EP0962736A2

EP0962736A2 - Corrugated fin for evaporator with improved condensate removal

Info

Publication number: EP0962736A2
Application number: EP99201397A
Authority: EP
Inventors: Steven R. Falta; Mohinder Singh Bhatti; Shrikant Mukund Joshi
Original assignee: Delphi Technologies Inc
Current assignee: Delphi Technologies Inc
Priority date: 1998-06-01
Filing date: 1999-05-04
Publication date: 1999-12-08
Also published as: EP0962736A3; JP2000028228A

Abstract

An improved corrugated heat conduction fin (20) for an evaporator (10) includes an integral feature for reducing the critical width of condensed water films retained in the internal channels formed between diverging pairs of fin walls (22). Aligned pairs of notches (26) are cut through the diverging fin walls (22), localized within the channels and between adjacent pairs of louver banks (24). What would otherwise be a long, continuos water film (F) formed in the fin channel is thereby broken up into a series of discontinuous individual films (F1, F2 and F3), each located over a respective louver bank (24). The individual, shorter films drain more efficiently through the louver banks (24), thereby growing to a significantly smaller critical width within the channels and blocking less air flow over and through the fin walls (22).

Description

TECHNICAL FIELD

This invention relates to condensation formation in evaporators in general, and specifically to a novel feature in the corrugated heat transfer fins which improves condensate drainage.

