Patent Publication Number: US-2020300548-A1

Title: Evaporative heat exchange apparatus with finned elliptical tube coil assembly

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to improvements in tubes in a coil assembly for use in an evaporative heat exchange apparatus in which the coil assembly is to be mounted in a duct or plenum of the apparatus in which external heat exchange fluids, typically a liquid, usually water, and a gas, usually air, flow externally through the coil assembly to cool an internal heat transfer fluid passing internally through the tubes of the coil assembly. The improvements concern the use of tubes or segments of the tubes having a generally elliptical cross-section, in combination with tube orientation, arrangement and spacing, and fin spacing, height and thickness, all of which must be carefully balanced, to provide increased heat transfer coefficients with an unexpected relatively low air pressure drop that produces high air volume that together produces very high heat exchange capacity. 
     Preferably, though not exclusively, the finned tube coil assembly of the present invention using tubes that have finned segments with generally elliptical cross-sections, is most effectively mounted in a counterflow evaporative heat exchanger so that water flows downwardly and externally through the coil assembly while air travels upwardly and externally through the coil assembly. The coil assembly of the present invention can be used also in a parallel flow evaporative heat exchanger in which the air travels in the same direction over the coil assembly as the water, as well as in a crossflow evaporative heat exchanger, where air travels over the coil in a direction transverse to the flow of the water. The evaporation of the water cools the coil assembly and the internal heat transfer fluid inside the tubes forming the coil assembly. 
     The tubes may be used in any type of evaporative heat exchange coil assembly made of an array of several, and preferably, many tubes that can have a variety of arrangements. The tubes are preferably arranged in generally horizontal rows extending across the flow path of the air and water which flow externally through the coil assembly, whether the air and water are in counterflow, parallel flow or crossflow pathways. The ends of the tubes may be connected to manifold or headers for appropriate distribution of the internal heat transfer fluid. The internal heat transfer fluid may be a heating fluid, a cooling fluid or a processing fluid used in various types of industrial processes, where the temperature of the internal heat transfer fluid needs to be modified, typically but not exclusively by cooling, and often but not exclusively by condensing, as a result of the heat transfer through the walls of the tubes by the external heat exchange fluids. 
     Typically, evaporative heat exchange apparatus use a number of serpentine tubes for the coil assemblies, and such serpentine tubes are often the preferred type of tubes used due to the ease of manufacture of effective coil assemblies from such tubes. While other types of tubes of the present invention useful for the evaporative heat exchange apparatus of the present invention, the tubes and coil assemblies of the present invention will primarily be described, without limitation, with respect to the preferred serpentine tubes. The following background information is provided to better understand the relationship of the tube and coil assembly components using serpentine tubes. Each serpentine tube comprises a plurality of two different types of portions, “segments” and “return bends.” The segments are generally straight tube portions which are connected by the return bends, which are the curved portions, sometimes referred to as “bights,” to give each tube its serpentine structure. In a preferred embodiment of the coil assembly of the present invention, the tubes, which may be generally straight in structure (referred to hereinafter as “straight tubes”), or the segments of each of the serpentine tubes, are generally elliptical in cross-section and the return bends can be any desired shape and are typically generally circular, generally elliptical, generally kidney-shaped or some other shape in cross-section. The generally horizontal maximum dimension of the generally elliptical segments is usually equal to or smaller than the generally horizontal cross-sectional dimension of the return bends, especially if the return bends have a circular cross-section. If desired, the return bends can have an elliptical cross-section, or a kidney-shaped cross-section, but it is usually easier to make the return bends with a circular cross-section. The segments of horizontally adjacent serpentine tubes are spaced from each other by the larger horizontal cross-section of the return bends when the return bends are in contact with each other, or may be spaced by vertically-oriented spacers between the return bends, depending on the design characteristics of the evaporative heat exchange apparatus in which the coil assemblies are used. 
     In the coil assemblies, the straight tubes or the segments of the serpentine tubes are preferably arranged in generally horizontal rows extending across the flow path of the air and water which flow externally through the coil assembly, whether the air and water are in counterflow, parallel flow or crossflow pathways. 
     Evaporative heat exchangers using coil assemblies using serpentine tubes having segments with generally elliptical cross-sections are also known, for example as disclosed in U.S. Pat. Nos. 4,755,331 and 7,296,620, the disclosures of which are hereby incorporated herein in their entireties, which are assigned to Evapco, Inc., the assignee of the present invention. These patents do not disclose or contemplate the use of finned tubes in the coil assembly in the evaporative heat exchange environment. 
     Finned tubes used in coil assemblies of dry (non-evaporative) heat exchangers are known and are used in view of the greater surface area provided by the fins to dissipate heat by conduction when exposed to air flowing externally through the coil assembly of the dry heat exchanger. Generally, the fins in such dry heat exchangers do not materially adversely affect the flow of air through the coil assembly of the dry heat exchanger. Finned coils are also used extensively in coil assemblies of products like home refrigerators to dissipate the heat to the ambient air. 
     Examples of coil assemblies for dry heat exchangers made using fins in the form of sheets or plates having holes though which segments having generally elliptical cross-sections pass are disclosed in Evapco, Inc.&#39;s U.S. Pat. Nos. 5,425,414, 5,799,725, 6,889,759, and 7,475,719. However, such coil assemblies are not useful with evaporative heat exchangers, since the sheets or plates would adversely affect the mixing and turbulence of the air and water involved with evaporative heat exchange that must pass externally through the coil assembly. 
     Evapco, Inc. and others have used finned tube coil assemblies in evaporative heat exchangers where the segments of the tubes in the coil assemblies have circular cross-sections that include fins extending along the length of the individual segments of the tubes. The segments having circular cross-sections are relatively easy to provide with fins, such as by spirally wrapping the segments with strips of metal forming the fins. These finned tubes have been used in evaporative heat exchangers, but in limited circumstances and with limited success. First, round tube coils with fins have been employed in heat exchangers to enhance dry cooling capacity in cold weather applications when not much capacity is needed and when using water as an external heat exchange liquid could result in freezing and other problems. Such uses were rather rare and were provided to deal with a problem, as opposed to a way to improve the primary function of evaporative cooling according to the present invention. Second, though round tube coils with fins have also been employed to improve evaporative cooling, this has not been successful. While the presence of the fins increases the heat transfer coefficient, in prior attempts the increases were offset because the fins also caused decreased air flow over the coil, thus resulting in lower performance. 
     The finned tube coil assembly of the present invention provides a number of significant advantages. The combination of the shape of the tubes, the spacing of the tubes, the height of the fins, and the number of fins per inch have resulted in exceptional and unexpected increases in evaporative thermal performance. The geometry of the tubes and their orientation and arrangement with a coil assembly play an essential part in the turbulent mixing of the air and water. The generally elliptical cross-sectional shape of the segments provides the advantages of a large amount of surface area of the tubes in a coil assembly, effective flow and heat transfer of process fluid internally within the tubes and enhanced external air and water flow characteristics. With the present invention, the surprising result of less resistance to the air and water passing externally through the coil assembly allows the use of higher air volume that provides additional thermal capacity compared to the prior art systems without adding any fan energy. The finned tubes provide an enhanced surface area for conductive heat exchange with the tubes and aid in turbulent mixing of the air and water externally flowing through the coil assembly, enhancing convective heat exchange between the air and the water. The finned tubes take up space that may impede the water and air flow and thereby would be expected to cause a very significant air side pressure drop, with the need for stronger motors for fans to move the air through the coil assembly in the heat exchanger. However, the finned tubes with generally elliptical cross-sections having the characteristics of the present invention not only provide a careful balance of enhanced coil assembly surface area for conductive heat exchange with any fluid flowing within the interior of the tubes and mixing and turbulence of the air and water for the convective heat exchange but also provide a surprising reduction in the air side pressure drop through the coil assembly, while retaining a very large increase in external heat transfer coefficient. 
     The overall capacity of the coil assembly of the present invention and evaporative heat exchangers containing it are greatly improved at nominal, or in certain circumstances even reduced cost, compared to the increase in capacity. For example, the cost per cooling ton may be reduced by, for instance, replacing a coil assembly using more non-finned tubes with a coil assembly using fewer finned tubes of the present invention. Additionally, an evaporative heat exchanger of a given size using non-finned tubes of the prior art could be replaced with a smaller evaporative heat exchanger according to the present invention that achieves the same or better thermal performance. Moreover, using a coil assembly having the finned tubes of the present invention could significantly reduce required fan energy, and therefore overall power consumption, as compared to a non-finned coil assembly of the same size. 
     Various types of heat exchange apparatus are used in a variety of industries, from simple building air conditioning to industrial processing such as petroleum refining, power plant cooling, and other industries. Typically, in indirect heat exchange systems, a process fluid used in any of such or other applications is subject to heating or cooling by passing internally through a coil assembly made of heat conducting material, typically a metal, such as aluminum, copper, galvanized steel or stainless steel. Heat is transferred through the walls of the heat conducting material of the coil assembly to the ambient atmosphere, or in a heat exchange apparatus, to other heat exchange fluid, typically air and/or water flowing externally over the coil assembly where heat is transferred, usually from hot processing fluid internally within the coil assembly to the cooling heat exchange fluid externally of the coil assembly, by which the internal processing fluid is cooled and the external heat exchange fluid is warmed. 
