Heat damper for a waste heat recovery unit and waste heat recovery unit equipped with a heat damper

Abstract

The present disclosure relates to a waste heat recovery unit (10) comprising a main heat exchanger (13) configured to exchange heat between an exhaust fluid from a heat source and a working fluid of a waste heat recovery system, the waste heat recovery unit (10) comprising an additional heat exchanger (12) configured to exchange heat between the exhaust fluid and, alternatively, a cooling fluid or part or all of the working fluid during transient conditions.

Description

[Technical field]

[0001] The present disclosure relates to a heat damper for a waste heat recovery unit and a waste heat recovery unit comprising a heat damper. The embodiments disclosed herein relate in particular to an improved waste heat recovery unit for a thermodynamic machine, such as, but not limited to, a gas turbine and / or engine generator or mechanical drive application, the waste heat recovery unit comprising a heat damper, more particularly a preheater for the working fluid of the waste heat recovery system. [Background technology]

[0002] Waste heat is generated in almost all mechanical and thermal processes. Sources of waste heat include, for example, hot combustion gases exhausted into the atmosphere, hot water discharged into the environment, heated products from industrial processes, and heat transferred from hot equipment surfaces. Thus, waste heat sources differ with respect to their collection 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] Waste heat recovery can be done through a variety of 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 the heat source via an exhaust fluid to a waste heat recovery system. The 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 a working fluid of the waste heat recovery system.

[0005] For example, the residual heat of a machine such as a thermodynamic system, i.e. the heat that is ultimately rejected by the system through the flue gas and the portion of the heat source that was not utilized by the system, often still has a sufficiently high enthalpy content and can be effectively converted into mechanical energy using a thermodynamic cycle. According to such exemplary cases, the waste heat recovery system typically includes not only a heat exchanger configured to transfer the heat stored in the flue gas from the machine to a working fluid, but also includes an expansion unit / s and compression unit / s of a Brayton cycle system and / or a Stirling cycle system, and / or an expansion unit / s of a Rankine cycle system, or includes a heat exchanger for further transferring the residual heat to an additional medium.

[0006] 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 during transient conditions and excessive cooling of their coldest section. Indeed, if a heat exchanger starts to empty of working fluid when hot flue gas starts to enter the heat exchanger itself, thermal shocks, i.e. internal stresses of the material of the heat exchange surfaces, occur due to the rapid fluctuations in temperature when hot flue gas enters the heat exchanger. Thermal shocks can cause cracks, which result in a shortened lifespan of the heat exchanger material. Similarly, if the same heat exchanger, which is devoid of working fluid and heated by hot flue gas, starts to fill with cold working fluid, thermal shocks can occur in the sections that are initially filled. On the other hand, if the heat exchanger is already starting to fill with working fluid when the hot flue gas enters the heat exchanger itself, especially when the waste heat recovery unit is operating according to the above exemplary case, but also when the hot flue gas comes from a waste incineration facility, the temperature of the hot flue gas may drop very quickly and reach the acid limit, and the resulting liquid acid condensed from the flue gas may cause serious corrosion problems for the equipment used to collect, cool and discharge the exhaust flue gas, resulting in the service life of the waste heat recovery unit being affected and potentially shortened.

[0007] Waste heat recovery units with exhaust flue gas diverters do not directly address the total exhaust gas flow rate to the heat exchange surfaces, but only partially alleviate the thermal shock. Nevertheless, diverters do not provide an effective solution, since their use in partial opening affects the exhaust gas flow, causing vortexes and noise. Moreover, diverters are difficult to control, since even small variations in the diverter geometry, combined with the exhaust gas characteristics, lead to ineffective regulations.

[0008] Moreover, these drawbacks are becoming more and more critical since the market now demands production flexibility, which means an increase in start and stop cycles and transients such as load changes. The oil and gas market in particular requires frequent load changes, which also increase the number of transients. As a result, recovery systems are increasingly exposed to heat sources with high start / stop frequencies.

