Patent Grant
7413004
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.
(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.
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.
The embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein:
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
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.
Referring to
Referring now to
Referring to
In the embodiment depicted in
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.
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.
Referring to
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 468A. 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 468A, 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.
| Number | Name | Date | Kind |
|---|---|---|---|
| 166461 | Houghton | Aug 1875 | A |
| 2004252 | Sorensen | Jun 1935 | A |
| 2872164 | Wilson | Feb 1959 | A |
| 2893705 | Fennel | Jul 1959 | A |
| 3474636 | Bligh | Oct 1969 | A |
| 3785350 | McCormick | Jan 1974 | A |
| 3788281 | Van Lookeren Campagne | Jan 1974 | A |
| 3835816 | Ferrin | Sep 1974 | A |
| 3987761 | Downs et al. | Oct 1976 | A |
| 4055152 | Vidaleng | Oct 1977 | A |
| 4116270 | Marushkin et al. | Sep 1978 | A |
| 4623309 | Teigen | Nov 1986 | A |
| 6167846 | Ogino et al. | Jan 2001 | B1 |
| 20010018962 | Joshi et al. | Sep 2001 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20070199688 A1 | Aug 2007 | US |
