Patent Grant
7882809
1. Field of the Invention
The present invention relates to a heat exchanger that includes a counterflow evaporator.
2. Discussion of the Background
Heat exchangers that include evaporators heated by hot gases typically suffer from relatively large size and high cost. Further, evaporators that generate steam at a single pressure typically exhibit poor thermal efficiency because the hot gas contacts the tubing conveying the liquid being evaporated in a cross-flow or parallel flow configuration at a single temperature, the saturation temperature at the pressure of interest. While previous systems and methods have attempted to improve upon steam boiler control and construction, these systems and methods still suffer from the drawback of cross-flow contact between the heating gas and the evaporating liquid.
In an effort to eliminate the above drawbacks of related art heat exchangers that include evaporators, the inventors have constructed a heat exchanger that includes a counter-flow evaporator as described below.
The present invention advantageously provides an evaporator including a lower drum, an upper drum, and a plurality of tubes extending between the lower drum and the upper drum. The tubes have fluid passageways therein extending from the lower drum to the upper drum. A duct is provided having a heating gas passageway provided therein. The plurality of tubes extends through the heating gas passageway. The fluid passageways define an overall flow path from the lower drum to the upper drum extending in a direction substantially counter-current to an overall flow path defined by the heating gas passageway extending from a gas inlet of the heating gas passageway to a gas outlet thereof.
The present invention also advantageously provides a heat exchanger including, in addition to the above evaporator, a superheater having a superheater heating gas passageway therein extending from a superheater gas inlet to a superheater gas outlet, where the superheater has at least one superheater tube having a superheater fluid passageway therein extending from a superheater fluid inlet to a superheater fluid outlet. The at least one superheater pipe extends through the superheater heating gas passageway. Additionally, an economizer is provided having an economizer heating gas passageway therein extending from an economizer gas inlet to an economizer gas outlet, where the economizer has at least one economizer tube having an economizer fluid passageway therein extending from an economizer fluid inlet to an economizer fluid outlet. The at least one economizer pipe extends through the economizer heating gas passageway. Furthermore, the superheater heating gas outlet is connected to the heating gas inlet of the evaporator, the heating gas outlet of the evaporator is connected to the economizer heating gas inlet, the economizer fluid outlet is connected to the lower drum of the evaporator, and the upper drum of the evaporator is connected to the superheater fluid inlet.
The present invention further advantageously provides a method of generating steam including providing a fluid flowing from a lower drum through a plurality of tubes to an upper drum, and providing a heated gas flowing from a gas inlet of a heating gas passageway to a gas outlet of the heating gas passageway such that the heated gas contacts the plurality of tubes and heats the fluid within the plurality of tubes from liquid-phase to gaseous-phase. In this method, the fluid flows through the plurality of tubes in a substantially counter-current direction to an overall flow path of the heated gas flowing from the gas inlet of the heating gas passageway to the gas outlet of the heating gas passageway.
Furthermore, the present invention advantageously provides a method of super heating steam including providing an economizer having a fluid flowing within at least one economizer pipe from an economizer fluid inlet to an economizer fluid outlet, and providing a evaporator having a lower drum connected through a plurality of tubes to an upper drum, where the lower drum receives the fluid from the economizer fluid outlet, and the fluid flows from the lower drum through the plurality of tubes to the upper drum. The method also includes providing a superheater having at least one superheater pipe with a superheater fluid inlet and a superheater fluid outlet, where the superheater fluid inlet receives the fluid from the upper drum of the evaporator, and providing a heated gas flowing through a heating gas passageway extending through the superheater, the evaporator, and the economizer, such that the heated gas contacts the at least one superheater pipe, the plurality of tubes, and the at least one economizer pipe. In this method, the fluid flows through the plurality of tubes of the evaporator in a substantially counter-current direction to an overall flow path of the heated gas flowing through the evaporator.
A more complete appreciation of the invention and many of the attendant advantages thereof will become readily apparent with reference to the following detailed description, particularly when considered in conjunction with the accompanying drawings, in which:
Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. In the following description, the constituent elements having substantially the same function and arrangement are denoted by the same reference numerals, and repetitive descriptions will be made only when necessary.
As depicted in
Once the evaporating fluid 16 is heated by the economizer 30, then the heated evaporating fluid is transported via pipe 36 past duct section 28 to a second coil (referred to as a “evaporator”) 40. Alternatively, all of the heating from the temperature of the evaporating fluid supply 19 to the evaporator exit temperature may be achieved in the evaporator 40. The evaporator 40 includes a lower drum 42, which receives the heated evaporating fluid via pipe 36, an upper drum 44, and a series of tubes 46 having fluid passageways therein that extend between the lower drum 42 and the upper drum 44. In one embodiment, the evaporator 40 does not raise the temperature of the evaporating fluid to any large extent, but rather takes care of the phase change of the evaporating fluid from liquid to gas. In this embodiment, the economizer 30 raises the evaporating fluid temperature close to the saturation temperature. Preferably, the economizer 30 will have a lower evaporating fluid flow area than the evaporator 40, such that either fewer tubes flow in parallel in the economizer and/or those tubes are of a smaller diameter. This embodiment maximizes the heat transfer rate to the evaporating fluid in the economizer 30 and the evaporator 40, respectively.