[0009] To solve these problems of thermal shock and corrosion, according to the prior art, the waste heat recovery unit is made of materials such as Hi-Cr stainless steel, Ni alloy, etc., which have high corrosion resistance and good mechanical strength at high temperatures.

[0010] On the other hand, these materials are very expensive and do not completely solve these important problems. Summary of the Invention

[0011] According to the present disclosure, it is proposed that the waste heat recovery unit is provided with a thermal damper, configured as a small heat exchanger compared to the size of the main heat exchanger of the waste heat recovery unit and installed upstream of the waste heat recovery unit to absorb excess heat coming from the heat source, i.e. allowing the waste heat recovery unit to operate in a proper manner, since during transient conditions the waste heat recovery unit will endure less severe operating cycles. The thermal 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 to exchange heat with the hot fluid from the heat source in the thermal damper, and has the function of absorbing the most critical transient conditions of temperature, thus enduring thermal stresses and "relieving" the rest of the waste heat recovery unit, i.e. the main heat exchanger, from these conditions. Furthermore, by preheating the working fluid of the waste heat recovery system, the thermal damper avoids acid condensation in the coldest section of the main heat exchanger of the waste heat recovery unit.

[0012] As a result, only the heat damper needs to be manufactured from high quality materials with good mechanical strength at high temperatures, while the rest of the waste heat recovery unit is made from less noble materials and therefore cheaper.

[0013] Despite the fact that the heat damper is made of precious metals, it still has a shorter life span than the rest of the heat recovery unit, and as a result, the heat damper is designed to be easily replaced separately from the rest of the heat recovery unit.

[0014] Thus, in one aspect, the subject matter disclosed herein is directed to a thermal damper for a waste heat recovery unit. In particular, the thermal damper is configured as a preheater for a working fluid of the 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 functions by absorbing the most severe thermal shocks due to high temperatures and large temperature differences between the hot and cold fluids.

[0015] In another aspect, the subject matter disclosed herein is directed to a preheater for a waste heat recovery unit that allows for preheating the working fluid of the waste heat recovery system before it enters the inlet section of the heat exchanger of the waste heat recovery unit, thus preventing acid condensation in the coldest parts of the heat exchanger due to the presence of aggressive components in the exhaust fluid from the heat source.

[0016] According to yet another aspect, the subject matter disclosed herein is directed to a method of operating a preheater for a waste heat recovery unit to vary the flow rate of a working fluid through the preheater according to the temperature of the exhaust fluid, enabling an appropriate reduction in the temperature of the exhaust fluid reaching the heat exchanger of the waste heat recovery unit, thus limiting the surface temperature of the hottest outlet section of the heat exchanger of the waste heat recovery unit.

[0017] The subject matter disclosed herein is therefore directed to a thermal damper, in particular a working fluid preheater for a waste heat recovery unit, and a method of operating a preheater for a waste heat recovery unit that allows the waste heat recovery unit to be made from less expensive materials, limiting the need for expensive, high performance materials to the preheater, thus allowing for an overall savings in the cost of materials used.

[0018] According to another aspect, the subject matter disclosed herein is directed to a thermal damper, in particular a working fluid preheater for a waste heat recovery unit, and a method of operating a preheater for a waste heat recovery unit that reduces the possibility of damage to the main heat exchanger while at the same time increasing the availability of the entire waste heat recovery unit by concentrating any critical conditions to the thermal damper, where the thermal damper is an easily maintainable and / or replaceable device. [Brief description of the drawings]

