Steam Condensers

Abstract

A direct sub-atmospheric steam condenser having horizontal condenser tubes arranged in pairs of banks of tubes. The banks in each pair are inclined to each other and converge upwardly, and the steam inlet header for each bank is connected to a steam manifold which is arranged below the tops of the banks of tubes.

Description

This invention relates to steam condensers of the type in which steam at sub-atmospheric pressure is passed through tubes having extended external surfaces over which pass cooling air. This type of condenser is known and will hereinafter be referred to as a "direct, sub-atmospheric, steam condenser."

Known condensers of this type which are arranged to occupy a minimum plot area, have the tubes arranged with their centre lines in generally vertical planes and grouped in pairs of banks. The tubes in each bank of a pair are parallel and inclined to the tubes in the other bank of the pair so that the tubes in the respective banks converge upwardly and at their upper ends are connected to a steam manifold common to the banks of the pair. A condenser will normally include a number of pairs of banks, such a pair being known as an A-frame, all or some of which are fed from the same steam manifold.

This known type of direct, sub-atmospheric steam condenser has been commercially successful and has been used where the plot area to be occupied by the condenser is limited. However, such condensers suffer from the following disadvantages:

1. The use of longer finned tubes of larger diameter than those at present in use and which would result in simpler and cheaper design of the steam distribution manifold system is not practicable since their use would increase the overall height of the unit. Such height increase would lead to a considerable increase in cost of the support structure for the steam manifolds and banks of tubes. Such additional cost would reduce or nullify any benefit brought about by the use of such longer tubes of larger diameter.

2. Since the economic limit of tube length for the above reason is less than 20 ft. or thereabouts and since it is normal when using force draft to use fans as large as possible (up to diameters of 20 ft. or thereabouts) it is not economically possible with this arrangement to achieve a design which ensures that each tube of the condenser is served by air from more than one fan whilst using fans of optimum size. This means that, if a fan has to be shut down for any reason, the whole of a tube bank will become inoperative.

3. Since it is not regarded as good design practice to allow the condensate to fall back into the entering steam it is necessary to connect the manifold distribution system to headers at the upper ends of the tubes. This leads to long manifolds and to an expensive support structure for the manifolds due both to the large diameter of the manifolds and to their positions at or near to the highest point of the condenser.

4. Since the size and shape of a pair of tube banks forming an A-frame unit is such as to preclude the possibility of shop assembling such a unit for easy road or rail transport expensive field assembly is required. Also since the steam distribution manifolds are mounted on the tops of the tube banks the manifolds cannot be placed in position until the tube banks are in position.

5. Each bank of tubes normally consists of a number of tube rows and during winter operation there is danger of freezing of the condensate in the tubes due to the fact that the condensate is mainly formed in the lower rows of tubes which are exposed to the cooler part of the air flow over the tubes.

It is an object of the invention to provide a direct, sub-atmospheric steam condenser in which the first four of the above disadvantages are avoided and in which, if desired, simple means may be provided for overcoming the fifth disadvantage.

According to the invention we provide a steam condenser having horizontal steam condenser tubes through which steam is passed at sub-atmospheric pressure while cooling air passes over the external surfaces of said tubes; comprising at least one pair of banks of tubes, the banks in each pair being mutually inclined and converging upwardly; a steam inlet header connected to one end of each bank and a further header connected to the other end of each bank; suction means connected to said headers; drain means arranged to drain condensate from at least one of said headers for each bank of tubes; and a steam manifold connected to said steam inlet headers, the manifold being arranged below the tops of the banks.

When we say that the tubes are substantially horizontal, we mean that the tubes make an angle of no more than 5.degree. with the horizontal. A slight slope of this nature may be desirable to assist the condensate to flow out of the tubes into the condensate headers.

A condenser embodying the invention has the following advantages as compared with the known type of steam condenser referred to above:

1. The use of larger diameter and hence longer tubes is quite practicable and this enables shorter and more compact steam distribution manifolds to be used. Thus the larger the diameter of a tube in a condenser of this type the longer it can be and the less manifolding will be required due to the reduced number of tubes. Also since the manifolds are mounted at a level below the tops of tube banks (if desired below the banks) this reduces the length of the steam manifolds which lead up from a turbine mounted at a lower level than the condenser and whose exhaust is fed to the condenser.

