BR112014016949B1 - heat exchanger, and method for repairing, inspecting, cleaning or improving a heat exchanger - Google Patents

Abstract

HEAT EXCHANGE, E, METHODS FOR CLEANING OR REPAIRING A HEAT EXCHANGER AND FOR REPAIRING, INSPECTING, CLEANING OR IMPROVING A HEAT EXCHANGE. A modular plate and shell heat exchanger in which welded pairs of heat transfer plates are spaced in line (tandem) and coupled in parallel between an inlet and outlet conduit to form a heat transfer assembly. The heat transfer assembly is placed in the hull to transfer heat from a secondary fluid to a primary fluid. Modules of one or more of the heat transfer plates are detachably connected using gaskets in the inlet and outlet conduits that connect to a primary fluid inlet and a primary fluid outlet nozzle. The heat transfer assembly is supported by a frame that rests on an inner rail that attaches to the hull and facilitates removal of the heat transfer plates. The shell and plate heat exchanger has a removable head integral with the shell for removal of the heat transfer assembly for inspection, maintenance and replacement.

Description

FOUNDATION 1. Field

[0001] The present invention relates generally to heat exchangers and more particularly to modularization for stacked plate heat exchangers.

2. Description of the related technique

[0002] Feed water for steam generators in nuclear power plants is typically preheated before being introduced into the secondary side of the steam generators. Similarly, feedwater is preheated before being introduced into boilers for non-nuclear power plant applications. Feedwater heat exchangers are typically used for this purpose. Conventionally, heat exchanger designs are divided into two generic classes: heat exchangers with a plate structure and those with a shell and tube structure. The main difference in the two classes with regard to both construction and heat transfer is that the heat transfer surfaces are mainly plates on one structure and tubes on the other.

[0003] The plate and shell heat exchanger in numerous feedwater heater applications employs a tubular horizontal or vertical shell that has hemispherical or flat ends. The interior of the horizontal hull is divided into sections by a tube plate that is normal to the axis of the hull. More specifically, at one end of the hull a water chamber section is defined on one side of the tube sheet which includes a water inlet chamber which has a water inlet opening and a water outlet chamber which has a water inlet opening. outflow of water. In a U-tube shell and tube heat exchanger, the plurality of heat transfer tubes are bent at their intermediate portions into a U-shape and extend from the other side of the tube sheet along the axis of the tube. hull. These tubes are fixed to the tube sheet at both ends such that one end of each of the tubes opens into the water inlet chamber while the other end opens into the water outlet chamber. Another type of shell and tube heat exchanger employs straight tubes with an inlet chamber and an outlet chamber, respectively, at opposite ends of the tubes. The heat transfer tubes are supported by a plurality of tube support plates spaced at an appropriate pitch in the longitudinal direction of the tubes. A steam inlet opening and a drain inlet and outlet are formed in the hull where the tubes extend.

[0004] In operation, the feedwater coming into the feedwater heater from the water inlet chamber flows through the U-shaped heat transfer tubes and absorbs heat from the heating steam coming into the feed water heater from the steam inlet opening to condense the steam. Condensate is collected at the bottom of the hull and discharged to the outside through a drain at the bottom of the hull. Thanks to the cylindrical shape of the hull and heat exchange tubes, the structure is well suited as a pressure vessel, and thus tube and shell heat exchangers have been used in extremely high pressure applications.

[0005] The most significant disadvantage of tube and shell heat exchangers is their weight, heavy when compared to the surface area of the heat transfer surfaces. Due to this, tube and shell heat exchangers are usually large in dimension. It is also difficult to design and manufacture tube and shell heat exchangers when heat transfer, flow characteristics, and expense are taken into account.

