WO2016036732A1

WO2016036732A1 - Frost tolerant microchannel heat exchanger for heat pump and refrigeration applications

Info

Publication number: WO2016036732A1
Application number: PCT/US2015/047925
Authority: WO
Inventors: Michael F. Taras
Original assignee: Carrier Corporation
Priority date: 2014-09-05
Filing date: 2015-09-01
Publication date: 2016-03-10

Abstract

A heat exchanger is provided including a fluidly coupled first heat exchanger slab and second heat exchanger slab. The first heat exchanger slab includes at least one first tube segment and the second heat exchanger slab includes at least one second tube segment. The second heat exchanger slab is arranged downstream from the first heat exchanger slab relative to airflow. A plurality of first fins having a first fin density extends from the first heat exchanger slab. A plurality of second fins having a second fin density extends from the second heat exchanger slab. The first fin density is different than the second fin density.

Description

FROST TOLERANT MICROCHANNEL HEAT EXCHANGER FOR HEAT PUMP AND REFRIGERATION APPLICATIONS

BACKGROUND

[0001] This invention relates generally to heat pump and refrigeration applications and, more particularly, to a microchannel heat exchanger configured for use in a heat pump or refrigeration system.

[0002] Heating, ventilation, air conditioning and refrigeration (HVAC&R) systems include heat exchangers to reject or accept heat between the refrigerant circulating within the system and surroundings. One type of heat exchanger that has become increasingly popular due to its compactness, structural rigidity, and superior performance, is a microchannel or minichannel heat exchanger. A microchannel heat exchanger includes two or more containment forms, such as tubes, through which a cooling or heating fluid (i.e. refrigerant or a glycol solution) is circulated. The tubes typically have a flattened cross-section and multiple parallel flow channels. Fins are typically arranged to extend between the tubes to assist in the transfer of thermal energy between the heating/cooling fluid and the surrounding environment. The fins have a corrugated pattern, incorporate louvers to boost heat transfer, and are typically secured to the tubes via brazing.

[0003] Conventional microchannel heat exchangers commonly have substantially identical fins throughout the heat exchanger core. In the heat pump and refrigeration applications, when the microchannel heat exchanger is utilized as an evaporator, moisture present in the airflow provided to the heat exchanger for cooling may condense and then freeze on the external heat exchanger surfaces. The ice or frost formed may block the flow of air through the heat exchanger, thereby reducing the efficiency and functionality of the heat exchanger and HVAC&R system. Microchannel heat exchangers tend to freeze faster than the round tube and plate fin heat exchangers and therefore require more frequent defrosts, reducing useful heat exchanger utilization time and overall performance. Consequently, it is desirable to construct the microchannel heat exchanger with improved frost tolerance and enhanced performance.

SUMMARY OF THE INVENTION

[0004] A heat exchanger is provided including a fluidly coupled first heat exchanger slab and second heat exchanger slab. The first heat exchanger slab includes at least one first tube segment and the second heat exchanger slab includes at least one second tube segment. The second heat exchanger slab is arranged downstream from the first heat exchanger slab relative to airflow. A plurality of first fins having a first fin density extends from the first heat exchanger slab. A plurality of second fins having a second fin density extends from the second heat exchanger slab. The first fin density is different than the second fin density.

[0005] In addition to one or more of the features described above, or as an alternative, in further embodiments the first fin density is lower than the second fin density.

[0006] In addition to one or more of the features described above, or as an alternative, in further embodiments the first fin density is between about 6 fins per inch and about 18 fins per inch.

[0007] In addition to one or more of the features described above, or as an alternative, in further embodiments the second fin density is between about 12 fins per inch and 23 fins per inch.

[0008] In addition to one or more of the features described above, or as an alternative, in further embodiments the second heat exchanger slab is arranged at an angle to the first heat exchanger slab.

[0009] In addition to one or more of the features described above, or as an alternative, in further embodiments the second heat exchanger slab is arranged at a 180° angle to the first heat exchanger slab.

[0010] In addition to one or more of the features described above, or as an alternative, in further embodiments the first heat exchanger slab includes a first tube bank and the second heat exchanger slab includes a second tube bank.

[0011] In addition to one or more of the features described above, or as an alternative, in further embodiments the first tube segments and the second tube segments are integrally formed.

