Abstract:
The heat exchanger with a radial heat source has a first header, a second header, first tubes and second tubes. The first header is configured to allow liquid to enter and exit the heat exchanger. The second header is spaced from the first header and has at least one lower baffle provided therein. The first tubes extend from the first header to the second header, with the first tubes being spaced proximate to the radial heat source. The second tubes extend from the first header to the second header, with the second tubes being spaced from the radial heat source a greater distance than the first tubes. An enhancement device may be positioned in respective tubes of the first tubes to create a water vortex in the first tubes wherein boiling of the water in the first tubes is prevented.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to heat exchangers, and in particular to radially fired heat exchangers with multiple rings of tubes. 
       BACKGROUND OF THE INVENTION 
       [0002]    For many years, commercial water heaters have been constructed using burners and heat exchanger water flow tubing. Commercial water heaters must be capable of producing and heating water with tens of thousands, and even hundreds of thousands, of BTUs. Further, in modern commercial applications, the emission standards for water heaters are strictly regulated. Complete burning of fuel is controlled so that hydrocarbon emissions are very low. In many existing commercial water heaters, natural gas is burned in an environment of forced air. 
         [0003]    Many direct-fired, commercial water heating systems are known in the industry. One commercially available system, disclosed in U.S. Pat. No. 4,261,299, utilizes a horizontal combustion chamber around which water flows through a double-walled shell that is wound repeatedly around the combustion chamber with spaces between each successive winding to accommodate a countercurrent flow of exhaust gases. 
         [0004]    Another system, disclosed in U.S. Pat. No. 4,938,204, utilizes a dual tank design. One tank contains the primary heat exchanger in which a horizontally mounted conventional burner heats water flowing through two-pass, U-bend fire tubes. Exhaust gases that exit the primary heat exchanger at 350 degrees Fahrenheit to 400 degrees Fahrenheit are routed to a secondary heat exchanger where they are passed countercurrent to ambient makeup water to preheat the water before entering the primary exchanger. Makeup air is preheated to over 200 degrees Fahrenheit by passing it through ductwork which surrounds the exhaust gases exiting the secondary exchanger. 
         [0005]    Some of the newer prior art systems utilize primary exchanger sections comprising a vertically-disposed, radially-directed, cylindrical burner in combination with a plurality of fixed length, copper-finned tubes arranged vertically around the burner. Water flows through the tubes, which are typically connected to headers located above and below the combustion zone, either in single or double-pass configurations. In some heaters, the copper-finned tubes are intermeshed and completely surround the burner to enhance heat transfer. Difficulties have been experienced with these heaters, however, because of the length of the tubing required to allow for effective heat exchange and the limited amount of expansion or contraction that can be accommodated with the fixed tube design. 
         [0006]    U.S. Pat. No. 5,687,678 discloses a commercial water heater apparatus, including a housing, a radial-fired burner within the housing, a single continuous, multiple-loop, finned coil tubing heat exchanger for circulating water around the burner, having at least a first set of inner coils forming a coil trough therebetween and a second set of outer coils nested within the coil trough formed by the inner set of coils, the outer set of coils forming a second coil trough around the exterior thereof, and a coil baffle interposed in the second exterior trough for deflecting heat adjacent to the second set of coils. 
         [0007]    Highly efficient transfer of heat energy from the burned fuel to the water has been an object of commercial water heater design for a number of years. In accomplishing the high efficiency heat transfer from the combustion products to the circulated water, in many systems a certain amount of water vapor in the combustion gases will be condensed from the combustion gas. This condensate is typically highly acidic, having PH values in the range of between 2 to 5, depending upon the chemical constituents of halogenated hydrocarbon in the natural gas and air mixture. For example, increased halogen content of the natural gas and air mixture can greatly increase the acidity of the condensate. Therefore, various commercial water heaters are simply designed to operate below the efficiency at which large quantities of condensate are likely to form so that the acidic vapors are discharged in vapor form in high temperature exhaust gas. 
         [0008]    Notwithstanding the systems disclosed in the prior art, it would be beneficial to have a radial-fired heat exchanging apparatus which has a compact configuration and which can quickly and efficiently transfer heat to water passing through the tubes. 
