Natural gas serves as the energy source for much of the currently generated electricity. To this end, the gas undergoes combustion in a gas turbine which powers an electrical generator. However, the products of combustion leave the gas turbine as an exhaust gas quite high in temperature. In other words, the exhaust gas represents an energy source itself. This energy is captured in a heat recovery steam generator (“HRSG”) that produces superheated steam that powers another electrical generator.
Such exhaust gas includes carbon dioxide and water in the vapor phase, but also includes traces of sulfur in the form of sulfur dioxide and trioxide. Those sulfur compounds, if combined with water, produce sulfuric acid which is highly corrosive. As long as the temperatures of the heating surfaces remain above the acid dew point temperature of the exhaust gas, SO2 and SO3 pass through the HRSG without harmful effects. But if any surface drops to a temperature below the acid dew point temperature, sulfuric acid will condense on that surface and corrode it.
Dew point temperatures vary depending on the fuel that is consumed. For natural gas the temperature of the heating surfaces should not fall below about 140° F. For most fuel oils it should not fall below about 235° F.
Generally, an HRSG comprises a casing having an inlet and an outlet and a succession of heat exchangers—namely a superheater, an evaporator, and a feedwater heater arranged in that order within the casing between the inlet and outlet.
Such heat exchangers for an HRSG can have multiple banks of coils, the last of which in the direction of the gas flow can be a feedwater heater. Surfaces vulnerable to corrosion by sulphuric acid do exist on the feedwater heater. The feedwater heater receives condensate that is derived from low-pressure steam discharged by the steam turbine, and elevates the temperature of the water. Then the warmer water from the feedwater heater flows into one or more evaporators that convert it into saturated steam. That saturated steam flows on to the superheater which converts it into superheated steam. From the superheater, the superheated steam flows to the steam turbine.
In this process, by the time the hot gas reaches the feedwater heater at the back end of the HRSG, its temperature is quite low. However, that temperature should not be so low that acids condense on the heating surfaces of the feedwater heater.
Generally, in the above-discussed process, most HRSGs produce superheated steam at three pressure levels—low pressure (LP), intermediate pressure (IP) and high pressure (HP). Further, an HRSG can have what are termed an LP Evaporator, an HP Economizer, and an IP Economizer. The feedwater heater typically discharges some of the heated feedwater directly into an LP evaporator.
A feedwater heater, or preheater, in a steam generator extracts heat from low temperature gases to increase the temperature of the incoming condensate before it goes off to the LP evaporator, HP economizer, or IP economizer. Multiple methods have been used to increase the temperature of the condensate before it enters any part of the preheater tubes within the gas path (e.g., recirculation pump, external heat exchanger). These methods are used to prevent the exhaust gas temperature from dropping below the acid dew point and causing sulfuric acid corrosion.
Prior systems and methods have been limited in application because the feedwater temperature was not high enough to protect against dew point corrosion of all fuels. The movement of the heat transfer coils to the hotter regions provides for higher differentials in the heat exchanger.
In the present disclosure, an external water-to-water heat exchanger heats the lower temperature inlet condensate with the source of heat being hot water that is exiting the first stage of the feedwater heater. The condensate flow first enters the external heat exchanger. Thereafter preheated condensate leaves the external heat exchanger and enters the feedwater heater. Water energy exiting the preheater is used to preheat the incoming condensate. The present disclosure places a section of a preheater surface into a hotter section of the gas flow, upstream of the LP evaporator, to achieve the beneficial result of increasing source inlet temperature and directly increasing the outlet temperature of the preheated condensate exiting the external heat exchanger. This arrangement allows the use of an external heat exchanger in designs with higher dew points in the cold end. The present system and method can thus create a larger temperature differential in the external water-to-water heat exchanger. This larger temperature differential than present in the prior art, yields a higher outlet temperature and protects the HRSG from cold end condensation corrosion from fuels with higher acid dew points.
The foregoing and other features and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings.
Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings.
The following detailed description illustrates the claimed invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the disclosure, describes several embodiments, adaptations, variations, alternatives, and uses of the disclosure, including what is presently believed to be the best mode of carrying out the claimed invention. Additionally, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The disclosure is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.
