Abstract:
Exhaust hood methods, devices, and systems are disclosed herein that improve the performance of short-circuit hoods. Such improvements can be provided relatively inexpensively by utilizing existing features of the structure of short-circuit hoods. The resulting enhancement in performance may exceed that of a regular exhaust hood while avoiding the pitfalls associated with make-up air injection into the exhaust hood recess. The components of the short-circuit system that inject make-up and/or conditioned air into the exhaust hood recess can be converted into a combination of horizontal and vertical jets at a lower edge of the exhaust hood. A retrofit jet generator can be installed into the short-circuit system outlets. A series of openings in the jet generator can generate the combination of horizontal and vertical jets at a lower edge of the exhaust hood when air from the make-up air or conditioned air source is supplied thereto.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/505,520, filed Jul. 7, 2011, which is hereby incorporated by reference herein in its entirety. 
     
    
     FIELD 
       [0002]    The present disclosure relates generally to exhaust systems, and, more particularly, to systems, methods, and devices for retrofitting short-circuit exhaust hoods to improve performance thereof. 
       BACKGROUND 
       [0003]    Short-circuit exhaust hoods direct air from an air source toward an exhaust air intake within the recess of the hood to be exhausted together with a load from a fume source. The short-circuit system involves an air source that supplies and directs conditioned air, unconditioned air (typically referred to as make-up air), or a combination thereof into the exhaust hood recess in order to enhance capture and energy efficiency. Short-circuit exhaust hoods have been widely adopted in commercial kitchens due to at least two different motivations. First, it is believed that short-circuit exhaust hoods are more energy efficient than their non-short-circuit counterparts because part of the required exhaust air supply is satisfied with unconditioned air. Second, codes that required a minimum amount of air (in cubic feet per minute (cfm)) to be exhausted from kitchens could be circumvented by the use of short-circuit hoods. 
         [0004]    However, most short-circuit hoods simply do not work very well, especially in view of their complexity and cost. In particular, short-circuit hoods have at least two air passages (e.g., one for exhausting fumes and another for introducing make-up air into the exhaust recess) and potentially more than two air passages (e.g., an additional one for introducing conditioned air into the vicinity of the exhaust hood to enhance employee comfort). Despite this added complexity, short-circuit systems have not been able to reduce the volume of conditioned air needed to achieve full capture and containment of a fume load under certain conditions. In fact, a short-circuit system may actually increase the amount of conditioned air that is exhausted. To operate effectively, an exhaust blower or fan for the exhaust hood must operate at a higher speed than if the short-circuit system was not present due to the need to remove not only the effluent-laden air but also the make-up air from the short-circuit supply. Make-up air may also increase turbulence in the vicinity of the fume source, which may increase the volume of conditioned air that is entrained in the effluent, thereby increasing the amount of exhaust required. 
       SUMMARY 
       [0005]    Exhaust hood methods, devices, and systems are disclosed herein that improve the performance of short-circuit hoods. Such improvements can be provided relatively inexpensively by utilizing existing features of the structure of short-circuit hoods. The resulting enhancement in performance may exceed that of a regular exhaust hood while avoiding the pitfalls associated with make-up air injection into the exhaust hood recess. In embodiments of the disclosed subject matter, the components of the short-circuit system that inject make-up and/or conditioned air into the exhaust hood recess can be converted into a combination of horizontal and vertical jets at a lower edge of the exhaust hood. Such conversion may be accomplished by installing a retrofit jet generator or plenum into the short-circuit system outlets. The jet generator may include a sheet metal blank with a series of openings that generate the combination of horizontal and vertical jets at a lower edge of the exhaust hood when air from the make-up air or conditioned air source is supplied thereto. 
         [0006]    In embodiments, a closed circuit exhaust hood can have an exhaust-buffering recess and a make-up air discharge configured to direct make-up air from a source directly into the hood recess. A method for improving the performance of a closed circuit exhaust hood can include lowering a total volume of air injected into the hood recess by blocking discharge of the make-up air from the source into the hood recess except through at least one opening having a width of no more than 8 mm, and regulating a pressure in air channels leading to the at least one opening to a pressure of approximately 0.2 to 0.5 in water gauge. An initial velocity of the make-up air discharged from the at least one opening can be at least 4 m/s. 
         [0007]    In embodiments, a kitchen exhaust can have an exhaust-buffering recess. A make-up air discharge can be configured to direct make-up air directly into the hood recess. A conditioned air discharge can be configured to generate a conditioned air curtain at a forward edge of the hood. The make-up air discharge can have a make-up air discharge opening connected to a make-up air plenum that receives air from a make-up supply through a make-up air intake collar. The conditioned air discharge can have a conditioned air discharge opening connected to a conditioned air plenum that receives air from a conditioned air supply through a conditioned air intake collar. The make-up air and conditioned air intake collars can have volume control dampers fitted thereto. A method for modifying the kitchen exhaust can include removing grills covering the make-up air and conditioned air discharge openings, and affixing a jet generator over the conditioned air and make-up air discharge openings. The jet generator can have vertical and horizontal faces and can define a jet plenum portion that, once affixed, extends into the hood recess in a horizontal direction to permit air from the make-up air plenum to flow to a lower end of the hood. Each of the vertical and horizontal faces can have at least one opening therein. The at least one opening in the horizontal face can face downwardly and be located at the lower end of the hood once the plenum is affixed. The openings in the vertical and horizontal faces can be located to receive air from the jet plenum portion such that vertical and horizontal jets are generated from the received air. 