[0006] A typical plate heat exchanger is composed of ribbed or grooved rectangular plates that are pressed together by means of end plates which, in turn, are fastened to the ends of the plate stack by means of tie rods or screws of tension. The spaces between the plates are closed and sealed with seals with bands on their outer circumference, and seals are also used in the flow channels. Since the bearing capacity of thin plates is poor, they are reinforced with grooves that are usually arranged transversely in adjacent plates, in which they also improve the pressure resistance of the structure when the crests of the grooves are supported by each other . However, the most important aspect is the importance of the grooves for heat transfer; the shapes of the grooves and their angle with respect to the flow affect heat transfer and pressure losses. In a conventional plate heat exchanger, a heat supply medium flows into every other space between the plates and a heat receiving medium flows into the remaining spaces. In alternating pairs of plates the flow is conducted between the plates through holes located in the vicinity of the corners of the plates. Each space between plates in alternating pairs of plates always contains two holes with closed edges and two other holes that act as input and output channels for the space between the plates. Plate heat exchangers are usually constructed of relatively thin plates when a small, lightweight structure is desired. Since the plates can be profiled to any desired shape it is possible to make the heat transfer properties suitable for almost any type of application. The ultimate weakness in conventional plate heat exchangers is the seals that limit the pressure and temperature resistance of the heat exchangers. In several cases the seals have hindered the possibility of being used as a corrosive medium for supplying heat or receiving heat.

[0007] Attempts have been made to improve the construction of the plate heat exchanger by deleting all seals and replacing them with brazed joints or brazed seams. Plate heat exchangers manufactured by brazing or soldering usually resemble those equipped with seals. The most significant external difference is the absence of tension screws between the ends. However, the brazed or brazed structure makes it difficult, if not impossible, to disassemble without destroying such heat exchangers for cleaning.

[0008] Attempts have been made to combine the advantages of the shell and tube heat exchanger and the plate heat exchanger in heat exchangers, whose construction partially resembles both of these basic types. One such solution is disclosed in US Patent 5,088,552, in which circular or polygonal plates are stacked on top of one another to form a stack of plates which is supported by means of end plates. The stack of plates is surrounded by a hull whose sides are provided with inlet and outlet channels for corresponding flows of heat-supplying and heat-receiving media. Differing from the conventional plate heat exchanger, all fluid flows into the spaces between the plates are directed from outside the plates. When the heat exchanger according to the publication is closed by welding, it is possible to achieve the same pressures as when using a shell and tube heat exchanger, with the heat transfer properties of a plate heat exchanger.

[0009] The International Publication WO 91/09262 aims to present an improvement on the preceding Publication, which more prominently presents typical aspects of both plate exchangers and tube and shell heat exchangers. The circular plates are brought together in pairs by welding them together through the edges of holes that form an inlet and outlet channel. By welding the pairs of plates manufactured in the above manner together across the outer perimeters of the plates, a closed circuit is achieved for flowing a heat transfer medium. Differing from the conventional plate heat exchanger, this structure is welded and there are only two holes in the plates. Flow from another heat transfer medium is directed to each other space between the plates by means of a hull that surrounds the stack of plates. To prevent runoff from occurring between the slab stack and the hull, seals are used, which are primarily used as deflectors for runoff. Obviously, pressure resistance is not required of the deflectors. Due to the structure of the slab stack it is difficult to implement the seals. Elastic rubber gaskets are suggested for the seals so that it is possible to disassemble the heat exchanger, eg for cleaning purposes.

[00010] The shell and tube heat exchanger currently used in nuclear power plants has a common design flaw, that when tube degradation occurs, in an effort to minimize leakage, the only option is to plug the damaged tube, resulting in a loss of thermal efficiency. Loss of thermal yield in the feedwater system is costly for nuclear power plants and eventually requires replacement of the shell and tube feedwater heater. Another limitation of pipe hull design is that hull-side inspection is typically limited to small manholes and manholes and, as a result, corrosion and erosion damage is difficult to detect. Significant corrosion/erosion has been sustained by internal baffles which can lead to: (1) flow contouring and thermal performance degradation, and (2) tube wear due to flow-induced vibration. Significant corrosion/erosion has also been observed on the inner hull surface of the hull and tube feedwater heater design.

[00011] Therefore, a new feedwater heater design is desired for long-term sustainable thermal load, and for long-term improved component integrity over current shell and tube feedwater heater design. Preferably, long-term sustainable thermal loading will be achieved by replacing or repairing the heat transfer surfaces as needed, rather than requiring the heat transfer surface to be taken out of service. Additionally, it is desired to be able to increase the heat transfer capacity of the feedwater heater to accommodate power plant re-ratings (improvements) without replacing the entire feedwater heater.

