Abstract:
A heat exchanger in a cabinet space for use in a stationary air conditioning system, wherein the heat exchanger has a pair of substantially horizontal manifold tanks engaged to a plurality of substantially vertically oriented flow tubes bent into substantially an L shape having a bend angle sufficiently steep that condensate runs along the surface under the force of gravity. Furthermore, the heat exchanger has improved fins and is angled in the horizontal direction for less inhibited drainage of condensate.

Description:
CROSS REFERENCE TO RELATED APPLICATION: 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 60/779,630 filed on Mar. 6, 2006. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to heat exchangers used in stationary or residential air conditioning systems, and particularly to a heat exchanger used in an outdoor cabinet of the type having a generally rectangular prismatic shape of limited height, depth and frontal area. 
       BACKGROUND OF THE INVENTION 
       [0003]    Stationary air conditioning systems employ an outside heat exchanger that exchanges heat with the outside or ambient air, and an indoor heat exchanger that exchanges heat with the indoor air. In systems used strictly for cooling, the inside heat exchanger operates always as an evaporator (and generally only during warm months), while the outside heat exchanger operates only as a condenser. As such, the inside heat exchanger (evaporator) is subject to water condensation on its cold outer surfaces, while the outside heat exchanger is not. Some provision must be made to drain the indoor evaporator, but that is not an issue with the outdoor condenser. In so called heat pump applications, where the refrigerant flow and heat exchanger function is switched from cold to warm months, the outside heat exchanger acts as an evaporator in colder months. As such, surface water condensation is an issue, and a more critical issue than with the indoor heat exchanger, since it is subject to frosting or icing. This is especially critical when a defrosting operation is run on the outside heat exchanger. Failure to drain off the melted surface water can result in a harder freeze once the defrosting operation is halted. 
         [0004]    Possible surface condensation drainage schemes are affected by the type of heat exchanger construction, and by the available installation space for the heat exchanger. Historically, the typical heat exchanger construction has been expanded copper tubing with flat, so called plate fins. Flat plate fins drain relatively easily when oriented vertically (with the tubes horizontal). A simple trough under the bottom edge of the core serves to collect the drainage. With inside heat exchangers, installation is often in a duct which is limited in vertical space (parallel to the direction in which gravity acts), but not as limited in horizontal space. Also, with duct mounted indoor heat exchangers, the fan source of forced air flowing through the duct is generally more remote from the inside heat exchanger, so its shape and installation orientation are generally determined strictly by questions of drainage and available space, with no need to accommodate the shape of a proximate fan or blower. In order to get more heat exchanger capacity within a duct volume of limited vertical space, it is known to bend the core into a symmetrical V shape, presenting two wings to the air flow, which together comprise more surface area than just a single vertical slice of the duct volume would provide. When the core construction is the plate fin and expanded tube type, the V may be oriented with the apex vertical, as seen in U.S. Pat. No. 5,062,280, and a V shaped trough below would serve for drainage. If the core construction is instead a so called tube and center type, with flat flow tubes fed at opposite ends by manifold tanks, and corrugated fins brazed between the tubes, then it is the flat tubes which become the more convenient drainage surface, as opposed to the flat fins. The core can be simply oriented with the tubes vertical, and the tanks above and below, with a trough below the lower tank. If it is desired to pack more heat exchange capacity into the duct space, a symmetrical V shape can be created by bending the tubes in the middle (which is far easier than bending the tanks) and the resulting V shape oriented with the bend at the bottom, and both tanks in a plate above the bend, as disclosed in U.S. Pat. No. 5,279,360. A lower trough below the bend collects the condensate. The angle of the V bend is sharp, less than 90 degrees, so that the tubes are still close to a vertical orientation to drain well under the force of gravity. 
         [0005]    Some indoor heat exchangers are incorporated not in ducts with remote air supply, but in wall mounted units with proximate squirrel cage type fans. Examples may be seen in U.S. Pat. Nos. 5,918,666 and 4,958,500. In these units, horizontal space is more limited than the available vertical installation space. Consequently, a tube and flat plate fin type core is bent in a much more shallow V, and oriented with one tank above, one below, so that the flat fins are, if not vertical, at least oriented in vertical planes to enhance drainage. Again, a trough below the lower edge of the core collects condensate. 