[0019] A more complete understanding of the embodiments of the present invention, and many of the anticipated advantages thereof, will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which: [Figure 1]FIG. 1 shows a schematic diagram of a new and improved waste heat recovery unit of a waste heat recovery system according to a first embodiment, the waste heat recovery unit including a thermal damper, in particular a working fluid preheater. [Diagram 2] FIG. 2 shows a schematic diagram of a new and improved waste heat recovery unit of a waste heat recovery system according to a second embodiment, the waste heat recovery unit including a thermal damper, in particular a working fluid preheater. [Diagram 3] FIG. 3 shows a simplified embodiment of a thermal damper, in particular a working fluid preheater, for a waste heat recovery unit of the system of FIG. 1 or FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] According to one aspect, the present subject matter is directed to a waste heat recovery unit including a thermal damper, in particular a working fluid preheater, configured as a compact heat exchanger (coil) made of high-grade materials (high chromium steel, Ni alloys, etc.) and supplied on one side with a variable flow rate of exhaust fluid from a heat source and on the other side with a variable flow rate of cooling fluid, preferably the working fluid of the waste heat recovery system (CO2, boiler feed water / steam, organic fluids).

[0021] According to another aspect, an exhaust fluid flow control device (diverter) may be placed downstream or upstream of the thermal damper.

[0022] According to yet another aspect, by varying the flow rate through the thermal damper, in particular the working fluid preheater, the exhaust fluid temperature reaching the heat exchanger of the waste heat recovery unit is appropriately reduced, thus limiting the surface 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, thus preventing acid condensation due to aggressive components in the exhaust fluid.

[0023] In order to improve the surface temperature control system, a control device such as a temperature indicator 3, 28 can be installed on the main coil of the waste heat recovery unit as a feedback of the control temperature of the thermal damper.

[0024] Further, according to another aspect, in the case of an ORC system, i.e. a waste heat recovery system using an organic fluid as the working fluid, configured to directly heat the organic fluid (i.e. without an intermediate fluid), the thermal damper is supplied with a cooling fluid selected from among safe fluids (H2O, CO2, etc.) to have the warmest temperature that can occur only in the coil of the thermal damper during a transient condition, so that the organic fluid can be directly heated in the coil of the main heat exchanger of the waste heat recovery unit without the problem of thermal degradation of the organic fluid due to high temperatures. The heat absorbed by the safe fluid can be used to preheat the organic working fluid itself.

[0025] Finally, according to an alternative exemplary embodiment, the waste heat recovery unit provided with a thermal 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) using either natural or forced circulation.

[0026] Referring now to the drawings, FIG. 1 illustrates a waste heat recovery unit that operates, inter alia, as a working fluid preheater and includes a thermal damper as shown in accordance with an exemplary embodiment of the present invention.

[0027] In one particular embodiment shown with reference to FIG. 1, the waste heat recovery unit 10 includes a body 11 including three parts, a thermal damper 12, also referred to herein below as a working fluid preheater 12, a main heat exchanger 13, and a bypass duct 14. Both the main heat exchanger 13 and the bypass duct 14 are arranged downstream of the preheater 12. In addition, 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 for controlling the exhaust gas flow rate to the main heat exchanger 13 and a diverter bypass section 16 for controlling the exhaust gas flow rate to the bypass duct. The diverter heat exchanger section 15 and the diverter bypass section 16 are mechanically linked such that the same actuator 7 simultaneously operates the opening of one and the closing of the other, allowing the passage of a constant flow rate through the diverter. Alternatively, the diverter heat exchanger section 15 and the diverter bypass section 16 can be operated by different actuators.

[0028] 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 (typically two-phase flow) of a Rankine cycle waste heat recovery system, but is also suitable for operation with single-phase flow, as described herein below.

[0029] In the exemplary embodiment shown in FIG. 1, the valve group 9 is configured to regulate the working fluid flow rate through the working fluid supply line 18. The working fluid supply line 18 is branched into a bypass flow line 19 connected to the preheater 12 by the intervention of a valve 20 and a main flow line 21 connected to the main heat exchanger 13 through the main heat exchanger supply line 2 by the intervention of a valve 22. The preheated fluid flow line 23 is directed from the preheater 12 to a separator 24 when the working fluid flow is a two-phase flow, in particular when the working fluid is a working fluid of a Rankine cycle waste heat recovery system. The separator 24 is configured to separate the two-phase preheated fluid into a preheated liquid fraction and a preheated vapor fraction. The preheated liquid flow line 25 and the preheated vapor flow line 26 are configured to collect the preheated liquid fraction and the preheated vapor fraction from the separator 24, respectively. The preheated vapor flow line 26 is passed through the main heat exchanger supply line 2 to the main heat exchanger 13. A superheated fluid flow line 27 is directed from the outlet from the main heat exchanger 13 to the collector 8 .