2. Since it is practicable to use tubes of 30 ft. or more in length it is quite feasible to arrange for each tube to be fed by air from two or more fans whilst still using optimum fan sizes. This means that at least 50 percent of each tube bank can still be operated whilst any one fan is out of action.

3. Since the manifolds are mounted at a level beneath the tops of the tube banks this enables a more economical support structure to be used, and also enables the manifolds to be erected prior to the erection of the tube banks.

4. Since the tubes are horizontal it is also quite feasible to arrange pairs of banks into A-frames whose base width is of the order of 12-14 ft. and which can be completely shop assembled and transported by road or rail ready for directly mounting on to a simple support structure. This reduces erection time and the complexity and the cost of the support structure. This is possible, if, e.g., the capacity of an electric generating station including the condenser is between 6 and 30 Megawatts whereas for larger stations it may be necessary to assemble the tubes into the headers on site.

The steam manifolding may be arranged in the lower half of the condenser above the bottoms of the banks. Alternatively, the steam manifolding may be arranged below the banks with upwardly extending connections between the manifolds and the headers.

The cooling air may be caused to flow over the tubes either by forced or natural draught. If forced draught is used then there is provided, beneath the banks, a plurality of fans rotatable about vertical axes. If natural draught is used, there is provided a tower with peripherally extending means between the lower end of the tower and the ground so that said lower end is spaced above the ground. Air thus flows into the base of the tower between the bottom of the tower and the ground and then upwardly through the banks of tubes.

Various pass arrangements are possible with a condenser embodying the invention. In a first, or partial co-current, pass arrangement steam passes first along the tubes of a lower part of each bank and then flows back along an upper part of the bank. In this arrangement, the inlet header may be divided by a pass plate or other means. Since the final stages of the condensation take place in the second pass, i.e., in the upper part of each bank, the danger of freezing is reduced since these upper parts of the banks are heated by the warmer air passing through the condenser.

This partial co-current arrangement constitutes a major advantage over a single pass arrangement particularly when using the condenser in very cold climatic conditions. As compared with known condensers of the type specified cheaper control systems are possible for maintaining the desired vacuum in the condenser and savings in the power required to drive the fans (where provided) may be achieved during operation at low temperatures. During the final stages of condensation, air which is unavoidably drawn into the system at the turbine represents an appreciable volume of the gas flowing through the condenser and the known phenomenon of "vapour blanketing" takes place in which an insulating layer of non-condensable gas flows adjacent to the tube wall thus reducing the rate of condensation. A substantial temperature difference may build up across this insulating layer so that the tube wall may fall to freezing temperatures whilst the bulk steam temperature of the gas flowing in the tube is in excess of the freezing point. For this reason it is a considerable advantage not to allow very cold air in contact with the outside of the tube whilst condensing under these conditions.

Prevention of tube blockage due to frozen condensate may be prevented by simple control schemes with this pass design and in moderate climatic conditions only fan switching may be required.

Alternatively, the steam may make a single pass through each bank and then proceed through a secondary bank at another part of the condenser. Thus there will be a number of primary banks through which the steam first flows and a number of secondary banks fed from the primary banks. Normally a number of primary banks will feed a single secondary bank.

If desired, each header may be formed as a rolled box section each having secured thereto two rows of tubes. The tubes are externally fillet welded to the headers. If it is desired to make each bank of three or more rows of tubes then each header may be formed of a plurality of box sections, two in the case of three rows of tubes, one header supporting the third row of tubes. Connections will be provided between the parts of the same header which are constituted by different rolled sections.

Alternatively fabricated box sections may be used with plugs opposite each tube or removable cover plates for tube fixing, inspection and maintenance.