[0012] In addition to one or more of the features described above, or as an alternative, in further embodiments the at least one first tube segment and the at least one second tube segment are microchannel tubes having a plurality of discrete flow channel formed therein.

[0013] In addition to one or more of the features described above, or as an alternative, in further embodiments at least one of the first tube segment and second tube segment includes a first heat exchanger tube and a second heat exchanger tube connected by a web extending therebetween.

[0014] A microchannel heat exchanger is provided including a first manifold, a second manifold separated from the first manifold, and a plurality of tube segments arranged in a spaced parallel relationship and fluidly coupling the first and second manifold. Each of the plurality of tube segments includes a bend defining a first section and a second section of each tube segment. The second section is arranged at an angle to the first section. A plurality of first fins having a first fin density extends from the first section of the tube segments. A plurality of second fins having a second fin density extends from the second section of the tube segments. The first fin density is different than the second fin density.

[0015] In addition to one or more of the features described above, or as an alternative, in further embodiments the first fin density is lower than the second fin density.

[0016] In addition to one or more of the features described above, or as an alternative, in further embodiments the first fin density is between about 6 fins per inch and about 18 fins per inch.

[0017] In addition to one or more of the features described above, or as an alternative, in further embodiments the second fin density is between about 12 fins per inch and 23 fins per inch.

[0018] In addition to one or more of the features described above, or as an alternative, in further embodiments the bend is formed about an axis perpendicular to a longitudinal axis of the plurality of tube segments.

[0019] In addition to one or more of the features described above, or as an alternative, in further embodiments the bend of each tube segment includes a ribbon fold.

[0020] In addition to one or more of the features described above, or as an alternative, in further embodiments each of the plurality of tube segments is a microchannel tube having a plurality of discrete flow channels formed therein.

[0021] In addition to one or more of the features described above, or as an alternative, in further embodiments each of the plurality of tube segments includes at least a first heat exchanger tube and a second heat exchanger tube connected by a web extending

therebetween.

[0022] In addition to one or more of the features described above, or as an alternative, in further embodiments the first section and the second section are substantially different in length.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: [0024] FIG. 1 is a schematic diagram of an example of a vapor refrigeration cycle of a refrigeration system;

[0025] FIG. 2 is a side view of a microchannel heat exchanger according to an embodiment of the invention prior to a bending operation;

[0026] FIG. 3 is a cross-sectional view of a tube segment of a microchannel heat exchanger according to an embodiment of the invention;

[0027] FIG. 4 is a cross-sectional view of a tube segment of a microchannel heat exchanger according to an embodiment of the invention;

[0028] FIG. 5 is a perspective view of a microchannel heat exchanger according to an embodiment of the invention;

[0029] FIG. 6 is a perspective view of a microchannel heat exchanger according to another embodiment of the invention;

[0030] FIG. 7 is a cross-sectional view of a microchannel heat exchanger according to yet an embodiment of the invention; and

[0031] FIG. 8 is a cross-sectional view of a microchannel heat exchanger according to yet an embodiment of the invention.

[0032] The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION

[0033] Referring now to FIG. 1, a vapor compression refrigerant cycle 20 of an air conditioning or refrigeration system is schematically illustrated. Exemplary air conditioning or refrigeration systems include, but are not limited to, split, packaged, chiller, rooftop, supermarket, and transport refrigeration systems for example. A refrigerant R is configured to circulate through the vapor compression cycle 20 such that the refrigerant R absorbs heat when evaporated at a low temperature and pressure and releases heat when condensed at a higher temperature and pressure. Within this cycle 20, the refrigerant R flows in a counterclockwise direction as indicated by the arrow. The compressor 22 receives refrigerant vapor from the evaporator 24 and compresses it to a higher temperature and pressure, with the relatively hot vapor then passing to the condenser 26 where it is cooled and condensed to a liquid state by a heat exchange relationship with a cooling medium (not shown) such as air. The liquid refrigerant R then passes from the condenser 26 to an expansion device 28, wherein the refrigerant R is expanded to a low temperature two-phase liquid/vapor state as it passes to the evaporator 24. The low pressure vapor then returns to the compressor 22 where the cycle is repeated. It has to be understood that the refrigeration cycle 20 depicted in FIG. 1 is a simplistic representation of an HVAC&R system, and many enhancements and features known in the art may be included in the schematic. In particular, the heat pump refrigerant cycle includes a four- way valve disposed downstream of the compressor with respect to the refrigerant flow that allows reversing the refrigerant flow direction throughout the refrigerant cycle to switch between the cooling and heating mode of operation for the environment to be conditioned.

