LANCED SINE-WAVE HEAT EXCHANGER

Abstract: A sine-wave like plate fin (12) for a finned tube heat exchanger coil is provided having an impro­ ved enhanced heat transfer area between adjacent pairs of holes (16) in the plate fin (12). The enhanced heat transfer area includes a plurality of raised lance elements (36,38) disposed thereon generally at the peaks and troughs of the sine-wave wherein the raised elements (36) at the peaks are concave and the raise elements (38) at the troughs are convex.

Description: The present invention relates to heat transfer plate fins for transferring heat between the fin and a fluid flowing over the fin comprising the features as indicated in the precharacterising part of claim 1. The present invention also relates to the use of heat transfer plate fins in a plate fin tube heat exchanger. Such plate fins are known from JP-A-60-223 995. They are utilized in the air conditioning and refrigeration industry and 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 a mechanical and thermal connection 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 as well as surface variations on the fin and openings through which the tubular members are inserted. Generally, the HVAC industry presently forms a plurality of rows of fins simultaneously from a section of plate fin stock. These rows of 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. Prior art fins are provided with a variety of surface variations or enhancements to improve the transfer of heat energy between the fluids passing through the tubular members and over the plate fin surfaces. These enhanced fins are either 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. Wavy fins, in addition to having raised lances, may also have louvered enhancements. A louver is defined a section of fin stock having one elongated slit wherein the surface of the fin stock on one side of the slit is raised from the surface of the stock. Generally, enhanced wavy fins either have a raised lance or a louver at both the leading and trailing edges. Enhanced fins with raised lances at the edges are weak and non-rigid along the edges due to the surface enhancement thereon. Enhanced wavy fins with louvers at the leading and trailing edges have very steeply inclined surfaces at the edges and cause excessive pressure drop due to the steep angle of inclination. Thus, there is a clear need for an enhanced plate fin surface which has strong leading and trailing edges, and also eliminates the very deep trough which contributes to excessive pressure drop. In JP-A-60223995 there is described a heat transfer plate fin according to the preamble of claim 1. More specifically, JP-A-60223995 discloses a plate fin for transferring heat between the fin and a fluid flowing thereover in a heat exchanger, the plate fin having opposite facing upper and lower surfaces, fin edges and a plurality of fin collars disposed in a row that is parallel to the fin edges and defining holes for tubes. The plate fin comprises a corrugation having a cross-section defining a sine-like wave line of predetermined height along the upper and lower surfaces; the sine-like wave line having curved peaks at a maximum of the wave heights and curved troughs at a minimum of the wave heights, and a plurality of enhanced heat transfer sections, each section comprising a group of elongate raised lance elements and being disposed between a pair of adjacent fin collars. The fin edges comprise leading and trailing edges upstream and downstream of the direction of flow of fluid flowing over the surfaces of each of the enhanced heat transfer sections respectively, the fin edges being free from raised lance elements. The aforementioned JP-A-60223995 does not address the specific problems underlying the present invention with respect to the minimisation of pressure gradients across the fin and the elimination of a continuous build-up of boundary layer stagnation along the surface of the fin. JP-A-60223995 discloses curved wavy fins to improve the transfer of heat between the fluid and the fin by using raised lanced elements which extend generally transverse to the peaks and troughs and by bending the flow of fluid along the curvature of the fin from the leading edge to the trailing edge. In the prior art, though surface enhancements, such as raised lances, generally disrupt boundary layers, these enhancements were always accompanied by an increase in pressure drop through the fin. It is an object of the present invention to alleviate the problems due to boundary layer stagnation and pressure drops across the fin. The present invention also seeks to improve the strength and rigidity of the leading and trailing edges of the fin in single row fin coils. To achieve this, the heat transfer plate fin of the invention is characterized by the features set forth in the characterizing portion of claim 1. According to the invention, there is provided a sine-like wave line in a plane that is perpendicular both to the fin edges and to the upper and lower surfaces, each of the elongate raised lance elements in an enhanced heat transfer section having a longer dimension that is parallel both to the fin edges and to the peaks and troughs, and being raised above the upper surface at a point on the corrugation that is at a maximum amplitude of the sine-like wave line, the raised lance elements being disposed only along a selected number of peaks and troughs in each section disposed between a pair of adjacent fin collars. Advantageous embodiments of the invention are claimed in the subclaims. In a specific embodiment, the present invention provides a multirow fin coil with an enhanced plate fin having the trough between tubes of a basic sine-wave pattern to reduce pressure drop through the heat exchanger coil. It is yet a further feature of the present invention to provide a multirow enhanced plate fin which may be cut into single-row fins for use in heat exchanger coils. For a better understanding of the invention, its operating advantages and specific objects obtained by its use, the invention is more fully described with reference to the accompanying drawings 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 top plan view of a preferred embodiment of the present invention; Figure 3 is a sectional view taken along line iii-iii of figure 2; Figure 4 is an elevational view of a single-row plate fin incorporating a preferred embodiment of the present invention; and Figure 5 is a fragmentary elevational view of a single-row coil incorporating a plurality of the preferred embodiments of the present invention. Referring to figure 1, there is illustrated a plate finned 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 