       SUMMARY OF THE INVENTION 
       [0009]    An exemplary embodiment is directed to a heat exchanger having a radial heat source. The heat exchanger has a first header, a second header, first tubes and second tubes. The first header is configured to allow liquid to enter and exit the heat exchanger. The second header is spaced from the first header and has at least one lower baffle provided therein. The first tubes extend from the first header to the second header, with the first tubes being spaced proximate to the redial heat source. The second tubes extend from the first header to the second header, with the second tubes being spaced from the radial heat source a greater distance than the first tubes. Liquid with the lowest velocity enters the second header through the second tubes proximate the lower baffle to provide for the shortest return path through the first tubes to equalize the flow rate through each first tube. 
         [0010]    Another exemplary embodiment is directed to a heat exchanger having a radial heat source. A first header of the heat exchanger has a first chamber for receiving a liquid as the liquid enters the heat exchanger and a second chamber for receiving the liquid prior to the liquid exiting the heat exchanger. A second header is spaced from the first header. First tubes extend from the second chamber of the first header to the second header, with the first tubes being spaced proximate to the radial heat source. Second tubes extend from the first chamber of the first header to the second header, with the second tubes being spaced from the radial heat source a greater distance than the first tube. The circumferential spacing between the first tubes provides a gap allowing for the proper heating of the first tubes while allowing sufficient heat to reach the second tubes to properly heat the second tubes. 
         [0011]    Another exemplary embodiment is directed to a heat exchanger having a radial heat source. The heat exchanger has a first header through which liquid enters and exits the heat exchanger. A second header is spaced from the second header and has at least one lower baffle provided therein. First tubes extend from the first header to the second header, with the first tubes being spaced proximate to the radial heat source. An enhancement device is positioned in respective tubes of the first tubes. The enhancement device creates a water vortex in the first tubes wherein a high velocity water stream which flows through the first tubes is in contact alternately with a hot side and then a cooler side of the first tubes, wherein boiling of the water in the first tubes is prevented. 
         [0012]    Most copper-fin radially-fired heat exchangers in the market today obtain increased capacity by using longer tubes or increasing the number of tubes in a single ring. Using multiple rings of tubes as described herein effectively lengthens the tube linear distance without increasing the height of the heat exchanger. Consequently, the heat exchanger is half the size of a comparable single-ring heat exchanger. 
         [0013]    Another exemplary added benefit of multiple rings is the increased heat transfer coefficient on the gas side of the tubes. This is due to the increased velocity of the gas since the flow area is reduced because the heat exchanger is shorter. Higher efficiency with less material is achieved. 
         [0014]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is an isometric view of one embodiment of the heat exchanger assembly of the present invention, the heat exchanger being enclosed by a shell. 
           [0016]      FIG. 2  is a cross-sectional view of the heat exchanger assembly of  FIG. 1 , taken along the line  2 - 2  of  FIG. 1 , showing finned inner tubes with enhancement device positioned therein. 
           [0017]      FIG. 3  is an exploded perspective view of the heat exchanger assembly of  FIG. 1 . 
           [0018]      FIG. 4  is an exploded perspective view of a two-pass heat exchanger housed in the heat exchanger assembly of  FIG. 1 . 
           [0019]      FIG. 5  is a cross-sectional view of the inner and outer tubes of the heat exchanger taken along line  5 - 5  of  FIG. 4 . 
           [0020]      FIG. 6  is a top view of the heat exchanger of  FIG. 4 . 
           [0021]      FIG. 7  is a cross-sectional view of the heat exchanger of  FIG. 4 , taken along the line  7 - 7  of  FIG. 8 , showing outer tubes in cross-section. 
           [0022]      FIG. 8  is a top isometric view of a top header of the heat exchanger of  FIG. 4 . 
           [0023]      FIG. 9  is a top view of the top header of the heat exchanger of  FIG. 4 . 
           [0024]      FIG. 10  is a bottom isometric view of the top header of the heat exchanger of  FIG. 4 , showing chambers through which the liquid flows. 
           [0025]      FIG. 11  is a cross-sectional view of the top header of the heat exchanger of  FIG. 4 , taken along the line  11 - 11  of  FIG. 9 , showing the inlet pipe and the inner and outer chambers. 