The inventive disclosures are now provided for a heat exchanging system and method for use in an HRSG. An overall illustration of a system which features use in a heat-recovery steam generator (HRSG) appears in U.S. Pat. No. 6,508,206 B1 (hereafter “'206 patent”). The '206 patent is hereby incorporated by reference in this application as if fully set forth herein.
The disclosure of the present inventive features of the present application show an HRSG 50 with an arrangement of heat exchangers and flow channels that provide improvements over the prior art.
With reference to
The casing 53 generally will have a floor 61 over which the heat exchangers are supported, and sidewalls that extend upwardly from the floor 61. Typically the top of the casing 53 is closed by a roof 63. The floor 61 and roof 63 extend between the sidewalls so that the floor 61, sidewalls and roof 63 help to form the duct 54. From outlet 59 the gas can flow through flu 67.
Generally, the heat exchangers comprise coils that have a multitude of tubes that usually are oriented vertically and arranged one after the other transversely across the interior of the casing 53. The coils are also arranged in rows located one after the other in the direction of the hot gas flow depicted by the arrows in
Now attention is directed to the arrangement of the heat exchangers shown in
Downstream from the Upstream Coils 70, the novel arrangement has a preheater booster 74. As will be discussed, the preheater booster 74 provides for a feedwater heater presence in a hotter region of the HRSG to facilitate return feeding therefrom to a heat exchanger that feeds water to other parts of the feedwater heater.
Continuing the description from upstream to downstream, left to right in
Now, with more specific reference to the schematic view of
As seen in the
The feedwater heater 80 has two sections 103 and 106, which can be arranged side by side in the duct 54, as shown in
Focusing now on the flow of water among aforementioned components of the arrangement, a water-to-water heat exchanger 125 is illustrated as located to the exterior of the duct 54. The condensate pump 52 discharges feedwater into a supply pipe 127, which delivers that feed water into the inlet of the low temperature path 130 of heat exchanger 125. The feedwater leaves the low temperature path 130 in exchanger 125 at its outlet and flows into a connecting pipe 132 which acts as a conduit. Pipe 132 delivers the feedwater to the tubes at the downstream face 114 of section 106. The water leaves the section 106 at its upstream face 110 and flows through a transfer pipe 135 which serves as a conduit to connect with the inlet of the preheater booster 74 coil at its downstream face 93. The water flows thence through preheater booster coil 74 toward the upstream side thereof to exit the preheater booster coil 74 at its upstream face 90. From there, it flows into a transfer pipe 138 which acts as a conduit to connect with the inlet of the high temperature path 140 of heat exchanger 125.
Within the high temperature path 140 of heat exchanger 125 the temperature of the water decreases since it loses heat to water in the low temperature path 130. At the outlet of the high temperature path 140, the water enters transfer pipe 143 which acts as a conduit to be delivered to the section 103 at its downstream face 112. The water thence flows through section 103 to exit therefrom at its upstream face 108 whereby the temperature of the water is raised, to thence pass through a discharge pipe 150. Pipe 150 acts as a conduit and extends to connect with the LP Evaporator 77 at its downstream face 100. From the upstream face 96 of LP Evaporator 77, the water can flow, for example, to the HP Economizer.
Now the system will be discussed with exemplary temperatures. The exhaust gases from the gas turbine “G”, enter the upstream face 153 of the last of the Upstream Coils 70, here designated, for example, as a high pressure (HP) economizer 155. The gases enter the HP Economizer upstream face 153 at a temperature of about 500° F. The exhaust gases exit the downstream face of HP Economizer 155 at a temperature of about 380° F., and enter the upstream face 90 of preheater booster 74 at about that same temperature.
Water from the condensate pump 52 discharges water at about 120° F., which enters the heat exchanger 125 through pipe 127 at about the same temperature.
Now a review of the temperatures of the water flowing into and leaving the feedwater heater sections 103 and 106 is given.
Turning now to the feedwater heater section 103, the temperature of water exiting the heat exchanger high temperature path 140 enters pipe 143 at about 230° F. From there it enters the downstream face 112 of section 103 at about 230° F.
Thus the water temperature entering both downstream faces 112 and 114 of sections 103 and 106 is about 230° F.
