WO2008079132A1

WO2008079132A1 - Multi-channel heat exchanger with improved condensate drainage

Info

Publication number: WO2008079132A1
Application number: PCT/US2006/049290
Authority: WO
Inventors: Michael F. Taras; Alexander Lifson
Original assignee: Carrier Corporation
Priority date: 2006-12-26
Filing date: 2006-12-26
Publication date: 2008-07-03
Also published as: US20100012305A1; EP2097708A4; CN101600932A; EP2097708A1; CN101600932B

Abstract

A heat exchanger includes a first generally vertical header and a second generally vertical header and a generally vertical array of a plurality of generally flat heat exchange tubes extending in a horizontal direction therebetween. Each heat exchange tube has a plurality of channels extending longitudinally in parallel relationship from its inlet end to its outlet end, each channel defining a discrete refrigerant flow path. A plurality of fins extends between parallel-arrayed tubes. To facilitate drainage of the collected condensate from the external surfaces of the flat heat exchange tubes, the tubes are aligned at a slight angle with respect to the horizontal so that the trailing edge of each tube is positioned lower than the leading edge of each tube.

Description

MULTI-CHANNEL HEAT EXCHANGER WITH IMPROVED CONDENSATE DRAINAGE

Field of the Invention

[0001] This invention relates generally to refrigerant vapor compression system heat exchangers having a plurality of parallel, flat tubes extending between a first header and a second header with fins positioned between these tubes, and more particularly, to providing for improved drainage of condensate collecting on the external surfaces of the flat tubes and fins.

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 beverage products with 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 vapor compression cycle employed. 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 some refrigerant vapor compression systems, the evaporator is a parallel tube heat exchanger having a plurality of tubes extending longitudinally in parallel, spaced relationship between a first generally vertically extending header or manifold and a second generally vertically extending header or manifold, one of which serves as an inlet header/manifold. The inlet header receives the refrigerant flow from the refrigerant circuit and distributes the refrigerant flow amongst the plurality of parallel flow paths through the heat exchanger. The other header serves to collect the refrigerant flow as it leaves the respective flow paths and to direct the collected flow back to the refrigerant line for return to the compressor in a single pass heat exchanger or to a downstream bank of parallel heat exchange tubes in a multi-pass heat exchanger. In the latter case, this header is an intermediate manifold or a manifold chamber and serves as an inlet header to the next downstream bank of parallel heat transfer tubes.

[0005] Historically, such parallel tube heat exchangers used in refrigerant vapor compression systems have used round tubes, typically having a diameter of 1/2 inch, 3/8 inch or 7 millimeters. More recently, flat, typically rectangular or oval in cross-section, multi-channel tubes are being used in heat exchangers for refrigerant vapor compression systems. Each multi-channel tube generally has a plurality of flow channels extending longitudinally in parallel relationship the entire length of the tube, each channel providing a relatively small flow area refrigerant flow path. Thus, a heat exchanger with multi-channel 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. Sometimes, such multi-channel heat exchanger constructions are called microchannel or minichannel heat exchangers as well. [0006] Commonly, fins are positioned between heat transfer tubes for heat transfer enhancement, structural rigidity and heat exchanger design compactness. The heat transfer tubes and fins are permanently attached to each other (as well as to the manifolds) during furnace braze operation. The fins may have flat, wavy, corrugated or louvered design and typically form triangular, rectangular, offset or trapezoidal airflow passages.

[0007] When a heat exchanger is used as an evaporator in a refrigerant vapor compression system, moisture in the air flowing through the evaporator and over the external surface of the refrigerant conveying tubes and associated fins of the heat exchanger condenses out the air and collects on the external surface of the those tubes and fins. In general, condensate naturally drains well from refrigerant vapor compression system evaporators having round heat transfer tubes and plate fins due to the cylindrical outer surface of a round tube and vertically extended plate fins. FOT evaporator heat exchangers having the flat tubes and serpentine fins arranged in a vertical orientation extending between a pair of horizontally disposed headers, such as, for example, the heat pump evaporator/condenser heat exchanger disclosed in U.S. Patent 5,826,649, the condensate depositing on the heat transfer tubes and associated heat transfer fins inherently drains down the vertically extending tubes under the influence of gravity. The draining condensate is typically collected in a drain pan disposed beneath the heat exchanger.

