Heat recovery steam generator with parallel tube bundles

Abstract

The present disclosure relates to a heat exchanger, in particular to a waste heat recovery device for generating steam. In particular, the heat exchanger is provided with two or more separate tube bundles, each tube bundle being arranged in a separate section of the housing. Additional separate bypass sections are arranged in the middle of the housing. A first heat exchange fluid, in particular a waste heat fluid, flows through the housing and is distributed between individual sections of the housing, and a second heat exchange fluid, in particular water, flows through each tube bundle and exchanges heat with the first heat exchange fluid. Alternatively, at least one tube bundle is flowed through by a different heat exchange fluid.

Description

[0001] manual Technical Field

[0002] The present disclosure relates to a waste heat recovery device, and in particular to a waste heat recovery device for generating steam, namely a heat recovery steam generator. Background Art

[0003] Waste heat occurs in almost all mechanical and thermal processes. Waste heat sources include, for example, hot combustion gases discharged into the atmosphere, hot water released into the environment, heating products present in industrial processes, and heat transferred from thermal equipment surfaces. Therefore, waste heat sources differ in the state of aggregation (mainly fluid and gaseous), temperature range and the frequency of their occurrence. The most significant amounts of waste heat are lost in industrial and energy generation processes.

[0004] Recovering waste heat can be done through various waste heat recovery technologies depending on the waste heat temperature to provide a valuable energy source and reduce overall energy consumption.

[0005] Typically, waste heat is transferred from a heat source to a waste heat recovery system via an exhaust fluid. The waste heat recovery system typically comprises a waste heat recovery device, 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 are kept separated by a solid wall, allowing heat to be transferred from one side of the wall to the other side while preventing the fluids from mixing. In practice, according to one possible embodiment, one fluid flows inside one or more tubes forming a tube bundle, while another fluid flows outside the tubes in a volume confined within the shell (such a heat exchanger is considered a shell and tube heat exchanger).

[0006] In particular, within the framework of the present disclosure, the surplus heat of a machine (such as a thermodynamic system), i.e. the heat which is ultimately discharged by the system through the flue gases together with a portion of the heat source not utilized by the system, is used to heat water to produce steam which can be used in a process (cogeneration) or for driving a steam turbine. A common application of a heat recovery steam generator (hereinafter also identified with the acronym HRSG) is in combined cycle power plants, in which the hot exhaust gases from a gas turbine are fed to the HRSG to produce steam which in turn drives a steam turbine.

[0007] However, the disadvantage of heat recovery steam generators is that they are difficult to adapt to changes in the working fluid flow rate, and start-up and maintenance take a long time. The adjustability of heat recovery steam generators is low. In the case of once-through steam generators, at low loads (usually less than 30% of the rated load), the steam generator becomes unstable.

[0008] These disadvantages are becoming more and more important as the market now demands production flexibility, which means an increase in temporary states, such as start and stop cycles and load changes. The oil and gas market in particular requires frequent load changes, which also increases the number of temporary states. Therefore, heat recovery systems are increasingly affected by heat sources with high start / stop frequencies.

[0009] Another problem is related to the maintenance of the heat recovery system. For example, scale may be deposited on the surface of the wall, keeping the heat exchange fluid separated, thereby reducing the heat transfer coefficient and thus the overall efficiency of the heat recovery system. In order to remove the scale or for any other maintenance needs, the heat exchanger is stopped. In order to speed up the maintenance operation, the wall separating the heat exchange fluid (for example, the tubes in the tube bundle) may be removable to allow easier access to the surface to be treated. However, the tube bundle of a large heat exchanger may be very difficult to handle due to its weight. Summary of the invention

[0010] According to the present disclosure, a heat exchanger, in particular a waste heat recovery device for producing steam, is proposed, provided with two or more separate tube bundles, each tube bundle being arranged in a separate section of a shell, wherein an additional separate section of the shell not containing any tube bundle is arranged in the middle of the shell. In particular, a first heat exchange fluid (usually a hot fluid) flows through the shell and is distributed between the separate sections of the shell. Each tube bundle is flowed through by a second heat exchange fluid, usually a fluid heated by exchanging heat with the first heat exchange fluid. Alternatively, at least one tube bundle is flowed through by a different heat exchange fluid.

