air dampers  30  and the return air dampers  32  are interlocked for each exhaust stack  28 . When the discharge air damper  30  is open, the return air damper  32  is closed. When the discharge air damper  30  is closed, the return air damper  32  is open. The return air dampers  32  are located on return air ducts  34 . The return air ducts  34  prevent leakage air to the environment from the stacks  28 . 
     A manual damper  36  is preferably installed in the return air duct  34 . The manual dampers  36  are used to set up and adjust the return airflow. 
     A static pressure sensor and transmitter  38  is located between the fan  24  and the exhaust stacks  28  and measures the static pressure of the exhaust exiting the fan  24 . The static pressure sensor and transmitter  38  is adapted to transmit a signal proportional to the static pressure of the exhaust exiting 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. 
     The set point of the static pressure of the exhaust exiting the fan  24  is set to maintain a minimum required exit velocity and momentum of the exhaust from the stacks  28 . The controller  34  modulates the fan speed to maintain the static pressure set point of the exhaust exiting the fan  24 . If the static pressure is lower than the set point, the controller  34  will speed up the fan  24 , and vice versa. 
     The set point of the static pressure at exhaust header  16  (before the fan  24 ) depends on the requirements of the fume hoods  12  and layout of the exhaust system ductwork. The controller  34  modulates a makeup air damper  36  to maintain the static pressure set point at exhaust header  16 . If the pressure is lower than the set point, the controller will open makeup air damper  36 , and vice versa. 
     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 . Further, the controller  34  received and processes a signal from the static pressure sensor and transmitter  38  proportional to the static pressure between the fan  24  and the exhaust stacks  28 . The controller may be programmed with a variety of desired set points, including various static pressure set points, total pressure set points, and design stack exhaust flow rates. The controller  34  is adapted to transmit a signal to a 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 electro-mechanical drive (e.g. an eddy current drive or viscous drive) commonly used and well known to those skilled in the art. The controller is also adapted to transmit a signal to the adjustable discharge air damper  30 , the adjustable return air damper  32 , and the make-up air damper  36 . 
     The measured toxic exhaust flow rate should be less than the sum of the multiple stack design flow rates calculated under conditions of the air dampers being open. Preferably, the total stack toxic exhaust flow rate is close to or equal to the toxic exhaust flow rate entering from the fume hoods. 
     From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects herein above set forth together with other advantages which are obvious and inherent to the formulation. 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 is within the scope of the claims. Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth is to be interpreted as illustrative and not in a limiting sense.