Coil-wound heat exchangers (“CWHE”) are often a preferred type of heat exchanger used in natural gas liquefaction systems. In a CWHE, the fluid(s) to be cooled are circulated through many layers of tubes that are wrapped around a central mandrel, separated by axial spacers, and contained within a shell space. The assembly of tubes, mandrel and spacers forms a tube bundle, or bundle. Refrigeration is provided by a flow of an expanded refrigerant (often a mixed refrigerant) through the shell space. A common problem with CWHEs is temperature maldistribution of the refrigerant between concentric zones in the shell space, meaning that there is a radial temperature gradient between zones in a particular location between the warm and cold ends of the bundle.
Attempts have been made to correct such radial temperature maldistribution by “zoning” the tube sheets—meaning routing tubes that are connected to each of the cold end and warm end tube sheets through a single zone. This configuration is described in greater detail herein in connection with
Such configurations increase the cost of building the CWHE because the number of tube sheets required at both the cold and warm ends is a function of the number of zones, which often results in a greater number of tube sheets than required to accommodate the number of tubes in the bundle.
Therefore, there is a need for a CWHE configuration that enables flow adjustments to correct radial temperature maldistribution with less of the incremental cost and complexity associated with prior art solutions to radial maldistribution.
Several specific aspects of the systems and methods of the subject matter disclosed herein are outlined below.
Aspect 1: A coil-wound heat exchanger comprising:
a shell;
a first bundle comprising
a first group of tube sheets located at the first bundle end, each of the first group of tube sheets being in fluid flow communication with one of the plurality of tube sets at the first tube end;
a plurality of valves, each of the plurality of valves being in fluid flow communication with each of the first group of tube sheets and located at the first bundle end; and
a second group of tube sheets located at the second bundle end, at least one of the second group of tube sheets being in fluid flow communication with more than one of the plurality of tube sets at the second tube end.
Aspect 2: The coil-wound heat exchanger of Aspect 1, wherein the first bundle end is a cold end of the first bundle and the second bundle end is a warm end of the first bundle.
Aspect 3: The coil-wound heat exchanger of any of Aspects 1-2, wherein each of the second group of tube sheets is in fluid flow communication at the second tube end with at least one of the plurality of tubes from each of the plurality of tube sets.
Aspect 4: The coil-wound heat exchanger of any of Aspects 1-3, wherein the second bundle end comprises a plurality of sectors circumferentially arranged around the mandrel, each of the second group of tube sheets being in fluid flow communication with second tube ends originating from a single one of the plurality of sectors.
Aspect 5: The coil-wound heat exchanger of any of Aspects 1-4, further comprising a temperature sensor located in each of the plurality of zones.
Aspect 6: The coil-wound heat exchanger of Aspect 5, wherein the warm bundle has a bundle height extending from the cold bundle end to the warm bundle end and each of the temperature sensors is located within a middle 50% of the bundle height.
Aspect 7: The coil-wound heat exchanger of Aspect 5, wherein the warm bundle has a bundle height extending from the cold bundle end to the warm bundle end and each of the temperature sensors is located within a middle 20% of the bundle height.
Aspect 8: The coil-wound heat exchanger of any of Aspects 1-7, further comprising a first inlet conduit in fluid flow communication with the first group of tube sheets and the second group of tube sheets and a second inlet conduit in fluid flow communication with a third group of tube sheets and a fourth group of tube sheets.
Aspect 9: The coil-wound heat exchanger of Aspect 8, wherein the third group of tube sheets is located at the first bundle end, each of the third group of tube sheets being in fluid flow communication with more than one of the plurality of tube sets at the first tube end and the second group of tube sheets is located at the second bundle end, each of the second group of tube sheets being in fluid flow communication with more than one of the plurality of tube sets at the second tube end.
Aspect 10: The coil-wound heat exchanger of any of Aspects 1-9, wherein the plurality of zones comprise an innermost zone and an outermost zone, wherein at least one of the innermost zone and the outermost zone each contains between 10 and 20 percent of the plurality of tubes.
Aspect 11: The coil-wound heat exchanger of any of Aspects 1-10, wherein the plurality of zones comprise an innermost zone and an outermost zone, wherein at least one of the innermost zone and the outermost zone each contains less than 10 percent of the plurality of tubes.