         [0008]    In embodiments, a kitchen exhaust can have an exhaust-buffering recess. A make-up air discharge can be configured to direct make-up air directly into the hood recess. A conditioned air discharge can be configured to generate a conditioned air curtain at a forward edge of the hood. The make-up air discharge can have a make-up air discharge opening connected to a make-up air plenum that receives air from a make-up supply through a make-up air intake collar. The conditioned air discharge can have a conditioned air discharge opening connected to a conditioned air plenum that receives air from a conditioned air supply through a conditioned air intake collar. The make-up air and conditioned air intake collars can have volume control dampers fitted thereto. A method for modifying the kitchen exhaust can include disconnecting the make-up air intake collar from the make-up air supply, removing the make-up air volume control damper, fitting a fan to the make-up air collar, removing the make-up air and conditioned air discharge grills, and affixing a jet generator over the conditioned air and make-up air discharge grills. The fan can be arranged such that air is drawn thereinto and such that ambient conditioned air is supplied through the make-up air intake collar into the make-up air plenum. The jet generator can have vertical and horizontal faces and can define a jet plenum portion that, once affixed, extends into the hood recess in a horizontal direction to permit air from the make-up air plenum to flow to a lower end of the hood. Each of the vertical and horizontal faces can have at least one opening therein. The at least one opening in the horizontal face can face downwardly and can be located at the lower end of the hood once the plenum is affixed. The openings in the vertical and horizontal faces can be located to receive air from the jet plenum portion such that vertical and horizontal jets are generated from air supplied by the fan. 
         [0009]    In embodiments, a method for modifying a short-circuit exhaust hood can include exchanging a jet generator for a make-up air discharge register, which covers an outlet of an air supply plenum and through which air from the air supply plenum enters into a recess of the short-circuit exhaust hood. The exchanging can include removing the discharge register and sealing upper and lower portions of the jet generator around the air supply plenum outlet. The jet generator together with surfaces of the short-circuit exhaust hood can form a plenum that conveys air from the air supply plenum outlet to a plurality of first and second openings in the jet generator. The first openings can be constructed to form horizontally directed jets at a lower end of the recess. The second openings can be constructed to form vertically directed jets at the lower end of the recess. The jet generator can have a protruded shape in cross-section such that both the first and second openings are spaced from the upper portion of the jet generator and the air supply plenum outlet in a horizontal direction. 
         [0010]    In embodiments, a jet generating apparatus can be used in a short-circuit exhaust hood having a make-up air supply plenum with an outlet in fluid communication with a recess of the short-circuit exhaust hood. The jet generating apparatus can include a first portion, a second portion, an outlet portion, and an intermediate portion. The first portion can extend in a substantially vertical direction. The second portion can extend in a substantially horizontal direction. The outlet portion can be provided at a horizontal end of the second portion and include a plurality of first and second openings therein. The intermediate portion can connect the first portion to the outlet portion. The first openings can be constructed to form first jets in the horizontal direction, and the second openings can be constructed to form second jets in the vertical direction. The first and second portions can be constructed to seal the air supply plenum outlet when installed in the short-circuit exhaust hood. The outlet portion can have a protruded shape in cross-section such that both the first and second openings are spaced from the first portion in the horizontal direction. 
         [0011]    In embodiments, a method for modifying a short-circuit hood can include replacing a make-up air system or conditioned air supply system, which provides a flow of air at a front of the short-circuit hood via a respective outlet, with a jet generator that forms a combination of horizontally and vertically directed jets along a lower portion of a recess of the short-circuit hood at said front. 
         [0012]    In embodiments, a short-circuit hood can include a plenum adjacent the hood, the plenum having an interior volume separated from an interior volume of the hood recess by a wall. A method of modifying the short-circuit hood can include removing a portion of the wall to communicate at least a portion of the internal volume of the plenum with the interior volume of the hood recess, thereby expanding the interior volume of the recess. A curved wall can be affixed in position to replace the removed portion of the wall. The curved wall may form a curved angled lip portion at a lower edge of the hood that curves toward a fume intake facing an interior of the hood recess. A jet generator can be affixed to the hood in order to generate generally horizontal and generally vertical jets along the lower edge of the hood. The jet generator may include an integrated or separate controller and sensor for detecting an exhaust load. The controller can be connected to the exhaust system and a rate of flow of the exhaust of the exhaust system may be regulated by the controller responsively to a signal from the sensor. 
         [0013]    Objects and advantages of embodiments of the disclosed subject matter will become apparent from the following description when considered in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]    Embodiments will hereinafter be described with reference to the accompanying drawings, which have not necessarily been drawn to scale. Where applicable, some features may not be illustrated to assist in the illustration and description of underlying features. Throughout the figures, like reference numerals denote like elements. 
           [0015]      FIG. 1  shows a side cross-section of a short-circuit exhaust hood installed over a cooking appliance. 