[0030] Referring to FIG. 1, when the working fluid stream is that of a Rankine cycle waste heat recovery system, the operation of the thermal damper 12 for a waste heat recovery unit 10 according to the present disclosure is as follows.

[0031] The working fluid through the working fluid supply line 18 is liquid. When the waste heat recovery system is started, a controlled flow rate of working fluid, reduced relative to the nominal value of the working fluid flow rate from the working fluid supply 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 heat exchange with the hot exhaust gas stream 17 and then directed through the preheated fluid stream line 23 to the separator 24. In the separator 24, the preheated working fluid stream is separated into a preheated liquid fraction and a preheated vapor fraction. The amount of the preheated liquid fraction in the separator 24 is controlled via a level indicator 4 operating a valve 5 in the preheated liquid stream line 25. Typically, the preheated liquid stream in the preheated liquid stream line 25 can be recovered in a thermal cycle. The preheated vapor fraction is directed through preheated vapor stream line 26 and main heat exchanger feed line 2 to main heat exchanger 13 for further heat exchange with exhaust gas 17 and collected in collector 8 as a superheated vapor stream.

[0032] The function of the separator 24 is essential when the waste heat recovery unit of FIG. 1, which is part of a Rankine cycle waste heat recovery system, is started up. Indeed, when the system is started up, the hot exhaust gas stream is present on one side of the coils of the main heat exchanger 13 and the stream of the preheated vapor fraction is present on the other side of the coils. The contact with the preheated vapor fraction causes a lower cooling of the temperature of the main heat exchanger 13, compared to the cooling that could be caused by the liquid stream, due to the lower heat exchange coefficient. Furthermore, the temperature of the preheated vapor fraction is higher than the liquid fraction. Thus, the preheated vapor reduces the thermal shock to the coils of the main heat exchanger 13. A further advantage is due to the fact that the steam quickly fills all the main heat exchangers 13, reducing the time required to complete the start-up.

[0033] As the start-up phase progresses, the pressure of the vapor fraction inside the main heat exchanger 13 and the main heat exchanger supply line 2 increases, as does the temperature. A possible backflow from the main heat exchanger 13 to the separator 24 is prevented by the check valve 260 arranged on the preheated vapor flow line 26. In addition, if a higher pressure is present downstream, the pressure inside the separator 24 may also increase, so that a possible backflow from the separator 24 is prevented by the check valve 230 arranged on the preheated flow line 23. Upon measuring a set temperature, the temperature indicator 28 operates the valve 22 to gradually increase the amount of liquid working fluid flowing through the liquid working fluid supply line 18, directed through the mainstream line 21, mixed with the preheated vapor in the preheated vapor flow line 26, and then sent through the main heat exchanger supply line 2 to the main heat exchanger 13.

[0034] As long as the start-up continues, in order to obtain a smoother change in temperature along the main heat exchanger supply line 2 and the main heat exchanger 13, when the temperature indicator 28 measures the set temperature, the control level on indicator 4 is removed and valve 5 is closed, so that the separator 24 is filled with the preheated liquid fraction, which is then sent through the main heat exchanger supply line 2 to the main heat exchanger 13. Then, valve 22 is opened, so that a gradually increasing amount of liquid working fluid flows from the liquid working fluid supply line 18 to the mainstream line 21, where it is mixed with the preheated liquid fraction and is subsequently sent through the main heat exchanger supply line 2 to the main heat exchanger 13. The supply line 2 therefore contains even less vapor fraction than liquid fraction, taking into account both the mixture with the liquid coming from the line 21 through the valve 22 and the rising pressure.