     In evaporative indirect heat exchange apparatus in which the finned tube coil assembly of the present invention is used, heat is transferred using indirect evaporative exchange, where there are three fluids: a gas, typically air (accordingly, such gas will usually be referred to herein, without limitation, as “air”), a process fluid flowing internally through a coil assembly of tubes, and an evaporative cooling liquid, typically water (accordingly, such external heat exchange or cooling liquid will usually be referred to herein, without limitation, as “water”), which is distributed over the exterior of the coil assembly through which the process fluid is flowing and which also contacts and mixes with the air or other gas flowing externally through the coil assembly. The process fluid first exchanges sensible heat with the evaporative liquid through indirect heat transfer between the tubes of the coil assembly, since it does not directly contact the evaporative liquid, and then the air stream and the evaporative liquid exchange heat and mass when they contact each other, resulting in more evaporative cooling. 
     In other embodiments, direct evaporative heat exchange may be used together with the indirect evaporative heat exchange involving the finned tube coil assembly of the present invention, as explained in more detail hereinafter, to provide enhanced capacity. In direct evaporative heat exchange apparatus, air or other gas and water or other cooling liquid may be passed through direct heat transfer media, called wet deck fill, where the water or other cooling liquid is then distributed as a thin film over the extended fill surface for maximum cooling efficiency. The air and water contact each other directly across the fill surface, whereupon a small portion of the distributed water is evaporated, resulting in direct evaporative cooling of the water, which is usually collected in a sump for recirculation over the wet deck fill and any coil assembly used in the apparatus for indirect heat exchange. 
     Evaporative heat exchangers are commonly used to reject heat as coolers or condensers. Thus, the apparatus of the present invention may be used as a cooler, where the process fluid is a single phase fluid, typically liquid, and often water, although it may be a non-condensable gas at the temperatures and pressures at which the apparatus is operating. The apparatus of the present invention may also be used as a condenser, where the process fluid is a two-phase or a multi-phase fluid that includes a condensable gas, such as ammonia or FREON® refrigerant or other refrigerant in a condenser system at the temperatures and pressures at which the apparatus is operating, typically as part of a refrigeration system where the process fluid is compressed and then evaporated to provide the desired refrigeration. Where the apparatus is used as a condenser, the condensate is collected in one or more condensate receivers or is transferred directly to the associated refrigeration equipment having an expansion valve or evaporator where the refrigeration cycle begins again. 
     The present invention uses a finned tube coil assembly where the claimed combination of factors of tube shape, orientation, arrangement and spacing, and fin spacing, height and thickness, all of which must be carefully balanced, to provide increased heat transfer coefficients with an unexpected relatively low air pressure drop that produces high air volume. The combination of increased heat transfer coefficients with high air volume produces very high heat exchange capacity. 
     Definitions 
     As used herein, the singular forms “a”, “an”, and “the” include plural referents, and plural forms include the singular referent unless the context clearly dictates otherwise. 
     Certain terminology is used in the following description for convenience only and is not limiting. Words designating direction such as “bottom,” “top,” “front,” “back,” “left,” “right,” “sides,” “up” and “down” designate directions in the drawings to which reference is made, but are not limiting with respect to the orientation in which the invention and its components and apparatus may be used. The terminology includes the words specifically mentioned above, derivatives thereof and words of similar import. 
     As used herein, the term “about” with respect to any numerical value, means that the numerical value has some reasonable leeway and is not critical to the function or operation of the component being described or the system or subsystem with which the component is used, and will include values within plus or minus 5% of the stated value. 
     As used herein, the term “generally” or derivatives thereof with respect to any element or parameter means that the element has the basic shape, or the parameter has the same basic direction, orientation or the like to the extent that the function of the element or parameter would not be materially adversely affected by somewhat of a change in the element or parameter. By way of example and not limitation, the segments having a “generally elliptical cross-sectional shape” refers not only to a cross-section of a true mathematical ellipse, but also to oval cross-sections or somewhat squared corner cross-sections, or the like, but not a circular cross-section or a rectangular cross-section. 
     Similarly, an element that may be described as “generally normal” to or “generally parallel to” another element can be oriented a few degrees more or less than exactly 90° with respect to “generally normal” and a few degrees more or less than exactly perfectly parallel or 0° with respect to “generally parallel,” where such variations do not materially adversely affect the function of the apparatus. 
     As used herein, the term “substantially” with respect to any numerical value or description of any element or parameter means precisely the value or description of the element or parameter but within reasonable industrial manufacturing tolerances that would not adversely affect the function of the element or parameter or apparatus containing it, but such that variations due to such reasonable industrial manufacturing tolerances are less than variations described as being “about” or “generally.” By way of example and not limitation, “fins having a height extending from the outer surface of the segments a distance of substantially 23.8% to substantially 36% of the nominal tube outside diameter” would not allow variations that adversely affect performance, such that the fins would be too short or too tall to allow the evaporative heat exchanger to have the desired enhanced performance. 
     As used herein, the term “thickness” with respect to the thickness of the fins, refers to the thickness of the fins prior to treatment after the fins are applied to the tubes to make the finned tubes, such as galvanizing the tubes or the coil assembly using the finned tubes, as such treatment would likely affect the nominal thickness of the fins, the nominal fin height and the nominal spacing of the fins. Thus, all of the dimensions set forth herein are of the finned tubes prior to any later treatment of the finned tubes themselves or of any coil assembly containing them. 
     As used herein, where specific dimensions are presented in inches and parenthetically in centimeters (cm), the dimensions in inches controls, as the centimeter dimensions were calculated based on the inches dimensions by multiplying the inches dimensions by 2.54 cm per inch and rounding the centimeter dimensions to no more than three decimal places. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to an improvement in an evaporative heat exchanger comprising a plenum having a generally vertical longitudinal axis, a distributor for distributing an external heat exchange liquid into the plenum, an air mover for causing air to flow in a direction through the plenum in a direction generally countercurrent to, generally parallel to, or generally across the longitudinal axis of the plenum, and a coil assembly having a major plane and being mounted within the plenum such that the major plane is generally normal to the longitudinal axis of the plenum and such that the external heat exchange liquid flows externally through the coil assembly in a generally vertical flow direction, wherein the coil assembly comprises inlet and outlet manifolds and a plurality of tubes connecting the manifolds, the tubes extending in a direction generally horizontally and having a longitudinal axis and a generally elliptical cross-sectional shape having a major axis and a minor axis where the average of the major axis length and the minor axis length is a nominal tube outside diameter, the tubes being arranged in the coil assembly such that adjacent tubes are generally vertically spaced from each other within planes generally parallel to the major plane, the adjacent tubes in the planes generally parallel to the major plane being staggered and spaced with respect to each other generally vertically to form a plurality of staggered generally horizontal levels in which every other tube is aligned in the same generally horizontal level generally parallel to the major plane, and wherein the tubes are spaced from each other generally horizontally and generally normal to the longitudinal axis of the tube. 
     The improvement comprises the tubes having external fins formed on an outer surface of the tubes, wherein the fins have a spacing of substantially 1.5 to substantially 3.5 fins per inch (2.54 cm) along the longitudinal axis of the tubes, the fins having a height extending from the outer surface of the tubes a distance of substantially 23.8% to substantially 36% of the nominal tube outside diameter, the fins having a thickness of substantially 0.007 inch (0.018 cm) to substantially 0.020 inch (0.051 cm), the tubes having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the tubes of substantially 100% to substantially 131% of the nominal tube outside diameter, and the horizontally adjacent tubes having a generally vertical center-to-center spacing of substantially 110% to substantially 300% of the nominal tube outside diameter. 
     Preferably, the tubes are serpentine tubes having a plurality of segments and a plurality of return bends, the return bends being oriented in generally vertical planes, the segments of each tube connecting the return bends of each tube and extending between the return bends in a direction generally horizontally, the segments having a longitudinal axis and a generally elliptical cross-sectional shape having a major axis and a minor axis where the average of the major axis length and the minor axis length is a nominal tube outside diameter, the segments being arranged in the coil assembly such that the segments of adjacent tubes are generally vertically spaced from each other within planes generally parallel to the major plane, the segments of adjacent tubes in the planes generally parallel to the major plane being staggered and spaced with respect to each other generally vertically to form a plurality of staggered generally horizontal levels in which every other segment is aligned in the same generally horizontal level generally parallel to the major plane, and wherein the segments are spaced from each other generally horizontally and generally normal to the longitudinal axis of the segment connected to the return bend. 
