Heat recovery steam generator having parallel tube bundles

Abstract

The present disclosure relates to a heat exchanger, and more particularly to a waste heat recovery unit used to generate steam. In particular, the heat exchanger is provided with two or more separate tube bundles, each tube bundle being arranged in a separate part of the shell. An additional separate bypass section is arranged in the center of the shell. A first heat exchange fluid, particularly a waste heat fluid, flows through the shell and is distributed between separate sections of the shell, and a second heat exchange fluid, particularly water, flows through each tube bundle and exchanges heat with the first heat exchange fluid. Alternatively, different heat exchange fluids are flowed through at least one tube bundle.

Description

Technical Field

[0001] The present disclosure relates to waste heat recovery units, and more particularly to waste heat recovery units used to generate steam, i.e., heat recovery steam generators.

Background Art

[0002] Waste heat is generated in almost all mechanical and thermal processes. Examples of waste heat sources include high-temperature combustion gases discharged into the atmosphere, warm water released into the environment, heated products from industrial processes, and heat transferred from the surfaces of high-temperature equipment. Therefore, waste heat sources vary with respect to their aggregate state (mainly fluids and gases), temperature range, and their frequency of occurrence. Most of the waste heat is lost in industrial and energy generation processes.

[0003] Recovery of waste heat can be carried out through various waste heat recovery techniques depending on the waste heat temperature to provide a useful energy source and reduce overall energy consumption.

[0004] Typically, waste heat is transferred from a heat source to a waste heat recovery system through an exhaust fluid. A waste heat recovery system typically includes a waste heat recovery unit, i.e., a heat exchanger configured to transfer the residual enthalpy of the exhaust fluid of the heat source to the working fluid of the waste heat recovery system. The heat exchange fluids remain separated by a solid wall, which allows heat transfer from one side of the wall to the other while preventing mixing of the fluids. In practice, according to one possible embodiment, one fluid flows inside one or more tubes forming a tube bundle, and the other fluid flows outside the tubes within a space confined inside a shell (this type of heat exchanger is identified as a shell-and-tube heat exchanger).

[0005] In particular, within the framework of the present disclosure, the residual heat of a machine such as a thermodynamic system, i.e., the heat that is ultimately released by the system through the flue gas together with a part of the heat source not utilized by the system, is used to heat water to generate steam, and that steam can be used in a process (cogeneration) or to drive a steam turbine. A common use of a heat recovery steam generator (also identified hereinafter by the acronym HRSG in this specification) is in a combined cycle power generation plant where the high-temperature exhaust from a gas turbine is supplied to the HRSG to generate steam, and that steam then drives a steam turbine.

[0006] However, in addition to the fact that it is difficult for a heat recovery steam generator to adapt to changes in the flow rate of the working fluid, it has the drawback of requiring a long time for startup and maintenance. The rangeability of a heat recovery steam generator is low. In the case of a once-through steam generator, at low loads (typically less than 30% of the nominal load), the steam generator becomes unstable.

[0007] Currently, the market demands production flexibility, which means an increase in transient conditions such as start-up and stop cycles and load fluctuations, so these drawbacks are becoming increasingly significant. The oil and gas markets in particular require frequent load changes and also increase the number of transient conditions. As a result, heat recovery systems are increasingly exposed to heat sources with a high start-up / stop frequency.

[0008] A further problem relates to the maintenance of the heat recovery system. For example, scale adheres to the surface of the wall, and the heat exchange fluid becomes separated from the wall, which may reduce the heat transfer coefficient and, as a result, reduce the overall efficiency of the heat recovery system. To remove the scale or for any other maintenance required, the heat exchanger is shut down. To speed up the maintenance work, the heat exchange fluid separation walls, such as the tubes of the tube bundle, may be removable to allow easier access to the surface to be treated. However, the tube bundles of large heat exchangers can be very difficult to handle due to their weight. SUMMARY OF THE INVENTION

[0009] According to the present invention, it is proposed that a heat exchanger, particularly a waste heat recovery unit used to generate steam, is provided with two or more separate tube bundles, each tube bundle being arranged within a separate section of the shell, and an additional separate section of the shell without a tube bundle is arranged in the center of the shell. In particular, a first heat exchange fluid, which is generally a high-temperature fluid, flows through the shell and is distributed between separate sections of the shell. Each tube bundle has a second heat exchange fluid flowing therethrough, which is generally a fluid heated by exchanging heat with the first heat exchange fluid. Alternatively, different heat exchange fluids are flowed through at least one of the tube bundles.