           [0016]      FIG. 2  is a simplified schematic diagram of a modified short-circuit exhaust hood, according to one or more embodiments of the disclosed subject matter. 
           [0017]      FIG. 3A  shows a side cross-section of the short-circuit exhaust hood of  FIG. 1  modified to generate horizontal and vertical jets, according to one or more embodiments of the disclosed subject matter. 
           [0018]      FIG. 3B  shows a bottom view of the modified hood of  FIG. 2 . 
           [0019]      FIG. 4  shows a side cross-section of a jet generator for modifying a short-circuit hood, according to one or more embodiments of the disclosed subject matter. 
           [0020]      FIG. 5  shows a side cross-section of a short-circuit exhaust hood with conditioned air curtain. 
           [0021]      FIG. 6  shows a side cross-section of the short-circuit exhaust hood of  FIG. 5  modified to generate horizontal and vertical jets, according to one or more embodiments of the disclosed subject matter. 
           [0022]      FIG. 7  is a simplified process flow diagram for a method of modifying a short-circuit exhaust hood to generate horizontal and vertical jets, according to one or more embodiments of the disclosed subject matter. 
           [0023]      FIG. 8A  is a generalized illustration of a short-circuit range hood according to the prior art. 
           [0024]      FIG. 8B  shows a modification of the hood of  FIG. 8A  according to embodiments of the disclosed subject matter in which a conditioned air supply device operates to seal an unused make-up air register opening and positions jet generators to provide vertical and/or horizontal jets, according to respective embodiments. 
           [0025]      FIG. 8C  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which a conditioned air supply device operates to seal an unused make-up air register opening and positions jet generators to provide vertical and/or horizontal jets, according to respective embodiments and in which the make-up air capacity of the short-circuit air supply is redirected and utilized. 
           [0026]      FIG. 8D  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which a conditioned air supply device operates to seal an unused make-up air register opening and positions jet generators to provide vertical and/or horizontal jets, according to respective embodiments and in which an excess capacity of the hood is throttled using a higher pressure drop or adjustable grease filter. 
           [0027]      FIG. 8E  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which an interior volume of the hood is increased by modifying a forward inside wall of the hood which displaces unused volume of the make-up air plenum of the short-circuit air supply. 
           [0028]      FIG. 8F  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which an interior volume of the hood is increased by modifying a forward inside wall of the hood which displaces unused volume of the make-up air plenum of the short-circuit air supply and in which jet generators to provide vertical and/or horizontal jets. 
           [0029]      FIG. 8G  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which an interior volume of the hood is increased by modifying a forward inside wall of the hood which displaces unused volume of the make-up air plenum whilst redirecting the make-up air through a separate register of the short-circuit air supply. 
           [0030]      FIG. 8H  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which an interior volume of the hood is increased by modifying a forward inside wall of the hood which displaces unused volume of the make-up air plenum whilst redirecting the make-up air through a separate register of the short-circuit air supply and in which jet generators to provide vertical and/or horizontal jets. 
           [0031]      FIG. 8I  shows a bottom view of adjacent jet generator plenums at a corner of the hood according to embodiments of  FIGS. 8A through 8H . 
           [0032]      FIG. 8J  is a view from the rear illustrating a jet generator plenum that wraps along a side of the hood and under it to distribute jets along a lower edge of the hood. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]      FIG. 1  shows a short circuit hood  101  that has a recess  102  to buffer exhaust fumes  103  emanating from a cooking appliance  120 . The recess  102  can have an aspect ratio in cross-section of approximately one, but may be any depth. The depth of the hood (i.e., D in  FIG. 1 ) is significant and ensures that there is an amount of air retained in recess  102  that can be displaced by pulses of rapid buoyancy-driven hot air (“plug flows” as they are sometimes called) to prevent driving fumes into the ambient space surrounding the hood  101 . An exhaust plenum  104  draws gas from the recess  102  and into an exhaust duct  110  drawn by a fan (not shown). Such a fan may be installed remote from the exhaust hood  101 , for example, on the roof of a building housing a kitchen where the hood is located. Filter  148  can be arranged between the recess  102  and the exhaust plenum  104  such that fumes pass through the filter en route to the exhaust duct  110 . The exhaust hood  101  may also have sidewalls with flat, inwardly facing side surfaces at opposite sides of the hood that bound the recess  102  and define the lateral extent thereof. Such sidewalls  100  may extend the full depth of the hood or only a partial depth of the hood. 
         [0034]    Make-up air flows from a supply duct  106  into a supply plenum  108  and out through a discharge grill or register  114 . The make-up air may also be supplied by a fan (not shown) located remote from the exhaust hood, for example, on the roof of the building. The discharge grill  114  diffuses and directs a large volume of air directly into the recess. The discharge grill  114  may extend only partly along a width of the hood  101 . Note that the width of the hood  101  is taken to be the dimension of the hood in the direction perpendicular to the plane of the cross-section illustrated in  FIG. 1  (i.e., into the page of  FIG. 1 ). Optionally, a conditioned air duct  118  may be provided at the front edge  116  of the exhaust hood  101 . The conditioned air duct may convey conditioned air from an HVAC duct (not shown) to discharge  117  to form a supply of relatively slow moving air. This conditioned air discharge could be used to improve the comfort level of employees working in the vicinity of the hood, for example, to keep cooks and other employees working with cooking appliance  120  cool and comfortable. 
