Patent Publication Number: US-2010107675-A1

Title: Heat exchanger with improved condensate removal

Description:
FIELD OF THE INVENTION 
     This invention relates generally to heat exchangers for cooling air and, more particularly, to providing for improved removal of condensate accumulating on the external surfaces of the heat exchange tubes and any heat transfer fins associated with the heat exchange tubes. 
     BACKGROUND OF THE INVENTION 
     Refrigerant vapor compression systems are well known in the art. Air conditioners and heat pumps employing refrigerant vapor compression cycles are commonly used for cooling or cooling/heating air supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used for cooling air, or other secondary media such as water or glycol solution; to provide a refrigerated environment for food items and beverage products within display cases in supermarkets, convenience stores, groceries, cafeterias, restaurants and other food service establishments. 
     Conventionally, these refrigerant vapor compression systems include a compressor, a condenser, an expansion device, and an evaporator serially connected in refrigerant flow communication. The aforementioned basic refrigerant vapor compression system components are interconnected by refrigerant lines in a closed refrigerant circuit and arranged in accord with the employed vapor compression cycle. The expansion device, commonly an expansion valve or a fixed-bore metering device, such as an orifice or a capillary tube, is disposed in the refrigerant line at a location in the refrigerant circuit upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser. The expansion device operates to expand the liquid refrigerant passing through the refrigerant line running from the condenser to the evaporator to a lower pressure and temperature. The refrigerant vapor compression system may be charged with any of a variety of refrigerants, including, for example, R-12, R-22, R-134a, R-404A, R-410A, R-407C, R717, R744 or other compressible fluid. 
     In many refrigerant vapor compression systems, the evaporator is a parallel tube heat exchanger having a plurality of round heat exchange tubes extending longitudinally in a horizontal direction in parallel, spaced relationship, the heat exchange tubes being interconnected at their respective ends by so-called hairpin return bends to form a serpentine coil within each evaporator circuit. In many cases, hairpin configurations are used, instead of straight tube arrangements, with the return bends required only on one side of the hairpin-configured heat exchange tubes to form an evaporator serpentine refrigerant circuit. Typically, a plurality of serpentine evaporator circuits is employed to flow refrigerant downstream in a parallel manner. In particular, in evaporator applications, a number of parallel refrigerant circuits, either of identical configuration throughout an evaporator or of a divergent towards the downstream end configuration, are used. One end of each serpentine coil (or circuit) is connected to the refrigerant cycle so as to receive refrigerant flow from the refrigerant cycle and the other end of each serpentine coil (or circuit) is connected to the refrigerant cycle so as to return refrigerant flow to the refrigerant cycle. The upstream receiving end of each serpentine coil is typically connected to a refrigerant cycle through a distributor or an inlet manifold, while the downstream returning end of each serpentine coil is connected to a refrigerant cycle through an outlet manifold. 
     In some refrigerant vapor compression systems, the parallel tube evaporator is a parallel flow heat exchanger (also often called a microchannel or minichannel heat exchanger) having a plurality of flattened heat exchange tubes extending longitudinally in a horizontal direction in parallel, spaced relationship between a pair of spaced headers (or manifolds). In this case, for multi-pass evaporator configurations, the return bends are substituted by intermediate manifolds or manifold chambers, while a number of parallel circuits is defined by a number of parallel heat transfer tubes within each pass. 
     In either round tube or flattened tube heat exchangers, external heat transfer fins are commonly positioned between heat exchange tubes for heat transfer enhancement, structural rigidity and heat exchanger design compactness. The heat exchange tubes and heat transfer fins are permanently attached to each other, typically, by a mechanical contact, for round tube and plate fin heat exchangers, or by a furnace brazing operation, for parallel flow heat exchangers. The heat exchange tubes may have internal heat transfer and structural enhancement elements as well. 
     When a heat exchanger is used as an evaporator in a refrigerant vapor compression system for cooling air, moisture in the air flowing through, the evaporator and over the external surfaces of the refrigerant conveying tubes and associated fins of the heat exchanger condenses out the air and accumulates on the external surface of the those tubes and fins. Typically, the condensate accumulating on the external surfaces of the heat exchange tubes and associated fins will gradually flow under the force of gravity and drain into a drain pan disposed beneath the heat exchanger. However, with many heat exchanger constructions, particularly those having flattened tubes disposed horizontally and extending longitudinally in a horizontal direction, condensate accumulating on the heat exchange tubes and associated fins does not always drain quickly therefrom. 