[0034] Referring now to FIG. 2, an example of a heat exchanger 30 configured for use in the vapor compression system 20 is illustrated in more detail. The heat exchanger 30 may be used as either a condenser 24 or an evaporator 28 in the vapor compression system 20. The heat exchanger 30 includes at least a first manifold or header 32, a second manifold or header 34 spaced apart from the first manifold 32, and a plurality of tube segments 36 extending in a spaced, parallel relationship between and connecting the first manifold 32 and the second manifold 34. In the illustrated, non-limiting embodiments, the first header 32 and the second header 34 are oriented generally horizontally and the heat exchange tube segments 36 extend generally vertically between the two headers 32, 34. However, other

configurations, such as where the first and second headers 32, 34 are arranged substantially vertically are also within the scope of the invention.

[0035] Referring now to FIG. 3, an example of a cross-section of a heat exchange tube segment 36 is illustrated. The tube segment 36 includes a flattened microchannel heat exchange tube having a leading edge 40, a trailing edge 42, a first surface 44, and a second surface 46. The leading edge 40 of each heat exchanger tube 36 is upstream of its respective trailing edge 42 with respect to an airflow A passing through the heat exchanger 36. The interior flow passage of each heat exchange tube segment 36 may be divided by interior walls into a plurality of discrete flow channels 48 that extend over the length of the tubes 36 from an inlet end to an outlet end and establish fluid communication between the respective first and second manifolds 32, 34. The flow channels 48 may have a circular cross-section, a rectangular cross-section, a trapezoidal cross-section, a triangular cross-section, or another non-circular cross-section. The heat exchange tubes 36 including the discrete flow channels 48 may be formed using known techniques and materials, including, but not limited to, extruded or folded.

[0036] In one embodiment, illustrated in FIG. 4, each of the plurality of tube segments 36 includes at least a first heat exchange tube 50 and a second heat exchange tube 52 connected by a web 54 extending at least partially there between. The configuration of the plurality of second heat exchange tubes 40, such as the width thereof or the number and shape of discrete flow channels for example may be substantially identical to, or

alternatively, may be different from the plurality of first heat exchange tubes.

[0037] The heat exchanger 30 has a multi-pass configuration relative to airflow A. To achieve a multi-pass configuration, in one embodiment illustrated in FIG, 5, the heat exchanger 30 includes a fluidly coupled first tube bank 60 and second tube bank 62, where each tube bank 60, 62 includes at least one heat exchange tube segment 36. The second tube bank 62 is disposed behind the first tube bank 60 and is downstream with respect to the airflow A passing through the heat exchanger 30. The first and second tube banks 60, 62 may be fluidly coupled by piping, one or more manifolds (as shown), or another fluid joint for example. The first tube bank 60 forms a first slab 70 of the heat exchanger 30 relative to airflow A and the second tube bank 62 forms a second slab 72 of the heat exchanger 30 relative to airflow A.

[0038] in another embodiment, shown in FIG. 6, the multi-pass configuration is achieved by forming at least one bend 80 in each tube segment 36 of the heat exchanger 30. The bend 80 is formed about an axis extending substantially perpendicular to the longitudinal axis of the tube segments 36. In the illustrated embodiment, the bend 80 is a ribbon fold; however other types of bends are within the scope of the invention. . The bend 80 at least partially defines a first section 82 and a second section 84 of each of the plurality of tube segments 36, wherein in the bent configuration, the first section 82 forms a first slab 70 of the heat exchanger 30 relative to airflow A and the second section 84 forms a second slab 72 of the heat exchanger 30 relative to airflow A. In the illustrated, non-limiting embodiment, the bend 80 is formed at an approximate midpoint of the tube segments 36 between the opposing first and second manifolds 32, 34 such that the first and second sections 82, 84 are generally equal in size. However, other embodiments, such as shown in FIG. 8, where the first section 82 and the second section 84 are substantially different in length are within the scope of the invention.