SUMMARY

[00012] The foregoing objectives are achieved by means of a plate and hull modular feedwater heater, in which brazed heat transfer plate pairs are placed in a hull to transfer heat from the steam drain flow of extraction for feedwater in a nuclear power plant. The heat transfer plate pairs or groupings soldered or otherwise bonded of heat transfer plate pairs, i.e. modules of heat transfer plate pairs are arranged in tandem, and at least some of the modules are connected using gaskets, and share in parallel a common inlet conduit and outlet conduit which are respectively connected to feedwater inlet and outlet nozzles. The inlet and outlet conduits of heat transfer plate pairs form a heat transfer assembly that is preferably supported by a structure that rests on, and is movable along, an inner rail attached to the interior of the hull, which facilitates removal of heat transfer plates from the hull. The shell and plate modular feed water heater has a removable head integral with the shell for removing heat transfer plates for inspection, repair or replacement. Preferably, the entry and exit locations are sealed up to and extend through the removable head.

[00013] Preferably the heat exchanger provided herein includes a means to increase the heat exchange capacity of the unit over time to accommodate improvements to the plant in which the heat exchanger is installed. In one embodiment the inlet and outlet conduits include a number of additional connection points for pairs of heat transfer plates which are initially buffered. In another embodiment the inlet and outlet conduits can be expanded by connecting additional pairs of heat transfer plate or modules. In the latter mode the heat exchanger may initially be provided with a spacer module which has a relatively negligible heat transfer capacity, which is supported in tandem with the heat transfer plate modules. A plate heat transfer module can later be replaced by the spacer module to increase the heat transfer capacity of the heat exchanger. Desirably, at least some of the couplings between the heat transfer plate pairs or modules or bonded pairs of heat transfer plates are detachable for ease of repair and replacement. Preferably, tie rods connect the modules and, in the embodiment where the inlet and outlet conduits extend between modules, the tie rods provide compressive force to pressure seals at the interface of conduit segments of the modules which interface to form a tight seal.

[00014] Preferably the heat transfer assembly is removed from the hull with the removable head. Alternatively, a manhole inlet is provided in the hull to provide access to the interior of the hull, to disconnect the feedwater inlet nozzle from the feedwater inlet conduit and to disconnect the feedwater outlet conduit from the nozzle. feed water outlet, or both options can be provided.

[00015] Desirably, the modules have support panels at each end, between which the tie rods extend. The heat transfer plate pairs are sandwiched between the support panels and, in one embodiment, the primary fluid inlet conduit and the primary fluid outlet conduit pass through the modules. Preferably, the support panels are thicker than the heat transfer plates. In one embodiment the heat transfer plates between the support panels are welded together and to the support panels, and adjacent support panels are mechanically connected to each other.

[00016] The invention also provides a method of cleaning or repairing the feedwater heater, which includes the steps of: accessing the interior of the pressure vessel hull; removing at least one pair of heat transfer plates from the heat transfer plate heat transfer assembly; clean, repair or replace the removed pair of heat transfer plates and reconnect the cleaned, repaired or replaced pair of heat transfer plates to the heat transfer assembly. Preferably the step of accessing the interior of the pressure vessel hull includes removing the detachable head, and the step of removing at least one pair of heat transfer plates comprises removing the one pair of heat transfer plates from the inlet conduit. feed water and the feed water outlet conduit.

[00017] The invention further includes a method of repairing, inspecting, cleaning or improving the feedwater heater, in which the pressure vessel has a detachable head. The method comprises the steps of: removing the detachable head, or otherwise accessing the interior of the pressure vessel hull; and disconnecting the feedwater inlet conduit and the feedwater outlet conduit from the feedwater inlet nozzle and the feedwater outlet nozzle, respectively, while the heat transfer assembly is in the pressure vessel . This method further includes the step of replacing a defective pair of heat transfer plates, as well as the step of increasing the number of pairs of heat transfer plates after the feedwater heater has been put into service to improve the feed water heater.

BRIEF DESCRIPTION OF THE DRAWINGS

[00018] Another understanding of the invention can be obtained from the following description of preferred embodiments when read in conjunction with the accompanying drawings, in which:

[00019] Figure 1 is an elevation view of the feedwater heater of an embodiment of this invention;

[00020] Figure 2 is a top view of the feedwater heater shown in Figure 1;

[00021] Figure 3 is a perspective view of another embodiment of the feedwater heater of this invention, with the heat transfer assembly separated into modules and partially removed from the hull;

[00022] Figure 4 is a perspective view of one of the end modules of pairs of heat transfer plates of the modality shown in Figure 3;