     If the condensate accumulating on the external surfaces of the heat exchange tubes and associated fins becomes excessive, overall performance of the refrigerant vapor compression system is adversely impacted. For example, excessive condensate retention on the external surfaces of the heat exchange tubes can result in increased air side pressure drop through the evaporator, which causes increased fan power consumption, and reduced heat transfer through the heat transfer tubes, which negatively affects evaporator capacity. Also, since in many air conditioning applications, the indoor air is continuously circulated through the air conditioning system, even when the refrigerant is not circulating through the evaporator heat exchanger, condensate accumulating on the external surfaces of the heat transfer tubes and associated fins of the evaporator may be undesirability recaptured, either through re-evaporation or by re-entrainment, by the air passing through the evaporator. This recaptured condensate may even be carried back into the conditioned space, which increases the humidity in the conditioned environment, potentiality adversely impacting the comfort of occupants within that conditioned environment. 
     It should to be noted that cooling heat exchangers of air handling equipment, utilizing cold water or glycol solutions to cool and dehumidify air supplied to the conditioned environment, face an identical problem of condensate blow-off, which causes similar undesired consequences. 
     SUMMARY OF THE INVENTION 
     An evaporator heat exchanger having an arrangement of generally horizontally extending heat exchange tubes and associated generally vertically extending heat transfer fins is provided with airflow guide vanes for directing airflow across the external surfaces of the heat exchange tubes and associated heat transfer fins to facilitate drainage of condensate from the external surfaces of the heat exchange tubes and associated heat transfer fins. 
     In one embodiment, the heat exchanger includes a plurality of heat exchange tubes arranged in a parallel array and extending longitudinally in a horizontal direction, a plurality of associated generally vertically extending heat transfer fins, and a plurality of airflow guide vanes disposed at the air side inlet to the heat exchange tube and heat transfer fin arrangement for directing the airflow passing into the heat exchange tube array so as to flow more along the condensate accumulating surfaces of each heat transfer fin to enhance drainage of condensate form the heat exchanger external surfaces. The heat exchange tubes may be, for example, of a round, or flat rectangular or flattened oval cross-section. Additionally, at least one partition may be selectively positioned with respect to the heat exchange tube array to locally accelerate the airflow passing therethrough along the heat transfer surfaces of the fins to enhance shedding of condensate therefrom. 
     In another embodiment, a plurality of airflow guide vanes may be disposed at the airflow outlet of the heat exchange tube array. The outlet airflow guide vanes may be used in addition to or instead of the inlet airflow guide vanes for re-directing the airflow passing out of the heat exchange tube array, and also affecting the airflow passing through the heat exchange tube array similarly to the inlet airflow guide vanes. 
     In one embodiment, the evaporator heat exchanger is a flattened multi-tube parallel-flow heat exchanger (also frequently called a microchannel or minichannel heat exchanger) with the heat exchange tube array generally oriented vertically and having adjoining heat transfer serpentine fins positioned therebetween. In this configuration, the heat exchange tubes, rather than plate fins, provide main condensate draining surfaces. In one embodiment of a round tube and plate fin heat exchanger, the heat exchange tubes are positioned generally vertically and heat transfer fins provided with louvers to promote condensate drainage paths. Any of inclined configurations for both heat exchanger types are also within the scope of the invention and can take advantages of at least one of generally horizontal and generally vertical heat exchange tube orientations. 
