US 3804159 A
A heat exchanger in the form of a finned tube configuration for transferring heat between the contents of the tube and the atmosphere surrounding the finned tube. The tube may actually be made up in numerous multi-path configurations. The fins preferably extend outwardly from the tube and are attached to the tube in good heat-conducting relationship. The fins are perforated in a predetermined pattern to provide openings through which the outside atmosphere is directed as jets upon portions of the fins in such a fashion as to disrupt boundary layers normally existing adjacent the surfaces upon which the jets impinge.
Description (OCR text may contain errors)
United States Patent Searight et al.
[451 Apr. 16, 1974 JET IMPINGEMENT FIN COIL  Inventors: Edward F. Searight, Harvard; Pierre J. Brosens, Blemont, both of Mass.
 Assigneei Thermo Electron Corporation,
 Filed: June 13, 1972  Appl. No.: 262,235
 US. Cl.. 165/109, 165/181, 165/182  Int. Cl F28f 13/02, F28f 1/38  Field of Search 165/181, 182,183,109
 References Cited UNITED STATES PATENTS 3,033,536 5/1962 Guzzmann r 165/182 X 3,540,530 11/1970 Kritzer 165/181 X 3,450,199 6/1969 Warrell 165/154 X 1,983,549 12/1934 Krackowizer 165/182 X 3,509,867 5/1970 Brosens et a1 126/116 R 3,416,011 12/1968 Lyczko 165/154 X 3,205,147 9/1965 Foure et a1. 1.65/1 X 1939 Germany 165/181 1931 Great Britain 165/181  ABSTRACT A heat exchanger in the form of a finned tube configuration for transferring heat between the contents of the tube and the atmosphere surrounding the finned tube. The tube may actually be made up in numerous multi-path configurations. The fins preferably extend outwardly from the tube and are attached to the tube in good heat-conducting relationship. The fins are perforated in a predetermined pattern to provide openings through which the outside atmosphere is directed as jets upon portions of the fins in such a fashion as to disrupt boundary layers normally existing adjacent the surfaces upon which the jets impinge.
14 Claims, Drawing Figures JET IMPINGEMENT FIN COIL BACKGROUND OF THE INVENTION To enhance heat transfer from tubes, pipes or coils through which a heated or cooled fluid is passing, it is commonplace to utilize fins or other heat-conducting extensions which serve effectively to increase the surface area of the heating or cooling element in contact with the surrounding atmosphere. The use of fans or blowers to channel and drive the surrounding atmosphere over the finned elements is equally well-known.
Despite the use of various configurations and extensions to increase the surface area, even under forced draft conditions, the efficiency of heat transfer in finned tube structures remains surprisingly low.
One reason for the relatively inefficient heat transfer in structures such as those described is the presence of what isknown as a boundary layer of fluid. This boundary layer is a generally stationary thin layer of fluid which adheres to the heat-exchanging surfaces and acts as an insulating cushion. The existence of such boundary layers has not gone unnoticed and some efforts have been made to remove or disrupt it in order that heat transfer might be improved. Generally, such efforts as have been made, have been directed toward the creation of turbulence in the medium surrounding the heat-exchanging surfaces. One specific structure for achieving turbulence utilizes fins in which louvers are formed. The turbulence as fluid is forced through the louvers diminishes the boundary layer primarily along the fin in' which the louvers are formed, but not sufficiently to achieve maximum heat transfer. In such a structure the effectiveness increases as a function of increase in louver openings.
It is, therefore, the principal object of the present invention to maximize heat transfer between finned tube structures and the atmosphere surrounding such structures.
It is a further object of the present invention to lower the cost of finned tube heat transfer structures by reducing the amount of material required for efficient heat transfer.
It is a still further object of the present invention to apply the principles of jet impingement to finned tube heat transfer structures.
It is another object of the present invention to simsecond fluid and the fin surface, A, is the fin area per fin, n is the fin effectiveness, h, is the heat transfer resistance between the surface of the tube and the fin root, A, is the contact area per fin between the tube and the fin root, k is the thermal conductivity of the tube wall, t is the tube wall thickness, h, is the heat transfer coefficient between the first fluid and the inside tube wall, r, and r,, are the inside and outside radii of the tube, respectively, and T T T T, and T, are the temperatures of the fin root, the second fluid, the outer and inner tube surfaces, and the first fluid, respectively.