         [0006]    In the case of outside heat exchangers used in heat pump applications, as noted, condensate drainage is even more critical, but the installation and mounting considerations are different, and drainage is likewise dependent on the available installation space and the type of core construction. One typical configuration is barrel shaped or cylindrical, with a central fan and a cylindrical heat exchanger surrounding the central fan. With an expanded tube and plate fin type of core, the entire core is bent into a cylindrical shape, and the plate fins are oriented vertically, in planes that basically radiate from the central axis of the cylinder. Being vertically oriented, drainage is fairly good. Replacing the cylindrical or barrel shaped core with a manifold tank and flat tube construction presents two alternatives, straight vertical tanks with tubes bent into a C shape and lying in horizontal planes, or straight, vertical tubes with the manifold tanks bent into a C shape. Flat tubes lying in horizontal planes are obviously poor candidates for surface condensate drainage, while it is difficult to bend anything but simple manifold shapes out of a straight line. Cylindrical, one piece tubular manifolds may be fairly easily bent, but heavier, two piece box shaped manifolds would be difficult to bend. 
         [0007]    Another typical configuration for outside heat exchangers is shown in  FIG. 1 . A cabinet that is basically a rectangular prism has a blade type puller fan oriented close to a large area front grill of the cabinet, pulling air therethrough and exhausting it out of an opposed rear grill. One narrow side of the prism is occupied by various controls and componetry, and an opposed narrow side grill is available for air flow. In order to make maximum use of the available area and volume, a ninety degree bend, L shaped expanded tube and plate fin core is used, which has a short section occupying the available side of the volume. As with the cylindrical shaped core, orienting the flat fins in vertical planes provides for good drainage. And, as with the cylindrical core, the obvious replication of the core in a tank and flat tube brazed construction consists of horizontally oriented, bent manifolds, one above and one directly below, with vertically oriented flat tubes. An example may be seen in U.S. Pat. No. 5,826,649. There, the manifolds are bent in a U shape, rather than an L, although an L shape with a  90  degree bend could be provided by the same basic method. As with the barrel shape heat exchanger, bending the manifolds, in order to allow for a vertical tube orientation, is really only practicable with simple, thin walled tubular manifolds, which is what is disclosed in the patent. 
       SUMMARY OF THE INVENTION 
       [0008]    The subject invention provides a flat tube and tank type construction capable of being installed in a rectangular prism shaped outdoor cabinet, in which most of the tube length is oriented vertically for good drainage. The disclosed embodiment provides an equivalent capacity of an L shaped core, but the manifold tanks are straight and do not require bending. 
         [0009]    In the embodiment disclosed, the manifold tanks are both straight and substantially horizontally oriented, one at the top and one at the bottom, but the two tanks are not located directly in line with one above the other. Instead, one tank is offset relative to the other, with the flow tubes bent into a basic L shape as they extend from one tank to the other. The majority of the length of each tube is straight and vertical, so that most of the area of the core lies in a plane parallel to the front grill of the cabinet. Each shorter bent section of tube is bent not at a ninety degree angle, as in the prior art described above, but at a shallow angle relative to the horizontal that is just sufficient to allow condensed surface water to run along and drain of the shorter section of the tube. The angle necessary for the water to run is shallow enough that the vertical projection of the shorter section tube does not add unduly to the vertical height of the cabinet. In the embodiment disclosed, the offset tank is the lower tank, and the lower tank is also tilted slightly, so that condensed water running down the tubes and hitting the lower tank also runs along the length of the lower tank to an end thereof, concentrating at a point for easier removal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a drawing of a prior art expanded tube and plate fin heat exchanger core typically used in an outdoor cabinet of the type having a generally rectangular prism shape; 
           [0011]      FIG. 2  is a partially schematic view of a preferred embodiment of a heat exchanger according to the invention installed in a similar cabinet; 
           [0012]      FIG. 3  is a side view of the heat exchanger showing the outlines of the cabinet; 
           [0013]      FIG. 4  is a side view of a single tube showing the surface condensation water flow direction; 
