FIELD OF THE INVENTION
This invention relates generally to cooling systems, and more particularly to heat exchangers usable in such cooling systems.
BACKGROUND OF THE INVENTION
Typical refrigeration systems often utilize conventional fin-and-tube heat exchanger coils to dissipate heat from refrigerant passing through the heat exchanger coils. Usually, in large-scale cooling systems, a single, oftentimes large, conventional fin-and-tube heat exchanger coil 100, as depicted in prior art FIG. 3, is sized to dissipate or reject an amount of heat equal to the heat load of the refrigeration system. Multiple conventional fin-and-tube heat exchanger coils 100 might also be used. The fin-and-tube heat exchanger coil(s) is/are sized to dissipate the amount of heat in the refrigerant that was absorbed in other portions of the refrigeration system.
Usually, in large-scale cooling systems, the fin-and-tube heat exchanger coil(s) is/are positioned outside a commercial building, such as on a rooftop, to allow heat transfer between the fin-and-tube heat exchanger coil and the outside environment (i.e., to allow the heat in the refrigerant to dissipate into the outside environment). Further, natural or ram airflow may be augmented by a mechanical airflow that may be provided by a fan, for example, to assist in air-cooling the fin-and-tube heat exchanger coil.
Fin-and-tube heat exchanger coils, depicted generally at 100, in prior art FIG. 3, often display less than ideal efficiencies and relatively high cost in dissipating heat from the refrigerant passing through the coils as compared to newer technologies. Typically, fin-and-tube heat exchanger coils can be rather large for the amount of heat they can dissipate from the refrigerant as compared to newer technologies. Additionally, fin-and-tube heat exchanger coils use a great deal of copper in their construction. Presently, copper is a very expensive commodity. Further, the larger the heat exchanger coil becomes, the more refrigerant used in the refrigeration system, thus effectively increasing the risk of potential damage to the environment by an accidental atmospheric release. The efficiency of fin-and-tube heat exchanger coils is however not very dependent on the direction of the air flow relative to the coils the coils. This can be seen in prior art FIG. 3, the arrows indicating air flow over the various tubes 102.
A more recent form of heat exchanger is the microchannel coil (MCC) heat exchanger. Microchannel coil (MCC) heat exchangers are typically made of aluminum, replacing the costlier copper of the fin-and-tube heat exchanger coils. Further, in similar heat exchange applications, MCC heat exchangers can be made significantly smaller than fin-and-tube heat exchanger coils that effect similar heat exchanges. To date, however, microchannel coil (MCC) heat exchangers are known to be quite sensitive to the direction of airflow relative to the plane of the MCC heat exchanger. Efficiency drops off dramatically as the direction of airflow varies from the normal relative to the plane of the MCC heat exchanger.
Currently, the major application of microchannel coils is in the automotive industry. Microchannel coils 110 may be used as a condenser and/or an evaporator in the air conditioning system of an automobile. See prior art FIGS. 1 and 2. A microchannel heat exchanger coil, for example, in an automotive air conditioning system, is typically located toward the front of the engine compartment, where space to mount the heat exchanger coil is limited and where the direction of airflow is normal. Therefore, the microchannel heat exchanger coil, which is much smaller, lighter, and less costly than a conventional fin-and-tube heat exchanger coil that would otherwise be used in the automotive air conditioning system, is a suitable fit for use in an automobile.
Referring to FIGS. 2 and 7, the prior art MCC tube heat exchanger 110 includes an inlet header 116 and a spaced apart outlet header 118. Each of the headers 116, 118 has a fluid passageway 121 defied therein. The respective fluid passageways 121 are in fluid communication by means of the microchannel tubes 112 that extend between the headers 116,118. The headers 116,118 each have a known depth dimension 119. A heat exchanger plane 125 includes the longitudinal axes 126 of the headers 116, 118 and can be thought of as the windward face of the MCC tube heat exchanger 110. The plane 125 is usually presented normal to the incoming air flow.