[0008] U.S. Patent No. 5,279,360 discloses an evaporator heat exchanger having an array of parallel heat exchange tubes of flattened cross-section disposed in spaced relationship with V-shaped fins disposed between the facing flat surfaces of adjacent heat exchange tubes. Each heat exchange tube is bent into a V-shape and disposed in a vertical plane with its inlet end connected in fluid communication with a first horizontally extending header and its outlet end connected in fluid communication with a second horizontally extending header. The apexes of the arrayed V-shape-bent heat exchange tubes are aligned at a lower elevation than the headers, and a condensate trough is disposed therebeneath. Condensate collecting on the flattened heat exchange tubes and the fins therebetween drains downwardly along the respective fin-free edge surfaces of the flattened heat exchange tubes to the condensate trough.

[0009] However, with respect to prior art heat exchangers having tubes of flattened cross-section disposed horizontally and extending longitudinally in a horizontal direction between a pair of spaced, generally vertical headers, condensate collecting on the upper side of the tubes does not drain therefrom because of the horizontal disposition of the flat external surface of the tube. If the condensate collecting on the external surfaces of the heat exchanger tubes becomes excessive, overall performance of the refrigerant vapor compression system will be adversely impacted. For example, excessive condensate retention of 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. Also, condensate collecting on the external surfaces of the heat transfer tubes of the evaporator may be undesirability re-entrained in the air passing through the evaporator and transversely over the flattened tubes. Further, under certain conditions, excessive condensate retention promotes faster frost accumulation and undesirably requires more frequent defrost cycles.

Summary of the Invention

[0010] A heat exchanger having generally flattened heat exchange tubes extending longitudinally between a pair of spaced headers is provided wherein condensate collecting on the flat surfaces of the tubes from an air flow passing over the tubes inherently drains from the external flat surfaces of the flattened heat transfer tubes.

[0011] The heat exchanger includes first and second spaced apart and generally vertical longitudinally extending headers, and at least one heat exchange tube haying a generally flattened cross-section and defining at least one fluid flow path extending along a longitudinal axis thereof. The flattened heat exchange tube extends longitudinally in a horizontal direction between the first and second headers and has an inlet to the fluid flow path opening in fluid communication to the first header and an outlet to the fluid flow path opening in fluid communication to the second header. The flattened heat exchange tube has a transverse axis extending from its leading edge to its trailing edge, the leading edge being disposed upstream with respect to airflow of the trailing edge. The transverse axis of the flatted heat exchange tube is disposed at an acute angle with the horizontal with the leading edge preferably disposed vertically higher than the trailing edge. In one embodiment, the transverse axis of the flattened heat exchange tube is disposed at an acute angle with the horizontal in the range of from about 5 degrees to about 10 degrees. [0012] In an embodiment, the heat exchanger includes a plurality of flattened heat exchange tubes disposed in parallel, spaced relationship in a generally vertical array. Additionally, the heat exchanger may include a plurality of heat transfer fins extending between adjacent tubes of the parallel tube array. In an embodiment, the plurality of fins extends from a position aft of the leading edges of adjacent tubes of the parallel tube array to a position forward of the trailing edges of adjacent tubes of the tube array. In an embodiment, the plurality of fins extends from a position aft of the leading edges of adjacent tubes of the parallel tube array to a position aft of the trailing edges of adjacent tubes of the tube array and that portion of each of the fins extending aft of the trailing edges of adjacent tubes of the tube array may include a lip portion extending behind the trailing edge of tube of the parallel array of tubes lying subadjacent the fin. In an embodiment, the plurality of fins may comprise a plurality of generally vertical plate-like fins extending between adjacent tubes of said parallel tube array. Alternatively, corrugated serpentine fins may be disposed between the tubes. The fins may have a flat, wavy, offset strip or louvered design and form triangular, rectangular, or trapezoidal airflow passages. [00131 In an embodiment of the heat exchanger, each flattened heat exchange tube defines a plurality of parallel fluid flow paths extending parallel to a longitudinal axis thereof, with each fluid flow path of the plurality of parallel fluid flow paths having an inlet to the fluid flow path opening in fluid communication to the first header and an outlet to the fluid flow path opening in fluid communication to the second header. The plurality of the channels defining the flow paths within each heat transfer tube may be of circular, oval, rectangular, triangular or trapezoidal cross-section. In an embodiment, each of the fluid flow paths may comprise a refrigerant flow path.