     Where the tubes are serpentine tubes, the improvement comprises the segments having external fins formed on an outer surface of the segments, wherein the fins have a spacing of substantially 1.5 to substantially 3.5 fins per inch (2.54 cm) along the longitudinal axis of the segments, the fins having a height extending from the outer surface of the segments a distance of substantially 23.8% to substantially 36% of the nominal tube outside diameter, the fins having a thickness of substantially 0.007 inch (0.018 cm) to substantially 0.020 inch (0.051 cm) %, the segments having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the segments of substantially 100% to substantially 131% of the nominal tube outside diameter, and the horizontally adjacent segments having a generally vertical center-to-center spacing of substantially 110% to substantially 300% of the nominal tube outside diameter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
         FIG. 1  is an isometric view of one embodiment of a serpentine finned tube according to the present invention used with other such finned tubes in a coil assembly of an evaporative heat exchange apparatus. 
         FIG. 2  is an enlarged view of a portion of the serpentine tube of  FIG. 1 , showing the area in  FIG. 1  within the circle designated “ FIG. 2 .” 
         FIG. 3  is a vertical cross-section view taken along lines  3 - 3  of the embodiment of  FIG. 2 . 
         FIG. 4  is an end elevation view taken along the left-hand end of  FIG. 1 , showing a serpentine tube having a generally vertical plane extending 90° into the plane of the drawing sheet. 
         FIG. 5A  is a first embodiment view, partly in end elevation and partly in vertical cross-section, of a portion of four tubes of a plurality of serpentine tubes of a coil assembly, taken along lines  5 - 5  of the embodiment of  FIG. 1 , showing the generally elliptical segments having their major axes generally vertically aligned and generally parallel to the plane of the return bends when the tubes are generally vertically oriented as shown with respect to the tube in  FIG. 4 . 
         FIG. 5B  is a second embodiment view, partly in end elevation and partly in vertical cross-section, of a portion of four tubes of a plurality of serpentine tubes of a coil assembly, taken along lines  5 - 5  of the embodiment of  FIG. 1 , showing generally elliptical segments having their major axes of adjacent tubes on different levels angled in opposite directions with respect to each other and to the plane of the return bends as shown in  FIG. 4 . 
         FIG. 6  is an isometric view of one embodiment of an exemplary coil assembly made using the finned tubes of the present invention. 
         FIG. 6A  is a schematic side elevation drawing of the embodiment of the exemplary coil assembly of  FIG. 6  made using serpentine finned tubes of the present invention. 
         FIG. 6B  is a schematic side elevation drawing of an alternative embodiment of an exemplary coil assembly made using the finned tubes of the present invention. 
         FIG. 6C  is a schematic side elevation drawing of another alternative embodiment of an exemplary coil assembly made using the finned tubes of the present invention. 
         FIG. 7  is a schematic, vertical cross-section view of a first embodiment of an induced draft, counterflow, evaporative heat exchanger including an arrangement of two finned tube coil assemblies of the present invention within the evaporative heat exchanger. 
         FIG. 8  is a schematic, vertical cross-section view of an embodiment of a forced draft, counterflow, evaporative heat exchanger including an arrangement of two finned tube coil assemblies of the present invention within the evaporative heat exchanger, with some typical components removed for the sake of clarity. 
         FIG. 9  is a schematic, vertical cross-section view of an embodiment of an induced draft evaporative heat exchanger including an arrangement of a finned tube coil assembly of the present invention located directly below a direct contact heat transfer media section including wet deck fill within the evaporative heat exchanger, with some typical components removed for the sake of clarity. 
         FIG. 10  is a schematic, vertical cross-section view of another embodiment of an induced draft evaporative heat exchanger including an arrangement of a finned tube coil assembly of the present invention located directly above a direct contact heat transfer media section including wet deck fill within the evaporative heat exchanger, with some typical components removed for the sake of clarity. 
         FIG. 11  is a schematic, vertical cross-section view of an embodiment of an induced draft, counterflow evaporative heat exchanger including an arrangement of a finned tube coil assembly of the present invention located in a spaced configuration below fill within the evaporative heat exchanger, with some typical components removed for the sake of clarity. 
         FIG. 12  is a graph of results of testing of various embodiments of an evaporative heat exchanger using coil assemblies of the present invention as compared to other types of coil assemblies under equivalent conditions using test procedures as explained hereinafter. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described with reference to the drawings, where like numerals indicate like elements throughout the several views, and initially with reference to  FIGS. 1-4, 5A and 5B  showing embodiments of a finned tube, together with  FIGS. 6, 6A, 6B and 6C , showing various embodiments of a coil assembly made using a number of the finned tubes, as well as  FIG. 7 , showing one embodiment of an exemplary evaporative heat exchange apparatus containing the coil assembly of the finned tubes of the present invention. 
     While the preferred embodiments of the invention use finned tubes of the present invention for all of the tubes in a coil assembly of an evaporative heat exchange apparatus to provide the greatest advantages and benefits of the invention, and are the embodiments described in detail hereinafter, other embodiments of the invention include using at least one finned tube of the present invention in a coil assembly together with other, non-finned tubes in such a coil assembly. Preferably a plurality of finned tubes, such that at least some, more preferably the majority, and most preferably as mentioned above, all of the tubes in a coil assembly for an evaporative heat exchange apparatus are the finned tubes of the present invention. When finned tubes are used in such a coil assembly together with non-finned tubes, the finned tubes are used in any desired arrangement of finned and non-finned tubes, but preferably and without limitation, the finned tubes may usually be arranged to be on the top portion of a coil assembly and the non-finned tubes may be on the bottom portion of the coil assembly. 
     The basic component of the present invention is a finned tube  10 , preferably but not exclusively in the form of a serpentine tube best seen in  FIGS. 1-4 , formed to provide the advantages of the invention when combined with other such finned tubes into a coil assembly  24  (see  FIGS. 6 and 6A ). The coil assembly  24  has a major plane  25 , that in turn is used in an evaporative heat exchange apparatus, such as evaporative heat exchanger  26 , for example (see  FIG. 7 ). When the finned tube  10  is in the preferred form of a serpentine tube, it has a plurality of generally straight segments  12  that have a longitudinal axis  13  and which are interconnected by return bends  16 . The tubes  10  may be made of any heat-conductive metal, such as galvanized steel, stainless steel, copper, aluminum or the like. Stainless steel and galvanized steel, where the zinc is applied to the steel to form galvanized steel after tubes are assembled into a coil assembly  24 , are the presently preferred materials for the tubes  10  for most evaporative heat exchange applications. 
     The return bends  16  may be integrally and unitarily formed with the segments  12  to form the tubes  10 . Alternatively, the fins can be included on the segments  12  and the return bends  14 , having connector end portions  16  can be connected to connector end portions  18  of the segment  12  after fins  20  are formed on the outer surface of the segments  12 . The connecting end portions  16  of the return bend  14  match the shape and are typically slightly larger in cross-sectional area than the connecting end portions  18  of the segments  12 , such that the connecting end portions  18  of the segments fit within the connecting end portions  16  of the return bend  14 , and may be conveniently substantially sealed in a substantially liquid-tight and preferably substantially gas-tight manner, such as by welding the connecting end portions  16  and  18  together. Alternatively, the connecting end portions  16  of the return bends  14  match the shape and may be slightly smaller in cross-sectional area than the connecting end portions  18  of the segments  12 , such that the connecting end portions  18  of the segments fit over the connecting end portions  16  of the return bend  14 , and may be conveniently substantially sealed in a substantially liquid-tight and preferably substantially gas-tight manner, such as by welding the connecting end portions  16  and  18  together. The connecting end portions  16  and  18  may have a generally elliptical or other cross-sectional shape. Preferably, for ease of manufacture and handling, the connecting end portions  16  and  18  have a generally circular cross-sectional shape, such that it is easier to orient and connect together the connecting end portions  16  and  18 , and so that uniform return bends  14  can be used that preferably have a generally circular cross-sectional shape throughout their curved length from one connecting end portion  16  to the opposite connecting end portion  16 . However, if desired, such as for creating a more tightly packed coil assembly of a plurality of generally horizontally arranged tubes  10 , the return bends may have a generally elliptical cross-sectional shape, where major axes of the ellipses of the body of the return bends  14  between the connector end portions  16  are oriented in a generally vertical direction, for most applications within an evaporative heat exchanger. Alternatively, the return bends  14  may have a kidney-shaped cross-section throughout their length, with or without kidney-shaped connecting end portions  16  if the connecting end portions  18  of the segments  12  have matching kidney-shaped cross-sections. It is preferred to connect the return bends  14  to the segments  12  after the fins  20  have been applied to the segments, for ease of manufacture. 
     The tubes  10  are assembled into a coil assembly  24 , best seen in  FIGS. 6 and 6A , where the tubes  10  are serpentine tubes. Typically, a coil assembly  24  has a generally rectangular overall shape retained in a frame  28 , and is made of multiple serpentine tubes  10 , where the segments  12  are generally horizontal and closely spaced and arranged in levels in planes generally parallel to the major plane  25  of the coil assembly  24 . The coil assembly  24  has an inlet  30  connected to an inlet manifold or header  32 , which fluidly connects to inlet ends of the serpentine tubes  10  of the coil assembly, and an outlet  34  connected to an outlet manifold or header  36 , which fluidly connects to the outlet ends of the serpentine tubes  10  of the coil assembly. Although the inlet  30  is shown at the top and the outlet  34  is shown at the bottom of the coil assembly  24 , the orientation of the inlet and outlet could be reversed, such that the inlet is at the bottom and the outlet is at the top, if desired. The assembled coil assembly  24  may be moved and transported as a unitary structure such that it may be dipped, if desired, if its components are made of steel, in a zinc bath to galvanize the entire coil assembly. 