The MCC tube heat exchanger 110 includes a plurality of microchannel tubes 112. Each microchannel tube 112 has a length dimension 114 extending from header 116 to header 118, as depicted in prior art FIG. 7. The two edges 120, 122 are joined by two spaced apart, parallel relatively long sides 124 defining a chord 123 of the microchannel tube 112. In cross section, the edges 120, 122 and sides 124 of the microchannel tube 112 define a very thin rectangle with an interior fluid passage 113. The fluid coupling of the headers 116, 118 is by means of the plurality of microchannel tubes 112. In the prior art MCC tube heat exchanger 110, the chord 123 of each of the microchannel tubes 112 is disposed orthogonally with respect to the plane 125 of the MCC tube heat exchanger and the length of the chord 123 is therefore limited to a maximum equal to the depth 119 of the respective headers 116, 118.
The prior art MCC tube heat exchanger 110 further includes fins 130, as depicted in prior art FIGS. 8 and 9. The fins 130 are typically formed of a single metallic ribbon that is compressed at series of bends 131, the bends 131 being formed in alternating directions. The ribbon of the fins 130 is affixed at the alternating bends 131 to respective adjacent microchannel tubes 112 in a heat conducting joint. The fins 130 include heat exchange surfaces 132. The plane of each of the heat exchange surfaces 132 is, in the prior art, usually disposed generally orthogonal with respect to a respective side 124 of an adjacent microchannel tube 112, adjacent heat exchange surfaces 132 being disposed in a parallel array. The height dimension 134 of the heat exchange surfaces 132 is absolutely limited by the distance 134 between adjacent microchannel tubes 112.
The plane of adjacent fins 130 of some prior art MCC heat exchangers 110 is angled with respect to one another. The ribbon forming the fins 130 has very sharp bends. See U.S. Pat. No. 6,988,538. Such alternate angling reduces the number of heat transferring heat exchange surfaces 132 that can be included in a given length 114 of the microchannel tube 112 and the sharp bends 131 provide for only a minimal heat conducting joint with the respective microchannel tube 112. For these reasons the alternating angling disposition is not favored as being less efficient than the parallel array disposition of prior art FIGS. 8 and 9.
For most efficient heat exchange in the prior art, the flow of air through the MCC tube heat exchanger 110 is normal to the plane 125 of the heat exchanger 110, as depicted in prior art FIGS. 1 and 2. Efficiency of the known MCC tube heat exchanger 110 depends on the flow of air being substantially parallel with the chord 123 and across the two sides 124, as depicted in prior art FIG. 2, and past the orthogonally disposed fins 130. For this reason, the most efficient of all known uses of the MCC technology has been with normal air flow relative to the plane of the MCC heat exchanger, where the leading edge 120 of each microchannel tube 112 is presented to the air flow and the air flow proceeds down both sides 124 to the trailing edge 122.
Angling the known MCC tube heat exchanger 110 to the direction of airflow results in known and calculable reductions of efficiency, as compared to normal airflow with the same MCC tube heat exchanger 110. Such angling is noted in Prior art FIGS. 4-6. Angling of the MCC tube heat exchanger 110 reduces the footprint of the heat exchanger unit, which in turn reduces cost. However, such angling disadvantageously sacrifices efficiency of the heat exchanger unit because the MCC tube heat exchanger 110 is angled with respect to the fan 140 and airflow through the MCC tube heat exchanger 110 is then not normal, but is turned. A resulting issue with heat exchanger units that are mounted on a rooftop is that the airflow is typically turned 90 degrees as it flows through the heat exchanger. This results from a horizontally mounted fan 140 drawing in a generally horizontal flow of air 142 and expelling the airflow 142 in a generally vertical direction. Such flow path change results in reduced efficiency of the MCC tube heat exchanger 110. For example a representative reference local loss coefficient of the orientation depicted in prior art FIG. 4 is 0.83. The coefficient is directly applied to a local mass flow, resulting in a significantly diminished mass flow.
The above reduction of mass flow has led engineers to angle known MCC heat exchangers 110 to the relative airflow, such as the 60 degree angle of prior art FIGS. 5 and 6 in which the direction of air flow 130 is not normal to the plane 125 of the MCC tube heat exchanger 110. Such angling advantageously results in a greater height dimension of the MCC heat exchangers 110 relative to the overall height of the heat exchanger unit. Further, angling the MCC heat exchangers 110 results in a reduced footprint of the heat exchanger unit, thereby reducing cost. While cost is reduced, efficiency is reduced as a result of the MCC heat exchangers 110 being angled with respect to the fan 140. In this case, the exemplary reference local loss coefficient of the angled orientation is 0.38, a considerable reduction as compared to the orientation of FIG. 4, but still a significant source of energy loss. Even this reduced loss generates a significant loss in airflow velocity and loss in efficiency of the MCC tube heat exchanger 110.