           [0014]      FIG. 5  is an enlarged plan view of a pair of tubes and air fin corrugations; 
           [0015]      FIG. 6  is a detail of a portion of the surface of the bent section of a tube, showing the water flow in relation of the corrugated fin brazed to the tube surface; 
           [0016]      FIG. 7  is a front view of the heat exchanger that illustrates adds a small horizontal tilt on the bottom tank. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    Referring first to  FIG. 2 , the environment for a heat pump system outdoor heat exchanger made according to the invention is a cabinet, indicated generally at  50 , that has a generally rectangular prism shape, with a pre determined height H and depth D. The width is not as significant to a description of the invention, but it is also, obviously, a pre determined limit on what can be installed inside. Top and bottom of cabinet  50 , labeled “T” and “B,” are defined relative to the force of gravity, acting from top to bottom. Also, vertical should be understood as parallel to the force of gravity, and, obviously, horizontal is understood as normal to vertical. A front grill  52  admits ambient air pulled in by a blade type fan  54  that spins in a plane substantially parallel to the front grill  52 , exhausting air out a rear grill  56 . If provision were made, air could also be pulled in through the top, sides, or bottom of the cabinet  50 , but for the embodiment disclosed, that would not be needed. As such, cabinet  50  is very similar to that described above and illustrated in  FIG. 1 . A preferred embodiment of an outdoor heat exchanger made according to the invention is indicated generally at  58 . Generally, heat exchanger  58  fits within cabinet  50 , in the available space between fan  54  and front grill  52  and below fan  54 . As the outdoor heat exchanger in a heat pump system, it is subject to the water surface condensation issues discussed above. 
         [0018]    Referring next to  FIGS. 2 ,  3  and  4 , heat exchanger  58  is the so called tube and fin or tube and center type, sometimes inaccurately referred to as a “parallel flow” configuration, although it is, in fact, a cross flow heat exchanger. Heat exchanger  58  is of the brazed, all aluminum type, rather than the less expensive, but less effective, expanded tube and plate fin type typically used. Hollow manifold tanks, an upper tank  60  and lower tank  62  (differing only in location, not construction) act to feed refrigerant into and/or out of a plurality of parallel flat flow tubes  64 , the ends of which extend into each tank  60  and  62 . While the refrigerant flow path contemplated here is single pass (flow from one tank  60  to the other  62 , without direction change), it would not be accurate to refer to one tank as a feed tank and the other a return tank, since that function will switch as the refrigerant flow direction is changed. Each tank  60  and  62 , as disclosed, is of the general type described in co owned U.S. Pat. No. 5,062,476, hereby incorporated by reference. Such a tank, a brazed, two piece construction with a heavy, thicker gage tank base (often extruded of non braze clad aluminum) and a separate, stamped and clad aluminum slotted header plate that is brazed thereto, presents a number of performance and manufacturing advantages over a one piece, cylindrical tubular manifold tank, despite its apparently greater complexity. However, such a design would be very difficult to bend out of its straight line shape, either before or after the core brazing operation, and would thus not lend itself well to the kind of tank bending proposed in U.S. Pat. No. 5,826,649 discussed above. Tubes  64  are typically aluminum extrusions, as well, and while not as easy to bend as cylindrical copper tubing, are easier to bend (even to bend in the same plane as their width) than the tanks, as disclosed generally in U.S. Pat. No. 5,279,360. The symmetrical, sharply angled V shape disclosed there would not work in the environment involved here, however. Brazed between the tubes  64  are corrugated fins  66 , described in more detail below. The heat exchanger  58  of the invention takes advantage of the space available within cabinet  50 , with no appreciable change thereto, while providing good surface condensate drainage. This is done by bending each tube  64  into a general L shape, with a longer, straight section  64 S and a shorter bent section  64 B. As a practical matter, this would likely be done by bending the already brazed core as a unit, since typical tube and center stackers and braze ovens are designed to handle flat cores. The longer straight tube sections  64 S correspond to a flat section of the core which fits easily between the fan  54  and front grill  52 . The shorter, bent tube sections  64 B correspond to an additional section of the core which provides extra refrigerant capacity and heat transfer, and fits within the available depth D of the cabinet  50  and below the fan  54 . 