[0010] Accordingly, in one aspect, the subject matter disclosed herein relates to a heat exchanger having a shell divided into a plurality of separate shell sections arranged in parallel and a plurality of tube bundles arranged in parallel. In the framework of the present disclosure, the definition of "shell sections arranged in parallel", "parallel shell sections", or "parallel sections" means that each section is separated from all other parallel sections in such a way that the same heat exchange fluid is distributed between each parallel section. Further, in the framework of the present disclosure, the definition of "tube bundles arranged in parallel", "parallel tube bundles", or "parallel bundles" means that each tube bundle arranged in parallel is separated from all other parallel tube bundles, the fluid flowing inside any parallel tube bundle can be the same or different, and there is no connection between the outlet of a tube bundle and the inlet of any other tube bundle.

[0011] In another aspect, the subject matter disclosed herein is directed to a heat exchanger in which each tube bundle is disposed within a separate shell section that is accessible from the outer wall of the heat exchanger.

[0012] In one aspect, the plurality of parallel tube bundles makes it possible to limit the weight of each tube bundle. At the same time, each parallel tube bundle is light in weight and accessible from the outer wall of the heat exchanger, so that it can be removed or even replaced in case of failure.

[0013] In one additional aspect, an additional separate section of the shell located at the center of the shell and having no tube bundles is configured as a bypass duct surrounded by other separate sections, limiting the heat insulation and thickness of the heat insulating material. This configuration enables a circular shape, better external insulation, and improved strength against external loads.

[0014] In another aspect, the parallel tube bundles enable continuous operation and increase overall availability even when one or more tube bundles are damaged.

[0015] In yet another aspect, the plurality of parallel tube bundles are enabled to operate with different fluids or at different pressure levels. Thus, combinations of different fluids and / or pressures are possible, including water, oil, and organic fluids. For example, using a heat exchanger operating as a once-through steam generator having different pressure levels in different sections, steam can be obtained from one tube bundle and hot water or hot oil can be obtained from different tube bundles simultaneously.

[0016] In addition, the heat exchanger can be used with a smaller heat exchanger disposed upstream to absorb excess heat coming from the heat source, i.e., during transient conditions, enabling the waste heat recovery unit to withstand less severe operating cycles. The smaller heat exchanger can be configured as a working fluid preheater, i.e., the working fluid of the waste heat recovery system can be used as a cooling fluid that exchanges heat with the high-temperature fluid from the heat source, thus avoiding the condensation of acid in the coldest section of the main heat exchanger of the waste heat recovery unit.

Brief Description of the Drawings

[0017] A more complete understanding of the embodiments of the present invention and many of the advantages expected therefrom will be readily obtained by referring to the following detailed description of the invention and considering it in association with the accompanying drawings, in which they will be better understood.

Figure 1

Figure 2

Figure 3

Figure 4

Detailed Description of the Invention

[0018] Referring now to the drawings, FIG. 1 shows a heat exchanger, particularly a cross-flow type heat recovery steam generator, and illustrates a heat exchanger according to an exemplary embodiment of the present invention.

[0019] In one specific embodiment shown with reference to FIG. 1, the heat exchanger 10 includes a shell 11, and the shell 11 includes an inlet 12 for a first heat exchange fluid, particularly a heating fluid, and an outlet 13 for the first heat exchange fluid. Inside the shell 11, an internal space is defined which is divided by a wall 14 into separate sections 15, 16, 19. Specifically, the internal space of the shell 11 is divided into a first heat exchange section 15, a second heat exchange section 16, and a bypass section 19.