[00023] Figure 5 is a perspective view with a portion cut away and removed from the heat transfer assembly partially shown in Figures 3 and 4;

[00024] Figure 6 is a schematic of the primary fluid flow through the feedwater heater modality illustrated in Figures 3 to 5;

[00025] Figure 7 is a side view of a pair of heat transfer plates;

[00026] Figure 8 is a schematic view of one embodiment of a heat transfer plate module described herein below;

[00027] Figure 9 is a schematic view of a second embodiment of a heat transfer plate module described herein below;

[00028] Figure 10 is a sectional view of a spacer module described here below; and

[00029] Figure 11 is a partially sectioned side view of a tie rod segment that can be employed to couple two heat transfer plate modules.

DESCRIPTION OF THE PREFERRED MODALITY

[00030] Current feedwater heater designs employed in nuclear power plants utilize a shell and tube heat exchanger arrangement. Another generic type of heat exchanger that has been around since 1923 is the skeleton plate heat exchanger. The latter is characterized by compact design, high heat transfer coefficients, high fluid pressure drop within the plates, and is generally limited to low pressure fluids. The modalities described here provide a plate and shell feedwater heater that combines and optimizes the aspects of a plate and skeleton heat exchanger and the traditional shell and tube heat exchanger that is conveniently serviceable and serviceable. easily changed in a relatively cost-effective way to increase its heat transfer capability where desired.

[00031] An embodiment of the feed water heater 10 of the claimed inventions hereinafter is illustrated in the elevation view shown in Figure 1 and in the top view shown in Figure 2. Two heat transfer plates 12 and 14 are welded together to form a pair of brazed plates 16 which form between them a flow path for feedwater fluid as in a traditional plate heat exchanger. In one embodiment the pair of heat transfer plates 16 is detachably connected, such as with gaskets 18 and bolted flange gaskets 20, and in direct communication with an inlet head tube 22 at one end of the pair of transfer plates of brazed heat transfer plates 16 and an output head tube 24 at the other end of the pair of brazed heat transfer plates 16. A number of these pairs of brazed heat transfer plates 16 are stacked in a spaced tandem arrangement, each coupled between the input head and output head, to form a heat transfer assembly that has a parallel flow path. Such an arrangement is shown in Figure 2. Alternatively, it should be appreciated that a number of pairs of heat transfer plates 16 can be coupled in series with the ends of the series arrangement releasably secured in a manner similar to the head tube. inlet 22 and to the outlet header tube 24. In either mode, the terminal ends of the heat transfer plate pairs 16 are connected, either directly or indirectly, to the inlet header tube 22 and the output header tube 24. The tube inlet head 22 and outlet head tube 24 are respectively connected to a feed water inlet and a feed water outlet nozzle 26 and 28, preferably using a bolted closure with gaskets, in a manner similar to that described for releasably secure the pair of heat transfer plates 16 to the inlet and outlet head tubes 22 and 24, although it should be appreciated that other connection means Removable can be used.

[00032] In the modality shown in Figures 1 and 2, the head tubes 22 and 24 are supported by a skeleton structure 30 that rests on an inner rail 32 attached to the lower portion of the cylindrical hull 34 that forms a pressure vessel that surrounds the assembly heat transfer plate 36. Rail 32 and wheels 33 on skeleton frame 30 facilitate removal of the heat transfer plate assembly from the hull for repair, cleaning or upgrading. In one embodiment the hull has an integral hemispherical end 38 on one side and a removable hemispherical head 40 on the other side, to completely enclose and seal the heat transfer assembly 36 within the pressure vessel formed by the cylindrical hull 34, hemispherical end 38 , and removable head 40. However, it should be appreciated that the ends need not be hemispherical to take advantage of this invention, although hemispherical ends are preferable for high pressure applications. Removable head 40 has feedwater inlet nozzle 26 and feedwater outlet nozzle 28 extending therethrough, as shown in Figures 1 and 2. Alternatively, hemispherical end 38 can be constructed to be removable to the head. Instead of the head 40, or pumps can be connected by bolted flange connections to the hull 34 for added flexibility in gaining access to the interior of the hull 34 to hold the heat transfer plate assembly 36. The hull 34 is also equipped with an inlet of extraction steam 42, drain inlets 44 and 46 and drain outlets 48 and 50.