           [0026]      FIG. 12  is an isometric view of a bottom header of the heat exchanger of  FIG. 4 . 
           [0027]      FIG. 13  is a top view of the bottom header of  FIG. 12 . 
           [0028]      FIG. 14  is a bottom view of the bottom header of  FIG. 12 , showing a baffle provided therein to deflect the liquid to allow the bottom header to provide a reverse return configuration. 
           [0029]      FIG. 15  is a cross-sectional view of the bottom header of the heat exchanger, taken along the line  15 - 15  of  FIG. 14 . 
           [0030]      FIG. 16  is an isometric view of a top tube sheet of the heat exchanger of  FIG. 4 . 
           [0031]      FIG. 17  is an isometric view of a bottom tube sheet of the heat exchanger of  FIG. 4 . 
           [0032]      FIG. 18  is an isometric view of an enhancement device which is inserted into the inner tubes of the heat exchanger. 
           [0033]      FIG. 19  is a top isometric view of an exemplary alternate top header of the heat exchanger, the alternate header having baffles to allow the liquid to make four passes through the tubes. 
           [0034]      FIG. 20  is a bottom isometric view of the alternate top header of the heat exchanger, showing chambers and baffles which control the flow of the liquid. 
           [0035]      FIG. 21  is a bottom view of the alternate top header of  FIG. 20 . 
           [0036]      FIG. 22  is a bottom isometric view of an alternate bottom header of the heat exchanger, the alternate header having baffles to allow the liquid to make four passes through the tubes. 
           [0037]      FIG. 23  is a top isometric view of the alternate bottom header of the heat exchanger. 
           [0038]      FIG. 24  is a bottom view of the alternate bottom header of  FIG. 22 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    The radially-fired heat exchanger  10  of the present invention can be used in a gas-fired hot water boiler. In such a hot water boiler, air and fuel are pre-mixed and ignited through the radial-fired burner  8 . The closed-loop heat exchanger  10  is designed for counter-flow operation to optimize heat transfer. 
         [0040]    In general, when heat is required (as indicated by water temperature), an operating temperature control switch signals to a micro-processor-based flame safeguard programmer. The programmer energizes a blower motor and an air-flow differential pressure switch, providing a specific prepurge time. This allows the boiler to purge any residual gas. 
         [0041]    After the purge is complete and correct air flow is established, the programmer powers an ignition transformer, and a gas pilot is spark-ignited. When the pilot flame is detected by a UV sensor, a signal is sent to the programmer which then opens both main gas valves. The main burner  8  ignites and the pilot is de-energized. Alternatively, the radially-fired heat exchanger may use direct light technology. When the desired water temperature is reached, the operating control switch opens and the programmer closes both main gas valves. 
         [0042]    When the water temperature is reduced by the load on the system, the operating temperature control switch will close again. This sequence recycles automatically to the start of the cycle provided that the limits on water flow and gas pressure are met. 
         [0043]    A radial-fired, fan-assisted burner  8  with a screen-type diffuser fits vertically into the circular heat exchanger  10 . This vertical burner/heat exchanger  10  design produces a higher thermal efficiency than is possible with any conventional horizontal gas-fired boiler. Flame distribution is controlled by the pre-calculated free area of the screen. The fuel mixture is controlled by calibrated injection ports and an adjustable air shutter to produce a clean-burning blue flame. The burner  8  can be quickly and easily removed from the exchanger  10  for cleaning or inspection. 
         [0044]    The radial-fired burner is designed to provide uniform radial jets of flame, the tips of which jets of flame are adjacent to but spaced apart from the innermost portions of the heat exchanger  10 . The heated gases from the flames flow generally upward, primarily radially outward, but also with a component of upward flow due to heat expansion at the flames and then subsequently a downward flow after the heated exhaust gas exchanges its heat to the heat exchanger tubing such that the exhaust gases move downward along the exterior of the heat exchanger tubing  12 ,  14  to exhaust gases toward the lower end of the tubes and radially outward therefrom. Because of the completeness of the burning, the exhaust gases may be generally discharged with minimal impact on the environment, or, if additional purification is required by any particular governmental standards, may be further treated prior to discharge. 
