Abstract:
In devices known in the art, “conventional firetube” and “waste heat recovery” boilers each require many small tubes making successive passes within the boiler. In one embodiment of the invention, however, an enhanced conduit replaces numerous conventional small tubes. In some embodiments, the enhanced conduit incorporates a plurality of fins, each of which extends through a wall of the conduit. In other embodiments, the enhanced conduit incorporates a plurality of tubes along its outer surface, through which a heat transfer medium flows. Both designs enhance the heat transfer relationship between the hot fluid and the heat transfer medium by providing a continuous heat transfer relationship with the heat transfer medium, increasing the surface area involved in the heat transfer relationship and enhancing convection/conduction couples. For some applications, all of the tube banks of other devices in the art can be replaced by one continuous enhanced conduit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The current application claims the benefit of PCT Patent Application No. PCT/US2004/027812, filed Aug. 27, 2004, and U.S. Provisional Application No. 60/498,486 filed Aug. 28, 2003, which are hereby incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   (1) Technical Field 
   The present invention relates generally to a heat exchanger, and more specifically to a “direct-fired” or “indirect-fired” boiler for generating steam, hot water, hot oil, and hot molten metals. 
   (2) Related Art 
   All boilers operate according to the physical sciences of thermodynamics and heat transfer. Essentially, forced hot gas is cooled within the boiler by transferring heat to a heat transfer medium, often water, to generate steam or hot water. Depending upon system requirements, direct-fired boilers and/or indirect-fired boilers are commonly placed in service to produce steam and hot water. In the case of a direct-fired boiler, a fueled burner or combustor is fired into the boiler, generating heat within the boiler itself. The fueled burner establishes a flame, producing a hot fluid, which is in heat transfer relation with a cooler heat transfer medium. A temperature differential between the hot fluid and the heat transfer medium drives the heat transfer process by way of conduction, convection, and radiation. 
   In a similar manner, a “waste heat recovery” or indirect-fired boiler makes use of residual heat from an isolated thermodynamic process. However, radiation heat transfer is a less significant heat transfer mechanism for the indirect-fired boiler. For boilers of either direct-fired or indirect-fired construction, the heat transfer medium is usually water and/or steam, due in large part to their widespread availability and substantial heat capacity. Another advantage of water/steam heat transfer media is that it presents no imminent environmental threat. 
   A conventional type of direct-fired boiler, commonly called a “firetube” boiler, employs a fueled burner to generate heat. The burner is fired into a single main tube, called the firetube. This firetube absorbs the majority of the radiation emitted from the combustion process. In addition, convective/conductive couples drive heat transfer between the hot fluid and the heat transfer medium throughout the device. Conventional firetube boilers typically contain one to three additional banks of significantly smaller tubes, called passes. For example, a firetube boiler design that includes two banks of tubes in addition to the firetube is termed a “three-pass firetube boiler,” elicited from the path of the hot fluid. The course of flow for the “three-pass firetube boiler” occurs after the fueled burner generates hot gas inside the firetube, which is then driven through a first bank of smaller tubes flowing opposite the firetube, and then diverted through a second bank of smaller tubes flowing parallel to the firetube. A channel, called the “turn-around pass,” is located between each pass, wherein the hot gas reverses direction. The hot gas cools while flowing through the tube passes of the firetube boiler by transferring energy to the heat transfer medium. For either design, all tube banks, less the “turn-around pass,” are in heat transfer relationship with the heat transfer medium. In a similar manner, although a “waste heat recovery” or indirect-fired boiler does not require a firetube, the hot gas does flow sequentially from tube bank to tube bank as required to enact the heat transfer. As a result, heat transfer to the heat transfer medium is largely dependent upon the total length of the tubes it contacts. This can result in larger and more expensive devices. 
   Accordingly, a need exists for a heat exchange device capable of greater efficiency in the transfer of heat from its fluid to its heat transfer medium. 