     In another embodiment, the heat exchanger is a cooling heat exchanger of air handling equipment, utilizing cold water or glycol solutions to cool and dehumidify air supplied to the conditioned environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following detailed description of the invention, reference will be made to and is to be read in connection with the accompanying drawing, where: 
         FIG. 1  is a schematic diagram of a refrigerant vapor compression system incorporating a heat exchanger as an evaporator; 
         FIG. 2  is a side elevation view, partly sectioned, of a first exemplary embodiment of an evaporator heat exchanger equipped with flow guide vanes; 
         FIG. 3  is a side elevation view, partly sectioned, of a second exemplary embodiment of an evaporator heat exchanger equipped with flow guide vanes; 
         FIG. 4  is a side elevation view, partly sectioned, of a further exemplary embodiment of an evaporator heat exchanger equipped with flow guide vanes; and 
         FIG. 5  is a plan view, partially sectioned, of the heat exchanger of  FIG. 2  taken along line  5 - 5  illustrating partitions disposed within the guide vane array and protruding inside the heat exchanger. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The heat exchanger of the invention will be described herein in use as an evaporator, in connection with a simplified air conditioning cycle refrigerant vapor compression system  100 , as depicted schematically in  FIG. 1 . Although the exemplary refrigerant vapor compression cycle illustrated in  FIG. 1  is a simplified air conditioning cycle, it is to be understood that the heat exchanger of the invention may be employed in refrigerant vapor compression systems of various designs, including, without limitation, heat pump cycles, economized cycles, cycles with tandem components such as compressors and heat exchangers, chiller cycles, cycles with reheat and many other cycles including various options and features. Also, it has to be recognized that although the blow-off phenomenon is described in connection to evaporators of refrigerant systems operating in a vapor compression cycle, cooling heat exchangers of air handling equipment, utilizing cold water or glycol solutions to cool and dehumidify air supplied to the conditioned environment, face an identical problem and can equally benefit from the invention. 
     The refrigerant vapor compression system  100  includes a compressor  105 , a condenser  110 , an expansion device  120 , and a heat exchanger  10 , functioning as an evaporator, connected in a closed-loop refrigerant circuit by refrigerant lines  102 ,  104  and  106 . The compressor  105  compresses refrigerant from a lower suction pressure to a higher discharge pressure and circulates this hot, high pressure refrigerant vapor through discharge refrigerant line  102  into and through the heat exchange tubes of the condenser  110 , wherein the hot refrigerant vapor is desuperheated, condensed to a liquid and typically subcooled, as it passes in heat exchange relationship with a cooling fluid, such as ambient air, which is blown over the heat exchange tubes of the condenser  110  by the condenser fan  115 . The high pressure, liquid refrigerant leaves the condenser  110  and thence passes through the liquid refrigerant line  104  to the evaporator heat exchanger  10 , traversing the expansion device  120 , wherein the refrigerant is expanded to a lower pressure and temperature to form a refrigerant liquid/vapor mixture. 
     The now lower pressure and lower temperature, expanded refrigerant passes through the heat exchange tubes  40  of the evaporator heat exchanger  10 , wherein the refrigerant is evaporated, and typically superheated, as it passes in heat exchange relationship with air to be cooled, and typically dehumidified, which is passed over the heat exchange tubes  40  and associated heat transfer fins  50  by the evaporator fan  15 . The refrigerant, predominantly in a vapor thermodynamic state, passes from the evaporator heat exchanger  10  through the suction refrigerant line  106  to return to the compressor  105 . As the airflow traversing the evaporator heat exchanger  10  passes over the heat exchange tubes  40  and associated heat transfer fins  50  in heat exchange relationship with the refrigerant flowing through the heat exchange tubes  40 , the air is cooled and the moisture in the air flowing through the evaporator heat exchanger  10  and over the external surfaces of the refrigerant conveying tubes  40  and heat transfer fins  50  of the evaporator heat exchanger  10  condenses out of the air and collects on the external surface of these heat exchange tubes  40  and associated heat transfer fins  50 . A drain pan  45  is provided beneath the evaporator heat exchanger  10  for collecting condensate that drains from the external surfaces of the heat exchange tubes  40  and associated heat transfer fins  50 . 
     The parallel flow heat exchanger  10  will be described herein in general with reference to the illustrative exemplary embodiment of a section of the heat exchanger  10  depicted in  FIGS. 2-4 . The heat exchanger  10  includes a heat exchange tube circuit arrangement or bundle  12  having an airflow inlet at an upstream end of the heat exchange tube circuit arrangement  12  and a plurality of airflow guide vanes  60  disposed in association with this heat exchange tube circuit arrangement  12 , and positioned slightly upstream of the heat exchange tube bundle  12 , with respect to the airflow. In the exemplary embodiments depicted in  FIGS. 2 and 3 , the heat exchange tube arrangement  12  includes a plurality of round heat exchange tubes  40  arranged in a parallel array, each tube extending in a generally horizontal direction along its longitudinal axis and being interconnected to another tube by a hairpin return bend (not shown) to form at least one serpentine circuit. Typically, the round heat exchange tubes  40  have a diameter of ½ inch, ⅜ inch or 7 millimeters. The at least one serpentine tube circuit of the heat exchanger  10  has an inlet end connected in refrigerant flow communication to refrigerant line  104 , through a distributor or inlet manifold (not shown), for receiving refrigerant flow from the refrigerant cycle and an outlet end connected in refrigerant flow communication to refrigerant line  106 , through an outlet manifold (not shown), for returning refrigerant flow to the refrigerant cycle. 