[0039] As shown in the FIGS, the heat exchanger 30 can be formed such that the first slab 70 is positioned at an obtuse angle with respect to the second slab 72. Alternatively, or in addition, the heat exchanger 30 can also be formed such that the first slab 70 is arranged at either an acute angle or substantially parallel (FIG. 7) to the second slab 72. As a result of the coupling or bend 80 between the first and second slabs 70, 72, the heat exchanger 30 may be formed having a conventional A-coil or V-coil shape. Forming the heat exchanger 30 by bending the tube segments 36 results in a heat exchanger 30 having a reduced bending radius, such as when configured with a 180° bend for example. As a result, the heat exchanger 30 may be adapted to fit within the sizing envelopes defined by existing air conditioning and refrigeration systems.

[0040] The heat exchanger may have any of a variety of configurations such that refrigerant flows through at least a portion of the first slab 70 before passing through one or more tube segments 36 of the second slab 72. For example, the first slab 70 may function as a first pass relative to the airflow A, and the second slab 72 may be configured as a second pass relative to the airflow A. However, other multi-pass configurations, such as configurations having multiple passes within each slab 70, 72 are within the scope of the invention.

[0041] Referring again to FIGS. 2-5, a plurality of first fins 74 extend from the first slab 70 and a plurality of second fins 76 extend from the second section 72 of the heat exchanger. In embodiments where the heat exchanger 30 is formed by bending the plurality of tube segments 36, no fins are arranged within the bend portion 80 of each tube segments 36. The fins 74, 76 may be integrally formed with the tube segments 36 of each slab 70, 72, or alternatively, may be mounted, such as by brazing for example to a surface of the tube segments 36, One or both of the plurality of first fins 74 and second fins 76 may be formed of a fin material tightly folded in a ribbon- like serpentine fashion thereby providing a plurality of closely spaced fins that extend generally orthogonal to the flattened tube segments 36.

[0042] The plurality of fins 74 mounted to the first slab 70 of the heat exchanger 30 are substantially different from the plurality of fins 76 mounted to the second slab 72 of the heat exchanger 30. The fins 74 mounted to the first slab 70 of the heat exchanger 30 are configured with a lower fin density than the fins 76 mounted to the second slab 72 of the heat exchanger 30 relative to the airflow A. In one embodiment, the plurality of fins 74 mounted to the first tube bank 60 or first section 82 configured to form a first pass of the heat exchanger 30 relative to the airflow A has a fin density between about 6 fins per inch about 18 fins per inch, and preferably between 8 and 16 fins per inch. The plurality of fins 76 mounted to the second tube bank 62 or the second section 84 configured to form a second pass of the heat exchanger 30 relative to the airflow A has a fin density between about 12 fins per inch an about 23 fins per inch, and preferably between 16 and 21 fins per inch.

[0043] Heat exchange between the one or more fluids within the plurality of tube segments 36 and an air flow A, occurs through the exterior surfaces 46, 48 of the heat exchange tube segments 36, collectively forming a primary heat exchange surface, and also through the heat exchange surface of the fins 74, 76 which forms a secondary or extended heat exchange surface.

[0044] By positioning the fins 74 having a lower fin density on the first slab 70 of tube segments 36 relative to the airflow A, the heat transfer between airflow A and a fluid flowing through the discrete flow channels thereof is reduced. At the same time, this initial heat transfer between the fluid R and the airflow A causes a significant portion of the moisture within the airflow A to condense and collect on the front slab 70, particularly the fins 74 of the heat exchanger 30. However, because the amount of heat transfer in the front slab 70 is limited, the temperature of the airflow A does not drop appreciably, condensed moisture amount is limited and massive frost formation is delayed. Also, relatively wide spacing between the fins allows for lower performance degradation over time and prolonged intervals between defrosts. In addition, gravity and/or the pressure of the airflow A will cause condensation formed on the fins 74 to flow downward from the external surfaces of the heat exchanger 30. Further heat transfer then occurs as the airflow A passes over the second slab 72 of tube segments 36. Because a significant amount of the moisture has been removed from the airflow A prior to reaching the second slab 72, the heat transfer within the second slab 72 of the heat exchanger 30 may be increased, specifically by including fins 76 having a higher fin density than the fins 74 of the first slab 70.

