Patent Publication Number: US-9897111-B2

Title: Aspirating induction nozzle with flow transition

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. non-provisional patent application Ser. No. 13/067,269, filed May 20, 2011, and it also relates to U.S. non-provisional patent application Ser. No. 14/209,154, filed Mar. 13, 2014, the disclosures of both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of exhaust air systems for buildings and/or other enclosed areas, and more particularly, to exhaust discharge nozzles configured to be attached to the outlets of exhaust fans, exhaust ducts and/or stacks, and similar exhaust type equipment/devices and are specifically designed to be installed in the outdoor ambient. 
     Many commercial and industrial processes exist which introduce hazardous and/or noxious chemicals into the building exhaust. These chemicals originate from a host of commercial/industrial processes within critical environments such as research laboratories, chemical storage facilities, generator housing rooms, thermal oxidizers, exhaust chemical scrubbers, etc. It is of paramount importance that the proper precautions are taken to ensure that the effluent is effectively managed 100% of the time. Specifically designed, purpose built exhaust systems are required to mitigate hazardous concentrations of processes chemicals. As governed by the ASHRAE 2011 HVAC Applications Handbook, a comprehensive flow model of the building must be executed to determine critical fluid flow patterns based on the unique geometry and wind flow patterns for the site. Consideration for the location of near-by air building fresh air intakes is a critical factor which must be accounted for so as to avoid possible effluent re-entrainment into the facility in unprocessed concentrations. In order to be effective, the critical exhaust provisions must be properly designed and must achieve continuous rated performance in the real world dynamic environment where the system is to operate. Failure to meet any of the above criteria would jeopardize the safety of those working in and around the proximity of the critical environment and/or residents of surrounding communities. 
     An effective solution, as standardized by ASHRAE, is to propel exhaust gases upward to a critical height above the building roofline such that the effluent has exited the building boundary layer (envelope) and entered the atmospheric free stream. The atmospheric free stream essentially provides an effective mechanism to safely exhaust the effluent, by imparting sufficient plume dilution, thus reducing the concentrations of hazardous chemicals to levels deemed safe. If it is impossible or impractical to reach the atmospheric free stream, and thus plume “touchdown” is possible, then sufficient fresh air dilution must be imparted to the effluent prior to the point of discharge to ensure hazardous concentrations are sufficiently dispersed. This critical height above the building roofline is termed the “effective stack height.” In its simplest form, the effective stack height is the height at which a theoretical centerline of the building exhaust plume becomes completely horizontal due to the impact of the specified horizontal cross wind velocity. The effective stack height, h se  (ft), can be calculated from the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) HVAC Applications Handbook as:
 
 h   se   =h   s   +h   r   −h   d  
 
Where:
         h is the physical exhaust system height (ft)   h r  is the plume rise (ft)   h d  is the amount of stack wake downwash in (ft)
 
The plume rise component, h r , is the distance the exhaust plume will be propelled above the terminal discharge point of the physical equipment. Plume rise for momentum driven flow is calculated based on the recommendations of the ASHRAE. From as early as 1999 through 2010 the ASHRAE HVAC Handbook has stipulated the use of a special case of the Brigg&#39;s Equations to determine plume rise h r , which is defined as:
 
 h   r =3.0  d   e ( V   e   /U   H )
 
Where:
   d e  is the effective (hydraulic) diameter (ft) at the terminal discharge point of the system computed from: d e =(4 A e /π)^(½), where A e  is the cross-sectional area of the discharge opening   V e  is the equipment exit velocity (ft/min) at cross wind velocity   U H  is the cross wind velocity (ft/min) at the building roofline
 
This adaptation of the Briggs Equation is a function of dynamic variables. Equipment performance data must be acquired using dynamic testing parameters. Specifically, the equipment exit velocity, V e , must be measured with the specified design cross wind, U H , applied to the system. Moreover, it is a necessary condition that the effective diameter, d e , be determined for the location where the equipment exit velocity, V e , was measured. It is recommended that this location be final discharge point (i.e. terminal location) of the exhaust system to the atmosphere. For this form of the Briggs equation for plume rise to be applicable, the discharge velocity profile at the system discharge must be characterized as uniform. A uniform velocity profile is defined as having minimal velocity gradients in the transverse plane of system discharge.
       