BACKGROUND OF THE INVENTION

Air conditioning system evaporators, since they blow warm, humid air over cold metal heat transfer surfaces, are uniquely subject to the condensation of water films on those surfaces. While this is a plus in terms of dehumidifying the air, it is a detriment in terms of several possible effects on the evaporator and its efficient operation, especially in the case of automotive air conditioning system evaporators. Surface water can accumulate until it is actually blown out of the rear face of the evaporator core, the so called "spitting" phenomenon. This is generally prevented with screens on the rear face to retard the water, but this adds cost and represents an additional air flow obstruction. Wet cores are also more subject to microbial growth and odor, which can generally only be prevented with the addition of expensive anti microbial coatings. In addition, the mere physical presence of an adherent film of water can retard the free flow of air through the core, and increase the air pressure drop. This is especially true for films of water that adhere within the interior channels formed by the "V" shaped juncture of the divergent wall pairs that make up corrugated heat transfer fins, a very common type of fin.
Known approaches to removing condensate generally involve using gravity to drain it out through any existing drainage paths, sometimes coupled with altering the core structure to provide additional or more efficient drainage paths. The most obvious and simple approach is to simply orient the core so that its basic structure drains most efficiently. For so called tube and fin evaporators, which incorporate small diameter, round refrigerant flow tubes and flat, thin heat transfer fins perpendicular thereto, the obvious orientation to promote drainage places the refrigerant tubes horizontally and the fins vertically. Condensed water can easily drain down the vertical fin faces to drip from the bottom of the core. Such designs are not otherwise particularly thermally efficient, however.
A more efficient evaporator core design incorporates wide, flat flow tubes, formed either as runs of a continuous, serpentine tube, or as individual stamped plates brazed together in pairs. In either case, the flow tubes are oriented with their flat outer surfaces generally vertical, again, so that condensed water can drain easily downwardly. The heat transfer fins used with such designs are generally corrugated fins. The typical corrugated heat transfer fin is a series of folded fin walls, which diverge from a sharply angled "V" shaped crest. The outer, convex surfaces of the crests are brazed to the vertical, flat flow tube surfaces. The inner, concave channels formed by the diverging fin walls are horizontally oriented, aligned with the direction of air flow. The basic shape of corrugated fins is not conducive to condensate drainage. The fin walls are oriented generally horizontally, and run almost the full width of the flat flow tube surfaces to which they are brazed. Without more, the horizontal fin walls would totally block downward condensate drainage along the vertical flow tube surfaces, as well as blocking drainage from between the fin walls themselves. Fortunately, however, the corrugated fin walls typically have openings therethrough in the form of louvers which, though intended for other purposes, also coincidentally provide a downward drainage path.
Referring to Figures 1 through 3, an automotive air conditioning system evaporator of the general type described above is indicated generally at 10. The evaporator core is built up from a series of vertically oriented, regularly spaced pairs of parallel flow tubes 12, through which relatively cold refrigerant vapor is circulated in a U shaped flow pattern. To assist in the conduction of heat from the air to the flow tubes 12, a large corrugated fin, indicated generally at 14, is brazed between the opposed pairs of the flow tubes 12. Each fin 14 consists of a series of integrally folded pairs of divergent, rectangular fin walls 16. Basic dimensions of the fin walls 16 include a length of approximately fifty millimeters, and width of about ten millimeters, and a wall thickness of around 1 millimeter. Each pair of fin walls 16 are joined alternately at a sharply angled V, shown at β, with an internal angle of 15 degrees at most, and potentially less than that, since the fin walls 16 may be almost parallel to one another in the core, in order to achieve high fin densities. The external, convex crests of the fin 14 are brazed to the surfaces of flow tubes 12, making a much wider external angle that is, potentially, almost 90 degrees. The internal surface juncture of the divergent pairs of fin walls 16 is radiused at approximately a millimeter or less, rather than making a very sharp V point. Therefore, even if the interior angle β is essentially zero, there is still a tight, concave internal channel formed between the interior surfaces of the fin walls 16, with consequences described below. Each fin wall 16 is also pierced by four (or some even number of) rectangular banks of regularly spaced, conventional louvers 18, spaced apart by approximately 2 millimeters. Each bank 18 consists of a regular pattern of individual louvers (ten to twenty total in each bank 18) bent out of the plane of the fin wall 16. The angle of each individual louver relative to the fin wall 16 out of which it is bent is typically about 45 degrees, and the direction of the louver angle, positive or negative, alternates from one bank of louvers 18 to the next. The basic purpose of the louver banks 18, as is well known to those skilled in the art, is to break up the flow of air over the fin wall, and prevent the formation of efficiency reducing boundary layers that could otherwise occur. While the specific dimensions of each individual louver are not especially significant to the subject invention, what is significant is that each louver, regardless of its dimensions, does create a long, thin opening through the fin wall 16, because of the way it is formed. Each bank of louvers 18 has a width of approximately 7 mm, so that the ends of the individual louver openings approach very near the "bottom" of the channels, leaving an uncut fin wall width Δ of only approximately one to one and a half millimeter. In fact, for any fin design, louver length would be maximized, (and Δ consequently minimized), within the constraints of manufacture, so as to optimized air flow. This fact operates to help drain condensate in a fashion described in more detail next.
Referring next to Figures 4 and 5, the behavior of water condensate that forms on the fin 14 described above is illustrated. Initially, as moist air is blown across the horizontally oriented fin walls 16 (which is downwardly on the page, as shown by the arrows) it begins to condense. Surface tension forces draws the condensing water strongly into the crevice shaped internal channels, as best seen in Figure 6. A long, thin film forms, indicated at "F", which is shifted somewhat, by the force of the blown air, and is therefore thicker and wider toward the trailing edge of wall 16. Furthermore, the film F does not begin to form directly at the leading edge of the crest, since the air does not cool enough to condense until it has moved downstream slightly. However, the surface tension force is sufficient to retain the film F against being blown or "spit" off of the trailing fin wall edge, at least at this initial stage. The film thickness initially reaches a greatest width W, as shown in Figure 7, of approximately three millimeters, and is also thick enough to bridge and fill the channel completely out to that width W. The film F is therefore wide enough to overlap with the ends of the louver openings in the louver banks 18. At this point, however, the surface tension force predominates over the gravity force tending to pull it vertically down and through the open louver banks 18, so that drainage is minimal. The film F will continue to thicken. Conversely, on the outside of the fin walls 16, condensed water collects in a much wider "corner," that is, the external angle formed between the exterior of the fin wall 16 and the external surface of the flow tube 12. Film surface tension is much less effective in the wider corner, and condensed water can drain much more easily through the louver banks 18, without collecting and adhering as it does in the internal fin channels.
Referring next to Figures 8 and 9, as water continues to condense, the surface film F becomes both wider and thicker, with the width W growing eventually to a "critical" width of approximately six millimeters. At the critical width, the film F becomes less stable, and the surface tension forces no longer can prevent drainage down through the open banks of louvers 18. Water retained in the channels begins to drain down successively through successive fin walls 16, shrinking the film width, and eventually draining out from the bottom of the core. After draining, the film width and thickness again expand to the critical width, however, clogging the narrow channels formed between the diverging fin walls 16, reducing air flow and increasing air pressure drop. After re attaining the critical width, the process of instability, draining, shrinkage, and re expansion begins again, in a repeating cycle. The air flow blocking effect of the water films alone reduces thermal efficiency. The water film F also tends to insulate the metal conduction surfaces form the air flow, reducing conduction and convection efficiency.
Existing design approaches for enhancing condensate drainage from flat flow tube-corrugated fin type evaporators typically follow the obvious expedient of simply providing vertical drainage troughs in the external surfaces of the flow tubes, creating a gap relative to the external fin crest, while making no change in the fin design itself. For example, U.S. Patent 4,621,685 issued November 11, 1986 to Nozawa and U.S. Patent 4,966,230 issued October 30, 1990 to Hughes et al., both show vertical drain troughs pressed into the flow tube surface at a point about midway along the length of the fins. U. S. Patents 4,353,224 issued October 12, 1982 to Nonogaki et al. and 4,926,932 issued May 22, 1990 to Ohara et al, both show a vertical drain trough formed in the flat flow tube surface located near the trailing edge of both the flow tube and the fin, recognizing that that is where the film thickness is greatest. All that a drain trough in the flow tube surface can do, however, regardless of its location, is to help drain water from the external fin-to-tube-surface interface, which, as noted above, is not the primary problem. A trough in the flow tube external surface cannot have much, if any, effect on the internal fin channel, since it does not even directly open into the internal fin channel where the film collects.

