US3356135A

US3356135A - Once-through steam generator with means to provide saturated feed water

Info

Publication number: US3356135A
Application number: US421135A
Authority: US
Inventors: Robert K Sayre
Original assignee: Individual
Current assignee: Individual
Priority date: 1964-12-24
Filing date: 1964-12-24
Publication date: 1967-12-05
Anticipated expiration: 1984-12-05

Description

Dec. 5, 1967 R. K. SAYRE ONCETHROUGH STEAM GENERATOR WITH MEANS TO PROVIDE SATURATED FEED WATER Filed Dec. 24, 1964 M WAGE-T United States Patent O 3,356,135 ONCE-THROUGH STEAM GENERATOR WITH MEANS To PROVIDE SATURATED FEED WATER The present invention relates to an improved oncethrough steam generator and more particularly to a oncethrough steam generator having the design advantage of eliminating thermal stresses, boiling stability, and flow distribution difficulties between boiling channels located in parallel.

Several diiculties arise in the employment of the oncethrough steam generator, for example, as a component of a nuclear power plant. In such steam generators there is a normal operating thermal stress problem which is more severe then that of the recirculating boiler because there is no mixing of the cold feed water with the saturated recirculating water. The result of this is that the cold feed water in a once-through steam generator will normally come in direct contact with the steam generator shell and/or tube sheets, as a result there occur large thermal stresses in such shell or tube sheets.

There is also a problem of insuring boiling stability with once-through steam generators. Unless special design provisions are made, boiling channels with high heat fluxes land high vaporization fractions will oscillate when fed with subcooled water. In the once-through steam generator, of course, there is usually employed a maximum degree of feed subcooling (the feed enters the channel directly without mixing with recirculating water), and the vaporization fraction is 1.0. Hence, once-through steam generators are highly susceptible to oscillations.

Another difliculty in the design of once-through steam generators is brought about by what is called boiling disease. It has been shown that the pressure drop through the parallel boiling channels with subcooled feed is virtually independent of mass velocity over wide flow ranges. The result of this is that special design provisions must be made to equalize the flow through any parallel How multichannel boiling pathway with subcooled feed.

In carrying out the present invention, thermal stresses in pressure boundary members are minimized by avoiding direct Contact between the cold feed water and the pressure boundary members except the heat transfer tubing. Heat transfer tubing is best able to withstand the thermal stresses caused by this contact with the cold feed water because it contains a minimum of discontinuities, restraint, and bending stresses. Contact between the tube sheet, or shell, and feed water is prevented by leading the feed water directly into a small chamber or manifold in the tube bundle bounded by tightly fitting bales at the two ends of the chamber and a shell around the bundle covering the sides of the chamber. The novel feed heating arrangement according to the present invention also insures boiling stability. Generally, boiling systems fed with saturated water will not oscillate, nor will boiling systems wit-h a very low fraction of vaporization oscillate. According to the invention, the advantageous result of decreasing or eliminating oscillations is provided by splitting the boiler into two dynamically independent systems, the rst of which brings the feed water nearly to saturation or slightly beyond saturation, and the second of which carries the output of the rst to dryness. Thus both are independently stable.

More particularly, the method of splitting the preheating section from the boiling section according to the present invention is to make the inertia of the preheating section very high compared with that of the boiling section. The feed water heating flow is confined to annuli of small cross sectional area so that the feed now attains a high velocity and hence a high inertia. Means are also provided to couple the feed water heating section directly to the entire feed water system by the agency of a completely filled feed Water inlet manifold which contains the feed water.

The feed heating annuli are made of sufficient length to bring the feed approximately to saturation. The saturated feed is then brought to a condition of lower velocity in a baiiled boiling section. With the resulting large drop in velocity there will be a corresponding drop in inertia, thereby decoupling the two sections of the boiler.

vThe third problem of flow distribution is solved in two ways. First the boiling section is fed with saturated water. Parallel channels fed with saturated water do not suffer from flow distribution problems. Second the entire boiling section is in the form of a single channel so that the only problem is one of water distribution within the channel, a problem of mechanical mixing rather than a problem of basic flow characteristics.

It is accordingly, among the `objects of the present invention to provide:

An improved steam generator for high power plants having means of a novel design of reducing oscillations in the steam generator.

A once-through steam generator of improved design having means for reducing thermal stresses, and for enhancing boiling stability in flow distribution between boiling channels.

A once-through steam generator of improved design in which there is provided means for splitting the boiler portion of the system into two independent systems each of which is independently stable, thereby reducing oscillation.

A once-through steam generator of novel design having improved means for coupling the feed water heating section directly to the entire feed water system inertially.

