Abstract:
The high velocity and high dilution exhaust system uses a centrifugal fan provided with a tapered nozzle. The nozzle compresses the airstream exiting the fan to increase back pressure and velocity. The air flow from the fan enters a stack having a venturi further increasing the velocity and decreasing the pressure. The decrease in pressure causes a suction, allowing the introduction of ambient air to mix with and dilute the output of the fan. The total discharge from the exhaust stack has a high velocity resulting in a plume height and effective height of the exhaust before dispersion occurs.

Description:
BACKGROUND OF THE INVENTION 
   Industrial and institutional processes often produce fumes required to be exhausted and removed from the immediate area of the building. Exhaust systems include ducts, hoods, and exhaust fans to extract the contaminated fumes. Specific applications, such as laboratory or processing exhaust, are hazardous and must be exhausted to insure the safety of those working in close proximity to the source of the exhausted effluent. Safety concerns extend not only to those in the immediate area where the fumes are generated, but also to others located in the building as well as occupants in surrounding buildings. 
   Improperly designed exhaust systems that ineffectively discharge high concentrations of effluent can result in entrainment of the hazardous or noxious exhaust into the building air conditioning system, contaminating the fresh air brought into the building. 
   Problems are encountered in particular where the contaminated exhaust is heavier than air, is corrosive or has a foul odor. In these instances it is necessary to displace the exhaust at a height allowing dispersement to negate the possibility of concentration of the effluent at ground level. 
   In applications where exhaust needs to be displaced high above ground level, exhaust fans and stacks are typically placed on roof tops. To insure the displacement at levels high above ground level, it is known to use long exhaust stacks having an exit orifice at the desired height. Often, the stacks are so long as to be unstable and require the use of guy wires or other braces to ensure their stability, especially if high wind conditions are ever expected. 
   There is a need in the prior art for an improvement in the design of a fan and stack to deliver fumes to a maximum possible height, before dispersion of the exhaust within the environment occurs to allow complete dissipation and prevent concentration and contamination of the buildings at lower levels. 
   It is an object of the invention to provide an exhaust fan having a high plume height. 
   It is another object of the invention to have an exhaust fan having a compact configuration. 
   It is yet another object of the invention to provide a exhaust fan requiring low energy but having a high exhaust velocity. 
   It is another object of the invention to provide an exhaust fan allowing dispersement at a height preventing exhaust from reentering a building through an air conditioning system or other roof mounted equipment. 
   It is still another object of the invention to allow dispersement of exhaust eliminating costly corrosion caused by exhaust vapors. 
   It is another objective of the invention to provide an exhaust for diluting the exhaust before exiting the exhaust stack. 
   These and other objects of the invention will become apparent to one of ordinary skill in the art after reviewing disclosure of the invention. 
   SUMMARY OF THE INVENTION 
   The high velocity and high dilution exhaust system uses a centrifugal fan provided with a tapered nozzle. The nozzle compresses the airstream exiting the fan to increase back pressure and velocity. The air flow from the fan enters a stack having a venturi further increasing the velocity and decreasing the pressure. The decrease in pressure causes a suction, allowing the introduction of ambient air to mix with and dilute the output of the fan. The total discharge from the exhaust stack has a high velocity resulting in a plume height and effective height of the exhaust before dispersion occurs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is a end view of a prior art exhaust system; 
       FIG. 1   b  is a end view of the high velocity and high dilution exhaust system of the invention; 
       FIG. 2  is a side view of the exhaust system of the invention attached to a plenum; 
       FIG. 3  is a side view of a second centrifugal fan and second embodiment of the exhaust stack; 
       FIG. 4  is a side view of a centrifugal fan having a third embodiment of the exhaust stack; 
       FIG. 5  is a side view of a centrifugal fan having a fourth embodiment of the exhaust stack; 
       FIG. 6  is an end view of the centrifugal fan and fifth embodiment of the exhaust stack; 
       FIG. 7  is an side view of a centrifugal fan and a sixth embodiment of the exhaust stack; 
       FIG. 7   a  is a view of an alternative nozzle cap useable with the exhaust system of  FIG. 7 ; 
       FIG. 7   b  is a view of an alternative nozzle cap useable with the exhaust system of  FIG. 7 ; 
       FIG. 7   c  is a view of an alternative nozzle cap useable with the exhaust system of  FIG. 7 ; 
       FIG. 8  is a side view of a centrifugal fan and a seventh embodiment of the exhaust stack; 
       FIG. 9  is a side view of a centrifugal fan and an eighth embodiment of the exhaust stack; 
       FIG. 10  is a side view of a centrifugal fan and ninth embodiment of the exhaust stack; and 
       FIG. 11  is a side view of the centrifugal fan with a tenth embodiment of the exhaust stack. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1   a  shows a conventional exhaust system, as may be mounted on a roof. The fan  20 , such as a centrifugal fan, powered by motor  24 , receives exhaust from the ventilation system of the building and sends exhaust through exhaust stack  40 . Upon exiting the top of the exhaust stack  40 , the exhaust travels a short distance before dissipating within the ambient air. The total distance of the stack and distance traveled before dispersement is shown as the effective height. 