     The initial adaptation and application of the Briggs equation for plume rise did not effectively capture many critical site specific parameters, and the accepted method for calculating plume rise has been redefined in Chapter 45 of the ASHRAE 2011 HVAC Applications handbook using the Briggs equation for the vertical jet momentum of the exhaust versus downwind distance as:
 
 h   r =min {β h   x   ,βh   r }
         β is the stack capping factor, 1.0 without cap as in the present invention The plume rise verses downwind distance h x  in (ft) is obtained from:
 
 h   x =[(3 F   m   x )/(β j   2   U   H   2 )]^(⅓)
 
Where:
   F m  is the momentum flux (ft 4 /s 2 ) and is calculated as F m =V e   2 (d e   2 /4)   β j  is the jet entrainment coefficient computed as β j =⅓+(U H /V e )   x is the downwind distance
 
The final plume rise h f  in (ft) is determined from:
 
 h   f ={0.9[ F   m ( U   H   /U *)]^(½)}/( U   H β j )
       

     Where:
         U H /U* is the he logarithmic wind profile computed as U H /U*=2.5 ln(H/z 0 )   H is the building height above ground level (ft)   U* is the friction velocity (ft)   z 0  is the surface roughness length (ft) which can be obtained from the Atmospheric Boundary Layer Parameters Table in Chapter 45 of the ASHRAE 2011 HVAC Design Handbook.
 
The possibility of stack wake downwash, h d , is an essential component to evaluate when computing the effective stack height of an exhaust system. Stack wake downwash occurs where arrogance low velocity exhaust streams are pulled downward by negative pressures immediately downstream of the exhaust system discharge. The amount of stack wake downwash in (ft) can be obtained from h d =d e [3.0−β(V e /U H )]
       

     As specified in the ASHRAE 2011 standard, the cross wind velocity at the building roofline U H , as applied to all equations which require this parameter, is the maximum design wind speed at the building roof height at which air intake contamination must be avoided. As stated by ASHRAE, this maximum design speed must be at least as large as the hourly wind speed exceeded 1% of the time. Chapter 14 of the 2009 ASHRAE Fundamentals Handbook lists this value for many cities. When determining this 1% value, the height and terrain of the building in relation to the height of the anemometer and terrain of at the local meteorological station measuring wind speed must be corrected for the effects of boundary layer friction. This is accounted for according to the following expression which appears in Chapter 24 of the 2009 ASHRAE Fundamentals handbook:
 
 U   H   =U   met [(δ met   /H   met )^ a   met ]*[( H /δ)^ a ]
 
Where:
         U met  is the hourly wind speed as measured from a nearby meteorological station   δ met  is the atmospheric boundary layer thickness of the meteorological station which is assigned based on a terrain category which can be obtained from a table of atmospheric boundary layer parameters   H met  is the height of the anemometer (typically 33 feet)   a met  is the meteorological exponent corresponding to the terrain category of the meteorological station which is assigned based on a terrain category which can be obtained from a table of atmospheric boundary layer parameters.   H is the height at which the required windspeed U H  is being adjusted for; typically the discharge location of the exhaust system.   δ is the atmospheric boundary layer thickness of the exhaust system location which is assigned based on a terrain category which can be obtained from a table of atmospheric boundary layer parameters   a is the exponent corresponding to the terrain category of the location where the exhaust system is located which is assigned based on a terrain category which can be obtained from a table of atmospheric boundary layer parameters.       