Brief Description of the Drawings

[0014] 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: [0015] FIG. 1 is a schematic diagram of a refrigerant vapor compression system incorporating a heat exchanger as an evaporator; [0016] FIG. 2 is a perspective view of an exemplary embodiment of an evaporator heat exchanger in accordance with the invention;

[00171 FIG- 3 is a partially sectioned, elevation view taken along line 3-3 of

Fig. 2;

[0018] FIG. 4 is a partially sectioned, elevation view of another exemplary embodiment of an evaporator heat exchanger in accordance with the invention; and

[0019] FIG. 5 is a partially sectioned, elevation view of an alternate exemplary embodiment of an evaporator heat exchanger in accordance with the invention.

Detailed Description of the Invention

[0020] 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 cycles 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. [0021] The refrigerant vapor compression system 100 includes a compressor

105, a condenser 110, an expansion device 120, and the heat exchanger 10, functioning as an evaporator, connected in a closed loop refrigerant circuit by refrigerant lines 102, 104 and 106. The compressor 105 circulates hot, high pressure refrigerant vapor through discharge refrigerant line 102 into the inlet header of the condenser 110, and thence through the heat exchanger 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 passed over the heat exchange tubes by the condenser fan 115.

[0022] The high pressure, liquid refrigerant leaves the condenser 110 and thence passes through 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 thence passes through the heat exchanger 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, in many cases, 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, collects in the outlet header 30 of the evaporator heat exchanger 10 and passes therefrom through suction refrigerant line 106 to return to the compressor 105 through the suction port thereto. As the air flow traversing the evaporator heat exchanger 10 passes over the heat exchange tubes 40 and 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 surface of the refrigerant conveying tubes 40 and heat transfer fins 50 of the evaporator heat exchanger 10 condenses out the air and collects of the external surface of the tubes and fins. A drain pan 45 is provided beneath the evaporator heat exchanger 10 for collecting condensate that drains from the external surface of the tubes 40 and fins 50. [0023J The parallel flow heat exchanger 10 will be described herein in general with reference to the illustrative embodiments of the heat exchanger 10 depicted in FIGs. 2-4. The heat exchanger 10 includes a plurality of heat exchange tubes 40 arranged in a generally vertical array, each of which extends in a horizontal direction along its longitudinal axis between a generally vertically extending first header 20 and a generally vertically extending second header 30, thereby providing a plurality of refrigerant flow paths between the two headers. Although the refrigerant headers 20 and 30 are shown of a cylindrical configuration, the may be of a rectangular, half of cylinder or any other shape as well as have a single chamber or multi-chamber design, depending on the refrigerant path arrangement. Each heat exchange tube 40 has a first end mounted to the first header 20, a second end mounted to the second header 30, and a plurality of parallel flow channels 42 extending longitudinally, i.e. along the generally horizontally disposed longitudinal axis of the tube, the entire length of the tube, whereby the each of the individual flow channels 42 provides a flow path in refrigerant flow communication between the first header and the second header. The internal refrigerant pass arrangement may be a single-pass configuration or a multi-pass configuration, depending on particular application requirements.

[0024] Additionally, each multi-channel heat exchange tube 40 has a generally flattened cross-section, for example, a rectangular cross-section or oval cross-section, and defines an interior that may be subdivided to form a side-by-side array of independent flow channels 42. Each flattened multi-channel tube 40 may have a width as measured along a transverse axis extending from the leading edge 44 to the trailing edge 46 of, for example, fifty millimeters or less, typically from ten to thirty millimeters, and a height of about two millimeters or less, as compared to conventional prior art round tubes having a diameter of 1/2 inch, 3/8 inch or 7 mm. The tubes 40 are shown in the accompanying drawings, for ease and clarity of illustration, as having ten channels 42 defining flow paths having a circular cross- section. However, it is to be understood that in applications, each multi-channel tube 40 may typically have from about ten to about twenty flow channels 42. Generally, each flow channel 42 will have a hydraulic diameter, defined as four times ttie cross-sectional flow area divided by the "wetted" perimeter, in the range generally from about 200 microns to about 3 millimeters. Although depicted as having a circular cross-section in the drawings, the channels 42 may have a rectangular, triangular, oval or trapezoidal cross-section, or any other desired non- circular cross-section. Also, heat transfer tubes 40 may have other internal heat transfer enhancement elements, such as mixers and boundary layer destructors. [0025J As in conventional practice, to improve heat transfer between the air flowing through the heat exchanger 10 over the external surface of the heat transfer tubes 40 and the refrigerant flowing through the parallel flow channels 42 of 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 are brazed or otherwise securely 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 surface of the flat heat transfer tubes 40. Thus, the external surfaces of the heat transfer tubes 40 and the surfaces of the 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 air flow redirection to improve heat transfer characteristics. In the exemplary embodiment of the heat exchanger 10 depicted in Fig. 2, the fins 50 constitute a plurality of plates disposed in parallel, spaced relationship and extending generally vertically between the heat transfer tubes 40. However, it is to be understood that other fin configurations, such as, for example, generally corrugated serpentine wavy, offset or louvered fins forming triangular, rectangular, or trapezoidal airflow passages may be used instead of generally vertical fins in the evaporator heat exchanger of the invention.

