HVAC condensers presently use fin and tube coils, primarily with copper tubes and aluminum fins. A significant weight reduction of the overall unit could be accomplished if the tubes were also formed of aluminum and then brazed or glued to the fins. Small sized brazed aluminum heat exchangers as

microchannel tubing are used in the automotive industry.

However, the application and the sizes are distinct.

Automobile radiators are not as concerned about efficiency as the HVAC industry is. Also, simply resizing an automotive heat exchanger does not provide an optimum solution.

In order to accomplish this, the design of an aluminum heat exchanger with microchannel tubing must be analyzed and optimized.

U. S. Patent 4,998,580 to Guntly et al. and U. S.

Patent 5,372,188 to Dudley et al. are directed to a condenser with a small diameter hydraulic flow path where hydraulic diameter is conventionally defined as four times the cross sectional area of the flow path divided by the wetted perimeter of the flow path. The Guntly et al. patent requires hydraulic diameters of about 0.07 inches and less while the Dudley et al. patent requires a hydraulic diameter in the range of 0.015 to 0.040 inches. This technology is used in the automotive industry and is not optimum for an air cooled chiller application.

Summary of the Invention The present invention is directed to solving the problem in the prior art systems.

It is an object, feature and advantage of the present invention to provide an aluminum heat exchanger with multiple parallel flow paths for use in a large chiller for air conditioning purposes. It is a further object, feature and an advantage of the present invention to significantly reduce the weight of a large chiller.

It is an object, feature and advantage of the present invention to provide a heat exchanger with multiple parallel flow paths having a hydraulic diameter greater than 0.07 inches and less than 0.30 inches. It is a further object, feature and advantage of the present invention to provide a hydraulic diameter in the range greater than 0.07 inches and less than or equal to 0.26 inches. It is yet a further object, feature and advantage of the present invention to provide a hydraulic diameter in the range greater than 0.07 inches and less than or equal to 0.14 inches. It is a still further object, feature and advantage of the present invention to provide a hydraulic diameter in the range of 0.14 inches less than or equal to 0.26 inches. Finally, in the preferred embodiments of the present invention the hydraulic diameter is either 0.07 inches or 0.14.

The present invention provides a heat exchanger.

The heat exchanger comprises a first coil assembly including an inlet manifold, an outlet manifold parallel to and spaced from the inlet manifold; and a plurality of tubes each operably connected to and linking the inlet and the outlet manifolds.

Each tube has a multiplicity of flow paths and a hydraulic diameter in the range of 0.05 : to HD < 0. 30.

The present invention also provides an air conditioning system including a compressor, a first heat exchanger, a fan motivating air across the first heat exchanger, an expansion device and a second heat exchanger serially linked into an air conditioning cycle by tubing. The first heat exchanger includes an inlet manifold, an outlet manifold, and a multiplicity of adjacent flow paths surrounded by a common tube wall and interconnecting the inlet manifold with the outlet manifold.

The present invention further provides a method of manufacturing an air cooled chiller. The method comprises the steps of: forming a first heat exchanger to include a multiplicity of adjacent flow paths wherein the flow paths are sized and shaped to a preferred hydraulic diameter within the range of 0.7 zu the hydraulic diameter is zu 0.30 inches where hydraulic diameter = 4 times the cross sectional area divided by the total wetted perimeter; providing a fan to move air across the multiplicity of adjacent flow paths; providing a compressor, a second heat exchanger, and an expansion device; and linking the compressor, the first heat exchanger, the expansion device, and the second heat exchanger serially into an air conditioning cycle by tubing.

The present invention still further provides a method of transferring heat in a heat exchanger. The method comprises the steps of: forming a first heat exchanger to include a multiplicity of adjacent flow paths wherein the flow paths are sized and shaped to a preferred hydraulic diameter HD within the range of 0.7 < HD < 0.30 inches where hydraulic diameter HD as defined as four times a cross sectional area divided by a total wetted perimeter; and transferring heat thru a wall enclosing said flow paths and to a fluid contained therein.

Brief Description of the Drawings Figure 1 is a block diagram of an air cooled chiller system in accordance with the present invention.

Figure 2 shows a first preferred embodiment of the present invention taken along lines 2-2 of Figure 1.

Figure 3 is an alternative embodiment of the multi- path tubes shown in Figure 2.

Figures 4a and 4b are diagrams of fins used in the heat exchanger shown in Figure 1.

Figure 5 is a block diagram of a multiple coil assembly configuration as a preferred embodiment of Figure 1.

Detailed Description of the Drawing Figure 1 shows an air conditioning system 10 including a compressor 12, a first heat exchanger 14 functioning as a condenser, an expansion device 16 such as an expansion valve, and a second heat exchanger 18 functioning as an evaporator. The compressor 12, the first heat exchanger 14, the expansion device 16, and the second heat exchanger 18 are serially linked in an air conditioning cycle by tubing 20. The first heat exchanger 14 functions as a condenser in releasing heat from the system, while the second heat exchanger 18 functions as an evaporator in cooling a fluid transported to and from the heat exchanger 18 by means of conduit 22. Such systems are generally well known and are sold by The Trane Company, a Division of American Standard Inc., under the registered trademarks CenTraVac and Series R.

