Description

TECHNICAL FIELD

[1] The present disclosure concerns a waste heat recovery unit and, in particular, a waste heat recovery unit used to generate steam, i.e. a heat recovery steam generator.

BACKGROUND ART

[2] Waste heat occurs in almost all mechanical and thermal processes. Sources of waste heat include for example hot combustion gases discharged to the atmosphere, heated water released into environment, heated products existing industrial processes, and heat transferred from hot equipment surfaces. As such, waste heat sources differ regarding the aggregate state (mainly fluid and gaseous), temperature range, and frequency of their occurrence. The most significant amounts of waste heat are being lost in the industrial and energy generation processes.

[3] Recovering the waste heat can be conducted through various waste heat recovery technologies, depending on the waste heat temperature, to provide valuable energy sources and reduce the overall energy consumption.

[4] Typically, waste heat is transferred from a heat source to a waste heat recovery system through an exhaust fluid. Waste heat recovery systems typically include 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 a working fluid of the waste heat recovery system. The heat exchanging fluids are kept separate by solid walls, allowing for heat transfer from one side to the other of the wall, at the same time preventing mixing of the fluids. In practice, according to one possible embodiment, one fluid flows inside one or more tubes, forming a tube bundle, while the other fluid flows outside the tubes, in a volume confined inside a shell (this kind of heat exchangers being identified as shell and tube heat exchangers). [5] In particular, within the frame of the present disclosure, the remaining heat of a machine, such as a thermodynamic system, i.e. the heat discharged by the system through flue gases eventually along with a portion of the heat source not exploited by the system, is used to heat water to generate steam, which can be used in a process (cogeneration) or used to drive a steam turbine. A common application for a heat recovery steam generator (also identified herein below with the acronym HRSG) is in a combined-cycle power plant, where hot exhaust from a gas turbine is fed to the HRSG to generate steam that in turn drives a steam turbine.

[6] However, heat recovery steam generators have drawbacks due to the difficulty to adapt to changes in the operating fluids flow rate, together with long time needed for start-ups and maintenance. Range- ability of heat recovery steam generators is low. In the case of once- through steam generators, at low loads (typically lower than 30% of the nominal load) the steam generator becomes unstable.

[7] These drawbacks are getting more and more important, because, at present, the market requires production flexibility, which implies an increase of transitory states, such as starts and stops cycles and load variations. The oil and gas market in particular requires frequent load variations, also increasing the number of transitory states. As a consequence, heat recovery systems are more and more subject to heat sources with a high start/stop frequency.

[8] An additional problem is connected with the maintenance of heat recovery systems. For example, scale can deposit on the surface of the walls keeping heat exchange fluids separate, reducing heat transfer coefficient and, as a consequence, the overall efficiency of the heat recovery system. In order to remove scale, or for any other maintenance need, the heat exchanger is halted. In order to speed up maintenance operations, the heat exchange fluid separating walls, e.g. the tubes of the tube bundle, may be removable, to allow easier access to the surfaces to be treated. However, tube bundles of big heat exchangers can be very difficult to handle, because of their weight. SUMMARY

[9] According to the present disclosure, it is proposed that heat exchangers, and in particular waste heat recovery units used to generate steam, are 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, devoid of any tube bundle, is arranged in the middle of the shell. In particular, a first heat exchanging fluid, generally a hot fluid, flows through the shell and is distributed between the separate sections of the shell. Each tube bundle is flown by a second heat exchanging fluid, generally a fluid to be heated by exchanging heat with the first heat exchanging fluid. Alternatively, at least one tube bundle is flown by a different heat exchanging fluid.

[10] Thus, in one aspect, the subject matter disclosed herein is directed to a heat exchanger with a shell divided into a plurality of separate shell sections arranged in parallel and with a plurality of tube bundles arranged in parallel. In the frame of the present disclosure, the definitions “shell sections arranged in parallel”, “parallel shell sections” or “parallel sections” mean that each section is separate from all the others parallel sections, in the sense that a same heat exchanging fluid is distributed between each parallel section. Moreover, in the frame of the present disclosure, the definitions “tube bundles arranged in parallel”, “parallel tube bundles” or “parallel bundles” mean that each tube bundle that is arranged in parallel is separate from all the others parallel tube bundles, the fluid flowing inside any parallel tube bundle being the same or different, no connection being present between the outlet of a tube bundle and the inlet of any other.

[11] In another aspect, the subject matter disclosed herein is directed to a heat exchanger wherein each tube bundle is arranged in a separate shell section that is accessible from the external walls of the heat exchanger.