         [0035]    In embodiments of the disclosed subject matter, a short-circuit hood as described with respect to  FIG. 1  may be converted into a hood with horizontal and vertical jets at a lower edge of the hood. In particular, the conversion of the make-up air discharge into horizontal and vertical jets that coalesce into planar jets serve to enhance the capture and containment capability of the hood, thereby minimizing the amount of conditioned air required to be exhausted by the hood for adequate removal of produced effluent. The particular range of velocities, positioning, and direction of the jets in combination with a shape of the hood recess, are such as to create a large buffer zone that enhances capture as well as preventing captured fumes from escaping the hood, e.g., due to cross-drafts. 
         [0036]    By providing the series of jets at or near the lower edge of the exhaust hood recess, the vertically directed jets can form an air curtain that confines the entry of conditioned air into the exhaust stream to an effective aperture defined by the terminus of the air curtain while the horizontally directed jets can form a virtual barrier to prevent (or at least reduce the amount of) fumes from being drawn from the recess by external air movements. The large volume defined by the canopy interior, extended by the vertically directed jets, creates a large buffer zone to smooth out transients in plug flow. The horizontally directed jets ensure that fumes captured in the exhaust recess can be retained in the exhausted recess until removal by the exhaust plenum. This enhanced capture efficiency permits the exhaust fan (or blower) to operate at a slower speed while enforcing full capture and containment. This in turn minimizes the amount of conditioned air that must be extracted with a concomitant reduction in energy loss. 
         [0037]    In general, a conversion device  200  can be added to an already installed short-circuit exhaust system  201 , which may include an exhaust fan  208  for removing gas from an exhaust hood  206  and an air supply  210  for supplying air to a make-up air plenum  204  for introduction into the exhaust hood  206 . The conversion device  200  includes a jet generator  202  that can be installed between a make-up air plenum  204  and the exhaust hood  206 , as shown in  FIG. 2 , for example at a make-up air outlet of the plenum  204 . The jet generator  202  can include a blank (e.g., a sheet metal blank) with a plurality of openings arranged proximal to a bottom edge of the exhaust hood  206 . A first set of the openings can generate capture jets that emanate from the jet generator  202  in a horizontal direction. A second set of the openings can generate capture jets that emanate from the jet generator  202  in a vertical direction. The air supply  214 , which may provide make-up air and/or conditioned air, conveys air to the jet generator  202 , which restricts the air flow in order to form the horizontal and vertical jets. Other components can be provided to redirect air flow in the short-circuit hood  206  and/or increase the flow rate or pressure of air supplied to the blank in order to form the jets. 
         [0038]    Individual jets are directed either in a substantially vertical direction (i.e., from the hood toward the ground) or in a substantially horizontal direction (i.e., parallel to the ground and toward the exhaust outlet side of the hood). The jets directed in a particular direction may coalesce into a planar jet (e.g., a vertical curtain jet or an unattached horizontal planar jet) a short distance from the nozzles from which they originate. 
         [0039]    Multiple jets that have nozzles with smaller diameters and that propel air at a higher velocity are generally more effective than a single jet with one long and narrow nozzle or even multiple jets with much larger nozzles. The effectiveness of the air jets depends, in large part, on its output velocity. Air jets with larger nozzles must discharge air at a faster rate to achieve a comparable output velocity. On the other hand, smaller nozzles generally produce much smaller scale turbulence and tend to disturb the thermal flow created by the cooking surface to a lesser degree than larger scale turbulence. Smaller nozzles also require less air. Jets with lower output velocities create an air flow that dissipates more quickly due to loss of momentum to viscosity and may have a throw that is only a short distance from the nozzle. Operation and design of the plenums forming the jets can be such that the jets are formed with such dimensions and velocity that the jet air flow dissipates prior to or shortly after reaching the effluent plume  103  and/or the cooking appliance  120 . 
         [0040]    Each jet can be formed by a respective air nozzle formed in a jet generating plenum. Although not a requirement, the nozzles for generating jets in a particular direction along a particular lower edge of the exhaust hood can be positioned to form a substantially straight line. The nozzles may simply be perforations in a plenum defined by a metal blank of the conversion apparatus attached to the short-circuit system. Alternatively, they may be nozzle sections with a varying internal cross section that minimizes expansion on exit. The nozzles may contain flow conditioners such as settling screens and/or or flow straighteners. The initial velocities of the horizontal jets may be between 2 and 3.5 times the initial velocities of the vertical jets, the initial velocity in this case being the point at which individual jets coalesce into a single planar jet. 
         [0041]    The horizontal and/or vertical jets may be directed parallel with respect to each other. That is, horizontal jets may emanate from the plenum and proceed laterally across the lower portion of the recess in non-intersecting directions. Alternatively, some of the horizontal jets may be angled with respect to other horizontal jets. For example, horizontal jets located at corners of the hood, i.e., in or around the intersection of front edge of the hood with its sides may be angled toward the center of the hood where an exhaust vent is located. Thus, when the hood has a relatively large aspect ratio (i.e., width of the front edge of the hood as compared to the width of the side edge of the hood), the horizontal jets at the corners can be angled toward the center to enhance capture and containment of exhaust. Alternatively or additionally, the plenum that forms the jets may have an angled or curved geometry, as viewed in a top-down plan view of the hood, such that horizontal jets emanating in a direction substantially perpendicular to the plenum surface are automatically angled toward the center of the hood. 