There is a need in the industry for more efficient MCC tube heat exchangers in applications in which the flow of air is not normal to the plane of the heat exchanger. In this situation, the direction of airflow is altered from the intake side of the MCC tube heat exchanger to the exhaust side by as much as ninety degrees. As noted above, with known MCC tube heat exchangers, such non-normal air flow significantly diminishes the efficiency of the MCC tube heat exchanger as compared to normal air flow on both the intake and exhaust sides of the MCC tube heat exchanger.
SUMMARY OF THE INVENTION
The present invention substantially meets the aforementioned needs of the industry by providing a high efficiency MCC tube heat exchanger for use with air flows that are not normal to the MCC heat exchanger. The present invention provides, in one aspect, a heat exchanger assembly adapted to efficiently condense a refrigerant in a refrigeration system where the flow of air to the MCC tube heat exchanger is not normal. The MCC tube heat exchanger assembly includes at least one microchannel heat exchanger coil including an inlet header and an outlet header, each microchannel of the coil being angled with respect to the plane of the MCC heat exchanger.
The present invention provides, in a further aspect, a method of assembling a MCC tube heat exchanger assembly. The MCC tube heat exchanger assembly may be adapted to condense a refrigerant for use in a refrigeration system. The method includes forming a MCC tube heat exchanger assembly with angled microchannel tubes and/or fins that provide for increased efficiency of the MCC tube heat exchanger assembly when the MCC tube heat exchanger assembly is angled with respect to the direction of airflow, i.e. the air flow is not normal to the plane of the MCC tube heat exchanger assembly.
The present invention provides, in addition to microchannel tubes and fins oriented to non-normal air flow, a greater heat transfer area of the respective microchannels and fins for a given depth of the headers of the MCC heat exchanger. Efficiency of the device of the present invention is therefore improved by two means. The first is angling the microchannel tubes and fins into the airflow and the second is the greater heat changer area of the microchannel tubes and fins presented to the air flow that is made possible by the angling of the microchannels and/or fins.
The present invention is a heat exchanger that includes a plurality of MCC microchannel tubes, each microchannel tube of the plurality of microchannel tubes having at least one microchannel fluid passage defined therein and having a chord, the chord being the orthogonal distance from a leading edge to a trailing edge, each microchannel tube of the plurality of microchannel tubes being disposed such that the chord is less than orthogonally disposed relative to a heat exchanger plane. The present invention is further a method of forming such a heat.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top plan view of a prior art MCC heat exchanger;
FIG. 2 is a sectioned perspective view of a prior art MCC heat exchanger;
FIG. 3 is a perspective view of a sectioned prior art fin and tube heat exchanger;
FIG. 4 is a partially sectioned perspective view of a prior art MCC tube heat exchanger;
FIG. 5 is a perspective view of a of a prior art MCC tube heat exchanger;
FIG. 6 is a side elevational schematic of a prior art heat exchanger assembly employing prior art MCC tube heat exchangers;
FIG. 7 is a perspective view of a prior art heat exchanger as employed in the heat exchanger assembly of FIG. 6;
FIG. 8 is an elevational view of the microchannel tubes and fins of the prior art MCC tube heat exchanger of FIG. 7;
FIG. 9 is a perspective view prior art fins of FIG. 8;
FIG. 10 includes three-side elevational schematic depictions of heat exchanger assemblies in v-shape, single heat exchanger, and w-shape configurations;
FIG. 11 is a side elevational schematic of a v-shaped heat exchanger assembly employing two MCC tube heat exchangers of the present invention;
FIG. 12 is a perspective view of a MCC tube heat exchanger of the present invention as employed in the heat exchanger assembly of FIG. 11;
FIG. 13 is a front elevational depiction of the microchannel tubes and fins of the present invention;
FIG. 14 is a perspective view of the fins of the present invention;
FIG. 15 is a perspective view of a portion of a MCC tube heat exchanger of the present invention employing both angled microchannel tubes and angled fins;
FIG. 15
a is a perspective view of a microchannel tube of the present invention having curved sides;
FIG. 15
b is a perspective view of a microchannel tube of the present invention having an airfoil shape; and
FIG. 16 is a perspective view of a portion of a MCC tube heat exchanger of the present invention employing both angled microchannels and angled fins.
