FIELD OF THE INVENTION
The invention relates to design of a heat exchanger called Moisture separator and reheater whose function is to separate moisture & reheat expanded steam (called as cold reheat steam or CRH) from high pressure turbine, to a higher temperature before being admitted to Lower Pressure turbine of nuclear power plant. The hot steam from Moisture Separator is called hot reheat steam (or HRH). The moisture separation is done first and then the expanded steam (or CRH) is heated by getting extracted heat from live steam (extracted from steam generator) & bled steam (steam extracted from HP Turbine), the degree of re-heating or re-heat temperature is further raised (from 160 oC to 236 oC). The live and the bled steam are having sufficiently high degree of temperature i.e. temp 251 oC & 210 oC respectively. The heat exchanger design is such that the extracted steam flows through respective tube bundles of bled steam tubes (BST) & live steam tubes (LST) and cycle steam i.e. CRH flow over tube bundles of BST & LST through partition walls and a shell that enclose the bundle of tubes.
BACKGROUND OF THE INVENTION
Previously a design does exist for application which is closest to the application
for which the proposed invention is created.
The disadvantages associated with the past design are enlisted below:
Previously design was done manually treating each part separately.
Values of HTC, Pressure drop factor etc. were read manually.
Human intervention was more for such long hand calculations.
Error chances were there.
More iteration were not possible if done it would take a long time.

OBJECTS OF THE INVENTION
The objective of the invention was to develop the design in such a way that meets following objectives:
Heat exchanger to meet functional requirements like allowing maximum heat transfer between a fluid (live steam & bled steam as a heating fluid) flowing through tubes and other fluid (cold reheat from turbine exhaust as CRH) flowing over tubes.
Heat exchanger to be suitable for compact plat layout i.e. taking less ground space and having major portion in vertical direction.
Heat exchanger to be having minimum pressure drop limited within 0.4 Ata. Shell side.
Acceptance level of disturbing/damaging (to geometry) factors like flow-induces mechanical & acoustic out of its functioning, light layout requirements.
SUMMERY OF THE INVENTION
The design objectives which can be clubbed into a design with an extraordinary combination of following features:
- Use of Stainless Steel Integrated Low Fin Straight Tubes.
- Use of cladded plate & SS Plate.
- Use of Alloy Steel Hemispherical heads with man way access.
- Use of self-reinforced nozzles.

- Hanging at one end Tube bundles allowed for free thermal expansion.
- Use of snubbers for support in seismic activity as well as in operation.
- Use of RIGID STRUTS for vertical support.
– Arrangement of integral piping for reheater steam inlet, outlet, drain, & vent.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1- Shows a cross-sectional view of the Moisture Separator and Re-Heater assembly.
Fig.2- Shows the schematic view of the Moisture Separator and Re-Heater assembly.
Fig.3- Shows the schematic view of the Moisture Separator Portion.
Fig.4- Shows a view of the chevron separators along section I-I of Moisture Separator Portion.
Fig.5- Shows a cross-sectional view along section II-II of Moisture Separator Portion.
Fig.6- Shows the schematic view of the Reheater Portion (Bled & Live Steam).
Fig.7- Shows a plan view of the Moisture Separator and Re-Heater assembly.

DETAILED DESCRIPTION OF THE INVENTION
The aim of the invention is to provide a compactly designed heat exchanger suitable for a compact plant layout (refer Fig.1).
The flow on shell side is such that CRH i.e., shell side fluid enters at the inlet connection point through i.e. CRH inlet (101).Heavy steam flow through CRH inlet impinging the inner pipe (102) could create damaging effect, hence a specially shaped half pipe (103) is welded around the inner pipe exactly at the location where steam entering through CRH inlet (101) impinges the inner pipe (102) part of separator assembly. The half pipe not only reinforces the inner pipe it diverges the steam into coarse separator as well.
Because of sudden change in path and principle of loss of kinetic energy the moisture droplets of bigger size stick to the inside shell surface (104) & coarse separator section (105) components. Because of the gravity the bigger & heavier moisture droplets comes down to the drain connection (123).
Because of the pressure difference as the wet steam reaches to fine separator section (106) it has to pass through the fine separator assemblies (107) fitted with chevron type separators (108). Thus the residual moisture of steam is separated & drains out through drain collector (124). Using chevron type separators (108) forces wet steam to follow a restricted torturous path thus moisture droplets stuck in the pockets available in the separators and drain out and hence very high moisture separation efficiency is achieved.

