Abstract:
A system and method for controlling flow in filtering systems and for balancing the flow through fluid systems employs flow control devices that minimize suspended matter precipitation. Several embodiments are included. In a first embodiment, a smooth-walled flow control device ( 410 ) with no abrupt transitions is provided in a flow conduit section. In a second embodiment, a filter ( 305 ) acts as a flow control device. A variation of the latter locates a flow control device ( 300 ) immediately adjacent to the filter ( 305 ) and upstream of it. In other embodiments, a control system ( 950 ) detects the real time status of the load to provide on the fly critical balancing.

Description:
This application claims benefit of provisional No. 60/224,123 filed Aug. 10, 2000, Ser. No. 60/226,953 filed Aug. 22, 2000 and Ser. No. 60/263,557 filed Jan. 23, 2001. 

   FIELD OF THE INVENTION 
   The present invention relates generally to flow-volume control devices. More specifically, the present invention relates to flow control devices that may be used for balancing fluid flow in a context where suspended particles are entrained in the fluid and their precipitation must be avoided, in free-flowing parts of a flow system, except during filtration. 
   BACKGROUND 
   Exhaust hoods are used to remove air contaminants close to the source of generation located in a conditioned space. For example, one type of exhaust hoods, kitchen range hoods, creates suction zones directly above ranges, fryers, or other sources of air contamination. Exhaust hoods tend to waste energy because they must draw some air out of a conditioned space in order to insure that all the contaminants are removed. As a result, a perennial problem with exhaust hoods is minimizing the amount of conditioned air required to achieve total capture and containment of the contaminant stream. 
   Referring to  FIG. 1 , a typical prior art exhaust hood  90  is located over a range  15 . The exhaust hood  90  has a recess  55  with at least one vent  65  (covered by a filter  60 ) and an exhaust duct  30  leading to an exhaust system (not shown) that draws off contaminated air  45 . The vent  65  is an opening in a barrier  35  defining a plenum  37  and a wall of the canopy recess  55 . The exhaust system usually consists of external ductwork and one or more fans that pull air and contaminants out of a building and discharge them to a treatment facility or into the atmosphere. The recess  55  of the exhaust hood  90  plays an important role in capturing the contaminant because heat, as well as particulate and vapor contamination, are usually produced by the contaminant-producing processes. The heat causes its own thermal convection-driven flow or plume  10  which must be captured by the hood within its recess  55  while the contaminant is steadily drawn out of the hood. The recess creates a buffer zone to help insure that transient, or fluctuating, surges in the convection plume do not escape the steady exhaust flow through the vent. The convection-driven flow or plume  10  may form a vortical flow pattern  20  due to its momentum and confinement in the hood recess. The Coanda effect causes the thermal plume  10  to cling to the back wall. The exhaust rate in all practical applications is such that room air  5  is drawn off along with the contaminants. 
   Referring now also to  FIG. 2 , exhaust hoods  90 , such as illustrated in  FIG. 1 , vary in length and can be manufactured to be very long as illustrated in FIG.  2 . Here multiple vents  65  can be seen from a straight-on view from the vantage of a worker  80 . The length can present a problem because the perimeter along which capture and containment must be achieved is longer near the ends than in the middle. In the middle, there is only one perimeter, the one along the forward edge indicated at  70  in FIG.  1 . At the ends, this perimeter includes the side edge as well which is indicated at  75  in FIG.  1 . The additional perimeter length that must be accommodated at the ends may be called an “end effect.” In other words, the hood cannot be approximated as a two-dimensional configuration because of its finite length. As a result of the increased perimeter at the ends, more air must be exhausted in the vicinity of the ends of the hood than in the middle because the perimeter at the ends consists of both the forward edge  70  of the hood adjacent the worker and end edges  75 , which are perpendicular to the forward edge  70 . 
   If the minimum exhaust rate for the entire hood is to be achieved, then less air should be exhausted near the middle section than near the ends. Otherwise, an excess rate of air exhaust will occur near the middle section to insure the rate at the ends is sufficient. Thus, as a result of the end effects and the requirement of full capture and containment, more air must be drawn through the middle section than necessary. In addition, a higher volume of effluent may be generated at some parts of a hood than at others. This variability leads to the same result: some parts of the hood may require a greater exhaust rate than others. 
   Referring to  FIG. 3 , a similar problem occurs when multiple hoods are connected to a single exhaust system. For example, the hoods may be connected to a common exhaust duct  191 . Each hood must be balanced against the others so that each exhausts at the minimum rate that ensures full capture and containment of the contaminants. Again, ducts carrying grease aerosol should not have dampers because of the hazard caused by grease precipitation. 
   It is known in the prior art to make fixed modifications to the flow requirements of a long hood to address the end effects. 
   SUMMARY OF THE INVENTION 
   Briefly, flow control devices located either within a duct or inside a hood recess enable the control of the relative exhaust volume flow rates through sections of a long exhaust hood or through separate hoods linked to a single exhaust fan. In one embodiment, flow control devices are located inside the ducts. Although using flow control devices in a duct is known, conventional flow control devices, known as “dampers,” cause aerosol precipitation in exhaust ducts of kitchen ventilation systems and are therefore not used. To address this problem, the latter flow control devices are shaped to minimize steady and quasi-steady flow effects associated with the precipitation of grease from the aerosol state. In another embodiment, the flow control devices are located within the hood recess so that any precipitation that occurs as a result of the steady flow structures will remain within the recess and can be cleaned easily. Both types of flow control devices must be designed differently from conventional dampers. Flow control devices within the ducts are designed to restrict flow without forming flow effects that result in the precipitation of grease. Flow control devices within the canopy recess are designed such that they do not interfere with the vortical flow effect caused by the thermal convection plume. Each device may be adjustable or fixed, but preferably they are adjustable in applications where perfectly uniform negative pressures in the building exhaust hookups cannot be guaranteed. 
   A first type of flow control device provides a smooth transition or transitions that do not create regular (stationary or periodic) flow effects associated with precipitation of grease. These are located in the exhaust duct. Flow-volume control dampers with smooth flexible walls are described in detail in U.S. patent application Ser. No. 60/226,953, filed on Aug. 22, 2000 entitled FLOW-VOLUME CONTROL DEVICE, which is hereby incorporated by reference in its entirety as if fully set forth herein. A second type is located in the hood recess and is designed not to intrude into the recess in such a way as to interfere with the vortical flow therein. One category of the second type are dual-function grease filters that control the flow rate and perform the grease-separation function simultaneously. 
   Another strategy for throttling flow without introducing a separate cause of grease (or other suspended particulate) is to make a filter, often present in such systems, that functions as a flow throttling device itself. This may be done in various ways by modulating the size of apertures that are integral with the filter cartridge without interfering with the cartridge&#39;s ability to filter out particulates. 
   According to still other embodiments, the invention provides a control system to provide real time control of the balancing devices. To that end, various sensor inputs may be employed to determine when a hood is as close as possible to a minimum flow rate and prior to a breach. Infrared camera imaging, temperature sensors, and other detection devices may be used to classify the real time status of the load and to control the balance accordingly. 