         [0042]    The horizontal and vertical jets can be formed on the front edge of the exhaust hood (i.e., an edge opposite the exhaust outlet and/or closest to a working edge of the cooking appliance). In addition, the horizontal and vertical jets can be formed at other edges of the exhaust hood, such as, but not limited to side or rear edges of the exhaust hood. A common plenum can be used to form the jets on the front and additional edges of exhaust hood, although multiple separate plenums are also possible according to one or more contemplated embodiments. 
         [0043]      FIGS. 3A-3B  show an exhaust hood  101  after modification (hood  101  shown in  FIG. 1  prior to the modification) according to an embodiment of the disclosed subject matter.  FIG. 3A  shows a side cross-section view of the hood  101  while  FIG. 3B  shows the hood  101  as viewed from the cooking appliance  120  below the hood. The make-up discharge grill  114  can be removed from the short-circuit supply plenum  108 , thereby leaving outlet  168  open. A jet generator  150  can be attached over outlet  168  and sealed thereto. An upper portion  152  of the jet generator may attach to the exhaust hood over an upper end of the make-up air outlet  168  while a lower portion  155  may attach to the exhaust hood over a lower end of the make-up air outlet  168  (e.g., by extending over a portion of the lower edge  116  of exhaust hood). The upper portions  152  and lower portions  155  of the jet generator may seal the jet generator  150  to the make-up air outlet  168  such that air from the short-circuit supply plenum  108  is forced into plenum  153  formed by the jet generator and out through openings  142 ,  160  therein to form respective jets. 
         [0044]    The jet generator  150  can be, for example, an angled sheet metal blank. First openings  142  and second openings  160  can be provided in the angled sheet metal blank. Alternatively or additionally, the first and/or second openings may be formed by interaction of portions of the jet generator  150  with respective portions of the hood  101  and/or supply plenum  108 . The shape of the jet generator  150  can be such that at least a portion thereof extends away from the plenum  108  into the area of the recess  102 . For example, the jet generator  150  may have a nose shape or other protruded shape in side view. The “nose” shape of the jet generator  150  can include a bevel  151  that directs air at a front portion of the recess  102  back toward the center of the hood, as indicated by the arrow  154 . 
         [0045]    The jet generator  150  can be formed to extend around the side edges of the hood  101  as well as the front edge thereof. Thus, the jet generator  150  can be constructed to have laterally extending side portions to form a U-shape in top down plan view (see, for example,  FIG. 4 ). The jet generator  150  attaches to the inside face of the sides  100  along the sides to form a sealed longitudinal plenum that feeds holes  142  and  160  extending along the sides. The arrangement thus forms a small plenum  153  that wraps around the inside facing perimeter of the hood, attaching to the inward face of the sides  100 , as shown in  FIG. 3B . The make-up air supply plenum  108  may only be provided at the front edge of the exhaust hood; however, the interaction between the outlet  168  and the jet generator  150  at the front edge of the hood may be sufficient to supply air to the portions of the jet generator along the front and side edges via plenum  153 . The interior of the jet generator  150  forming plenum  153  may be constructed such that the air flow out of each of the horizontal and vertical jets is substantially the same as other horizontal and vertical jets. In other words, the plenum  153  may be configured such that jets on the side portions far away from the make-up air outlet  168  have the same or similar velocities and flow rates as the respective jets on the front portions of the jet generator  150  close to the make-up air outlet  168 . Alternatively, separate blanks can be provided for the side edges, which may be inserted into make-up air supply plenums on side edges (not shown) or portions of the make-up air supply plenum at the front edge. 
         [0046]    The openings  142  and  160  allow air from the supply plenum  108  to escape into the recess of the exhaust hood, but the volume of supply air is substantially reduced from the rate of the original short-circuit system of  FIG. 1 . Openings  142  in jet generator  150  can generate jets  140  directed in a substantially horizontal direction (i.e., parallel to the ground) while openings  160  can generate jets  146  directed in a substantially vertical direction (i.e., perpendicular to the ground). Note that in the figures, only a portion of the jets have been shown for clarity of illustration. Openings  142  in jet generator  150  can be spaced apart a range of suitable distances and have diameters of various magnitudes to provide substantially a free planar jet whose throw is effectively about 30 to 90 percent of the dimension of the hood across the direction they face (front to back or side to side). The “throw” may be a distance of an isothermal jet when it reaches a terminal velocity of 5 feet per minute (fpm). The vertical holes  160  may be constructed to produce jets having a throw of about 20 to 90 percent of the vertical spacing between the appliance  120  and the lower edge  116  of the hood  101 . 
