Patent Publication Number: US-6213867-B1

Title: Venturi type air distribution system

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an air handling system for a building and, in particular, such a system employing one or more induction units adapted to mix two air flows. 
     A variety of air handling systems for both large and small buildings are already known in the air handling industry. Air handling systems both for residential and commercial buildings can include the use of a central heating system that includes a fan unit capable of blowing heated air through air ducts that deliver the air to the various rooms of the building. When this system is used in conjunction with a central air conditioner, it is also capable of providing cool air to the various rooms through the air ducts. A relatively large fan is generally required for a large commercial or industrial building. Air silencers can be installed on both the inlet side and outlet side of these large fans to reduce the noise levels created by the operation of such fans. 
     It is also known to provide so called induction units that employ the venturi effect to mix together both return air from a building and primary air. The two air flows are mixed in a mixing chamber located adjacent an elongate air plenum with a primary air inlet at one end. Tapered nozzles extend into the mixing chamber and are connected to a wall of the air plenum. The return air from serviced space enters the mixing chamber which is flanked by the induction unit&#39;s coils on one side and five sides of the enclosure of the unit. There is an opening on the sixth side of the enclosure for entry of the return air. These units can typically be mounted on a wall of a room with the air plenum section located near the floor and the air outlet located at the top of the unit. Such induction units have at least several advantages including the ability to operate at very low noise levels since they do not employ any fans or similar air circulating devices. They can also be used in conjunction with both high pressure as well as low pressure air duct systems and they provide for a reasonably efficient mixing of the primary air and the return air. 
     Systems for delivering treated air to a room through an outlet located in the ceiling are already known. For example, U.S. Pat. No. 4,672,887 which issued Jun. 16, 1987 to Fred Sproul Sr. describes an air delivery system located above a horizontal ceiling in a dwelling. The air duct system delivers treated air to a valance/diffuser air system that can be located adjacent one wall of the dwelling. The conditioned or treated air is forced into the air delivery system by a blower of a conditioning unit such as a forced air furnace. At the wall the air is initially distributed lengthwise along an elongate horizontal chamber and then distributed through apertures in a downwardly direction. However, this known system does not use air induction units for mixing return air and primary air. In this known system the return air system is located beneath the floor of the dwelling. 
     More recent U.S. Pat. No. 5,577,958 issued to Mitsubishi Denki Kabushiki Kaisha in November, 1996 describes a ceiling-embedded cassette type air conditioner located above a decorative grate or panel through which return air can pass. A blower is located centrally in this air conditioner and it forces the return air through two or more heat exchangers located on the perimeter of the blower. The conditioned air is returned to the room through two or more outlets located at the ceiling level. Air directing plates can be positioned in the air outlets and these can direct the outflowing air to flow into the room at an angle to the horizontal. This known air conditioning system does not employ any induction unit that relies on the venturi effect and, because it employs a blower, it will be quite noisy when it is operating. 
     It is an object of the present invention to provide an air handling system for a building which employs at least two induction units and which is capable of mixing return air and primary air efficiently and quietly. 
     It is a further object of the present invention to provide an air handling apparatus for a building that includes two induction units, which apparatus can be manufactured and installed at a reasonable cost and can be operated and maintained at a low cost. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, an air handling system for a building having a horizontally extending ceiling and an enclosed space below this ceiling includes two induction units adapted for mounting adjacent the ceiling, each unit having an air mixing section forming a relatively long air mixing chamber and an elongate horizontally extending air plenum section mounted at an upper end of the air mixing chamber and having a primary air inlet formed therein. Air nozzles extend into the air mixing chamber of each unit and are mounted on a side of the air plenum section. Each air nozzle has an inlet end that is open to an interior chamber of the air plenum section. The air mixing section has an air outlet formed at a lower end thereof and a side air inlet for permitting return air to flow through a side of the air mixing section and into the air mixing chamber. Supporting members are also provided for mounting the two induction units so that each air mixing section extends at a substantial acute angle to the ceiling and is located adjacent the ceiling. During use of this system, the return air is drawn by the venturi effect created by the nozzles into each air mixing chamber. Each induction unit is capable of delivering a mixture of primary air, that passes through the plenum section and the nozzles, and return air through its air outlet to the enclosed air space. 