         [0019]    Referring next to  FIGS. 4 ,  5  and  6 , the drainage of condenser surface water along the surface of a tube  64  is illustrated. The corrugated fins  66  brazed between the tubes  64  are of the type generally disclosed in co assigned U.S. Pat. No. 5,669,438, also incorporated herein by reference. Unlike flat, plate fins typical in a conventional residential outdoor heater core, corrugated fins like  66 , having a crest brazed to and crossing the surface of the tube  64  every few millimeters, as best seen in  FIG. 6 . These brazed crests could act like serial dams to block downward flow, and are therefore not inherently conducive to efficient surface condensate drainage, even when the tubes  64  themselves are oriented vertically. However, the use of unique fin geometry and longer louver lengths, as disclosed in co assigned U.S. Pat. No. 6,439,300, has significantly improved drainage of water condensation out of the fin itself. In addition, the longer louver length, while primarily directed toward more efficient convection heat transfer to the air, also provides an easier drainage path over and through a brazed fin crest, as seen in  FIG. 5 , even when that brazed fin crest is oriented so as to block the natural drainage flow direction. 
         [0020]    Still referring to  FIG. 6 , once water has flowed down the straight sections  64 S, it must “turn the corner” if it is to be collected and removed readily from the cabinet  50 . Several considerations determine the shape of the corner, that is, the size of the angle θ at which the tube sections  64 B are bent, as measured downwardly from the horizontal. A zero degree bend angle from the horizontal (equivalent to a 90 degree L) would be the most compact in terms of fitting within the available vertical height H of cabinet  50 , but water would not drain well along the length of a totally horizontal section of tube, and would tend to drip off unpredictably. Somewhat surprisingly, it has been found that a bend angle of as little as 10 degrees allows water that has drained down the longer length  64 S to “turn the corner” and run along and down the shorter length of tube  64 B to the surface of the lower manifold tank  62 , which will stop the condensate from running farther and allow it to drip off in a predictable line and collected in a trough or similar receptacle. As can be seen in  FIG. 6 , as the tube  64  is bent and transitions from one section to the other, the crests of the fins  66 , which were horizontal (and therefore normal to the direction of condensed water flow along the tube straight section  64 S) now fan out and transition to a slanted orientation, so that water can easily run down the line of the brazed crest of the fins  66 . In addition, as disclosed in co owned U.S. Pat. No. 5,669,438 noted above, the louvers formed in the walls of each fin  66  are stamped with a pattern of angled lead louvers  68 L and trailing angled louvers  68 T, which change angular direction to either side of a turn around rib  70 . The primary purpose, historically, of the louvers  68 L and  68 T has been to break up the flow of air over the walls of the fin, and thereby preventing the kind of laminar air flow that would inhibit efficient heat transfer. Here, by orienting the fins  66  so that the lead louvers  68 L are oriented in the same general direction as the natural direction of the water flow (down, and to the right, in the drawing), it is thought that water flow is less inhibited in its flow through the louvers  68 L, to the extent that it does still drain through the louvers  68 . Water will certainly tend to flow through the louvers  68 L more readily than through the louvers  68 T, given their relatively lower position, regardless of the louver slope direction. 
         [0021]    Referring again to  FIGS. 1 and 2 , it can be see how the embodiment of the tube and tank, flat tube brazed heat exchanger  58  of the invention, compared to the traditional expanded round tube and plate fin core, provides a comparable area of core face that fits within the same available space or volume in cabinet  50 . Since the brazed core construction is inherently more efficient, even just the portion of the core represented by the tube straight sections  64 S would likely as much or more heat exchange capacity than the conventional, plate fin and round tube core. With the addition of the extra core area and volume provided by the bent tube sections  64 B, even more capacity is provided with no diminution in the inherently superior drainage potential of a vertically oriented tube. 
         [0022]    Referring next to  FIG. 7 , an additional feature can be added to the orientation of the core of the invention to enhance drainage. The entire heat exchanger  58  or the lower tank  62  can be given a slight horizontal tilt t of around 5 degrees. This does not disturb the still substantially vertical orientation of the tube straight sections  64 S to any significant degree, but does allow the line drainage along the lower tank  62  to concentrate at the lowest corner, almost a point drainage, in effect. 
         [0023]    Variations in the disclosed embodiment could be made. The bent tube sections  64 B could be placed at the top of the available space, the entire heat exchanger  58  being flipped vertically, in effect, providing essentially the same capacity in the same volume. Or, an additional bent section could be provided at the top, providing more capacity, which occupying slightly more vertical space. Regardless, most of the core area and tube length would be vertically oriented, while the bent sections would provide more capacity without detracting from the outer surface condensate drainage.