[0011] Thus, 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 having a plurality of tube bundles arranged in parallel. In the framework of the present disclosure, the definition "shell sections arranged in parallel", "parallel shell sections" or "parallel sections" means that each section is separate from all other parallel sections, meaning that the same heat exchange fluid is distributed between each parallel section. Furthermore, in the framework of the present disclosure, the definition "tube bundles arranged in parallel", "parallel tube bundles" or "parallel bundles" means that each tube bundle arranged in parallel is separate from all other parallel tube bundles, the fluid flowing inside any parallel tube bundle is the same or different, and there is no connection between the outlet of a tube bundle and the inlet of any other.

[0012] In another aspect, the subject matter disclosed herein relates to a heat exchanger, wherein each tube bundle is arranged in a separate shell section accessible from an outer wall of the heat exchanger.

[0013] In one aspect, multiple parallel tube bundles allow limiting the weight of each tube bundle. At the same time, each parallel tube bundle is light and accessible from the outer wall of the heat exchanger, allowing removal or even replacement in the event of a malfunction.

[0014] In an additional aspect, an additional separate section of the shell arranged in the middle of the shell without any tube bundle is configured as a bypass duct surrounded by other separate sections, thereby limiting the thickness of heat removal and insulation. This arrangement allows a rounded shape, better external insulation and improved strength against external loads.

[0015] On the other hand, parallel tube bundles allow for continued operation even in the event of damage to one or some of the tube bundles, thereby increasing overall availability.

[0016] In yet another aspect, multiple tube bundles in parallel allow operation with different fluids or at different pressure levels. Thus, different fluid and / or pressure combinations are possible, including water, oil, and organic fluids. For example, a heat exchanger operating as a once-through steam generator with different pressure levels in different sections can be used to simultaneously obtain steam from one tube bundle and hot water or hot oil from another tube bundle.

[0017] In addition, the heat exchanger can be used with a smaller heat exchanger arranged upstream to absorb excess heat from the heat source, that is, during temporary conditions, allowing the waste heat recovery device to undergo less severe working cycles. The smaller heat exchanger can be configured as a working fluid preheater, that is, the working fluid of the waste heat recovery system can be used as a cooling fluid that exchanges heat with the hot fluid from the heat source, thereby avoiding acid condensation in the coldest section of the main heat exchanger of the waste heat recovery device. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A more complete understanding of the present invention and its many intended advantages will be readily obtained by referring to the following detailed description, as embodiments thereof become more readily understood when considered in conjunction with the accompanying drawings, in which:

[0019] Figure 1 shows a schematic diagram of a new and improved heat exchanger according to a first embodiment;

[0020] Figure 2 A schematic diagram showing the working fluid distribution of a novel heat exchanger according to a second embodiment, wherein a single working fluid is distributed to two parallel tube bundles;

[0021] Figure 3 shows a plan view of a novel heat exchanger according to a third embodiment, wherein the housing of the heat exchanger is divided into two separate parallel sections; and

[0022] Figure 4 Shown according to Figure 1 Schematic diagram of the control and regulation system of the novel improved heat exchanger of the embodiment. DETAILED DESCRIPTION

[0023] Now referring to the accompanying drawings, Figure 1 A heat exchanger is shown, which is particularly used as a once-through heat recovery steam generator, and is described according to an exemplary embodiment of the present invention.

[0024] In reference Figure 1 In a specific embodiment shown, the heat exchanger 10 comprises a housing 11, which comprises an inlet 12 for a first heat exchange fluid (particularly a heating fluid) and an outlet 13 for the first heat exchange fluid. An internal space is defined within the housing 11, which is divided into separate sections 15, 16, 19 by a wall 14. In particular, the internal space of the housing 11 is divided into a first heat exchange section 15, a second heat exchange section 16 and a bypass section 19.

[0025] 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, while 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, and 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. The first heat exchange section inlet 151, the second heat exchange section inlet 161 and the bypass section inlet 191 are each provided with a corresponding flow regulating device 20. In particular, Figure 1 The heat exchanger is used as a once-through heat recovery steam generator, and the first heat exchange fluid is a heating fluid, specifically a waste heat fluid, such as hot combustion gases from a gas turbine.

