HEAT EXCHANGER WITH BLOW-OFF CONDENSATE COLLECTING SCREEN

Field of the Invention

[0001] This invention relates generally to heat exchangers for cooling air, and more particularly, to direct expansion evaporator heat exchangers of refrigerant vapor compression systems and cooling heat exchangers of air handling equipment.

Background of the Invention

[0002] 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 heverage products within display cases, bottle coolers or other similar equipment in supermarkets, convenience stores, groceries, cafeterias, restaurants and other food service establishments.

[0003] 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, connecting 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.

[0004] 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. [0005] 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.

[0006] In either round tube or flattened tube heat exchangers, external heat transfer fins are commonly positioned between heat transfer tubes for heat transfer enhancement, structural rigidity and heat exchanger design compactness. The heat

transfer tubes and fins axe permanently attached to each other, typically, by a mechanical contact, for round tube and plate fin heat exchangers, and by furnace brazing operation, for parallel flow heat exchangers. The heat transfer tubes may have internal heat transfer enhancement elements as well.

[0007] 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 of the air and collects on the external surface of the those tubes and fins. Typically, the condensate collecting on the external surfaces of the heat exchange tubes and associated fins will gradually drain off the tubes under the force of gravity and drip into a drain pan disposed beneath the heat exchanger. However, with many heat exchanger constructions, especially having flattened tubes disposed horizontally and extending longitudinally in a horizontal direction, condensate collecting on the heat exchange tubes and associated fins does not always drain quickly therefrom. [0008] If the condensate collecting on the external surfaces of the heat exchanger becomes excessive and the velocity of the airflow passing through the heat exchanger and over these external surfaces is high enough, the collected condensate may be stripped off or blown off the external surfaces of the heat exchange tubes and associated fins and carried downstream the air passage by the momentum of the airflow, potentially creating leakage problems. In some cases, a portion of this entrained condensate may even be carried back into the conditioned space, which increases the humidity of that environment, potentiality adversely impacting the comfort of occupants within the conditioned space. [00091 Although heat exchanger drainage characteristics depend on heat exchanger design and orientation, at certain airflow velocities, condensate blow-off phenomenon can be experienced in a vast majority of operating conditions. It has to be noted that condensate amount collected on the external evaporator surfaces increases from the top to the bottom, promoting higher airflow velocities and creating more favorable conditions for the condensate blow-off within the bottom portion of the heat exchanger. Consequently, evaporator heat exchangers are typically designed to keep the velocity of the airflow through the heat exchanger coil

below the level at which condensate would be blown off the external surfaces of the heat exchange tubes and associated fins, even though operation at higher airflow velocities would be desirable to provide a higher recirculation rates of air from the condition space and sufficient penetration and mixing of the conditioned airflow in the conditioned space, as well as for refrigerant system capacity adjustment and supplied air temperature control.

[0010] 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

[0011] A heat exchanger for cooling a flow of air passing therethrough includes a heat transfer surface including a plurality of refrigerant conveying heat exchange tubes and associated fins, and having a condensate collecting screen disposed downstream of the heat exchanger, with respect to airflow, to capture condensate entrained in the airflow. In one embodiment, the condensate collecting screen may comprise a wire mesh screen. In another embodiment, the condensate collecting screen may comprise a screen having a honeycomb mesh structure. Condensate flow channels may be formed in the surface of the condensate collecting screen facing upstream, with respect to the airflow.

Brief Description of the Drawings

[0012] 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:

[0013] FIG. 1 is a schematic diagram of a refrigerant vapor compression system having an evaporator heat exchanger with a condensate collecting screen;

[0014] FIG. 2 is a perspective view of a section of an exemplary embodiment of an evaporator heat exchanger including a condensate collecting screen;

[0015] FIG. 3 is an exploded front view of a section of the condensate collecting screen of the evaporator heat exchanger of FIG. 2;

[0016] FIG. 4 is a sectioned, elevation view taken along line 4-4 of FIG. 2; and

[0017] FIG. 5 is a side elevation view, partly sectioned, of another exemplary embodiment of the evaporator heat exchanger of FIG. 2.