     Upon examination of the equation for effective stack height it becomes evident that the most critical parameters affecting a system&#39;s ability to achieve this specification are discharge geometry (d e ), discharge velocity (V e ), and the design wind speed (U H ) where the system is to operate. Furthermore, the American National Standards Institute/American Industrial Hygiene Association ANSI/AIHA Z9.5 2012 Laboratory Ventilation standard mandates a minimum discharge velocity of 3000 ft/min be constantly maintained in order to be in compliance. Standard Z9.5 2012 also specifies that the physical exhaust system height, h s , be a minimum of 10 ft. above adjacent roof lines and air intakes and in a vertical up direction. 
     It should be noted that standard industry testing methods, at the present time do not incorporate cross winds into the testing protocol. The Air Movement and Control Association (AMCA) has developed AMCA Standard 260-07 Laboratory Methods of Testing Induced Flow Fans for Rating and is generally accepted as the industry standard. However, while this test does certify discharge flow volume of an induction exhaust system, it does not include dynamic testing with the influence of a cross wind. Therefore, using outlet flow data to calculate system exit velocities measured according to AMCA standard 260-07 can lead to erroneous discharge velocity ratings. Furthermore, if static system exit velocities (i.e. no cross wind present during measurement) are used in the special case Briggs Equation, which is a function of dynamic variables only, to determine plume rise, the prediction of performance will be physically incorrect. Plume rise (i.e. the quotient) determined in this manner would always be mathematically undefined (i.e infinite asymptote) due to the 0 ft/min cross wind velocity devisor; which is an impossible physical phenomena to achieve. However, if the AMCA standard 260-07 were modified to incorporate cross wind, then the Briggs equations would be a mathematically valid method of calculating plume rise, provided that the velocity profile at the discharge was uniform. Additionally, an advanced engineering approach is to use computational fluid dynamics (CFD) software to calculate system performance; the AMCA  260  test can be simulated with the cross wind component included to develop real world performance data. The Briggs equation is valid for calculating plume rise using the CFD data; however this only applies to systems with a uniform discharge profile. Additionally, the most current methodology of calculating plume rise as defined by ASHRAE should always be used. 
     The application of discharge nozzles at the exit point of exhaust systems enhances the performance capability by increasing discharge velocity. Increased discharge velocity provides a plume rise component of the exhaust stream with the specific goal of maximizing the exhaust/effluent dispersion and dilution of the hazardous/contaminated air and/or effluent gases and vapors from buildings, rooms, and other enclosed spaces by reaching the atmospheric free stream If it is impossible or impractical to reach the atmospheric free stream, and thus plume “touchdown” is possible, then sufficient fresh air dilution must be imparted to the effluent prior to the point of discharge to ensure hazardous concentrations are sufficiently dispersed. Discharge nozzles may be of the non-inducing or fresh air inducing type. Inducing type discharge nozzles have the unique ability of leveraging physics to draw in fresh ambient air downstream of the primary air mover (fan), while non-inducing type nozzles must draw the fresh air in through a mixing plenum and process the air through the fan, thus requiring a comparatively higher electrical power consumption. Properly designed nozzles are capable of propelling high velocity plumes of exhaust gases to heights sufficient to prevent stack wake downwash and disperse the effluent over a large upper atmospheric area, so as to avoid exhaust contaminant re-entrainment into building ventilation intake zones. Discharge nozzles are able to provide a superior alternative to conventional tall exhaust stacks which do not increase the velocity of the exhaust and thus must be significantly taller than systems discharging at a comparatively higher velocity through the use of discharge nozzles. Tall exhaust stacks are costly to construct, may be prohibited by zoning height restrictions, are visually unattractive by today&#39;s architectural standards, and may detract from community relations due to the inherent industrial connotation. 
