Description

TECHNICAL FIELD

[1] The present disclosure concerns a heat damper for a waste heat recovery unit and a waste heat recovery unit comprising a heat damper. Embodiments disclosed herein specifically concern improved waste heat recovery units for thermodynamic machines such as but not limited to gas turbines and/or engine power generators or mechanical drive applications, wherein the waste heat recovery unit is provided with a heat damper and more in particular with a preheater of a working fluid of a waste heat recovery system.

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 exiting 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.

-i- [5] For example, 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, often has still sufficiently enthalpy content and may be validly converted into mechanical energy using a thermodynamic cycle. According to such exemplary case, a waste heat recovery system typically includes not only a heat exchanger configured to transfer the heat stored in the flue gases from the machine to a working fluid, but also includes an expansion unit/group and a compression unit/group of a Brayton cycle system and/or a Stirling cycle system and/or an expansion unit/group of a Rankine cycle system or include a heat exchanger to further transfer the residual heat to an additional medium.

[6] However, waste heat recovery units, and in particular heat exchangers of waste heat recovery units have drawbacks due to possible overheating of their hottest section and excessive cooling of their coldest section during transitory states. In fact, if the heat exchanger starts empty of working fluid when the hot flue gases begin to enter the heat exchanger itself, then the entrance of the hot flue gases into the heat exchanger generates a thermal shock, i.e. an internal stress of the material of the heat exchanging surfaces, due to a quick variation in temperature. Thermal shocks may produce cracks and, as a result, life of the heat exchanger’s material is shortened. In the same way, if the same heat exchanger, which is empty of working fluid and heated by the hot flue gases, starts to be filled with cold working fluid, a thermal shock can occur in the first filled sections. On the other hand, if the heat exchanger starts already filled with a working fluid when the hot flue gases enter the heat exchanger itself, in particular if the waste heat recovery unit is operating according to the exemplary case above, but also in case hot flue gases come from a waste incineration facility, then the hot flue gases temperature is lowered very quickly and can reach the acid due point, the resulting liquid acid that is condensed from the flue gas possibly causing serious corrosion problems for the equipment used in collecting, cooling and discharging the exhaust flue gas. As a consequence, the service life of waste heat recovery units is affected, being potentially reduced. [7] Waste heat recovery units equipped with an exhaust flue gas diverter do not directly address the full exhaust gas flowrate to the heat exchanging surfaces, partially mitigating the thermal shock. Nevertheless, diverters do not offer a valid solution, because their use in partial opening affects the exhaust gases flow, causing swirls and noise. Moreover, diverters are difficult to control, since even small variations of a diverter geometry, combined with exhaust gases properties, lead to ineffective regulations.

[8] Moreover, 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, recovery systems are more and more subject to heat sources with a high start/stop frequency.

[9] In order to solve these problems of thermal shock and corrosion, according to the prior art, waste heat recovery units are made of materials, such as Hi-Cr Stainless Steel, Ni-Alloy, or the like that have high corrosion resistance and good mechanical strength at high temperatures.

On the other hand, these materials are very expensive and do not completely solve these criticalities.

SUMMARY

[10] According to the present disclosure, it is proposed that waste heat recovery units are provided with a heat damper, configured as a small heat exchanger, compared to the size of the main heat exchanger of the waste heat recovery unit, installed upstream the latter to absorb excessive heat coming from the heat source, namely during a transitory state, allowing the waste heat recovery unit to be operated in a proper manner, since the latter shall withstand less severe operating cycles. The heat damper can be configured as a working fluid preheater, i.e. the working fluid of the waste heat recovery system is used as a cooling fluid exchanging heat with the hot fluid from a heat source in the heat damper, has the function of absorbing the most critical transients of temperature and therefore it bears the thermal stresses, “freeing” the rest of the waste heat recovery unit, namely the main heat exchanger, from these conditions. Furthermore, by preheating the working fluid of the waste heat recovery system, the heat damper avoids the acid condensation in the coldest section of the main heat exchanger of the waste heat recovery unit.

As a consequence, only the heat damper has to be manufactured with high grade materials with good mechanical strength at high temperatures, while the rest of the waste heat recovery unit is made of less noble material and therefore less expensive.

[11] Notwithstanding the fact that it is made of a noble material, nevertheless the heat damper is still subject to a shorter life than the rest of the waste heat recovery unit. As a consequence, the heat damper is designed to be replaced easily, separately from the rest of the waste heat recovery unit.

[12] Thus, in one aspect, the subject matter disclosed herein is directed to a heat damper for a waste heat recovery unit. In particular, the heat damper is configured as a preheater of a working fluid of a waste heat recovery system. Additionally, the subject matter disclosed herein is directed to a method of operating a preheater for a waste heat recovery unit in which the preheater will work by absorbing the most severe thermal shocks due to high temperatures and large temperature differences between hot and cold fluid.

[13] In another aspect, the subject matter disclosed herein is directed to a preheater for a waste heat recovery unit allowing for preheating the working fluid of the waste heat recovery system before entering the inlet section of the heat exchanger of the waste heat recovery unit, so preventing acid condensation in the coldest portion of the heat exchanger due to the presence of aggressive components in the exhaust fluid from the heat source.

[14] According to still another aspect, the subject matter disclosed herein is directed to a method of operating a preheater for a waste heat recovery unit in order to vary the flowrate of the working fluid through the preheater according to the temperature of the exhaust fluid, allowing for the temperature of the exhaust fluid reaching the heat exchanger of the waste heat recovery unit to be properly lowered, so limiting the skin temperature of the hottest outlet section of the heat exchanger of the waste heat recovery unit.

[15] Thus, the subject matter disclosed herein is directed to a heat damper, in particular a working fluid preheater for a waste heat recovery unit and to a method of operating a preheater for a waste heat recovery unit allowing for the waste heat recovery unit to be made in a cheaper material and limiting the need of expensive and high performances materials to the preheater, thus allowing for overall savings in the cost of materials to be used.

[16] According to another aspect, the subject matter disclosed herein is directed to a heat damper, in particular a working fluid preheater for a waste heat recovery unit and to a method of operating a preheater for a waste heat recovery unit allowing for increasing the availability of the whole waste heat recovery unit, by reducing the possibility of damage to the main heat exchanger and at the same time concentrating any critical condition on the heat damper, the last being an easily maintainable and/or replaceable device.

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 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;

Figure 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 Figure 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 Figure 1 or 2.

DETAILED DESCRIPTION OF EMBODIMENTS

[18] 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 (CO2, boiler feed water/steam, organic fluid) of a waste heat recovery system.

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

[20] 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.

[21] 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.

[22] 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 (H2O, CO2 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.

[23] 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.

[24] Referring now to the drawings, Figure 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.

[25] In one specific embodiment, shown with reference to Figure 1 , the waste heat recovery unit 10 comprises a main body 1 1 , 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.

[26] The waste heat recovery unit 10 of Figure 1 is configured to exchange heat between a hot exhaust gas stream and a working fluid stream of a Ran- kine 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.

[27] In the exemplary embodiment shown in Figure 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 exchangerfeed 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.

[28] Making reference to Figure 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.

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.

[29] The function of the separator 24 is essential when the waste heat recovery unit of Figure 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.

[30] 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 mam heat exchanger 13 through the main heat exchanger feed line 2.

[31] 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.

[32] 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.

[33] 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.

[34] Always making reference to Figure 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 singlephase 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 singlephase 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.

[35] 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 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 (H2O, CO2 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. [36] Referring to Figure 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 Figure 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.

[37] 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 Figure 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.

[38] 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.