     FIG. 1   b  shows a centrifugal fan having one of the exhaust stacks usable with the invention. The exhaust leaves the stack  50  with high velocity and stream integrity and has a plume height giving an effective height equal to that of prior art devices having a high stack. The invention has the advantage of diluting the effluent with a compact configuration. 
     FIG. 2  shows the centrifugal fan and exhaust stack as part of a ventilation system. Exhaust is received through a duct  30  which terminates at the inlet plenum  36 . The inlet plenum  36  is provided with an ambient by-pass  32  having by-pass damper with louver  34 . Within the plenum, the exhaust from duct  30  and ambient air through by-pass  32  forms the inlet fan flow which enters centrifugal fan  20  through isolation damper  38 . Motor  24  powers centrifugal fan to spin the inlet fan flow and produce pulsed turbulent flow. Fan  20  is provided with a nozzle, to be described later, to stabilize the pulsed turbulent flow exiting the centrifugal fan  20 . 
   Exhaust exiting the fan nozzle enters first stage  52  of the exhaust stack  50 . The first stage  52  has an inward taper as the exhaust moves upwardly to the top end. A second stage  54  of the exhaust stack connects to the first stage  52  by struts  56 . The spaces between the struts allows the inlet of ambient air as will be described. The exhaust transitioning from the first stage  52  to the second stage  54  exits a small orifice and enters the second stage  54  having a diameter larger than the exit orifice, creating a venturi. Whenever gas flows through a venturi, the narrow portion of venturi causes an increase in velocity and decrease in pressure. The decrease in pressure creates suction causing induced flow of ambient air into the bottom of second stage  54  entering between the struts  56 . The result is an exhaust from the top of second stage  54  having high velocity and dilution of the inlet fan flow that entered fan  20  through plenum  36 . 
     FIG. 3  shows a second embodiment of an exhaust stack attachable to centrifugal fan  20 . Nozzle  26  attached to the fan  20  is shown in phantom, the nozzle  26  having an inward taper to produce an outlet having a smaller diameter. The exhaust is compressed, producing a back pressure stabilizing the pulsed, turbulent flow produced by the fan  20 . Exhaust exiting the nozzle  26  enters first stage  60 , having a diameter greater than the outlet of the nozzle  26 . This causes a venturi effect and induces flow through apertures  62  provided in the first stage  60  below the outlet of the nozzle. This causes dilution of the inlet fan air. The exhaust exits the first stage  60  and enters second stage  64 . Second stage  64  has an initial inward taper to a minimum diameter then a slightly outward taper until the outlet. This venturi shape induces a second flow of ambient air entering through the bottom of the second stage  64  to further dilute the effluent. The amount of ambient air added to the exhaust entering the fan is measured as percent dilution. Percent dilution is the amount of ambient air relative to the inlet fan flow present in the exhaust from the top of the exhaust stack. If the same amount of ambient air is added to the inlet fan flow, therefore, there would be 100 percent dilution. Test conducted with this exhaust stack have shown  16  percent dilution. 
     FIG. 4  shows the exhaust stack of  FIG. 3  but with the addition of an outwardly extending flange  66  extending from the first stage  60  above the apertures  62 . The addition of this flange  66  increases the percent dilution to 20 percent. Likewise,  FIG. 5  is similar to the exhaust stack of  FIG. 3  but having an additional cylindrical extension  68  extending from second stage  64 . This additional height of the exhaust stack grants time and distance for the entrainment of ambient air entering the bottom of the second stage  64 . Tests have shown that percent dilution increases to 42 percent for this embodiment. 
     FIG. 6  shows another embodiment of the exhaust stack attachable to a centrifugal fan  20 . The exhaust has an outwardly flaring flange  70  provided with apertures to allow induced flow and supporting an inwardly extending flange  72  transitioning to a first stage  74  having a slight outwardly taper and joining to a second stage  76 . The venturi effect created by the inwardly extending flange  72  and outwardly tapering first stage  74 , induces flow of ambient air through the apertures in the outwardly extending flange  70  to dilute the exhaust coming through the nozzle of the centrifugal fan  20 . Tests have shown 40 percent dilution. 