     A further development of the exhaust nozzle design is the type nozzle that employs the Venturi effect to draw additional ambient air into the primary effluent stream. The venturi type nozzle can further be described as an aspirating, or induction type, as related to conventional technological description for this type nozzle. The additional induced air volume dilutes the primary exhaust gases at/near the nozzle as the combined mixed air volumes are released into the atmosphere. Also, with this exhaust-air mixture volume increase, the discharged gas is expelled at a higher velocity, achieving a greater plume height. The underlying effect of greater volume at greater discharge velocity is an increased effluent momentum, which assists with the effluent disbursement into the atmosphere. 
     The features and functions of induction nozzles are described in greater detail in U.S. patent application Ser. No. 13/067,269, the disclosure of which is incorporated herein by reference. 
     High plume lift is particularly critical with regard to exhaust gases from potentially contaminated sources, such as laboratories and other facilities in which chemical processes produce noxious fumes. To insure that potentially contaminated exhaust reaches a minimum altitude to avoid downwash, many environmental and building code standards specify a minimum discharge velocity from an exhaust nozzle. For example, ANSI Z9.5 2012 currently requires a minimum discharge velocity of 3000 feet per minute (FPM) at the outlet of a lab exhaust nozzle. Therefore, the nozzle must be designed to ensure that the discharge velocity is always at or above the governing guidelines. To achieve this, the flow rate from the fan at the inlet of the nozzle must be accelerated. 
     In the present invention, a frusto-conical transitional flow impinger provides a mechanism to effectively control the flow velocity in the region from the discharge of the fan impeller through the nozzle body. It is important to note, that any decrease in velocity in the region of the discharge of the fan impeller to the discharge of the nozzle body would require a subsequent re-acceleration of the air stream to initiate and sustain the fresh air inducing venturi effect at the fresh air induction ports within the nozzle. Deceleration and subsequent re-acceleration is very inefficient and increases pressure loss through the nozzle, thereby requiring more fan power, larger fan/motor sizes, and limiting usable range of the device. The addition of the impinger provides a mechanism to ensure that flow velocities are always constant or increasing until the discharge plane of the nozzle body, thereby offering a means to optimize the design of the nozzle for the given flow and/or operational pressure drop requirements. 
     Fans have a characteristic performance curve for a given impeller rotational speed, whereby the available static pressure is a function of flow rate. As the nozzle can be considered an accessory in the air stream, each characteristic nozzle design has an inherent initial loss at a given flow rate. The pressure drop is the consequence of the additional energy required to accelerate the airstream to the minimum acceptable discharge velocities per ASHRAE/ANSI. As more flow is introduced to the nozzle inlet, the loss (pressure drop) through the nozzle increases according to a square relationship with flow. Thus, one specific nozzle will require all of the available static pressure capacity a specific fan can provide at a given rpm well before the entire unobstructed flow range of the fan is provided. By adjusting the impinger and induction port dimensions, various nozzles that would mount to the same fan, with the same nominal exterior housing dimensions, can be efficiently designed. Thus additional flow can be introduced into the variant nozzle for the same pressure drop and corresponding minimum discharge velocity requirement. The net effect is a re-indexing of the nozzle operating range, such that additional capacity can be provided. This re-indexing strategy can be repeated as many times as necessary to completely cover a given flow range. This flow range can apply to retrofit applications or be leveraged to design nozzle sizes for a specific fan series. Moreover, fans are typically characterized in industry by their nominal impeller diameter which constitutes a given fan series. As the impeller diameter deviates from standard, which may be due to a specific performance goal or application, so will the exterior fan housing dimensions. The present invention has the unique ability to be modified so as to maintain the same engineered velocity and flow profile throughout the nozzle body and plume development chamber, effectively making true application-specific design possible 
     SUMMARY OF THE INVENTION 
     The predominant application for the nozzle is mounting to a tubular inline fan. However, the nozzle can be mounted to any devices and/or ductwork through the use of transitions, etc. As depicted in  FIGS. 1A and 1B , the typical tubular inline fan, whether belt-driven or direct drive, has an annular discharge area. This annular discharge space generally begins at the immediate discharge of the fan impeller. The outer ring of the annulus is created by the overall fan housing, and the inner ring of the annulus is created by the motor chamber for direct drive or the sheave chamber for belt drive. Guide vanes, which serve to straighten the discharging air stream are often positioned in this annular space. The true outlet velocity of a fan must be measured in this annular region. The preferred design will have a motor enclosure or sheave enclosure which terminates on the same plane as the fan housing. 