[00033] During operation, the introduced feed water passes through the inlet nozzle 26, the inlet head tube 22, the pairs of welded heat transfer plates 16 where it is heated by the drain flow and extraction steam, of the outlet head tube 24 and outlet nozzle 28. The extraction steam entering the feedwater heater through the extraction steam inlet 42 is distributed by the steam sacrifice plate 52 and passes through the upper hull region where it mixes with the drain flow entering from the drain flow inlet nozzles 44 and 46. The extraction steam and the drain flow then pass between the welded pairs of heat transfer plate 16 where it is cooled by the feed water and condenses to the lower hull region where it exits through drain flow outlet nozzles 48 and 50.

[00034] During a plant shutdown, an inspection of the heat transfer plates and inner surface of the hull can be performed using the following steps. First, the hull end 38 is bolted to the flange 54 and removed. Head tubes 22 and 24 can then be disconnected from inlet and outlet nozzles 26 and 28. A manhole 56 on head 40 can be used to gain access to the connection between inlet and outlet head tubes 22 and 24 of the head nozzles. inlet and outlet 26 and 28. Alternatively, when head 40 is removed on flange 58 head 40 can be moved outwardly with heat transfer assembly 36 sliding onto rail 32 so that access can be gained for the connection. between inlet and outlet heads 22 and 24 and feedwater inlet and outlet nozzles 26 and 28. Spool tubing (not shown) will need to be removed from inlet and outlet nozzles 26 and 28 before moving head 40. Then the heat transfer plate assembly 36 can be moved as a unit along the tracks 32 located at the bottom of the hull 34, to a point where the individual heat transfer plates 12 and 14, and the interior of the hull 34 can be inspected how much to damage. The individual heat transfer plate pairs 16 can then be cleaned or, if necessary, repaired or replaced. If repair or replacement is necessary, the pair of heat transfer plates 16 that needs attention can be bolted to the inlet header tube 22 and the output header tube 24 and replaced with a new or repaired pair of heat transfer plates 16 screwed into place. The outlet head tube 24 and the inlet head tube 22 are also provided with one or more additional openings 60, which are initially sealed by plugs. These additional openings may be unsealed to accommodate additional heat transfer plate pairs 16 if future improvement is desirable.

[00035] Removable plate design allows heat transfer surface replacement and mass production of heat transfer plates and gaskets results in relatively low cost for critical spares. Employing this design makes it possible to increase the number of plates and thus the heat transfer area to accommodate energy improvements and provides improved inspection of the hull side.

[00036] Although specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to these details could be developed in light of the overall teachings of the disclosure. For example, although separate inlet and outlet header tubes, or conduits, are shown in the embodiment illustrated in Figures 1 and 2, any other structure that performs its described function may also be used without departing from the spirit of this invention. For example, the embodiment of heat transfer assembly 36 shown in Figures 3, 4 and 5 shows segments of inlet and outlet conduits 22 and 24 as integral parts of the pairs of heat transfer plates 16. In Figures 3, 4 and 5 components corresponding to those shown in Figures 1 and 2 are given equal reference characters. The heat transfer plate assembly 36 in the embodiment shown in Figures 3, 4 and 5 is formed of a number of heat transfer plate modules 17, four such heat transfer plate modules are visible in Figure 5. Each such module 17 is formed from a number of pairs of tandemly spaced heat transfer plates 16 which are connected together as an integral unit. Each of the modules 17 shown in Figures 3, 4 and 5 has approximately 10 such pairs of heat transfer plates, although it should be appreciated that any number of such pairs of heat transfer plates 16 can be used, with the consequence that the more pairs of heat transfer plates 16 for a module 17 the more expensive the module will be to replace. Alternatively, the more modules there are, the more will be spent on gaskets and closure equipment. An optimal range of the number of boards per module should be determined on a specific application basis based on economic considerations. Also the number of modules 17 in heat transfer assembly 36 can vary depending on the number of pairs of heat transfer plates 16 per module, and the heat transfer requirements of the application in which the heat exchanger will be employed.