   In this embodiment, the nozzle of the centrifugal fan is provided with a nozzle cap. As seen in  FIG. 6 , the nozzle cap has a solid central U-shaped trough extending across the opening forming two exit apertures  27 . The result is to reduce the volume of the exhaust plume exiting the centrifugal fan while maximizing the surface area. It is at the boundary of the fast moving exhaust stream and ambient air within the exhaust stack that drags the ambient air and induces flow through apertures in the outwardly extending flange  70 . 
     FIG. 7  shows another embodiment of the exhaust stack having an outwardly extending apertured flange  170  attached to the fan and supporting an inwardly extending flange  172 . A first stage  174  attaches to the inwardly extending flange and has a slight inward taper at a rate much less than the inward taper of the flange  172 . Extending from the first stage  174  is a cylindrical second stage  176 . The thin nozzle has the same nozzle cap used in the embodiment of  FIG. 6  and tests have shown a 58 percent dilution percent. 
     FIG. 7   a  shows a second type of nozzle cap that may be used with the embodiment of  FIG. 7 . This nozzle cap has a cross-shaped aperture reducing the volume of the plume exiting the nozzle while trying to maximize the surface area to induce flow of ambient air into the exhaust stack through the apertured flange  170 . By changing the configuration of the nozzle cap, tests have shown an increase in dilution percent to 75 percent. 
     FIG. 7   b  shows a third type of nozzle useable with the exhaust stack of  FIG. 7 . This nozzle has a central plug attached to the perimeter of the nozzle cap by struts. Test with this nozzle cap have shown a dilution of 55 percent. 
     FIG. 7   c  shows a fourth type of nozzle useable with the embodiment of  FIG. 7  having a six vein cross rather than a four veined cross shown in  FIG. 7   a . Tests with this type of nozzle cap have shown a dilution of 61 percent. 
     FIG. 8  shows another embodiment of the exhaust stack attachable to a centrifugal fan  20  having the outwardly tapering apertured flange  70  with a first stage  82  having a first section attached to and extending upwardly from the apertured flange  70  and a second section having a slightly smaller taper. Attached to the first stage is second stage  84  having a first inwardly tapering section, having the same taper as the second section of the first stage, and a second cylindrical section. Each of the first and second stages has a venturi effect inducing flow through the aperture flange  70  and from underneath the second stage  84 , respectively. 
   The embodiment shown in  FIG. 8  has a nozzle cap with a central conical plug and test have shown a dilution of  76  percent. Replacing this nozzle cap with the cross shaped nozzle cap of  FIG. 7   a  has shown dilution of 105 percent. 
     FIG. 9  shows an embodiment similar to  FIG. 8  but having a second stage  85  which is a simple cylinder extending upwardly from the first stage  82 . This embodiment has shown dilution of 92 percent in tests. 
     FIG. 10  is similar to  FIG. 8  but having the addition of an intermediate stage  86  between the first stage  82  and second stage  84 . The intermediate stage  86  is a short conical piece having the same taper as the second section of the first stage and the bottom section of the second stage. Induced flow can enter the exhaust stack from the space underneath the intermediate stage. This stack has shown a 105 percent dilution. 
     FIG. 11  shows a embodiment having the first stage  82  and intermediate stage  86 . However, in this embodiment the second stage is replaced with two additional stages identical to the intermediate stage. Flow can be induced through the apertured flange  70  and underneath each of the stages  86 ,  88 ,  89  above the first stage. Tests have shown an 86 percent dilution with this output stack. 
   Each of the embodiments uses the venturi effect within the exhaust stack to induce flow of ambient air and increase velocity. Needless to say, increased dilution increases the mass air flow exiting the exhaust stack. The greater the mass air flow, the lower the velocity due to the greater weight of the exhaust being moved. With the various configurations of the exhaust stack, an appropriate dilution rate and exhaust speed can be chosen for any application. The result is an exhaust that has a greater plume height than prior art devices enabling dispersion of exhaust from a compact low energy configuration. 
   While the invention has been described with reference to preferred embodiment, various variations and modifications would be apparent to one of ordinary skill in the art. The invention encompasses such variations and modifications. The stacks may be used with any type of fan, such as centrifugal, a belt driven axial fan or a direct drive axial fan.