A finned tube heat exchanger, comprising:
- a plurality of closely spaced parallel fin plates;
- at least one row of heat transfer tubes passing through the fin plates generally perpendicular thereto and secured in heat conductive relationship thereto to provide heat transfer between a first fluid circulating through the tubes and a second fluid flowing across and between the fin plates, the second fluid flowing in a nominal fluid flow direction, and flowing in local flow directions as it passes around the circumference of the tubes, the nominal fluid flow direction defined as a single direction irrespective of the local flow directions, the tubes defining tube centers;
- said fin plates having fin collars around the heat transfer tubes and groups of louvers formed in the areas generally between adjacent tubes of a row through which the second heat exchange fluid flows;
- said groups of louvers comprising elongate strips systematically formed between a number of slits formed in the fin perpendicular to the nominal fluid flow direction of flow of the second fluid;
- selected ones of said strips offset from the nominal plans of the fin plate by end portions of the strips which are formed to position the intermediate portions of the strips parallel to the fluid direction and to the nominal plane of the fin plate, with individual strips offset in different directions and amounts, in the direction parallel to the axes of the tubes, to produce a staggered pattern of flat louvers;
- the end portions of said strips adjacent the tubes being formed at angles to be substantially tangent to local flow streamlines of the second fluid as it passes around the circumference of the tubes systematically according to a formula of substantially constant U/V, where the formula is U/V=(Y-R.sup.2 ((Y-Z)/(X.sup.2 +(Y-Z).sup.2))+((Y+Z)/(X.sup.2 +(Y +Z).sup.2)))), where R equals the outside radius of the fin collars, Z equals half the distance between the heat transfer tube centers, X denotes a coordinate of points in the fin where X increases in the direction of airflow and Y denotes a coordinate of points in the fin where Y increases in a direction perpendicular to the airflow and to the X direction, where the origin (X=O, Y=0) is defined as being directly between tube centers of two adjacent tubes within the same row, and where the formula is solved for values of X, Y of constant U/V at which to locate points along the end portions of said strips;
- and
- said fin plates having sloped portions adjacent the areas thereof which are between tubes of a row and which have said louvers, said sloped portions extending in upstream and downstream directions, respectively, with respect to the direction of flow of said second fluid so that the leading edge of the upstream sloped portion and the trailing edge of the downstream sloped portion are offset from the nominal plane of the fin plate in the area between the tubes so as to provide an overall corrugation to the fin to increase its stiffness.