It is clear from the equation that a reduction in size and weight of the heat exchanger is possible by increasing the heat transfer coefficient, hf between the second fluid and finned surfaces. The precise effect of a change in h; is difficult to formulate because a change in h; shifts the temperature distributions among the various fin and coil elements. Also, as h; increases, the fin effectiveness, 1 decreases and some of the benefit of an increase in h; is lost. Nevertheless, the heat transfer coefficient between the second fluid and the fin (h is generally the controlling resistance in the system and, plainly, reduction of the resistance can result in substantial reductions in the size and weight of the heatexchanger. Causing the second fluid to impinge as jets upon the film disrupts and dissipates the film, increases the heat transfer coefficient (hf), and significantly increases the efficiency of heat transfer. The present invention operates in a particular advantageous manner when the present fluid is a condensing vapor or an evaporating liquid. The second fluid is frequently air.
For a better understanding of the present invention, its objects, features, and advantages, reference should be made to the following detailed description of a preferred embodiment of the invention, that embodiment being illustrated in the appended drawing in which:
plify the geometry of finned tube heat transfer structures.
'It is still another object of the present invention to reduce the size and weight of finned tube heat exchangers.
GENERAL DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION Considering a single tube to which fins are attached in good heat-conducting relationship, a first fluid being present within the tube at a given temperature and a second fluid passing over the fin, sensible heat flow, to or from the second fluid per fin, is given by:
PREFERRED EMBODIMENTS OF THE INVENTION In FIG. 1, a tube 12 preferably of copper or aluminum, which may form a part of a coil or battery of tubes passes through the convolutions of a formed or joined fin arrangement 14 which forms a series of heat transfer surfaces. The formed fin arrangement 14 may be formed or otherwise provided with flanges 16 along its solid fins length, the fin being in any case in good heat-' conducting relationship with the flanges 16. The flanges 16, in turn, are welded, brazed, mechanically expanded or otherwise attached to the tube 12 also in good heat-conducting relationship. As is indicated by the lower vertical arrows, ambient air is caused to flow over the finned tube accordion structure in a direction which is preferably substantially perpendicular to the plane defined by the lines along which the fins 14 are folded and must flow along a path which causes all, or at least a major portion, of the fluid to which the fins 14 are exposed to pass through openings 18 formed in the fins 14. The structure may be appropriately enclosed along its sides and ends, as shown in FIG. 4, to cause such major portion of the fluid to pass through the openings 18.
In each fin, the openings 18 are formed according to a convenient selected pattern. For most applications, an advantageous hole pattern is one wherein holes 18 are substantially evenly distributed across the surface of each fin, with allowances for the passage therethrough of the tube 12, and the openings in each fin are staggered with respect to holes in adjacent fins so fluid passing through the holes will be directed onto solid target surfaces of the adjacent fins. In preferred structures, less than 20 percent of the fin area is open. Optimum results lie in the 2 percent to percent range for most applications. The preferred range of opening sizes lies between 0.030 inch and 0.125 inch. However, with some loss in performance, open area and opening sizes outside these ranges may be used and still provide results superior to finned tube constructions without jet forming openings.
In operation, incoming air is diverted by certain of the folded edges 17 of the fin structure 14 into spaces formed by adjacent fins and thence along crooked paths through the openings 18. The openings 18 form jets which impinge upon solid portions of adjacent fins. This action is indicated by arrows in FIG. I. Finally, exhaust air leaves the finned tube heat transfer structure as indicated by the vertical arrows above that structure.
I In FIG. 2, the illustrated structure is very similar to that of FIG. 1. Again, there is a vapor tube 12, folded fins 14, attached to the tube by means of flanges l6, and includingjet-forming openings 18. However, in the embodiment of FIG. 2, the openings 18 are limited to one set of fin members in such a fashion that fluid, such as air, is directed against the unperforated surfaces of a second set of fin members. Thus, there is provided additional surface against which the jets are directed, the unperforated surfaces preventing interaction between jets.