     As shown in  FIGS. 2A, 2B and 3 , the aspirating nozzle mounts to the discharge flange of the fan which is (preferably) coterminous with the fan housing and motor/sheave enclosure. The frusto-conical transitional flow impinger is designed to mount to the motor/sheave enclosure to continue the annular area of the fan into the nozzle body. The presence of the frusto-conical transitional flow impinger ensures that the exhaust flow area will either stay the same or converge as the flow transitions from the fan to the nozzle; thus velocity is always maintained or increased from the moment it leaves the fan impeller through the nozzle body. It is essential to note that no fresh induction air is permitted to mix with the exhaust flow volume from the fan until the exhaust flow has exited the discharge plane of the “nozzle body” (defined as the plane of the circular top of the frusto-conical transitional flow impinger). Thus, actual exhaust nozzle velocity can be determined and engineered such that the venturi effect can be maximized as the flow exits the discharge plane of the nozzle body and fresh air is induced (aspirated). 
     As illustrated in  FIGS. 2A, 2B and 3 , the transitional flow impinger is mounted to the motor/sheave enclosure. The impinger can have a straight tubular extension to extend into the tubular fan housing if the motor/sheave enclosure is not coterminous with the fan discharge. The exhaust flow volume enters a tubular fan, and energy is imparted via the rotating impeller. As the flow volume discharges the impeller passageways, it enters an annular space within the fan. The annular space is defined as the space between the overall fan tubular housing and the tubular internal motor/sheave enclosure of a lesser diameter. The area from the impeller discharge to the location where the motor/sheave enclosure terminates is fan constant, and consequently so is the velocity of the exhaust air stream. Ideally, the annular area created by the fan housing and the motor/sheave enclosure continues to the fan discharge; if not, the impinger can have a tubular extension that completes the annular area. 
     As the flow discharges the annular fan area, it enters the nozzle inlet. The impinger is engineered such that the area through the nozzle body either remains the same or is decreasing. The “nozzle body” is defined as the area encompassed from the inlet of the nozzle to the top of the frusto-conical transitional flow impinger. Therefore, the nozzle body is the flow passageway through the nozzle where the flow volume is entirely made up of fan exhaust flow. The nozzle body discharge velocity can therefore be optimized to ensure that exhaust velocities of the potentially contaminated airstream are always at or above the guidelines set forth by many governing bodies (ANSI/ASHRAE). It is also important to note that the windband is the actual discharge location of the overall nozzle assembly. And it is the discharge velocity from this location that must be measured to certify overall system compliance. 
     In the present invention, the frusto-conical transitional flow impinger provides a mechanism to effectively control the flow velocity in the region from discharge of the fan impeller through the nozzle body. It is important to note, that any decrease in velocity in the region of the discharge of the fan impeller to the discharge of the nozzle body would require a subsequent re-acceleration of the air stream to initiate and sustain the fresh air inducing venturi effect at the fresh air induction ports within the nozzle. Deceleration and subsequent re-acceleration is very inefficient and magnifies the pressure drop requirements of the nozzle, thereby requiring more fan power, larger fan/motor sizes, and limiting usable range of the device. The addition of the impinger provides a mechanism to ensure that flow velocities are always constant or increasing until the discharge plane of the nozzle body, thereby offering a means to optimize the design of the nozzle for the given flow and/or operational pressure drop requirements. 
     As seen in  FIG. 3 , typically, the triangular induction port outlet vertices are fixed with the top plane of the impinger (approximately ⅛ inch clearance). Therefore, the height of the impinger and its top diameter coupled with the height and base of the triangular outlets, can be varied to significantly optimize the operation of the aspirating nozzle for many inlet flow conditions or desired operational ranges of the device. 