[0035] 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, so that all the liquid working fluid flowing in the liquid working fluid supply line 18 is sent to the main heat exchanger 13 via the mainstream line 21 and the main heat exchanger supply line 2.

[0036] When preheating is no longer needed, the heat damper 12 is removed from the system by closing valve 20 on the bypass flow line 19 and valve 6 on the preheat fluid flow line 23, and by opening a vent / drain 29 located along the preheat fluid flow line 23 to force fluid out of the heat damper 12.

[0037] Always referring to FIG. 1 , the operation of the thermal damper 12 for the waste heat recovery unit 10 according to the present disclosure is operated by adjusting the valve 22 and opening the on-off valve 20 to split the working fluid from the working fluid supply line 18 into a bypass flow that is directed to the preheater 12 through the bypass flow line 19 and a main flow that is directed to the main heat exchanger 13 through the main flow line 21 and the main heat exchanger supply line 2. In the preheater 12, the working fluid is heated by heat exchange with the hot exhaust gas stream 17, and the preheated working fluid flow is then directed to mix with the main flow before entering the main heat exchanger 13. A separator 24 is traversed by the preheated working fluid flow (if the working fluid flow is a single-phase working fluid flow, the system may also not include the separator 24 and associated ancillary components). The mixed stream is directed through main heat exchanger supply line 2 to main heat exchanger 13 for further heat exchange with exhaust gas 17 and collected as a superheated fluid stream in collector 8. Thus, even when the working fluid stream is a single-phase working fluid stream, the thermal damper according to the present disclosure is important to prevent thermal shock and acid condensation by controlling the temperature of the working fluid stream directed to main heat exchanger 13.

[0038] With reference to FIG. 2, the same reference numbers are used for the same components of the embodiment shown with reference to FIG. 1, but the thermal damper 12 according to the present disclosure is also suitable for use with an organic Rankine cycle (ORC) system with organic fluid direct heating (i.e. without intermediate fluid). In such a case, the thermal damper 12 is realized as part of a separate circuit supplied with a safety fluid (H2O, CO2, etc.) from the service cooling fluid circuit 200. The safety fluid thus exchanges heat and cools the exhaust gas stream 17. The organic fluid is directly supplied to the main heat exchanger 13 and is heated directly in the coil of the waste heat recovery unit 10 by heat exchange with the lower temperature exhaust gas stream, thus without the problem of thermal degradation of the organic fluid due to excessively high temperatures. The heat absorbed by the safety 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 to both natural circulation boilers and forced circulation boilers used in Rankine cycle waste heat recovery systems.

[0039] Referring to FIG. 3 showing a simplified schematic diagram of a preheater for a waste heat recovery unit according to the present disclosure, the preheater 12 is configured as a removable part of the body 11 of the waste heat recovery unit 10 to facilitate maintenance and / or replacement of the preheater 12 in case of damage. In the exemplary embodiment of FIG. 3, the preheater 12 is configured of a tube bundle 30 supported by a frame 31. A flange 32 is configured to removably couple the cold fluid bypass flow line 19 to a cold fluid collector 33 and an inlet side of the tube bundle 30. The outlet side of the tube bundle 30 is arranged as a preheat fluid collector 34 and is connected to a flange 35 and directed to a flow 23 configured to removably couple the preheat fluid collector 34 to the preheat flow line 23.

[0040] It should be noted that by locating the preheater 12 upstream of the diverter, the preheater 12 can reduce the temperature of the hot exhaust gas flow 17 even if the hot exhaust gas flow 17 is completely directed to the bypass duct 14. As a result, the heat exchange surface 13 can be made of cheaper materials even if the bypass duct 14 is integrated with the body 11 as in the embodiment shown in FIG. 1. Indeed, even if the flow downstream of the bypass duct 14 is at least partially redirected to the heat exchange surface 13, its temperature is not so high as to cause a thermal shock of the heat exchange surface 13. However, for the same reason, since the exhaust gas cannot bypass the preheater 12 by closing the diverter damper 15, the preheater 12 must be designed to withstand the extreme thermal shock when the working fluid enters the preheater after the waste heat recovery unit is started. If the bypass duct is not integrated with the body 11 but is realized as a separate body, the location of the preheater 12 can be downstream of the diverter.