   While the invention will now be described in connection with certain preferred embodiments and examples and in reference to the appended figures, the described embodiments are not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the scope of the invention as defined by the appended claims. Thus, the following description and examples of the preferred embodiments of the invention are only intended to illustrate the practice of the present invention. The particular embodiments are shown by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention. While the embodiments are described in relation to a metal-air battery cell, the invention is not limited solely to this type of battery cell. Parts of the invention can also be applied to alkaline and other primary battery cells. Prism-shaped metal-air battery cells are illustrated in the description of the invention because the metal-air battery cells are particularly suitable for describing many of the features of the invention. While the embodiments are described in relation to a rectangular shaped battery cell, the invention is not limited to battery cells having rectangular casings. Instead, the invention covers all prism-shaped battery cells, including but not limited to hexagonal, octagonal, and other cells having casings with relatively straight side walls. 
   The particular embodiments are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description, taken with the drawings, makes it apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a side view of a canopy style wall hood according to the prior art. 
       FIG. 2  is a front view of a long canopy style hood with multiple vents. 
       FIG. 3  is a front view of multiple hoods attached to a common exhaust system. 
       FIG. 4  is a side section view of a canopy style hood according to embodiment of the invention. 
     FIG.  5 A. is a section view of a canopy style hood according to the embodiment of FIG.  4 . 
     FIG.  5 B. is a perspective view of a shutter with an actuator mechanism according to embodiment of the invention. 
       FIG. 6  is a front view of a canopy style hood with multiple vents including the shutter mechanism of FIG.  5 B. 
       FIG. 7  is a front view of multiple canopy style hoods connected to a common exhaust in which respective vents of the hoods are controlled by shutter mechanisms according to embodiment of the invention. 
       FIG. 8  is a section view of a canopy hood with a shutter according to another embodiment of the invention. 
       FIG. 9A  is a side view of a centrifugal style cartridge filter used for grease extraction. 
     FIG.  9 B. is a section view of a canopy style hood with a flow control mechanism according to another embodiment of the invention. 
       FIG. 10  is a side view of a canopy style hood with the flow °control mechanism according to still another embodiment of the invention. 
       FIG. 11  is a front view of vents of a canopy hood or back shelf hood with rolling shutters according to yet another embodiment of the invention. 
       FIG. 12  is a section view of rolling shutter mechanism according to an embodiment of the invention. 
       FIG. 13  is a partial section view all long hood with multiple exhaust vents and corresponding flow of throttling devices according to an embodiment of the invention. 
       FIG. 14  is a sectional side view of the embodiment of FIG.  13 . 
       FIG. 15  is the perspective cut away of a shutter mechanism according to embodiment of the invention. 
       FIG. 16  a perspective cut away of a shutter mechanism according to another embodiment of the invention. 
       FIG. 17  is a sectional view of a combination filter/flow throttling device according to embodiment of the invention. 
       FIG. 18  is a sectional view of a combination filter/flow throttling device according to embodiment of the invention. 
       FIG. 19  is a sectional view of a combination filter/flow throttling device of  FIG. 18  in a throttle-down position. 
       FIG. 20  is the face view of the filter of  FIGS. 18 and 19  shown partly in throttle-down position and partly in throttle-up position. 
       FIG. 21A  is a sectional view of a combination filter/flow throttling device according to yet another embodiment of the invention. 
     FIG.  21 B. is a sectional view of the filter/flow throttling device of  FIG. 21  a in the throttle-up position. 
     FIG.  21 C. is a front view of the filter of  FIGS. 21   a  and  21 B. 
       FIG. 22A  is a section view of a filter/flow throttling device according to another embodiment of the invention. 
     FIG.  22 B.  FIG. 22B  is a section view of the filter of  FIG. 22A  in a throttle-down position. 
       FIG. 22C  is a front view of the filter of FIGS.  22 A. and  22 B. 
       FIG. 23A  is a alternative embodiment of the device of  FIGS. 22A through c . 
     FIG.  23 B. is an alternative embodiment of the device of  FIGS. 22   a  through  22 C. 
     FIG.  24 A. is a section view of a canopy hood with a flow throttling device including a cleaning fluid according to embodiment of the invention. 
     FIG.  24 B. is a section view of the flow throttling device of  FIG. 24  day in the throttle-down position. 
     FIG.  24 C. is a top view of the embodiments of  FIGS. 24A and 24B . 
       FIG. 25A  is a section view of a flow throttling device also using A cleaning fluid according to embodiment of the invention. 
       FIG. 25B  is a section view of the flow throttling device of  FIG. 25  a in a throttle-down position. 
       FIG. 26  is a section view of a canopy hood showing a flow throttling device in which apply them is contracted according to embodiment of the invention. 
       FIG. 27  is a section view of the embodiment of  FIG. 26  in throttle-down position. 
       FIG. 28A  is a business section view of the canopy hood showing a flow throttling device employing an expandable bladder according to embodiment of the invention. 
       FIG. 28B  is a section view of the flow throttling device of  FIG. 28  a in throttle-down position 
       FIG. 29  to section view of a canopy hood with a flow throttling device employing a flexible back wall of a plenum according to embodiment of the invention. 
       FIG. 30  is a section view of a canopy hood with a flow throttling device using a ball bowel arrangement according to embodiment of the invention. 
       FIG. 31  is a section view of a canopy hood with the flow throttling device of  FIG. 30  in throttle-down position. 
       FIGS. 32A and 32B  are side views of an alternative bowel arrangement suitable for use in the embodiment of  FIGS. 30 and 31 . 
       FIG. 33  is a section view of a flow throttling device for a hood and a throttle-up position according to an embodiment of the invention. 
       FIG. 34  is a section view of the flow throttling device of  FIG. 33  in a throttle-down position. 
       FIG. 35  is a front view all long hood with multiple vents and multiple duct sections which may be selectively blocked according to embodiment of the invention. 
       FIG. 36  is a section side view of the embodiment of FIG.  35 . 
       FIG. 37  is a perspective view of a cylindrical module of a combination filter/flow throttling device according to an embodiment of the invention. 
       FIG. 38  is a perspective view of a combination filter/flow throttling device employing the module of  FIG. 37 and a  rotating assembly. 
       FIG. 39  is a perspective view of the embodiment of  FIG. 38 and a  throttle-up position. 
       FIG. 40  is a section view of a canopy style hood sensors to gather data about cooking conditions. 
       FIG. 41  is a blocked side man of the controller with sensors for controlling the balance of one or more kitchen exhaust hoods. 
       FIG. 42  is a perspective view of a cooking appliance and hood showing various camera angles. 
       FIG. 43A  is a side view of a hood and cooking appliance with a plume in which the exhaust rate is higher than necessary. 
       FIG. 43B  is a side view of a hood and cooking appliance with a plume in which the exhaust rate is set at an optimal rate. 
       FIG. 43C  is a side view of a hood and cooking appliance with a plume in which the exhaust rate is set to low. 
       FIG. 44  is a perspective view of a canopy quoted and cooking appliance showing a plume escaping containment. 
       FIG. 45  is a Schlerian photograph of the thermal plume rising from a cooking appliance into a canopy hood. 
       FIG. 46  is a section view of a canopy hood with a shutter and an actuator mechanism according to embodiment of the invention. 