Detailed Description of the Invention

[0018] 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. [0019] 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.

[0020] 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 and associated fins. A drain pan 45 is provided beneath the evaporator heat exchanger 10 for collecting condensate that drains from the external surfaces of the heat transfer tubes 40 and associated fins 50.

[0021] The 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 12 and a blow-off condensate collecting screen 60 disposed downstream of the heat exchanger 10, with respect to the airflow. The heat exchange tube circuit arrangement 12 includes a plurality of round heat exchange tubes 40 arranged in a parallel array, each tube extending in a horizontal direction along its longitudinal axis and being interconnected to another tube by a hairpin return bend 41 to form a serpentine circuit. Although a single circuit is shown in Figure 2 for simplicity purposes, multi-circuit arrangements are utilized quite often in the art and are within the scope of the invention. Also, different circuits may not necessarily be of equal length, and, in evaporator applications, a number of parallel refrigerant circuits is either identical throughout an evaporator or

diverges towards the downstream end. Further, in many cases, hairpin configurations are used, instead of straight tube arrangements, with the return bends required only on one side of the hairpins to form each evaporator serpentine refrigerant circuit. Typically, the round heat exchange tubes 40 have a diameter of 1/2 inch, 3/8 inch or 7 millimeters. In Figure 2, the serpentine tube circuit 12 of the heat exchanger 10 has an inlet end connected in refrigerant flow communication to refrigerant line 104, through a distributor or an 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. [0022] 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 and the refrigerant flowing through the heat transfer tubes 40, the heat exchanger 10 includes a plurality of external heat transfer fins 50 extending between each set of the parallel-arrayed tubes 40. The fins 50 are brazed or otherwise securely (e.g., mechanically) attached to the external surfaces of the adjoining tubes 40 to establish heat transfer contact, by heat conduction, between the fins 50 and the external surfaces of the heat transfer tubes 40. Thus, the external surfaces of the heat exchange tubes 40 and the surfaces of the associated fins 50 together form the external heat transfer surface that participates in heat transfer interaction with the air flowing through the heat exchanger 10. The external heat transfer fins 50 also provide for structural rigidity of the heat exchanger 10 and quite often assist in airflow redirection and velocity increase to improve heat transfer characteristics. In the exemplary embodiment of the heat exchanger 10 depicted in Fig. 2, the heat transfer fins 50 constitute a plurality of plates disposed in parallel, spaced relationship and extending generally vertically between the heat exchange tubes 40 positioned horizontally. However, it is to be understood that other fin configurations, such as, for example, wavy or louvered fins may be used instead in the evaporator heat exchanger of the invention.

[0023] As noted hereinbefore, condensate collecting on the external surfaces of the heat exchange tubes 40 and associated fins 50 may be stripped away from the tube surface and be re-entrained in the airflow passing over the heat exchange tubes

40 and associated fins 50, rather than draining from the external surfaces of the heat exchange tubes 40 and associated fins 50 to flow under the force of gravity to the drain pan 45. If condensate doesn't drain quickly under the force of gravity, at certain airflow velocities and operating conditions, some amount of the collected condensate can be blown off external evaporator surfaces and re-enter an air stream downstream of the evaporator heat exchanger. Further, the amount of condensate collected on the external evaporator surfaces increases from the top to the bottom, promoting higher airflow velocities and creating more favorable conditions for the condensate blow-off within the bottom portion of the heat exchanger. [0024] To eliminate condensate re-entrained in the airflow from passing into a supply air duct or even farther into the conditioned space, a condensate collecting screen 60 is disposed downstream, with respect to the airflow, as shown in Figure 2. As the airflow leaving the heat exchanger 10 traverses the downstream condensate collecting screen 60, the airflow passes through the openings 65 formed by the lattice structure 62 of the condensate collecting screen 60. Therefore, the condensate still entrained in the airflow downstream of the heat exchanger 10 is captured upon the lattice structure 62 of the condensate collecting screen 60, thereby removing the condensate from the airflow.