         [0045]    The centrally located burner  8  has a cylindrical burner surface, which is preferably formed of a thin sheet of pressed high-temperature metal fibers having perforations uniformly therethrough so that the forced gas and air mixture is forced out of the perforations through cylindrical burner surface where it is ignited and burns to produce heat, which is transferred to the tubes  12 ,  14  of the heat exchanger  10  both by convection of the heated gases and also by radiation. 
         [0046]    The heat exchanger  10  has integral tubes  12 ,  14 , arranged vertically with removable cylindrical headers  16 ,  18 . This tube configuration provides a high heat transfer ratio and a fast response to load requirements. Since the tubes  12 ,  14  completely surround the burner  8 , ambient losses are eliminated. All the hot gases are forced over the tubes, maximizing heat transfer and producing the high efficiency. 
         [0047]    With reference to  FIGS. 1 through 18 , an exemplary first embodiment of the heat exchanger  10  is shown. The heat exchanger  10  has a top header  16 , a bottom header  18 , a first ring of tubes  12 , a second ring of tubes  14 , a top tube sheet  20  and a bottom tube sheet  22 . As best shown in  FIGS. 2 through 7 , the first ring of tubes  12  and the second ring of tubes  14  extend between the top header  16  and the bottom header  18 . The top tube sheet  20  and the bottom tube sheet  22  cooperate with the tubes  12 ,  14  to maintain the tubes  12 ,  14  in position relative to each other. 
         [0048]    As best shown in  FIGS. 1 through 3 , shell halves  24 ,  26  cooperate with reinforcing/fastening ribs  28 , flanges  30 , gaskets  32  and gaskets  34  to encase the heat exchanger  10 , thereby providing a sealed tight shell which retains the heat from the burner  8  and allows water or other liquids to flow through the headers  16 ,  18  and tubes  12 ,  14 . 
         [0049]    The exemplary heat exchanger  10  shown has two rings of tubes  12 ,  14  through which water or other liquid flows. In the embodiment shown, the tubes  12 ,  14  are made from copper, but other material having the appropriate strength and heat stability and transfer characteristics can be used, such as, but not limited to, copper nickel, aluminum, stainless steel and alloys thereof. While two rings of tubes  12 ,  14  are shown, any number of multiple rings may be used without departing from the scope of the invention. 
         [0050]    The tubes  12 ,  14  may have radially extending fins to allow for more efficient transfer of heat. As is shown in the drawings, the tubes  12 ,  14  extend radially about an opening  36  in which the burner  8  is positioned. The inner tubes  12  are closer to the opening  36  and the burner  8 , while the outer tubes  14  are spaced further from the opening  36 . The location of the rings of tubes  12 ,  14  is not arbitrary, but designed to provide maximum efficiency. If the diameter D 1  of the first ring is too small, the tubes  12  will be too close to the burner  8 , which will cause combustion problems, i.e. high carbon monoxide (CO). It is, therefore, not desirable to have the flames of the burner  8  contact any surface of the inner tubes  12  or the outer tubes  14 , but rather have the heated gases from the flames surround the tubes  12 ,  14 , as previously described. 
         [0051]    Referring to  FIG. 5 , the circumferential tube spacing S 1 , S 2  from one tube  12 ,  14  to another is critical for pressure design and gas flow design. If the gap or spacing S 1  between the inner tubes  12  is too wide, the inner tubes  12  would not be properly heated, resulting in an underperforming design. If the gap or spacing S 1  between the inner tubes  12  is too narrow, the outer tubes  14  would not be properly heated, again resulting in an underperforming design. Stated differently, the circumferential spacing between first tubes provides a gap which allows for the proper heating of the first tubes while allowing sufficient heat to reach the second tubes to properly heat the second tubes. 