[0045] Reducing the amount of heat transfer between an airflow A and a fluid R in a first slab 70 of a heat exchanger 30 allows the moisture condensed from the airflow A to be more easily removed. As a result, the formation of frost, and therefore a required number of defrost cycles required to maintain the operational efficiency of the heat exchanger 30 are reduced. Because the operational efficiency of the heat exchanger 30 is improved (due to a lower number of defrost cycles and increased heat transfer in the second slab), the size of the heat exchanger 30 required for a desired application may also be reduced. Alternatively, size of other components, such as a compressor may be reduced, which in turn would cause even higher evaporation temperature and further reduction of defrost cycles as well as the system performance boost.

[0046] While the present invention has been particularly shown and described with reference to the exemplary embodiments as illustrated in the drawing, it will be recognized by those skilled in the art that various modifications may be made without departing from the spirit and scope of the invention. Therefore, it is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as, but that the disclosure will include all embodiments falling within the scope of the appended claims. In particular, similar principals and ratios may be extended to the rooftops applications and vertical package units.

Claims

CLAIMS: What is claimed is:

1. A heat exchanger comprising:

a first heat exchanger slab having at least one first tube segment;

a second heat exchanger slab fluidly coupled to the first heat exchanger slab, the second heat exchanger slab being arranged downstream from the first heat exchanger slab relative to an airflow, the second heat exchanger slab including at least one second tube segment;

a plurality of first fins having a first fin density extending from the first heat exchanger slab, and

a plurality of second fins having a second fin density extending from the second heat exchanger slab, wherein the first fin density and the second fin density are different.

2. The heat exchanger according to claim 1 , wherein the first fin density is lower than the second fin density.

3. The heat exchanger according to claim 1, wherein the first fin density is between about 6 fins per inch and about 18 fins per inch.

4. The heat exchanger according to claim 1 , wherein the second fin density is between about 12 fins per inch and about 23 fins per inch.

5. The heat exchanger according to claim 1, wherein the second heat exchanger slab is arranged at an angle to the first heat exchanger slab.

6. The heat exchanger according to claim 5, wherein the angle between the second heat exchanger slab and the first heat exchanger slab is about 180 degrees.

7. The heat exchanger according to claim 1, wherein the first heat exchanger slab includes a first tube bank and the second heat exchanger slab includes a second tube bank.

8. The heat exchanger according to claim 1, wherein the at least one first tube segment and the at least one second tube segment are integrally formed.

9. The heat exchanger according to claim 1 , wherein the at least one first tube segment and the at least one second tube segment are microchannel tubes having a plurality of discrete flow channels formed therein.

10. The heat exchanger according to claim 9, wherein at least one of the first tube segment and the second tube segment includes a first heat exchanger tube and a second heat exchanger tube connected by a web extending there between.

11. A heat exchanger comprising:

a first manifold; a second manifold separated from the first manifold;

a plurality of tube segments arranged in spaced parallel relationship and fluidly coupling the first manifold and the second manifold, the plurality of tube segments including a bend defining a first section and a second section of each tube segments, the second section being arranged at an angle to the first section;

a plurality of first fins having a first fin density extending from the first section of the tube segments, and

a plurality of second fins having a second fin density extending from the second section of the tube segments, wherein the first fin density and the second fin density are different.

12. The heat exchanger according to claim 11, wherein the first fin density is lower than the second fin density.

13. The heat exchanger according to claim 11, wherein the first fin density is between about 6 fins per inch and about 18 fins per inch.

14. The heat exchanger according to claim 11, wherein the second fin density is between about 12 fins per inch and about 23 fins per inch.

15. The heat exchanger according to claim 11, wherein the bend is formed about an axis arranged perpendicular to a longitudinal axis of the plurality of tube segments.

16. The heat exchanger according to claim 11, wherein the bend of each tube segment includes a ribbon fold.

17. The heat exchanger according to claim 11, wherein the angle between the second section and the first section is about 180 degrees.

18. The heat exchanger according to claim 11, wherein each of the plurality of tube segments is a microchannel tube having a plurality of discrete flow channels formed therein.

19. The heat exchanger according to claim 18, wherein each of the plurality of tube segments includes at least a first heat exchanger tube and a second heat exchanger tube connected by a web extending there between.

20. The heat exchanger according to claim 11 , wherein the first section and the second section are substantially different in length.