   SUMMARY OF THE INVENTION 
   In devices known in the art, “conventional firetube” and “waste heat recovery” boilers each require many small tubes making successive passes within the boiler. In one embodiment of the invention, however, an enhanced conduit replaces numerous conventional small tubes. In some embodiments, the enhanced conduit incorporates a plurality of fins, each of which extends through a wall of the conduit. In other embodiments, the enhanced conduit incorporates a plurality of tubes along its inner surface, through which a heat transfer medium flows. Both designs enhance the heat transfer relationship between the hot fluid and the heat transfer medium by providing a continuous heat transfer relationship with the heat transfer medium, increasing the surface area involved in the heat transfer relationship and enhancing convection/conduction couples. For some applications, all of the tube banks of other devices in the art can be replaced by one continuous enhanced conduit. In other applications, the heat transfer fluid flows through the enhanced conduit while the hot fluid flows along an outer surface of the enhanced conduit. 
   The High-Efficiency Enhanced Boiler (HEEB) of the present invention offers improvements over conventional designs. A first improvement is a continuous heat transfer relation by surrounding the enhanced conduit with heat transfer medium. A second improvement is the possibility of substantial turndown ratios. A third improvement is the feasibility of manufacturing devices for applications requiring steam pressures in excess of 21.4 atmospheres absolute, whereas conventional firetube boilers have practical limitations. Finally, the HEEB is readily configurable to generate superheated steam. 
   Therefore, a first objective of the present invention is to provide a High Efficiency Enhanced Boiler capable of generating superheated steam or steam/hot water output. A second objective of the present invention is to provide an effective method for direct-fire or indirect-fire heat transfer to a molten metal heat transfer medium. A third objective of the present invention is to provide a High Efficiency Enhanced Boiler for “waste heat recovery” or indirect-fired boiler applications. A fourth objective of the present invention is to provide a boiler with an enhanced conduit capable of removing heat from the burner flame by proximally located fins. 
   A first aspect of the invention is directed toward a device for transferring heat from a fluid to a heat transfer medium comprising a vessel for containing the heat transfer medium, a conduit extending through a wall of the vessel, the conduit having a first surface in contact with the heat transfer medium and a second surface in contact with a fluid within the conduit, and a plurality of fins, each fin extending through a wall of the conduit, contacting the heat transfer medium and the fluid, wherein heat is transferred from the fluid to the heat transfer medium via the plurality of fins. 
   A second aspect of the invention is directed toward a device for transferring heat from a fluid to a heat transfer medium comprising a vessel containing the heat transfer medium, a conduit extending through a wall of the vessel, the conduit having a first surface in contact with the heat transfer medium and a second surface in contact with a fluid within the conduit, and at least one tube, wherein the heat transfer medium flows within the tube and the fluid flows around the tube. 
   A third aspect of the invention is directed toward a device for transferring heat from a fluid to a heat transfer medium comprising a vessel containing the heat transfer medium, a first conduit extending through a wall of the vessel, the first conduit having a first surface in contact with the heat transfer medium and a second surface in contact with a fluid within the first conduit, a plurality of fins, each fin extending through a wall of the first conduit, wherein heat is transferred from the fluid to the heat transfer medium via the plurality of fins, and at least one tube, wherein the heat transfer medium flows within the tube and the fluid flows around the tube, and wherein heat is transferred from the fluid to the heat transfer medium via the tube. 
   The foregoing and other features of the invention will be apparent from the following more particular description of embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
       FIG. 1  shows a side-view of one embodiment of the invention. 
       FIG. 2  shows a front-view of one embodiment of the invention. 
       FIG. 3  shows a side elevational view of one embodiment of the invention. 
       FIG. 4  shows a cross-sectional view of one embodiment of the invention. 
       FIG. 5  shows a side elevational view of the device of  FIG. 4 . 
       FIG. 6  shows a side elevational view of the device of  FIG. 4 . 
       FIG. 7  shows a cross-sectional view of one embodiment of the invention. 
       FIG. 8  shows a top-view of the device of  FIG. 7 . 
       FIG. 9  shows a front-view of the device of  FIG. 7 . 