     The impinger can also be leveraged to control where the vena contracta occurs. As the discharging air volume is accelerated in the nozzle body, the vena contracta is essentially the point in the discharging air stream where the hydraulic diameter is the least and the velocity is at its maximum, characterized by a contraction coefficient, which is defined as the ratio of the area of the exhaust stream (i.e. jet) and the discharge area of the nozzle body (i.e. orifice). The height of the top of the impinger has an intrinsic influence on the vena contracta characteristics. The exhausting air volume will tend to separate from the top of the impinger and converge at a downstream distance which is dependent on impinger height and its top diameter. When the flow converges, the velocity is at a maximum and the vena contracta is evident. The maximized velocity at this point serves to enhance the venturi effect and further improve the induction capacity of the system. This effect is unique to the present invention and serves to optimize plume characteristics and system performance. 
     The location between the discharge plane of the nozzle body and the windband discharge is known as the “plume development chamber”. The plume development chamber is also described as the only space in the overall discharge nozzle where the primary exhaust stream experiences a temporary increase in area. This increased area occurs at a controlled location downstream of the discharge plane of the nozzle body, and serves to accommodate the volume of fresh air being induced by the sustained venturi effect. Once the fresh air is introduced into this space, energy is transferred between the high velocity primary exhaust stream and the fresh air induction flow stream, resulting in a more uniform velocity flow profile of the overall exhaust stream. This “developed” stream is then accelerated to the specified velocity and discharged from the device. 
     The key principle of the aspirating induction nozzle is that, because of all of the features outlined, the present invention has the unique ability to establish and maintain a venturi effect in the presence of an ambient crosswind. Thus, the system is able to successfully perform over a vastly expanded range of ambient crosswinds with minimal degradation in performance. Prior art in the industry leverages static ambient testing methods with no crosswinds and legacy equations to de-rate performance for real world dynamic condition, which may not physically apply. The frusto-conical transitional flow impinger provides a mechanism to optimize the venturi effect in the presence of a cross wind and thus enables a real world dynamic performance rating to be specified. Discharge velocities and fresh air entrainment are accurately optimized for a given nozzle inlet flow and operating range requirement. 
     The present invention provides the ability to modify key interior nozzle components to optimize for a specific range of flows while maintaining the same outer physical appearance. It enables application specific design or the ability to design to predefined flow ranges and provide solutions for a given series of fans. The series of fans and/or nozzle is typically defined by the size of the inlet. 
     Fans have a characteristic performance curve for a given impeller rotational speed whereby the available static pressure is a function of flow rate. As the nozzle can be considered an accessory in the air stream, each characteristic nozzle design has an inherent initial loss at a given flow rate. The pressure drop is the consequence of the additional energy required to accelerate the airstream to the minimum acceptable discharge velocities per ASHRAE/ANSI. As more flow is introduced to the nozzle inlet, the loss (pressure drop) through the nozzle increases according to a square relationship with flow. Thus, one given nozzle will require all of the available static pressure capacity a specific fan can provide at a given rpm, well before the entire unobstructed flow range of the fan is provided. By adjusting the impinger and port dimensions, various nozzles that would mount to the same fan, with the same nominal exterior dimensions, can be efficiently designed. Thus additional flow can be introduced into the variant nozzle for the same pressure drop and corresponding minimum discharge velocity requirement. The net effect is a re-indexing of the nozzle operating range such that additional capacity can be provided. This, re-indexing strategy can be repeated as many times as necessary to completely cover a given flow range. This flow range can apply to retrofit applications or be leveraged to design nozzle sizes for a specific fan series. 
     The foregoing summarizes the general design features of the present invention. In the following sections, specific embodiments of the present invention will be described in some detail. These specific embodiments are intended to demonstrate the feasibility of implementing the present invention in accordance with the general design features discussed above. Therefore, the detailed descriptions of these embodiments are offered for illustrative and exemplary purposes only, and they are not intended to limit the scope either of the foregoing summary description or of the claims which follow. 