       FIG. 6B  is a schematic side elevation drawing of another alternative embodiment of an exemplary coil assembly  24  made using the finned tubes  10  of the present invention, where the finned tubes  10  are generally straight tubes that extend across the major plane  25  (not shown). In this embodiment, an inlet  30  for the internal heat transfer or process fluid is connected to an inlet manifold or header  32 . The internal fluid flows from the inlet manifold or header  32  into a plurality of finned tubes  10  that are fluidly connected at one end to the inlet manifold or header  32  at an upper level and into a second, upper manifold or header  33 A to which the opposite ends of the upper level finned tubes  10  are fluidly connected. The internal fluid then flows from the second, upper manifold or header  33 A through a lower level of finned tubes  10  fluidly connected at one end to the second, upper manifold or header  33 A into a third, intermediate manifold or header  33 B to which the opposite ends of the finned tubes  10  are fluidly connected. From the third, intermediate manifold or header  33 B, the internal fluid flows into a still lower level of finned tubes  10  which are fluidly connected at one end to the third, intermediate manifold or header  33 B to a fourth, lower manifold or header  33 C to which the opposite ends of the finned tubes  10  are fluidly connected. Then the internal fluid flows from the fourth, lower manifold or header  33 C to which the one end of the lowest level of the finned tubes  10  are fluidly connected to an outlet manifold or header  36  to which the opposite ends of the finned tubes  10  are fluidly connected. An outlet  34  for the internal heat transfer or process fluid is connected to the outlet manifold or header  36 . As described above regarding the embodiment of  FIGS. 6 and 6A , if desired for particular uses, the flow of the internal fluid can be reversed, such that the described inlet  30  would be an outlet and the described outlet  34  would be the inlet. 
       FIG. 6C  is a schematic side elevation drawing of an alternative embodiment of an exemplary coil assembly  24  made using the finned tubes  10  of the present invention, where the finned tubes  10  are generally straight tubes that extend across the major plane  25  (not shown) and fluidly connect directly at respective opposite ends to an inlet manifold or header  32  and to an outlet manifold or header  36 . An inlet  30  for the internal heat transfer or process fluid is connected to the inlet manifold or header  32 . An outlet  34  for the internal heat transfer or process fluid is connected to the outlet manifold or header  36 . As described above regarding the embodiment of  FIGS. 6, 6A and 6B , if desired for particular uses, the flow of the internal fluid can be reversed, such that the described inlet  30  would be an outlet and the described outlet  34  would be the inlet. 
     The segments  12  of the finned tubes  10  shown in  FIGS. 6 and 6A  and the generally straight finned tubes  10  as shown in  FIGS. 6B and 6C  have external fins  20 , which are preferably spiral fins, that contact the outer surface of the segments  12 . The fins may be serrated, may have undulations or corrugations or may be of any other desired well-known structure. If desired, collars  22  may be integrally and unitarily formed with the fins  20 , where the collars  22  provide a direct and secure contact with the surface of the tubes  10  or segments  12  over a greater surface area than if only the edges of the fins  20  were in contact with the outer surface of the tubes  10  or segments  12 . The fins  20  and collars  22  may be formed simultaneously on the tubes  10  or segments  12  using commercially available equipment in a manner known to those involved with producing filmed tubes, and especially spiral finned tubes. Alternatively, the fins  20 , with or without collars  20  may be applied individually onto the outer surface of the tubes  10  or segments  12 , and then secured, such as by welding, into place, but this is an expensive and labor intensive manner of applying the fins  20  to the tubes  10  or segments  12 . 
     Preferably, the fins  20  are applied spirally in a continuous manner to the tubes  10  or segments  12  by conventional equipment. The fins  20  are formed from a band of metal of the same type as used in for the tubes  10 , and the band is fed from a source of the band at a rate and in a manner to spirally wrapped around the tube  10  or segment  12  as the tube  10  or segment  12  is advanced longitudinally along and rotated around its longitudinal axis  13  through the spiral fin forming equipment. As the fins  20  are wrapped around the tube  10  or segment  12 , the inner radius of the fins  20  buckles while the outer radius does not, which creates minor corrugations or indentations in the fins themselves. This buckling occurs in a regular, repeating process in a left-to-right pattern to form undulations in and out of the plane of the material used to form the fins, not shown in  FIGS. 2 and 3 . 
     If collars  22  are desired, the band of metal of the same type as used in for the tubes  10 , is fed from a source of the band at a rate and in a manner to be bent longitudinally to provide a flat portion that becomes the collars  22  and an upstanding portion that becomes the fins  20 . The bent metal band is spirally wrapped around the segments  12  as the segments  12  are advanced longitudinally along and rotated around their longitudinal axis  13  through the spiral fin forming equipment. When the strip of metal is spirally applied to the segments to form the fins  20  with collars  22 , the fins  20  typically have undulations in and out of their plane, rather than straight as shown in  FIGS. 2 and 3  for the ease of illustration, while the collars  22  are flat against the surface of the segments  12 , resulting from the metal deformation during the application of the strip of metal to the advancing and rotating segments. 
       FIGS. 5A and 5B  show respective first and second embodiments, partly in end elevation and partly in vertical cross-section, of a portion of four serpentine tubes  10 A or  10 B, for  FIGS. 5A and 5B , respectively, of a plurality of tubes  10  of a coil assembly  24 , taken along lines  5 - 5  of the embodiment of  FIG. 1 . As shown, starting from the left-hand side of each of  FIGS. 5A and 5B , the second and fourth tubes are shown in a preferred orientation as being staggered in height, or vertically (as shown, lower), with respect to their next generally horizontally adjacent first and third tubes.  FIGS. 5A and 5B  also illustrate alternative embodiments of orientations of the major axes of the generally elliptical segments  12 A of serpentine tubes  10 A in  FIG. 5A  and the generally elliptical segments  12 B of serpentine tubes  10 B in  FIG. 5B . Otherwise, the embodiments of  FIGS. 5A and 5B  are similar to each other. In  FIGS. 5A and 5B , the cross-section of  FIG. 1  was selected such that the fins are not shown or described for the sake of clarity, but the orientations of the major and minor axes of the generally elliptical segments should be understood as relating to the entire length of the finned segments  12  until they connect with or are unitarily formed with the return bends  14 A and  14 B. Although each of the return bends  14 A and  14 B is shown as having a circular cross-sectional shape, as explained above, the return bends  14 A and  14 B may alternatively have a generally elliptical cross-sectional shape, a generally kidney-shaped cross-sectional shape, or other cross-sectional shape. For ease of explanation, the orientation of the major axes of the generally elliptical finned segments  12 A and  12 B will be described in the preferred embodiment of the serpentine tubes  10  as shown in the embodiment illustrated in  FIGS. 6 and 6A , but in principle, the same orientation can be and, preferably, is provided for the generally straight and generally elliptical finned tubes  10  used in a coil assembly such as the coil assemblies shown in  FIGS. 6B and 6C . 
     In both  FIGS. 5A and 5B , the segments  12 A or  12 B of adjacent tubes are generally vertically spaced from each other within planes generally parallel to the major plane  25  of the coil assembly  24  at respective upper generally horizontal levels L 1 A and L 1 B and respective lower generally horizontal levels L 2 A and L 2 B. Thus, the segments  12 A or  12 B of adjacent tubes  10 A or  10 B are in planes generally parallel to the major plane  25  and are staggered and spaced with respect to each other generally vertically to form a plurality of staggered generally horizontal levels in which every other segment is aligned in the same generally horizontal level generally parallel to the major plane  25 . 
     In the first embodiment of  FIG. 5A , the generally elliptical segments  12 A have their major axes generally vertically aligned and generally parallel to the plane of the return bends  14 A when the tubes  10 A are generally vertically oriented as shown with respect to the tube  10  in  FIG. 4 . This alignment or orientation is regardless of whether the segments are on an upper generally horizontal vertical level L 1 A or a lower horizontal level, such as the next adjacent generally horizontal level L 2 A. 
     In the second embodiment of  FIG. 5B , the generally elliptical segments  12 B have their major axes of the tubes  10 B on the different, next adjacent generally horizontal levels L 1 B and L 2 B, angled in opposite directions with respect to the plane of the return bends  14 B when the tubes  10 B are generally vertically oriented as shown with respect to the tube  10  in  FIG. 4 . As shown in  FIG. 5B , in a preferred embodiment where the major axes of the segments  12  are oriented in opposite directions on adjacent horizontal levels, the angle of all of the major axes on a first generally horizontal level L 1 B is about 20° from the plane of the return bends and the angle of all of the major axes on the next adjacent generally horizontal level L 2 B is about 340° from the plane of the return bends. In this configuration, each horizontal level L 1 B, the major axes of all of the segments  12 B are oriented in the same angled direction and on the next adjacent lower level L 2 B, the major axes of all the segments are oriented in the same angled direction, but in an opposite angled orientation from the angled orientation of the major axes in level L 1 B. Where the major axes are angled in opposite directions on adjacent horizontal levels, they are sometimes known as a “ric-rac” arrangement or orientation, and this term is used in the Table below to designate this type of arrangement or orientation. If desired, however, on each level L 1 B or L 2 B, the major axes of the segments within the same generally horizontal level may be angled in opposite directions. 
     Thus, as represented in  FIGS. 5A and 5B , the major axes of the finned segments  12 A or  12 B on a first generally horizontal level L 1 A or L 1 B, respectively, may be 0° to about 25° degrees from the plane of the return bends and the angle of the major axes of the finned segments  12 B or  12 A, respectively, on the next adjacent generally horizontal level L 2 B or L 2 A, respectively, may be about 335° to 360° from the plane of the return bends.  FIG. 4  shows the oppositely angled major axes of the finned segments  12  as described with respect to  FIG. 5B  for a complete serpentine tube  10 . 