are maintained together by oppositely disposed tube sheets 18 having holes therethrough in axially alignment with holes 16. A plurality of hairpin tubes 20 are laced through select pairs of holes 16 as illustrated and have their open ends joined together in fluid communication by return bins 22, which are secured to the hairpin tubes 20 by soldering or brazing or the like. In operation, a first fluid to be cooled or heated flows through hairpin 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 hairpin tubes 20 and plate fins 14 to or from the other fluid. The fluids may be different types, for example, the fluid flowing through tubes 20 can be a refrigerant and the fluid flowing between plate fins 14 and over the tubes 20 can be air. As illustrated in figure 1, plate fin tube heat exchanger coil 10 is a staggered two-row coil since each plate fin 14 has two rows of staggered holes therein for receiving hairpin tubes. The present invention contemplates a heat exchanger coil of only one row of tubes, or more than two rows of tubes, and with holes 16 of one row in staggered relation with holes 16 of an adjacent row. Also, multirow coils can be formed either from a plurality of multirow single plate fins or a composite of a plurality of single row coils. Referring now to figures 2-3, another embodiment is illustrated wherein plate fin 12 is a staggered three-row fin type having three rows of staggered holes 16 with enhanced heat transfer sections 24 disposed between adjacent holes 16. Collars 17 are formed about holes 16 during fin manufacture for receiving tubes 20 therein to insure good physical and thermal contact. The plate fins generally have two complete sine-like wave patterns per row of tubes. Referring primarily to figure 3, the cross-section of plate fin 12 taken in a plane generally transverse to fin 12 illustrates a double wavy sine-like wave pattern along the surface line 50 of the fin 12. Generally elongate lanced elements 36, 38 are raised upwardly relative to the original surface along surface line 50. Lanced elements 36, 38 also maintain an original convex or concave shape, respectively, in the plane of the cross-section. Further, the raised lanced elements 36, 38 are positioned only at the maximums and minimums, or peaks and troughs respectively, of the sine-like wave patterns. Further, the raised lance elements 36, 38 occur only just oppose the tube hole 16. Thus, the trough 56 between adjacent tube rows have no raised lances therein. The absence of raised lances in trough 56 allows for slitting a multirow fin into single row fins whereby the leading and trailing edges do not contain a portion of a raised lance. Thus, in single row fins, as will be fully described herein, the leading and trailing edges are not fragile and subject to deformation. In summary, figures 2 and 3 illustrate an embodiment of the present invention having a double wavy pattern per tube row, and accordingly there are three raised lances per double wavy pattern. Generally elongate raised lance elements 36, 38 are parallel to edges 32 of plate fin 14 and are positioned between adjacent holes 16 in each tube row. Further, elongate raised lance elements 36, 38 are cut or lanced on both sides thereof to define a pair of oppositely disposed openings 46 with the openings on opposite sides of the peaks and troughs. It should also be noticed relative to the raised lance elements 36, 38 that they are generally concave in the troughs between adjacent tubes, and convex at the peaks between adjacent tubes, but there are no raised lance elements in the troughs between adjacent rows of tubes. Thus, the cross-sectional shapes of elements 36, 38 are curved and generally either convex or concave depending on the original wave line 50. As described above, raised lanced elements 36, 38 increase the ability of plate fin 12 to absorb or dissipate heat as required. Referring now to figure 4, there is illustrated a cross-sectional elevational view of a single row plate fin 12. The single row plate fin 12 is generally cut from a multiple row plate fin sheet, but may be individually manufactured as a single row plate fin. The fin 12 illustrates a double wavy sine-like wave pattern along the surface of line 50 wherein each sine-wave has a length (W). Thus, each single row plate fin 12 have raised lanced elements 36, 38 at each peak 52 and trough 54 between adjacent tube holes 16. It should be noted that raised lanced elements 36, 38 are vertically offset from the surfaced line 50 only in the plane between adjacent tube holes 16. Thus, the edges 32 of plate fin 12, even though they are at a minimum or trough of the wave line 50, are free from raised lanced elements. The absence of raised lanced elements at the edges of the plate fins provide rigidity to the plate fins and prevent a ragged or cluttered appearance due to the shredding or twisting of lanced elements at the edges. Moreover, non-enhanced edges 32 eliminate problems caused by steeply inclined surfaces when the edges have raised louvers or damaged fins when the edges have raised lances or portions of raised lances. Further, because of the double wavy sine-like pattern formed by raised elements 36, 38 along surface line 50, the pressure drop across fins 12 is minimized, which further increases the heat transfer efficiency thereof. Referring now to figure 5, there is illustrated a transverse cross-sectional elevational view of a plurality of spaced-apart fins 12 with a tube received through respective axially aligned holes 16. Collars 17 are formed about holes 16 during fin manufacture for receiving tubes 20 therein and for properly spacing adjacent plate fins. Arrow A indicates the direction of fluid flow, such as air flow, over and between plate fins 12 and around tube 20. As the fluid flows between fins 12, raised lanced elements 36, 38 cause the fluid to follow a toruous path to either absorb or dissipate heat energy with fins 12. A toruous path followed by the fluid through plate fins 12 virtually eliminates a continuing buildup of boundary layer stagnation along the surface of fins 12. Boundary layer buildup is particularly undesirable since boundary layers on heat transfer surfaces decrease the rate of heat transfer, and if the boundary layer is not disrupted, it gradually increases in depth along its length, which further degrades heat transfer. Also, the positioning of the raised lanced elements 36, 38 only at the peaks 52 and trough 54 of surface line 50 minimizes the pressure drop across plate fins 12, which further increases the heat transfer efficiency thereof. Plate fins 12 and tubes 20 can be made of aluminium, cooper, or other suitable materials.

