Patent Application
20200141568
The present invention is related to the field of heat exchangers, in particular heat exchangers such as evaporators, superheaters, reheaters, and economizers intended to be used in thermal fluid steam generators such as Molten Salt Steam Generators (MSSG) of Concentrated Solar Power plants (CSP).
Background It is known that the CSP tower plants generally comprise one or more solar receivers which are situated at the apex of a central tower. These solar receivers are heated by concentrated incident solar rays and they generate a hot fluid that will be further used to produce high-pressure steam capable of driving a turbine and of producing electricity.
More specifically the CSP tower plant has as main components, namely, at least a heliostat solar field, a solar receiver installed on the top of the tower, a steam generator, a steam turbine and a storage system. In molten salt technology, the molten salt is heated typically to 565° C. in the solar receiver and stored in the hot storage tank. When a production of electricity is required, the hot salt flows from the hot tank to the Molten Salt Steam Generator (MSSG) to generate steam which will be injected into the steam turbine.
So-called “shell and tube” heat exchangers refer in prior art to a class of heat exchanger designs suitable for higher pressure applications. This type of heat exchanger is consisting of a large pressure vessel called a “shell” having a set of tubes, called “bundle”, inside it. A first fluid runs through the tubes while a second fluid flows inside the shell over the tubes, the first and the second fluid having different temperatures, with the aim of transferring heat from the second fluid to the first fluid or vice versa.
There are many variations on the shell and tube design. As an example,
To provide an improved heat exchange between the two fluids, the flow path of the second fluid is often determined by intermediate baffles 28 forming respective passages so that the second fluid flow changes its direction in passing from one passage to the next one. The baffles are usually under the form of partial circular segments or annular rings and disks, installed perpendicular to the longitudinal axis of the shell 22 to provide a zigzag flow of the second fluid.
A prior art alternative of the above design, depicted in
Different concepts of steam generator are already known. A synthesis of these different concepts is reported in the Sandia report 93-7084 “Investigation of thermal storage and steam generator issues, Bechtel Corporation”, in which are listed advantages and drawbacks of the existing steam generators.
In order to improve efficiency of the heat transfer in the heat exchangers, it is known since the 1920s that baffles mounted in the shell can have a specific shape intended to guide the fluid in a helical path. Moreover, with a continuous helical baffle, the heat transfer rate increases of about 10% compared with that of conventional segmental baffles for the same shell-side pressure drop (J. Heat Transfer (2007), Vol. 129(10), 1425-1431). This pattern allows to reduce leakage streams occurring in segmental baffles and further to increase the heat transfer coefficient greatly (J. Heat Transfer (2010), Vol. 132(10), 101801). Also, the flow stratification and stagnant zone are avoided (according to calculations), which allows a complete draining and decreases fouling susceptibility (lower fouling resistance and lower heat transfer area).
Document WO 2009/148822 discloses baffles mounted in the shell to guide the fluid into a helical flow pattern, with different helix angles when the baffle is proximate the inlet and the outlet respectively. Documents U.S. Pat. Nos. 2,384,714, 2,693,942, 3,400,758, 4,493,368 and WO 2005/019758 each disclose each different kinds of baffles, but with the same aim of providing a helical flow pattern of the fluid. Document U.S. Pat. No. 1,782,409 discloses a continuous helical baffle.
The current solutions are not satisfactory for example in terms of thermal gradient flexibility, efficiency (pressure drop, heat transfer coefficient), drainability, natural circulation, etc. and newly designed steam generator and/or individual heat exchangers thereof should meet technical requirements such as:
Moreover forced-recirculation evaporator material and manufacturing costs are higher than those for natural-circulation evaporators due to the recirculation pump capital cost.