[0025] In an embodiment, the condensate collecting screen 60 may comprise a wire mesh screen made of aluminum, stainless steel, plastic or other rust-resistant material wire. Referring now to FIGs.2 and 3, in particular, in the exemplary embodiment there illustrated, the condensate collecting screen 60 comprises a screen having a honeycomb mesh lattice structure 62 defining hexagonally-shaped openings 65 through which airflow passes. The porosity of the screen 60, that is the open area through which the airflow passes, should be large enough that the screen 60 presents a relatively small air-side pressure drop. On the other hand, the lattice structure 62 should have sufficiently large surface area as to present an effective barrier for capturing the condensate entrained in the airflow from passing through the condensate collecting screen 60. Those skilled in the art will appreciate that the specific condensate collecting screen porosity desirable for any particular heat exchanger arrangement will depend upon the heat exchanger design and orientation as well as the particular operating conditions to which the heat exchanger is

exposed. Preferably, however, the condensate collecting screen 60 will have porosity, defined as the overall open cross-section area provided by the openings 65 as a percentage of the total face area of the screen 60, in the range of from about 60 percent to about 85 percent. Further, the lattice structure 62 of the condensate collecting screen 60 may have pentagonal, square, triangular or any other cells, instead of than hexagonal cells shown in FIG. 3. [0026] Referring now to FIG. 4, in an embodiment, a network of interconnected condensate flow channels 64 is formed on the face 66 of the mesh screen lattice structure 62 facing upstream, with respect to airflow leaving the heat exchanger 10. The condensate flow channels 64 facilitate drainage of condensate depositing on the face 66 of the screen 60 downwardly, under the influence of gravity, into the drain pan 45. The depth of the condensate flow channels and particular shape of the channels are within the skill of the ordinary artisan to select and primarily depend on the particular heat exchanger design and specific operating conditions. Also, the depth of the condensate flow channels 64 may increase towards the bottom portion of the condensate collecting screen 60 to accommodate large condensate amounts drained under the force of gravity. [0027] Although the heat exchanger 10 shown in the exemplary embodiment depicted in FIGs. 1 and 2 is illustrated as a round tube and plate fin heat exchanger, it is to be understood that the condensate collecting screen 60 may be used in connection with any type of evaporator heat exchangers used in refrigerant vapor compression systems. For example, as depicted in FIG. 5, instead of round heat exchange tubes, the evaporator heat exchanger 10 could have multi-channel, flattened tubes 140, for example of rectangular or oval cross-section, extending longitudinally in parallel relationship between a pair of spaced headers or manifolds 150 and 160 for distributing refrigerant from the refrigerant cycle amongst the heat exchange tubes 140 and collecting refrigerant from the tubes 140 for return to the refrigerant cycle. For example, each flattened multi-channel heat exchange tube 140 might have 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 defines a plurality of parallel flow-channels 142, that can 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 multichannel heat exchange tubes extending in parallel relationship between the inlet and outlet headers of the heat exchanger will have a relatively large number of small flow area refrigerant flow paths extending between the two headers. Quite often, such multi-channel heat exchanger constructions are called microchannel or minichannel heat exchangers as well. As mentioned above, in case of flattened tube multi-channel 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. Naturally, for single-pass evaporator arrangements, there are only inlet and outlet manifolds present. Further, heat transfer fins 150 may be of different serpentine design here, may have louvers or offset strips, and typically define rectangular, triangular, trapezoidal, etc. airflow passages within the evaporator heat exchanger. [0028] While the condensate collecting screen 60 has been particularly shown and described with reference to the exemplary embodiments of the heat exchangers as illustrated in the drawings, it is to be understood that the condensate collecting screen is not limited in application to heat exchangers having horizontally disposed heat exchange tubes, but may be used in connection with heat exchangers having heat exchange tubes disposed so as to extend longitudinally in a vertical direction or other non-horizontal (inclined) direction. Thus, although heat exchanger drainage characteristics depend on heat exchanger design and orientation, at certain airflow velocities, condensate blow-off phenomenon can be experienced for any heat exchanger configuration in a vast majority of operating conditions that the present invention would address. Also, as mentioned above, 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, take advantage and equally benefit from

the invention. Further, it is to be understood that one skilled in the art may make various changes in detail in the present invention without departing from the spirit and scope of the invention as defined by the claims.