       FIG. 47  is a section view of a canopy hood with a shutter and an actuator mechanism according to another embodiment of the invention. 
       FIG. 48A  is a perspective view of expandable scroll module which functions as a filter/flow throttling mechanism according to an embodiment of the invention. 
       FIG. 48B  is a perspective view of a set of the expandable scroll modules of  FIG. 48A  attached to each other such that they can expand and contract as a unit. 
       FIG. 49  is a section view of the embodiment of  FIG. 48  in a throttle-up position. 
       FIG. 50  is a section view of the embodiment of  FIGS. 48 and 49  in a throttle-up position. 
       FIG. 51  is a perspective view of the embodiment of  FIG. 48  showing a supporting framework and actuator mechanism. 
       FIG. 52  is a section view of the embodiment of  FIG. 51  showing a support feature of that embodiment. 
       FIG. 53  is a perspective view of an embodiment similar to the embodiment of  FIGS. 48A and 48B  in which flow exits from a central position between divided sets of scroll modules. 
       FIG. 54  shows a support structure for the embodiment of FIG.  53 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Referring to  FIG. 4 , a kitchen hood  125  has a canopy  145  positioned over a heat/contaminant source  175  (such as a grill) to capture a thermal convection plume  170  produced by the heat/contaminant source  175 . The canopy  145  defines a recess  140 , having an access  155 . An exhaust fan (not shown) draws a flue stream  105  through an exhaust plenum  180 . Negative pressure in the exhaust duct  180  in turn draws gases residing in the recess  140  through a vent  130 . In the vent  130  is a mechanical grease filter  115 , set in a boundary wall  120  that defines part of the recess  140 . The filter reduces the mass of suspended grease particles in the resulting flue stream. The grease filter  115  may be an impingement filter or one based on cyclone type separation principles. The thermal convection plume  170  carries pollutants and air upwardly into the canopy recess  140  by buoyancy forces combined with forced convection resulting from the suction created by the exhaust fan. A combined effluent stream comprising the thermal convection plume  170  and conditioned air drawn from the space  165  in which the hood  125  is located, flows into the vortex  135 . This flow is extracted from the canopy recess  140  steadily forming the effluent stream  110 , which becomes the flue stream  105 . 
   The kitchen hood  125  may have multiple vents  130 , each connected to the exhaust plenum  180 . Alternatively, multiple exhaust plenums  180  may be connected to a single exhaust duct header (not shown but as indicated at  191  in  FIG. 3 ) supplied by a single fan (not shown) as will be appreciated by those skilled in the relevant art. The exhaust rate through the exhaust plenum  180  or exhaust duct header determines the rate of extraction of effluent and indoor air  165  by the hood  125 . The determination of the optimal flow rate involves a tradeoff between energy conservation and a requirement called capture and containment. Capture and containment is the state where no pollutant from the thermal plume  170  or the buffered volume in vortex  135  escapes into the conditional space. 
   Full capture and containment requires the exhaust of at least some air  165  from the space in which the hood  125  is located. To conserve energy, the exhaust rate should be set at the lowest possible rate that still provides full capture and containment. This setting must account for the variability of the thermal plume  170 , which varies with the cooking load, stage of cooking (e.g., rendering of fat which causes dripping and attendant smoke), and random variation (e.g., random dripping from fatty foods) or steam generation. Thus, not only does the exhaust load vary along the canopy  125  (in the direction into the plane of the drawing), as discussed in the background section, it also varies with time. The prior art approach has been one of setting the flow rate according to the peak expected load. This approach insures that the bulk exhaust rate is high enough to provide full capture and containment by the hood, or hood portion, requiring the greatest volume of exhaust to achieve it (capture and containment), at the times of maximum instantaneous load. 
   Again, the load can vary along the length of a long hood or from hood to hood and the balancing problem is analogous in balancing from hood portion to hood portion as it is for balancing from hood to hood. 
   In the present system, a flow control system is employed to permit modulation of the exhaust from one hood  125  to another or from one vent  120  to another along a single long hood  110 . In addition, the potential exists to provide this flow control system, to be discussed hereon, with real-time control. Thus, according to the inventive system, the exhaust rate may be controlled to achieve the lowest local (“local” referring generically to the respective hood portion or the respective each hood linked to a common exhaust) exhaust rate required for the current local, instantaneous load. This is achieved by controlling the local exhaust rate by an active flow control device  120  linked to a real-time control (discussed in greater detail much later in the present specification). 
   Referring now also to  FIGS. 5A ,  5 B, and  6 , to balance flow across a single hood  145  (FIG.  6 ), or across multiple hoods connected to a single exhaust system (see FIG.  7 ), a flow control device  120  selectively blocks a portion of an exhaust vent  130  in a boundary wall  190  of the hood  145 . The flow control device  120  has a flat plate  112  partially covering the vent  130  defining an aperture  185 . The flat plate  112  is selectively moved across the vent  130  which makes the aperture  185  variable-sized. The flat plate  112  may be moved by a linear actuator  119  such as a linear motor with a driver  118  and stator  117 . The flat plate  112  may be guided by linear bearings  113 . Note that the shape of the flow control device  120  is generally flat so that its impact on the shape of the canopy recess  140  is minimal. Thus, the flow control device  120  does not interfere with the vortical flow pattern  135 . Where canopy  145  is of great length (again, “length” referring to the dimension perpendicular to the plane of the  FIG. 5A  drawing and best illustrated by FIG.  6 ), where multiple vents  130  are linked to a common exhaust duct  205 , the respective flow control devices  120  may be set to provide a larger aperture  185  for the vents  130  close to the ends of the canopy  145  and to provide a smaller aperture  185  for the vents  130  near the middle of the canopy  145 . Alternatively, if the type of cooking appliance or load varies along the length of the hood, the flow control devices  120  may be set accordingly. Referring now also to  FIG. 7 , in multiple hoods  230  linked to a common exhaust header  220  the flow control device  120  may be set to restrict flow more in those canopies  145  protecting lower loads and to restrict flow less in canopies  145  protecting higher loads. Further, real-time control, which is discussed later in the present specification, may be used to control each flow control device  120  according to an instantaneous load sensed by a smoke, temperature, image, and/or other sensor system as described below. 
   Referring to  FIG. 8 , the canopy recess  140  acts as a buffer to dampen the effects of temporal variability in the load. The thermal plume  170  rises at a rate that is faster than the mean rate of exhaust. In wall-type hoods as illustrated, the flow  135  circulates within the canopy recess  140  dissipating its energy in a turbulent cascade whilst the plume  170  and room air  165 , drawn by negative pressure created by the exhaust fan (not shown), are tapped from the canopy recess  140  as indicated figuratively by the arrow  245 . The shape of the canopy recess  140  augments the vortical pattern by guiding it in a circular path as illustrated at  135 . The vortical pattern may not be present in all hoods, but all hoods have some capacity to buffer temporal variability in the load whether a stable vortex is formed or not. More complex flow patterns may arise in other hoods, depending on the load, the hood shape and other variables. 