Claims: A heat transfer plate fin (12) for transferring heat between said fin and a fluid flowing over said fin, in a plate fin and tube heat exchanger (10), said plate fin (12) having opposite facing upper and lower surfaces, the fluid flowing over the surfaces, fin edges (32) and a plurality of fin collars (17) disposed in a row that is parallel to said edges (32) and defining holes (16) for tubes (20), said plate fin (12) comprising: a corrugation (50) in said fin (12), said corrugation (50) having a cross-section defining a sine-like wave line (50) of predetermined height along the upper and lower surfaces, said sine-like wave line (50) having curved peaks (52) at a maximum of said wave heights and curved troughs (54,56) at a minimum of said wave heights, and a plurality of enhanced heat transfer sections (24), each section (24) comprising a group of elongate raised lance elements (36,38) and being disposed between a pair of adjacent fin collars (17), said fin edges (32) comprising leading and trailing edges (32) upstream and downstream of the direction of flow (A) of said fluid flowing over the surfaces of each of said enhanced heat transfer sections (24) respectively, said fin edges (32) being free from raised lance elements (36,38), characterized in that said sine-like wave line (50) is in a plane that is perpendicular both to said fin edges (32) and to said upper and lower surfaces, and in that each of said elongate raised lance elements (36,38) in an enhanced heat transfer section (24) has a longer dimension that is parallel both to said fin edges (32) and to said peaks (52) and troughs (54,56), and is raised above said upper surface at a point on said corrugation that is at a maximum amplitude of said sine-like wave line (50), said raised lance elements (36,38) being disposed only along a selected number of said peaks (52) and troughs (54) in each section (24) disposed between a pair of adjacent fin collars (17). A heat transfer plate fin as set forth in claim 1, characterized in that said sine-like wave line (50) is a double wavy pattern having two peaks (52) each with a raised lance element (36) and one trough (54) with a raised lance element (52). A heat transfer plate fin as set forth in claim 2, characterized in that said raised lance elements (36) at the peaks (52) have a concave cross-sectional shape and said raised lance elements (38) at the troughs (54) have a convex cross-sectional shape. The use of heat transfer plate fins (12) according to any of claims 1 to 3 in a plate fin tube heat exchanger (10) comprising: a plurality of said heat conductive plate fins (12) having a plurality of said holes (16) therein, said fins (12) being disposed parallel to each other at predetermined intervals whereby said fluid is a first fluid flowing over said surfaces between adjacent fins (12), 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, characterized in that said sine-like wave line (50) of each of said convoluted plate fins (12) is in a plane generally parallel to the flow (A) of said first fluid, whereby said peaks (52) and troughs (54,56) extend along said corrugation in a direction transverse to the flow (A) of said first fluid, each of said plate fins (12) having said enhanced heat transfer sections (24) disposed between adjacent said holes (16), said enhanced heat transfer sections (24) having a group of said successively adjacent generally elongate raised lance elements (36,38) only at said curvilinear peaks (52) and troughs (54), each of said enhanced heat transfer sections (24) having said leading edge (32) upstream in the direction of flow (A) of the first fluid and said trailing edge (32) downstream in the direction of flow (A) of the first fluid.

Inventors: SACKS, PAUL SYMONDS
Applicants: CARRIER CORPORATION
Application year: 1988
Main classification: F28F1
IPC codes: F28F1