[00037] In the embodiment shown in Figures 3, 4 and 5, the outer surface, i.e. the front and rear of each pair of heat transfer plates 16 has two openings on each side, with corresponding openings substantially aligned with one with another, and to which incremental segments 23 of inlet and outlet conduits 22 and 24 are connected, such as by soldering, brazing or any other suitable connection that forms a substantially rigid durable gasket that is substantially impermeable to fluids flowing in the and around inlet and outlet conduits 22 and 24 in the area between pairs of heat transfer plates 16. The incremental segments of inlet and outlet conduits 22 and 24 passing between pairs of heat transfer plates 16 and the outer surface of adjacent pairs of heat transfer plates 16 provide a flow path between pairs of heat transfer plates 16 for extraction steam and flow. drain passes. The outer end of segments 23 of input and output conduits 22 and 24 formed through each module 17 preferably has a flange onto which the corresponding flange of an adjacent heat transfer plate module segment 23 can be connected; preferably with a gasket pressed between the flanges. Outer segments 23 in each module 17 can then be secured to a corresponding segment 23 on the outer side of an adjacent module with a gasket in between using tie rods 64 shown in Figures 3, 4 and 5, although other forms of mechanical attachment may be used in place of the tie rods. In the embodiment shown in Figures 3,4 and 5 the modules 17 are held in position by front and rear face structures or plates 62 which are brought together by tie rods 64. The face plate 62 in front of the heat transfer plate assembly has openings for the inlet and outlet conduits 22 and 24 so that the flanges on the outer segments 23 can be respectively attached to the inlet and outlet nozzles 26 and 28 shown in Figure 2. The outer segments 23, i.e. both inlet and heat outlet 36 are plugged to close the feed water drain loop or the rear heat transfer plate is made without the inlet and outlet holes.

[00038] A schematic of the primary fluid flow through the heat transfer plate assembly of the modalities described above that has a parallel flow path through the pairs of heat transfer plates 16 is illustrated in Figure 6. Figure 7 shows the construction of heat transfer plate pairs. As shown in Figure 7, a weld bead 66 extends around each of the incremental segments 23 of the inlet conduit 22 at corresponding openings in the heat transfer plates 12 and 14 and forms a fluid-tight seal at the interface. Similarly, a weld bead 68 extends around the incremental segments 23 of the outlet conduit 24 in corresponding openings in the heat transfer plates 12 and 14 and forms a fluid-tight seal at the interface. In addition, a waist weld 70 extends around the entire circumference of the pair of heat transfer plates 16. As shown in Figure 7, the primary fluid penetrates into the inlet 72 of the inlet conduit 22 of each pair of plates. of heat transfer 16 connecting it to adjacent pairs or support plates. A portion of the fluid flows down between the heat transfer plates 12, 14 where it absorbs heat from the extraction steam and drain flow passing over the outside of the heat transfer plate pairs, and exits at the outlet 78 to outlet conduit 24 where it joins with the upstream flow of primary fluid from other pairs of heat transfer plates that has entered through the inlet of outlet conduit 76 to pair of heat transfer plates 16. In except for the last pair of heat transfer plates 16 at end 80 (Figure 5) of heat transfer plate assembly 36, the remainder of primary fluid entering inlet 72 does not flow between heat transfer plates 12 and 14 of a given pair of heat transfer plates 16 exits through the inlet conduit outlet 74 to the next pair of heat transfer plates 16. All of the primary fluid passing through the inlet conduit to the ex The tremor 80 of heat transfer plate assembly 36 is conducted through the last pair of heat transfer plates 12 and 14 where it exits through outlet conduit 24 as shown in Figure 6. It is irrelevant whether water flows upward as shown in Figure 6 or down as described here, or sideways through the pairs of heat transfer plates 16, as long as the flow extends from the inlet conduit 22 to the outlet conduit 24.

[00039] Figure 8 is a schematic of one embodiment of a heat transfer plate module 17. The module 17 is shown with four pairs of heat transfer plate 16, although as previously described, the number of plate pairs 16 heat transfer rate may vary. The heat transfer plate pairs 16 have relatively thin heat transfer plates 12 and 14 as compared to the outer support plates 82 which are thicker than the inner heat transfer plate pairs 16. The support plates 82 are referred to. as support plates and are longer than the others and extend after the others to accommodate the stays shown in Figures 3, 4 and 5, although it should be appreciated that this embodiment is slightly different than the embodiment shown in Figures 3, 4 and 5. However, the manner in which the modules are fastened together is the same, although it should be appreciated that other means of fastening the modules together, eg continuous threaded rods, screws, etc. could also be used. The inner heat transfer plates are soldered together with the incremental segments of conduit 23 shown in Figure 4 extending therebetween with the solder extending around the circular openings in the incremental segments of the inlet conduit 22 and outlet conduit 24 and the outer edges, by means of circumferential plate welds 70. Gasket grooves 84 are provided around inlet conduit 22 and outlet conduit 24 openings in support plates 82 for gaskets, to seal the openings at the interface with support plates corresponding 82 of adjacent modules 17.