     The return bends  14 ,  14 A and  14 B are shown as being generally circular in cross-section. The outside diameter of the circular cross-section of the return bends substantially equals the nominal tube outside diameter that is an average of the lengths of the major and minor axes of the segments  12 ,  12 A and  12 B having a generally elliptical cross-section. Preferably, but without limitation, the outside diameter of the return bends and the nominal tube outside diameter are about and preferably substantially 1.05 inches (2.67 cm), where the wall thickness of the tubes forming the segments  12  and the return bends  14  is about 0.055 inch (0.14 cm). The minor axis of the generally elliptical tube  10  or segments  12 ,  12 A and  12 B is about 0.5 to about 0.9 times, and preferably about 0.8 times the nominal tube outside diameter. Thus, the generally elliptical straight tubes  10  and segments  12 ,  12 A and  12 B having a nominal tube outside diameter of 1.05 inches (2.67 cm), would have a minor axis length of about and preferably substantially 0.525 inch (1.334 cm) to about and preferably substantially 0.945 inch (2.4 cm), and preferably about and preferably substantially 0.84 inch (2.134 cm). Tubes  10  with these dimensions have been found to have a good balance among an appropriate inner diameter or dimensions to allow the processing fluid in the form of any desired gas or liquid to easily flow within the tubes  10 , proximity of such processing fluid to the tube wall for good heat transfer through the walls of the tubes with the elliptical cross-sectional shape that has a large effective surface area, and ability to provide an appropriate number of tubes  10  to be packed into a coil assembly  24 . The tubes are strong, durable and when in serpentine form, able to be readily worked, including connecting the segments  12  and return bends  14  and placement within a coil assembly  24 . Depending on the environment and intended use of the evaporative heat exchangers, such as the evaporative heat exchanger  26 , in which the finned tubes  10  of the present invention are placed, the dimensions and cross-sectional shape of the tubes  10  may be varied considerably. 
     The spacing and orientation of the tubes  10  having the generally elliptical cross-sectional shape or segments having the generally elliptical cross-sectional shape within a coil assembly  24  are important factors for the performance of the evaporative heat exchanger containing the coil assembly  24 . If the spacing between segments  12  is too tight, air and water flow through and turbulent mixing within the coil assembly will be adversely affected and fans with greater horsepower will be needed and there will be an increased pressure drop. If the spacing between segments  12  is too great, then there will be less tubes per surface area of the major plane  25  of the coil assembly  24 , reducing the heat transfer capacity, and there may be inadequate, as in insufficient for example, mixing of the air and water, adversely affecting the degree of evaporation, and thereby heat exchange. The orientation of the segments  12 , particularly with respect to the angle of the major axes of the segments, also affects the heat exchange ability of an evaporative heat exchanger with which they are used. 
     The spacing of the fins  20  around the outer surface of the segments  12  is critical. If the fin spacing is too close (too many fins per inch, for example), the ability of the external heat exchange liquid and the air to effectively mix turbulently is adversely affected and the fins  20  may block the space externally of the coil assembly  24 , such that greater air mover power is needed. Similar concerns involve the critical determination of the height of the fins (the distance from the proximal point where the base of the fins  20  contact the outer surface of the segments  12  and the distal tip of the fins). While higher fins have greater surface area which the evaporating water may coat, longer fins may block the air passage. Thicker fins  20  also have similar critical concerns. Thicker fins are more durable and are better able to withstand the forces of water and air, as well as other material that may be entrained in either as they pass through a coil assembly, but thicker fins may also block the flow of water or air through the coil assembly and would be more expensive to manufacture. All of these factors adversely affect performance. 
     If the fin spacing is too great (not enough fins per inch, for example), the advantages of a sufficient number of fins  20  for the evaporative water to coat would not be present and there may be an adverse effect on the desired mixing of the water and air responsible for efficient evaporation. Similar concerns are present when the fin height is too low, as there is not enough structure of the fins to be coated with the water, and there may be less mixing of the water and air. Thinner fins may not be sufficiently durable to withstand the hostile environment to which they are subject in evaporative heat exchangers and if the fins are too thin, they could be bent during operation as they are subject to the forces of both the water and air impacting them, adversely affecting flow of both the water and air. In addition, and more significantly, thinner fins transfer less heat. 
     The present invention was conceived and developed in view of the foregoing factors of tube shape, orientation, arrangement and spacing, and fin spacing, height and thickness, all of which must be carefully balanced, and which was a difficult task requiring considerable testing and experimentation. Based on such work, the appropriate parameters of tube shape, arrangement, orientation and spacing, as well as fin spacing, height and thickness were determined. 
     The orientation and spacing, within a coil assembly  24  and an evaporative heat exchanger, of the tubes  10  with their segments  12  and return bends  14  will be described primarily with reference to  FIGS. 5A and 5B . The center-to-center spacing D H  generally horizontally (which will be generally parallel to the major plane  25  in  FIG. 6 ) and generally normal to the longitudinal axis  13  of the segments  12 ,  12 A and  12 B is substantially 100% to substantially 131%, preferably substantially 106% to substantially 118%, and more preferably substantially 112% of the nominal tube outside diameter. The vertical straight tube or segment spacing D V  generally is not as critical to the performance of an evaporative heat exchanger as the horizontal tube or segment spacing D H . The segments  12 ,  12 A and  12 B have a generally vertical center-to-center spacing of substantially 110% to substantially 300% of the nominal tube outside diameter, preferably substantially 150% to substantially 205% of the nominal tube outside diameter, and more preferably, substantially 179% of the nominal tube outside diameter. This generally vertical center-to center spacing is indicated by the distance D V  between the upper generally horizontal levels L 1 A and L 1 B and the lower generally horizontal levels L 2 A and L 2 B, respectively. 
     These parameters may be applied as follows to the presently preferred embodiment, where the nominal tube outside diameter is substantially 1.05 inches (2.67 cm). The center-to-center spacing D H  of the finned straight tubes  10  or segments  12 ,  12 A and  12 B of the serpentine finned tubes  10  would be substantially 1.05 inches (2.67 cm) to substantially 1.38 inches (3.51 cm), preferably substantially 1.11 inches (2.82 cm) to substantially 1.24 inches (3.15 cm), and more preferably substantially 1.175 inches (2.985 cm). The finned tubes  10  or the finned segments  12 ,  12 A and  12 B would have a generally vertical center-to-center spacing D V  of substantially 1.15 inches (2.92 cm) to substantially 3.15 inches (8.00 cm), preferably substantially 1.57 inches (3.99 cm) to substantially 2.15 inches (5.46 cm), and more preferably substantially 1.88 inches (4.78 cm). In some embodiments, the major axes of the finned tubes  10  or the finned segments  12 ,  12 A are oriented substantially vertically, so that they are generally parallel to the plane of the return bends  14  as shown in  FIG. 4 . In other embodiments, the major axes of the finned tubes  10  or the finned segments  12 B may be greater than 0° to about 25°, and preferably about 20°, from the plane of the return bends  14  and the angle of the major axes of the finned tubes  10  or the finned segments  12 B on the next vertically adjacent generally horizontal level, may be about 335° to less than 360°, and preferably about 340° from the plane of the return bends  14 , such that the major axes of the finned tubes  10  or the finned segments  12  are oriented in opposite directions on vertically adjacent horizontal levels. 
     The parameters relating to the fins  20 , namely fin spacing along the longitudinal axis  13  of the segments  12 , the fin height from the outer surface of the segments  12  and the fin thickness are as follows according to the present invention. 
     The fins  20  are preferably spiral fins and have a spacing of substantially 1.5 to substantially 3.5 fins per inch (2.54 cm) along the longitudinal axis  13  of the segments  12 , preferably substantially 2.75 to substantially 3.25 fins per inch (2.54 cm) and more preferably substantially 3 fins per inch (2.54 cm). Expressed alternatively, the center-to-center distance between the fins is therefore, respectively, substantially 0.667 inch (1.694 cm) to substantially 0.286 inch (0.726 cm), preferably substantially 0.364 inch (0.925 cm) to substantially 0.308 inch (0.782 cm), and more preferably substantially 0.333 inch (0.846 cm). 
     The fins  20  have a height of substantially 23.8% to substantially 36% of the nominal tube outside diameter, preferably substantially 28% to substantially 33% of the nominal tube outside diameter, and more preferably substantially 29.76% of the nominal tube outside diameter. These parameters may be applied as follows to the presently preferred embodiment, where the nominal tube outside diameter is substantially 1.05 inches (2.667 cm). In this embodiment, the fins  20  have a height of substantially 0.25 inch (0.635 cm) to substantially 0.375 inch (0.953 cm), preferably substantially 0.294 inch (0.747 cm) to substantially 0.347 inch (0.881 cm), and more preferably 0.3125 inch (0.794 cm). 
     The fins  20  have a thickness of substantially 0.007 inch (0.018 cm) to substantially 0.020 inch (0.051 cm), preferably substantially 0.009 inch (0.023 cm) to substantially 0.015 inch (0.038 cm), and more preferably substantially 0.01 inch (0.025 cm) to substantially 0.013 inch (0.033 cm). As noted above in the “Definitions” section, dimensions for the thickness of the fins are for the fins on the finned tubes prior to any later treatment of the finned tubes themselves or of any coil assembly containing them. Where the finned tubes or coil assembly are subjected to a later treatment, typically by galvanizing steel finned tubes or more typically, galvanizing the entire coil assembly containing them, the thickness of the fins increases by the thickness of the zinc coating applied during galvanization. Also typically, the fins after galvanization are thicker at a base proximal to the outer surface of the tube than at a tip of the fins distal from the outer surface of the tube. Because the fins are thicker after galvanizing, the spacing between the fins is reduced accordingly. Usually this is not of concern concerning the thermal performance or heat capacity of the evaporative heat exchangers and the rust or other corrosion inhibition of the galvanizing is important in providing the finned tubes and coil assemblies with greater longevity than if they were not galvanized. 
     The coil assembly  24  of any desired configuration, such as shown in any of  FIG. 6, 6A, 6B or 6C , is then installed into an evaporative heat exchanger apparatus, such as evaporative heat exchanger  26 , as shown in  FIG. 7 . Evaporative heat exchangers have many varied configurations, and several are shown schematically in  FIGS. 7-11 . Typical evaporative heat exchangers in which the coil assembly  24  of the present invention may be used are, for example without limitation, any of several available from Evapco, Inc., such as Models ATWB or ATC, which may include the components and operate as disclosed in Evapco, Inc.&#39;s U.S. Pat. No. 4,755,331. Evaporative heat exchange apparatus, though they many variations, have the basic structure and operation described below, initially with reference to  FIG. 7 . 