   Referring now to  FIGS. 9A ,  9 B, and  10 , another type of flow control device provides variable control of the flow rate through certain types of filters  305 . Referring momentarily to  FIG. 9A  in particular, in certain types of filters  305 , the raw effluent stream enters as indicated at  246  and leaves at the ends of the filters as indicated at  307 . Examples of this type of filter are described in U.S. Pat. No. 4,872,892, which is hereby incorporated by reference in its entirety as if fully set forth herein. Focussing again on  FIG. 9B , the exit flows  307  are selectively blocked by movable plates  300  thereby providing a variable exit passage  325 . In the embodiment of  9 B, the plates  300  translate as indicated by arrows  308 . In the embodiment of  FIG. 10 , movable plates  330  are pivotably mounted by hinges  335  and pivoted to provide variable exit passages  340 . 
   Referring now to  FIGS. 11 and 12 , another embodiment of a flow control device employs scroll shutters  360  that unroll from spools  385  inside a covered compartment  265 . Each shutter  360  selectively blocks a vent  370  on the canopy recess side thereby providing a variable aperture  350  respective of each vent  370 . Each vent  370  may be separated by a partition portion  380  from one or two adjacent vents  370 . Suitable guides and drive mechanisms are available from the field of movable shutters and may be employed to actuate the present embodiment. 
   Referring to  FIGS. 13 and 14 , a flow control device such as described in U.S. patent application 60/226,953 may be employed in a duct leading from the respective vents  420  of a single hood or from groups of vents in one or more hoods all linked to a common exhaust (not shown in this drawing). In the embodiment of FIGS.  13  and  14 , a single hood is shown. A wall  425  of the recess has three vents  420  each leading to a respective plenum  430 . Each plenum is connected to a duct containing a flow control device  410  having smooth walls as described in the above US Patent Application. Each flow control device  410  then leads to a common plenum  400  from which effluent is drawn through a common exhaust  415 . By regulating each flow control device  410  separately, the flow through the respective vents  420  can be optimized as discussed above. A similar configuration may be used to balance respective hoods connected to a common exhaust. 
   Referring to  FIG. 15 , another type of flow control device  510  selectively blocks flow through a vent  505  (in a wall of a canopy  525 ) using a vertical-blind type mechanism. Louvers  515  of the flow control device  510  pivot in a manner analogous to window blinds. The louvers  515  may be oriented with their pivot axes parallel to the tangent of the vortex  135  formed within a canopy recess  500 . In this orientation, the louvers  515  generate less resistance to the vortical flow. To vary the flow through the flow control device  510 , the louvers  515  are pivoted about their axes in concert to vary the net flow area through the vent  505  in the canopy wall  525 . Referring to  FIG. 16 , in flow control device  530 , which is similar to that of  FIG. 15 , louvers  535  are located over only a portion of the vent  505 , since the flow may not need to be cut off 100%. Alternatively, the louvers  515  may be as in  FIG. 15 , but not close 100%. 
   Referring to  FIG. 17 , the structure of an impingement filter  545  is varied to modulate flow therethrough. The drawing shows a split view of a single filter in two configurations. On the left side of the drawing, the concave-back plates  550  and concave forward plates  555  are close together narrowing the flow passage between the inlets  570  and the outlets  580 . In the right side of the drawing, the separation distance is increase providing a larger flow passage that is correspondingly less resistant to flow therethrough. The separation distance may be varied progressively or step-wise, depending on design choice, by any suitable mechanism. 
   In the example shown, adjustable standoffs  560  are used to separate the plates  550  and  555 . For example, the adjustable standoffs could be screws  560  with idle clips  565  that hold one end of the screws  560  at a fixed position along it length and threaded holes  566  that traverse the lengths of the screws  560  when it is turned. The separation device may be automatic or manual, as required. 
   Referring to  FIGS. 18 ,  19 , and  20 , in a configuration of a grease filter of a type similar to those described in U.S. Pat. No. 4,872,892, modulation of the flow of exhaust through a vent of a range hood is afforded. In this embodiment, a filter is formed substantially as described in the above patent. That is, air flows into slots  620  along a face of the filter as indicated at  632  (all similar slots—only one is labeled) and exits through the ends of tubular sections  610  as indicated by the outward-facing-flow symbol  633 . While travelling through each chamber tubular section  615 , the flow swirls helically due to the tangential entry of the flow at each slot  620 . The aperture of the slots  620  is varied by bending a flexible wall  630  of each slot by a gang pull-rod  635 . When the gang pull-rod is moved as illustrated in  FIG. 19 , the flexible walls  630  bend narrowing the slots  620  and restricting the flow.  FIG. 20  is a split view showing two configurations of the filter. The open configuration of  FIG. 18  is illustrated on the left side of FIG.  20  and the closed configuration of  FIG. 19  is illustrated on the right side of FIG.  20 . The aperture  620  may be varied progressively or in steps. 
   Note that while in the embodiment of  FIGS. 18-20 , the inlet slots  620  are varied in flow area by bending a wall that forms the tubular chambers  615 , it is possible to accomplish a similar result using separate blocking plate with a hinge. That is, the wall  630  may be a separate element pivotably attached to the rest of the modules  610 . 
   Referring to  FIGS. 21A ,  21 B, and  21 C, based on a filter design similar to those of U.S. Pat. No. 4,872,892, flow entering the filter is selectively blocked by a movable shutter plate  660 . Again each tubular chamber  650  receives air through a respective slot-shaped flow aperture  655  and delivers it through exits  649  of each of a plurality of modules  648  as indicated by the arrows  656  and  657 . When the shutter plate  660  is in a relatively open position as shown in  FIG. 21A , each flow aperture  655  is relatively large in area. When the shutter plate  660  is in a relatively closed position as shown in  FIG. 21B , the flow aperture  655  is relatively small in area. Thus, the shutter plate  660  position may be used to control the pressure drop across the filter and consequently the flow rate across the filter. 
   All of the filters that are able to control flow may be used for hood balancing. If each filter is controlled independently, the flow rate through each vent of one or more hoods can be controlled independently. Each filter may be controlled in each hood of a system to flow-balance longer hoods and to balance hoods against each other. Alternatively, a single filter of a hood with multiple vents can be controlled leaving the other filters uncontrolled. This may allow the balancing of the entire hood against other hoods. In a longer hood, this solution may be less desirable because it would vary the exhaust rate across the length of the hood, which may produce inefficiencies as discussed above. 
   Referring to  FIGS. 22A ,  22 B, and  22 C, based on a more conventional type of filter cartridge known as an impingement filter (also discussed above), a shutter plate  653  is moved to vary the size of flow apertures  657 . Effluent flows from the inlet flow apertures  657  to respective outlets  658 . The selective variation of the flow apertures  657  varies the pressure drop through the flow apertures  657 . Note that although in this embodiment, a shutter plate  653  is used to selectively block the aperture  657 , it is clearly possible to use a shutter plate to selectively block the outlets  658  or both to achieve the same effect. 
   The shutter plate of  FIGS. 21A-C  and  22 A-C are illustrated as having rectangular openings. Referring to  FIGS. 23A and 23B , it is possible to employ other shapes to good effect. For example, in the embodiment of  FIG. 23A , a shutter plate  680  has openings  675  with a curved border such that access to the middle section of the filter is blocked more than the ends. In the embodiment of  FIG. 23B , the opposite is true. In the latter embodiment, a shutter plate  681  has openings  676  with a curved border such that access to the end sections is blocked more than the middle section. Either embodiment may be used with either type of filter cartridge or others not described herein, but the embodiment of  FIG. 23B  may be more favorable in a filter such as described in U.S. Pat. No. 4,872,892 because it favors a longer travel path of the air along the flow modules providing greater grease separation in the process. 