     As used in the following claims, the terms “above” and “below” are defined relative to the direction of the primary effluent flow, so that a feature that is “above” another feature is relatively in the downstream direction, and a feature which is “below” another feature is relating in the upstream direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an isometric view of a typical in-line fan (direct drive shown, but applies to belt-driven as well); 
         FIG. 1B  is an isometric section view of the in-line fan of  FIG. 1A  taken along the plane A-A; 
         FIG. 2A  is an isometric view of a fan/nozzle assembly according to one embodiment of the present invention; 
         FIG. 2B  is an isometric section view of the fan/nozzle assembly of  FIG. 2A  taken along the plane B-B; 
         FIG. 3  is a front section view of the fan/nozzle assembly of  FIG. 2A  taken along the plane B-B; 
         FIG. 4A  is an isometric view of a nozzle with a transitional flow impinger engineered for minimum design inlet flow for a given fan series/model size; 
         FIG. 4B  is an isometric section view of the nozzle of  FIG. 4A  taken along the plane C-C; 
         FIG. 5A  is a front section view of the nozzle of  FIG. 4A  taken along the plane C-C; 
         FIG. 5B  is a plan section view of the nozzle of  FIG. 5A  taken along the plane D-D; 
         FIG. 6A  is an isometric view of a nozzle with an transitional flow impinger engineered for maximum design inlet flow for a given fan series/model size; 
         FIG. 6B  is an isometric section view of the nozzle of  FIG. 6A  taken along the plane E-E; 
         FIG. 7A  is a front section view of the nozzle of  FIG. 6A  taken along the plane E-E; 
         FIG. 7B  is a plan section view of the nozzle of  FIG. 7A  taken along the plane F-F; 
         FIG. 8A  is an isometric view of a nozzle with induction ports above the top of the transitional flow impinger engineered for reduced exhaust flow volume; 
         FIG. 8B  is an isometric section view of the nozzle of  FIG. 8A  taken along the plane G-G; 
         FIG. 9A  is a front section view of the nozzle of  FIG. 8A , taken along the plane G-G; and 
         FIG. 9B  is a plan section view of the nozzle of  FIG. 9A  taken along the plane H-H. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1A and 1B , a typical in-line fan assembly  20  comprises a tubular outer fan housing  21 , a tubular inner motor/sheave enclosure  22  and an impeller  23 . Between the fan housing  21  and the motor/sheave enclosure  22  is formed an annular discharge plenum  24 , through which flows a primary exhaust flow from the impeller  23 . Multiple radial guide vanes  25  within the discharge plenum  24  serve to straighten the primary effluent flow  16 A. The distal end of the fan housing  21  has a discharge flange  26 , to which the aspirating induction nozzle assembly  10  of the present invention is mounted. 
     Referring to  FIGS. 2A, 2B and 3 , the nozzle assembly  10  comprises a frusto-conical nozzle body  11 , multiple tetrahedral induction ports  12 , and a frusto-conical windband  13 . The nozzle body  11  has a proximal annular inlet  14 , which surrounds the base  17  of a frusto-conical transitional flow impinger  15  and communicates with a primary effluent passage  16 . 
     Referring to  FIGS. 4A through 9B , the induction ports  12  radially converge and slope centripetally from the base  17  to the top  18  of the transitional flow impinger  15 , such that the cross-sectional area  16 B of the primary effluent passage  16  remains constant or diminishes from the nozzle body inlet  14  to the discharge plane  19  of the nozzle body  11 . At the nozzle body discharge plane  19 , the cross-section  16 B of the primary effluent passage  16  comprises a grid of alternating splayed radial arms  27  between the induction ports  12 , as best seen in  FIGS. 5B, 7B and 9B . The cross-sectional area  16 B of the primary effluent passage  16  at the nozzle body&#39;s discharge plane  19 , which is the sum of the areas of the radial arms  27 , constitutes the nozzle body discharge area  28  (the hatched area of  FIGS. 5B, 7B and 9B ). The nozzle body discharge area  28  is engineered so that, for a given primary exhaust flow rate (cfm), a minimum flow velocity of 3000 ft/min or greater is achieved at the windband discharge  29 . 
     As best seen in  FIGS. 4B, 5A, 6B, 7A, 8B and 9A , each of the port outlets  30  has a port outlet base  12 B, which is positioned at a port elevation distance  12 C above the nozzle body discharge plane  19 . The areas of the triangular outlets  30  of the induction ports  12  can be varied, along with the height of the transitional flow impinger  15  and the diameter of the impinger&#39;s top  18 , so as to optimize the operation of the aspirating induction nozzle  10  for different inlet flow conditions and/or desired operational ranges. 