The present invention is directed to an improved condenser 14. This improved condenser 14 is preferably formed of aluminum and has an inlet manifold 30 receiving hot gaseous refrigerant from the conduit 20 and the compressor 12. This hot gaseous refrigerant is distributed by the inlet manifold 30 to a plurality of tubes 32. These tubes 32 conduct the hot gaseous refrigerant from the inlet manifold 30 through the

tubes 32 to an outlet manifold 34. In the process, the hot gaseous refrigerant is condensed and returns to the conduit 20 as a liquid where it is modulated through the expansion device 16 to the second heat exchanger 18. The tubes 32 are preferably microchannel or parallel flow tubing. Microchannel tubing is shown by applicant's U. S. Patent 5,967,228 to Bergman et al. which is assigned to the assignee of the present invention and hereby incorporated by reference.

Air is moved over the tubes 32 by an air moving device 36 such as a fan either to or away from the fan 36 as indicated by arrow 38. To enhance heat transfer from the tubes 32, fins 40 are provided to enhance the heat transfer. These fins 40 will be subsequently described with reference to Figure 4.

The preferred embodiment of the tubes 32 is shown in Figure 2 and an alternative embodiment is shown in Figure 3.

The heat transfer tube 32 shown in Figure 2 includes a multiplicity of adjacent flow paths 40,42,44,46 and 48 throughout the length of the tube 32 and surrounded by a common tube wall 50. The adjacent flow paths 40 through 48 are separated by barrier walls 52,54,56 and 58 respectively.

In Figure 2, the flow paths 40 and 48 are of similar shape and cross sectional area and the flow paths 42, 44 and 46 are of similar shape and cross sectional area. The flow paths 40,42,44,46 and 48 are sized and shaped to form a preferred hydraulic diameter HD within the range of: 0.7 < HD < 0.30 inches.

Hydraulic diameter is conventionally calculated according to the following formula: Hydraulic Diameter (HD) = cross sectional area X 4 total wetted perimeter Empirical study shows that a 100 ton air cooled chiller should have a hydraulic diameter of at least 0.07 whereas a 240 ton air cooled chiller should have a hydraulic diameter of about 0.14 inches. Linear extrapolation shows that a 480 ton air cooled chiller should have a hydraulic diameter of about 0.26 inches. Thus, the preferred range of hydraulic diameters is 0.07 < HD < 0.30 with an intermediate range of 0.07 < HD < 0.26. An optimum range appears to be 0.07 < HD < 0.14, with preferred hydraulic diameter of 0.14.

In determining the hydraulic diameter, the total cross sectional area of the flow paths 40,42,44,46 and 48 is either measured or calculated, and the total wetted perimeter for those same flow paths is determined in a similar manner.

For the sake of expediency, exemplary calculations are performed for the alternative embodiment shown in Figure 3.

In this Figure 3, like reference numerals are used to denote like elements.

In the tube 32 shown in Figure 3, each of the multiplicity of flow paths has an identical size and shape 60.

The cross sectional area for these multiplicity of flow paths 60 can be determined by taking an individual flow path 60a, determining a height 62 and a width 64, and multiplying the height 62 and width 64 together to determine an area for a

single flow path 60a. The total cross sectional area for the tube 32 is determined by multiplying by the number of flow paths, in this case 5, by the cross-sectional area per flow path leading to the calculation that the total cross sectional area equals 5 times the height 62 time the width 64.

The wetted perimeter for any individual flow path 60 can be calculated as two heights (62) plus two widths (64).

Total wetted perimeter can be determined by multiplying the wetted perimeter for any particular flow path by the number of individual flow paths 60, in this case 5, to result in a total wetted perimeter of 5 times (2H plus 2W). This results in a hydraulic diameter according to the following formula: HD = 10 (HXW) X 4/20 (H+W) which reduces to: HD = 2H X W/ (H + W) Figure 4a shows a first fin embodiment where a corrugated fin 40a is used. Similarly, Figure 4b shows the use of a sinusoidal fin 40b.

Figure 5 is directed to a multiple coil assembly embodiment of the invention in contrast to Figure 1 which shows a single coil assembly 70. In practice, multiple coil assemblies 70,72,74 and 76 might be used. The arrangement shown in Figure 5 is described in applicant's previous U. S.

Patent 5,067,560 to Carey et al. which is assigned to the assignee of the present invention and hereby incorporated by

reference. The control of such a condenser is described in applicant's U. S. Patent 5,138,844 to Clanin et al. which is assigned to the assignee of the present invention and also incorporated by reference.

In Figure 5, the first coil assembly 70 is basically perpendicular to ground and a second coil assembly 76 is spaced from the first coil assembly 70 and is generally arranged in a parallel plane. A third coil assembly 72 is positioned between the first and second coil assembly 70,76 and lying in a plane which is not parallel to the planes of first and second coil assemblies 70,76. A fourth coil assembly 74 also lies between the first and second coil assembly 70,76 at a line in a plane which is not parallel to the planes of the first and second coil assembly 70,76. The fourth coil assembly 74 preferably is at a complimentary angle to the third coil assembly 72. The potential airflow paths are shown by arrows 80.

What has been described is a condenser for use in the large air cooled chiller. It will be apparent to a person of ordinary skill in the art that many alterations and modifications are readily apparent. Such modifications include varying the material from aluminum to other light weight materials having a good heat transfer coefficient as well as modifying the number and shape of the multiple flow paths within each tube. All such modifications and alterations are contemplated to fall within the spirit and scope of the following claims.