In FIG. 3, there is shown an embodiment of the invention which includes a tube 12 similar to that discussed above, to which straight fins 22 are attached. Here, as in the case of the other embodiments of the invention, flanges may also be used to insure good heatconducting contact between the fins and the tube, although they are not shown. Alternate pairs of the fins 22v are joined or capped at the input and the output sides, as, for example, at 24 and 26, respectively. These closures may be made with the square fins, as illustrated, The closures may be formed, for example, by press fitting, cementing, bending or welding ends of adjacent fins together or by applying individual caps which engage adjacent fin ends along their length. Once again, input air is indicated by the vertical arrows at the bottom of the figure, and the fin members are perforated to form apertures through which the input air is' directed in the form of jets which impinge upon the solid surface of an adjacent fin. The design of FIG. 3 may be of particular interest when relatively wide fin spacing is desired.
FIG. 4 illustrates a small portion ofa system in which the finned tube of the invention may be incorporated.
A duct member 32, which may be of any desired length I is penetrated by tubes such as the tube 12 which may be independent or a part of a coil or other configuration. The tubes are spaced along the length of the set of fins 34. The set of fins extends from side to side across the width of the duct. Input air, or other fluid, may be forced through the duct as indicated by the arrows at the bottom of FIG. 4. The air is diverted from its original path and formed into generally transverse jets by the openings 38 formed in the fins. It will be noted that each opening confronts an unperforated area of an adjacent fin in order that the jets impinge upon a fin surface to break up stagnant boundary layers formed on such surfaces, thereby to enhance heat transfer.
Another fluid flows through the tubes 12 and it, of course, is at a different temperature than that of the fluid forced through the duct 32. The other fluid may also be forced through the tubes 12 or it may be a part of a system in which no applied force is necessary.
For the purpose of illustrating the relationship 'between a particular heat transfer coefficient (h,) and certain other design parameters, a typical design procedure will be discussed. It has been shown above that causing fluid to impinge as jets upon the fins increases their heat transfer coefficient and thus their efficiency.
In terms of specific dimensions for a practical embodiment of the invention such as shown in FIG. 1, it can be stated that heat transfer (0,) between a refrigerant in the tube 12 and air passing over the structure is:
where h, denotes the heat transfer coefficient between the inner wall of the copper tubes and the refrigerant, h, that between the air and the fins, A, the outside area of the copper tubes, 1;, the fin effectiveness, A, the fin area, and t t and t,, the copper tube, the refrigerant and the air temperatures, respectively, and r, and r, the inner and outer radii of the tube.
By way of example, assume a heat transfer coil has a length of 54 in. a fin depth of 1.25 in. a fin height of 37.5 in. thirty three eighths inch diameter tubes on 1.25 in. vertical spacing, and heat to be dissipated of 103,000 Btu/hr. Therefore:
0- =-(Q./h,- A.) (r,/r.) (103,000 /1,200 x 13.3 x 0.91 7F where h, is assumed to be 1,200 Btu/hr ft F (based on the high velocity refrigerant flow normally used in finned coils) and a tube thickness of 0.016 in. Therefore, in such a jet impingement coil, the effective initial temperature difference between the air and the fin base for a design inlet air temperature (t,,) of and a condensing temperature (2,) of will be:
( r a) r (u) (a) u (12595)7=z a, r,,
r r,, 23F
The values used in determining the jet impingement transfer coefficient, h are based on such initial temperature differences.
Further, the jet impingement transfer coefficient, h; is a function of the ratio A P/D, where A Pis the pressure drop across the orifice and D is the diameter of the jet at the vena contracta. The actual correlation requires, of course, that a number of other factors such as orifice-to-target distance, cross-flow velocity, etc., be held within certain limits to be valid. The correlation does, however, illustrate the desirability of using as small a jet as can be tolerated for the specific application. It will be assumed that, for the case being examined, a relatively large diameter of 0.1 in. is used, and A P is 0.2 in H O. Under the assumed conditions, the average of the h, values available for the inlet and target side of the jet plates will be taken as 18 Btu/hr ft F.