       FIG. 7  is a schematic, vertical cross-section view of an embodiment of an induced draft, counterflow, evaporative heat exchanger  26 , where water flows generally vertically downwardly and air flows generally vertically upwardly through the plenum and coil assembly, including an arrangement of two finned tube coil assemblies  24  of the present invention within the evaporative heat exchanger. The evaporative heat exchanger  26  has a housing  38  enclosing a plenum  40  having a generally vertical longitudinal axis  42 . One or more coil assemblies  24  are mounted within the plenum  40  such that the major plane  25  of each coil assembly is generally normal to the longitudinal axis  42  of the plenum. In this way, the generally vertical plane of the return bends  14  in the preferred embodiment using serpentine tubes  10 , as shown in  FIG. 4  and as indicated by the generally vertical alignment of the tubes  10  in the coil assemblies as shown in  FIG. 7 , are also generally normal to the major plane  25  of the coil assemblies  24  and parallel to the longitudinal axis  42  of the plenum. Based on this alignment, the finned segments  12 , with their longitudinal axes  13 , of the tubes  10  are also in generally horizontal staggered planes parallel to the major plane  25  of the coil assemblies  24  and generally normal to the longitudinal axis  42  of the plenum  40 . If generally straight finned tubes  10  are used as shown in  FIGS. 6B and 6C , then the finned tubes with their longitudinal axes also are in generally horizontal staggered planes parallel to the major plane  25  of the coil assemblies  24  and generally normal to the longitudinal axis  42  of the plenum  40 . 
     Air flows from the ambient atmosphere around the heat exchanger  26  via air inlets  44  which may, and preferably do, have louvers, or more preferably, selectively openable and closeable air inlet dampers  45  that may be closed or partially or fully opened based on various atmospheric and operating conditions, in a well-known manner, and to protect the plenum  40  from inclusion of unwanted objects. In the embodiment of  FIG. 7 , air is drawn into the plenum  40 , passes though the coil assemblies  24  and exits an air outlet  46  by the action of an air mover located in an air outlet housing  50 . The air mover in this embodiment is shown as a fan  48 , in the form of a propeller fan, which is preferred for use as an induced draft fan to draw air from the ambient atmosphere. Other types of fans, such as centrifugal fans, could be, but usually are not used as induced draft fans. A grating or screen (not shown) is placed over the fan  48  for safety and to keep debris away from the fan  48  and out of the evaporative heat exchanger  26 . 
     A bottom wall of the evaporative heat exchanger  26 , together with the adjoining front, back and side walls, defines a sump  52  for the water or other external heat exchange liquid. If desired, a drain pipe with an appropriate valve and a fill pipe with an appropriate valve (none of which is shown) may be included for draining and filling or replenishing the sump  52 . Water in the sump  52  is circulated to a liquid distributor assembly  54 , which when turned on distributes, via spray nozzles, orifices in a pipe or via other known devices and techniques, the water as the evaporative heat transfer liquid above the coil assemblies  24 . The distributor assembly  54  is connected to one end of a conduit  56  in fluid connection at the other end to the water in the sump. The distributor assembly  54  is activated or turned on typically when a pump  58  is turned on to pump water from the sump  52  to the distributor assembly  54  through the conduit  56 . 
     The evaporative heat exchanger  26  also preferably includes drift eliminators  60  above the liquid distributor assembly  54  and below the fan  48  and air outlet  46 . The drift eliminators very significantly reduce water droplets or mist entrained in the air exiting the outlet  46 . Many drift eliminators of various materials are available commercially. The presently preferred drift eliminators are PVC drift eliminators available from Evapco, Inc. as disclosed in Evapco, Inc.&#39;s U.S. Pat. No. 6,315,804, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     In operation, as air is drawn into the plenum  40  through the air inlets  44  and any associated louvers or dampers  45 , it is also drawn through the coil assemblies  24 . Water is distributed over the coil assemblies  24  by the liquid distributor  54 . As the air travels upwardly through the coil assemblies  24  it is mixed with the water, with an appropriate degree of turbulence as provided by the orientation and arrangement of the finned segments  12  having the fins  20  with the characteristics, dimensions and parameters disclosed above. The water coats the outer surfaces of the tubes  10 , including the segments  12  having the generally elliptical cross-sectional shape, as well as the fins  20 . The air causes the water to evaporate, thereby cooling the water, such that the cooled water exchanges heat with the tubes  10  of the coil assembly and the process fluid contained internally within the tubes  10 . Water ultimately passes through the coil assemblies  24  and is collected in the sump  52 , and recycled into the liquid distributor  54  through the conduit  56  by the pump. The air with any entrained water is drawn upwardly through the drift eliminators  60 , whereby most, and preferably almost all, of the water is removed from the air stream, before the air is exhausted through the air outlet  46  by the fan  48 . 
     As noted above, the coil assemblies  24  having the finned tubes  10  of the present invention may be used in a large variety and types of evaporative heat exchange apparatus.  FIGS. 8-11  schematically illustrate a small sample of such various evaporative heat exchangers, with some typical components shown in  FIG. 7  removed for the sake of clarity. In  FIGS. 8-11 , components that are shown and that are the same as those in  FIG. 7  are not described again, but are identified by like numerals used in  FIG. 7 , except that a letter designation common to the embodiments of each of  FIGS. 8-11  is used, where, for example, the coil assemblies  24 A are used in the evaporative heat exchanger  26 A of  FIG. 8 , the coil assembly  24 B is used in the evaporative heat exchanger  26 B of  FIG. 9 , the coil assembly  24 C is used in the evaporative heat exchanger  26 C of  FIG. 10  and the coil assembly  24 D is used in the evaporative heat exchanger  26 D of  FIG. 11 . Any new components not used in a previous Fig. are identified by a different numeral. 
       FIG. 8  is a schematic, vertical cross-section view of an embodiment of a forced draft, counterflow, evaporative heat exchanger  26 A including an arrangement of two finned tube coil assemblies  24 A of the present invention within the plenum  40 A of the evaporative heat exchanger. Here, compared to the induced draft evaporative heat exchanger  26  of  FIG. 7 , instead of using a propeller fan  48  mounted in an air outlet housing  50 , the forced draft evaporative heat exchanger  26 A of  FIG. 8  uses a centrifugal fan  62  type of air mover to force air, entering the plenum  40 A within the housing  38 A through a screen  47  covering the air inlet. The air is then forced generally vertically upwardly and through the coil assemblies  24 A, through which water is flowing generally vertically downwardly. Thereafter, the air moves through the drift eliminators  60 A and out of the evaporative heat exchanger  26 A through the air outlet  46 A. The centrifugal fan  62  is typically mounted within a lower portion at one side of the housing  38 A adjacent an air inlet typically covered by a screen  47 . The sump for the water is not shown in  FIG. 8 , but would be present below the coil assemblies  24 A such that the water in the sump is blocked from reaching the centrifugal fan  62 . 
       FIG. 9  is a schematic, vertical cross-section view of an embodiment of an induced draft evaporative heat exchanger  26 B including an arrangement of a finned tube coil assembly  24 B of the present invention located directly below a direct contact heat transfer media section including wet deck fill  64 , described below, within the plenum  40 B of the evaporative heat exchanger. In the evaporative heat exchanger  26 B of  FIG. 9 , air is drawn into the plenum  40 B through an air inlet  44 B and any associated louvers or dampers  45 B, where the air inlet  44 B is laterally adjacent to the coil assembly  24 B. The evaporative heat exchanger  26 B of  FIG. 9  differs in a first respect from the evaporative heat exchanger  26  of  FIG. 7 , in that the air is drawn through the coil assembly  24 B in a direction generally normal, transverse or horizontally with respect to the generally vertical downwardly flow of water externally through the coil assembly  24 B, known in the industry as a crossflow arrangement. The mixing and turbulence of the air and water externally through the coil assembly  24 B in a crossflow arrangement is somewhat different than but still quite effective, compared to the mixing and turbulence of the air and water externally through the coil assembly  24  of  FIG. 7  in a counterflow arrangement. 
     The evaporative heat exchanger  26 B of  FIG. 9  differs in a second respect from the evaporative heat exchanger  26  of  FIG. 7  in that the evaporative heat exchanger  26 B of  FIG. 9  includes a direct contact heat exchange section containing wet deck fill  64  below the liquid distributor  54 B and above the coil assembly  24 B, which provides direct, evaporative heat exchange when the air flow and the evaporative water or other cooling liquid come into direct contact with each other and are mixed with some desired degree of turbulence within the wet deck fill  64  resulting in additional evaporative cooling. The turbulent mixing of the air and water in the wet deck fill  64  allows for greater heat transfer between the air and water, but the benefits of the increased turbulent mixing in the wet deck fill  64  should not be overcome by potential adverse effects on the energy requirements of a larger fan motor or fan size or air flow reduction. As noted above, there is a fine balance among these factors when deciding whether and what type of wet deck fill heat transfer media to use. That is why the use of the wet deck fill  64  is optional in evaporative heat exchangers using the coil assembly of the present invention. The wet deck fill may be any standard fill media, such as plastic fill, typically PVC, as well as wood or ceramic fill media, or any other fill media known in the art. The presently preferred fill media is Evapco, Inc.&#39;s EVAPAK® PVC fill, disclosed in Evapco, Inc.&#39;s U.S. Pat. No. 5,124,087, the disclosure of which is hereby incorporated by reference herein, in its entirety. When wet deck fill  64  is used, it may be located above the coil assembly  24 B as shown in  FIG. 9 , or below the coil assembly  24 C as shown in  FIG. 10 , since in either location, the additional heat transfer in the wet deck fill  64  will further evaporatively cool the water draining into the sump  52 B or  52 C. 
     In the embodiment of  FIG. 9 , louvers  65  are built into the inlet side of the wet deck fill  64 , such that the air may be drawn through the louvers  65  into the wet deck fill in a crossflow manner as described above with respect to the crossflow arrangement concerning the coil assembly  24 B. 
     The embodiment of the evaporative heat exchanger  26 B of  FIG. 9  operates as follows. Ambient air in the environment of the evaporative heat exchanger is drawn into the plenum  40 B through the air inlets  44 B and any associated louvers or dampers  45 B, and in a crossflow manner externally through the coil assembly  24 B, though which water, pre-cooled in the wet deck fill  64  of the direct contact heat exchange section, externally flows generally vertically downwardly. Ambient air is also drawn into the wet deck fill  64  in a crossflow manner with respect to the water flowing generally vertically downwardly through the louvers  65 , where the water is evaporatively cooled before it contacts the coil assembly  24 B below the wet deck fill  64 . The air is then drawn from the wet deck fill  64  into the plenum  40 B. 