[0020] The first heat exchange section 15 has a first heat exchange section inlet 151 for the first heat exchange fluid and a first heat exchange section outlet 152 for the first heat exchange fluid. The second heat exchange section 16 has a second heat exchange section inlet 161 for the first heat exchange fluid and a second heat exchange section outlet 162 for the first heat exchange fluid. The bypass section 19 has a bypass section inlet 191 for the first heat exchange fluid and a bypass section outlet 192 for the first heat exchange fluid. Flow regulating means 20 are provided at the first heat exchange section inlet 151, the second heat exchange section inlet 161, and the bypass section inlet 191, respectively. In particular, the heat exchanger of FIG. 1 operates as a cross-flow type heat recovery steam generator, and the first heat exchange fluid is a heating fluid, particularly a waste heat fluid, for example, high-temperature combustion gas from a gas turbine.

[0021] The first tube bundle 17 is disposed inside the first heat exchange section 15, and the second tube bundle 18 is disposed inside the second heat exchange section 16. The first tube bundle 17 has a first tube bundle inlet 171 and a first tube bundle outlet 172, and the second tube bundle 18 has a second tube bundle inlet 181 and a second tube bundle outlet 182. The second heat exchange fluid, particularly the fluid to be heated, flows inside the first tube bundle 17. The same or different fluid, particularly the second fluid to be heated, flows inside the second tube bundle 18. In particular, in the heat exchanger of FIG. 1 operating as a once-through type heat recovery steam generator, the fluid to be heated is water, and cold water is supplied to both the first tube bundle inlet 171 and the second tube bundle inlet 181, and is heated inside the tube bundles 17 and 18 to obtain steam, and the steam exits the heat exchanger 10 from both the first tube bundle outlet 172 and the second tube bundle outlet 182.

[0022] Referring to FIG. 2, the same reference numerals are used for the same components of the embodiment shown with reference to FIG. 1. The water supplied to the first tube bundle inlet 171 and the second tube bundle inlet 181 has a common origin, that is, from the water inlet 21. The steam exiting the heat exchanger 10 from the first tube bundle outlet 172 and the second tube bundle outlet 182 is directed to a common appliance through the steam outlet 22. The distribution of water to the first tube bundle inlet 171 and the second tube bundle inlet 181 is adjusted by a water distribution circuit 211. The water distribution circuit 211 connects the water inlet 21 to the first tube bundle inlet 171 and the second tube bundle inlet 181 and includes a plurality of valves 212. The collection of steam from the first tube bundle outlet 172 and the second tube bundle outlet 182 is operated by a steam collection circuit 221. The steam collection circuit 221 connects the first tube bundle outlet 172 and the second tube bundle outlet 182 to the steam outlet 22 and includes a plurality of valves 222.

[0023] Referring to FIG. 3 showing the plant diagram of the heat exchanger 10 according to Embodiment 3, it is better shown that the wall 14 divides the internal space inside the shell 11 of the heat exchanger into a first heat exchange section 15, a second heat exchange section 16, and a bypass section 19 that are separated from each other. In particular, since the bypass section 19 is surrounded by the first heat exchange section 15 and the second heat exchange section 16, heat dissipation is restricted, and as a result, the thickness of the heat insulation material becomes thinner.

[0024] According to another aspect of the present disclosure, in order to absorb excess heat coming from a heat source, a heat damper (not shown) is arranged upstream of the heat exchanger 10. A smaller heat exchanger can be configured as a preheater for the fluid to be heated, that is, the fluid heated in the heat exchanger can be used as a cooling fluid that exchanges heat with the high-temperature fluid from the heat source. Such a heat damper, which is a smaller heat exchanger, can be configured as a removable part inside the shell 11 of the heat exchanger 10 upstream of the flow regulating means 20 that distributes the heating fluid between the separate sections 15, 16, 19.