[12] In one aspect, a plurality of parallel tube bundles allows to limit the weight of each tube bundle. At the same time, each parallel tube bundle being less heavy and accessible from the external walls of the heat exchanger allows removal or even replacement in case of failure. [13] In one additional aspect, the additional separate section of the shell, devoid of any tube bundle, arranged in the middle of the shell, is configured as a by-pass duct that, being surrounded by the other separate sections, limits the heat rejection and thickness of the insulation. This arrangement allows for a circular shape, better external insulation, and improved strength against external loads.

[14] In another aspect, the parallel tube bundles allow continuous operation even in case of damage of one or some tube bundles, increasing the overall availability.

[15] In yet another aspect, the plurality of tube bundles in parallel allows working with different fluids or at different pressure levels. Accordingly, different fluid and/or pressure combinations are also possible, including water, oil and organic fluids. For example, a heat exchanger working as a one-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 a different tube bundle.

[16] Additionally, the heat exchanger can be used together with a smaller heat exchanger, arranged upstream to absorb excessive heat coming from the heat source, namely during a transitory state, allowing 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 exchanging heat with the hot fluid from a heat source, thus avoiding the acid condensation in the coldest section of the main heat exchanger of the waste heat recovery unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[17] A more complete appreciation of the embodiments of the invention and many of the expected advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Figure 1 illustrates a schematic of a new, improved heat exchanger according to a first embodiment; Figure 2 illustrates a schema of a working fluid distribution of a new heat exchanger according to a second embodiment, wherein a single working fluid is distributed into two parallel tube bundles;

Figure 3 illustrates a plant view of a new heat exchanger according to a third embodiment, wherein the shell of the heat exchanger is divided into two separate parallel sections; and

Figure 4 illustrates a schematic of a control and regulation system of a new, improved heat exchanger according to the embodiment of figure 1.

DETAILED DESCRIPTION OF EMBODIMENTS

[18] Referring now to the drawings, Figure 1 shows a heat exchanger, which operates in particular as once-through heat recovery steam generator, and which is illustrated in accordance with an exemplary embodiment of the invention.

[19] In one specific embodiment, shown with reference to Figure 1 , a heat exchanger 10 comprises a shell 1 1 , comprising an inlet 12 for a first heat exchange fluid, in particular a heating fluid, and an outlet 13 for the first heat exchange fluid. Within the shell 1 1 is defined an internal room that is divided by a wall 14 into separate sections 15, 16, 19. In particular, the internal room of the shell 1 1 is divided into a first heat exchange section 15, a second heat exchange section 16, and a bypass section 19.

[20] 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 by-pass section 19 has a by-pass section inlet 191 for the first heat exchange fluid and a by-pass 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 by-pass section inlet 191 are each provided with respective flow regulating means 20. In particular, the heat exchanger of figure 1 operating as a once-through heat recovery steam generator, the first heat exchange fluid being a heating fluid and in particular a waste heat fluid, for example hot combustion gases from a gas turbine.

[21] A first tube bundle 17 is arranged inside the first heat exchange section 15 and a second tube bundle 18 is arranged 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. A second heat exchange fluid, in particular a fluid to be heated, is flown inside the first tube bundle 17. The same or a different fluid, in particular a second fluid to be heated is flown inside the second tube bundle 18. In particular, the heat exchanger of figure 1 operating as a once-through heat recovery steam generator, the fluid to be heated is water, cold water being fed both to the first tube bundle inlet 171 and to the second tube bundle inlet 181 , heating up inside the tube bundles 17 and 18, steam being obtained and exiting the heat exchanger 10 from both the first tube bundle outlet 172 and the second tube bundle outlet 182.

[22] Making reference to Figure 2, the same reference numbers being used for the same components of the embodiment shown with reference to Figure 1 , the water fed to the first tube bundle inlet 171 and to the second tube bundle inlet 181 has a common origin, from a water inlet 21 , while 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 a steam outlet 22. Distribution of water to the first tube bundle inlet 171 and to the second tube bundle inlet 181 is regulated by a water distribution circuit 211 , connecting the water inlet 21 with the first tube bundle inlet 171 and with the second tube bundle inlet 181 and including a plurality of valves 212. 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 , connecting the first tube bundle outlet 172 and the second tube bundle outlet 182 with the steam outlet 22 and including a plurality of valves 222.

