HEAT DAMPER FOR A WASTE HEAT RECOVERY UNIT AND WASTE HEAT RECOVERY UNIT COMPRISING A HEAT DAMPER

Abstract

The disclosure concerns a waste heat recovery unit comprising a main heat exchanger configured to exchange heat between an exhaust fluid from a heat source and a working fluid of a waste heat recovery system, wherein the waste heat recovery unit comprises an additional heat exchanger configured to exchange heat between the exhaust fluid and alternatively a cooling fluid or a portion or the whole of said working fluid during transitory states.

Claims

1. A waste heat recovery unit comprising a main heat exchanger configured to exchange heat between a hot exhaust fluid stream from a heat source and a working fluid stream of a waste heat recovery system, wherein the waste heat recovery unit comprises an additional heat exchanger arranged upstream said main heat exchanger, the additional heat exchanger being configured to exchange heat between the hot exhaust fluid stream and a cooling fluid stream.

2. The waste heat recovery unit according to claim 1, wherein a diverter is arranged downstream said additional heat exchanger, the diverter being configured to control the amount of the hot exhaust fluid stream directed to said main heat exchanger and to a bypass duct.

3. The waste heat recovery unit according to claim 1, wherein a diverter is arranged upstream said additional heat exchanger, the diverter being configured to control the amount of the hot exhaust fluid stream directed to said main heat exchanger and to a bypass duct.

4. The waste heat recovery unit according to claim 1, wherein said additional heat exchanger is made with a high grade material.

5. The waste heat recovery unit according to claim 1, wherein said additional heat exchanger is configured as a removable portion of the waste heat recovery unit.

6. The waste heat recovery unit according to claim 1, wherein the waste heat recovery unit comprises a working fluid feed line, a bypass stream line connected at a first end to the working fluid feed line and at a second end to the additional heat exchanger and a main stream line connected at a first end to the working fluid feed line and at a second end to a first end of a main heat exchanger feed line; the waste heat recovery unit additionally comprising a preheated working fluid line connected at one end to a working fluid exit of the additional heat exchanger and at a second end to the first end of the main heat exchanger feed line, the second end of the main heat exchanger feed line being connected to the main heat exchanger.

7. The waste heat recovery unit according to claim 6, wherein a separator is arranged downstream the preheated working fluid line and upstream the main heat exchanger feed line.

8. The waste heat recovery unit according to claim 7, wherein a vapor stream line is connected at a first end with an upper portion of the separator and at a second end with the first end of the main heat exchanger feed line and wherein a liquid stream line is connected with a lower portion of the separator.

9. The waste heat recovery unit according to claim 6, wherein the main stream line comprises a valve connected to a temperature sensor arranged along the main heat exchanger feed line.

10. The waste heat recovery unit according to claim 1, wherein the waste heat recovery unit comprises a cooling fluid feed line connected at a first end to a service cooling fluid circuit and at a second end to the inlet of the additional heat exchanger and a cooling fluid outlet line connected at a first end to the outlet of the additional heat exchanger and at a second end to the service cooling fluid circuit, and wherein the waste heat recovery unit comprises a working fluid feed line connected to the main heat exchanger.

11. The waste heat recovery unit according to claim 10, wherein a heat exchanger is configured to exchange heat between the cooling fluid downstream the additional heat exchanger and the working fluid upstream the main heat exchanger.

12. The waste heat recovery unit according to claim 1, wherein said waste heat recovery unit is a once through heat recovery steam generator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 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:

[0019] FIG. 1 illustrates a schematic of a new, improved waste heat recovery unit of a waste heat recovery system according to a first embodiment, the waste heat recovery unit including a heat damper, in particular a working fluid preheater;

[0020] FIG. 2 illustrates a schematic of a new, improved waste heat recovery unit of a waste heat recovery system according to a second embodiment, the waste heat recovery unit including a heat damper, in particular a working fluid preheater; and

[0021] FIG. 3 illustrates a simplified embodiment of a heat damper, in particular a working fluid preheater for the waste heat recovery unit of the system of FIG. 1 or 2.

