Claims
- 1. A tube and fin heat exchanger comprising a plurality of tubes arranged in spaced side-by-side relationship, a folded strip forming a plurality of louvered fins arranged in spaced side-by-side relationship and between and in heat transfer relationship with adjacent ones of said tubes, said fins having a thickness and stacked density such as to constitute not more than about 12% nor less than about 2.5% of the space between the adjacent tubes, said fins further having a total louvered area not more than about 60% nor less than about 40% of the total fin area, and said fins having a length along their air flow facing edge per unit of area between the tubes divided by said unit area that as measured in millimeters is not more than about 1.2 mm.sup.-1 nor less than about 0.68 mm.sup.-1.
- 2. A tube and fin heat exchanger comprising a plurality of tubes arranged in spaced side-by-side relationship, a folded strip forming a plurality of louvered fins arranged in spaced side-by-side relationship and between and in heat transfer relationship with adjacent ones of said tubes, said fins having a thickness and stacked density such as to constitute not more than about 12% nor less than about 2.5% of the space between the adjacent tubes, said fins further having a total louvered area not more than about 60% nor less than about 40% of the total fin area completely located directly between the adjacent tubes, and said fins having a length along their air flow facing edge per unit of area between the tubes divided by said unit area that as measured in millimeters is not more than about 1.2 mm.sup.-1 nor less than about 0.68 mm.sup.-1.
- 3. A tube and fin heat exchanger comprising a plurality of flat tubes arranged in spaced side-by-side relationship, a folded strip forming a plurality of louvered fins arranged in spaced side-by-side relationship and between and in heat transfer relationship with adjacent ones of said tubes, said fins having a thickness and stacked density such as to constitute not more than about 12% nor less than about 2.5% of the space between the adjacent tubes, said fins further having a total louvered area not more than about 60% nor less than about 40% of the total fin area, said fins having a length along their air flow facing edge per unit of area between the tubes divided by said unit area that as measured in millimeters is not more than about 1.2 mm.sup.-1 nor less than about 0.68 mm.sup.-1, and said fins further having at least one strengthening beam of reverse bent cross-sectional configuration formed integral therewith and extending thereacross between the adjacent flat tubes for strengthening the fins against crushing from external forces and the flat tubes against ballooning from internal forces.
TECHNICAL FIELD
This application is a continuation-in-part of application Ser. No. 776,419, filed Sept. 16, 1985 which is a continuation of application Ser. No. 642,100, filed Aug. 20, 1984 now abandoned.
This invention relates to tube and fin heat exchangers and more particularly to louvered fin arrangements therefor.
In flat tube and fin heat exchangers such as used as radiators in vehicle engine cooling systems, the design and geometry of the fin (also called air center) has progressed from a plain surface with arbitrary dimensions to various louver arrangements and dimensions tailored from experience to improve the heat transfer efficiency and thereby decrease the necessary core size. Illustrative of the various louvered fin designs that have evolved are those disclosed in U.S. Pat. Nos Modine 1,726,360; Saunders U.S. Pat. No. 2,063,757; Simpelaar U.S. Pat. No. 2,703,226; Simpelaar U.S. Pat. No. 2,789,797; Morse U.S. Pat. No. 3,003,749; Rhodes et al U.S. Pat. No. 3,250,325; Nickol et al U.S. Pat. No. 3,265,127; Przyborowski U.S. Pat. No. 3,298,432; Jentet U.S. Pat. No. 3,305,009; Brown U.S. Pat. No. 3,521,707; Rhodes 3,993,125; Verhaegle et al U.S. Pat. No. 4,311,193; Cheong et al U.S. Pat. No. 4,328,861 and Hiramatsu U.S. Pat. No. 4,332,293. Typically, compact heat exchangers utilize extended fin surface area to increase spatial efficiency with the heat transfer performance tending to be enhanced as the fin density (number of fins per unit length along the tubes) increases provided sufficient working fluid (air) mass flow rate is maintained across the extended fin surface. As seen in the above louvered fin designs, various arrangements of consecutive multiple louvers have been utilized to create turbulence in the working fluid; the theory being that such added turbulence increases the convection heat transfer coefficient resulting in increased heat transfer by insuring maximum temperature differential between the extended surface and the working fluid (air). Although the louvers do increase the heat transfer performance of an extended surface fin at a constant air mass flow rate, such increase is typically gained at the expense of air pressure drop.
Various louver spacing, louver angle and fin density combinations give different trade off relationships but it has been found that certain identifiable important trends do evolve whose positive attributes can best be utilized as disclosed later in the present invention by not simply adding more louvers or altering their geometry within a pressure drop constraint as has been the normal practice. As to such trends, it has been found for example that at constant air mass flow, the heat rejection increases with fin density if the internal geometry (i.e. that of the louvers) remains unchanged. Furthermore, the heat transfer actually becomes less sensitive to louver angle as the fin density increases. On the other hand, the air side pressure drop and therefore the air flow rate becomes more sensitive to louver angle as the fin density increases. Moreover, both the heat transfer and the air side pressure drop become more significant as core depth increases along with the fin density. However, increase in core depth can be avoided with increased fin density but then the louvers must be specially tailored to avoid excessive air side pressure drop. And then there are also vehicle performance factors to consider related to radiator performance. For example, the air side pressure drop limitations may be determined by whether an electric fan or an engine driven fan forces the air flow past the fins in the radiator combined with vehicle ram air effectiveness and functional interrelationships between engine heat rejection and vehicle operating condition. Then there are the affects of air flow variations on air conditioning system performance as well as the variance in engine cooling requirements between diesel and gasoline engines. In addition, there are various manufacturing considerations such as the sensitivity of louver formation and thus performance to tool settings, fin handling and core stacking and their attendant costs, flux/degreaser entrapment and fin resistance to crushing along with column strength to prevent ballooning of the flat tubes from internal pressure.