The lower drum 42 preferably includes a temperature sensor 41 for use in monitoring and controlling the operation of the system, and a blowdown port 43. The upper drum 44 also preferably includes a temperature sensor 45 for use in monitoring and controlling the operation of the system according to the method of U.S. Pat. No. 7,017,529, which is incorporated herein in its entirety. The system can, optionally, be provided with one or more level sensing means connected to one or more of the drums 42 and 44 for control according to traditional methods. In the embodiment where little temperature change occurs across the evaporator 40, a liquid recirculation means can also be provided to transport evaporating fluid from the upper drum 44 to the lower drum 42, in order to assure a constant level and temperature of liquid in the evaporator tubes 46.
The tubes 46 of the evaporator 40 extend through duct section 26, which has a heating gas passageway therein to carry the heating gas 14 that extends from a heating gas inlet adjacent the upper drum 44 to a heating gas outlet adjacent the lower drum 42. Under typical operation, the evaporating fluid is in the liquid phase in the lower drum 42 and in the gas phase in a discharge pipe 56 from the upper drum 44. The evaporating fluid is present within the tubes 46, and absorbs heat from the heating gas 14 traveling over the outside of the tubes 46. The flow of the heating gas 14 through the evaporator 40 is in an overall counter-current direction as compared to the flow of the evaporating fluid 16 traveling through the evaporator. In other words, the heating gas 14 is traveling through the evaporator 40 in a downward vertical direction in the embodiment in
In one embodiment of the present invention, the baffles 48 are spaced to maintain a maximum heating gas velocity through the tube bundle greater than 3 meters per second. In another embodiment, the baffles 48 are spaced to maintain a maximum heating gas velocity through the tube bundle greater than 6 meters per second.
In one embodiment of the present invention, the evaporating fluid 16, which is now in the gaseous form, is transferred from the upper drum 44 to a third coil (referred to as a “superheater”) 50 via a pipe 56. The superheater 50 brings the evaporating fluid to its final temperature, which can be any temperature above the saturation temperature, but below the maximum service temperature of the superheater materials of construction. In the superheater 50, the evaporating fluid enters through an inlet 51 and travels through a series of tubes 52 that extend across a portion of duct 20 of the heat exchanger adjacent an inlet 22 of the duct 20 carrying the heating gas 14 from a heat source. The tubes 52 extend across the duct 20 in an array 54, and the tubes can extend in a single pass arrangement or in a multi-pass serpentine manner back and forth across the superheater, in order to achieve the desired heat exchange between the heating gas and the evaporating fluid. Likewise, one continuous evaporating fluid path can exist between the inlet and outlet, or two or more paths can be provided in parallel. The configuration of tubes 52 used preferably provide an overall counter flow arrangement between the flow direction of the heating gas flowing through the superheater 50 (bottom to top in
Overall the three coils of the embodiment depicted in
The evaporator 40 preferably includes a structure for removing droplets from the evaporating fluid exiting the evaporator 40. The present invention includes a mist eliminator within the upper boiler drum 44, as depicted in
The mist eliminator further includes a mist eliminator pipe 70 that is provided within the housing 60 in an inclined manner such that a lower inlet end 72 is within the housing and pipe 70 extends through an opening 64 in the housing 60 such that an upper outlet end 74 is outside of the housing 60. The mist eliminator pipe 70 has packings 76 (only one packing is shown for clarity) fully packed therein. Alternatively, the packings 76 can be replaced by a structured media, as in the case of the packings 66. In the embodiment employing individual packings 76, the mist eliminator pipe 70 preferably is provided with a mesh or perforated plate 73 welded to the lower inlet end 72 in order to retain the packings 76 within the pipe 70, and a mesh or perforated plate 75 welded to the upper outlet end 74 in order to prevent the evaporating fluid flow from carrying the packings 76 out of the pipe 70. The velocity of the steam evaporating fluid is typically well below fluidization velocity of the packings; however, it is preferable to provide such mesh or perforated plates in order to prevent the packings from being carried out by the steam evaporating fluid flow.