SUMMARY OF THE INVENTION

An improved corrugated heat conduction fin in accordance with the subject invention is characterised by the features specified in Claim 1. The subject invention takes a very different design approach to enhancing condensate drainage. No vertical troughs are formed in the flow tube surface. No change is made to the basic dimensions of the fin or fin walls, or to the banks of louvers. A very slight change is made to the areas of the fin walls located between the louver banks. This area is altered in such a way that the surface tension forces cannot create a film. Consequently, the typical long and continuous film is broken up into a series of shorter films, each located directly over a respective louver bank, but with no film forming in the area between louver banks. These individual, discontinuous films, for reasons not perfectly understood, are less stable, and therefore drain more frequently and efficiently down through the louver banks over which they form. The critical film width is significantly less, as is the air pressure drop across the core. Efficiency is conversely enhanced.
In the preferred embodiments disclosed, the fin wall areas between louver banks are rendered incapable of supporting a water film by the very simple expedient of piercing through the fin walls at their channel forming juncture with an aligned pair of small, semi circular notches. Water film cannot form on a surface that does not exist, so the continuos film is broken at each notch, between each bank of louvers. While the aligned notches in consecutive fins would appear to effectively form vertical drainage troughs opening through the fin channels, drainage does not, surprisingly, appear to take place through the aligned notches to any significant extent. Instead, what apparently happens is that drainage still occurs primarily through the louver banks, but with the critical thickness of the various individual films greatly reduced. Two possible embodiments of the notches are disclosed, with the same basic shape and effect, but manufactured differently.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will appear from the following written description, and from the drawings, in which:
Figure 1 is a front view of an evaporator of the general type in which the improved fin of the invention is incorporated;
Figure 2 is an enlarged circled portion of Figure 1;
Figure 3 is an exploded perspective view of two flow tubes and one fin;
Figure 4 is a plan view of the length of one fin wall of the conventional fin described above;
Figure 5 is a cross section of the fin taken along the line 5-5 in Figure 3;
Figure 6 is a view like Figure 4, after a water film has started to form;
Figure 7 is a cross section taken along the line 7-7 of Figure 6;
Figure 8 is a view like Figure 6, but after the film has reached its critical width;
Figure 9 is a cross section taken along the line 9-9 of Figure 8;
Figure 10 is a plan view of the length of one fin wall of a first embodiment of a fin made according to the invention;
Figure 11 is a cross section taken along the line 11-11 of Figure 10;
Figure 12 is a plan view like Figure 10, after a water film has started to form;
Figure 13 is a cross section taken along the line 13-13 of Figure 12;
Figure 14 is a view like Figure 12, after the film has reached its critical width;
Figure 15 is a cross section taken along the 15-15 of Figure 14;
Figure 16 is a perspective view of the fin in the same condition as in Figure 14;
Figure 17 is a perspective view of just a pair of the discontinuous water films from Figure 16;
Figure 18 is a perspective view of an alternate embodiment of the fin; and
Figure 19 is a cross section taken along the line 19-19 of Figure 18.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to Figures 10 and 11, a first embodiment of an improved fin made according to the invention is indicated generally at 20. Fin 20 has essentially every feature that the conventional fin 14 described above has, and with the same dimensions. Fin walls 22 identical to fin walls 16 diverge from relatively sharp, integral internal channels with the same small included angle and radius. Fin wall width and length are the same, and the louver banks 24 have the same number, size, spacing, orientation, and dimensions for the individual louvers. This is significant, as the louver banks 24 have a shape and size intended to optimize air flow, not to optimize condensate drainage, even though they do coincidentally provide condensate drainage out of the channels. In fact, were the dimensions of the louver banks themselves changed so as to enhance condensate drainage, that would likely involve an increase in the louver openings' width, louver angles or the like, which could negatively affect their primary function of air flow enhancement. The subject invention instead alters a different area of the fin in a way that cooperates with conventional louver banks 24 to enhance condensate drainage. Specifically, in the flat area of each pair of diverging fin walls 22 that is located between each pair of adjacent louver banks 24, a localized void in the form of an aligned pair of generally semi circular notches 26 is cut completely out. In terms of manufacturing, this would most conveniently be done concurrently with the cutting of the louver banks 24, and a feature to do so would be incorporated within the same cutting and forming tool. For each fin wall 22, one notch between each adjacent pair of four louver banks 24 amounts to three notches 26 total. The radius of notch 26 is approximately one millimeter, making it wide enough to take up much of the unused area between adjacent louver banks 24, and deep enough to exceed Δ as defined above. The notches 26, when viewed from the perspective of Figure 10, would all be vertically aligned, taking on the aspect of a vertical trough. Surprisingly, however, the aligned notches 26 do not act as, and are not intended to act as, open vertical drainage paths, as is described next.