Aspect 12: A method of making a coil-wound heat exchanger, the method comprising:
(a) forming a warm bundle having a warm end and a cold end by winding a plurality of tubes around a mandrel to form a plurality of tube layers, the plurality of tube layers being divided among a plurality of zones, the plurality of zones being concentrically arranged throughout the warm bundle;
(b) providing a shell that defines a shell space between the shell and the mandrel;
(c) connecting each of a first group of tube sheets to a first subset of the plurality of tubes, each first subset comprising tubes located in a plurality of zones, the first group of tube sheets located at one selected from the group of the warm end and the cold end of the warm bundle;
(d) connecting each of a second group of tube sheets to a second subset of the plurality of tubes, each of the second subset comprising tubes located in one zone of the plurality of zones, the second group of tube sheets located at a different one selected from the group of the warm end and the cold end of the warm bundle than the first group of tube sheets; and
(e) providing a valve in downstream fluid flow communication with each of the second group of tube sheets.
Aspect 13: The method of Aspect 12, further comprising:
(f) forming a cold bundle within the shell space, the cold bundle being in fluid flow communication with at least some of the plurality of tubes.
Aspect 14: The method of any of Aspects 12-13, further comprising:
(g) placing a temperature sensor in each of the plurality of zones.
Aspect 15: The method of any of Aspects 12-14, further comprising:
(h) placing a temperature sensor in each of the plurality of zones within a middle 50% of a warm bundle height, the warm bundle height extending from the warm and of the warm bundle to the cold end of the warm bundle.
Aspect 16: The method of any of Aspects 12-15, further comprising:
placing a temperature sensor in each of the plurality of zones within a middle 20% of a warm bundle height, the warm bundle height extending from the cold end to the warm end.
Aspect 17: A system for liquefying a feed gas, the system comprising:
a coil-wound heat exchanger comprising a warm bundle, a shell, and a shell space contained within the shell, the warm bundle comprising:
a feed circuit having a feed stream conduit, a plurality of warm end tube sheets located at the warm end, a plurality of cold end feed tube sheets located at the cold end, and a product conduit, the plurality of warm end feed tube sheets and the plurality of cold end feed tube sheets being in fluid flow communication with a first group of the plurality of tubes, the feed stream conduit, the plurality of warm end feed tube sheets, the plurality of cold end feed tube sheets, and the product conduit all being in fluid flow communication;
a refrigerant circuit comprising a closed loop, the at least one refrigerant circuit comprising:
wherein each tube sheet of a first selected from the group of the warm end feed tube sheets and cold end feed tube sheets is in fluid flow communication with only one of the plurality of tube sets and each tube sheet of a second selected from the group of the warm end feed tube sheets and cold end feed tube sheets is in fluid flow communication with more than one of the plurality of tube sets.
Aspect 18: The coil-wound heat exchanger of Aspect 17, further comprising a temperature sensor located in each of the plurality of zones.
Aspect 19: The coil-wound heat exchanger of Aspect 18, wherein the warm bundle has a bundle height extending from the cold bundle end to the warm bundle end and each of the temperature sensors is located within a middle 50% of the bundle height.
Aspect 20: The coil-wound heat exchanger of Aspect 18, wherein the warm bundle has a bundle height extending from the cold bundle end to the warm bundle end and each of the temperature sensors is located within a middle 20% of the bundle height.
Aspect 21: A method of operating the coil-wound heat exchanger of any of Aspects 1-20, the method comprising:
(a) measuring a zone temperature in each of the plurality of zones; and
(b) reducing a difference between the zone temperatures of two zones of the plurality of zones by adjusting a position of at least one of the plurality of valves.
The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the invention. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention.
In order to aid in describing the invention, directional terms may be used in the specification and claims to describe portions of the present invention (e.g., upper, lower, left, right, etc.). These directional terms are merely intended to assist in describing and claiming the invention and are not intended to limit the invention in any way. In addition, reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features.
In the claims, letters are used to identify claimed steps (e.g. (a), (b), and (c)). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims.