     For example, as shown in  FIGS. 4B and 5A , for minimum design inlet flow, the impinger  15  is relatively long and the induction port outlets  30  are relatively smaller, as compared to the shorter impinger cone  15  and larger inductor port outlets  30  shown in  FIGS. 6B and 7A , which are configured for maximum design inlet flow. As can be seen by comparing  FIG. 5B  with  FIG. 7B , the area of the cross-hatched radial arms  27 , which together constitute the nozzle body discharge area  28 , is greater for the maximum design inlet flow ( FIG. 7B ) then for the minimum design inlet flow ( FIG. 5B ). 
     As shown in  FIGS. 5A, 7A and 9A , the primary effluent flow  16 A converges to a vena contracta point  34  above the nozzle body discharge plane  19 , at which the primary effluent flow  16 A achieves a maximum velocity. As depicted in  FIGS. 4B, 5A, 6B and 7A , the vertices  30 A of the triangular induction port outlets  30  are typically aligned (within a port clearance offset  30 B of approximately ⅛ inch) with the circumference of the impinger top  18 . In an alternate design, depicted in  FIGS. 8A, 8B, 9A and 9B , the vertices  30 A of the port outlets  30  are located at a port clearance offset  30 B slightly above the impinger top  18 . The annular primary exhaust flow does not converge immediately after the impinger top  18 , but continues downstream in a gradually converging annular pattern above the impinger top  18 . Therefore, if the port outlet vertices  30 A extend into the region above the impinger top  18 , which is a “dead” flow space, the nozzle body discharge area  28  can be slightly reduced to accommodate lesser primary effluent flows  16 A without reducing the flow velocity at the windband discharge  29 . 
     Referring to  FIGS. 4A, 4B, 5A, 6A, 6B, 7A, 8A, 8B and 9A , the full-length windband  13  extends annularly around the central nozzle body  11  from the windband inlet  29 A at or below the port inlet opening  12 A to shield the induction ports  12  from performance-degrading cross-winds. The windband also creates a protected plume development zone  31  between the nozzle body  11  and the windband discharge  29 , where the primary exhaust flow and fresh induction air merge into a single airstream as they are accelerated to the required discharge velocity. The net effect is a near uniform transverse discharge velocity profile, which serves to maximize plume integrity and minimize turbulence. 
     As shown in  FIGS. 4B, 5A, 6B, 7A, 8B and 9A , the inlet baffle  32  serves to straighten the flow of induction air entering the induction ports  12 , thus ensuring that the flow is laminar and avoiding noise-generating turbulence. Full length windband brackets  33  are used to fasten the windband  13  to the interior nozzle assembly  10  and to secure the inlet baffle  32 . Additionally, the brackets  13  serve as full-length guide vanes with three distinct aerodynamic purposes. First, they segregate each induction port  12 , thereby minimizing the impact of crosswind infiltration. Second, they extend below the windband  13  and serve to eliminate potential flow attachment to the tubular nozzle body  11 . Third, they extend into the plume development chamber  31  to straighten the discharging exhaust volume. 
     The nozzle  10  can be designed with an acoustic treatment provision which is integral to the induction ports  12 . This feature is integral to the design of the nozzle  10  and provides sound attenuation with no additional system height or equipment. As an example of this feature, perforated stock can be formed around each prevailing surface of the induction ports  12  on the interior of the nozzle  10 . The perforated material is then mounted to each induction port  12  so as to form a cavity to be packed with the acoustic fill. Mineral wool, or similar acoustic media, is then packed into the cavity. As the airflow passes the perforated and packed cavities on the port  12 , turbulent eddies are reduced and excessive sound waves are absorbed. Thus the sound power level of the device is reduced. 
     Similar to the acoustic treatment of the induction ports  12 , the impinger  15  can be outfitted with a perforated sheet material layer forming a cavity on all sides. This cavity can then be packed with acoustic attenuation media. As an alternative, the impinger  15  itself can be constructed of the perforated material, and filled entirely with acoustic attenuation media. Again, as the airflow passes the perforated and packed pod, turbulent eddies are reduced and excessive sound waves are absorbed. Thus the sound power level of the device is reduced. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications and substitutions are possible, without departing from the scope and spirit of the present invention.