WAVY PLATE-FIN

Abstract: A plate fin (12) for a fined tube heat exchanger coil is provided having an improved enhanced heat transfer area between adjacent pairs of tube holes (16) in the plate fin (12). The enhanced heat transfer area includes a plurality of aper­ tures (40) disposed through the fin at the maximum and minimum of a sine-wave like pattern forming the surface of the fin.

Description: The present invention relates to a plate fin comprising the features as indicated in the pre-characterising part of claim 1 and to the use of such a plate fin in a finned tube heat exchanger. Such a plate fin is known from JP-A-60 202 294. The fins are utilized in the air conditioning and refrigeration industry and are normally manufactured by progressively stamping a coil of plate fin stock and then cutting the stamped fin to the desired length. The fins are then collected in the proper orientation and number in preparation for forming a coil. Previously formed hairpin tubes are then inserted through openings within the fins and thereafter expanded to form mechanical and thermal connections between the tubes and fins. The open ends of the hairpin tubes are fluidly connected by way of U-shaped return bends, and subsequently the return bends are soldered or brazed in place. The plate fins are typically manufactured in either a draw or drawless die to form both the fin shape, and the surface variations on the fin and openings through which the tubular members are inserted. Generally, the industry presently forms a plurality of rows of fins simultaneously from a single section of plate fin stock. These multi-row fins are cut to the desired number of rows for the coils and are then collected on stacking rods or within a box or some other means to form a pile or stack of fins ready to be laced with hairpin tubes to form the coil. It is known to those skilled in the art that a fundamental contributor to the limiting of local convective heat transfer is the establishment and persistence of thick hydrodynamic boundary layer on the plate fins of heat exchangers. For this reason, prior art fins are provided with a variety of surface variations or enhancements to restart or disrupt the boundary layer and thus increase the transfer of heat energy between the fluid passing through the tubular members and the fluid passing over the plate fin surfaces. These enhanced fins are generally either enhanced flat fins or wavy fins. Flat fins are generally enhanced by manufacturing raised lances therein. A raised lance is defined as an elongated portion of fin formed by two parallel slits whereby the stock between the parallel slits is raised from the surface of the fin stock. In addition to having raised lances, enhanced fins may also have louvered enhancements. A louver is defined as a section of fin stock having one or two elongated slits wherein the stock moved from the surface of the fin stock always has at least one point remaining on the surface of the fin stock. These lances and louvers promote restarting or thinning of the hydrodynamic boundary layer, thus increasing the local heat transfer coefficient. However, generally large numbers of lances or louvers are added to a surface to improve the heat transfer which is accompanied by a significant and undesirable increase in air pressure drop through the coil. Further, such lanced and louvered fins are structurally weakened by the slitting operation, and as well, may be difficult and costly to manufacture. Typical of the abovementioned prior art fins are those shown in JP-A-60 202 294. This plate fin comprises corrugated wall means having opposite facing first and second surfaces respectively for transferring heat between the wall means and a fluid flowing over the surfaces, the wall means having a sine-like wave pattern of predetermined height along the first and second surfaces in a direction with the flow of the fluid flowing over the surfaces, the sine-like wave pattern having curved peaks at the maximum of said wave heights of the pattern and curved troughs at the minimum of said wave heights of the pattern whereby the peaks and troughs extend along the corrugated wall means generally transverse to the direction of flow of fluid flowing over the surfaces, and enhanced heat transfer means consisting of apertures in the corrugated wall means. The aforementioned JP-A-60 202 294 shows plate fins having sine-like wave surfaces with a plurality of small holes provided on the whole fin surfaces. Due to the plurality of holes and the straight line path of flow along the surfaces between fins in one channel, the momentum of the flow causes the fluid to flow through the holes in order to cause mixing or turbulence in the adjacent channels, thereby restarting or disrupting boundary layer. It is an object of the present invention to reduce boundary layer separating downstream of the peaks and troughs of the plate fin and eliminate the recirculation without altering the general length-wise streamlines of the fluid. To achieve this, the plate fin of the invention is characterized by the features set forth in the characterizing part of claim 1. According to the invention, there is provided a plate fin having apertures disposed only within 45 sine-wave degrees of the peaks and troughs while the remaining sections of the wall means are free from apertures, whereby the fluid flows through the apertures generally only at the peaks and troughs. Advantageous embodiments of the invention are claimed in the subclaims. It has been found desirable to provide a plate fin having an enhanced surface which results in a more favourable balance between heat transfer and pressure loss. With the plate fin according to the present invention viscous losses of the fluid flowing between two adjacent wary fins of a heat exchanger are reduced by delaying of eliminating boundary layer separation downstream of the peaks and thus reducing of eliminating recirculation in the troughs. The plate fin will result in a heat transfer improvement while maintaining of lowering the air pressure loss at a given face velocity. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be made to the accompanying drawings and descriptive matter in which there is ellustrated and described a preferred embodiment of the invention, in which; Figure 1 is a perspective view of a plate fin heat exchanger incorporating the enhanced plate fin of the present invention; Figure 2 is a perspective view of a multi-row plate fin according to a first preferred embodiment of the present invention; Figure 3 is a perspective view of a multi-row plate fin according to a second preferred embodiment of the present invention; Figure 4 is a transverse cross-sectional view of a conventional wavy fin; and Figure 5 is a sectional view taken along line V-V of figure 3. The embodiments of the invention described herein are adapted for use in condensing or evaporating heat exchangers used in heating, ventilating, and air conditioning systems, although it is to be understood that the invention finds like applicability in other forms of heat exchangers. Plate fin heat exchangers are generally used in conventional direct expansion vapor compression refrigeration systems. In such a system, the compressor compresses gaseous refrigerant, often R-22, which is then circulated through a condenser where is is cooled and liquified and then through an expanding control device to the low pressure side of the system where it is evaporated in another heat exchanger as it absorbs heat from the fluid to be cooled and changes phase from a partial liquid and partial vapor to a superheated vapor. The superheated vapor then flows the compressor to complete the cycle. Typically, a plate fin heat exchanger is assembled by stacking a plurality of parallel fins, and inserting a plurality of hair pin tubes through the fins and mechanically expanding the tubes to make physical contact with each fin. The heat transfer characteristics of the heat exchanger are largely determined by the heat transfer characteristics of the individual plate fins. Referring now to the drawings, figure 1 illustrates a fin tube heat exchanger coil 10 incorporating a preferred embodiment of the present invention. Heat exchanger coil 10 comprises a plurality of spaced-apart fin plates 12, wherein each plate fin 12 has a plurality of holes 16 therein. Fin plates 12 may be any heat conductive material, e.g. aluminum. Fin plates 12 are maintained together by oppositely disposed tube sheets 18 having holes therethrough (not shown) in axial alignment with holes 16. A plurality of hair pin tubes 20 are laced through selected pairs of holes 16 as illustrated and have their open ends joined together in fluid communication by return bends 22, which are secured to hair pin tubes 20 by soldering or brazing or the like. The hair pin tubes may be any heat conductive material, e.g. copper. In operation, a first fluid to be cooled or heated flows through hair pin tubes 20 and a cooling or heating fluid is then passed between fin sheets 12 and over tubes 20 in a direction indicated by arrow A. Heat energy is transferred from or to the first fluid through hair pin tubes 20 and plate fins 12 to or from the other fluid. The fluids may be different types, for example, the fluid flowing through tubes 20 can be refrigerant and the fluid flowing between plate fins 12 and over the tubes can be air. As illustrated in figure 1, finned tube heat exchanger coil 10 is a staggered two-row coil since each plate fin 12 has two rows of staggered holes therein for receiving hair pin tubes 20. The holes 16 of one row are arranged in either staggered or in-line relation with the holes 16 of an adjacent row. Also, the heat exchanger can be a composite heat exchanger made from a plurality of single row heat exchangers. Referring now to figures 2-3, a multi-row plate fin is illustrated each having rows of tube holes 16 with enhanced heat transfer sections 24 between respective adjacent pairs of holes 16. Plate fin 12 also includes leading and trailing edges 26, 28 which may have a plurality of serrations thereupon to add rigidity to the plate fin edges. Collars 14 are formed about holes 16 during fin manufacture for receiving tubes 20 therein and spacing adjacent plate fins. In figures 2-3, only the plate fin 12 is shown and the tubes that would pass through the collars are omitted for simplicity. An example of a prior art plate fin heat exchanger is shown in figure 4. The heat exchanger 10 has wavy fins, so that the heat transfer from the tube 20 through the collar 14 to the plate fin 12 is increased over that of the ordinary flat plate fin. The fluid flowing in direction of arrow A, e.g. air, supplied by means of a fan or the like passes along the plate fins 12 and transfers heat to or from the surfaces of the plate fins of a temperature different from that of the air thereby allowing a heat exchanging operation to be performed continuously between the first fluid flowing over the plate fins and the second fluid flowing through the tubes. In the heat exchanger of figure 4, flow channel 30 is formed between two adjacent plate fins 12. The fluid passing between adjacent plate fins 12 in the channels 30 forms a hydrodynamic boundary layer along the top 32 and bottom 34 surfaces of the plate fin 12. However, the boundary layer separates downstream of the peaks 36 on the top surface 32 and the peaks 38 of the bottom surface 34 and recirculates or forms eddies (shown by the flow arrows a between adjacent plate fins) in the next adjacent downstream trough. An adverse pressure gradient is responsible for the formation of the eddies. The adverse pressure gradient is caused by the streamline divergence and subsequent deceleration of the length-wise free stream fluid in the vicinity of the downstream portion 46 of peak 36 of top surface 32 and downstream portion 48 of the peak 38 of the bottom surface 34. The deceleration of the free stream fluid causes