High capacity finned tube heat exchanger

[57] ABSTRACT

The fins of a finned tube heat exchanger extend in spaced parallel fashion across a plurality of fluid carrying tubes. The fins are bent into shallow angulated convolutions in the direction of flow of an external fluid passing between the fins and around the tubes. The fins carry a plurality of aligned openings within each convolution wall to increase heat transfer capacity with little increase in the fluid pressure drop of the external fluid flowing through the exchanger.

Claims, 3 Drawing Figures

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PATENTEBBMR12IBM

3^796^258

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FIC3

3,796,258

HIGH CAPACITY FINNED TUBE HEAT EXCHANGER

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to finned tube heat exchangers, and more particularly to a fin construction and configuration for increasing the heat transfer efficiency of the heat exchanger.

2. Description of the Prior Art

Finned tube heat exchangers consist essentially of a plurality of closely spaced sheet metal fins with a plurality of refrigerant carrying tubes passing through the fins generally at right angles thereto. A high degree of heat transfer occurs between the exterior fluid such as air which generally is in forced flow across the surface of the fins and at right angles to the axis of the tubes and the fluid within the tubes by way of the fins. Such heat exchangers employ various tube arrangements and fin shapes in an attempt to achieve maximum heat transfer. Basically, the fin surfaces are designed to break the boundary layer formed on the surface of the fin and to create turbulence in the air and thus increase heat transfer between the refrigerant within the tubes and the exterior fluid passing over the fin surfaces. Unfortunately, the heat transfer capacity of the heat exchanger increases at the expense in air pressure drop to the heat exchanger. One attempt at providing an increased heat transfer capacity by modification of the fin structure is set forth in U.S. Pat. No. 3,631,922, wherein openings are provided within the fin by striking a plurality of tangs outwardly from the fins from one side thereof and towards the adjacent fin, the tangs projecting through the air flow and creating a desired turbulence to air flow through the heat exchanger and at right angles to the multiple refrigerant carrying tubes. The presence of the tangs constitutes a lint trap and accelerates clogging of the coil. Further, the presence of the tangs actually inhibit passage of the air through the holes resulting from the struck away tangs and their presence, while substantially increasing the turbulence of the air stream passing therethrough for increased heat transfer capacity, materially increase the pressure drop of the air stream across the heat exchanger.

SUMMARY OF THE INVENTION

The improved fin structure and configuration of the fin and tube heat exchanger of the present invention has for its object a provision of a fin surface which has higher outside film coefficient with little increase in air pressure drop through the heat exchanger by diffusing and mixing the air as it flows through the heat exchanger. The fin is constructed such that the air flowing through the exchanger changes direction to remove more heat from the fin surface. The length of the flow path for the air flowing through the heat exchanger is increased to increase heat transfer between the fluids. It permits higher face velocities without condensate blow off problems where the heat exchanger is used within a dehumidifying application, while further providing a fin which is lighter in weight which acts to reduce the weight of the heat exchanger as a whole.

These objects are met in a preferred embodiment of a fin and tube heat exchanger in which a plurality of fluid carrying tubes pass through aligned holes within a plurality of closely spaced, generally parallel fins formed of conventional thin metal sheet stock. Specifically, the fins are bent into convoluted form in the flow direction of the external fluid and carry a plurality of openings or holes within respective convolution walls 5 which are in flow aligned to permit the air flow to change direction and to pass alternately along its flow path in contact with respective opposite surfaces of the same fin as it moves across and through the convolutions. Preferably, the fins are bent or folded in accor10 dion pleated fashion in terms of shallow, angulated convolutions with each convolution wall carrying a plurality of holes elongated in a direction at right angles to the flow path of the air stream. Where the fluid carrying tubes are arranged in rows in offset fashion, the air 15 passage holes within the fins lie intermediate of the tubes for each row to create air turbulence in line with the tube row.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary elevational view of a finned tube heat exchanger utilizing the improved fin construction of the present invention.

FIG. 2 is a plan view of a portion of one of the fins of the heat exchanger of FIG. 1.

FIG. 3 is an enlarged, sectional view of a portion of the fin illustrated in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT ³⁰ Referring to the drawings, there is illustrated in FIG. 1 a vertically oriented heat exchanger 10 such as that forming an evaporator coil or a condenser coil for an air conditioning system humidifier, or the like, in which case, a plurality of elongated refrigerant carrying tubes ³⁵ 12 are connected in serpentine fashion by means of return bend tube portions 14 into a vertically oriented assembly and held in place by frame members at the top and bottom such as frame member 16 and constituting a relatively rigid structural array. Only a fragmentary ⁴⁰ portion of the heat exchanger is illustrated, the heat exchanger 10 being coupled by conventional means to a source of refrigerant which passes through the refrigerant carrying tubes in series or parallel flow paths to effect heat exchange with a secondary external fluid ⁴⁵ which in this case is air, passing at right angles to the axis of the tubes 12 and to the plane of the paper illustrating the same, FIG. 1. In this respect, the frame member 16 may be provided with annular flanges 18 ₅θ through which the individual tubes pass and in the illus^υ trated portion of the heat exchanger, the return bends may be integral with or joined to straight tubes 12 approximately at the location of the flared flanges 18.