DETAILED DESCRIPTION OF EMBODIMENTS

[0022] According to one aspect, the present subject matter is directed to a waste heat recovery unit including a heat damper, in particular a working fluid preheater, the heat damper being made in high grade material (high Chromium Steel, Ni alloy, etc.) and being configured as a small heat exchanger (coil) fed on one side by a variable flowrate of an exhaust fluid from a heat source and on another side by a variable flowrate of a cooling fluid, preferably a working fluid (CO.sub.2, boiler feed water/steam, organic fluid) of a waste heat recovery system.

[0023] According to another aspect, an exhaust fluid flow rate control device (diverter) can be located downstream or upstream said heat damper.

[0024] According to still another aspect, by varying the flowrate through the heat damper, in particular the working fluid preheater, the exhaust fluid temperature reaching the heat exchanger of the waste heat recovery unit is properly lowered, so limiting the skin temperature of the hottest outlet section of the heat exchanger of the waste heat recovery unit, and the working fluid is preheated before entering the inlet section of the heat exchanger of the waste heat recovery unit, so preventing acid condensation due to aggressive components in the exhaust fluid.

[0025] To enhance the control system skin temperature, control instruments such as temperature indicators 3, 28 can be installed on the main coil of the waste heat recovery unit as feedback of the control temperature on the heat damper.

[0026] Additionally, according to another aspect, for an ORC system, i.e. a waste heat recovery system using an organic fluid as working fluid, the system being configured with direct heating of the organic fluid (i.e. without intermediate fluid), the heat damper is fed with a cooling fluid chosen amongst safe fluids (H.sub.2O, CO.sub.2 or the like), in order to have the warmest temperatures that could occur during transient conditions on the coil of the heat damper only, so allowing to have the organic fluid direct heating on the coil of the main heat exchanger of the waste heat recovery unit without the issue of Organic fluid thermal degradation due to high temperatures. The heat absorbed by the safe fluid can be used to preheat the organic working fluid itself.

[0027] Finally, according to alternative exemplary aspects, the waste heat recovery unit provided with a heat damper, in particular a working fluid preheater can be used in a once through heat recovery steam generator (OTSG) or in a heat recovery steam generator (HRSG) either with natural or forced circulation.

[0028] Referring now to the drawings, FIG. 1 shows a waste heat recovery unit including a heat damper, which operates in particular as a working fluid preheater, and which is illustrated in accordance with an exemplary embodiment of the invention.

[0029] In one specific embodiment, shown with reference to FIG. 1, the waste heat recovery unit 10 comprises a main body 11, comprising three portions, a heat damper 12, also called herein below working fluid preheater 12, a main heat exchanger 13 and a by-pass duct 14. The main heat exchanger 13 and the by-pass duct 14 are both arranged downstream the preheater 12. Additionally, a diverter is arranged along the connection between the preheater 12 and the main heat exchanger 13, the diverter comprising a diverter heat exchanger section 15 to control the exhaust gas flow rate to the main heat exchanger 13 and a diverter bypass section 16 to control the exhaust gas flow rate to the bypass duct. The diverter heat exchanger section 15 and the diverter bypass section 16 are mechanically linked, so that the same actuator 7 operates a simultaneous opening of one and closing of the other, allowing for the passage of a constant flow through the diverter. Alternatively, the diverter heat exchanger section 15 and the diverter bypass section 16 can be operated by different actuators.

[0030] The waste heat recovery unit 10 of FIG. 1 is configured to exchange heat between a hot exhaust gas stream and a working fluid stream of a Rankine cycle waste heat recovery system, generally a two-phase flow, but is also suitable for operating with a single-phase flow, as explained herein below.

[0031] In the exemplary embodiment shown in FIG. 1, a valve group 9 is configured to regulate a working fluid flow rate through a working fluid feed line 18. The working fluid feed line 18 is split into a bypass stream line 19, connected to the preheater 12 by interposition of a valve 20 and a main stream line 21, connected to the main heat exchanger 13 through a main heat exchanger feed line 2 by interposition of a valve 22. A preheated fluid stream line 23 is directed from the preheater 12 to a separator 24 when the working fluid stream is a two-phase stream, in particular in case the working fluid is a working fluid of a Rankine cycle waste heat recovery system. The separator 24 is configured to separate a two-phase preheated fluid into a preheated liquid fraction and a preheated vapour fraction. A preheated liquid stream line 25 and a preheated vapour stream line 26 are configured to respectively collect the preheated liquid fraction and the preheated vapour fraction from the separator 24. The preheated vapour stream line 26 is routed to the main heat exchanger 13 through the main heat exchanger feed line 2. A superheated fluid stream line 27 is directed from the exit from the main heat exchanger 13 to a collector 8.