One of the problems encountered in direct steam condensers is corrosion of the tubes. If the tubes of the normal external diameter used, e.g. 1 - 11/2, have their walls thickened to resist corrosion then the flow area for the steam is reduced below a desirable value.

An advantage is gained if tubes which have an increased wall thickness and external diameter are used so that the tubes still provide sufficient flow area for the steam and sufficient wall thickness to resist corrosion. We prefer to use tubes having an external diameter of between 11/2 and 3 inches.

The extended surface of the tubes will normally be provided by fins. These fins may be secured to the tube in any of the known methods for example by first grooving the tube and then inserting an edge of a helically wound strip of material into the groove and closing the groove to grip the strip.

Alternatively, a helically wound strip of L-section may be secured to the external surface of the tube.

Embodiments of the invention will now be described in detail by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a side view of a bank of tubes in a condenser embodying the invention;

FIGS. 2 and 3 are views in the directions of the arrows X and Y in FIG. 1 of the return and inlet headers respectively of the bank of FIG. 1;

FIGS. 4, 5 and 6 are plan, side and end views respectively of a condenser embodying the invention;

FIG. 7 is a plan view of a condenser having a different arrangement of banks of condenser tubes;

FIGS. 8 to 11 are detail views of the ends of banks of tubes employed in the arrangement of FIG. 7;

FIG. 12 is a vertical sectional view taken on line 12--12 in FIG. 13 of a natural draught cooling tower having a steam condenser according to the invention;

FIG. 13 is a plan view of the steam condenser mounted in the cooling tower of FIG. 12; and

FIG. 14 is a schematic view of a steam operated plant.

Referring now to the drawings, a direct, subatmospheric steam condenser embodying the invention comprises a rectangular frame 10 which is supported above either the ground, or the roof of a building such as a turbine house, by means of a plurality of upright columns 11. Mounted on the top of rectangular frame 10 are a number of pairs 13 of banks 14 of tubes 15. Each bank is similar and comprises three rows of finned tubes (FIGS. 2 and 3) which are arranged with their centre lines generally horizontal, the rows being inclined to the vertical. The tubes preferably have an outside diameter of between 11/2 and 3 inches for the reasons set forth above. At each end, each tube is secured to a header 16 or 17 by conventional tube sheet practice. The two banks 14 of a pair converge upwardly as shown in FIG. 5 and the tubes lie parallel to two opposite sides 18 of the rectangular framework 10. Substantially the whole area of the framework is covered with longitudinally extending rows of pairs 13 of banks of tubes arranged in four lateral rows 19, 20, 21 and 22.

Two steam manifolds 23 are provided each extending parallel to one of the other sides 24 of the framework and arranged below, but aligned with the gap between, the rows of each outer pair of rows 19, 20 and 21, 22. The manifolds are fed from their one ends 25 and each has outlets or feed pipes 26 (see FIGS. 1 and 3) extending upwardly to the headers 16 at those ends of the banks adjacent the manifold. The outlets connect to nozzles in the headers which feed the steam from the manifold into headers.

Suspended below the framework are a number of fans 27 with driving motors 28 and which are rotatable about vertical axes and serve to entrain air and pass it upwardly over the tubes of the banks. It will be seen from FIGS. 4 and 5 that two fans such as 27a supply air to each bank of each pair 13 so taht if one fan should fail or be turned off the other fan still impels air over the tubes of the banks 13.

Various pass arrangements may be provided. Assuming that, as shown in FIGS. 1 to 3, the first arrangement described above is used, the steam passes first along the tubes in a major part of each bank from the header 16 to the header 17 and then back from the header 17 to the header 16 along the tubes in an upper part of the bank. The inlet headers 16 which are connected to the steam manifolds 23 are divided by pass plates 29 so that the steam is directed into the tubes in the lower major part of each header. Condensate drain means 30 and 31 are respectively provided in each return header 17 and in the part 32 of each inlet header 16 separated from the main part thereof by the pass plate 29. The drain means 30 communicate with condensate headers 33 shown in FIGS. 1, 2, 4, 5 and 6 and the drain means 31 with condensate headers 34 shown in FIGS. 1, 3, 5 and 6. The parts 32 of the inlet headers 16 are connected to vent manifolds 35 connected to a vacuum source to remove the non-condensable gases.