     Instead of round tubes, the evaporator heat exchanger  10  could have multi-channel, flattened tubes  140 , for example, of rectangular or oval cross-section, arranged in parallel spaced relationship in a vertical array, as depicted in  FIG. 4 . The multi-channel, flattened tubes  140  extend longitudinally in a horizontal direction between a pair of spaced headers or manifolds (not shown) for distributing refrigerant received from the refrigerant cycle amongst the heat exchange tubes  140  and collecting refrigerant from the heat exchange tubes  140  for return to the refrigerant cycle. Each flattened multi-channel heat exchange tube  140  might have, for example, a width of fifty millimeters or less, typically from ten to thirty millimeters, and a height of about two millimeters or less. Each flattened heat exchange tube  140  may define a plurality of parallel refrigerant flow channels  142 , that may be of round, rectangular, trapezoidal, triangular or other cross-section, typically from about ten to about twenty in number, extending longitudinally the entire length of the tube. Each channel provides a refrigerant flow path of relatively small cross-sectional area and having a hydraulic diameter, defined as four times the cross-sectional flow area divided by the “wetted” perimeter, in the range generally from about 200 microns to about 3 millimeters. Thus, a heat exchanger with multi-channel tubes extending in parallel relationship between the inlet and outlet headers of the heat exchanger has a relatively large number of small flow area refrigerant flow paths extending between the two headers. Sometimes, such multi-channel heat exchanger constructions are called microchannel or minichannel heat exchangers as well. 
     As in conventional practice, to improve heat transfer between the air flowing through the heat exchanger  10  over the external surfaces of the heat exchange tubes  40 ,  140  and the refrigerant flowing through the heat exchange tubes  40 ,  140 , the heat exchanger  10  includes a plurality of external heat transfer fins  50 ,  150  extending between each set of the parallel-arrayed tubes  40 ,  140 . The plate fins  50  may be of flat or wavy configuration, may have louvers and are typically mechanically or otherwise securely attached to the external surfaces of the adjoining heat exchange tubes  40 . The fins  150  are of serpentine configuration, may have louvers or offset strips, generally form rectangular, triangular or trapezoidal air passages and are typically furnace brazed to the external surfaces of adjoining flattened heat exchange tubes  140 . In both cases, heat transfer contact is established respectively between heat exchange tubes  40  and  140  and heat transfer fins  50  and  150 , by heat conduction. Thus, the external surfaces of the heat exchange tubes  40 , 140  and the surfaces of the heat transfer fins  50 ,  150  together form the external heat transfer surface that participates in heat transfer interaction between the refrigerant flowing inside the heat exchange tubes  40  and  140  and the air flowing through the heat exchanger  10  over its external heat transfer surfaces. The external heat transfer fins  50 ,  150  also provide for structural rigidity of the heat exchanger  10  and quite often assist in air flow redirection and alignment to improve heat transfer characteristics. As mentioned above, in the exemplary embodiments of the heat exchanger  10  depicted in  FIGS. 2-3 , the heat transfer fins  50  constitute a plurality of plates disposed in parallel, spaced relationship and extending generally vertically between the generally horizontally extending heat exchange tubes  40 . 
     As noted hereinbefore, condensate accumulates on the external surfaces of the heat exchange tubes  40 ,  140  and the associated heat transfer fins  50 ,  150 , during operation of the heat exchanger  10  in an air cooling mode, either as an evaporator of the refrigerant system  100  or as an air cooling heat exchanger of an air handler. Also, for a generally horizontal orientation of the heat exchange round tubes  40 , condensate accumulated on external surfaces of the heat exchanger  10  drains primarily along the plate fins  50  under the force of gravity. On the other hand, if the flattened heat exchange tubes  140  are oriented generally horizontally, condensate drainage becomes more problematic, since the condensate can only drain, under the force of gravity, along the leading edges  152  and trailing edges  154  of the heat exchange tubes  140 , and most likely, from the trailing edges  154 , due to airflow momentum pushing condensate along the width of the heat exchange tubes  140  from the leading edge  152  to the trailing edge  154 . 