[0026] The first tube bundle 17 is arranged in the first heat exchange section 15, and the second tube bundle 18 is arranged in 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. A second heat exchange fluid, in particular a fluid to be heated, flows in the first tube bundle 17. The same or different fluid, in particular a second fluid to be heated, flows in the second tube bundle 18. In particular, Figure 1 The heat exchanger is used as a direct-through heat recovery steam generator. The fluid to be heated is water. Cold water is fed to the first tube bundle inlet 171 and the second tube bundle inlet 181, and is heated in the tube bundles 17 and 18. Steam is obtained from the first tube bundle outlet 172 and the second tube bundle outlet 182 and leaves the heat exchanger 10.

[0027] refer to Figure 2 , the same reference numerals are used for reference Figure 1The same components of the embodiment shown, the water fed to the first tube bundle inlet 171 and the second tube bundle inlet 181 have a common source from the water inlet 21, while the steam leaving the heat exchanger 10 from the first tube bundle outlet 172 and the second tube bundle outlet 182 is directed to a common device through the steam outlet 22. The distribution of water flowing to the first tube bundle inlet 171 and the second tube bundle inlet 181 is regulated by a water distribution circuit 211, which connects the water inlet 21 with 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, which connects the first tube bundle outlet 172 and the second tube bundle outlet 182 with the steam outlet 22 and includes a plurality of valves 222.

[0028] refer to Figure 3 , which shows a plan view of a heat exchanger 10 according to a third embodiment, better showing a wall 14, which divides the internal space within the housing 11 of the heat exchanger into a first heat exchange section 15, a second heat exchange section 16 and a bypass section 19 which are separated from each other. In particular, the bypass section 19 is surrounded by the first heat exchange section 15 and the second heat exchange section 16, limiting heat removal, and therefore the thickness of the thermal insulation is low.

[0029] According to another aspect of the present disclosure, a heat absorber (not shown) is arranged upstream of the heat exchanger 10 to absorb excess heat from the heat source. A smaller heat exchanger can be configured as a preheater for the fluid to be heated, i.e. the fluid to be heated in the heat exchanger can be used as a cooling fluid that exchanges heat with the hot fluid from the heat source. Such a heat absorber as a smaller heat exchanger can be configured as a removable part within the housing 11 of the heat exchanger 10, upstream of the flow regulating device 20 that distributes the heating fluid between the individual sections 15, 16, 19.

[0030] It should be noted that even if the hot exhaust gas flow is entirely directed to the bypass section 19, the position of the heat absorber located upstream of the flow regulating device 20 allows the heat absorber to reduce the temperature of the hot exhaust gas flow, so that even if the flow downstream of the bypass section 19 is at least partially redirected to the heat exchange sections 15 and 16, the temperature of the hot exhaust gas flow will not be high enough to cause thermal shock to the tube bundles 17 and 18.

[0031] refer to Figure 4 , which shows that according to Figure 1Schematic diagram of a control and regulation system of a heat exchanger 10 of an embodiment of the invention. In particular, a control system 30 simultaneously controls the flow of a 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 by a pressure difference indicator 31 according to the pressure difference in each heating fluid section 15, 16. In the event of an imbalance being detected, the flow in the individual sections 15, 16, 19 is regulated by a flow regulation device 20, which can be operated by a corresponding actuator 32.

[0032] As far as the regulation and control of the fluid to be heated is concerned, the main parameter to be controlled is the fluid flow rate. The total fluid flow inlet is measured by a flow transmitter 33, while the fluid flow rate in each tube bundle 17, 18 is measured by a respective flow transmitter 34. The temperature of these fluids is also controlled at the outlet of the respective tube bundle 17, 18 in order to fine-tune the fluid flow rate in the tube bundles 17, 18. In particular, the temperature at the outlet of the respective tube bundle 17, 18 is controlled by a temperature indicator 32 and is related to the fluid flow inlet measured by the flow transmitter 33 and to the fluid flow rate in each tube bundle 17, 18 measured by the respective flow transmitter 34. In the event of a detected imbalance, the flow rates in the tube bundles 17, 18 are regulated independently by valves 35, which can be operated by respective actuators 36.

[0033] The following table shows two different control principles according to the disclosed embodiment of the present invention. In particular, according to the first principle represented by the data shown in Table 1, where the flow percentage in each tube bundle is represented with reference to the total nominal flow of the fluid to be heated by the heat exchanger, when a decrease in the total flow of the fluid to be heated is detected, the fluid in one of the tube bundles remains unchanged, while the fluid in the other tube bundle is reduced.