Referring now to FIGS. 7 to 11, there is shown a construction of steam condenser employing pairs of banks of condenser tubes which are somewhat similar to the banks of tubes previously described, and corresponding parts will be designated by the similar references. However, the connections to the headers of the banks of tubes and the flow paths for the steam are somewhat different. Referring to FIG. 7, it will be seen that the steam condenser comprises the lateral rows of banks of tubes 19, 20, 21 and 22, and also includes 16 longitudinal rows of pairs of banks of tubes. The pairs of banks of tubes in each longitudinal row are spaced axially from each other to provide the four lateral rows 19 to 22 and the two steam manifolds 23 extend between rows 19 and 20 and 21 and 22 in similar manner as described with reference to FIG. 4.

In FIG. 7, the reference B and A designate respectively sections of primary and secondary banks of tubes. The banks of tubes in the section B constitute the primary banks fed with steam from the manifolds 23 and the banks of tubes in Sections A constitute the secondary banks which are fed with uncondensed steam from the primary banks. The corresponding parts in the construction of the bank of tubes in sections A and B to the members described with reference to FIGS. 1 to 6 will be designated with the letter A or B accordingly.

Referring now to FIGS. 8 and 9, there are shown end views of a bank of tubes as employed in a section B. The staem inlet header is shown in FIG. 9 and is designated by reference 16 B and is supplied with steam from steam manifold 23 through outlets or feed pipes 26B. However, steam inlet header 16B does not have a pass plate or drain means as in the previously described construction.

Turning now to FIG. 8, this shows the header 17B at the other end of the bank of tubes and this has drain means 30B which communicates with condensate header 33B. Also header 17B has a steam outlet 38 by which uncondensed steam from the primary bank (Section B) may pass to the secondary bank (Section A) by way of an interconnecting manifold 39.

Referring now to FIGS. 10 and 11, there are shown the headers at each end of a bank of tubes as used in a Section A. The steam inlet header 16A is supplied with uncondensed steam from the primary bank by way of inter-connecting manifold 39 and outlets or feed pipes 26A. The uncondensed steam from Section B passes down the secondary bank comprised by Section A and is condensed therein and passes to header 17A (FIG. 11) at the other end of the bank of tubes. Condensate drains off by way of drain means 30A and condensate header 33A. A vent manifold 35A is connected to the header 17A and serves to place the steam under sub-atmospheric pressure throughout the condenser.

Returning now to FIG. 7, the flow path for the steam sas it passes from the two steam manifolds 25 is indicated by the arrows in the figure from which it will be apparent that the steam passes initially through the pairs of primary banks in the Sections B where it undergoes a partial condensation, and then passes to the pairs of secondary banks in Section A where the condensation is completed. The arrangement may be provided with fan units similar to the fan units 27 to provide forced draught cooling, or the arrangement may be mounted in a natural draught cooling tower.

Referring now to FIGS. 12 and 13, there is shown a steam condenser according to the invention mounted in a natural draught cooling tower. The structure of the cooling tower is indicated generally by reference numeral 40 and the steam condenser indicated generally by reference numeral 41 is mounted above the ground in the lower portion of the tower. The supporting framework 10 of the condenser is supported by struts 11 and the steam condenser 41 is indicated only schematically but comprises pairs of banks of tubes 13. The construction and arrangement of the steam condenser may be as described with reference to FIGS. 1 to 6 though of course without the provision of the fan units 27 to provide the forced draught, or the arrangement described with reference to FIGS. 7 to 11 may be employed.

Referring to FIG. 14, there is shown schematically a steam powered plant comprising a steam generator 42 which supplies steam under pressure to a steam turbine 43. Exhaust steam is fed from the turbine to a steam condenser 44 which then recycles condensed steam to the steam generator. The steam condenser shown in FIG. 14 may comprise a forced draught steam condenser installation. as shown in FIGS. 1 to 6 or FIGS. 7 to 11, or may comprise a natural draught tower and steam condenser installation as shown in FIGS. 12 and 13.