In this bed within a bed configuration, the mixed-phase evaporating fluid enters the housing 60 from the tubes 46, enters the lower inlet end 72 of the mist eliminator pipe 70, and then exits the upper outlet end 74, which is fluidly connected to pipe 56. The packings 66 are intended to intercept and coalesce the majority of liquid-phase droplets that may be present within the evaporating fluid exiting from the tubes 46 of the evaporator. The packings 76 within the mist eliminator pipe 70 provide for further capture and elimination of droplets that may have made it passed the first set of packings.
In one embodiment of the present invention, the cross sectional area of the pipe 70 is smaller than the cross sectional area available for fluid flow in the housing (or shell) 60. In one embodiment of the present invention, all of the packings 76 and 66 are similar in characteristic size. In another embodiment of the present invention, the packings 66 possess a larger characteristic size than the packings 76. In another embodiment of the present invention, the packings 76 possess varying characteristic size from the inlet end 72 to the discharge end 74. In one embodiment of the present invention, the velocity of the gas phase evaporating fluid through the pipe 70 is below the droplet entrainment velocity (or “superficial velocity,” which is a velocity of flow if the pipe were empty (i.e., no media), and at which droplet shear within the pipe occurs) for the evaporating fluid in question. In another embodiment of the present invention, when the evaporating fluid is water, the velocity in the pipe 70 is below 5 m/sec. In another embodiment of the present invention, when the evaporating fluid is water, the velocity in the pipe 70 is below 3 m/sec. Such velocities may be necessary to prevent droplet shear within the pipe 70 in conjunction with a desired maximum velocity of heating gas through the bundle of tubes 46.
The first coil (referred to as an “economizer”) 130 heats the evaporating fluid 16, which begins in a liquid phase, to a temperature below the boiling temperature. The evaporating fluid 16 is pumped to the economizer 130 via a supply pipe 19 by a pump 18, and the evaporating fluid travels through a series of tubes 132 that extend across a portion of duct 125 of the heat exchanger upstream of an outlet 124 of the duct 125 carrying the heating gas 14 from a heat source. The configuration of tubes 132 used preferably provide an overall counter flow arrangement between the flow direction of the heating gas flowing through the economizer 130 (bottom to top in
Once the evaporating fluid 16 is heated by the economizer 130, then the heated evaporating fluid is transported via pipe 136 to a second coil (referred to as an “evaporator”) 140. The evaporator 140 includes two lower drums 142A and 142B, which receive the heated evaporating fluid via pipe 136, and an upper drum 144. Alternatively, it should be appreciated that a single lower drum 142 and multiple upper drums 144 can be provided. In fact, embodiments having a number of upper and lower drums operated in parallel are possible with greatly-reduced height compared to the embodiment depicted in
In the embodiment of
The tubes 146A and 146B of the evaporator 140 extend through duct sections 126A and 126B, respectively, which carry the heating gas 14. Under typical operation, the evaporating fluid is in the liquid phase in the lower boiler drums 142A and 142B and in the gas phase exiting the upper drum 144. The flow of the heating gas 14 through the evaporator 140 is in an overall counter flow direction as compared to the evaporating fluid 16 traveling through the evaporator. In other words, the heating gas 14 is traveling through the evaporator 140 in an overall downward direction in the embodiment in
The evaporating fluid 16, which is now in the gaseous form, is transferred from the upper drum 144 to a third coil (referred to as a “superheater”) 150 via a pipe 156. The superheater 50 brings the evaporating fluid to its final temperature above the saturation temperature. In the superheater 150, the evaporating fluid enters through an inlet 151 and travels through a series of tubes 152 that extend across a portion of duct 125 of the heat exchanger adjacent an inlet 22 of the duct 125 carrying the heating gas 14. The configuration of tubes 152 used preferably provide an overall counter flow arrangement between the flow direction of the heating gas flowing through the superheater 150 (bottom to top in
Once the evaporating fluid 16 is heated by the superheater 150 to its final temperature, then the heated evaporating fluid is discharged from outlet 158.
The embodiment depicted in
The embodiment in
The present invention provides a system that allows for efficient heat transfer due to the overall counter-current flow. The present invention also allows for minimized size by controlling the Reynold's number of the heating gas across the liquid-conveying tubes of the evaporator independent of the tube array depth or total heat transfer area. The present invention also allows for minimized depth of the tube array (number of rows of tubes in the array) as well as more uniform temperatures in the tubes, thus advantageously reducing thermal stresses as compared to an overall cross-flow configuration.
The present invention can be constructed using a housing and seal configuration as taught in U.S. Pat. No. 6,957,695 in order to further accommodate thermal expansion with a sealing ductwork.
It should be noted that the exemplary embodiments depicted and described herein set forth the preferred embodiments of the present invention, and are not meant to limit the scope of the claims hereto in any way.
Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
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| Number | Date | Country | |
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| 20080104960 A1 | May 2008 | US |