[0032] Making reference to FIG. 1, in case the working fluid stream is a working fluid stream of a Rankine cycle waste heat recovery system, the operation of the heat damper 12 for a waste heat recovery unit 10 according to the present disclosure is the following.

[0033] The working fluid through the working fluid feed line 18 is liquid. When the waste heat recovery system is started, a controlled flow-rate of working fluid, which is reduced with respect to the nominal value of working fluid flow-rate from the working fluid feed line 18, is directed to the preheater 12, by closing the valve 22 and opening the valve 20. In the preheater 12, the working fluid is heated by exchanging heat with the hot exhaust gas stream 17, and is subsequently directed to the separator 24 through the preheated fluid stream line 23. In the separator 24, the preheated working fluid stream is separated into a preheated liquid fraction and a preheated vapour fraction. The amount of the preheated liquid fraction in the separator 24 is controlled through a level indicator 4, operating a valve 5 of a preheated liquid stream line 25. Typically, the preheated liquid stream of the preheated liquid stream line 25 can be recovered in the thermal cycle. The preheated vapour fraction is directed to the main heat exchanger 13 through the preheated vapour stream line 26 and the main heat exchanger feed line 2, to additionally exchange heat with the exhaust gas 17 and to be collected as a superheated vapour stream in the collector 8.

[0034] The function of the separator 24 is essential when the waste heat recovery unit of FIG. 1, part of a Rankine cycle waste heat recovery system, is started. In fact, when the system is started, the hot exhaust gas stream is present on one side of the coil of the main heat exchanger 13 and the preheated vapour fraction stream is present on the other side of the coil. The contact with the preheated vapour fraction causes a lower cooling of the temperature of the main heat exchanger 13, compared to the cooling that could be caused by a liquid stream, because of the lower thermal exchange coefficient. Additionally, the temperature of the preheated vapour fraction is higher than the liquid fraction. Thus, the preheated vapour reduces the thermal shock on the coil of the main heat exchanger 13. An additional advantage is due to the fact that the vapour rapidly fills all the main heat exchanger 13, reducing the required time to complete the start-up.

[0035] As the start-up phase progresses, the pressure of the vapour fraction inside the main heat exchanger 13 and the main heat exchanger feed line 2 is ramped up and the temperature increases as well. A possible counter flow from the main heat exchanger 13 to the separator 24 is prevented by a nonreturn valve 260 arranged on the preheated vapour stream line 26. Additionally, since also the pressure inside the separator 24 could increase if a higher pressure is present downstream, a possible counter flow from the separator 24 is prevented by a non-return valve 230 arranged on the preheated stream line 23. When the temperature indicator 28 measures a set temperature, it operates the valve 22 to allow a progressively increasing of the amount of liquid working fluid to flow through the liquid working fluid feed line 18 to be directed through the main stream line 21, to mix together with the preheated vapour of the preheated vapour stream line 26 and to be subsequently routed to the main heat exchanger 13 through the main heat exchanger feed line 2.

[0036] As long as the start-up continues, in order to obtain a smoother change of temperature along the main heat exchanger feed line 2 and the main heat exchanger 13, when the temperature indicator 28 measures the set temperature, the control level on the indicator 4 is excluded and the valve 5 is closed, so that the separator 24 is filled with the preheated liquid fraction. The preheated liquid fraction is consequently routed to the main heat exchanger 13 through the main heat exchanger feed line 2; the valve 22 is then opened to allow a progressively increasing amount of the liquid working fluid to flow from the liquid working fluid feed line 18 to the main stream line 21, to mix together with the preheated liquid fraction and to be subsequently routed to the main heat exchanger 13 through the main heat exchanger feed line 2; consequently, the feed line 2, considering both the mixing with liquid coming from line 21 through the valve 22, and the ramping up pressure, contains even less vapour fraction than liquid fraction.

[0037] Another alternative solution is that, when the temperature indicator 28 measures the set temperature, the valve 22 is opened and at the same time the valve 20 is closed. As a consequence, all the liquid working fluid flowing through the liquid working fluid feed line 18 is routed to the main heat exchanger 13 through the main stream line 21 and the main heat exchanger feed line 2.