It will be seen from the foregoing description that the condenser can be made of comparatively small height since the tubes are horizontal and the steam manifolds are arranged below the banks as shown. Thus the heights of the banks between horizontal planes containing the lines 36 and 37 in FIG. 5 is of the order of 12 to 14 ft. as compared with the corresponding height of approximately 25 ft. above a similar frame which is required by the known type of condenser referred to above.

In condensers embodying the invention a multiplicity of small A-frames will normally be employed where one large A-frame may have been used in previously known designs. A windshield may not be required in a condenser embodying the invention because only the outermost banks are seriously affected by wind. In previous designs a very much larger portion of the condenser could be subjected to the effect of wind. Various modifications may be made to the invention Thus instead of having three rows of tubes in each bank there may be two rows. The headers may be formed of rolled box sections. If there are only two rows of tubes per bank then one box section will serve both rows of tubes which may be externally welded by fillet welds to a wall of the header. If three rows of tubes are used, there will have to be two box sections which together form each header, the box sections being inter-connected by steam pipes. The pass arrangement may be as described above with primary and secondary banks.

Claims

What we claim then is:

1. A steam powered plant comprising a steam turbine; an air cooled, sub-atmospheric, steam condenser comprising a plurality of pairs of banks of tubes, the banks of each pair converging upwardly, each bank comprising two horizontally spaced apart, aligned headers and a plurality of straight substantially horizontal tubes connected between the headers, steam inlet chambers and condensate outlet chambers formed by the headers, flow directing means for directing the steam to flow through at least two tubes in series between a steam inlet chamber and a condensate outlet chamber, a steam manifold below said banks and connected to the outlet of said steam turbine, elongated feed pipes extending upwardly from the manifold to the steam inlet chambers, suction means connected to said condensate outlet chambers, fan means arranged below the banks and operable to cause a flow of air to the banks and operable to cause a flow of air upwardly over the banks, means to collect condensate from said condensate outlet chambers and a steam generator to receive said condensate and to generate steam and to feed the same to the steam turbine.

2. An air-cooled, sub-atmospheric, steam condenser comprising a plurality of pairs of banks of tubes, the banks of each pair converging upwardly and each bank comprising two horizontally spaced apart, aligned headers and a plurality of straight substantially horizontal tubes connected between the headers, steam inlet chambers and condensate outlet chambers formed by the headers, flow directing means for directing steam to flow through at least two tubes in series between a steam inlet chamber and a condensate outlet chamber, said flowing directing means comprising a divider in one header of each of at least some of the banks and serving to divide said header into a steam inlet chamber and a condensate outlet chamber, the tubes in each bank containing a divided header and connected to the condensate outlet chamber of said divided header being above the tubes of the bank connected to the steam inlet chamber of said divided header, a steam manifold below said banks, elongated feed pipes extending upwardly from the manifold to the steam inlet chambers, suction pipes connected to said condensate outlet chambers, and fan means arranged below the banks and operable to cause a flow of air upwardly over the banks.

3. A steam condenser according to claim 3, wherein, in each bank having a divided header, there is a greater number of tubes connected to the steam inlet chamber than to the condensate outlet chamber.

4. An air-cooled, sub-atmospheric, steam condenser comprising a plurality of pairs of banks of tubes including primary and secondary banks of tubes, the banks of each pair converging upwardly and each bank comprising two horizontally spaced apart, aligned headers and a plurality of straight substantially horizontal tubes connected between the headers, steam inlet chambers and condensate outlet chambers formed by the headers, a steam manifold below said banks, elongated feed pipes extending upwardly from the steam manifold to the steam inlet chambers, suction pipes connected to said condensate outlet chambers, fan means arranged below the banks and operable to cause a flow of air upwardly over the banks and interconnecting manifolds connecting each secondary bank to a plurality of primary banks whereby steam flows first through tubes of a primary bank and then flows through tubes of a secondary bank.