BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

[0001] The present invention relates to refrigeration system air-cooled condensers. DESCRIPTION OF THE BACKGROUND

[0002] A typical refrigeration system condenser consists of multiple, serpentine heat transfer fluid paths (or circuits) such that the superheated heat transfer vapor entering each circuit (path) will be condensed completely prior to leaving the heat exchange device. Figure 3 illustrates an example of a prior art condenser tube bundle. The condenser consists of approximately 50 serpentine tubes, with one inlet header and one outlet header. Vapor enters the upper header (inlet) and is dispersed into all 50 tubes, all having the same diameter. For the entire fluid flow path, the number of the tubes remains constant, and the cross-sectional area of each tube remains constant. At the bottom of the tube bundle, the condensed refrigerant is collected at the outlet header.

SUMMARY OF THE INVENTION

[0003] The overall heat transfer coefficient is primarily controlled by the external heat transfer coefficient and at other times by the internal film heat transfer coefficient. At each circuit entrance (or path), the entire volume exists in a gaseous (or vapor) state. The initial vapor velocity at each circuit entrance is significant resulting in a high internal pressure drop per incremental fluid circuit length which in turn provides a significant internal film heat transfer coefficient. The external heat transfer coefficient governs heat removal in this portion of each circuit. As heat transfer continues between the refrigerant and air along each circuit length and the heat transfer fluid (still in a vapor state) reaches saturation, the vapor begins to condense. As a result, and continuing along each circuit length, the vapor volume and velocity decrease. The vapor exit velocity for each circuit is virtually nil - the heat transfer fluid in liquid form exits the condenser. The continuous reduction in vapor velocity along each fixed cross sectional area circuit length decreases the internal film heat transfer coefficient. Moreover, the internal film heat transfer coefficient prior to approaching the exit region of each circuit limits the condenser's potential or overall heat transfer capability.

[0004] Applicant has observed certain deficiencies in the prior art, including that while the volume and velocity of vapor is a maximum at the entrance of the first pass, there is little or no vapor velocity in the last pass. The significant inlet vapor volume produces a high refrigerant pressure drop in the first pass due to the high vapor velocity. This in turn limits the refrigerant mass flow rate per tube (or circuit/path). Conversely, the very low vapor velocity in the last pass adversely affects the internal film heat transfer coefficient and thus reduces the condenser's total heat transfer capability.

[0005] The present invention ameliorates heat transfer deficiency of the prior art as well as high initial refrigerant pressure drop in the first pass by providing multi-cross sectional fluid paths (circuits) for condensation coupled with segmented headers in lieu of return bends. Thus at the entrance of each circuit when the vapor volume is significant, a larger cross-sectional area is provided for each circuit. The larger total initial cross sectional area reduces the internal pressure drop and the vapor velocity while maintaining the internal film heat transfer coefficient above the extemal heat transfer coefficient. As the vapor volume decreases along each circuit length as a result of condensation, the total cross sectional area is reduced to maintain a threshold internal film heat transfer coefficient that is equal to or greater than the extemal heat transfer coefficient. This decrease in total cross sectional area may be accomplished by incorporating a multiple pass circuit selection coupled with a greater total cross sectional area for the initial fluid path in comparison to later passes. This arrangement lowers the initial heat transfer fluid pressure drop per incremental circuit length with minimal heat transfer sacrifice in the first pass. Moreover, it significantly improves the condenser's heat transfer deficiency by increasing the internal film heat transfer coefficient in the later passes in comparison to the prior art single cross-sectional area circuit devices. Overall, the multi-cross sectional condenser of the invention provides greater heat rejection at a lower heat transfer fluid pressure drop. The multi-cross sectional fluid path condenser of the invention can be implemented using larger tubes in the first pass and smaller tubes in subsequent passes, or by using more tubes in the first pass and fewer tubes in subsequent passes, or by some combination of the two, that is reducing both the number of tubes and the cross-sectional area of the tubes in with each subsequent pass.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 is a cutaway perspective view of a evaporative refrigerant condenser.