As the moisture separated steam reaches to reheater portion (109) of Moisture Separator & Re-heater it passes over straight integrated finned tubes (110) and reheating takes place in two stages. Temp rise in Step-I is from 160 oC to 200 oC and Step-II is from 200 oC to 236 oC.
The re-heaters of Stage-I (111) and Stage-II (112) both in 02 no’s each placed at diagonally opposite circumferentially in shell assembly(113) of reheater assembly portion (100b) has two types extracted steam tube side also called as live steam (128) and bled steam (129) respectively which are responsible for cycle steam reheating. To have the most optimum mechanical design the fluid with higher pressure i.e., live steam/bled steam ( at pressure 21.3 & 41.5 kg/sq.cm(a) respectively was chosen to flow through tubes while the cycle steam as a pressure of 6.36 kg/sq.cm(a)) was chosen to flow in crossed flow manner over the tubes through shell assembly(113). The length sections of re-heaters of Stage-I (111) and Stage-II (112) supported by baffles (114). The live & bled steam flowing through straight finned tubes (110) releases heat by getting condenses within the straight finned tubes. Because of the vertical tube orientation no condensate falls back and interferes with the incoming live/bled steam. Designed for differential pressure of inlet (116) & outlet (117) channels the flow remains continuous from top to bottom at all times. For the differential thermal expansion of finned tube (110) the lower end of The re-heaters of Stage-I (111) and Stage-II (112) is kept freely hanging & guided with the help of lower support plate(115).Hemispherical type heads for the geometry of inlet (116) & outlet (117) channels of re-heaters lead to smallest design thickness in high internal pressure cases.

Hence, a hemispherical type channel was selected. A unique tub-shaped groove in tubesheet (118) has been designed to bear all mechanical loadings, besides providing a leak-tight joint between tube side and shell side fluids. The inner surface profile of tubesheet is such that it smoothly interfaces with inner profile of hemispherical channel, thereby eliminating any stress concentration on account of sharp changes in geometry sections. The live/bled steam enters from Live/bled steam inlet of inlet channel and leaves at outlet channel (117) through condensate connection (119).
Because of the heavy metal removal on the shell of moisture separator portion (100a) as well as re-heater portion (100b) of Moisture Separator Assembly for creating large sized inlet/outlet nozzle (120,121,122) holes (sizes up to 1880 NB, 800 NB) requires compensation which is normally provided by compensating pads which have diameters double the diameter of nozzle or hole being compensated. However, space limitation called for use of self-compensating nozzles and use of thick shell at this portion designed to carry all the reinforcement required.
For the maintenance purposes man-way access (125) are being provided moisture separator portion (100a) as well as re-heater portion (100b). The man-way access (125) is closed by means of a self-sealing cover (126).For the guidance & Flow of cycle steam partition Covers (127) & plates (115) are provided in such a way that no hindrance or reverse flow shall occur.
Aim of invention is to provide a heat exchanger with minimal Shell Side pressure drop for moisture separator portion (100a) & re-heater portion (100b) (within 0.30 kg/sq.cm Shell Side).

To minimize pressure drop especially on shell side for Moisture Separator Portion (100a) where flow (and thus, velocity) is very high of CRH steam, following design steps were taken:
(i) Internal components with adequate surface finish are placed in such a way
that most of them should not come to steam path thus reducing the
pressure drop.
(ii) Flow path for cycle steam is such that lesser cross flow velocities shall
generate thus minimizing the pressure drop (iii) Arrangement & layout of internal Piping (Heating Steam, Condensate &
Vent Piping) for easy flow of steam & condensate from reheater to shell
Nozzles.
To minimize pressure drop especially on shell side for Re-heater Portion (100b), following design steps were taken:
(i) Selection of Square Tube Layout for tube bundle led to improved results in
form of reduced pressure drops as compared to triangular pitches.(Figure-B)
(ii) For reheater assemblies use of partition plates & guide plates so that there
would be lesser cross flow velocities thus minimizing the pressure drop.(Figure-C)
(iii) Pipe layout & other internal components with adequate surface finish are placed in such a way that most of them should not come to steam path thus reducing the pressure drop.
Following design improvements were made as per the latest practices:

Use of cladded plate & SS Plate for coarse separator of moisture separator portion -
Because of detrimental effect of moisture present in cycle steam coming from HP Turbine the SS cladded plate is used for the construction of the shell (104) and SS Plate for other components of lower section of coarse separator portion.
Selection of SS integrated Finned tubes for improved heat transfer coefficient –
Since the fluids involved are vapor which have inherently low heat transfer coefficients so to have increased heat transfer co-efficient integrated finned tubes (110) has been used. However because of constraint of minimum tolerance on allowable pressure drop on shell it was evident to use FINNED TUBES with optimum no of fins per inch. This shall also minimize fouling of finned tube in the long run.
Lifting, Clearance and Accessibility -
The separator portion (101a) & as well as reheater portion (101b) are provided the lifting arrangements (140 & 141) to move & fix these assemblies. The coarse separator assembly of separator portion (101a), Shell and tube bundles of reheater portion (101b) is provided man-way accesses (125) for maintenance purposes. The man-way access (125) is closed by means of a self-sealing cover (14).