[0025] It should be noted that by arranging the heat damper upstream of the flow regulating means 20, even when the high-temperature exhaust gas flow is entirely directed to the bypass section 19, the heat damper can lower the temperature of the high-temperature exhaust gas flow, and as a result, even when the flow downstream of the bypass section 19 is at least partially redirected towards the heat exchange sections 15, 16, its temperature will not become so high as to cause a thermal shock to the tube bundles 17, 18.

[0026] Referring to FIG. 4, a schematic view of the control and adjustment system of the heat exchanger 10 according to the embodiment of FIG. 1 is shown. In particular, the control system 30 simultaneously controls the flow of the heating fluid in the heating fluid sections 15, 16 and the flow of one or more fluids to be heated in the tube bundles 17, 18. In particular, the flow of the heating fluid in the heating fluid sections 15, 16 is controlled as a function of the differential pressure in each heating fluid section 15, 16 by a differential pressure indicator 31. When an imbalance is detected, the flows in the separate sections 15, 16, 19 are adjusted via flow adjustment means 20 which can be actuated by respective actuators 32.

[0027] As far as the adjustment and control of the fluid to be heated are concerned, the main parameter to be controlled is the fluid flow. All the inflows of the fluid are measured by a flow transmitter 33, and the flow of the fluid in each of the tube bundles 17, 18 is measured by respective flow transmitters 34. Further, the temperature of these fluids is controlled at the outlets from the respective tube bundles 17, 18 to effect fine adjustment of the flow of the fluid inside the tube bundles 17, 18. In particular, the temperature at the outlet of each of the tube bundles 17, 18 is controlled by a temperature indicator 32 and is correlated with the inflow of the fluid measured by the flow transmitter 33 and the flow rate of the fluid in each of the tube bundles 17, 18 measured by respective flow transmitters 34. When an imbalance is detected, the flow rates in the tube bundles 17, 18 are independently adjusted via valves 35 which can be operated by respective actuators 36.

[0028] The following table shows two different control principles according to the disclosed embodiments of the present disclosure. In particular, according to the first principle represented by the data shown in Table 1, the flow percentage in each tube bundle is expressed with reference to the total nominal flow rate of the fluid heated by the heat exchanger, and when a decrease in the total flow rate of the fluid to be heated is detected, the fluid inside one of the tube bundles is maintained unchanged while the fluid inside the other tube bundle is decreased.

[0029]

Table 1

[0030] As shown in Table 1, referring to the once-through steam generator, when the decrease in the total flow rate of the fluid to be heated is small, the flow rate inside one of the tube bundles can be maintained constant, while the flow rate inside the other tube bundle can be decreased. When the total flow rate further decreases, in order to avoid the steam generator becoming unstable, it is necessary to completely close the flow inside one of the fluid bundles. The instability generally occurs when the flow in the tube bundle is less than 30% of the nominal load, corresponding to 15% of the total nominal flow rate of the heat exchanger with two parallel tube bundles of the same size. Therefore, Table 1 shows how the overall rangeability can be increased by arranging two parallel tube bundles in the heat exchanger.

[0031] Furthermore, according to the second control and adjustment principle represented by the data shown in Table 2, when a decrease in the total flow rate of the fluid to be heated is detected, the amount of fluid in both tube bundles can be decreased accordingly.

[0032]

Table 2

[0033] As shown in Table 2, regarding the once-through steam generator, when the decrease in the total flow rate of the fluid to be heated is small, the flow rate in the tube bundle can be decreased correspondingly in the same way in both tube bundles. However, also in this case, when the total flow rate further decreases, in order to avoid the steam generator becoming unstable, it is necessary to completely close the flow inside one of the fluid bundles. Nevertheless, Table 2 shows how the overall rangeability can be increased by arranging two parallel tube bundles in the heat exchanger.