The water entering section 103 exits at its upstream face 108 at the temperature of about 300° F. to pass through pipe 150 into LP Evaporator 77 at that temperature. Pipe 150 can also have a branches feeding off of it at 300° F. to the downstream face 157 of HP Economizer 155. Additionally, depending on the arrangement of coils of a particular HRSG, water feeding off the upstream face 108 of section 103 can also flow at 300° F. to the downstream face of other coils located upstream of preheater booster 74, such as to the downstream face of an intermediate pressure (IP) Economizer.
The temperature of the hot gas exiting the downstream face 100 of LP Evaporator 77 and entering at the upstream faces 108 and 110 of feedwater heater sections 103 and 106 is about 335° F. The temperature of the hot gas exiting the feedwater heater sections 103 and 106, at their respective downstream faces 112 and 114, is about 240° F.
Thus the surfaces of the tubes making up feedwater heater sections 103 and 106 are maintained to be about 240° F. or higher. This temperature is higher than the aforementioned dew point for condensation of sulphuric acid. Thus the condensation of sulfuric acid on the surfaces of the tubes making up the sections 103 and 106 will be resisted with the present design.
The gases leave the downstream preheater booster face 93 at a temperature of about 350° F., and enter the upstream face 96 of the LP Evaporator 77 at about that 350° F. temperature. The gases exit the LP Evaporator downstream face 100 at a temperature of about 335° F.
Feedwater from the condenser 51 can be discharged at approximately 120° F. through the supply pipe 127 into the low temperature path 130 of the heat exchanger 125.
The water leaving the heat exchanger 125 through the high temperature path exits at 230° F. and flows into section 103 at its downstream face 112 at a temperature of about 230° F.
With the present design the heat exchanger designated 125 does not require recirculation, and thus a recirculation pump and its attendant overhead and expense is not required for the heat exchanger. Further, with the present design there is no need to bypass any section of feedwater heater 80.
Also, with the present arrangement, the water temperature feeding into the LP Evaporator 77 from the feedwater preheater 80 enters at a temperature of 300° F. as compared to 250° F. with a temperature of water feeding into an LP Evaporator of a prior art system. Moreover, in the present system, water temperature of 300° F. feeding from the feedwater heater section 103 to the HP Economizer 155 or other economizer located upstream of the LP Evaporator, compares favorably to the water input temperature of 250° F. to HP Economizers and/or IP Economizers in a prior art design.
Now attention is directed to the modification of
Instead of the two feed water heater sections 103 and 106 described regarding
In
Pipe 232 delivers the feedwater to the downstream face 228 of feedwater heater section 216. The water leaves section 216 at its upstream face 226 to flow through a transfer pipe 246 to connect with the inlet of section 210 at its downstream face 220. The water flows through the coil of section 210 to thence leave its upstream face 218 to flow into a transfer pipe 252. From pipe 252, the water flows to preheater booster 74′ at its downstream face 93′. The water then passes through preheater heater booster 74′ to exit preheater booster stream face 90′ into a transfer pipe 255. Thence the water flows through pipe 255 to connect with the inlet of the high temperature path 258 of heat exchanger 125′.
Within the high temperature path 258 of heat exchanger 125′, the temperature of the water decreases since it loses heat to water in the low temperature path 231. At the outlet of the high temperature path 258, the water enters transfer pipe 261 to feed into feedwater heater section 213 at its downstream face 224. The water flows through section 213 to exit therefrom at its upstream face 222, whereby the temperature of the water is raised, to then pass into a discharge pipe 264. Pipe 264 extends to connect with LP Evaporator 77′ at its downstream face 100′, to be heated therein. From the LP Evaporator 77′, the water can flow from its upstream face 96′, to the HP Economizer, for example.
Now, as with the
Turning now to the most upstream of the feedwater heater sections, water leaves upstream face 218 of section 210, at a temperature of about 300° F. Then the water passes through pipe 252 to enter the downstream face 93′ of preheater booster 74′ at about 300° F. That water then passes through preheater booster 74′ to its upstream face 90′, to next exit through pipe 255 at about 340° F. The water then flows through pipe 255 into the high temperature path 258 of heat exchanger 125′ at a temperature of about 340° F.