DETAILED DESCRIPTION OF THE DRAWINGS
The heat exchanger assembly of the present invention is depicted generally at 10 in the figures. The heat exchanger assembly 10 may be used as a heat exchanger in a large-scale refrigeration system, such as that found in many commercial applications and multi-unit residences. In such a refrigeration system, the heat exchanger assembly 10 is frequently positioned outside the building, such as on the rooftop of the building, to allow heat transfer from the heat exchanger assembly 10 to the outside environment. The coils 14 of the heat exchanger assembly 10 may be advantageously disposed in a non-normal relationship relative to the incoming airflow. The usual role of the heat exchanger assembly 10 in the refrigeration system is to receive compressed, gaseous refrigerant from one or more compressors (not shown), condense the gaseous refrigerant back into its liquid form, and discharge the compressed, liquid refrigerant to one or more evaporators (not shown) located inside the store. The liquid refrigerant is evaporated when it is passed through the evaporators, and the gaseous refrigerant is drawn into the one or more compressors for reprocessing into the refrigeration system.
Refrigerants are typically given an R-XXX designation, such as “R-134a” or “R-22.” Additionally, other compounds, such as anhydrous ammonia, for example, may be used in such a refrigeration system to provide sufficient cooling to the refrigeration system. If any of the R-XXX designated refrigerants is used as the refrigerant of choice, the components of the refrigeration system in contact with the R-XXX may be made from copper, aluminum, or steel, among other materials. However, as understood by those skilled in the art, other refrigerants may not be compatible with some materials. If anyhydrous ammonia, for example, is used as the refrigerant of choice, copper components of the refrigeration system in contact with the anyhydrous ammonia may corrode. Alternatively, other refrigerants (including both two-phase and single-phase refrigerants or coolants) may be used with the heat exchanger assembly 10.
In addition to large refrigeration systems as noted above, the heat exchanger assembly 10 may also be used in various process industries, where the heat exchanger assembly 10 may be a portion of a fluid cooling system using a single-phase coolant (e.g., glycol). In such an application, the role of the heat exchanger assembly 10 in the fluid cooling system is to receive heated liquid coolant from one or more heat sources (e.g., a pump or an engine, for example), cool the heated liquid, and discharge the cooled liquid coolant to one or more heat sources. The cooled liquid coolant is again heated when it is put in thermal contact with the one or more heat sources, and the heated coolant is routed by the pump for re-processing into the fluid cooling system.
Further, the heat exchanger assembly 10 may be used with vehicles, including in applications using either two-phase or single-phase refrigerants or coolants. Such applications include, for example, air conditioning systems and the engine cooling system. The application of the heat exchanger assembly 10 is particularly desirable in low frontal profile vehicles where it may be desirable to angle the heat exchanger assembly 10 with respect to the incoming airstream, such angling being necessary to achieve sufficient cooling while maintaining a desired low frontal profile of the vehicle.
As depicted in FIGS. 10a and 11, the heat exchanger assembly 10 may include two microchannel heat exchanger coils 14a, 14b being supported by a frame 18. The frame 18 may be a freestanding structure. However, the frame 18 may comprise any number of different designs other than that shown. As such, the illustrated frame 18 is intended for illustrative purposes only.
FIGS. 10
a, 10b, and 10c depict exemplary embodiments of the heat exchanger assembly 10 of the present invention. Such embodiments are typical of rooftop installations where the heat exchanger assembly 10 is employed as a condenser. FIG. 10a is a V configuration employing two heat exchanger assemblies 10. FIG. 10b is configuration employing only a single heat exchanger assembly 10. FIG. 10c is W configuration employing four heat exchanger assemblies 10 and having the fans 15a, 15b set at an angled disposition as compared to the horizontal disposition of the fans 15 of FIGS. 10a, 10b.