     Preferably a heat exchanging coil unit is mounted adjacent to the side of at least one air mixing section so that return air flowing through the side air inlet first passes through the coil unit in order to be heated or cooled thereby. 
     According to a further aspect of the invention, there is provided a combination of a building structure having an enclosed space and an air handling system capable of providing a mixture of primary air and return air to the enclosed space. This combination includes a horizontally extending ceiling and walls forming the building structure and defining the enclosed space. Two induction units are mounted adjacent the ceiling and each unit has an air plenum section with a primary air inlet and an air mixing section forming an air mixing chamber and mounted on a side of the air plenum section. Air nozzles extend into each air mixing chamber and are mounted on the side of the air plenum section. These air nozzles each have an inlet end that is open to a primary air high pressure plenum chamber in the air plenum section. A side return air inlet in one side of the air mixing section permits return air to flow into the air mixing chamber from the enclosed space. Each air mixing section has an air outlet at an end thereof furthest from its plenum section. There are also supporting frame members mounting the two induction units adjacent the ceiling so that each air mixing section extends down from the air plenum section to its air outlet and extends at an acute angle to the ceiling. During use of this system, the return air from the enclosed space is drawn by a venturi effect created by the nozzles into each air mixing chamber and the two induction units deliver the mixture of primary air and return air through their air outlets to the enclosed space. 
     Further features and advantages will become apparent from the following detailed description of a preferred embodiment, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional elevation illustrating the preferred air handling system constructed in accordance with the invention; 
     FIG. 2 is a perspective view of an induction unit of the type that can be used in the air handling system of FIG. 1; 
     FIG. 3 is a schematic end view of the induction unit; 
     FIG. 4 is a cross-sectional view of the induction unit taken along the line IV—IV of FIG. 1; 
     FIG. 5 is a bottom view taken along the line V—V of FIG. 1 showing the ceiling of the enclosed space; 
     FIG. 6 is a schematic plan view illustrating how three pairs of induction units can be connected to a single air valve; and 
     FIG. 7 is a schematic cross-sectional elevation similar to FIG. 1 but illustrating another embodiment of the air handling system. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred air handling system  10  constructed in accordance with the invention is illustrated in FIG.  1 . This system is designed for a building  12  only a portion of which is shown for ease of illustration, this building having a generally planar ceiling  14  and an enclosed space  16 , for example, a room, below the ceiling. The preferred illustrated ceiling  14  is the type commonly referred to as a suspended ceiling that forms an enclosed space  18  between itself and a rigid structural or supporting ceiling  20  that may, for example, be made of concrete. The illustrated suspended ceiling is supported by vertically extending support wires  22  in a well known manner. The support wires  22  can extend up to the structural ceiling  20  and can be firmly attached thereto by any known mechanism, for example, the loop connector  24  shown. The wires  22  are commonly connected at the bottom end to a T-bar ceiling grid comprising a number of T-bar members  26 . Generally these T-bar members extend both longitudinally and widthwise of the room, although, for ease of illustration, the illustrated T-bars  26  are extending in only one direction. The T-bars support a number of standard ceiling panels  28  which can be of a standard length and width and, if necessary, cut to fit the required area. The outer perimeter panels  28  can be supported at their outer edges by any known means, such as by the illustrated angle members  30 , or by simply placing the edge of the panel on the top of the adjacent wall. 
     The preferred air handling system  10  includes two induction units  32  and  34 , each adapted for mounting above the ceiling  14 . Each induction unit  32 ,  34  has an air mixing section  36  forming a relatively long air mixing chamber  38 . As illustrated in FIG. 1, the length of the air mixing chamber is indicated by the distance marked L and is the length of the shorter of two parallel sidewalls  40  and  42 . It will be seen that the length L is relatively long compared to the narrow width W of the chamber. Preferably the air mixing chamber also has a substantial depth taken in a direction perpendicular to the cross-sectional plane illustrated in FIG.  1 . The substantial depth of the air mixing chamber can be more clearly seen from FIGS. 2 and 5. 
