Patent Application
20250067427
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
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.
According to another aspect, an exhaust fluid flow rate control device (diverter) can be located downstream or upstream said heat damper.
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.
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.
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.
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.
Referring now to the drawings,
In one specific embodiment, shown with reference to
The waste heat recovery unit 10 of
In the exemplary embodiment shown in
Making reference to
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.
The function of the separator 24 is essential when the waste heat recovery unit of
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.
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.
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.
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.
Always making reference to
Making reference to
Referring to
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
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102022000000296 | Jan 2022 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/025001 | 1/4/2023 | WO |