[0026] To facilitate drainage of the collected condensate from the external surfaces of the flat heat exchange tubes 40, the tubes 40 are aligned with their transverse axes at an slight angle with respect to the horizontal so that the trailing edge 46 of each tube 40 is positioned lower than the leading edge 44 of each tube 40. The leading edge 44 is the edge of the heat exchange tube 40 disposed at the air flow inlet side of the heat exchanger 10 and the trailing edge 46 is the edge of the heat exchange tube 40 disposed at the air flow outlet side of the heat exchanger 10. Under the influence of gravity and assisted by the airflow sheer force, with the trailing edge 46 of each tube 40 in the generally vertical array of horizontally extending tubes 40 of the heat exchanger 10 being positioned lower than the leading edge 44, condensate collecting of the external generally flat surfaces of the tubes 40 will flow transversely along the width of each tube 40 in the direction of the air flow across the generally flat surfaces of the tubes to pass off the respective trailing edges 46 of tubes 40 and drain into the drain pan 45. Condensate depositing on the surface of each of the fins 50 will drain downwardly unto the upper external surface of the tube 40 subjacent the lower end of the fin and likewise flow to the trailing edge of the tube and drain therefrom into the drain pan 45. Thus, with respect to the evaporator heat exchanger 10, both gravity and the airflow passing over the external surface of the heat exchange tubes 40 serve to facilitate drainage of condensate deposited on the external surfaces of the tubes 40. In an embodiment, the transverse axis of the flattened heat exchange tubes 40 is disposed at an acute angle with the horizontal in the range of from about 5 degrees to about 10 degrees, facilitating condensate drainage, while not compromising the airflow pattern. [0027] In the exemplary embodiments of the evaporator heat exchanger 10 depicted in Fig.4 and 5, the trailing edges 56 of the tins 50 extend beyond the trailing edges 46 of the respective heat exchange tubes 40. In these embodiments, the condensate may simply drain off the trailing edge 46 of each heat exchange tube 40 to drip into the drain pan 45, or the condensate may flow along the lower surface of the portion of the trailing edge 56 extending beyond the trailing edges 46 of the heat exchange tubes 40 to drip into the drain pan 45. In the exemplary embodiment depicted in Fig. 5, the trailing edge 56 of each of the fins 50 includes a lower extension 58 that extends downwardly aft of the trailing edge 46 of the heat exchange tube 40 subadjacent that fin to the fin 50 positioned next below. In this embodiment, the lower extension 58 further facilitates drainage of condensate by providing a downwardly extending surface along which the condensate will flow to the fin next below and eventually drain from the extension 58 of the lower most fin 50 into the condensate drain pan 45. Additionally, a lip 59 may be provided extending outwardly from the lower extension 58 and beneath the trailing edge 46 of the subadjacent tube 40 to provide a surface for directing condensate draining off the trailing edge 46 of that tube 40.

[0028] While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, 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.

Claims

We Claim:

1. A heat exchanger for cooling a flow of air passed therethrough comprising: first and second spaced apart and generally vertical longitudinally extending headers; and at least one heat exchange tuhe having a flattened cross-section and defining at least one fluid flow path extending along a longitudinal axis thereof, said at least one heat flattened exchange tube extending longitudinally in a horizontal direction between said first and second headers and having an inlet to said fluid flow path opening in fluid communication to said first header and an outlet to said fluid flow path opening in fluid communication to the second header, said at least one flattened heat exchange tube having a transverse axis extending from a leading edge of said at least one flattened heat exchange tube to a trailing edge of said at least one flattened heat exchange tube, said leading edge disposed upstream with respect to air flow of said trailing edge, the transverse axis of said at least one flatted heat exchange tube disposed at an acute angle with the horizontal with said leading edge disposed vertically higher than said trailing edge.

2. A heat exchanger as recited in claim 1 wherein said at least one flattened heat exchange tube comprises a plurality of flattened heat exchange tubes disposed in parallel, spaced relationship in a generally vertical array.

3. A heat exchanger as recited in claim 2 further comprising a plurality of fins extending between adjacent tubes of said parallel tube array.