In an embodiment, the present invention provides a hairpin heat exchanger, comprising: a first straight section; a second straight section; and a bent section linking the first straight section and the second straight section, wherein each straight section comprises a part of a first cylindrical shell or internal cylindrical shell and of a second cylindrical shell or external cylindrical shell, the first cylindrical shell being located inside the second cylindrical shell, both forming an intershell space enclosing a bundle of parallel U-bent tubes having each a first and a second straight part respectively located in the first and second straight section of the exchanger and a 180°-bent part located in the bent section of the exchanger, wherein, in use, a first fluid to be heated and vaporized is flowing, the external cylindrical shell comprising respectively at one end an inlet and at another end an outlet for a second fluid comprising a hot thermal fluid, so that, in use, the second fluid flows in the intershell space and cools down by exchanging heat with the first fluid flowing in the straight tubes, the intershell space enclosing baffles to guide the second fluid, and wherein the bundle of parallel U-bent tubes is extended out of the exchanger and connected, via bent tubes, respectively beyond an end of the internal shell and of the external shell at the first straight section to a first header configured to distribute the first fluid to the bundle of straight tubes and beyond an end of the internal shell and of the external shell at the second straight section to a second header configured to collect the first fluid, comprising liquid, vapor, or a mixture liquid/vapor, from the bundle of straight tubes.
The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. Other features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:
Aspects of the present invention overcome the drawbacks of the heat exchangers of prior art intended for steam generators.
In particular, embodiments of the invention obtain a reduced-size evaporator presenting high flexibility in terms of thermal gradient as well as improved efficiency thanks to optimized hydrodynamic salt flow leading to lower pressure drop, lower internal leakage (by-pass), improved heat transfer coefficient, lower tendency to foul, easily drainable molten salt, natural circulation (i.e. without circulation pump), long lifetime, and competitive cost.
Embodiments of the present invention avoid the utilization of thick components such as current tube sheets necessary in the shell-and-tube classical heat exchangers leading to the drawback that a high pressure zone is adjacent a low pressure zone.
A first aspect of the present invention relates to a hairpin heat exchanger having a first straight section, a second straight section and a bent section linking the first straight section and the second straight section, each straight section comprising a part of a first cylindrical shell or internal cylindrical shell and of a second cylindrical shell or external cylindrical shell, said first cylindrical shell being located inside said second cylindrical shell, both forming an intershell space enclosing a bundle of parallel U-bent tubes having each a first and a second straight part respectively located in said first and second straight section of the exchanger and a 180°-bent part located in said bent section of the exchanger, wherein, in use, a first fluid to be heated and vaporized is flowing, said external cylindrical shell being provided respectively at one end with an inlet and at another end with an outlet for a second fluid which is a hot thermal fluid, so that, in use, said second fluid is flowing in the intershell space and cooling down by exchanging heat with the first fluid flowing in the straight tubes, said intershell space enclosing also baffles to guide the second fluid, wherein the bundle of parallel U-bent tubes is extended out of the exchanger and connected, via bent tubes, respectively beyond an end of the internal shell and of the external shell at the first straight section to a first header distributing the first fluid to the bundle of straight tubes and beyond an end of the internal shell and of the external shell at the second straight section to a second header collecting the first fluid under the form of liquid, vapor or a mixture liquid/vapor from the bundle of straight tubes.
According to preferred embodiments of the invention, the hairpin heat exchanger also comprises one of the following characteristics or a suitable combination thereof:
A second aspect of the invention relates to the use of the hairpin heat exchanger as described above, as an evaporator.
A third aspect of the invention relates to the use of the hairpin heat exchanger as described above, as a superheater.
A fourth aspect of the invention relates to the use of the hairpin heat exchanger as described above, as a reheater or an economizer.
A fifth aspect of the invention relates to the use of the evaporator, the superheater, the reheater and economizer as described above, making at least one heat exchanger train in a molten salt steam generator (MSSG). Advantageously, the superheater, the reheater and/or the economizer are running counter-current, while the evaporator is running co-current.
Still under the scope of the present invention, the molten salt steam generator is a once-through or a forced circulation steam generator.
The present invention relates to a new design for a horizontal hairpin heat exchanger 1, as depicted in
The heat exchanger has a reciprocating flow between two fluids. A first fluid, generally a mixture of water and water steam, circulates through a first bundle of parallel horizontal straight tubes sections 2 located in the first straight part of the hairpin and further through a second bundle of parallel horizontal straight tubes sections 2 located in the second straight part of the hairpin. The tubes 2 of the first bundle are connected to the tubes 2 of the second bundle by 180° bent tube sections located in the head of the hairpin, forming thereby U-bent tube sections.
Supercritical carbon dioxide is another example of usable first fluid in the present invention.