_g5 The invention is particularly directed to the employment of a plurality of elongated, thin sheet metal fins 20 which extend longitudinally from one end of the heat exchanger 10 to the other in close, vertically spaced fashion to define a vertical array and through ₆₀ which the refrigerant carrying tubes 12 project to effect high efficiency heat transfer between the refrigerant within tubes 12 and forced air passing at right angles thereto, through the spaces between the fins 20. Each fin is formed of a generally rectangular strip of a high ₆₅ heat conducting metal by being stamped, for instance, from aluminum sheet stock or the like. Important to the completion of a low impedance heat conducting path between the refrigerant carrying tubes, which also may

3,796,258 be formed of aluminum, and the fins 20, is the provision of circular holes 22 for each tube, the holes 22 being created by punching material from the desired location to permit forming, as illustrated in FIG. 2, a number of rows of tubes as A, B, C and D to carry the 5 fins, the holes 22 of the rows of openings being alternately offset both laterally and longitudinally in the illustrated embodiment to achieve maximum heat transfer between the air flowing in the direction of arrows 24, FIG. 2, at right angles to the axis of the holes, and 10 the refrigerant or other confined fluid within the tubes 12. Further, in order to effect the conductivity between the fins and the tubes, which are received thereby, in the illustrated embodiment, preferably each tube receiving hole 22, is formed by an annular integral ring 15 26 formed during stamping, including a flange portion 28 at bend point 30 with the central tubular portion 32 receiving its tube 12 and forming a low impedance, heat conducting path between the ring 26 and the fin carrying the same. These tubular portions and rings are 20 formed by extruding the base sheet metal (fin stock). Intimate contact is then provided between the fin and the tube by expanding the tube about twenty thousandths inch (0.020 inch). The free edge 36 of the tubular portion 32 of ring 26 is flared outwardly to permit 25 the fins to readily receive the tubes and be stacked on the tubes during assembly of the heat exchanger.

An important aspect of the present invention resides in the provision of accordion pleats or angulated convolutions to the individual fins 20 by bending the fins 30 along fold lines 38, at right angles to the air flow path, indicated by arrow 24, to effect shallow angulated convolutions in which the adjacent convolution walls 40 extend obliquely in opposite directions as best seen in FIG. 3. Further, as illustrated in FIG. 3, convolution 35 walls 40 are provided with air passage openings or holes 42 intermediate of the fold lines which are generally in flow direction alignment and permit the air to pass from one side of the fin to the other at each convolution for increasing turbulence and heat transfer with- 40 out materially increasing the pressure drop for the air stream passing through the heat exchanger 10. As illustrated in FIG. 2, the air passage openings or holes 42 within the fins are elongated in a direction at right angles to the air flow path and are illustrated as being flat- 45 tened in configuration, although of course the size and configuration as well as the number of holes for each convolution wall may readily vary depending upon the design parameters for the particular heat exchanger application. Further, as illustrated in FIG. 2, the air pas- 50 sage openings or holes 42 form a series intermediate of tube openings 22 for a given tube row at all of the four tube location rows A, B, C, or D. As such, air flow impinging against a heat exchange refrigerant carrying tube 20 passes about the tube on each side thereof and 55 prior to impacting the tube downstream therefrom and in line therewith, passes through the four aligned air passage holes or openings 42 and thrice changes position relative to the surfaces of the fins to break the boundary layer formed on the surface of the fin and 60 create turbulence in the air passing through the heat exchanger. This causes the air to diffuse and mix for maximum heat transfer with the refrigerant carried within the individual tubes 20. While the fin structure preferably carries the rings 36 surrounding the tubes 20 65 and effects excellent heat transfer between the tubes and the fins 20, such members may be eliminated, in which case the edge of the fin defining the opening 20 in each case directly contacts the tube walls to achieve a high thermal conductivity joint therebetween.

From the above construction, it is noted that a substantial portion of the fin surface between adjacent tubes 20 carry the punched holes 42 to cause the air flowing through the exchanger to follow a long route relative to the surface of the fins over which the air passes and thereby increase the heat transfer therebetween. Where the heat exchanger of the present invention is employed in a dehumidifying function, the water removed from the air has a tendency to adhere to the fin surfaces and may accumulate within the punched holes 42. The punched holes 42 prevent the water blow out and thus the air passing therethrough may be driven at high face velocities without condensate blow off problems. Where the heat exchanger is employed in an environment in which weight is a material factor, since a substantial portion of the fin material is punched out to form the holes 42, simultaneously with bending the fins about fold lines 38 in accordion pleated fashion, the net result is the creation of a heat exchanger whose overall weight is reduced and permitting it to be used under high heat exchange efficiency in vehicle environments and other environments where weight is an important factor.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.