     For the generally vertical orientation of the heat exchange round tubes  40 , the heat transfer fins  50  become a barrier blocking condensate drainage, therefore, to create alternate paths for the condensate drainage, rather than leading edges  52  and trailing edges  54  of the heat transfer fins  50 , louvers or cutouts are typically made in the fins  50 . Generally, vertical orientation for the flattened heat exchange tubes  140  provides better condensate drainage, since the condensate primarily drains along vertically oriented heat exchange tubes  140 , and, if louvers or offsets are incorporated in the heat transfer fin design, through the louvers or offsets in the heat transfer fins  150 . In all cases, any of the inclined positions for the heat exchange tube bundle  12  of the heat exchanger  10  may possess any of the drawbacks and advantages of the generally horizontal and vertical heat exchange tube orientations. 
     Further, the amount of condensate accumulated on the external heat exchange surfaces increases from top to bottom creating more favorable conditions for condensate blow-off within the lower portion of the heat exchanger  10 . Additionally, during shutdown time intervals for a cooling media, such as a refrigerant of the vapor compression refrigerant cycle  100  or water or glycol solution for an air cooling heat exchanger of an air handler, while the airflow is still circulated over external surfaces of the heat exchanger  10 , the condensate accumulated on external surfaces of the heat exchanger  10  may re-evaporate and re-enter the airflow, or may even get carried away downstream of the air duct and into the conditioned environment. This is obviously undesirable, since water leakage problems and discomfort conditions in the climate controlled indoor environment may surface. Additionally, condensate accumulated on external surfaces of the heat exchanger  10  may promote frost, if the temperature falls bellow the freezing point. 
     Therefore, there is a need to effectively remove condensate from external surfaces of the heat exchanger  10 . To facilitate drainage of the accumulated condensate from the external surfaces of the round heat exchange tubes  40  or the flattened heat exchange tubes  140 , a plurality of guide vanes is disposed in operative association with these heat exchange tubes to direct air flow passing through the heat exchange tube arrangement  12  in a desired direction. The inlet guide vanes  60 , positioned slightly upstream of the heat exchange tube arrangement  12 , function to direct the incoming air to flow in a desired direction through the heat exchange tube arrangement  12 . Each inlet guide vane  60  comprises a longitudinally elongated member extending across the inlet to the heat exchange tube arrangement  12  and forming a certain angle of attack, in relation to incoming airflow, to force the incoming airflow in a desired direction. 
     In the exemplary embodiment depicted in  FIG. 2 , the inlet guide vanes  60  are aligned in parallel spaced relationship so as to direct the incoming air flow generally horizontally that could be associated with the operation when there is no need in the condensate removal enhancement. In this case, the airflow direction generally is not changed, although some benefits may be obtained due to airflow streamlining and potentially lower power consumption for the associated fan  15 . Similarly, in the exemplary embodiment depicted in  FIG. 4 , the inlet guide vanes  60  are aligned in parallel spaced relationship so as to direct the incoming air flow generally horizontally along the upper and lower surfaces of each flattened heat exchange tube  140 , each of which extends transversely in a generally horizontal direction. 