4. A heat exchanger as recited in claim 3 wherein said plurality of fins extends from a position aft of the leading edges of adjacent tubes of said parallel tube array to a position forward of the trailing edges of adjacent tubes of said tube array.

5. A heat exchanger as recited in claim 3 wherein said plurality of fins extends from a position aft of the leading edges of adjacent tubes of said parallel tube array to a position aft of the trailing edges of adjacent tubes of said tube array.

6. A heat exchanger as recited in claim 5 wherein the portion of each of said plurality of fins extending aft of the trailing edges of adjacent tubes of said tube array includes a lip portion extending behind the trailing edge of tube of said parallel array of tubes lying subadjacent said fin.

7. A heat exchanger as recited in claim 3 wherein said plurality of fins comprises a plurality of generally vertical plate-like fins extending between adjacent tubes of said parallel tube array.

8. A heat exchanger as recited in claim 3 wherein said plurality of fins comprises serpentine-like corrugated fins extending between adjacent tubes of said parallel tube array.

9. A heat exchanger as recited in claim 8 wherein said a serpentine-like corrugated fins extending between adjacent tubes of said parallel tube array are forming one of generally triangular, rectangular or trapezoidal airflow passages.

10. A heat exchanger as recited in claim 3 wherein said plurality of fins are at least one of louvered, wavy, offset strip or flat plate configurations.

11. A heat exchanger as recited in claim 1 wherein the transverse axis of said at least one flattened heat exchange tube is disposed at an acute angle with the horizontal in the range of from about 5 degrees to about 10 degrees.

12. A heat exchanger as recited in claim 11 wherein said at least one flattened heat exchange tube comprises a plurality of flattened heat exchange tubes disposed in parallel, spaced relationship in a generally vertical array.

13. A heat exchanger as recited in claim 11 further comprising a plurality of fins extending between adjacent tubes of said parallel tube array.

14. A heat exchanger as recited in claim 13 wherein said plurality of fins extends from a position aft of the leading edges of adjacent tubes of said parallel tube array to a position forward of the trailing edges of adjacent tubes of said tube array.

15. A heat exchanger as recited in claim 13 wherein said plurality of fins extends from a position aft of the leading edges of adjacent tubes of said parallel tube array to a position aft of the trailing edges of adjacent tubes of said tube array.

16. A heat exchanger as recited in claim 15 wherein the portion of each of said plurality of fins extending aft of the trailing edges of adjacent tubes of said tube array includes a lip portion extending behind the trailing edge of tube of said parallel array of tubes lying subadjacent said fin.

17. A heat exchanger as recited in claim 13 wherein said plurality of fins comprises a plurality of generally vertical plate-like fins extending between adjacent tubes of said parallel tube array

18. A heat exchanger as recited in claim 13 wherein said plurality of fins comprises serpentine-like corrugated fins extending between adjacent tubes of said parallel tube array.

19. A heat exchanger as recited in claim 18 wherein said a serpentine-like corrugated fins extending between adjacent tubes of said parallel tube array are forming one of generally triangular, rectangular or trapezoidal airflow passages.

20. A heat exchanger as recited in claim 13 wherein said plurality of fins are at least one of louvered, wavy, offset strip or flat plate configurations.

21. A heat exchanger as recited in claim 1 wherein said at least one flattened heat exchange tube defines at least one refrigerant flow path extending along a longitudinal axis thereof.

22. A heat exchanger as recited in claim 1 wherein said at least one flattened heat exchange tubes defines a plurality of parallel refrigerant flow paths extending parallel to a longitudinal axis thereof, each refrigerant flow path of said plurality of parallel refrigerant flow paths having an inlet to said refrigerant flow path opening in fluid communication to said first header and an outlet to said refrigerant flow path opening in fluid communication to the second header.

23. A heat exchanger as recited in claim 22 wherein said plurality of parallel fluid flow paths form at least one of rectangular, triangular, trapezoidal, circular or oval channels for refrigerant flowing herethrough.

24. A heat exchanger as Tβcited in claim 1 wherein said at least one flattened heat transfer tube has internal heat transfer enhancement elements.

25. A heat exchanger as recited in claim 1 wherein said at least one flattened heat transfer tube has one of rectangular or oval cross-section.

26. A heat exchanger as recited in claim 1 wherein said heat exchanger is a refrigerant system evaporator.

27. A heat exchanger as recited in claim 1 wherein said. heat exchanger has a single-pass configuration.

28. A heat exchanger as recited in claim 1 wherein said heat exchanger has a multi-pass configuration.