[23] Referring to Figure 3, illustrating a plant view of a heat exchanger 10 according to a third embodiment, the wall 14 is better shown, dividing the internal room inside the shell 11 of the heat exchanger into a first thermal exchange section 15, a second thermal exchange section 16 and a bypass section 19, separate from each other. In particular, bypass section 19 being surrounded by the first thermal exchange section 15 and by the second thermal exchange section 16, heat rejection is limited and thickness of the insulation is consequently low.

[24] According to another aspect of the present disclosure, a heat dumper (not shown) is arranged upstream the heat exchanger 10, to absorb excessive heat coming from the heat source. The smaller heat exchanger can be configured as a preheater of the fluid to be heated, i.e. the fluid to be heated in the heat exchanger can be used as a cooling fluid exchanging heat with the hot fluid from a heat source. Such a heat dumper, being a smaller heat exchanger, can be configured as a removable portion inside the shell 11 of the heat exchanger 10, upstream the flow regulating means 20 distributing the heating fluid between the separate sections 15, 16, 19.

[25] It is noted that the position of the heat dumper upstream the flow regulating means 20 allows the heat dumper to lower the temperature of the hot exhaust gas stream even if it is totally directed to the by-pass section 19, so that, even in case the flow downstream the by-pass section 19 is at least partially redirected to the heat exchange sections 15, 16, its temperature is not so high to cause a thermal shock of the tube bundles 17, 18.

[26] With reference to figure 4, it is shown a schematic of a control and regulation system of the heat exchanger 10 according to the embodiment of figure 1 . In particular, a control system 30 simultaneously controls the heating fluid flow inside the heating fluid sections 15, 16 and the flow of the fluid or the fluids to be heated inside the tube bundles 17, 18. In particular, the heating fluid flow inside the heating fluid sections 15, 16 is controlled as a function of the differential pressure inside each heating fluid section 15, 16, by means of differential pressure indicators 31 . In case an unbalance is detected, the flow inside the separate sections 15, 16, 19 is regulated through the flow regulating means 20, which can be operated by respective actuators 32.

[27] As far as the regulation and control of the fluids to be heated is concerned, the main parameter to be controlled is the fluid flow. The total fluid flow inlet is measured by a flow transmitter 33, while the fluid flow inside each tube bundle 17, 18 is measured by respective flow transmitters 34. Also the temperature of these fluid is controlled at the outlet from the respective tube bundles 17, 18, in order to perform a fine tuning of the fluid flow inside the tube bundles 17, 18. In particular, the temperature at the outlet of the respective tube bundles 17, 18 is controlled by means of temperature indicators 32 and is correlated with the fluid flow inlet, measured by a flow transmitter 33, and with the fluid flow inside each tube bundle 17, 18, measured by respective flow transmitters 34. In case an unbalance is detected, the flow inside the tube bundles 17, 18 is independently regulated through the valves 35, which can be operated by respective actuators 36.

[28] The following tables show two different control philosophies according to the disclosed embodiments of the present disclosure. In particular, according to a first philosophy, represented by the data shown in table 1 , where the flow percentage in each tube bundle is expressed with reference to the total nominal flow of the fluid to be heated by the heat exchanger, when a decrease of the total flow 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 lowered.

[Table follows on next page] Table 1

[29] As it is shown in table 1 , referring to a once-through steam generator, if the decrease of the total flow of the fluid to be heated is low, the flow inside one of the tube bundles can be maintained unchanged, while the flow inside the other tube bundle is lowered. When the total flow further decreases, the flow inside one of the fluid bundles needs completely closed, to avoid that the steam generator becomes unstable. Instability generally occurs when the flow inside the tube bundle is lower than 30% of the nominal load, corresponding to 15% of the total nominal flow of the heat exchanger comprising two parallel tube bundles having the same size. Thus, table 1 shows how, by arranging two parallel tube bundles in the heat exchanger, overall rangeability can be increased.

[30] Moreover, according to a second control and regulation philosophy, represented by the data shown in table 2, when a decrease of the total flow of the fluid to be heated is detected, the fluid inside both tube bundles can be correspondingly lowered.

[Table follows on next page] Table 2

[31] As it is shown in table 2, always referring to a once-through steam generator, if the decrease of the total flow of the fluid to be heated is low, the flow inside the tube bundles can be correspondingly lowered in the same way in both tube bundles. However, also in this case, when the total flow further decreases, the flow inside one of the fluid bundles needs to be completely closed, to avoid that the steam generator becomes unstable. Nevertheless, also table 2 shows how, by arranging two parallel tube bundles in the heat exchanger, overall rangeability can be increased.

[32] While aspects of the invention have been described in terms of various specific embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without departing form the spirt and scope of the claims.

Barzand & Zanardo Roma S.p.A.