AU654536B2

AU654536B2 - High-pressure feedwater preheater

Info

Publication number: AU654536B2
Application number: AU14047/92A
Authority: AU
Inventors: Mustafa Dr. Youssef
Original assignee: ABB Asea Brown Boveri Ltd; Asea Brown Boveri AB
Current assignee: Alstom SA
Priority date: 1991-04-15
Filing date: 1992-04-03
Publication date: 1994-11-10
Anticipated expiration: 2012-04-03
Also published as: ATE123129T1; DE59202292D1; CH682692A5; KR920020178A; EP0509298B1; AU1404792A; KR100205274B1; EP0509298A1

Abstract

In a regenerative high-pressure feedwater preheater of upright construction with a U-shaped tube bank (9, 10), with built-in heat removal zone (4), with integrated, submerged undercooling zone (18) and with a water chamber (27, 28) lying below, the heat removal zone and the undercooling zone are each shielded with a cover (6 and 20 respectively) against the condensation zone. In the heat removal zone (4), a chamber (23), which is not flowed through by vapour, is arranged around the tube bank (10). <IMAGE>

Description

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AUSTRALIA

Patents Act 1990 654536 ASEA BROWN BOVERI LTD

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ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Ti't'le: HIGH-PRESSURE FEEDWATER PREHEATER S. C C Ce Ce..

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CS CC C S C The following Zstatement is a full description of this invention including the best method of performing it known to us:-

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e Technical field The invention relates to a regenerative highpressure feedwater preheater of upright construction e..e with a U-shaped tube bundle, with an incorporated desuperheating zone, with an integrated flooded subcooling zone and with a water chamber located at the bottom, the desuperheating zone and the subcooling zone each being screened by a cover from the condensation *zone.

e State of the art Such a preheater is known from the trade article "The effect of condensate reheat on mean temperature difference in feedwater heater subcooling e* zones", published in the literature reference "Transactions of the ASME", October 1957, pages 1494- 1500, Figure 3a.

In such preheaters with an integrated and flooded subcooling zone, steam cannot pass under any operating conditions from the condensation zone into o0 the subcooling zone. Nevertheless, the heattransferring zones must be designed with extreme care, since external and internal leakages must also be minimised. Thus, the ingress of condensate streams into the subcooling zone is kept small by appropriately 2 thick final baffles of the subcooler, by small flowrelated pressure drops in the inlet part and by small annular gaps. Internal leakages are minimised by baffles welded tightly to the subcooler shell and by Ssmall annular gaps at the tube passages.

In this type of preheaters, the flooding height of the subcooler forms a pressure reserve which makes revaporisation of the condensate difficult. Flooding is ensured by an appropriate design of the condensate out- >O flow control and of the level control range. In this case, the highest controlled condenate (sic) level must be located below the final baffle of the desuperheater, so that no condensate can flow back into the desuperheating zone.

On the other hand, correct sizing of the desuperheating zone is also of greatest importance. The superheated bleed steam is passed in the desuperheater at a defined velocity in counter current or in crossflow to the feedwater. As long as the outer walls of o0 the tubes subjected to superheated steam is (sic) dry, the steam-side heat transfer to the tube takes place convectively. If the desuperheating zone is sized too large, condensation can already start at a point at which the outer tube wall reaches the local saturation S" temperature. In this case, the laws of condensation apply to the steam-side heat transfer to the tube and erosion/corrosion must be expected. For reasons of operational safety,\this state ma*s be avoided over the entire load range of the equipment.

0- Owing to the abovementioned necessary height difference between the final baffle of the desuperheating zone and the highest possible flooding level above the subcooling zone, wrong sizing of thd desuperheating zone can consequently result. This applies particularly in the case that, for example, the subcooling zone must be enlarged in order to lower the condensate outlet AL1 j temperature and hence also the steam flow. Such a design leads as a rule to a subcooling zone which is 3 subdivided by means of baffles into a plurality of chambers and thus has a greater height in the longitudinal direction of the equipment.

Description of the Invention The present invention provides a regenerative high-pressure feedwater preheater of upright construction for heating feedwater with steam, said preheater comprising a U-shaped tube bundle to carry water through a subcooling zone flooded with condensed steam, a condensation zone in which steam condenses on the tube bundle and a desuperheating zone in which steam is first contacted with said tube bundle, a pair of water chambers *being provided below the U-shaped tube bundle to feed unheated water to said tube bundle and remove the heated 15 feedwater from said tube bundle, wherein said preheater further comprises a chamber with no free steam flow therein, said chamber being arranged around the tube bundle in the desuperheating zone and sized to prevent condensation nuclei forming in the desuperheating zone.