[0034] Although aspects of the present invention have been described with respect to various specific embodiments, it will be apparent to those skilled in the art that many modifications, variations, and omissions are possible without departing from the spirit and scope of the claims. Barzano & Zanardo Roma S.p.A.

Claims

1. A heat exchanger (10) comprising: a shell (11) defining an internal space, an inlet (12) for a first heat exchange fluid, and an outlet (13) for the first heat exchange fluid; the internal space comprising at least one wall (14) dividing the internal space into separate sections (15, 16); the separate sections including at least one first heat exchange section (15) and at least one second heat exchange section (16); the first heat exchange section (15) having a first heat exchange section inlet (151) for the first heat exchange fluid and a first heat exchange section outlet (152) for the first heat exchange fluid; the second heat exchange section (16) having a second heat exchange section inlet (161) for the first heat exchange fluid and a second heat exchange section outlet (162) for the first heat exchange fluid; at least one first tube bundle (17) disposed inside the first heat exchange section (15) and at least one second tube bundle (18) disposed inside the second heat exchange section (16); the first tube bundle (17) having a first tube bundle inlet (171) and a first tube bundle outlet (172), and the second tube bundle (18) having a second tube bundle inlet (181) and a second tube bundle outlet (182); the first tube bundle inlet (171) being connected to a first supply line of a second heat exchange fluid, and the second tube bundle inlet (181) being connected to a second supply line of the second or further heat exchange fluid; at least one additional separate section (19) disposed within the internal space between the first heat exchange section (15) and the second heat exchange section (16); the additional separate section (19) having an additional separate section inlet (191) for the first heat exchange fluid and an additional separate section outlet (192) for the first heat exchange fluid; the additional separate section (19) being a first heat exchange fluid bypass section (19); A heat exchanger, characterized in that each heat exchange section (15, 16, 19) is provided with respective flow regulating means (20).

2. The heat exchanger (10) according to claim 1, wherein the flow regulating means (20) of each heat exchange section (15, 16, 19) is independent of the flow regulating means (20) of the other heat exchange sections (15, 16, 19).

3. The heat exchanger (10) according to claim 1 or claim 2, wherein the first heat exchange fluid is a heating fluid.

4. The heat exchanger (10) according to claim 3, wherein the heating fluid is exhaust gas.

5. The heat exchanger (10) according to claim 3, wherein the second heat exchange fluid and any additional heat exchange fluids are fluids to be heated.

6. The heat exchanger (10) according to claim 5, wherein the fluid to be heated is water, oil, or an organic fluid.

7. The heat exchanger (10) according to claim 6, wherein the fluid to be heated in the first supply line is different from the fluid to be heated in the second supply line and is water, oil, or an organic fluid.

8. The heat exchanger (10) according to claim 5, wherein the fluid to be heated in the first supply line and the fluid to be heated in the second supply line are the same fluid, and the first supply line and the second supply line are arranged in parallel.

9. The heat exchanger (10) according to claim 1, wherein each heat exchange section (15, 16, 19) can be separated by other heat exchange sections, and each tube bundle (17, 18) can be separated by other tube bundles.

10. The heat exchanger (10) according to claim 1, wherein at least the first tube bundle (17) and / or the second tube bundle (18) is composed of two or more sections arranged in series.

11. The heat exchanger (10) according to claim 1, wherein the first tube bundle inlet (171) and the first tube bundle outlet (172) and / or the second tube bundle inlet (181) and the second tube bundle outlet (182) are arranged in counterflow with respect to the inlet (12) for the first heat exchange fluid and the outlet (13) for the first heat exchange fluid.

12. The heat exchanger (10) according to claim 1, wherein an additional heat exchanger is arranged upstream of the separate sections (15, 16, 19), and the additional heat exchanger is smaller than the heat exchanger (10).

13. There is a control system (30), and the control system includes differential pressure indicators (31) for the flows of respective separate sections (15, 16) each provided with a respective tube bundle (17, 18), the heat exchanger (10) according to claim 1.

Images