         [0047]    Another way to describe the configuration of the holes  142  and  160  is in terms of the equivalents of isothermal planar free jets that are formed by holes  142  (producing horizontally directed jets) and holes  160  (producing vertically directed jets). For example, holes  142  can each have a diameter of 5 mm and can be provided linearly in the jet generator  150  at a spacing of 32 mm on centers, and holes  160  can each have a diameter of 3.4 mm and can be provided linearly in the jet generator  150  at a spacing of 32 mm on centers. The holes can be simply punched into sheet metal of about 12 gauge, for example. The air supply to the holes  142  and  160  in the jet generator  150  can be at a pressure of 0.2 to 0.4 in water gauge, for example, 0.265 to 0.31 in water gauge. However, other configurations, including hole sizes, spacings, plenum pressures, elongated linear slots, hole shapes, etc., for generating curtain jets are also possible according to one or more contemplated embodiments. 
         [0048]    In a typical short-circuit exhaust hood, the air flowing through the make-up grill  114  is about 50 cfm per linear foot of the hood (in a direction of the width of the exhaust hood), although wide variations in this number are possible. However, after insertion of the jet generator  150  according to embodiments of the disclosed subject matter, the resulting air flow emanating as horizontal and vertical jets is between 5 and 15 cfm per linear foot. 
         [0049]    Additionally or alternatively, other changes to the short-circuit hood may be made to reconfigure the make-up air or conditioned air flow into the hood as horizontal and vertical jets. Referring to  FIGS. 5-6 , a short-circuit hood is shown in unmodified ( FIG. 5 ) and modified ( FIG. 6 ) configurations.  FIG. 7  is a process flow diagram of a method for modifying the short-circuit hood shown in  FIG. 5 . 
         [0050]    In the configuration of  FIG. 5 , the short-circuit hood  500  includes air volume control dampers  506 ,  508  that regulate respective flows of make-up air (i.e., through make-up air collar  504  to the supply plenum  108 ) and conditioned air (i.e., through HVAC unit collar  510  to the conditioned air duct  118 ). Conditioned air supplied to the conditioned air duct  118  is used to create a vertical curtain of fresh air at the front of the hood. Thus, air from the duct  118  is discharged at outlet  516  through a grill or diffuser  517 . Meanwhile, make-up air (e.g., outside air) is provided via make-up air collar  504  to supply plenum  108  and then to the recess  102  of the hood through a grill or diffuser  114 . Fumes and other gases in the recess  102  can be exhausted by a fan connected to exhaust duct collar  502 , which withdraws the fumes from the recess  102  by way of filter  148  and exhaust plenum  104 . 
         [0051]    Referring now to  FIG. 7 , the retrofit of short-circuit exhaust hood  500  can begin at  702  with disconnecting the make-up air intake collar  504  from the make-up air supply (not shown). At  704 , the make-up air volume control damper  508  can be removed. An optional fire damper  602  can be installed in the collar  504  or the upper portion of plenum  108  after removal of damper  508 . Proceeding to  706 , a fan  604  can be attached to the make-up air intake collar  504 . The fan  604  can draw air from the ambient space (e.g., a hung ceiling space) as opposed to outdoor air or air directly from a duct of an HVAC system. 
         [0052]    At  708 , the discharge grill  114  in the supply plenum  108  can be removed, leaving behind a supply plenum outlet  614 . At  710 , a jet generator  650  can be installed at the supply plenum outlet  614  so at replace the discharge grill  114  with the plenum  615 . As noted herein, the jet generator  650  can have a plurality of openings or holes  642 ,  660  therein. At  712 , unused air flow paths of the short-circuit hood can be capped and/or blocked. For example, the air curtain duct  118  can be capped by cap  606  at the HVAC unit collar  506 . Additionally or alternatively, the diffuser  517  can be removed, leaving behind an air curtain outlet  516 , which can then be blocked by sealing portion  616 , as shown in  FIG. 6 . 
         [0053]    At  714 , air can be provided to the supply plenum  108  by running fan  604 , which in turn feeds holes  642  and  660  in jet generator  650  to form the horizontal jets  640  and vertical jets  646 , respectively. Jet generator  650  can have a nose structure with a forward edge portion  650   a  and side edge portions  650   b  that wrap around the inside surfaces of the hood  500  so as to create vertical and horizontal facing openings  642 ,  600  along the lower edge of the hood  500 . The fan can be controlled so as to regulate the pressure in the air supply plenum  108  to between 0.2 and 0.5 in water gauge and to provide a total volume flow rate of between 10 and 25 cubic feet per second (cps). 
         [0054]      FIGS. 8A through 8H  show views from the side and, in parts, in section. 
         [0055]      FIG. 8A  is a generalized illustration of a short-circuit range hood  801  according to the prior art. Outside air taken through a roof  818  penetration via a duct  804  is moved by a fan  814  to pressurize a short-circuit plenum  802  to provide make-up air to a short-circuit discharge register  810  and a ventilation register  812 . Fumes, make-up air, and conditioned air are drawn into a recess  800  by an exhaust system  816  (only the duct is shown but the exhaust system may include one or more fans, flow control dampers, and other elements). The fumes, make-up air, and conditioned air are drawn from the recess  800  via a filter plenum  808  which may hold a grease filter (not shown) as illustrated by arrows  811 . In variations of the basic design, one of the registers  810  and  812  may not be present. Where there is no register  810 , vanes may direct the flow of register  812  toward the hood  801  inlet. 