Water from the condensate pump 52 discharges water at about 120° F. into the heat exchanger 125′ through pipe 227 at about that same temperature. Now a review of the temperatures of the water as it leaves the heat exchanger 125′ is given. The water from the low temperature path 231 of heat exchanger 125′ feeds into the pipe 232 at a temperature of about 230° F. From there, the water at about 230° F. enters the most downstream of the feedwater heater sections, section 216, at its downstream face 228. The water then passes through section 216 to enter its upstream face 226 into discharge pipe 246 at about 250° F. Through pipe 246 the water then enters feedwater section 210 at its downstream face 220 at about 250° F. The water then flows through section 210 and exits at its upstream face 218 through pipe 252 at a temperature of about 300° F.
The water exits heat exchanger 125′ through its high temperature path 258 to enter pipe 261 at a temperature of about 230° F. The water flows through pipe 261 to enter the downstream face 224 of feedwater heater section 213 at about 230° F. The water exits section 213 at its upstream face 222 at a temperature of about 285° F. to pass through pipe 264 into LP Evaporator 77′ at that temperature. Pipe 285 can also have a branch feeding off of it at 285° F. to the downstream face 157′ of HP Economizer 155′.
Further, depending upon the arrangement of coils of a particular HRSG, water feeding off the upstream face 222 of section 213 can also flow at 285° F. to the downstream face of other coils located upstream of preheater booster 74′, such as to the downstream face of an intermediate pressure (IP) economizer.
The temperature of the hot gas exiting the downstream face 100′ of LP Evaporator 77′ and entering at the upstream face 218 of feedwater heater section 210, is at about 335° F. The temperature of the hot gas exiting the feedwater heater section 210 at its downstream face 220 is about 295° F. The temperature of the hot gas exiting feedwater heater section 213 at its downstream face 224 is about 260° F. Finally, at the downstream face 228 of the farthest downstream feedwater section 216, the hot gas exits at about 240° F. Hence with the
As for the
Further, with the present arrangement, the water temperature feeding into the LP Evaporator 77′ from the feedwater preheater 80′ enters at a temperature of 285° F. as compared to 250° F. for the temperature of water feeding into an LP Evaporator of a prior art system. Moreover, with the
In
From the upstream face 110″ of the feed water heater 80″, the water passes through pipe 135″ to enter the downstream face 93″ of preheater booster 74″ at about 300° F. That fluid leaves the preheater booster upstream face 90″ through pipe 138″ at about 340° F. Through pipe 138″, the water then flows into the high temperature path 140″ of heat exchanger 125″ at about 140° F.
Other designs employing the inventive features can be embodied with feedwater heaters having more than three sections such as in
Further, the embodiments have been illustrated with the entry of the water into the various heat exchangers being preferably at the downstream faces of the sections. However, less preferably the water could enter father upstream in the heat exchanger. Likewise the water is shown preferably as exiting various heat exchangers at a point at the upstream face of the heat exchanger, while less preferably the water could enter farther downstream from the upstream face.
The preheater booster coils versions 80, 80′ and 80′ have been illustrated in
However, the preheater booster coils can also be located upstream of the HP Economizer and provide higher temperature water to the infeed of the water to water heat exchangers such as illustrated at 125, 125′ and 125″. In such a case, the differential of the temperature of the gas surrounding the preheater booster coils to the water temperature inside the preheater booster coils would be higher than for the systems specifically illustrated in
The connections of the various discussed pipes have been described as preferably at the downstream or upstream faces of the heat exchangers such as the feedwater heater sections, the preheater booster, the LP Evaporator and the HP Economizer. However less preferably the connections of the various pipes can be otherwise near the downstream face or upstream face of such components.
Changes can be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
This application is related to and claims priority to U.S. Provisional Application No. 61/882,991 filed on Sep. 26, 2013, with named inventor Daniel B. Kloeckener, and International Application No. PCT/US2014/057005 filed Sep. 23, 2014, and published under International Publication No. WO 2015/048029 for “Heat Exchanging System and Method for a Heat Recovery Steam Generator” both of which are incorporated by reference herein.
Filing Document | Filing Date | Country | Kind |
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PCT/US2014/057005 | 9/23/2014 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2015/048029 | 4/2/2015 | WO | A |
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Number | Date | Country | |
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20160245127 A1 | Aug 2016 | US |
Number | Date | Country | |
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61882911 | Sep 2013 | US |