     Water is distributed over the wet deck fill  64  by the liquid distributor  54 B where it is initially cooled evaporatively by mixing with the air flowing through the wet deck fill  64  before draining into the coil assembly  24 B where it is turbulently mixed with the air and thereafter is drained from the coil assembly  24 B and collected in the sump  52 B. The water is recycled from the sump  52 B into the liquid distributor  54 B through the conduit  56 B by the pump  58 B. The air, with any entrained water, in the plenum  40 B is drawn upwardly through drift eliminators  60  (not shown in  FIG. 9 ) by the fan  48 B in the air outlet housing  50 B, before the air is exhausted through the air outlet  46 B. 
       FIG. 10  is a schematic, vertical cross-section view of another embodiment of an induced draft evaporative heat exchanger  26 C including an arrangement of a finned tube coil assembly  24 C of the present invention located directly above a direct contact heat transfer media section including wet deck fill  64 C within the plenum  40 C of the evaporative heat exchanger. The embodiment of the evaporative heat exchanger  26 C of  FIG. 10  operates as follows. One portion of ambient air in the environment of the evaporative heat exchanger is drawn into the apparatus through an inlet  44 C at the top of the apparatus aligned above the coil assembly  24 C and flows downwardly externally through the coil assembly in a generally vertical direction concurrent with the flow of water distributed over the coil assembly by the liquid distributor  54 C. Another portion of ambient air is also drawn into apparatus through the direct contact heat exchange section containing the wet deck fill  64 C through the optional louvers  65 C. The air traveling through the wet deck fill  64 C moves in a crossflow manner to water draining generally vertically from the coil assembly  24 C. 
     Water is distributed over the coil assembly  24 C by the liquid distributor  54 C where it is mixed with the concurrently flowing air, thereby being cooled evaporatively in the coil assembly, exchanging heat with the coil assembly  24 C, before draining into and through the wet deck fill  64 C. In the wet deck fill  64 C, the water is further turbulently mixed with the cross-flowing air where it is further evaporatively cooled, and thereafter is drained from the wet deck fill  64 C and collected in the sump  52 C. The water is recycled from the sump  52 C into the liquid distributor  54 C through the conduit  56 C by the pump  58 C. The air with any entrained water is drawn into the plenum  40 C and then upwardly through drift eliminators  60  (not shown in  FIG. 10 ) by the fan  48 C in the air outlet housing  50 C, before the air is exhausted through the air outlet  46 C. 
       FIG. 11  is a schematic, vertical cross-section view of an embodiment of an induced draft, counterflow, evaporative heat exchanger  26 D including an arrangement of a finned tube coil assembly  24 D located in a spaced configuration below wet deck fill  64 D within the plenum  40 D in the housing  38 D in the evaporative heat exchanger. 
     The embodiment of the evaporative heat exchanger  26 D of  FIG. 11  operates as follows. Air in the environment of the evaporative heat exchanger is drawn into the plenum  40 D through the air inlets  44 D and any associated louvers or dampers  45 D, and then is drawn into the wet deck fill  64 D in a counterflow manner with respect to the water flowing generally vertically downward through the wet deck fill  64 D. The liquid distributor  54  (not shown in  FIG. 11 ), located above the wet deck fill  64 D and below the drift eliminators (not shown in  FIG. 11 ), distributes the water over the wet deck fill  64 D where it is turbulently mixed with the air, thereby being cooled evaporatively. Then, the cooled water drains over the coil assembly  24 D, exchanging heat with the coil assembly  24 D, before draining into and being collected in the sump  52 D. If desired, the water draining from the wet deck fill  64 D may be concentrated to flow directly over the coil assembly  24 D as disclosed in Evapco, Inc.&#39;s U.S. Pat. No. 6,598,862, the disclosure of which is hereby incorporated by reference herein, in its entirety, to more efficiently direct the cooled water to the coil assembly  24 D. The water is recycled from the sump  52 D into the liquid distributor  54  through the conduit  56  (not shown in  FIG. 11 ) by the pump  58  (not shown in  FIG. 11 ). The air with any entrained water is drawn upwardly through drift eliminators by the fan  48 D in the air outlet housing  50 D, before the air is exhausted through the air outlet  46 D. 
     The performance of evaporative heat exchange apparatus is measured by the amount of heat transfer, typically but not exclusively during cooling. The measurements are affected by several factors. First, the measurements are affected by the amount and temperature of the process fluid flowing internally though the tubes  10  of the apparatus coil assembl(ies)  24  and the water or other cooling liquid flowing externally through the coil assembly. The flow rates are measured using flow meters and the temperature is measured using thermometers. The rate and temperature of the air flowing through the system is also significant, as well as the force required to drive the air mover  48  that moves the air through the apparatus. The air flow is typically measured by an anemometer in feet per minute through a tube, although other well-known air flow measuring devices could also be used, and is typically determined by the rating of the motor driving the fan of the air mover, usually expressed in horsepower (HP). 
     In one embodiment of the evaporative heat exchange apparatus using the coil assemblies  24  having the finned tubes  10  of the present invention, typically, but without limitation, the process fluid, in the form of water, is pumped into the inlet  30  and flows internally through the coil assembly at a rate of approximately 0.75 gpm to approximately 16.5 gpm per tube present in the coil assemblies, and preferably approximately 10 gpm per tube. The amount and rate of water that passes externally through the coil assembl(ies)  24  supplied through the water supply conduit  56  as distributed by the liquid distributor  54  is approximately 1.5 gpm/sq. ft. to approximately 7 gpm/sq. ft. of coil plan area determined with respect to the major plane  25 , and is preferably approximately 3 gpm/sq. ft. to approximately 6 gpm/sq. ft. Evaporative heat exchange apparatus using the coil assemblies  24  having the finned tubes  10  of the present invention typically, but without limitation, have an air flow rate of approximately 300 feet per minute to approximately 750 feet per minute, and preferably approximately 600 feet per minute to approximately 650 feet per minute. The power of the fan motors is dependent upon the size of the evaporative heat exchanger housing, the size of the coil assemblies used, the number and configuration of tubes in the coil assemblies, the number of coil assemblies used, the presence and orientation of any optional wet deck fill, the size and type of fan used, and several other factors, so no absolute values can be presented for the power of the fan motors required. In general, and without limitation, the power of the fan motors varies within a very broad range, such as approximately 0.06 HP to approximately 0.5 HP per square foot of plan area of the coil assemblies used in the evaporative heat exchangers, corresponding to the area of the major plane  25  coextensive with the length and width of the coil assembly. 
     In evaporative heat exchange apparatus using the finned tube coil assemblies  24  of the present invention, performance has been shown to be enhanced by an increased air flow rate even compared to similar coil assemblies using tubes having segments  12  with a generally elliptical cross-sectional shape but not containing fins  20  as in the present invention. In view of the space occupied by the fins  20  on the segments  12  of the tubes  10  used in coil assemblies  24  of the present invention, it would have been expected that the air flow rate would have decreased, as the fins  20  would have been expected to block the flow of both air and water, so that it was unexpected and surprising when the air flow rate increased. The increase in air flow rate provided a surprising enhancement of the thermal performance in evaporative heat exchange apparatus using the coil assemblies with the finned tubes  10  of the present invention. 
     The enhanced thermal performance of evaporative heat exchange apparatus using the coil assemblies  24  having finned tubes of the present invention will be described in greater detail with respect to the following non-limiting test procedure whereby various coil assemblies were tested, including those of the present invention, under equivalent test conditions. 
     The test procedure included mounting various single coil assemblies in an Evapco, Inc. Model ATWB induced draft, counterflow, evaporative cooler in a test facility. The general arrangement of the Model ATWB induced draft, counterflow, evaporative cooler is shown in  FIG. 7 , except that only one coil assembly  24  was used, instead of two coil assemblies  24  as shown in  FIG. 7 . The tested coil assemblies all had a plan area of 6 feet (1.83 m) long (corresponding to serpentine tubes having segments with return bends fitting within frames of this length with the appropriate spacing) by 4 feet (1.22 m) wide (corresponding to 37 adjacent tubes that were packed within frames of this width with the appropriate spacing) and had ten generally horizontal rows of segments  12  with generally elliptical cross-sectional shapes connected by return bends having a circular cross-sectional shape, where the major axes of segments were arranged in various orientations. All tested coil assemblies used tubes with return bends having an outside diameter of substantially 1.05 inches (2.67 cm) and segments having a nominal tube outside diameter of substantially 1.05 inches (2.67 cm), with a substantially horizontal center-to-center spacing D H  of 1.0625 inches (2.699 cm) (designated “Narrow” in the Table below) or 1.156 inches (2.936 cm) (designated “Wide” in the Table below) and a substantially vertical center-to-center spacing D V  of about 1.875 inches (4.763 cm). One tested coil assembly had no fins  20  on the segments (Test ID “A” in the Table below and the graph of  FIG. 12 ) and represented a base line against which other finned coil assemblies were compared. Other tested coil assemblies identified in the Table below and the graph of  FIG. 12  had spiral fins  20  with the parameters of fin spacing and height as described and claimed herein, and some had spiral fins  20  but not having the parameters of fin spacing and height as described and claimed herein. All of the coil assemblies including fins used fins of the same thickness, namely, 0.013 inch (0.033 cm), which is within the range of fin thickness described and claimed herein. Certain other coil assemblies, namely, those having the parameters associated with the Test ID “B” and “C” (tested in a different rig) and Test ID “D” (tested using 5 HP motor) in the Table below and the graph of  FIG. 12 , were tested in a different manner, but the performance data presented in the graph of  FIG. 12  were derived using industry calculations for standardizing performance data from apparatus of different configurations. The performance of the coil assemblies was tested over varying water flow rates internally through the coils of 60 gpm to 360 gpm, water flow rates externally through the coils of approximately 5.9 gpm per square foot, and air flow rates of 300 feet per minute (91.44 meters per minute) to 750 feet per minute (228.6 meters per minute), generated by a fan driven by a 3 HP motor (except as noted above regarding Test ID “C”). The coil assemblies tested had the parameters as set forth in the following Table: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                   
                 Major Axes 
                 D H  Tube 
                   