For the purpose of calculating fin effectiveness, 1 assume fins measuring 1.25 by 1.25 in. with a threeeighths .in. OD tube running through their geometric center, the configuration is equivalent to one with circular fins having a ratio of diameters of 4.0. The fin effectiveness, 1;, is given by an accepted correlation as a function of the parameter (a):
it is known that: l: (4 r, r,);= 3 r 0.047. ft for aluminum k 1 l8 Btu/hr ft F and h l8 Btu/hr ft F it t 0.008 in. Then:
q= 0.047 [2 X18 12/118 X 0.008] =0.92 and by interpolation:
From the heat transfer equation previously cited, it is now possible to calculate the required finned surface area:
A,= Qo/ t... ramh 103,000/23 x 0.63 1s w 395 ft The effective surface area per fin is 0.61 ft' /fin.
Thus the total number of fins required is:
395 ft /0.6l ft /fin 650 fins,
and the number of fins per inch is:
650 fins/54 in. 12 fins/in.
The resulting coil design can then be summarized as:
Capacity 103,000 Btu/hr Condensing temperature 125F Fins/inch l2 Finned length 54in.
Fin height 37.5 in.
Fin depth 1.25 in.
Tube diameter in.
Fin thickness 0.008 in.
This design indicates a material saving of approximately 25 percent as compared to conventional finned tubes.
What is claimed is:
1. A finned tube heat exchanger comprising a tube for containing a first fluid at a first temperature, a plurality of fins attached in heat conducting relationship to said tube, means integral with each of said fins forming the target areas of adjacent fins and staggered with respect to the perforations in adjacent fins.
2. A finned tube heat exchanger according to claim 1 wherein the sum of the areas of said perforations in each fin is less than 20 percent of the fin area.
3. A finned tube heat exchanger as defined in claim 1 wherein said fin structure comprises a continuous accordion structure.
4. A finned tube heat exchanger as defined in claim 3 wherein said second fluid flows in a direction having a substantial directional component normal to a plane defined by reverse turns in said accordion structure.
5. A finned tube heat exchanger for producing heat transfer between first and second fluids comprising a conduit for confining said second fluid at a second temperature under forced draft conditions, a tube extending across said conduit for containing a first fluid at a first temperature, a plurality of fins attached in heatconducting relationship to said tube, means forming perforations in each of said fins, means integral with each of said fins forming target areas in opposition to said perforations, the perforations in each fin being aligned with the target areas of adjacent fins and staggered with respect to the perforations in adjacent fins, means for causing substantially all of said second fluid to pass through said perforations to form fluid jets and for directing said jets of said second fluid onto said-target areas of adjacent fins whereby boundary layers of said second fluid normally adhering to said fins are disrupted and heat exchange between said first fluid and said second fluid is enhanced.
6. A finned tube heat'exchanger according to claim 5 wherein the sum of the areas of said perforations in each fin is less than 20 percent of the fin area.
7. A finned tube heat exchanger according to claim 5 wherein said first fluid comprises an evaporating liquid.
8. Afinned tube heat exchanger as defined in claim 5 wherein said first fluid comprises a condensing vapor.
9. A finned tube heat exchanger as defined in claim 8 wherein said second fluid comprises air.
10. A finned tube heat exchanger as defined in claim 8 wherein said tube is composed of copper and said fin structure is composed of aluminum. I 11. A finned jet heat exchanger as defined in claim 5 wherein said fins comprise a continuous accordion structure, intersections of adjacent fins, with said fins and said conduit, forming said means for forming and directing jets of said second fluid.
12. A finned jet heat exchanger according to claim 11 wherein said accordion structure is of unitary, folded configuration.
13. A finned jet heat exchanger as defined in claim 11 wherein said second fluid flows in a direction having a substantial directional componentperpendicular to a plane defined by reverse turns in said accordion structure.
14. A finned jet heat exchanger as defined in claim 13 wherein said second fluid flows along a path substantially perpendicular to such plane.