As shown in FIGS. 11 and 12, each microchannel heat exchanger coil 14a, 14b includes an inlet manifold or header 22 and an outlet manifold or header 26. The headers 22 and 26 are fluidly connected by a plurality of microchannel tubes 30. One or more baffles (not shown) may be placed in the manifolds 22, 26 to cause the refrigerant to make multiple passes through the microchannel tubes 30 for enhanced cooling of the refrigerant. Each of the headers 22, 26 has a known depth dimension 24. A heat exchanger plane 40 is defined including the longitudinal axes 44 of the respective headers 22, 26 and an orthogonal line 46 extending between the respective axes 44.
Each of the microchannel tubes 30 extends between a respective header 22 and respective header 26. As depicted in FIG. 12, each microchannel tube 30 has a length dimension 32 defining the distance between the two headers 22, 26 to which the respective microchannel tube 30 are coupled. Each microchannel tube 30 has a leading edge 34 and a trailing edge 36. A pair of opposed sides 38 extend between leading edge 34 and the trailing edge 36. The area sides 38 of the microchannel tube 30 comprise the bulk of the heat transfer of the microchannel tube 30. An interior fluid passage 31, as depicted in FIG. 16, is defined in each of the microchannel tubes 30. The fluid passage 31 is in fluid communication with the fluid passage 25 of the respective headers 22, 26. Each of the microchannel tubes 30 further has a height dimension 33 and a chord dimension 32. The height dimension 33 extends between the respective outer margins of the two sides 38. The chord 35 extends orthogonally between the leading edge 34 and the trailing edge 36 and is one dimension in the determination of the area sides 38. The longer the chord 35, the greater the heat exchange area of the sides 38. Advantageously, the microchannel tubes 30 of the present invention are angled at an angle of less than 90 degrees with respect to the plane 40 of the MCC tube heat exchanger 14 as depicted in FIGS. 11, 12, 15 and 16 and preferably between 10 and 60 degrees. Such angling increases the length dimension of the chord 35 as compared to the chord 123 of the prior art. The advantage of this angling where the angle of the incoming airflow is less than normal with respect to the MCC tube heat exchanger 14 is twofold and is noted below.
Referring to FIGS. 15, 15a, 15b, and 16, the microchannel tubes 30 may be formed to include a single internal passageway 31 or may be formed to include multiple internal passageways, or microchannels 42, that are much smaller in size than the internal passageway of the coil in a conventional fin-and-tube heat exchanger coil. The microchannels 42 may be round in cross section, as depicted in FIG. 15b, or rectangular, as depicted in FIG. 16. Other shapes may as well be used, as depicted in FIG. 15a where the sides 38 are curved. The microchannels 42 allow for more efficient heat transfer between the airflow passing over the microchannel tubes 30 and the refrigerant carried within the microchannels 42, as compared to the airflow passing over the coil of the conventional fin-and-tube heat exchanger coil.
The microchannel tubes 30 may be separated into about twenty or less microchannels 42, with each microchannel 42 being about 0.5-2.0 mm in height and about 0.5-2.0 mm in width, compared to a diameter of about 9.5 mm (⅜″) to 12.7 mm (½″) for the internal passageway of a coil in a conventional fin-and-tube heat exchanger coil. However, in other constructions of the flat microchannel tubes 30, the microchannels 42 may be as small as 0.4 mm by 0.4 mm, or as large as 4 mm by 4 mm.
Referring to FIG. 15a, in cross section, the microchannel tubes 30 may be curved to further enhance the passage of airflow 16 through the microchannel heat exchanger coil 14 where the direction of the airflow is changing in the passage. In cross section, the microchannel 30 has curved sides 38 preferably having the same radius so that the distance between the sides 38 is constant across the chord 32. Preferably, the edges 34, 36 are parallel.
Referring to FIG. 15b, in cross section, the microchannel tubes 30 may be airfoil shaped to even further enhance the passage of airflow 16 through the microchannel heat exchanger coil 14 where the direction of the airflow is changing in the passage. In cross section, the microchannel 30 has curved sides 38 preferably having the different radii to achieve the traditional air foil shape with a radiused leading edge 34 and tapering to a juncture of the sides 38 at the trailing edge 36. Preferably, the edges 34, 36 are parallel. The chord extends from the forwardmost point on the radiused leading edge 34 to the trailing edge 36.