         [0052]    Once the proper diameter D 1  and proper spacing S 3  ( FIG. 7 ) of the inner tubes from the burner  8  is determined, and once the proper spacing S 1  between the inner tubes  12  is determined, the number of inner tubes  12  needed can be determined, as the diameter D 1  of the inner tube circle and the spacing S 1  determines the number of tubes  12  in the inner ring. In addition, once the proper spacing S 4  ( FIG. 7 ) of the outer tubes  14  from the inner tubes  12  is determined, and once the proper spacing S 2  between the outer tubes  14  is determined, the number of outer tubes  14  can be determined, as the diameter D 2  of the outer tube circle and the spacing S 2  determines the number of tubes  14  in the outer ring. The diameter D 2  of the second ring of tubes is dependent upon the diameter D 1  of the first ring of tubes. The circumference of each ring increases by about 3 times the diameter increase. The number of tubes provided in each additional ring is calculated using a similar method. The diameters of the inner tubes  12  and outer tubes  14  may be the same or may be different depending upon the flow characteristics required. 
         [0053]    Referring to  FIGS. 16 and 17 , once the proper spacing is determined, openings  38 ,  39  are formed in the top tube sheet  20  and the bottom tube sheet  22 . The openings  38 ,  39  are spaced to correspond to the spacing of the inner and outer tubes  12 ,  14 . The tubes  12 ,  14  are inserted into the openings  38 ,  39  and are maintained in position relative thereto. 
         [0054]    The number of tubes  12 ,  14  in each ring determines the water velocity through them. This velocity must be high enough to prevent boiling and scaling problems, but low enough to prevent erosion. Therefore, when designing a multiple-ring radially-fired heat exchanger  10 , it is important to properly space the tubes  12 ,  14  to obtain the optimum velocity of the liquid to facilitate maximum efficiency. As more tubes  14  are provided in the second ring, the velocity of the liquid in the tubes  12 ,  14  becomes an issue. Consequently, the velocity in both rings must be adequate to allow for the proper heat transfer in both rings. If additional rings are provided, the system must be designed to allow for all tubes in all rings to have adequate velocity of the liquid. In the exemplary embodiment show, the optimum velocity is between 3 ft/s to 8 ft/s, although other flows are possible. 
         [0055]    As shown in  FIGS. 8 through 11 , the top or upper header  16  has an inlet pipe  40  which allows liquid to flow into an outer chamber  42  of the header  16 . An outlet pipe  44  extends from an inner chamber  46  to allow liquid to flow from the inner chamber  46  out of the heat exchanger  10 . In the exemplary embodiment shown in  FIGS. 8 through 11 , the top header  16  is cast from material having the appropriate strength and heat resistant characteristics, such as, for example, cast iron. Because the top header  16  is cast, the transition  48  between the inlet pipe  40  and the outer chamber  42  and the outlet pipe  44  and the inner chamber  46  can be configured to have smooth surfaces and to optimize their geometry to reduce the pressure drop as the flow of the liquid is directed through these areas. All the surfaces of the top header  16  can be controlled to allow minimal pressure drop. In addition, as the inlet and outlet pipes  40 ,  44  are cast, they may be made to have an oblong or oval configuration. This configuration also reduces the pressure drop associated with the moving liquid. Each of the multiple chambers  42 ,  46  of the top header  16  must be configured to meet the flow requirements of the system, i.e., ensure adequate flow rate and velocity while minimizing pressure drop. 
         [0056]    The top header  16  has openings or sensor wells  50  which extend into the outlet pipe  44  or other locations along the top header  16 . The wells  50  may have sensors  52  positioned therein for sensing water temperature, water level, flow rate, or any other relevant properties. As the top header  16  is cast, the wells  50  may be molded into the outlet pipe  44  to provide a direct path for the sensors  52  to be inserted at meaningful locations of the heat exchanger  10 , i.e., directly into the burner compartment. 
         [0057]    While the top header  16  is shown as a cast, single piece, components of the top header may be manufactured as separate pieces and assembled together by welding or the like. 
         [0058]    As shown in  FIGS. 12 through 15 , the bottom header  18  has a chamber  54  and a baffle  56 . The bottom or lower header  18  is also cast from material having the appropriate strength and heat resistant characteristics, such as, for example, cast iron. Because the bottom header  18  is cast, all surfaces of the chamber  54  can be configured to have smooth surfaces and to optimize their geometry to reduce the pressure drop as the flow of the liquid is directed through these areas. The chamber  54  of the bottom header  18  must be configured to meet the flow requirements of the system, i.e., ensure adequate flow rate and velocity while minimizing pressure drop. 