The present invention runs counter to accepted practice in providing a unique but simple geometrical solution to the air side pressure drop problem following on the discovery of certain critical or controlling parameters. As will be shown in various embodiments, with a relatively few strategically located louvers and a prescribed fin density the advantages of both multilouver and plain fin designs are effectively combined so as to substantially reduce the air side pressure drop from the current conventional multilouver-caused level but without a significant reduction in the constant mass flow heat transfer performance. This new fin design, because it combines the advantages of both a multilouver fin and a plain fin, will be referred to as a "hybrid" fin or air center.
In the making of this invention, flat tube and fin radiators with the early conventional plain fins (without louvers) were tested and it was found that encouraging but less than adequate heat transfer performance occurred at low (less than typical) air mass flow rates when compared to equivalent size radiators with the current conventional multilouver fins (louvers over most of the fin area) and higher fin densities (e.g. greater than 14 fins per inch) found adequate for the typical encountered air flow rates of 60-90 pounds per minute per square foot of radiator frontal area. The approach taken in the present invention was to vary the internal geometry of the heat exchanger, i.e. the louvering versus plain surface of the fin, to reduce the associated air side pressure drop at a given rate of heat transfer and mass flow as compared to conventional multilouvered fins. In pursuit of this objective to alter the air flow restrictivity only, the inventor was cognizant that in heat transfer applications where the rate of fluid flow over an extended surface is sufficiently sensitive to the restrictivity of the heat exchanger, the overall heat exchanger performance can actually be reduced where the extended surface, i.e. that of the fins, is increased to a point where it adds excessive core restrictivity. And since the heat transfer mechanism between the extended surface and the working fluid (air) is essentially convective, the governing heat transfer equation is
Moreover, it was discovered that each of the fins could then further be provided with an integral beam extending thereacross between the adjacent tubes so as to strengthen the fins against crushing from external forces and in the case of flat tubes also strengthen same against ballooning from internal forces or pressure. The net result was a compact highly efficient tube and fin heat exchanger ideally suited for use as a radiator in a vehicle's engine cooling system wherein with the prescribed new hybrid fin design and fin density, the various vehicle performance factors and manufacturing considerations previously identified are effectively accounted for in a cost efficient manner.
Considering the application of the present invention to multiple tube row arrangements, i.e. two or more rows of tubes as compared to a single row application, it was found that the typical multi-louvered pattern of the fin between the rows of tubes acted in effect as a heat conductivity restriction. As a result, heat transferring to areas on the extended fin surface between successive tubes was then hindered by the louver interruptions in the conductive flow path. The hybrid fin design of the present invention, on the other hand, with its stragetic location of fewer louvers and extended plain areas between successive tubes provides an increased conductive flow path between the tubes and thereby effectively reduces the temperature gradient in the fin. This improves the heat transfer efficiency because more of the fin mass then approaches the base temperature effectively increasing the T between the extended fin surface and the working fluid (air).
Summarizing then the significant advantages of the hybrid fin arrangement of the present invention, it will be appreciated that this new design arrangement operates to maintain the performance of the heat exchanger in nonrestriction sensitive systems while increasing performance where the working fluid flow rate is restriction sensitive. Moreover, the hybrid fin makes it possible to increase the fin density substantially without a significant reduction in fluid flow for restriction sensitive systems while improving the effectiveness with which the extended fin surface is utilized and thereby increase the cooling capabilities within a given spatially constrained heat exchanger. For instance, the core content may then be reduced by increasing fin density to reduce core volume. On the other hand, within the same geometry, the hybrid fin concept acts to control restrictivity to an acceptable level while actually matching heat transfer characteristics of the more material intensive core. This unique combination also provides for improved manufacturability with less flux/degreaser entrapment and lower tooling costs with fewer cutting tools (typically discs). Furthermore, it will be appreciated that there is provided improved performance reliability with the reduction in the number of louvers because the fin's internal geometry variations such as louver angle have less of an affect on core performance. And particularly important with respect to the downsizing of cars, there is provided the capability of substantially reducing the core depth by replacing it with increased fin density where core face area is restricted and the airflow rate is particularly sensitive to pressure drop. Furthermore, the lower hood lines for improved aerodynamics leads to low aspect ratio radiators and resultantly lower fan area/core face ratios and therefore a need to increase ram air effectiveness as provided by the present invention to maximize the core utilization in those portions not covered by the fan and shroud.
US Referenced Citations (3)
Continuations (1)
|
Number |
Date |
Country |
| Parent |
642100 |
Aug 1984 |
|
Continuation in Parts (1)
|
Number |
Date |
Country |
| Parent |
776419 |
Sep 1985 |
|
Patent Grant
4693307