[0007] Figure 2 shows the principal of operation of an evaporative refrigerant condenser.

[0008] Figure 3 shows a prior art evaporative refrigerant condenser tube bundle.

[0009] Figure 4a is a plan view photograph of a mockup of a multi-cross sectional area tube bundle (also referred to herein as "heat exchange bundle") according to an embodiment of the invention;

[0010] Figure 4b is a formal drawing corresponding to the photograph of Figure 4a

[0011] Figure 5a is a perspective view photograph of the mockup shown in Figure 5a.

[0012] Figure 5b is a formal drawing corresponding to the photograph of Figure 5a

[0013] Figure 6a is the plan view photograph of Figure 2a, with arrows to show the refrigerant flow path.

[0014] Figure 6b is a formal drawing corresponding to the photograph of Figure 6a.

[0015] Figure 7a is a labeled version of the photograph of Figure 5a.

[0016] Figure 7b is a formal drawing corresponding to the photograph of Figure 7a. [0017] Figure 8 is a sketch of an embodiment of the invention having four condenser sections.

[0018] Figure 9 is a perspective view of an embodiment of the invention having three condenser sections arranged so that the inlet header, outlet header and intermediate headers are all on the same side of the device.

DETAILED DESCRIPTION

[0019] This invention relates particularly to condenser coil bundles used in refrigerant condensers, and particularly (although not exclusively) in evaporative refrigerant condensers 10 of the type shown in Figures 1 and 2 configured to indirectly transfer heat between a superheated refrigerant and ambient air, operative in a wet mode or a dry mode as described below depending on ambient atmospheric conditions, such as temperature, humidity and pressure.

[0020] The apparatus 10 includes a fan 100 for causing air to flow through the apparatus, and as shown schematically in FIG. 1, sitting atop housing 15. At normal ambient atmospheric conditions where freezing of the cooling liquid, typically water, is not of concern, air is drawn into the plenum 18 of the apparatus via air passages at the bottom of the unit through the open air intake dampers , and enters the evaporative heat transfer section 12 where heat transfer takes place involving the distribution of water from a water distribution assembly 90 driven by a pump 96. When the ambient temperature and the temperature of the cooling liquid fall to indicate a concern of freezing the cooling liquid, the distributor assembly of cooling liquid is turned off.

[0021] Prior art refrigerant coil assemblies 20 have a generally parallelepiped overall shape of six sides retained in a frame 21 and has a major/longitudinal axis 23, where each side is in the form of a rectangle. The coil assembly 20 is made of multiple horizontal closely spaced parallel, serpentine tubes connected at their ends to form a number of circuits through which the refrigerant flows. Each individual circuit within the coil assembly is a single, continuous length of coil tubing that is subjected to a bending operation which forms the tubing into several U-shaped rows that are in a generally vertical and equally-spaced relationship from each other, such that each circuit has a resultant serpentine shape.

[0022] The coil assembly 20 has an inlet 22 connected to an inlet manifold or header 24, which fluidly connects to inlet ends of the serpentine tubes of the coil assembly, and an outlet 26 connected to an outlet manifold or header 28, which fluidly connects to the outlet ends of the serpentine tubes of the coil assembly. The assembled coil assembly 20 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.

[0023] The refrigerant gas discharges from the compressor into the inlet connection of the apparatus. Heat from the refrigerant dissipates through the coil tubes to the water cascading downward over the tubes. Simultaneously, air is drawn in through the air inlet louvers at the base of the condenser and travels upward over the coil opposite the water flow. A small portion of the water evaporates, removing heat from the system. The warm moist air is drawn to the top of the evaporative condenser by the fan and discharged to the atmosphere. The remaining water falls to the sump at the bottom of the condenser where it recirculates through the water distribution system and back down over the coils.