                 Fin Spacing 
                 Fin Height 
               
               
                 Test ID 
                 Orientation 
                 Spacing 
                 Fins 
                 (Fins/Inch) 
                 (Inch) 
               
               
                   
               
             
            
               
                 A 
                 20° &amp; 340° 
                 Wide 
                 No 
                 — 
                 — 
               
               
                   
                 Ric-rac 
               
               
                 B 
                 0° 
                 Wide 
                 Yes 
                 3 
                 0.25 
               
               
                 C 
                 20° &amp; 340° 
                 Wide 
                 Yes 
                 1.5 
                 0.3125 
               
               
                   
                 Ric-rac 
               
               
                 D 
                 0° 
                 Narrow 
                 Yes 
                 3 
                 0.3125 
               
               
                 E 
                 20° &amp; 340° 
                 Wide 
                 Yes 
                 3 
                 0.3125 
               
               
                   
                 Ric-rac 
               
               
                 F 
                 0° 
                 Wide 
                 Yes 
                 3 
                 0.3125 
               
               
                 G 
                 20° &amp; 340° 
                 Wide 
                 Yes 
                 1.5 
                 0.5 
               
               
                   
                 Ric-rac 
               
               
                 H 
                 20° &amp; 340° 
                 Wide 
                 Yes 
                 3 
                 0.5 
               
               
                   
                 Ric-rac 
               
               
                   
               
            
           
         
       
     
       FIG. 12  is a graph of results of testing of the coil assemblies identified in the Table in the evaporative heat exchanger under the same conditions set forth in the procedure described above, with respect to preferred internal process fluid (water) flow rates from 6 to 9.8 gpm per tube (where each tube is identified as a “circuit” in the x-axis legend on the graph. The graph show curves based on the heat transferred as measured in thousands of BTU/hour (MBH) versus the water flow internally through the coil assembly in gallons/minute/tube (GPM). Each curve A to H in  FIG. 12  corresponds to the respective coil assembly A to H of the above Table. 
     With reference to  FIG. 12 , the baseline performance of Curve A relates to coil assembly A, with a 20° to 340° ric-rac major axes segment orientation and no fins. Curves B to F above Curve A indicate that at the indicated internal water flow rate along the X-axis, such curves have a better thermal performance than the baseline performance, with increasingly better thermal performance from Curve B to Curve F. 
     Test ID “G” and “H” with a 20°-340° ric-rac major axes orientation, respective fin spacing of 1.5 and 3 fins/inch (2.54 cm) and fin height of 0.5 inch (1.27 cm) (outside the fin height parameter of the present invention) had consistently lower thermal performance (MBH) as indicated by Curves G and H, respectively. 
     In general, the test results show that an orientation of the major axes of the generally elliptical finned segments in a generally vertical direction (0°) provides better thermal performance than a ric-rac orientation of the major axes for tubes having the same fin height and fin spacing. Nevertheless arranging the major segments in a ric-rac orientation still provides a very considerable increase in thermal performance of a coil assembly having all of the other parameters within the scope of the present invention. For tubes having the same angle of orientation, namely a ric-rac or generally vertical orientation of the generally elliptical segments, fins having a height of 0.3125 inch (0.794 cm) provided the better thermal performance. For tubes having the same orientation angle of their major axes and fin height, less spacing within the parameters of the present invention provide better thermal performance. 
     The practical effect of the results shown in  FIG. 12  is that coil assemblies made using the finned tubes of the present invention, having the combination of factors of tube shape, orientation, arrangement and spacing, and fin spacing, height and thickness, all of which must be carefully balanced, provide a dramatic increase in thermal capacity and performance compared to other coil assemblies having the same footprint (plan area). Thus, based on the present invention, among the other benefits and advantages described above, a significantly more cost-effective coil assembly can be produced by providing a smaller coil assembly that results in the same heat capacity demand. This is important not only for increased initial commercial sales, but also for later more cost-effective operation of evaporative heat exchange apparatus using the coil assemblies of the present invention. For coil assemblies of the same plan area, the graph of  FIG. 12  very significantly shows enhanced thermal performance, for the embodiments tested and the results shown in  FIG. 12 , up to about an 18.3% increase in MBH, comparing the results of Curve F to the baseline Curve A, as measured at a rate of flow of internal process fluid (water) of 8 gpm per tube (calculated as 504−426=78/426×100=18.3%). 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.