A feature of the present invention is the provision of an improved manner of llow distribution. Means arev provided so that parallel channels do not have discontinuous flow distribution and so that the entire boiling section is formed of a single channel in which water distribution may be carried out within the channel by mechanical mixing.

The -foregoing objects as well as other Objects, features and advantages of the present invention will be better understood by referring to the following description and the accompanying drawings in which:

FIG. 1 is a modied side view of a once-through steam generator according -to one version of the invention; and

FIG. 2 is a view in section of a portion of the details of the feed heating section of the steam generator according to the invention.

Referring to FIG. 1, the steam generator depicted therein has an outer boiler wall or shroud 11 surrounding the units of the generator, at one end of which is a header section 13 having an inlet 15 and an outlet 17 for the purpose of providing heating lluid for the unit. The heating fluid is passed through the generator by means of a tube bundle 19 consisting of a plurality of spaced parallel tubes 21 forming `a U with each end of the U fluidly coupled to the header. The generator has a preheating section 22 and a boiling section 23 through which the tube bundle 19 passes. The steam output is provided at an outlet 24 which may be located conveniently at the top of the unit.

Referring to FIGS. 1 and 2 together, the feed water from which the steam is to be generated is passed under suitable pressure into the preheating section 22 via an 3S inlet 25. The preheating -section has a chamber 27 which is defined by

walls

29, 31, 33 and 34. The

walls

29, 31, 33 and 34 are preferably constructed as shown so that they are isolated fro-m the shroud 11. The walls of the chamber are held in place essentially by being affixed in any suitably manner to the tubes 21.

The bundle of tubes 19 passes through the chamber 27 so that the feed water comes in contact with each of the spaced tubes of the bundle. For each of the tubes 21 passing through the chamber 27 the-re -is provided a sleeve 3S surrounding each tube. The sleeve is of larger internal diameter than the external diameter of the tube. Thus each of the sleeves and its associated tube (together) define an elongated annular space 37 therebetween. Each of the sleeves 35 extends from a position within the chamber 27 continuously along the tube to a suitable predetermined position in the boiling section 23 downstream of the chamber 27.

The steam generator unit has a longitudinal baffle 39 located centrally of the unit connected to the end wall 29 but otherwise spaced from the walls of the chamber 27. The baffle 39 separates one leg of the U-shaped tube bundle 19 from the other leg of the U-shaped tube bundle.

In the boiling section 23 the longitudinal baffle 39 has extending outwardly therefrom a plurality of spaced transverse baffles 41. There is also a plurality of spaced baflles extending inwardly from the shroud 11 and staggered in relation to the baffles 41. The groups of baffles 41 and 43 provide in the boiling section a zig-Zag path for enhancing mechanical mixing of the feed water after it passes from the preheating section through the boiling section.

At the portion of the baffle adjacent to header unit 13 there is provided :an aperture 45 for passing the steam into a chamber 47. The chamber 47 bounds the wall 29 of chamber 27. Steam entering chamber 47 passes into contact with the wall 29 of the chamber 27 to enhance heating of the feed water. A vent 49 for expelling steam is provided in the chamber 47 for insuring steam flow.

The steam generating unit operates as follows:

When the feed water is passed through the inlet 25 into the chamber 27, the annular -spaces 37 defined by the sleeves 35 adjacent to respective tubes 21 provide a plurality of discrete high area-to-volume heat transfer regions through which the feed water passes. As the feed water passes through the annular spaces 37 it comes into close confined contact with each of the tubes 21 so that the feed water is preheated rapidly to a point at or near saturation or even slightly above saturation. Since the feed water in the annuli is heated while confined the feed achieves a high velocity and therefore substantially increases its inertia.

The essentially saturated feed is released at the egress end of the annuli 37 to the boiling section in which the feed water is mechanically mixed as it passes freely among the tubes 21 in a zig-zag path caused by the groups of baffles 41 and 43. As the feed water leaves the annuli 37 there is a considerable drop in velocity of the saturated feed Water due to expansion, The inertia of the feed water also drops, the effect of which is to decouple the cooler preheating section from the warmer boiler section of the unit.

In that the boiling section is in effect fed with essentially saturated water, and since the saturated feed in that portion of the unit is flowing essentially in a single mixing channel, the water distribution within the channel is determined primarily by its mechanical mixing.

