Patent Application
20120012292
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.'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.
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.
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.
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.
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
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
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
The segments 12 of the finned tubes 10 shown in
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
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
In both
In the first embodiment of
In the second embodiment of
Thus, as represented in
The return bends 14, 14A and 14B 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, 12A and 12B 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, 12A and 12B 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, 12A and 12B 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
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 DH of the finned straight tubes 10 or segments 12, 12A and 12B 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, 12A and 12B would have a generally vertical center-to-center spacing DV 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, 12A are oriented substantially vertically, so that they are generally parallel to the plane of the return bends 14 as shown in
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
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
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.'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.
The evaporative heat exchanger 26B of
In the embodiment of
The embodiment of the evaporative heat exchanger 26B of
Water is distributed over the wet deck fill 64 by the liquid distributor 54B where it is initially cooled evaporatively by mixing with the air flowing through the wet deck fill 64 before draining into the coil assembly 24B where it is turbulently mixed with the air and thereafter is drained from the coil assembly 24B and collected in the sump 52B. The water is recycled from the sump 52B into the liquid distributor 54B through the conduit 56B by the pump 58B. The air, with any entrained water, in the plenum 40B is drawn upwardly through drift eliminators 60 (not shown in
Water is distributed over the coil assembly 24C by the liquid distributor 54C where it is mixed with the concurrently flowing air, thereby being cooled evaporatively in the coil assembly, exchanging heat with the coil assembly 24C, before draining into and through the wet deck fill 64C. In the wet deck fill 64C, 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 64C and collected in the sump 52C. The water is recycled from the sump 52C into the liquid distributor 54C through the conduit 56C by the pump 58C. The air with any entrained water is drawn into the plenum 40C and then upwardly through drift eliminators 60 (not shown in
The embodiment of the evaporative heat exchanger 26D of
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
With reference to
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
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.