       FIG. 10  shows a cross-sectional view of one embodiment of the invention. 
       FIG. 11  shows a side elevational view of the device of  FIG. 10 . 
       FIG. 12  shows a side elevational view of the device of  FIG. 10 . 
       FIG. 13  shows a cross-sectional view of one embodiment of the invention. 
       FIG. 14  shows a cross-sectional view of one embodiment of the invention. 
       FIG. 15  shows a top view of the device of  FIGS. 13 and 14 . 
       FIG. 16  shows a cross-sectional view of one embodiment of the invention. 
       FIG. 17  shows a side elevational view of the device of  FIG. 16 . 
       FIG. 18  shows a side elevational view of the device of  FIG. 16 . 
       FIG. 19  shows a side elevational view of an enhanced conduit apparatus according to the invention. 
       FIG. 20  shows a housing enclosing the apparatus of  FIG. 19 . 
       FIG. 21  shows a cross-sectional view of the apparatus of  FIG. 19 . 
       FIG. 22  shows a side elevational view of an alternate embodiment of an enhanced conduit apparatus according to the invention. 
       FIG. 23  shows a side cross-sectional view of the apparatus of  FIG. 22 . 
       FIG. 24  shows a front cross-sectional view of the apparatus of  FIG. 22 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 6  depict a boiler  1  of the present invention, which includes a vessel  10  for containing a heat transfer medium. In some embodiments, vessel  10  is pressurized internally and designed according to American Society of Mechanical Engineers (ASME) codes for boilers and pressure vessels. The ASME codes are one of a few fabrication standards honored worldwide. Typically, internal design pressures for this class of vessel range from 1.1 to 21.4 atmospheres absolute, although there are vessels in existence that exceed pressures of 21.4 atmospheres absolute. For reasons of safety and reliability, the ASME codes and others restrict the materials and fabrication methods for vessels with internal design pressures over 2.0 atmospheres absolute. Therefore, only code recognized materials, such as, but not limited to, SA516 GR70, SA240 304, SA312 TP304, and SA106 B, are acceptable for fabrication of vessel  10 . In addition, the adherence to a Code infers that only a facility skilled in the art can fabricate a device such as vessel  10 . Additionally, insulation (not shown) covers the exterior surface of vessel  10  for reasons of efficiency and safety. 
   Four basic penetrations are commonly made to vessel  10 . In actuality, and commonly known to those of ordinary skill in the art, several penetrations of vessel  10  are required. Process and policy require penetrations for boiler inspection, boiler drainage, pressure relief, and sensing/gauging. Although the previously mentioned compulsory penetrations are not shown, it is assumed that these requirements are met in the final or code-authorized design. 
   The sump  20  proximal to the top of vessel  10  is indicative of a steam boiler. By design, sump  20  is known to moderate surging, a problem associated with steam production. Consequently, in order to maintain a sufficient level of a heat transfer medium (e.g., water in the case of a steam boiler), a feedwater inlet  30  is located near the bottom of vessel  10 . Any steam having left sump  20  continues upstream to deliver the stored energy and then returns downstream as condensate to feedwater inlet  30 , thus completing the cycle. This process is typical of a closed steam/water system. In reality, system losses require that provisions be made to replenish the heat transfer medium (e.g., make-up water). Furthermore, deaerators and water treatments are meant to protect the system components from oxidation and chemical attack. However, since deaeraters and chemical treatments are known to those of ordinary skill in the art, further explanation will not be given. 
   The final two penetrations shown in the vessel  10  are the hot fluid inlet  40  and the flue outlet  50  of enhanced conduit  60 . Situated entirely within vessel  10 , enhanced conduit  60  forms a non-communicating pressure boundary between a hot fluid contained within it and a heat transfer medium within vessel  10 . Thus, enhanced conduit  60  is entirely in heat transfer relation with the hot fluid and the heat transfer medium. Often, the hot fluid is hot air generated from a burner, although other fluids or liquids may be used. For example, it may be desirable to cool a molten metal or salt. In such a situation, the molten meal or salt may be passed through enhanced conduit  60 , transferring its heat to a heat transfer medium. 