[0024] The invention constitutes a change and improvement over the prior art wherein instead of tube bundles comprising a single cross-sectional area throughout the entire refrigerant flow path through the coil, the indirect heat exchange section has multiple sections, each having different cross-sectional areas, decreasing as the refrigerant travels through the heat exchange section.

[0025] Figures 4a, 5a, 6a and 7a are photographs of a mockup of a multi-cross- sectional area refrigerant condenser according to an embodiment of the invention. Figures 4b, 5b, 6b and 7b are formal drawings corresponding to Figures 4a, 5a, 6a and 7a, respectively. A first condenser section 103 includes plurality of straight tubes 105 having a first total cross- sectional area. While round tubes are shown in the mock-up, tubes of any shape, size and feature may be used according to the invention, Indeed, any passage capable of permitting refrigerant flow and heat exchange may be adapted for use in connection with the invention in the place of the tubes shown in the Figures, including microchannel plates and other conduit structures. For the sake of the description of the invention with reference to the mock-ups shown in Figures 4a, 5a, 6a and 7a, the term "tube" will be used, but it should be understood that the words "passage," or "conduit" may be substituted for the word "tube" in the description herein, whatever the construction, provided that it can convey refrigerant and permit heat exchange between refrigerant inside and air outside.

[0026] As used herein, the term "total cross-sectional area" refers to the sum of the cross-sectional areas of the individual tubes in a condenser section. The term "total cross- sectional area" as used herein is not calculated to include the area between tubes in a condenser section. The cross-sectional area of each straight tube 105 in first condenser section 103 may be the same as or different from one-another, but the sum of the cross- sectional areas of all straight tubes 105 in first condenser section 103 equals the first total cross-sectional area. The tubes in first condenser section 103 are preferably finned. Each straight tube 105 in the first condenser section 103 terminates at one end at inlet header or manifold 107 and at terminates at a second end at intermediate header or manifold 109.

[0027] A second condenser section 1 11 includes a second plurality of straight tubes 113 having a second total cross-sectional area. The cross-sectional area of each straight tube 113 in second condenser section 1 11 may be the same as or different from one-another, but the sum of the cross-sectional areas of all straight tubes 113 in second condenser section 1 11 equals the second total cross-sectional area. The second total cross-sectional area is less than the first total cross-sectional area. The cross-sectional area of each straight tube 113 in the second condenser section may be the same or different from the cross-sectional area of each straight tube 105 in the first condenser section, but the cross-sectional area of each straight tube 1 13 in the second condenser section is preferably less than cross-sectional area of each straight tube 105 in the first condenser section. The number of tubes in the second condenser section may be the same or different from the number of tubes in the first condenser section, but is preferably less. The length of the tubes in the second condenser section may optionally be shorter than the length of the tubes in the first condenser section (as shown for example in Figures 4a and 4b). The tubes in second condenser section 1 11 are preferably finned.

[0028] The second condenser section receives refrigerant from the first condenser section via intermediate header or manifold 109. As shown, for example in Figures 4a and 4b, each straight tube 113 in the second condenser section terminates at one end at intermediate header or manifold 109 and terminates at a second end at outlet header or manifold (not shown).

[0029] Alternatively, third, fourth and fifth or more condenser sections may be present. Figure 8 is a representation of an embodiment of the invention having four condenser sections. According to these embodiments, a second intermediate header or manifold 1 15 directs refrigerant to a third condenser section 117, and each of said third 1 17, fourth 1 19, and fifth or more condenser sections are each constructed of a plurality of straight tubes, and each of said third, fourth, and fifth or more condenser sections each have a total cross-sectional area that is less than a cross-sectional area of an immediately upstream condenser section.

[0030] Each of the straight tubes in said third, fourth, and fifth or more condenser section is connected at one end to an immediately upstream condenser section by an intermediate header or manifold, and at a second end to another intermediate header or manifold 121 (if there is a subsequent condenser section) or to an outlet header or manifold 123.