Directional terms may be used in the specification and claims to describe portions of the present invention (e.g., upper, lower, left, right, etc.). These directional terms are merely intended to assist in describing exemplary embodiments and are not intended to limit the scope of the claimed invention. As used herein, the term “upstream” is intended to mean in a direction that is opposite the direction of flow of a fluid in a conduit from a point of reference. Similarly, the term “downstream” is intended to mean in a direction that is the same as the direction of flow of a fluid in a conduit from a point of reference.
The term “fluid flow communication,” as used in the specification and claims, refers to the nature of connectivity between two or more components that enables liquids, vapors, and/or two-phase mixtures to be transported between the components in a controlled fashion (i.e., without leakage) either directly or indirectly. Coupling two or more components such that they are in fluid flow communication with each other can involve any suitable method known in the art, such as with the use of welds, flanged conduits, gaskets, and bolts. Two or more components may also be coupled together via other components of the system that may separate them, for example, valves, gates, or other devices that may selectively restrict or direct fluid flow.
The term “conduit,” as used in the specification and claims, refers to one or more structures through which fluids can be transported between two or more components of a system. For example, conduits can include pipes, ducts, passageways, and combinations thereof that transport liquids, vapors, and/or gases.
The term “circuit”, as used in the specification and claims, is intended to refer to a group of conduits and other equipment through which a particular fluid flows. In an open circuit, all of the fluid that enters the circuit at an upstream end will also exit the circuit at a downstream end, allowing for losses due to leakage. In closed circuit, all of the fluid in the circuit (again allowing for losses due to leakage) circulates a closed loop, through a group of conduits and other equipment.
The subcooled MRL stream 117 is reduced in pressure to form the expanded MRL stream 118, while the cooled feed stream 116 and cooled MRV stream 119 are further cooled to around −150° C. in the cold bundle 113 of the CWHE 114 to form a product stream 120, comprising liquid natural gas (“LNG”), and a subcooled liquid MRV stream 122 which is reduced in pressure and sent to the shell side of the cold bundle 113 where it is vaporized to provide refrigeration.
A vaporized mixed refrigerant stream 124 exits the shell side of the CWHE 114 at the warm end 174, is compressed to 40-70 bar, then cooled to form the mixed refrigerant stream 102, thereby completing the refrigeration loop.
It should be understood that the natural gas liquefaction system 100 shown in
In each of the subsequent embodiments disclosed herein, elements shared with the first embodiment (system 100) are represented by reference numerals increased by factors of 100. For example, the warm bundle 112 shown in
At the warm end 274 of the warm bundle 212, the pre-cooled feed stream 206 is split into multiple sub streams 225, 227, which feed warm end tube sheets 226, 228 respectively. The tube sheets 226, 228 each feed multiple process tubes 229a-c, 231a-c, respectively. Tube sheets are, in essence, manifolds that distribute fluid flow from a sub-stream 225, 227 into the process tubes 229a-c, 231a-c, which are wound around the mandrel 230 to form the warm bundle 212.
Although two tube sheets 226, 228 are shown in this example, any number of tube sheets could be used, depending on the number of process tubes in the circuit. Similarly, in the interest of simplifying the drawings, only three exemplary process tubes 229a-c, 231a-c are shown as being in fluid flow communication with each of the tube sheets 226, 228. For a typical LNG application, a tube bundle (meaning all of the process tubes in a section of a coil-wound heat exchanger) typically has thousands of tubes wound in 50-120 concentric tube layers wound around the mandrel 230, with layers being separated by axial spacers (not shown). A typical tube bundle has a diameter from 2-5 m and a length of 5-20 m.
At the cold end of the warm bundle 212, the process tubes 229a-c, 231a-c are consolidated into cold end tube sheets 232 and 234 with the cooled fluid being combined into the cooled feed stream 216. In order to show where each exemplary process tube 229a-c, 231a-c enters and exits the warm bundle 212, each is labeled at the warm end 274 and the cold end 276 of the warm bundle 212.
Having all of the process tubes for each tube sheet enter and exit each bundle in a single pie-shaped sector that is adjacent to the tube sheets enables the portions of the process tubes that connect the bundle to the tube sheet to be relatively short and enables the avoidance of process tubes crossing over one another. Accordingly, this configuration is preferred in many conventional implementations because it simplifies manufacture of the CWHE.