     Each induction unit also has an elongate, horizontally extending air plenum section  44  mounted at an upper end of the air mixing chamber and having a primary air inlet  46  formed therein at one end. The air inlet may be formed with a connecting flange  49  as illustrated in FIG.  2 . The plenum section forms an elongate, box-shaped plenum chamber  48 . 
     One or more rows of air nozzles  50  extend into the air mixing chamber  38  and are mounted on a side  52  of the air plenum section. Each air nozzle  50  has an inlet end  54  that is open to the primary air high pressure interior chamber or plenum chamber  48 . In the induction units  32  and  34  of FIG. 1 there is one row of the nozzles  50  in each unit but in the induction unit  34   b  of FIG. 2 there are two rows of nozzles arranged side-by-side. These rows extend horizontally and preferably the nozzles are arranged at an acute angle to the horizontal as illustrated in FIG. 1. A narrow passageway formed in each nozzle tapers inwardly from the inlet end  54  to a nozzle outlet  56 . In the preferred embodiment, the nozzles are made of plastic, for example polyethylene or they can be made of metal such as bronze. If they are made of plastic they should be capable of withstanding elevated temperatures of as much as 160° F. and more. The nozzle opening at the inlet end  54  in a preferred embodiment has a diameter of ¾″ and the discharge outlet of the nozzle has a diameter between ¼ and ⅜ths inch. This preferred nozzle causes only a low noise level during the operation of the induction unit. It will be appreciated that the size, shape, and number of nozzles in the induction unit can be varied by the system installer in order to meet the air handling requirements of the particular building. Furthermore, the nozzles in one of the induction units can be different from the nozzles of the other unit in order to provide different airflows from the two induction units. In other words, the system can be customized to suit and meet the requirements in the room above which the induction units are installed. 
     The air mixing section  36  has a long, narrow air outlet  58  formed at a lower end thereof. In one preferred embodiment, the width of the air outlet is approximately 4 inches whereas the length L o  indicated in FIG. 5 is about 4 feet. It will be appreciated that two or more pairs of the induction units as illustrated in FIG. 1 can be arranged along the length of the room, the number of pairs used depending upon the size and length of the room or enclosed space. These pairs of induction units can be arranged in one or more rows above the ceiling  14 . 
     A side air inlet  60  permits return air from the enclosed space or room  16  to flow through a side of the air mixing section  36  and into the air mixing chamber  38 . Arrows indicating the upward flow of return air RA through a perforated grate or panel  62  are shown in FIG.  1 . The panel  62  can be of standard, rectangular construction and can have reasonably large openings  64  formed therein for easy passage of the return air. It will be understood that, during use of this system, the return air is drawn by a venturi effect created by the nozzles  50  into each air mixing chamber  38 . In this way, the induction unit  32 ,  34  is capable of delivering a mixture of primary air, that passes through the plenum section  44  and the nozzles  50 , and return air through its air outlet  58  to the enclosed air space. 
     As indicated, the air handling system has two induction units  32  and  34  mounted above the ceiling  14  with the air mixing section  36  extending at a substantial acute angle to the ceiling  14  and at a substantial angle to the air mixing section of the other induction unit as shown in FIG.  1 . In the illustrated preferred embodiment, the substantial acute angle between the air mixing section  36 , in particular the two opposing side walls  40  and  42  thereof, and the ceiling is 45 degrees approximately, while the substantial angle between the two air mixing sections of the two induction units is 90 degrees approximately. Because of the 45 degree slope of each air mixing section, the airflow passing out through each air outlet  58  is generally downwardly and outwardly away from the center of the room. It will be appreciated that the centerline C of the room can be aligned with the center of the pair of induction units located at the apex point A. The centerline of the room is in a vertical plane midway between two opposing vertical walls  64  and  66  which are part of the building  12  and which define two of the vertical sides of the enclosed space  16 . However, if the vertical walls  64 ,  66  are located a reasonable distance from, for example two to three feet, from their adjacent respective air outlets  58 , then the downward airflow from the air outlet will flow out to the vertical wall and then be directed downwardly towards the floor by the vertical wall. This can result in a circulation pattern for the air which can provide for fresh conditioned air in all regions of the enclosed space or room. It will be appreciated that after the airflow passes down along the vertical wall  64  or  66 , it will then turn at the floor of the room and circulate back to the center of the room where it meets the opposite air flow and then passes upwardly to the ceiling and through the centrally located perforated panel  62 . It will be further appreciated that one of the walls  64  or  66  can be an outside wall with windows mounted therein while the opposite wall can be an inside wall. Thus the heating or cooling demands on one induction unit can be quite different from the demands on the other induction unit. Accordingly, the two induction units can be made differently so as to handle these different requirements. 