   Similarly, although the embodiments of the invention are often depicted as steam boilers, necessitating that the heat transfer medium be water, other fluids or liquids are also allowable. For example, the heat transfer medium may be any liquid, gas, or similar material with suitable heat transfer properties. 
   In a “single pass firetube boiler,” enhanced conduit  60  extends horizontally near a central axis of vessel  10 , as shown in  FIGS. 4 through 6 . A fuel-fired burner  70 , generates heat and energy, which are forced into enhanced conduit  60 . Burner fuel may include, for example, coal, distillate oil, natural gas, methanol, ethanol, propane, and liquefied petroleum gas. A forced draft subassembly (not shown) regulates the flow of gas to burner  70  so that the proper ratio of oxygen-to-fuel can be attained, and forces or drives the hot gas into enhanced conduit  60 . 
   Essentially, enhanced conduit  60  is under the same pressure as vessel  10 , except that the pressure is exerted on an internal surface of vessel  10  and an external surface of enhanced conduit  60 . Once again, the ASME code or other accepted design standard is invoked to comply with engineering requirements. In general, with respect to the length of enhanced conduit  60 , external pressure is more severe than internal pressure in terms of local stress. Generally, when external pressure applied to a conduit exceeds allowable stress limits, buckling or failure occurs. Accordingly, in one embodiment of the invention, the cross-sectional geometry of enhanced conduit  60  is circular. However, other shapes, including but not limited to square, rectangular, or ellipsoidal, are possible and within the scope of the present invention. 
   Within enhanced conduit  60 , a plurality of fins  80  extend intimately into the path of the hot fluid. Fins  80  establish a series of obstructions that force the hot fluid to assume a path around individual fins  80  in a manner that elicits turbulence, thereby enhancing heat transfer. Furthermore, a portion of each fin  80  extends through a wall of enhanced conduit  60  and contacts the heat transfer medium. Fins  80  thereby increase heat transfer through turbulent mixing of the hot fluid and by increasing the surface area exposed to the hot fluid and/or the heat transfer medium. Each fin  80  may be oriented through a wall of enhanced conduit  60  in any number of angles relative to the long and short axes of enhanced conduit  60 . As such, fins  80  may be oriented to direct the flow of the hot fluid and/or the heat transfer medium along a particular path. 
   Each fin  80  is fabricated from materials that demonstrate structural stability while providing good heat transfer characteristics. Possible fin  80  materials include, but are not limited to, generic steels, metals (including copper, molybdenum, etc.), ceramics, refractory materials, and engineered composites. A largely material-dependent objective of the present invention is the ability to extract heat by placing fins  80  in close proximity to the flame of burner  70 . One example (not shown) of a fin configuration capable of meeting this objective comprises a cylindrical generic steel body fitted with a spherical molybdenum tip. 
   For simplicity in depiction, cylindrical-shaped fins  80  are shown. However, other fin shapes or combinations of shapes are possible and considered to be within the scope of the present invention. Such shapes include, for example, square, elliptical, aerodynamic, rectangular, and spherical. In addition, such fins may be constructed with through holes, with threaded holes, with blind holes, and may be tapered or threaded. As an example (not shown) of a multi-geometric combination, the fin shape may be cylindrical at one end, tapered in the middle, and rectangular with blind holes toward its opposite end. Each fin  80  may be mechanically fastened to enhanced conduit  60  in an ASME code or other acceptable method, forming a pressure-rated joint. 