     However, in the exemplary embodiment depicted in  FIG. 3 , the inlet guide vanes  60  are aligned in parallel spaced relationship so as to direct the incoming airflow in a downward direction forming a certain angle of attack, in relation to incoming airflow, to force the incoming airflow to turn downwards to align more with the direction of the gravity force and towards the drain pan  45 . Likewise, in a flattened heat exchange tube embodiment of the heat exchanger  10 , whether the transverse axis of each flattened multi-channel heat exchange tube  140  of the heat exchange tube arrangement  12  depicted in  FIG. 4  were angled downwardly to a horizontal position with the trailing edge of each flattened tube  140  disposed downwardly from the leading each thereof, as taught in co-pending International Patent Application No. PCT/US06/(serial number to be assigned) (attorney docket 210 — 1042PCT), entitled “MULTI-CHANNEL HEAT EXCHANGER WITH IMPROVED CONDENSATE DRAINAGE” and subject to assignment to Carrier Corporation, the common assignee, or not, the inlet guide vanes  60  would also be tilted slightly downwardly in the direction of the airflow so as to direct the incoming airflow to flow generally parallel to the upper and lower external surfaces of the heat exchange tubes  140 . The downwardly directed airflow results in improved shedding of condensate from the external surfaces of the heat exchange tube arrangement  12 , since the sheer force created by the airflow momentum has a component aligned with the gravity force and can assist the gravity force in condensate drainage into the drain pan  45 . Reduced condensate accumulation on external surfaces of the heat exchange tube arrangement  12  promotes better heat transfer, lower pressure drop and fan power consumption, lower potential for condensate blow-off and associated leakage and discomfort problems, operation at higher airflow velocities, and lower probability for significant amount of frost accumulation. The guide vanes  60  may have various profiles along the airflow direction, such as a rectangular flat plate, a flat plate with rounded edges, a flat plate with diminishing thickness, an airfoil profile and many others. 
     In one embodiment of the heat exchanger  10 , the inlet guide vanes  60  are angularly adjustable so that the incoming airflow may be selectively diverted at a desired angle through the heat exchanger  10 . For example, each guide vane  60  could be pivotable about an axis through its trailing end, its forward end, or its mid-span, so as to adjust its angle of attack, in relation to the incoming airflow, depending on condensate drainage improvement required. Such an adjustment could be, for instance, a two-positional adjustment, a multi-positional adjustment or a continuous adjustment. As explained above, a person ordinarily skilled in the art would recognize at which applications and operating conditions a corresponding adjustment angle is required. 
     For instance, during shutdown time intervals for a refrigerant system and while the airflow is still blown over the evaporator heat exchanger and into the conditioned environment, it is desired to re-direct airflow more along the condensate accumulating surfaces by pivoting the guide vanes and hence improving condensate removal, in order to prevent condensate re-evaporation and re-entrainment into the airflow. On the other hand, at normal operation at relatively dry conditions, condensate removal enhancement is not required, so the guide vanes would preferably remain in their initial position. 
     Additionally, a plurality of outlet guide vanes  70  is disposed in operative association with the heat exchange tube arrangement  12  and positioned slightly downstream of the outlet to the heat exchange tube arrangement  12 . Each outlet guide vane  70  comprises a longitudinally elongated member extending in a horizontal direction across the outlet of the heat exchange tube arrangement  12 . The outlet guide vanes  70  function to redirect the airflow leaving the heat exchange tube arrangement  12  along a desired direction, but also, similarly to the inlet guide vanes  60 , affecting the upstream airflow passing through the heat exchange tube array. The outlet guide vanes  70  may be used in combination with the inlet guide vanes  60  or separately. 
     Furthermore, in some cases, the guide vanes may provide two additional benefits. One of them is re-direction of at least a portion of the condensate, blown off the evaporator external surfaces downstream the air passage, towards a drain pan. This would allow for operation at higher airflow velocities and avoidance of water leakage problems. Another advantage is associated with streamlining of the airflow entering an evaporator heat exchanger, in particular, while the guide vanes are in a normal position, improving system efficiency and reducing fan power consumption. 
     Referring now to  FIG. 5 , to further facilitate removal of the accumulated condensate from the external surfaces of the heat exchange tubes  40 , partitions  80  may be installed in operative association with the array of the guide vanes  60 , so as to cause a localized acceleration of the airflow along the surfaces of the heat transfer fins  50 . In the particular embodiment depicted in  FIG. 5 , the partitions  80  are positioned prior to entering, or at the entrance region of the heat exchange tube array  12  in association with localized regions where excessive condensate build-up is experienced or predicted. In particular, the partitions  80  may be positioned just upstream of the heat exchange tube array  12  or penetrating into the entrance region of the heat exchange tube array  12 . Internal portions of the partitions  80  may also be used to locally redirect the airflow to impinge upon a selected region of the surfaces of heat transfer fins  50  to further enhance shedding of accumulated condensate and prevent a locally excessive condensate build-up, for instance, in the region where refrigerant temperature is the lowest. 
     While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.