Vents, drains and sample connection -
For the evacuation of non-condensable and condensate fluids the connection (130 & 119) have been provided in both reheater portion (100b). Moisture separator portion (100a) also has the separate connection (123 & 124) for drain separation. These connections have been provided at most suited locations so that condensate or non-condensable fluid won’t fall back or create any blockages.
Stress Analysis -
The tube bundles are free for expansion and guides at bottom support plate (131.) so as to have negligible thermal stress. The layout of Pipe assemblies (132,133 & 134) are flexible so that there will be minimum thermal stress due to thermal expansions. For large connections in the shell of reheater portion and separator portion. Localized higher thick shell & nozzle pads are provided to bear nozzles stresses due to respective connection pipes.
Seismic proof design -
The entire assembly has been analyzed for seismic zone-5 i.e. highest risks zone. The structure has the provision of providing adequate supports in both vertically using thrust bearing (137) & horizontally using Rigid struts & STU (snubbers) (135 & 136) directions so as to keep the entire structure in its place in cases of any seismic vibration as well as in operation.
Flow-induced Vibration Control
A detailed analysis of flow-induced vibrations was carried out for different
assemblies.
Following choices emerged which led to betterment of flow-induced vibration
analysis results:

(i) Use of Half Pipe in Coarse Separator Portion -
The half pipe (103) not only dampers the central pipe (102) from flow induces vibrations it also reinforces the central pipe (102) from thermal movements. Half pipe (103) divides the stem entering the separator portion which is important for uniform flow and coarse separation of moisture from steam. (For layout see Fig. A)
(ii) Selection of Square Tube Layout for Tube Bundle –
Good results were obtained using rotated square tube layout as this, along with a larger pitch - tube OD ratio, helps in reducing the lift coefficient which is one of the factors increasing the Vortex Shedding amplitude. Larger amplitudes could lead to tube damages. (For layout see Fig. B and Flow see Fig. C)
Although a large tube pitch means compromise in heat transfer coefficient but this helps reduce cross-flow velocity past the tube and also reduce the pressure drop. If cross-flow velocity exceeds critical velocity, exceptionally high amplitudes of tube are resulted which could further lead to a seriously damaging phenomenon called fluid-elastic instability.
(iii) Use of stiffeners along with baffles in Tube Bundles –
Baffles (138) have been provided after a uniform gap of unsupported span. The position of baffles is in such a way that minimum pressure drop shall occur. To minimize the flow induces vibrations stiffeners (139) have been provided on both side of baffles alternatively stating from any end of tube bundle. This offers an advantage of assisting in creating structure to have minimal flow induced vibrations. (See Fig. D)

(iv) Threaded SS TUBE (Couplings) Holders in Tube Bundles –
SS tube holders are sandwiched between tube & baffles for pre-determined tubes. The thread profile of SS tube holders is kept as per fin profile of Tubes & welded with baffle circumferentially both side of the baffle (See Fig. E) this has been done for all baffles (138) through which that particular tube passes. This will result in a robust skeleton which not only helps in tube insertion and also for keeping the baffles (138) intact at desired location thus having a robust structure to minimize flow induces vibrations. (See Fig. F)

WE CLAIM
1. A dual function compact heat exchanger in a nuclear power plant delimiting pressure drop to a minimum level for moisture separator and reheater assembly, the heat exchanger comprises a lower half portion constituting a moisture separator and an upper half-portion operable as a re-heater, the portions being assembled supported vertically at the bottom over a pot bearing and fixed supports are being provided at least on two locations to restrict movement of the assembly in vertical as well as in horizontal planes, the moisture separator consisting of one each coarse and fine separator disposed such that the cycle steam respectively passing through the coarse separator and the fine separator, the reheater having at least four re-heater tube bundles formed of integral finned tubes located vertically to allow the heating steam traveling along the reheater tubes from top to bottom, wherein the moisture separator including the re-heater each configured as separate shells encased in a larger cylindrical shell having a plurality of self-compensating nozzles provided along the circumference to act as inlet port, outlet port, drain port and vent port for the cycle steam, wherein a plurality of segmented baffles supporting the finned tube bundles at different span of the tube bundles, wherein the finned tubes located in a square pattern and attached to a tub-shaped tube sheet having an inner surface profile enabled to smoothly interface with the inner profile of a hemispherical head housing channels for inlet and outlet of tube side fluid and Hemispherical type heads for the geometry of inlet & outlet channels of re-heaters lead to smallest design thickness in high internal pressure cases, wherein two nozzles acting as an inlet for live/bled steam as well as a condensate outlet of the tube-side fluid, wherein a plurality of flow restrictors provided for uniform tube side

steam distribution in the inlet of hemispherical head, wherein the inlet & outlet of the hemispherical head having a man-way accesses closed by at least one self-sealing cover, and wherein the heat is exchanged between the hot fluid flowing through the finned tubes and the cold fluid flowing over the finned tubes with minimal pressure drop, and flow-induced mechanical and acoustic vibrations.
2. The heat exchanger as claimed in claim 1, wherein the integral finned tubes are made Stainless Steel.
3. The heat exchanger as claimed in claim 1, wherein the number of self-compensating nozzle has been used.
4. The heat exchanger as claimed in claim 1, wherein the live/bled steam inlet of the inlet channel and leaves at the outlet channel through a condensate connection.