   In general, the heat transfer medium is water/steam, although molten metal (heat transfer salt) and hot oil systems are possible. As suggested earlier, widespread availability and substantial heat capacity are factors favoring water/steam as the most common heat transfer medium. At startup, vessel  10 , around the outside surface of enhanced conduit  60 , is filled with the heat transfer medium (e.g., water). Demand for steam signals burner  70  to ignite fuel into a combustible flame. The flame is directed at hot fluid inlet  40  of enhanced conduit  60 , whereby heat is drawn off by fins  80  located near the outer flame boundary. Fins  80  extract substantial energy from the flame by radiation/conduction/convection heat transfer to the heat transfer medium over the length of the flame. At the extreme boundary of combustion, where the flame ceases to exist, fins  80  remove heat from the hot fluid stream by convection/conduction couples. Additionally, the portion of each fin  80  extending within enhanced conduit  60  causes turbulence in the hot fluid stream, accelerating convection heat transfer, while the portion of each fin  80  extending outside enhanced conduit  60  provides more surface area for convective heat transfer to occur. More particularly, a balanced energy flow exists in the region of each fin  80 . The exhausted hot gas leaves enhanced conduit  60  through the flue outlet  50  on route to the stack (not shown). As the heat transfer medium (e.g., water) is heated, it evaporates and exits at sump  20 . From sump  20 , the steam goes to the load (not shown), where condensation occurs. The steam condenses to water and is pumped into inlet  30  in order to maintain a constant level of heat transfer medium within boiler  1 . 
   EXAMPLE 1 
   Referring to  FIGS. 7-12 , a direct-fired 3-pass 30-horsepower boiler  100  is shown, fabricated in accordance with the present design criteria for a pressure of 10 atmospheres and requiring a one million BTU (British thermal units) natural gas burner. Cylindrical vessel  110  has dimensions of 42-inches O.D. wide by 60-inches O.D. long, with ten-inch diameter enhanced conduit  160  winding through the interior of the vessel. Hot fluid enters boiler  100  through hot fluid inlet  140 , passes through enhanced conduit  160 , and exits through flue outlet  150 . Condensate returns to boiler  100  through feedwater inlet  130 . There are 280 ¾″ diameter fins  180  located circumferentially throughout enhanced conduit  160  in sets of ten. Fins  180  are mechanically fastened to enhanced conduit  160  by virtue of a self-locking taper and seal welding. The temperature of the exhausted flue gas is approximately 230° C. The thermal efficiency of such a design is increased, in part, due to the fact that “turn-around passes” are maintained in heat transfer relationship with the heat transfer medium within the boiler. 
   EXAMPLE 2 
   Referring now to  FIGS. 13-18 , a direct-fired boiler  200  is shown with a coiled enhanced conduit  260 . The long axis of cylindrical vessel  210  is oriented vertically, rather than horizontally as in Example 1. Rather than completing a series of reversals in direction as in Example 1, enhanced conduit  260  is coiled within vessel  210 , completing a total of three revolutions. Hot fluid enters boiler  200  through hot fluid inlet  240 , passes through enhanced conduit  260 , and exits through flue outlet  250 . As in Example 1, enhanced conduit  260  contains a plurality of fins  280  located around its circumference and along its length. Fins  280  may be fastened to enhanced conduit  260  by any of a number of means described above. 
   EXAMPLE 3 
   Referring to  FIGS. 19-21 , a 4-pass conduit  360  is shown. Unlike earlier-described embodiments, wherein a heat transfer medium sits within a vessel, the depicted embodiment incorporates a housing  360 A around the apparatus  360 . Housing  360 A directs a heat transfer medium along an outer surface of a pass  362 ,  364 ,  366 ,  368  as the hot fluid is directed along an inner surface of the same pass. In some embodiments, such as that shown in  FIG. 20 , the apparatus has a “reverse flow,” wherein as the hot fluid enters first pass  362  (often a firetube), the heat transfer medium enters through a heat transfer medium inlet  368 B at a distal end of the fourth pass housing  368 A, flows in a direction substantially opposite that of the hot fluid, and exits through a heat transfer medium outlet  362 B at a proximal end of the first pass housing  362 A. 