a local increase in the static pressure in the upper and lower surface troughs of the channel 30. Further, the undulating shape of the channel 30 gives rise to a positive pressure gradient in the direction of convex (peaks) to concave (troughs) surfaces at any point along the flow channel due to centrifugal effects. Thus, the prior art wavy plate fin heat exchanger has a higher pressure at the upper and lower surface troughs (as shown at B), while it has a lower pressure at the lower and upper surface peaks (as shown at C). The momentum of the length-wise fluid stream is not sufficient in the boundary layer near the surfaces of the fins to overcome the higher pressure at B, thus separation of the boundary layer occurs. Referring now to figure 5, there is illustrated a side elevational view of an embodiment of the present invention. There is shown a plurality of spaced-apart fins 12 with a tube 20 received through respective axial aligned holes 16. The wavy plate fins 12 have a sine-wave like pattern in cross section along the length-wise direction of fluid flowing over the upper surface 32 and lower surface 34. A plurality of orifice-like perforations 40 are punched, or the like, through the plate fins 12 at the maximums and minimums, or peaks 36 and troughs 34 of the plate fins. In figure 5, arrow A indicates the direction of fluid flow, such as air flow, over and between fin plates 12. As the fluid flows between fins 12 in channels 30, the pressure difference across a fin, in adjacent channels, causes the fluid to flow through perforations 40. A path followed by the fluid through the perforations 40 virtually eliminates recirculation fluid near the upper and lower troughs, and delays or eliminates separation downstream of the lower and upper surface peaks. Thus, a portion of the fluid will be passed between adjacent channels 30 from points B to C by virtue of the pressure difference between adjacent channels 30 at the peaks and troughs of a fin. The perforations 40 are sized so as to pass sufficient fluid therethrough to reduce or eliminate recirculation while not adversely altering the general length-wise stream lines of the fluid flowing in channel 30. In addition, the higher momentum fluid passing through the perforations 40 disrupt the boundary layer on the low pressure side of the plate fin and increase the rate of heat transfer even though the heat transfer surface area has been reduced by the perforations 40.

Claims: 1. A plate fin (12) comprising wall means having opposite facing first and second surfaces (32,34) respectively for transferring heat between the wall means and a fluid flowing over the surfaces (32,34); whereby the wall means are corrugated having a sine-like wave pattern of predetermined height along the first and second surfaces (32,34) in a direction with the flow of the fluid flowing over the surfaces (32,34), said sine-like wave pattern having curved peaks (36) at the maximum of said wave heights of the pattern and curved troughs (38) at the minimum of said wave heights of the pattern whereby said peak (36) and troughs (38) extend along said corrugated wall means generally transverse to the direction of flow of fluid flowing over the surfaces (32, 34); and enhanced heat transfer means consisting of apertures (40) in the corrugated wall means, characterized in that said apertures (40) are disposed generally along said peaks (36) and troughs (38) within 45 sine-wave degrees of said maximum of said peaks (36) and said minimum of said troughs (38) thereby forming an enhanced heat transfer section (24) generally along said peaks (36) and troughs (38), the remaining sections of said surfaces (32,34) being free from apertures, whereby generally only at said curved peaks (36) the fluid flowing over the surfaces (32,34) flows through said apertures (40) in a direction from the first surface (34) to the second surface (32) and whereby generally only at said curved troughs (38) the fluid flowing over the surfaces (32,34) flows through said apertures (40) in a direction from said the second surface (32) to the first surface (34). 2. A plate fin as set forth in claim 1, characterized in that said apertures (40) are elongated holes perpendicular to the direction of fluid flow. 3. A plate fin as set forth in claim 1, characterized in that said apertures (40) are circular holes. 4. The use of plate fins according to any of claims 1 to 3 in a finned tube heat exchanger (10) comprising a plurality or said heat conductive plate fins (12) having a plurality of holes (16) therein, said fins (12) disposed parallel to each other at predetermined intervals whereby a first fluid flows over said surfaces (32,34) between adjacent fins (12), and a plurality of heat transfer tubes (20) disposed in respective ones of said holes (16) in heat transfer relation with said plate fins (12), said heat transfer tubes (20) adapted to having a second fluid flowing therethrough whereby heat is transferred between said first and second fluids, each of said plate fins (12) having said sine-wave like shape in a plane generally parallel to the flow of said first fluid, said sine-wave like shaped plate fin (12) having a predetermined peak to trough amplitude, characterized in that each of said convoluted plate fins (12) has said enhanced heat transfer section (24) disposed between adjacent said holes (16), whereby said first fluid flowing over said surfaces (32,34) at said curvilinear peaks (36) and at said curvilinear troughs (38) flows through said apertures (40) due to a pressure difference there at between the surfaces (32,34). 5. The use of plate fins as set forth in claim 4, characterized in that the peak to trough amplitude is between about 0.5 and 1.5 times the distance between adjacent fins (12).

Inventors: BALLENTINE, PAUL HENRY, ESFORMES, JACK LEON, HAUGHT, ALAN FREDERIC, NASH, ERIC JAY, POLK, DONALD HENRY
Applicants: CARRIER CORPORATION
Application year: 1989
Main classification: F28F1
IPC codes: F28F1