   Referring to  FIGS. 24A and 24B , a canopy  717  has a recess  715  bounded, in part, by a flexible accordion wall  710 , a filter  720 , and a water tank  730 . The filter  720  is partly immersed in a pool of water or other liquid  735 , held by the tank  730 . The exposed face of the filter is limited by the immersion of part of the filter  720  in the pool of water  735  and thus the flow area is reduced. As a result, the flow area may be modulated by varying how deeply the filter  720  is immersed. By varying this flow area, the pressure drop between the recess  715  and a plenum  725  may be selectively varied to vary the exhaust flow. To vary the depth of immersion, the filter  720  may be translated. The flexible accordion wall  710  flexes to follow the filter  720 . The flexible accordion wall  710  may be made of steel or some other material. The filter may be held by a suitable engagement device (not shown) at the distal end of the flexible accordion wall  710 . Cleaning solution may be used in the tank  730 . During shutdown of the exhaust system, the filter  720  may be immersed more completely in the cleaning solution to clean the filter  720 . 
   Referring now also to  FIG. 24C , seal plates  723  prevent effluent gases from bypassing the filter  720  by going around it. The seal plates may extend from the top of the accordion wall  710  to the level of the liquid  735 . 
   Referring now to  FIGS. 25A and 25B , in another embodiment, a recess  745  is bounded in part by a fixed wall section  740  to which a filter  750  is connected at a distal end thereof. Seal plates (not shown) may be provided as in the embodiment of  FIGS. 24A-24C . The filter is immersed partly in a tank  755  filled with water or a cleaning solution or some other liquid  760 . Pressure drop between a suction-side plenum  765  and the recess  745  across the filter is governed by the level of the liquid  760  in the tank  755  which in turn controls the flow area available through the filter. In  FIG. 25A , the flow area is greater than the illustration of  FIG. 25B  because the liquid  760  level is higher in the latter figure. 
   Referring now to  FIGS. 26 and 27 , a recess  788  of an exhaust hood  789  is defined in part by a pivoting wall  781  that pivots at one end  790  and is connected by a flexible wall  781  at another end. The pivoting wall  781  also defines in part a suction side plenum  775  whose flow passage is reduced in flow area by the change in the angle of pivot of the pivoting wall  781 . The flow through each controlled vent  786  may be modulated by means of an independent apparatus as shown. Thus, for balancing flow through a single hood, two or more sets (“sets” may be single in number) of vents may lead into separately controlled plenums  775 . 
   Referring to  FIGS. 28A and 28B , a hood canopy  815  has a plenum  810  that receives exhaust air through a filter  820 . The pressure drop through the plenum  810  is modulated by varying the configuration of an obstruction  805 . The obstruction may, for example, be an inflatable bladder. The obstruction may be made of steel with an accordion type bellow integral thereto to permit its volume to vary. Alternatively, it may be of polymeric material or other suitable construction. The obstruction  805  is shown with a substantially pillow shape, but it is understood that it could have any shape. A shape that presents a face that is substantially parallel to the exit face of the filter  820  would be better than one that is at a substantial angle as shown so as not to favor one portion of the filter over another. Referring to  FIG. 29 , in a variation of the embodiments of  FIGS. 28A and 28B , wall of the plenum  810  has a face  808  and accordion ribbing  807  to permit the face  808  to be pushed into the plenum  812  to vary the flow channel area and thereby the pressure drop through the plenum. The same effect would be accomplished with an obstruction as in FIGS.  28 A and  28 B. That is, the face angled as face  808  could be formed in the obstruction  805 . 
   In the embodiments of  FIGS. 28A ,  28 B, and  29  separate plenums  810 / 812  may be provided for each modulated vent  814 / 811 . Alternatively, however, because the flow obstructor  805 / 808  may be made local to a respective vent  814 / 811 , all vents may share a common plenum  810 / 812  for a single hood while still providing the ability to balance a single long hood. That is, a separate and independently controllable flow obstructor  805 / 808  may be made respective to each vent  814 / 811  to control each controlled vent independently of the others. 
   Referring to  FIGS. 30 and 31 , a hood of substantially standard construction has a suction side plenum  835  which draws air through a filter  820 . An aperture  832  leads to an exhaust collar  800 . The aperture  832  is selectively blocked by a smooth obstruction  830  whose distance from the aperture  832  determines the flow area for exhaust flow through the aperture. In an embodiment, the flow obstruction  830  is in the shape of a sphere. Referring to  FIGS. 32A and 32B , an alternative shape for a flow obstruction  840  is a water-drop shape. For rectangular flow apertures  832 , other shapes may be used. Preferably, the shape of the flow obstruction is smooth so as not to generate stable and quasi-stable or periodic flow structures that result in undue precipitation of aerosols. 
   Referring to  FIGS. 33 and 34 , in a rectangular exhaust collar  850  fed from a suction side plenum  860  of an exhaust hood, flexible smooth flow obstructor plates  855  are provided. By varying the shape and area of a flow channel  857 , the pressure drop across the flow channel  857  is modulated providing the ability to balance suction side plenums  860  selectively. The shapes of the obstructor plates  855  may be varied by translating tongue segments  856  accordingly. The final actuator used to vary the shape and size of the flow channel  857  may be any suitable device. Note that one side only may be translated rather than both as indicated. 
   Referring to  FIGS. 35 and 36 , an exhaust hood has a suction side plenum  535 A divided into an upper part  535 A and a lower part  535 B. The upper and lower parts  535 A and  535 B are connected by a series of duct sections  547 / 548  that may be selectively covered with blanks  546  to vary the flow through each respective vent  566 . In the example situation shown in  FIG. 35 , two of the middle-most blanks are set to block flow through ducts  547  and permit free flow through ducts  548 . By selectively blocking some ducts  547  and permitting flow through other ducts  548 , the relative flow of the vents  566  is altered. For example, the flow through vent  566 ′ would be reduced relative to the flow through adjacent vents  566  because of the presence of the blanks  546 . Since no obstructions are added to a flow path, no mechanism is introduced that would cause undue precipitation. 
   Note that while in the embodiment of  FIGS. 35 and 36 , the blanks  546  are fixed in place, it would be possible to arrange for the blanks  546  to be selectively moved into place to provide real-time modulation of flow. Thus, in this embodiment, a movable blank  546  would either be in place blocking flow through a respective duct section  547  or it would be out of the way permitting free flow through the respective duct section  548 . Also, while in the embodiment described above, it was presumed that the configuration of the plenum  535 B was such that flow through the middle vent  566 ′ would be appreciably reduced relative to that through the other vents  566 , the latter plenum may be sufficiently generously sized that the only effect of reducing the aggregate flow area by blocking ducts  547  may be to reduce the total flow for the entire hood without redistributing the flow along the hood. Thus, this design may be used to balance multiple hoods or single hoods, as may all the previous embodiments. The advantage of using this technique rather than a single flow control, however, is that it does not create any obstruction around which fumes and air must flow. Thus, it avoids the attending precipitation problems. 
   Referring to  FIG. 37 , a cylindrical grease filter module  580  has in inlet  588  through which raw effluent and air are drawn and an outlet  592  from which the cleansed air is extracted. A guide van  582  causes an incoming stream  584  to be directed into a helical flow  590  so that grease and other airborne particulates precipitate on its interior walls. The exit flow  586  is directed at approximately a right angle to the incoming stream  584 . Functionally, the cylindrical grease filter module is similar in function to that of the filters described in U.S. Pat. No. 4,872,892. Its cylindrical walls, however, may provide lower resistance and improved cyclonic flow therewithin. 