According to one alternate embodiment, the straight tubes sections of the first bundle can discharge fluid into a bonnet through a thick(er) tube shell into which also end the straight tubes sections of the second bundle.
Thus, according to this particular embodiment the tubes have no U-bent tube sections.
According to the invention, the bundle of tubes 2 in each straight part is located between an internal cylindrical shell 3 and an external cylindrical shell 4, as represented in
The internal space 5 delimited by the two shells 3, 4 permits to hold a heat source, preferably a second fluid, within an annular flow path. This second fluid is a thermal fluid, for example molten salt(s) having been heated by the solar receivers at the apex of a CSP tower plant. The thermal fluid, by having its flow in contact with the bundle(s) of tubes 2, will transfer heat to the parallel-flowing first fluid running through the tubes 2. The first fluid and the second fluid can be co-current or counter-current, without departing from the scope of the present invention. Similarly the heat source or the second fluid can be any thermal fluid such as water, thermal oil, liquid sodium, fluidized bed, etc.
As illustrated by
Advantageously, as mentioned above, the thermal fluid is uniformly distributed on the shell at 360° (inlet, circulation, fluid temperature) thanks to a distribution jacket located at the inlet nozzle of the heat exchanger (see below).
In order to improve the efficiency of heat transfer, as shown in
According to one embodiment, the internal cylindrical shell 3 and the baffles 8 can be welded or bolted. Further a sealing device can be provided between the external shell 4 and the baffles 8 to avoid parasitic streams.
As shown on
More specifically, as shown on
Furthermore as shown on
In the shell-and-tube configuration, the first fluid, usually water, is generally under high pressure in quasi-spherical vessels or plenums. On the other side of the tube sheet, the salt flowing around the tube bundles is maintained under much lower pressure, requiring very thick tube sheets to withstand the pressure difference. The invention configuration provides prolongated tubes connected to standard headers (cylindrical, spherical, etc.) at the ends of the exchanger, in which the high pressure fluid is circulating. This allows to reduce the thickness of the tube sheets, if any, the pressure being limited. More specifically, in the rectangular section on
According to one embodiment of the present invention shown on
Today, increased speed for ramp-up and stop are often required by the client. Thick vessel walls or headers are not suited for accepting higher temperature gradients and are more subject to fatigue leading to shorter lifetime of the heat exchanger. In this context the present invention provides extended lifetime of the heat exchanger components.
The present invention is flexible and intended to be applied to a series of heat exchanger design used in MSSG technology, such as reheater, superheater, preheater and evaporator devices, wherein all the common components are made according to the generic heat exchanger design of the invention.
As mentioned above, a hot molten salt with decreasing temperature flows for example firstly in parallel through a reheater and a superheater to recombine and enter into the evaporator and further in the preheater/economizer in series.
In current embodiments, hot molten salt is entering the system at high temperature, for example 563° C. and certainly below 565° C. which is the degradation temperature for the usual molten salts. However it is under the scope of the present invention that the thermal fluid can withstand a temperature up to 700° C. All metal parts are advantageously made of stainless steel or noble metals which can withstand temperatures up to 600° C. and above.
Cold salt leaves the preheater at a temperature typically in the range of 290-300° C., or above a minimum temperature which is either the solidification temperature of the heat transfer fluid (as low as 240° C. for the molten salts such as sodium derivatives). Alternately any thermal fluid, e.g. thermal oil, can be used instead of molten salt with an operating temperature range in this case going for example from 80° C. (condensation and/or cristallization temperature) to 380° C. (example of degradation temperature).
Water at high pressure flows in tubes or pipes not in the shell side which allows lower thickness for the tube sheets and headers/shells and consequently a higher thermal gradient capability.
Although the design of the exchanger according to the present invention is optimized for natural circulation running, it could also be used in once-through or forced circulation steam generators.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.
The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.
| Number | Date | Country | Kind |
|---|---|---|---|
| 17172695.3 | May 2017 | EP | regional |
This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2018/062490, filed on May 15, 2018, and claims benefit to European Patent Application No. EP 17172695.3 and U.S. Provisional Patent Application No. 62/510,332, both of which were filed on May 24, 2017. The International Application was published in English on Nov. 29, 2018 as WO/2018/215239 under PCT Article 21(2).
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/062490 | 5/15/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62510332 | May 2017 | US |