[00040] A second embodiment of a heat transfer plate pair module 17 is shown in Figure 9. The embodiment shown in Figure 9 is very similar to that described above with respect to Figure 8, except that the heat transfer plates The outer casings have a gasket retaining ring 86 around the openings for the inlet conduit 22 and the outlet conduit 24. A single support plate is interposed between modules 17 and gaskets in the retaining rings 86 seal the openings 22 and 24 between each support plate and the heat transfer plates. Alternatively, grooves can be provided on one or both sides of the backing plates to retain the gaskets.

[00041] A spacer module 88 can be inserted in place of a heat transfer plate pair module 17 to preserve space for the latest addition of another heat transfer plate pair module 17 in the event of a future plant improvement in which the heat exchanger is installed requires additional heat transfer capability within the existing hull. One embodiment of such a spacer module 88 is illustrated in Figure 10. The spacer module 88 is preferably of the same size as the standard heat transfer plate pair module 17 for the heat exchanger unit 10 in which it is to be employed. The spacer module in this embodiment has two support plates 82 with gasket slots 84 as described above, which are separated by an upper support 96 and a lower support 98 with a secondary fluid drain 94. It should be appreciated that the upper support 96 and the support bottom 96 may, but need not, be part of a continuous support cylinder. The embodiment shown in Figure 10 is designed to be inserted between the heat transfer plate pair modules 17 in a tube 90 which is welded around its circumference at each support plate interface to form a hermetic seal. Tube 90 forms the portion of inlet conduit 22 that carries the primary fluid between the heat transfer plate pair modules 17 it connects. Similarly, a tube 92 is sealed to and covers the space between the support plates 82 of the spacer module 88 to carry primary fluid through the outlet conduit 24. If the spacer is used at the end of end 80 of the transfer plate assembly of heat 36, so the openings in the spacer module support plates 82 are unnecessary.

[00042] Figure 11 illustrates a modality of a tie rod arrangement that can be used to bring modules 17 and 88 together. Tie 64 is designed to extend between support plates 82 similar to the extensions between support structures 62 shown in Figure 5. In the embodiment shown in Figure 11, the links 64 have a reduced diameter end that has a circumferential thread 104. The circumferential thread 104 terminates in a support surface 106 that is sized to meet one side of a periphery of a module support plate around a hole, in which thread 104 is sized to extend through and out of the other side. The other end of tie rod 64 has an internal thread 100 that is sized to mate with an outer circumferential thread 84 in an adjacent tie rod 64 that is extended through a corresponding hole in an adjacent support plate 82. Preferably, the outer circumference 102 around the end of the tie rod which has the internal thread 100 has a square or hexagonal contour over which a torque can be easily applied.

[00043] As mentioned earlier, the heat transfer plate assembly 36 has wheels 33 that mount on the rail 32, described above, to facilitate maintenance of the heat transfer plate assembly. Maintenance is the same as described for the embodiment illustrated in Figures 1 and 2, except that to improve the heat transfer plate assembly, the spacer module 88 is removed and an additional heat transfer plate module 17 is attached to its place.

[00044] Additionally, although the preferred embodiment is described in an application for a feedwater heater, the invention can be employed with similar benefits in most other types of heat exchangers. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be provided shall receive the full scope of the appended claims and any of all their equivalents.

Claims (14)