   Referring to  FIGS. 38 and 39 , a filter cartridge  581  is formed from multiple cylindrical grease filter modules  580 . Each cylindrical grease filter module has a lever tab  604  which is tied to a rotator bar  602  which is used to rotate the cylindrical grease filter modules  580  in concert. By rotating the cylindrical grease filter modules  580 , the exposed area of the inlet  588  of each cylindrical grease filter module  580  is selectively altered. When the cylindrical grease filter modules  580  are in the positions shown in  FIG. 38  the flow through the filter cartridge  581  is restricted more than when they are in the positions shown in FIG.  39 . This is because the inlets  588  are increasingly blocked by partitions  606  as the cylindrical grease filter modules  580  rotate clockwise. Note that in an alternative embodiment, the cylindrical grease filter modules  580  may be set immediately adjacent to each other and the blocking function of the partition plate formed by the external surfaces of adjacent cylindrical grease filter modules  580 . In this way, the partition plates  606  may be avoided. 
   Referring to  FIGS. 40 and 41 , various sensor mechanisms may be used to provide real time control of the flow rate through one or more hoods. For example, a controller  950  may receive input signals from one or more input devices including one or more video cameras  961 , infrared video cameras  962 , opacity sensors  963 , temperature sensors  964 , audio transducers  965  (e.g., microphones), manual switches  966 , and flow rate sensors  967 . Based on one or more of these inputs signals the controller may control the setting of one or more output controllers  970  connected to any of the flow control devices described previously or described later in the present specification. Video or IR cameras may be located at any desired position, examples being indicated at  920  and  935  and as discussed later in connection with FIG.  42 . Opacity and temperature sensors may be located at any positions, two examples being indicated at  925 / 930 . 
   The technology in image processing is more than adequate to detect a change in a volume of smoke or heat resulting from an increased cooking load. Optical and/or infrared images may be captured and a cooking load indicator derived therefrom. For example, an IR image processing algorithm that simply indicates the percentage of the field of view that is above a temperature threshold may thereby indicate escape of a thermal plume from a hood; i.e., a loss of capture and containment due to the thermal plume rising in front of the external edge of the hood. As such a loss of containment is approached, the hot buffer zone tends to grow from deep within the recess until it breaches the capture zone. This growth of the buffer zone can be indicated in precisely the same way: by imaging a predefined field of view and recognizing the size and/or shape of the hot zone (the latter being defined as a zone in which the imaged temperature exceeds a predefined threshold). This is discussed further below. 
   The movement of a worker, the image of the food being cooked, the presence of smoke at particular locations (such as escape of containment at the edge of the hood), the temperature of air near the hood or within the canopy recess, the proximity of a worker, etc. may all be combined to form a classification input-vector from which a condition (e.g., percentage of full-load) classification may be derived. Algorithmic, rule-based methods may be used. Bayesian networks or neural network techniques may be used. Alternatively, just one sensory indicator of load may be used to determine the current load. For example, a gas rate flow sensor for a gas grill could provide the single input signal. Many possibilities are available with current sensor, machine-classification, and control technologies. 
   Referring to  FIG. 42 , various camera angles may be employed in a load-classifier that employs optical or IR images. For example, a camera  982  is positioned to image a side view of a canopy  972 , range,  984 , and a work area between and adjacent them. Referring also to  FIGS. 43A-43C  and  44 , in an IR-based camera, this side view can image a hot zone whose size and shape are dependent on effluent load (which includes heat) and exhaust rate.  FIG. 45  is a Schlerian image, but the shape of the hot plume is essentially the same as what would be provided by a thermal camera. As the exhaust rate falls below that necessary to provide capture and containment, a hot zone image provided by the camera  982  would expand progressively as illustrated in the series of  FIGS. 43A-43C . The hot zone changes from one associated with adequate capture and containment  990 , to one on the verge of breaching  992 , to one where capture and containment has been lost  994 . The changes in the images, the rate of change of images, and the history of change of the images may be employed in a control system as described to insure that capture and containment is maintained. 
   Referring now to  FIG. 44 , other camera angle views such as provided by camera  980  may provide more information about the particular location of the exhaust rate deficit along the canopy  972  edge  1003 . Illustrated in  FIG. 44  is an oblique view of a canopy and plume  1002  showing a spillover  1001  over an edge  1003  near an end of the canopy. This image may be used to provide an adjustment to exhaust flow rate favoring the portion of the canopy  972  close to an end thereof, as illustrated. The ability to detect spillover and its position along the edge  1003  may be obtained by positioning a camera  986  looking downwardly so that it captures the entire front edge  988 / 1003 . By taking multiple images, such as provided by cameras  974 ,  980 ,  982 , and  976 , it is possible to compare the shape of the three dimensional plume to determine an imminent spill. Thermal plumes have a characteristic waist  1005  that results from the increase in velocity and the draw of cooler air as they rise. This waist begins to bulge at the top as capture competency is lost. Again, the spillover can be detected as a three-dimensional model based on temperature or opacity. 
   The model or two-dimensional image(s) may be graded or thresholded. The image resolution need not be high since the structures are highly repeatable and their variability quite distinct. Thus, a relatively inexpensive imaging device may be employed with a small number of pixels. The classification process must include unrecognized classes and be capable of indicating same. For example, if the view of a camera is occasionally obstructed, the image processing process should be capable of recognizing the absence of an expected image and responding to it. Images that change suddenly or do not belong to a recognized plume shape may be classified as a bad image. The response to a bad image may be to ignore it or alternatively to ramp the exhaust rate to a design maximum until a recognized image is acquired again. Fiducial marks or particular features of the exhaust or cooking equipment may be employed to help determine if the camera view is obstructed. The lack of such features or fiducials in the image may indicate the loss of the image. 
   Activity can be indicated by live camera images, IR and optical. For example, the presence of an operator near the working area of a cooking appliance may be used as a signal indicating that the cooking load is increased. The particular activities in which the operator is engaged are likely to be highly repeatable events and readily classifiable by video classification methods as a result. For example, a particular stage of cooking may be characterized by the laying out of many pieces of meat on a hot grill. The movement of a worker&#39;s arms over the hot grill placing the meat is an activity that may be readily classified since it has distinct characteristics that distinguish it from other background activities such as cleaning or walking around the grill. Classifying the event of placing the meat on a grill may trigger a timer to anticipate when the load reaches a maximum. 
   Neural networks may be trained to classify the conditions in a kitchen using neural network techniques. The inputs from multiple devices may be combined to form a vector. The following are possible vectors. 
   1. Cameras 
   