         [0056]      FIG. 8B  shows a modification of the hood of  FIG. 8A  according to embodiments of the disclosed subject matter in which a conditioned air supply device operates to seal an unused make up air register opening and positions jet generators to provide vertical and/or horizontal jets, according to respective embodiments. The fan  814  is disconnected and the short-circuit plenum  804  is sealed by blank  834 , and a portion  836  of a jet generator plenum  830  is pressurized with conditioned air from the occupied space by a fan  832 . Fumes, make-up air, and conditioned air are drawn into the recess  800  by the exhaust system  816 . 
         [0057]    The jet generator plenum may have a dimension (indicated at  833 ) relative to the front-back dimension of the hood that is thin, for example, 50 to 150 cm and may extend across the forward face of the hood. The jet generator creates linear jets in a generally horizontal direction  828  and a generally vertical direction  829  along the forward edge of the hood. The linear jets may be formed by flowing air through openings  838 ,  840 , which may be an array of holes or a slit. The vertical and horizontal jets may also circumnavigate the hood lower edge on one or both lateral sides. In a canopy configuration, the vertical and horizontal jets may also circumnavigate the hood lower edge on all sides including the forward and back sides. In such configurations, separate plenums  830  may be used for each rectangular face of the hood  801  or the plenums may be combined. For example, a single plenum may wrap around two sides, three sides or all sides of the hood  801 . At the corners, the lower part of the jet generator plenum  830  indicated at  837  may be beveled to allow positioning against an adjacent plenum on another face of the hood  801  as illustrated in  FIG. 8I . 
         [0058]      FIG. 8C  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which a conditioned air supply device operates to seal an unused make-up air register opening and to provide vertical and/or horizontal jets, according to respective embodiments and in which the make-up air capacity of the short-circuit air supply is redirected and utilized. A jet generator plenum  848  is pressurized with conditioned air from the occupied space by a fan  832 . The fan  814  remains to function for the supply of make-up air, however, the short-circuit plenum is modified by forming a discharge register  844  which vents make-up air directly to a conditioned space outside the hood  801 . The short-circuit plenum  802  is sealed by blank  834  and a portion  848  of a jet generator plenum  842 . Fumes, make-up air, and conditioned air are drawn into the recess  800  by the exhaust system  816 . As above, in variations of the basic design, one of the registers  810  and  812  may not be present and where there is no register  810 , vanes may direct the flow of register  812  toward the hood  801  inlet. The jet generator plenum  842  wraps around the short-circuit plenum  802  not from the front toward the rear as in the embodiment of  FIG. 8B , but down along the side of the short-circuit plenum and then along the bottom as shown in  FIG. 8J . 
         [0059]    A further feature that may or may not be present in any of the embodiments of  FIGS. 8A through 8H  is an additional or replacement grease filter  846  that reduces the rate of exhaust by generating greater pressure drop than an original filter (not shown). By using a filter with a higher pressure drop, the grease capture effectiveness may be enhanced. For example, the filter may have a greater number of turns, sharper turns, or more tortuous turns therein as would be understood by those skilled in the design of grease filters. These higher efficiency grease filters may be readily designed and may be identified as inertial type grease filters. So the substitution is of a first inertial grease filter with a second one characterized by a higher pressure drop for a given volume flow rate and concomitantly higher capture effectiveness. As is known in the art, the higher capture effect is produced as a result of capturing smaller particles by the second as compared to the first. The replacement of the original filters may be a feature of any of the embodiments disclosed. The reduction of flow may correspond to the reduced amount of exhaust required for capture and containment as a result of the provision of the vertical and horizontal jets and the elimination of the short-circuit arrangement, which is less effective for a given net exhaust rate than a hood without. 
         [0060]    As above, the jet generator creates linear jets in a generally horizontal direction  828  and a generally vertical direction  829  along the forward edge of the hood. The linear jets may be formed by an array of holes or a slit. The vertical and horizontal jets may also circumnavigate the hood lower edge on one or both lateral sides with suitable arrangement of the plenum  848 . In a canopy configuration, the vertical and horizontal jets may also circumnavigate the hood lower edge on all sides including the forward and back sides. 
         [0061]      FIG. 8D  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which a conditioned air supply device operates to seal an unused make-up air register opening and provides vertical and/or horizontal jets, according to respective embodiments and in which an excess capacity of the hood is throttled using a higher pressure drop or adjustable grease filter. A jet generator plenum  848  is pressurized with conditioned air from the occupied space by the fan  832 . The fan  814  remains to function for the supply of make-up air with the short-circuit plenum modified as above to vent make-up air directly to a conditioned space outside the hood  801 . The short-circuit plenum  802  is sealed as above by blank  834  and a portion  848  of the jet generator plenum  842 . Fumes, make-up air, and conditioned air are drawn into the recess  800  by the exhaust system  816 . As above, in variations of the basic design, one of the registers  810  and  812  may not be present and where there is no register  810 , vanes may direct the flow of register  812  toward the hood  801  inlet  805 . The jet generator plenum  842  wraps around the short-circuit plenum  802  not from the front toward the rear as in the embodiment of  FIG. 8B , but down along the side of the short-circuit plenum and then along the bottom as shown in  FIG. 8J . 