[0034] [Table on next page]

[0035] Table 1

[0036] First tube bundle flow % Second tube bundle flow % Total traffic % 50 50 100 30 50 80 20 50 70 15 45 60 - 50 50 - 40 40 - 30 30 - 15 15

[0037] As shown in Table 1, referring to a once-through steam generator, if the reduction in the total flow of the fluid to be heated is low, the flow in one of the tube bundles can remain unchanged, while the flow in the other tube bundle is reduced. When the total flow is further reduced, the flow in one of the fluid bundles needs to be completely closed to avoid that the steam generator becomes unstable. Instabilities usually occur when the flow inside the tube bundle is lower than 30% of the nominal load, which corresponds to 15% of the total nominal flow for a heat exchanger comprising two parallel tube bundles of the same size. Therefore, Table 1 shows how the overall adjustability can be improved by arranging two parallel tube bundles in the heat exchanger.

[0038] Furthermore, according to the second control and regulation 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 fluids in the two tube bundles can be reduced accordingly.

[0039] [Table on next page]

[0040] Table 2

[0041] First tube bundle flow % Second tube bundle flow % Total traffic % 50 50 100 40 40 80 35 35 70 30 30 60 25 25 50 20 20 40 15 15 30 - 15 15

[0042] As shown in Table 2, still referring to the once-through steam generator, if the reduction in the total flow of the fluid to be heated is low, the flow in the tube bundles can be reduced accordingly in the same way in both tube bundles. However, also in this case, when the total flow is further reduced, the flow in one of the fluid bundles needs to be completely shut down to avoid that the steam generator becomes unstable. However, Table 2 also shows how the overall adjustability can be improved by arranging two parallel tube bundles in the heat exchanger.

[0043] While aspects of the present invention have been described in terms of various specific embodiments, it will be apparent to those skilled in the art that various modifications, variations, and omissions are possible without departing from the spirit and scope of the claims.

[0044] Barzanò and Zanardo Roma SpA.

Claims

1. A heat exchanger (10), comprising: a housing (11) defining an interior space, an inlet (12) for a first heat exchange fluid and an outlet (13) for said first heat exchange fluid, The internal space includes: dividing the internal space into separate sections (15, 16) of at least one wall (14), The separate sections include at least one first heat exchange section (15) and at least one second heat exchange section (16), 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, and 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, wherein at least one first tube bundle (17) is arranged in the first heat exchange section (15) and at least one second tube bundle (18) is arranged in 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 first tube bundle inlet (171) is connected to a first feed line of a second heat exchange fluid, and the second tube bundle inlet (181) is connected to a second feed line of the second heat exchange fluid or other heat exchange fluids, at least one additional separate section (19), said at least one additional separate section being arranged in said inner space between said first heat exchange section (15) and said second heat exchange section (16), The additional individual section (19) has an additional individual section inlet (191) for the first heat exchange fluid and an additional individual section outlet (192) for the first heat exchange fluid, The additional separate section (19) is a first heat exchange fluid bypass section (19), The invention is characterized in that each heat exchange section (15, 16, 19) is provided with a corresponding flow regulating device (20).

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

3. The heat exchanger (10) according to claim 1 or 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 or 4, wherein the second heat exchange fluid and any other 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 of the first feed line is different from the fluid to be heated of the second feed line, and the fluid to be heated is water, oil or an organic fluid.

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

9. A heat exchanger (10) according to any one of the preceding claims, wherein each heat exchange section (15, 16, 19) can be isolated from the others and each tube bundle (17, 18) can be isolated from the others.

10. The heat exchanger (10) according to any one of the preceding claims, wherein at least the first tube bundle (17) and / or the second tube bundle (18) consists of two or more sections arranged in series.

11. The heat exchanger (10) according to any one of the preceding claims, 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 countercurrently 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 any of the preceding claims, wherein an additional heat exchanger is arranged upstream of the individual sections (15, 16, 19), the additional heat exchanger being smaller than the heat exchanger (10).

13. Heat exchanger (10) according to any of the preceding claims, wherein a control system (30) is present, wherein the control system comprises a differential pressure indicator (31) of the flow rate of each individual section (15, 16) of the respective tube bundle (17, 18).

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