       
       
         
           a) Thresholded image (reduce to 1-bit map; all temperatures (radiative) or light levels above a threshold are one color and all temperatures or light levels another color. Process image to identify contiguous domains and form an area-number histogram by counting the number of domains falling within each of series of size ranges. The histogram values define a vector. The contiguous domains can be further processed to define feature points and their relationship mapped to a vector in a manner similar to optical character recognition techniques. 
           b) Thresholded image may be calibrated to provide high sensitivity to smoke or the range of radiative temperatures associated with a thermal plume characteristic of the cooking appliance. The image processing may be tuned to recognize and distinguish shapes characteristic of thermal plumes for the cooking processes being monitored. The output vector in this case would be a characterization of the particular plume state. 
           c) Camera may simply band-pass a color, luminosity, or radiative temperature range and cumulate the total of the image corresponding to that passed signal. This would be scalar. This could be done for a quad tree where the total band-passed image area for each quadrant of the image is passed as a component vector and this could be done down to multiple levels of a quad tree. 
           d) Spot temperatures of food and empty areas on a grill or other appliance may be used to predict the load. These may be derived from a single IR image and processed to report the total area, average temperature, or other lump parameters predictive of the load.
 
2. Opacity sensor
 
           a) Opacity may be monitored between two points to detect when a plume is swelling. For example, an opacity sensor may be positioned near the inside of the edge  1003  of the canopy  972  and the opacity at that point indicated. 
           b) The opacity near multiple points may be monitored and provided as a single vector from which it is possible to deduce the scale of turbulence induced by the thermal plume. (The opacity would be expected to vary over time at different locations along the edge in response to three-dimensional turbulent gusts giving rise to temporal and spatial variability in opacity that can be resolved using multiple opacity signals spaced apart and monitored synchronously.)
 