Referring next to Figures 12 and 13, an evaporator incorporating fin 20 would be identical to evaporator 10, with the same flow tubes 12, would be assembled in the same way, and would also be operated at all the same parameters, in terms of refrigerant flow rate, air flow rate and direction, and temperature. Water would condense on the internal channel forming surfaces of the fin walls 22, just as with the fin walls 16, but with a very significant difference. The notches 26, being incapable of forming a surface film, break the otherwise continuos film F into a series of discontinuous, individual films, noted at F1, F2 and F3, with F3 being the farthest downstream. Each of the separate films F1, F2 and F3 is located almost entirely over a respective louver bank 24, and displaced somewhat in the direction of air flow, which is downward on the page, as shown by the arrows. At the early point of film development shown in Figures 12 and 13, surface tension forces still predominate. The three films have grown wide enough to just begin overlapping with the ends of the openings of the individual louvers, but have not yet become unstable enough to begin to drain through. The film width, indicated at W', is measured in the same direction as the width W described above, and the film thickness, by definition, runs from inner surface to inner surface of the fin walls 22 between which the film is drawn and formed.
Referring next to Figures 14 through 17, the situation where the three individual films F1, F2 and F3 have reached their greatest width, that is, their critical width. The critical width W' is closer here to the initial film width, reaching approximately only three to three and a half millimeters, enough to overlap with the louver banks 24, but significantly narrower than that for the single film F on fin 14 noted above. Again, by critical width, it is meant that the films have become unstable and have begun to drain down through the louver banks 24 and shrink, before re expanding in a continuing cycle. The net drainage rate is thus greater than that achieved with conventional fins, and the significantly smaller critical film width is very beneficial in terms of allowing air to flow more freely through the fins 20, and the air pressure drop has been observed to be almost twenty five percent less in tests. The brazed juncture between the external crests of the folded fin walls 22 and the flow tube surface 12, indicated by the dotted plane in Figure 16, is critical to thermal conduction between air and refrigerant, since it is the closest contact of the fin wall 22 (air) to the surface of the flow tube 12 (refrigerant). Consequently, any blockage of air flow through the fin internal channels, which are the direct obverse of the fin external crests, is detrimental to that critical heat conduction path. As best seen in Figures 16 and 17, by decreasing the critical width of the water films that inevitably form in those internal fin channels, air flow and thermal efficiency are proportionately enhanced.
As already noted, the mechanism by which the invention works is not perfectly understood. It is clear that the single, wider film F is broken up into individual, discontinuous narrower films, It is clear that those individual films F1, F2 and F3 are less stable and less prone to surface tension forces, and that they drain through the louver banks 24 sooner and more efficiently, because they would not have such a significantly smaller critical width otherwise. But why they are less stable and better able to drain has not been fully analyzed at this point. What is clear, surprisingly, is that water is not draining significantly through the aligned notches 26 themselves, despite their coincidental similarity, when viewed vertically, to drain troughs. The trailing ends of the films F1, F2 and F3 "crowd up" to the leading edges of the notches 26, but have not been observed to be spilling any significant amounts of water over those edges and down through the notches 26. The notches 26 would not be able to drain a great deal water out of the individual films anyway, since they do not overlap with them, and do not constitute a significant amount of open area, as compared to all the pre existing louver openings. Instead, the notches 26 assist the pre existing ability of the louver banks 24 to act as drains.
An alternate embodiment of the invention, designated generally at 28, is illustrated in Figures 18 and 19. It works exactly the same way, and has the same basic dimensions as fin 20, but has some potential manufacturing and structural advantages. Fin 28 has identical fin walls 30 and louver banks 32, as compared to fin 20. However, instead of cutting out voids in the form of aligned pairs of semi circular notches 26 through each fin wall 30, a flap of wall material is lanced inwardly from and out of the crest, forming a generally vertical strut 38 in the internal channel and a single window 36 behind the strut 38. The single window 36 removes fin wall surface area from both fin walls 30, between the louver banks 32, effectively creating a pair of aligned notches in each pair of diverging fin walls 30 simultaneously. However, no individual scrap pieces of material are removed, as with the aligned pairs of notches 26. In addition, the vertical strut 38 acts to brace and strengthen the juncture of the fin walls 30 at the very point where the window 36 is formed. Being thin and basically aligned with the air flow, the struts 38 would not themselves significantly block air flow through the channels, while the windows 36 would break up the surface film of water in the same fashion as the notches 26.
Variations in the embodiments disclosed could be made, while achieving the same basic result. Fundamentally, what is done is to eliminate, or at least significantly reduce, the affinity of the inner surfaces of the fin walls for water film formation. However, that is not done over the entire surface area of the fin wall, as a complete hydrophobic coating would do, but only locally, just within the residual fin wall areas between the louver banks, and near the bottom of the fin channels. These are the areas where the single, continuous film would otherwise have bridged across individual louver banks. Means other than complete voids might be used to locally reduce the ability of a water film to form. Alternate means might include localized hydrophobic coatings, surface roughenings, or the like, applied to the same area as the notches disclosed above, in order to substantially reduce the ability of the surface to support and create a water film, but without removing the surface completely. Clearly, a void represents the ultimate in reduction of surface tension potential, since it has no surface at all. Voids with a shape other than the semi circular notch 26 could be used, such as square or triangular, although a semi circular is generally the easiest to punch out. Therefore, it will be understood that it is not intended to limit the invention to just the embodiments disclosed.