Portions of the warm bundle 212 not occupied by process tubes through which the pre-cooled feed stream 206 flows are occupied by tubes through which the MRV stream (not shown) or the MRL stream (not shown) flow. Such tubes typically have their own tube sheets. In the interest of simplifying the drawings, tubes and tube sheets for the MRV stream or the MRL stream are omitted.
This configuration results in fluid remaining separate throughout the process. For example, all of the fluid entering the warm bundle 312 through sub stream 344 exits the warm bundle through sub stream 356. In other words, each of the warm end tube sheets 326, 328, 333 is in fluid flow communication with only one of the cold end tube sheets 334, 332, 335.
The configuration of
This solution to the radial maldistribution problem has several drawbacks. Firstly, more tube sheets may be required to provide a tube sheet for each zone than would be required based purely on the number of tubes in the bundle. In addition, this solution requires additional valves to be positioned at the warm end of the warm bundle.
At the cold end 476, the process tubes 429a-c,431a-c are routed from the warm bundle 412 to the cold end tube sheets 432,434,435 so that each of the cold end tube sheets 432,434,435 is in fluid flow communication process tubes from a single zone. For example, each of the process tubes 429a,431a from the outer zone 454 terminate at cold end tube sheet 434. A control valve 462, 464 and 466 is located on each of the sub streams 460, 458, 456 at the cold end 476 of the warm bundle 412.
A temperature sensor 468, 470, 472 is provided in each of the zones 450, 452, 454 in the shell space of the warm bundle 412. The temperature sensors 468, 470, 472 are preferably located within the warm bundle 412 at an intermediate location, preferably within the middle 50% (more preferably within the middle 20%) of the height of the warm bundle 412. Alternatively, the temperature sensors 468, 470, 472 could be located at the cold end 476. An intermediate location is preferred because cold end temperatures may not always reflect radial maldistribution.
In the event that a temperature difference is detected between the temperature sensors 468, 470, 472, flow to the appropriate zone 450, 452, 454 can be adjusted using the control valve 462, 464 and 466 in a manner designed to reduce the temperature differential. For example, if the temperature sensor 472 reads significantly lower than temperature sensor 470, the temperature differential can be reduced by either incrementally opening control valve 466 or incrementally closing control valves 462, 464. Monitoring of the temperature sensors 468, 470, 472, and operation of the control valves 462, 464 and 466 can either be executed manually or with a controller (not shown). It is desirable that the control valves 462, 464 and 466 all be as open as possible, in order to maximize flow capacity of the system. Accordingly, if no radial maldistribution is detected, all of the control valves 462, 464 and 466 will normally be fully open. When radial maldistribution is detected, at least one of the control valves 462, 464 and 466 will normally be fully open.
While temperature measurements of the outlet sub streams 456, 458 and 460 could be used to guide the manipulation of the valves as in the prior art, using internal bundle temperatures (i.e., in the shell space) is preferable. Depending on the current operation, temperatures of the sub streams at the cold end may be very similar despite significant radial temperature gradients in the shell space at an intermediate location along the height of the warm bundle. For example, if the CWHE is operated with a high shell side refrigerant flow rate relative to the tube side flow rates, the exchanger may be “pinched” at the cold end, meaning the temperature difference between the shell side fluid and the tube side fluids are very small and the temperature difference between outlet sub streams also very small.
The configuration of
The exemplary embodiment shown in
It should be noted that the number of zones and relative size of each zone shown in
The preferred number of zones may also depend on the number of tubes in the circuit that is being divided. The number of tubes may dictate the minimum number of tube sheets, for example if three tube sheets are required it may be convenient to divide the exchanger into three zones, even if only two are needed to mitigate the expected maldistribution.
It should also be noted
Radial temperature gradients may indicate that there is a mismatch between the radial distribution of shell side refrigerant and radial distribution of tube side heat load. The invention allows the radial distribution of tubeside flow and therefore heat load to be adjusted to better match the radial distribution of shellside refrigerant, resulting in reduction of the radial temperature gradient.
It is preferable that at least one of the circuits have the cold and warm end tube sheet configuration of one of the embodiments of
As such, an invention has been disclosed in terms of preferred embodiments and alternate embodiments thereof. Of course, various changes, modifications, and alterations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention only be limited by the terms of the appended claims.
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