[0041] While aspects of the invention have been described with respect to various specific embodiments, it will be apparent to those skilled in the art that many modifications, changes, and omissions can be made without departing from the spirit and scope of the claims. Barzano & Zanardo Roma SpA

Claims

1. 1. A waste heat recovery unit (10), comprising a main heat exchanger (13) 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, said waste heat recovery unit (10) comprising an additional heat exchanger (12) arranged upstream of said main heat exchanger (13), said additional heat exchanger (12) configured to exchange heat between said hot exhaust fluid stream and a cooling fluid stream.

2. 2. The waste heat recovery unit (10) of claim 1, wherein a diverter is disposed downstream of the additional heat exchanger (12), the diverter being configured to control an amount of the hot exhaust fluid flow that is directed to the main heat exchanger (13) and a bypass duct (14).

3. 2. The waste heat recovery unit (10) of claim 1, wherein a diverter is disposed upstream of the additional heat exchanger (12), the diverter being configured to control an amount of the hot exhaust fluid flow that is directed to the main heat exchanger (13) and a bypass duct (14).

4. The waste heat recovery unit (10) according to claim 1, wherein the additional heat exchanger (12) is made of high-grade materials.

5. The waste heat recovery unit (10) of claim 1, wherein the additional heat exchanger (12) is configured as a removable part of the waste heat recovery unit (10).

6. 2. The waste heat recovery unit (10) of claim 1, comprising a working fluid supply line (18), a bypass flow line (19) connected at a first end to the working fluid supply line (18) and at a second end to the additional heat exchanger (12), and a mainstream line (21) connected at a first end to the working fluid supply line (18) and at a second end to a first end of a main heat exchanger supply line (2), the waste heat recovery unit (10) further comprising a pre-heated working fluid line (23) connected at one end to a working fluid outlet of the additional heat exchanger (12) and at a second end to the first end of the main heat exchanger supply line (2), the second end of the main heat exchanger supply line (2) being connected to the main heat exchanger (13).

7. 7. The waste heat recovery unit (10) of claim 6, wherein a separator (24) is disposed downstream of the preheated working fluid line (23) and upstream of the main heat exchanger supply line (2).

8. 8. A waste heat recovery unit (10) according to claim 7, wherein a vapor flow line (26) is connected at a first end to an upper portion of the separator (24) and at a second end to the first end of the main heat exchanger supply line (2), and a liquid flow line (25) is connected to a lower portion of the separator (24).

9. 7. The waste heat recovery unit (10) according to claim 6, wherein the main line (21) comprises a valve (22) connected to a temperature sensor (28) arranged along the main heat exchanger supply line (2).

10. 2. The waste heat recovery unit (10) of claim 1, comprising: a cooling fluid supply line (190) connected at a first end to a service cooling fluid circuit (200) and at a second end to the inlet of the additional heat exchanger (12); and a cooling fluid outlet line connected at a first end to the outlet of the additional heat exchanger (12) and at a second end to the service cooling fluid circuit (200), and the waste heat recovery unit (10) comprises a working fluid supply line (2) connected to the main heat exchanger (13).

11. 11. The waste heat recovery unit (10) of claim 10, wherein a heat exchanger (130) is configured to exchange heat between the cooling fluid downstream of the additional heat exchanger (12) and the working fluid upstream of the main heat exchanger (13).

12. The waste heat recovery unit (10) of claim 1, wherein the waste heat recovery unit (10) is a once-through heat recovery steam generator (OTSG).

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