The microchannel tubes 30 may be made from extruded aluminum to enhance the heat transfer capabilities of the flat tubes 30. In the illustrated construction, the flat microchannel tubes 30 are about 22 mm wide. However, in other constructions, the flat microchannel tubes 30 may be as wide as 50 mm, or as narrow as 10 mm. Further, the spacing between adjacent flat microchannel tubes 30 may be about 9.5 mm. In other constructions, the spacing between adjacent flat microchannel tubes 30 may be as much as 20 mm, or as little as 3 mm.
In distinction with respect to the prior art, the microchannel tubes 30 of the present invention are disposed at an angle of less than ninety degrees with respect to the plane 40 of the MCC tube heat exchanger 14, the plane 40 including the longitudinal axes 44 of the headers 22, 26 and an orthogonally disposed line 46 extending between axles 44. The microchannel tubes 30 are therefore disposed such that chords 32 are also disposed less than orthogonally with respect to the plane 40.
In distinction to the above prior art microchannel heat exchanger 110, the heat exchanger 10 of the present invention mounted as depicted in FIG. 11 at 60 degrees to the incoming airflow has an estimated loss coefficient that is less than 0.38, but greater than 0.05. This reduced loss coefficient provides for a more efficient heat exchanger assembly 10 where the airflow is changing direction as the airflow passes through the heat exchanger assembly 10. Further, the heat exchanger 10, by simply tilting the microchannel tubes 30 at 20 degrees from normal (as in the prior art) increases the area of the sides 38 by 6.4% as a result of the increased length of the chord 32. Such increased area increases the heat transfer ability of the microchannel tubes 30, thereby further contributing to increased efficiency of the present invention.
For further efficiency improvement of the present invention, the fin stock comprising the fins 50, as depicted in FIGS. 13 and 14 is tilted between 10 degrees and 45 degrees with respect to the respective adjacent microchannel tubes 30 between which the fins 50 are disposed. As noted in the background section above and depicted in prior art FIGS. 8 & 9, in many known applications of microchannel tube heat exchangers 110, including its birthplace, the automotive industry, the fin stock designs are generally straight, upright and parallel fin-to-fin. In such orientation, the plane of the radiator elements 132 is disposed orthogonal with respect to the plane of the sides 124 of the adjacent microchannel tubes 112 between which the fin 130 is disposed. In such disposition, the area of the radiator elements 132 is limited by the spacing dimension 134 between the adjacent microchannels 112.
The tilted fin stock arrangement of the present invention provides for a larger heat exchange surface 56 within the limited installation space defined between adjacent microchannel tubes 30. As depicted in FIGS. 13 & 14, for a microchannel spacing 58 that is equal to the microchannel spacing 134 in the prior art, the fins 50 angled at a preferred angle are of 20 degrees with respect to the plane of the side 38 of the microchannel 30 results in about a 15.6% increase in the surface area of the heat exchange surface 56 of the fin 50. Such increase in surface area significantly increases the efficiency of the tilted fins 50 with respect to the fins 130 of the prior art.
A further benefit of the tilted fins 50 of the present invention is that the bends 54 and the ribbon 52 comprising the fins 50 are formed of a flat section 60 formed between two smaller radius bends 62. The full area of the flat section 60 may be joined in a heat conductive joint to the sides 38 of the adjacent microchannel tubes 30. Such joint has a significantly greater area as compared to the joint defined between the bin 131 and the side 124 of the microchannel 112 in the prior art. A larger heat conducting joint results in greater transfer of heat from the microchannel 30 to the fin 50.
A depiction of FIGS. 15 & 16 shows both tilted microchannel tubes 30 and tilted fins 50. A user of a heat exchanger 10 as depicted in FIGS. 15 & 16 could expect a thermal performance improvement as follows:
Aerodynamic performance improved by 10-20%
Fin stock surface increased by 15.6%
Tube surface increased by 6.4%.
The total estimation of thermal improvement from both the tilted tube installation and the tilted fin stock installation is estimated to be greater than 12%.
The heat exchanger assemblies are described and shown for exemplary reasons only, and are not meant to limit the spirit and/or scope of the present invention.
Patent Application
20080277095