     There are supporting members for mounting each induction unit,  32 ,  34  so that the air mixing section  36  extends at a substantially acute angle to and down to the ceiling  14  and so that the air outlet  58  is positioned where the air mixing section  36  meets the ceiling  14 . It will be readily apparent to one skilled in the art that each induction unit can be supported rigidly in a variety of ways. In the illustrated embodiment of FIG. 1, there are vertically extending support frames  68  and  70  that extend down from the structural ceiling  20  and that are connected thereto. In FIG. 1, one of the frame members  70  is only shown in part for ease of illustration but it will be understood that it can be similar to the frame  68 . The bottom end of each frame member  68 ,  70  is connected by means of connecting flange  72  and bolts to the upper sidewall of the air mixing section  36 . Although only one of each of the vertical frame members  68  and  70  is shown in FIG. 1, it will be appreciated that there will normally be at least two of the frame members  68  and at least two of the frame members  70  with two of these frame members being located at opposite ends of the air mixing section. There is an additional frame support in the form of an elongate angle member  74  which not only joins together the two plenum sections  44  but also supports the induction units at this central location. It will be appreciated that the angle member  74  can either extend horizontally to vertical support walls (if sufficiently close) or it can in turn be connected by vertical frame members (not shown) to the structural ceiling  20 . Additional support frames can be provided if desired or if required in order to rigidly and securely support the two induction units. 
     Preferably a heat exchanging coil unit  76  is mounted adjacent to the downwardly facing sidewall  40  of each induction unit within the region of the side air inlet  60 . The length and width of the heat exchanging coil unit can correspond approximately to the length and width of the rectangular air inlet  60  in order to achieve the fill benefits of the heat exchanging coil unit but the coil unit can be made smaller if a larger one is not required to satisfy the heating or cooling requirements for that induction unit. The return air flowing through the side air inlet  60  first passes through the coil unit in order to be heated or cooled thereby. Each coil unit can per se be of known construction and can comprise a series of coolant pipes  78  that are arranged in a row and a number of closely spaced heat exchanging metal fins  80  that are parallel and that extend perpendicular to the sidewall  40 . The fins  80  are connected to the coolant pipes  78  for a good heat transfer therebetween. Thus the return air can easily flow between the fins or plates  80  to pass through the air inlet  60 . The heat exchanging unit  76  is mounted on the outside of the air mixing section in order not to interfere with the mixing of the air and the flow of air through the air mixing chamber. 
     In a preferred embodiment of the heat exchanging unit  76 , the coolant pipes are made of copper tubes and the thin plates or fins  80  are made of aluminum. The coolant tube should be suitable for a working pressure of up to 350 psig. Preferably there is provided at the lower end of each heat exchanging unit  76  a horizontally extending condensate pan or tray  82  in order to prevent condensate created by the heat exchanging unit from dripping down through the ceiling  14 . The pan  82  can either be non drainable or can be drained by a suitable tube connection (not shown). 