Of course, the heating fluid in the tubes 21 serves to heat toa desired degree of dryness the feed water passing among the ballles 41 and 43 and among the tubes 21 to the steam Ioutlet 24. Some of the steam will also pass through the aperture 45 into a chamber adjacent the wall 29 of the manifold. Flow of the steam via the aperture 45 is further enhanced by a provision of a vent 49 which communicates with the chamber 47. This steam adjacent the wall 29 serves to further heat or preheat the feed water as it is introduced into the chamber 27.

Thermal stresses in the shroud 11 and baffle 39 are minimized because the cool walls of chamber 27 are essentially isolated from contact therewith. Thermal stresses will occur to some extent in the tubing, but the tubing is normally capable of withstanding such stresses.

Since the action of the feed water passing through the annuli 37 while being heated produces high velocity, high inertia, saturated feed and further, since the high inertia feed is then brought to a much lower inertia by passing the steam from the annuli 37 into the larger single mixing channel indispersed with the tubes 21, the preheat'ing and boiling system are essentially decoupled thereby preventing oscillation.

The present arrangement is in direct contrast to previous units such as those having channels with high heat fluxes and 'nigh vaporization fractions which will oscillate when fed with subcooied water. This is especially true in once-through generators because of their use of a maximum amount of feed subcooling. ft is yet further appreciated that since there is a single mixing channel for the saturated feed, there is no difficulty or necessity for equalizing the flow through any of parallel flow type channel boiling pathways as in the previous known designs.

It is understood that the present generator may bc cmployed for other feed uids besides water. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. lt is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is.

1. A once-through heat exchanger comprising:

means for providing feed heating fluid for the exchanger;

enclosed chamber means, having Walls, for receiving feed to be converted into fluid of a higher energy state;

means defining a plurality of discrete spaced confined paths passing through and beyond said chamber;

said last named means having one end connected to receive the heating fluid and it's other end connected to discharge the heating fluid;

means surrounding each of said confined path means for defining at least one space therebetween of high area-to-volume ratio, cach of said last named means passing through one wall of said chamber;

channel defining means arranged for providing a mechanical mixing path for feed fluid in proximity t0 said confined path means for an extended distance in heat transferring contact with said confined path means;

outlet means for said unit, whereby when feed is introduced to said chamber under pressure, said feed enters the high area-to-volume space and is pre-heated therein, said feed being heated to approximately its saturation, said feed thereby obtaining high velocity and high inertia, said feed when saturated passing near the outlet of said space into a single mechanical mixing channel in close proximity t'o said confined path defining means whereby said saturated feed is brought to considerably lower inertia and velocity, said saturated feed thereafter being brought into its desired state of dryness by flowing in contact in said path with said path defining means and out of the system via said outlet.

2. A once-through steam generator having a feed inlet and a steam outlet comprising:

a shroud for the generator;

a tube bundle enclosed by the shroud and fluidly coupled at one end to receive heating fluid and fluidly coupled at its other end to discharge heating fluid, said tube bundle being the form of a U;

means for receiving the feed water, said means comprising a chamber having walls mounted isolatedly from the shroud; I

said tubes passing through said chamber;

a Sleeve for each tube of said tube bundle, one end of said sleeve extending through a wall of said chamber, the inside diameter of each of said sleeves being greater than the outside diameter of each tube of said tube bundle;

a central baille separating the legs of the U of said tube bundle;

said central baille having extending outward therefrom plurality of spaced transversed balles extending among said tube bundles;

said shroud having a plurality of spaced baille members extending inwardly towards said central baille.

3. A steam generator according to claim 2 wherein said central baille has an aperture therein;

said aperture defining the entrance to a chamber located adjacent at least one wall of said feed water chamber;

said chamber having a vent therein, whereby steam produced in said generator ilows thru said chamber for further preheating of said feed water.

4. A once-through steam generator having a feed inlet and a steam outlet comprising:

a shroud;

header means for providing heating iluid located at one end of said shroud;

a U-shaped tube bundle of spaced tubes located in said shroud and iluidly coupled at each end of the leg of said U to said header means for passing the heating fluid through said tubes;

baille means located centrally of said generator and dividing the legs of said U to form with the shroud a single channel enclosing said tube bundle;

means located within said shroud for receiving feed water, said means comprising a chamber having walls physically isolated from said shroud and from said baille, the walls of said chamber being arranged for support essentially by the tubes of said bundle;

means extending into said chamber and deilning an annular space around each tube of said bundle, said means extending Ialong each of said tubes outward of said chamber for a predetermined distance;

each of said last named means having an outlet for passing heated feed water into the channel defined by said central baille and said shroud;

whereby upon introduction of feed water into said chamber and into said annular spaces, said feed water is heated to essentially its saturation, said essentially saturated feed then being passed into said channel for mechanical mixing therein in the presence of said tube bundle, whereby said feed is brought to its desired degree of dryness.