[0038] Once the preheating is no longer necessary, the heat damper 12 is excluded from the system, by closing the valve 20 on the bypass stream line 19 and a valve 6 on the preheated fluid stream line 23 and by opening a vent/drain 29 arranged along the preheated fluid stream line 23, to drive out the fluid from the heat damper 12.

[0039] Always making reference to FIG. 1, in case the working fluid stream is a single-phase working fluid stream, the operation of the heat damper 12 for a waste heat recovery unit 10 according to the present disclosure is operated by splitting the working fluid from the working fluid feed line 18 into a bypass stream directed to the preheater 12 through the bypass stream line 19 and a main stream directed to the main heat exchanger 13 through the main stream line 21 and the main heat exchanger feed line 2, by regulating the valve 22 and opening the on-off valve 20. In the preheater 12, the working fluid is heated by exchanging heat with the hot exhaust gas stream 17, and the preheated working fluid stream is subsequently directed to mix with the main stream before entering the main heat exchanger 13. The separator 24 is crossed by the preheated working fluid stream (in case the working fluid stream is a single-phase working fluid stream the system can also comprise no separator 24 and relevant ancillaries). The mixed stream is directed to the main heat exchanger 13 through the main heat exchanger feed line 2, to additionally exchange heat with the exhaust gas 17 and to be collected as a superheated fluid stream in the collector 8. Therefore, also in case the working fluid stream is a single-phase working fluid stream, the heat damper according to the present disclosure is important to prevent thermal shocks and acid condensation by controlling the temperature of the working fluid stream directed to the main heat exchanger 13.

[0040] Making reference to FIG. 2, the same reference numbers being used for the same components of the embodiment shown with reference to FIG. 1, the heat damper 12 according to the present disclosure is also suitable to be used with an organic Rankine cycle (ORC) system with organic fluid direct heating (i.e. without intermediate fluid). In such a case, the heat damper 12 is realised as part of a separate circuit fed with a safe fluid (H.sub.2O, CO.sub.2 or the like) from a service cooling fluid circuit 200. Accordingly, the safe fluid exchanges heat and cools the exhaust gas stream 17. The organic fluid is fed directly to the main heat exchanger 13 to be directly heated in the coils of the waste heat recovery unit 10, by exchanging heat with the exhaust gas stream at a lower temperature, therefore without the issue of organic fluid thermal degradation due to excessively high temperature. The heat absorbed by the safe fluid can be recovered to preheat the organic working fluid in another external heat exchanger 130 before being directed to the main heat exchanger 13. This solution can be applied both to natural and forced circulation boiler used in Rankine cycle waste heat recovery systems.

[0041] Referring to FIG. 3, showing a simplified schematic of the preheater for the waste heat recovery unit according to the present disclosure, the preheater 12 is arranged as a removable portion of the main body 11 of the waste heat recovery unit 10, to ease maintenance and/or replacing of the preheater 12 in case of damage. In the exemplary embodiment of FIG. 3, the preheater 12 is composed of a tube bundle 30, supported by a frame 31. A flange 32 is configured to removably couple the cold fluid bypass stream line 19 with a cold fluid collector 33 and to an inlet side of the tube bundle 30. An outlet side of the tube bundle 30 is arranged as a preheated fluid collector 34, which is connected to a flange 35 and is directed to the stream 23 that is configured to removably couple the preheated fluid collector 34 to the preheated stream line 23.

[0042] It is noted that the position of the preheater 12 upstream the diverter allows the preheater 12 to lower the temperature of the hot exhaust gas stream 17 even if it is totally directed to the by-pass duct 14. As a consequence, the heat exchange surfaces 13 can be made with a less expensive material even if the by-pass duct 14 is integrated with the main body 11, as in the embodiment shown in FIG. 1. In fact, even in case the flow downstream the by-pass duct 14 is at least partially redirected to the heat exchange surfaces 13, its temperature is not so high to cause a thermal shock of the heat exchange surfaces 13. However, for the same reason, since the preheater 12 cannot be bypassed by the exhaust gas by closing the diverter damper 15, it has to be designed so it can withstand extreme thermal shocks when the working fluid enters the preheater after the waste heat recovery unit is started. If the bypass duct is not integrated with the main body 11, but is realized as a separate body, then the position of the preheater 12 can be downstream the diverter.

[0043] 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.