     Preferably, since the primary air entering the plenum section may be quite cool, one, two or more sides of the air plenum section  44  can be covered with a thick layer of insulating material  90 , for example, a flexible layer of neoprene. In the preferred embodiment illustrated in FIG. 1, the neoprene extends along the two upwardly facing sides of the air plenum section. In order to conduct primary air for each or both of the air plenum sections  44 , there can be provided an elongate air duct  92  which is connected to the primary air inlet  46 . For ease of illustration, only a portion of the air duct  92  is illustrated in FIG.  1 . The air duct can either be a flexible tube type duct (which may be required if the duct must pass around a number of obstacles) or it can be a rigid sheet metal air duct of known construction. It will be appreciated that the air duct extends to a source of primary air indicated generally at  94 . For example, it can extend to an outer wall of the building where an opening in the wall permits outside air to flow in. In order to supply two separate air plenum sections  44 , a Y-type connection can be provided in the air duct in the vicinity of the air plenum sections  44 . This arrangement has the advantage of providing a balanced supply of air to the two induction units resulting in an equal amount of primary air (and return air) flowing through the two induction units and out of the air outlets  58 . In the illustrated embodiment of FIG. 1, a variable air valve  96  which per se is of known construction is connected to the air duct and is capable of controlling the volume or primary air flowing into the two induction units. Preferably the air valve is a pressure independent type valve. Such a valve is shown and described in Canadian patent No. 1,237,359 issued May 31, 1988. The description and drawings of this Canadian patent are incorporated herein by reference. 
     It is also possible to provide a variable air volume control device located inside the induction unit itself and in particular inside the air plenum section  44 . An air control device of this type is illustrated in FIG. 2, the device including an adjustable air flow restricting plate  100 , the position of which is controlled by a control rod  102  that passes through an elongate, straight, slot  104  formed in the plate  100 . By reason of nuts threaded onto the rod  102  and located on opposite sides of the plate  100 , axial movement of the rod  102  can cause the plate  100  to pivot about hinges located at one end of the plate. By pivoting the plate  100  towards the sidewall  108  of the plenum section, the flow of primary air can be reduced and vice versa. Axial movement of the rod  102  can be accomplished manually or by means of a standard electrical linear actuator. 
     It will be seen from FIG. 5 that in the preferred, illustrated embodiment the two air outlets  58  form elongate, narrow slots and are parallel to one another. In one preferred embodiment they are spaced apart by a distance of at least three feet and in particular a distance of 3 feet, 4 inches. In this preferred embodiment, the height of the apex A (FIG. 1) above the ceiling  14  was approximately one foot six inches. It will be appreciated that because of the substantial slope of the two induction units, the overall height of the pair of induction units is reasonably small. The result is that the height of the space  18  above the hanging ceiling  14  need not be excessive, for example about two feet in the preferred embodiment. At the same time, the length of the air mixing chamber L can still be quite long permitting both good mixing of the two air flows and good static pressure regain. 
     It will also be noted that the aforementioned grill or panel  62  can be simply supported on two or more T-bars  26 . The grill can thus be readily removed to permit easy servicing of the induction units or the heat exchanger units. 
     One substantial advantage gained with the air handling system of the invention as described herein is the reduction in the primary air capacity that can be achieved. The amount of primary air required to supply a given size of enclosed space can be reduced by as much as 70% compared to a conventional air supply system. 
     It will be appreciated that although the configuration and arrangement of the induction units is preferably that illustrated in FIG. 1, it is also quite possible to arrange the induction units differently while still achieving some of the aforementioned advantages. 
     FIG. 6 illustrates in plan view a possible arrangement of three of the above described air handling systems with each system comprising a pair of induction units. The illustrated system may be suitable, for example, for a classroom area of the usual size. In FIG. 6 the three pairs of induction units are indicated at  120 ,  122  and  124 . Primary air is delivered to all three pairs of induction units through a single VAV valve  126  which again can be of known construction. This air valve delivers the primary air to a suitable distribution box  128  which, in a known manner, can contain baffles  160  in order to evenly and smoothly deliver the primary air to smaller air ducts  130  and  132 . As illustrated, there are two ducts  130 , one for each of the induction units of the air handling apparatus  120  and two ducts  132 , one for each of the induction units of the pair  122 . It will be appreciated that the pairs  124  and  122  of induction units are connected in series and that some of the air that is delivered to the air plenum sections of the pair  122  is passed on to the air plenum sections of the pair  124  by means of further air ducts  134 . Preferably, each of the air ducts  130 ,  132  is fitted with a standard, adjustable air damper indicated at  136 . Thus the amount of primary flow flowing through each of the ducts  130 ,  132  from the distribution box can be adjusted by the installer or maintenance staff, as required. It will also be understood that, in the usual case, twice the volume of air will be delivered to and passed through the air ducts  132  as will pass through the air ducts  130  in order to achieve a reasonably even distribution of a mixed airflow in the room above which these units have been installed. The distribution box  128  is preferably reasonably large. The construction of a distribution box of this type is per se well known in the air distribution industry and accordingly a detailed description thereof herein is deemed unnecessary. The size of the distribution box and the baffles are arranged so as to reduce the pressure loss in the distribution box. Because of the desirability of reducing pressure loss as much as reasonably possible, it will be understood that the distribution box or plenum  128  can be aerocoustically designed in a manner known in the air distribution art. 