5. A Once-through steam generator 'according to claim 3 wherein said generator further comprises transverse baille means located in said channel. p

6. A once-through steam generator according to claim 4 but further characterized by said central baille having aperture means therein, means defining a chamber iluidly coupled to said aperture means and located immediately adjacent said feed water chamber, said immediately adjacent chamber having a vent means therein whereby steam generated in said system may pass through said aperture into said adjacent chamber for additionally heating the feed water.

7. The once-through steam generator according to claim 5 wherein the tubes of said tube bundle are essentially parallel and spaced from each other.

8. A once-through steam generator having a feed inlet and steam outlet comprising:

a U-shaped tube 4bundle of spaced tubes;

a shroud surrounding said U-shaped tube bundle;

a central baille separating the legs of the U-shaped bundle;

a header section located at one end of said shroud and iluidly coupled to each leg of said U-shaped tube lbundle for supplying heating fluid for said tube bundle;

chamber means for receiving feed water, said chamber means being essentially thermally isolated from said shroud and from said central baille and essentially attached to said tube bundle;

means deilning an annular space for each tube of said tube bundle and extending into said chamber means and without said chamber means for predetermined distance along said tube bundle;

said annular space being of relatively small radial dimension in comparison with the diameter of said tube and thereby providing a space of large area-tovolume ratio through which said feed water may pass for heating said feed water to a high temperature and for providing high velocity inertia for said feed water as it becomes saturated in said annular space.

References Cited UNITED STATES PATENTS 178,244 6/1876 Koll 122-409 620,994 3/1899 Teste 122-242 653,3 72 7/ 1900 Altmann 122-242 673,908 5/1901 Jonsson 122-242 X 827,479 7/1906 Towne 165-159 FOREIGN PATENTS 741,988 12/ 1955 Great Britain.

ROBERT A. OLEARY, Primary Examiner. MEYER PERDN, Examiner. A. W. DAVIS, Assistant Examiner,

Claims

1. AT ONCE-THROUGH HEAT EXCHANGER COMPRISING: MEANS FOR PROVIDING FEED HEATING FLUID FOR THE EXCHANGER; ENCLOSED CHAMBER MEANS, HAVING WALL, FOR RECEIVING FEED TO BE CONVERTED INTO FLUID OF A HIGHER ENERGY STATE; MEANS DEFINING A PLURALITY OF DISCRETE SPACED CONFINED PATHS PASSING THROUGH AND BEYOND SAID CHAMBER; SAID LAST NAMED MEANS HAVING ONE END CONNECTED TO RECEIVE THE HEATING FLUID AND ITS OTHER END CONNECTED TO DISCHARGE THE HEATING FLUID; MEANS SURROUNDING EACH OF SAID CONFINED PATH MEANS FOR DEFINING AT LEAST ONE SPACE THEREBETWEEN OF HIGH AREA-TO-VOLUME RATIO, EACH OF SAID LAST NAMED MEANS PASSING THROUGH ONE WALL OF SAID CHAMBER; CHANNEL DEFINING MEANS ARRANGED FOR PROVIDING A MECHANICAL MIXING PATH FOR FEED FLUID IN PROXIMITY TO SAID CONFINED PATH MEANS FOR AN EXTENDED DISTANCE IN HEAT TRANSFERRING CONTACT WITH SAID CONFINED PATH MEANS; OUTLETS MEANS FOR SAID UNIT, WHEREBY WHEN FEED IS INTRODUCED TO SAID CHAMBER UNDER PRESSURE, SAID FEED ENTERE THE HIGH AREA-TO-VOLUME SPACE AND IS PRE-HEATED THEREIN, SAID FEED BEING HEATED TO APPROXIMATELY ITS SATURATION, SAID FEED THEREBY OBTAINING HIGH VELOCITY AND HIGH INERTIA, SAID FEED WHEN SATURATED PASSING NEAR THE OUTLET OF SAID SPACE INTO A SINGLE MECHANICAL MIXING CHANNEL IN CLOSE PROXIMITY TO SAID CONFINED PATH DEFINING MEANS WHEREBY SAID SATURATED FEED IS BROUGHT TO CONSIDERABLY LOWER INERTIA AND VELOCITY, SAID SATURATED FEED THEREAFTER BEING BROUGHT INTO ITS DESIRED STATE OF DRYNESS BY FLOWING IN CONTACT IN SAID PATH WITH SAID PATH DEFINING MEANS AND OUT OF THE SYSTEM VIA SAID OUTLET.