Claims

In a heat exchanger (10) of the type having generally vertically oriented, regularly spaced pairs of flat flow tubes (12) across which humid air subject to condensation is generally horizontally blown, and in which corrugated heat conduction enhancing fins (20) conductively joined between the flow tube pairs (12) have integral, diverging fin walls (22) the inner surfaces of which form generally horizontally oriented, internal channels within which condensed water is retained by surface tension forces in a continuos film (F) the width of which tends to retard the flow of air therethrough, said fin walls (22) also having a regularly spaced series of louver banks (24) opening therethrough that overlap partially with the channels in which said water film forms, characterized in that said fin walls (22) further include,
a series of discrete areas located in the channels and between the regularly spaced louver banks (24) having a reduced affinity for water film formation, whereby a series of discontinuous, individual condensation films (F1, F2, F3) is formed, each located at least partially over a respective louver bank (24), thereby enhancing drainage through the louver banks (24) and reducing the critical width of said discontinuous films (F1, F2, F3) to improve air flow between the fin walls (22).
A corrugated heat conduction enhancing fin (20) according to Claim 1, further characterised in that a series of voids in the form of aligned pairs of notches (26) are formed through the diverging fin walls (22) between adjacent pairs of louver banks (24) and located in the fin channels.
A corrugated heat conduction enhancing fin (28) according to Claim 2, further characterized in that aligned pairs of notches are effectively created by lancing an integral strut (38) inwardly of the fin channels and between adjacent pairs of louver banks (32), thereby leaving a window (36) opening through both diverging fin walls (30).