     It will be further appreciated by those skilled in the art that an air handling system constructed in accordance with the invention, for example a system designed for a school classroom, could be controlled with a known type of electrical control unit that provides for more than one mode of operation by the air handling system, for example, two different settings. The control unit can be set up so that there is a certain setting for when the room is occupied (in which case the air handling requirements will normally be greater) and another setting that would be used when the room is normally unoccupied. The electrical control unit can be set up to operate a two position air valve with each position of the valve representing one of these two settings. Again, the design of such a control system is well within the skill of those in the air handling industry and accordingly a detailed description herein is deemed unnecessary. 
     FIG. 7 of the drawings illustrates schematically how an air handling system  150  constructed in accordance with the invention can be set up with two different induction units  152 ,  154 . For example, as illustrated in FIG. 7, the series of nozzles  156  in the induction unit  154  can be made larger than the series of nozzles  158  in the induction unit  152 . By using larger nozzles in the unit  154 , it is possible to deliver a larger amount of primary air to the right induction unit  154  as compared to the amount delivered to the left unit. One situation where the use of different induction units constructed in this manner might be appropriate is when the right side unit  154  delivers its airflow to an exterior wall of the room which has windows extending along the wall while the left hand unit is delivering its airflow to an opposing inside wall of the room which is located in an area of the room that generally requires less heating or less cooling than provided by the right side unit  154 . FIG. 7 also illustrates the possibility of having different heat exchanging coil units  170  and  172  for the two induction units. As illustrated, the heat exchanging coil unit  170  for the induction unit  154  is larger in size and in heating or cooling capacity than the heat exchanging coil unit  172  of the induction unit  152 . The reason for this difference again can arise from the fact that different regions in the room can have quite different heating or cooling requirements. Generally these requirements can be estimated by the air handling engineer prior to the manufacture and installation of the air handling apparatus of the invention. Once these different requirements have been calculated, the manufacturer of the air handling apparatus can then readily design the two induction units to meet the particular requirements of the room or other enclosed space. 
     It will also be appreciated that in some cases there may be no need for a heating exchanging coil unit on one of the two inductions units while there is a need for a heat exchanging coil unit on the other induction unit. For example, the region of a room adjacent an interior wall may require very little or no additional heating or cooling to be provided by the air handling apparatus in the ceiling. The heating or cooling provided by a central heating or air conditioning unit of the building (from which the primary air is delivered) may be calculated to be quite sufficient for interior areas of the building including areas adjacent the interior walls. 
     In some applications, one of the induction units can be set up so that its heat exchanger unit will only provide cooling air, for example to an inner region of the room, while the other induction unit is provided with a heat exchanger capable of providing either cooled air or warmed air to another region of the room, for example, a perimeter area adjacent an exterior wall. Depending on expected outside climate conditions and other factors such as adjacent rooms, hallways, etc., the air handling engineer may determine that an interior region of the room will likely never require additional heating from the ceiling mounted air handling system but may, for example in mid-summer, require additional cooling to be provided by its respective induction unit. 
     Also, for some applications it may not be desirable or necessary to mount the one or more pairs of induction units along the centerline of the ceiling. In some applications, the air handling engineer may determine that the one or more pairs of induction units should be mounted closer to one wall of the room, for example, a wall having several windows mounted therein. Such an arrangement will provide a greater airflow in the region of the room adjacent the windows and a smaller airflow to an inner region of the room where it has been determined that less airflow will still be sufficient. 