[0031] Figure 9 shows an alternate embodiment of the invention in which the inlet header, outlet header and intermediate headers are all arranged on the same side of the device, and each condenser section contains two sets of straight lengths of tubes connected at an end opposite the header end by U-bends. Accordingly, inlet header 201 receives superheated refrigerant vapor and distributes it to first set of straight tubes 203 in a first condenser section 205. The first set of straight tubes 203 are connected at an opposite end to a second set of straight tubes 207 in said first condenser section by U-bends 209. The first and second set of tubes in the first condenser section have the same number of tubes and the tubes have the same diameter. U-bends 209 have approximately the same cross-sectional size/diameter as the first and second set of tubes in said first condenser section. The side of the second set of tubes in the first condenser section are connected at an end opposite the U-bend end to first intermediate header 211. First intermediate header then delivers the refrigerant to the second condenser section 213 having a second condenser first set of tubes 215 and a second condenser second set of tubes 217 connected at an opposite end from said intermediate header by another set of U-bends 219. The first and second set of tubes in said second condenser section have the same cross-sectional dimensions and are equal in number. The U- bends 219 connecting the first and second set of tubes in the second condenser section likewise have approximately the same cross-sectional dimensions as the first and second set of tubes they connect. The second condenser section second set of tubes 217 terminate at a second intermediate header 221. The second intermediate header 221 receives refrigerant from the second condenser section set of tubes 217 and direct it to the third condenser section 223. The third condenser section first set of tubes 225 are connected at a first end to the second intermediate header and at an opposite end to yet another set of U-bends (not shown) that are in-turn connected to a first end of third condenser section second set of tubes 227. The third condenser section second set of tubes 227 are connected at the header end to outlet header 229. The tubes of each condenser section are progressively smaller while (according to the embodiment shown in Figure 9) the number of tubes in each condenser section is equal. However, as with the embodiments described above, the size of the tubes could be left the same, and the number of tubes could be reduced, so that the total cross-sectional area of each condenser section is smaller than the first section, and is preferably smaller than each upstream section.

[0032] By increasing the number of circuits (tubes) in the first condenser section and increasing the cross-sectional area of each tube in the first condenser section the invention can reduce the inlet vapor velocity more than 50% and thus reduce the refrigerant pressure drop to less than 25% of the original value. Moreover, the entrance vapor velocity, per circuit, is sufficient to establish an internal film heat transfer coefficient greater than the external heat transfer coefficient while limiting the internal pressure drop for the heat rejection intended. The subsequent decrease in total cross sectional area will occur after the first path or even later in the heat transfer fluid path depending upon operating conditions. The number of tubes in the second condenser section may be adjusted to additionally lower vapor velocity which in turn reduces refrigerant pressure drop. The second group also exhibits a reduced total cross sectional area then the first group in this illustration and thus maintains vapor velocity prior to entering the last reduction in cross sectional area. A third condenser section may have further reduced cross sectional area to re-establish the vapor velocity prior to exiting the condenser. It is most preferred that each condenser section incorporate smaller or same as, cross sectional area paths in comparison to the initial circuits. In doing so, the fluid (vapor) velocity is re-established such that the associated internal film heat transfer coefficient is greater than that leaving the initial total cross sectional area provided coupled with initial circuit quantity. Multi-cross sectional interfaces are preferably utilized throughout the condenser as needed via segmented headers (see, e.g., Figs. 4a, 4b and Fig. 9) such that the heat transfer fluid (vapor) velocity can be maintained (on average) leading into the final pass. There are many permutations regarding the path cross sectional area coupled with number of paths per section that can be used with this invention to optimize performance. Iterative calculations can be performed depending upon the operating conditions, refrigerant and heat rejection requirements. There are other advantages with this invention including lower refrigerant inventory as well as better condenser efficiency due to reduced refrigerant pressure drop.