         [0059]    While the bottom header  18  is shown as a cast, single piece, components of the bottom header may be manufactured as separate pieces and assembled together by welding or the like. 
         [0060]    In the embodiment shown in  FIGS. 1 through 18 , the heat exchanger  10  is shown as a two-pass system. Relatively cool pressurized liquid enters the inlet pipe  40  and flows through the outer chamber  42  of the top header  16  into the outer ring of finned tubes  14 . The liquid is forced to flow into all of the tubes  14  of the outer ring. However, the pressure associated with the liquid entering the outer tubes  14  furthest from the inlet pipe  40  is less than the pressure associated with the liquid entering the outer tube  14  closest to the inlet pipe  40 . The liquid flows through the outer tubes  14  into the bottom header  18 . As the liquid flows through the outer tubes  14 , the heat generated by the burner  8  causes the temperature of the liquid to increase. 
         [0061]    Once the liquid enters the bottom header  18 , the pressure of the liquid forces the liquid through the chamber  54  of the bottom header  18  and through the inner tubes  12 . The baffle  56  of the bottom header  18  causes the liquid with the lowest velocity to have the shortest return path through the inner tubes  12  and the liquid with the highest velocity to have the longest return path. Because of the reverse return configuration, the flow rate through each tube  12  is equalized. The bottom header  18  is designed to provide adequate resistance to flow to prevent “short circuiting” of the flow. The path of least resistance is the return tube closest to the supply tube. 
         [0062]    The partially heated pressurized liquid is forced into all of the tubes  12  of the inner ring. The liquid flows through the inner tubes  12  into the inner chamber  46  of the top header  16  and out the outlet pipe  44 . As the liquid flows through the inner tubes  12 , the heat generated by the burner  8  causes the temperature of the liquid to continue to increase. As the inner tubes  12  are closer to the burner  8 , the change of temperature of the liquid in the inner tubes  12  is greater than the change of temperature of the liquid in the outer tubes  14 . 
         [0063]    As the temperature of the surfaces of the inner tubes  12  which are closer to the burner  8  can be significantly greater than the temperature of the surfaces of the inner tubes  12  away from the burner  8 , it is beneficial to have a method to “mix” the liquid as it flows through the inner tubes  12 . In order to accomplish this, enhancement devices  60 , as best shown in  FIGS. 2 and 18 , are used in the inner ring of tubes  12  to create a water vortex in the tubes  12 . This vortex ensures that there is a high velocity water stream in contact alternately with the hot side and then cooler side of the tube  12 . This action helps to prevent boiling of the water in the inner ring of tubes  12 . 
         [0064]    Referring to  FIGS. 19 through 24 , an alternate exemplary embodiment of a top header  116  and bottom header  118  is shown. In this embodiment, baffles  158  are provided in the outer chamber  142  of the top header  116  and baffles  156  are provided in the chamber  146  of the bottom header  118 , to convert the heat exchanger  110  from a two-pass to a four-pass. In this configuration, the inner and outer rings  112 ,  114  are divided in half, allowing the liquid to flow through only half of the tubes in any ring at any time. This allows the liquid to make four passes through the tubes  112 ,  114  rather than two as described above. Additional baffles may be added to alter the number of passes. 
         [0065]    Most copper-fin radially-fired heat exchangers in the market today obtain increased capacity by using longer tubes or increasing the number of tubes in a single ring. Using multiple rings of tubes as described herein effectively lengthens the tube linear distance without increasing the height of the heat exchanger. Consequently, the heat exchanger  10  is half the size of a comparable single-ring heat exchanger. 
         [0066]    An exemplary added benefit of multiple rings is the increased heat transfer coefficient on the gas side of the tubes. This is due to the increased velocity of the gas since the flow area is reduced as the heat exchanger  10  is shorter. Higher efficiency with less material is achieved. 
         [0067]    While the invention has been described with reference to a preferred exemplary embodiment, it will be understood by those skilled in the art that various changes, alterations and modifications may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the broadest interpretation of the appended claims to which the inventors are legally entitled.