     It is possible to connect more than one pairs of the induction units constructed in accordance with the invention either by means of a parallel connection or by means of a series connection. In the case of a parallel type connection where the primary airflow is delivered by separate air ducts to each pair, it is possible to connect any required number of the induction units in the ceiling and still deliver sufficient primary air to each pair of induction units. However, with a series type connection where the primary air flow is delivered through a single air duct arrangement to the pairs of induction units, the user of such a system is limited by the capacity of the air duct to deliver sufficient primary air to the pairs of induction units. In some cases, for example, it may only be possible to connect two pairs of induction units in series (see the arrangement of the induction units  122 ,  124  illustrated in FIG.  6 ). 
     Although FIGS. 1 and 7 illustrate a pair of induction units mounted above the level of the ceiling  14 , it will be appreciated that these induction units can also be mounted below but adjacent to the ceiling of the room, if desired. In the latter case, the space between the two outlet slots can either be left open or can be covered on the bottom by means of a suitable grill similar to that illustrated in FIGS. 1 and 7 or integrally connected to the bottom ends of the two induction units. 
     The air handling system of the present invention can also be constructed so that one of the two induction units provides a constant volume air supply with the possibility of varying the air temperature to a perimeter zone in a room while the other induction unit provides a constant temperature, variable air volume supply in order to make up for interior ventilation exhaust, for example, for a laboratory hood. Another possible arrangement for the air handling system is to have one of the induction units capable of providing a variable air volume air supply, the temperature of which may or may not be variable, to a perimeter area of the room while the other induction unit provides a constant volume air supply, the temperature of which can be adjusted, to the interior of the room which might, for example, be a conference or lecture room. 
     It will also be understood that with respect to the heat exchanging coil units  76 , in the usual situation these units will employ a number of secondary water coils through which water flows to either cool or heat the return air. It will be understood that these heat exchanging units which can per se be of standard construction can vary with respect to the number and arrangement of the secondary water coils and these coils can be piped in parallel or series. 
     It will further be appreciated by those skilled in this art that the air handling system of the invention can be used with induction changeover two pipe, induction non-changeover two pipe, or induction four pipe systems, all of which are known per se in the air handling art. In an induction changeover two pipe system, a change in the supply of water to the heat exchanging units is often carried out simply by closing or opening a suitable valve which can be done manually. After the changeover, for example in the fall, the heat exchanger units  76  can be used for heating while, after the changeover in the spring, the heat exchangers can be used for cooling. In the case of an induction non-changeover two pipe system, there is generally a central heating system that is capable of heating the air to a temperature in the range of 55 to 90 degrees F. and this central system that is capable of providing air of this temperature year round. In this system, the heat exchanger units on the induction units can simply be used to reheat the return air, when required. 
     In the four pipe system there are two separate heat exchanger units mounted on one or both of the induction units with one of the heat exchanger units providing heating when required and the other heat exchanger unit providing cooling. 
     In the majority of installations employing the air handling system of the present invention, the primary air supplied to the induction units will first be dehumidified and cooled in a central air apparatus installed at a suitable location in the building. The cooling-dehumidifying coil of this central air apparatus should precede the zone or building reheat coil. The latter may be required, depending on climatic conditions and the percentage of outside air. A humidifier may also be provided in the air supply system, preferably at the location of the central air apparatus. 
     In the case of air conditioned applications employing the present air handling system and a VAV valve, the valve controller should be deactivated by the user as a first step in providing for cool down and dehumidification after night shut-down of the system in order to avoid condensation problems. The VAV valve controller must be shut off as it is only temperature sensitive. 
     The present air handling system permits a wide range of possible arrangements of the two induction units permitting the present system to be adapted for various applications requiring a supply of air to an enclosed space. Suitable amounts of air can be provided through the horizontally extending slots of the two induction units at the required IAQ criteria for the particular room area being served. 
     It will be appreciated by those skilled in air handling systems that various modifications and changes can be made to the illustrated and described air handling system without departing from the spirit and scope of this invention. Accordingly, all such modifications of the air handling system as fall within the scope of the appended claims are intended to be part of this invention.