1. A heat exchanger (10), comprising: an elongated pressure vessel hull (34) having an axial dimension with a removable closure (40) at one end of the axial dimension, a primary fluid inlet (26), a primary fluid outlet (28), a secondary fluid inlet (42, 44, 46), a drain outlet (48, 50) and a heat transfer assembly (36) comprising: a primary fluid inlet conduit (22) extending into the pressure vessel (34) from the primary fluid inlet (26); a primary fluid outlet conduit (24) extending into the pressure vessel (34) from the primary fluid outlet (28); a plurality of pairs of heat transfer plates (16) supported in tandem with each of the pairs of sealed plates (70) around the periphery to define a primary fluid flow channel between a first and second heat transfer plate. heat (12, 14) of each pair, with each pair having a heat transfer plate inlet opening (72) fluidly connected either directly or indirectly to the primary fluid inlet conduit (22) and an outlet opening of heat transfer plate (78) fluidly connected either directly or indirectly to the primary fluid outlet conduit to form a parallel flow path flowing in the same direction through each of the pairs of heat transfer plates in a direction orthogonal to the axial dimension of the pressure vessel hull; and means for expanding a heat transfer capacity of the heat transfer assembly relative to an original heat transfer capacity that the heat exchanger has when the heat exchanger is first put into service; and wherein the plurality of pairs of heat transfer plates (16) are arranged in modules (17) with at least one of the modules, including at least one of the pairs of heat transfer plates, connected in tandem with an adjacent module or the primary fluid inlet or the primary fluid outlet with a non-destructively removable mechanical coupling (84); characterized by the fact that the means for expanding the heat transfer capacity of the heat transfer assembly includes a spacer module which has less heat transfer capacity than the heat transfer plate pair modules, with the spacer module connected in tandem with the heat transfer plate pair modules, the spacer module (88) being at least as long in axial dimension as the heat transfer plate pair modules and having an inlet duct passing axially through the same, fluidly connected either directly or indirectly to the primary fluid inlet conduit and an outlet duct passing axially therethrough, fluidly connected either directly or indirectly to the primary fluid outlet conduit. 2. Heat exchanger (10) according to claim 1, characterized in that at least some of the modules (17) include a plurality of heat transfer plate pairs (16) with the heat transfer plate pairs within the at least some of the modules supported together with a tie rod (64). 3. Heat exchanger (10) according to claim 2, characterized in that at least some of the modules (17) connected in tandem with an adjacent module are connected by coupling their respective rods (64). 4. Heat exchanger (10) according to claim 1, characterized in that the heat transfer assembly (36) is slidable out of the hull of the pressure vessel (34) when the removable closure (40) is open . 5. Heat exchanger (10) according to claim 1, characterized in that the heat transfer assembly (36) is movably supported on a rail (32) attached to an interior of the pressure vessel (34) so that the heat transfer assembly can be removed as a unit from the pressure vessel through the one end (40) by moving the heat transfer assembly along the track. 6. Heat exchanger (10) according to claim 5, characterized in that the heat transfer assembly (36) is supported on the rail (32) on wheels (33) that mount on the rail. 7. Heat exchanger (10) according to claim 1, characterized in that the primary fluid inlet (26) and the primary fluid outlet (28) extend from the removable closure (40). 8. Heat exchanger (10) according to claim 1, characterized in that the heat transfer assembly (36) is equipped with a number of extra couplings (60) configured to connect additional pairs of heat transfer plates (16), the extra couplings are initially plugged and are available for further improvement of the heat transfer capacity of the heat exchanger after the heat exchanger has been put into operation relative to an original heat transfer capacity, unplugging by the minus some of the extra couplings and connecting a number of the additional pairs of heat transfer plates. 9. Heat exchanger (10) according to claim 1, characterized in that the hull of the pressure vessel (34) is a cylindrical shape with hemispherical ends (40, 38). 10. Heat exchanger (10) according to claim 1, characterized in that at least some of the modules (17) comprise a plurality of pairs of heat transfer plates (16) with each of the pairs of transfer plates inside a module connected together in the tandem matrix through a welded coupling (23). 11. Heat exchanger (10) according to claim 1, characterized in that at least some of the modules (17) have a support plate (82) at a first and a second end, with the heat transfer plates ( 12, 14) between them, wherein the support plates are thicker than the heat transfer plates. 12. Heat exchanger (10) according to claim 1, characterized in that the modules (17) are supported in tandem by tie rods (64). 13. Method for repairing, inspecting, cleaning or improving a heat exchanger (10) as defined in claim 1, characterized in that it comprises the steps of: accessing the interior of the pressure vessel hull (34); disconnecting the primary fluid inlet conduit (22) and the primary fluid outlet conduit (24) from the primary fluid inlet (26) and the primary fluid outlet (28), respectively; and including the step of replacing a defective pair of heat transfer plates (16). 14. Method according to claim 13, characterized in that it includes the step of increasing the number of pairs of heat transfer plates (16) within the heat transfer assembly (36) after the heat exchanger (10 ) has been put into operation to improve the heat exchanger.

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