         [0062]    A further feature that may or may not be present in any of the embodiments of  FIGS. 8A through 8H  is a controller  846  configured to modulate the make-up air and, optionally also, the exhaust flow rates in order to match the make-up air supply in demand-based exhaust control. The controller may be integrated mechanically as part of the jet generator plenum  848 , fan  832 , and provided as a unitary device. Sensors such as infrared imaging sensors may be combined with such a unitary device and used to modulate the exhaust rate and make-up air rates according to a demand control scheme. Thus, controller  846  may be connected as a controller of the exhaust rate, which may be the exhaust fan speed or a damper (not shown), as well as connected to the fans  832  and  814  to modulate rates of exhaust and make-up air, according to demand control as indicated by the sensor and other inputs. For example, a scheme and component details may be provided according to International Publication No. WO 2010/065793, entitled “Exhaust Flow Control System and Method,” which is hereby incorporated by reference herein in its entirety. 
         [0063]    As above, the jet generator creates linear jets in a generally horizontal direction  828  and a generally vertical direction  829  along the forward edge of the hood. The linear jets may be formed by an array of holes or a slit. The vertical and horizontal jets may also circumnavigate the hood lower edge on one or both lateral sides with suitable arrangement of the plenum  848 . In a canopy configuration, the vertical and horizontal jets may also circumnavigate the hood lower edge on all sides including the forward and back sides. 
         [0064]      FIG. 8E  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which an interior volume of the hood is increased by modifying a forward inside wall of the hood which displaces unused volume of the make-up air plenum of the short-circuit air supply. In the modification, the short-circuit fan  814  and  804  may be blocked off (for example by blanks such as blank  812 ) or removed altogether. The forward wall of the interior of the hood  801  and the short circuit plenum  802  is removed or cut to enlarge the hood  801  recess  800 . In an embodiment, a curved wall as indicated at  852  may be secured in position to provide a smooth transitional configuration that causes buoyancy-driven fumes that are not immediately drawn into the filter plenum  808  to follow the wall  852  and deflected by a lip  854  back toward the hood recess  800 . 
         [0065]      FIG. 8F  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which an interior volume of the hood is increased by modifying a forward inside wall of the hood which displaces unused volume of the make-up air plenum of the short-circuit air supply and provides vertical and/or horizontal jets via a jet generator.  FIG. 8F  illustrates the similar features as  FIG. 8E , however a jet generator  842  and fan  832  may be arranged as described with reference to  FIG. 8C . 
         [0066]      FIG. 8G  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which an interior volume of the hood is increased by modifying a forward inside wall of the hood which displaces unused volume of the make-up air plenum whilst redirecting the make-up air through a separate register of the short-circuit air supply. In this variant, the short-circuit fan  814  and supply  804  are retained and used to feed a revised make-up air plenum  864  that utilizes part of the original short-circuit plenum volume, part of which is shared by a new interior volume  803  of the hood  801 . The curved wall  867  is emplaced in the manner described with respect to  FIG. 8E  but the shape permits a sufficient volume to be provided for the make-up air plenum  864 . A discharge register  862  discharges make-up air into a conditioned space. 
         [0067]      FIG. 8H  shows a modification of the hood of  FIG. 8A  according to further embodiments of the disclosed subject matter in which an interior volume of the hood is increased by modifying a forward inside wall of the hood which displaces unused volume of the make-up air plenum whilst redirecting the make-up air through a separate register of the short-circuit air supply and in which vertical and/or horizontal jets are provided by a jet generator.  FIG. 8H  illustrates similar features as  FIG. 8E , however a jet generator  832  and fan  842  may be arranged as described with reference to  FIG. 8C . 
         [0068]    For any of the embodiments of  FIGS. 8B through 8H , an embodiment of the disclosed subject matter can include kits which may be assembled to provide the describe supplemental components for modification of the prior art hood. 
         [0069]    It will be evident to those skilled in the art that the embodiments of the disclosed subject matter are not limited to the details disclosed herein, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. For example, while jets have been described as formed using a series of round nozzles, it is clear that it is possible to form jets using a single slot or non-round nozzles. Also, the source of air for the jets may be room air, outdoor air or a combination thereof. In addition, embodiments of the disclosed subject matter are applicable to any process that forms a thermal plume, not just a kitchen range. The principles disclosed and described herein can be applied to many different types and styles of exhaust hoods, including, but not limited to back shelf and canopy style hoods. 
         [0070]    Features of the disclosed embodiments may be combined, rearranged, omitted, etc., within the scope of the invention to produce additional embodiments. Furthermore, certain features may sometimes be used to advantage without a corresponding use of other features. 
         [0071]    It is thus apparent that there is provided, in accordance with the present disclosure, exhaust hood methods, devices, and systems. Many alternatives, modifications, and variations are enabled by the present disclosure. While specific embodiments have been shown and described in detail to illustrate the application of the principles of the present invention, it will be understood that the invention may be embodied otherwise without departing from such principles. Accordingly, Applicants intend to embrace all such alternatives, modifications, equivalents, and variations that are within the spirit and scope of the present invention.