   In the embodiment depicted in  FIG. 19 , three of the four passes  362 ,  364 ,  366  are enhanced, each containing a plurality of fins  380  extending through a wall of the pass. Optionally, one or more enhanced pass  362 ,  364 ,  366  may contain a helical member  390  along its outer surface. Located in such a manner, helical member  390  contacts or resides close to an inner surface of each enhanced pass housing  362 A,  364 A,  366 A of apparatus housing  360 A and directs the heat transfer medium along the surface of the pass  362 ,  364 ,  366 , effectively increasing contact between the pass and the heat transfer medium. Accordingly, in order to increase contact between fins  380  and the heat transfer medium, helical member  390  preferably lies parallel to the pattern of fins  380 . Such an arrangement effectively creates channels between the surface of a pass  362 ,  364 ,  366  and a pass housing  362 A,  364 A,  366 A, in which are situated a plurality of fins  380 . 
   Each pass  362 ,  364 ,  366 ,  368  is connected to another by a turn-around pass  363 ,  365 ,  367  which substantially reverses the direction of flow of the fluid within enhanced conduit  360 . For example, the fluid within enhanced conduit  360  initially flows through first pass  362  in direction A. Upon passage through first turn-around pass  363 , the fluid substantially reverses direction, entering second pass  364  in direction B. Similarly, upon passage through second turn-around pass  365 , the fluid again substantially reverses direction, entering third pass  366  in direction C. Finally, the fluid passes through third turn-around pass  367  and enters a non-enhanced pass  368  in direction D before flowing through flue outlet  350 . 
     FIG. 21  shows a side cross-sectional view of the apparatus in order to depict the obstructions within each enhanced pass  364 ,  366  created by the interior projections of fins  380 . Also depicted are the channels created between helical member  390  and enhanced pass housings  364 A,  366 A. 
   As depicted, only passes  362 ,  364 ,  366  contain fins  380  and, optionally, helical member  390 . However, it should be recognized that turn-around passes  363 ,  365 ,  367  may be enhanced with fins  380  and/or helical member  390  in addition to or instead of passes  362 ,  364 ,  366 . 
   EXAMPLE 4 
   Referring to  FIGS. 22-24 , a modified 4-pass enhanced conduit  460  is shown. Unlike the device in  FIG. 19 , wherein fourth pass  368  is an unenhanced conduit, modified enhanced conduit  460  includes a fourth pass  468  comprised of a plurality of tubes  494 . The plurality of tubes  494  is preferably arranged in a circular pattern, as depicted most clearly in  FIG. 24 , although other shapes are allowable. Similarly, while a plurality of tubes  494  is depicted, a single tube is also within the scope of the invention. 
   Heat transfer medium enters an opening  498  in an end of each tube  494  and flows through tube  494 , increasing the heat transfer from the hot fluid within fourth pass  468  to the heat transfer medium. Due to the transfer of heat from the hot fluid to the heat transfer medium, the difference in temperature between the hot fluid and the heat transfer medium is generally smaller along fourth pass  468  than along earlier passes  462 ,  464 ,  466 . Where such a smaller temperature difference exists, it has been found that such a plurality of tubes more efficiently transfers heat from the hot fluid to the heat transfer medium than does a plurality of fins  480  or a plurality of fins  40  and helical members  490 , such as those along earlier passes  462 ,  464 ,  466 . 
   Optionally, one or more baffles  496 ,  497  may be placed along the length of the plurality of tubes  494 . Such baffles may be outer baffles  496 , located around tubes  494 , or inner baffles  497 , located within the plurality of tubes  494 . Outer baffles  496  are preferably ring shaped so as to fit around a circular arrangement of the plurality of tubes  494 , although other shapes are allowable. Outer baffles  496  preferably contact or reside close to an inner surface of fourth pass housing  468 A. Inner baffles are preferably disc shaped so as to fit within a circular arrangement of the plurality of tubes  494 , although other shapes are allowable. Outer baffles  496  and inner baffles  497  disrupt the flow of the hot fluid within pass  468 . Inner baffles  497  force the hot fluid outside the plurality of tubes  494  to a location between the plurality of tubes  494  and fourth pass housing  468 A, while outer baffles  496  force the hot fluid in the opposite direction, i.e., into the center of the plurality of tubes  494 . This disruption of the flow of the hot fluid increases heat transfer from the hot fluid to the heat transfer medium. 
   While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.