3. Audio
 
           a) A simple frequency profile may be resolved into a histogram whose values correspond to the sound power in each of a series of ranges of audio frequency. The ranges need not be adjacent; they can amount to discrete band pass filters. Depending on the particular cooking process, the sound of frying, grilling meat, operator activity, etc. can make characteristic profiles. 
           b) A sound-signature classifier may be employed to add the temporal component to the sound classification. Depending on the type of load being monitored, certain audio signatures may be present and recognized using technology as employed in voice recognition. For example, the sound of a switch being turned on, the sounds of a spatula being used on a grill, etc. are discrete audio events that have temporal signatures that are characteristic to them.
 
4. Temperature
 
           a) Sensors placed at various locations may each provide components of a vector. 
           b) Sensors may be arrayed to provide a signal indicative of a spatial temperature profile which can be characterized by a more compact set of numbers than simply the whole series of temperatures. For example, the sharpest increases of temperature along respective dimensions may be reported to indicate the location of respective boundaries of the thermal plume  1003 .
 
5. Proximity
 
           a) The presence of food or other workpieces whose presence is predictive of load, may be sensed. The proximity sensor may be provided as a single signal or multiple signals may provided from multiple sensors. Alternatively, the distance of the object may be sensed using a proximity sensor. For example, something that grows while it is heated could indicate a stage of a varying load. 
           b) The presence of an operator and the duration of the operator&#39;s presence may be used to signal the load.
 
6. Motion
 
           a) The movement of a worker, tools, and/or workpieces may be predictive of the load. 
         
       
     
  
   Referring now to  FIGS. 46 and 47 , a great variety of different kinds of actuators may be employed to operate the various flow control devices described above. Preferably, designs which are tolerant of grease deposition from the effluent. A couple of embodiments are shown to illustrate the range of possibilities, but these by no means are these intended to represent an exhaustive range. The prior art relating to hermetic seals, motor and actuator seals, high temperature, high corrosion environments, etc. are rich with candidate devices that may be employed. In  FIG. 46  a lever formed by a first arm  1017  and a second arm  1018  connected through a top wall  1019  of a canopy. The top wall is corrugated to allow it to flex so that when an actuator  1013  pushes the first arm  1017  upwardly, the second arm  1018  moves downwardly actuating a blind mechanism  1010 . The embodiment of  FIG. 46  thereby provides a hermetic seal between the linear actuator  1013  and the blind mechanism  1010 , which provides flow control. In  FIG. 47 , another actuator embodiment has a motor and cam  1021  that are mounted externally from the canopy recess  1012  which moves a blind mechanism  1022  through a seal  1030  with a bellows  1022  and pushrod  1032 . Again the sensitive mechanisms are isolated outside the canopy recess  1012 . Many such mechanisms may be employed and a comprehensive discussion of them is not necessary since many suitable mechanisms are described in the machine mechanism prior art. 
   Referring now to  FIG. 48A , a scroll shaped module  1130  has an inlet  1132  through which air is admitted as indicated by arrows  1120 ,  1110  and  115 . The admitted air swirls as indicated by helical arrows  1117  and  1110  and exits as indicated by arrows  1125 . The helical motion is caused by the fact that the entry point  1132  is at a tangent to the cylindrical space  1131  defined by the scroll shaped module  1130 . The entry point  1132  is a gap between an inside distal edge  1136  and an outside distal edge  1137  defined by the scroll shape of the scroll shaped module  1130  and can be increased and reduced in width by flexing the scroll shaped module  1130 . 
   Referring to  FIG. 48B , the scroll shaped module&#39;s  1130  are connected to each other to form a filter cartridge  1140 . The outside distal edge  1137  of each module  1130  is connected to a middle portion  1138  of an adjacent module  1130  (except for a last module  1130 ′. Referring to  FIGS. 49 and 50 , the modules  1130  may be supported in any of a number of ways so that when they are drawn apart (as indicated by arrows  1171 ) as illustrated in  FIG. 49 , the inlet  1132  expands and the resistance to the inflow of air is reduced. When the modules  1130  are squeezed together as indicated in  FIG. 50  (the force being as indicated by arrows  11 ,  72 ), the inlet  1132  contracts and resistance to the inflow of air increases. As a result, the bank of cartridges  1147  forms a combination filter and flow throttling device. 
   Referring to  FIGS. 51 and 52 , a support mechanism with a back plate  1180  and L-shaped lower braces  1195  support scroll-shaped modules  1130  by tongues  1148  on each module. The latter fit into channels  1147  formed in the edges of back plate  1180 . A sliding L-shaped seal member  1185  is slidably attached to one of the L-shaped lower braces  1195  and moved relative to the back plate  1180  and lower braces  1195  to squeeze and expand the scroll-shaped modules  1130 . A tongue  1186  of one of the L-shaped lower braces  1185  is elongated to serve as a seal when the entire device is placed in an exhaust vent. 
   Referring to  FIGS. 53 and 54 , in an embodiment that is similar to the previous embodiments, a set of scroll shaped modules  1270  have exits  1255  in the center thereof. Thus, functionally, they are like the modules  1230  of the previous embodiments except that their outlets are toward the middle of the filter device  1299  rather than along its edges. As in the previous embodiment, the air enters tangentially as indicated by arrows  1265  and swirls in a helical motion until it exits as indicated by arrows  1255 . Because the air does not need to exit the sides, side panels  1285  may be incorporated in a support structure  1225 . A single opening  1220  may be formed in the back (downstream face) of the support structure for air to exit. A similar configuration  1235  to that described in connection with the embodiment of  FIG. 51  may be used to compress and expand the modules  1270 . 
   Note that although in the above embodiments, the discussion is primarily related to the flow of air, it is clear that principles of the invention are applicable to any fluid. 
   Although in the embodiments described above and elsewhere in the specification, real-time control is described, it is recognized that some of the benefits of the invention may be achieved without real-time control. For example, the flow control device  120  may be set manually or periodically, but at intervals to provide the local load control without the benefit of real-time automatic control.