20 The advantages of the invention are to be seen especially in the simplicity of the novel measure. It remains for the designer to integrate the volume, which is thus created and is rheologically and hence also thermically inactive, into the desuperheating zone as desired and, if appropriate, to subdivide it into a plurality of chambers.

It is particularly advantageous if both the cover and the bottom of the chamber are formed by tube support plates. Corresponding to the baffling of the desuperheating zone, to form the chamber, it is then only necessary to connect the support plates concerned by a simple metal sheet.

3a Brief Description of the Drawing An illustrative example of the invention is represented diagrammatically in the drawing, in which: Figure 1 shows a partial longitudinal section through a feedwater preheater, Figure 2 shows a cross-section along the cut line 2-2 in Figure 2, and Figure 3 shows a cross-section along the cut line 3-3 in Figure 1.

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4 Only the elements essential to the understanding of the invention are shown. The direction of flow of the working medium is marked by arrows.

Preferred embodiment of the invention The high-pressure feedwater preheater of upright construction is enclosed in an outer shell 1, at the lower end of which a steam inlet orifice 2 and a condensate outlet orifice 3 are arranged opposite each C> other. In the left-hand part of the equipment, a desuperheating zone 4 is provided. Upwards towards the condensation zone 5, this is provided with a cover 6 in the form of a final baffle. The interior of the desuperheating zone 4 is baffled by means of steam baffles 7, 7' and 7" which at the same time perform the function of tube support plates.

The upper end of the desuperheating zone 4 leads into a steam distribution shaft 8 which is bounded by closing walls 17. This shaft extends cen- 0)0 trally between a U-shaped tube bundle, the two arms of which form a right-hand cold tube bundle part 9 and the I left-hand warm tube bundle part 10. The shaft is arranged such that its mutually opposite longitudinal orifices act as a steam outlet. These longitudinal orifices are provided with perforated cover sheets 12 which have steam outlet orifices 11 in the shaft plan on the one hand and steaming holes 13 in the plane of the tubed zone on the other hand. In the central space in each of the tube bundle parts 9 and 10, a venting channel 15 is recessed, along which a vent pipe 14 passes. In the condensation zone 5, tube supports 16 are arranged in such a way that the condensate produced can trickle downwards.

In the right-hand part of the equipment, a baffled subcooling zone 18 is provided. The baffles 19 acting as tube support plates subdivide the subcooling zone into a plurality of chambers and force the condensate into a cross/counterflow to the feedwater which 5 flows in the tubes of the tube bundle. This subcooling zone is likewise provided upwards towards the condensation zone 5 with a cover 20 in the form of a final baffle. In the condensate outflow line not shown, a c control element 21 is provided, by means of which the outflow stream is controlled. Above the final baffle the flooding level of the condensate is marked 22.

29 marks the level control range.

The cold tube bundle part 9 leads on the under IO side of the preheater via a tube plate 26 into a collection chamber 27, through which the feedwater flows upwards into the tubes. After it has been heated up, the feedwater is discharged from the equipment via the collection chamber 28, into which the warm arm 10 of 1 the tube bundle leads.

To this extent, three-zone feedwater preheaters are known. It can then happen that, according to the specifications of the operator, the condensate outlet temperature must be lowered to a particularly low level Q0 so that, inter alia, the steam rate drawn in can be reduced. A lowering of the condensate outlet temperature leads to an increase in the heat exchange area in the subcooling zone. At a given external diameter of the equipment, this means an enlargement of the sub-

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1 cooling zone in the longitudinal direction of the equipment, that is to say in the height. In order then to meet the condition that the highest possible flood- .ing level must in any case be below the final baffle in the desuperheater, this inevitably also leads to a raising of this final baffle and hence to an undesired increase in the heat exchange area in the desuperheating zone, unless countermeasures are taken.

This is where the invention intervenes: To reduce the heat exchange area in the desuperheating zone in spite of the increased total volume to a tolerable level, a thermically inactive zone is created therein. In the simplest way, this is achieved by providing a chamber 23 not bearing a steam flow is pro- 6 vided within the desuperheating zone around the warm tube bundle part 10. According to the illustrative example shown, the steam baffles 7' and 7" can be used for this purpose. These plates which are anyway welded on one side to the partition 30 between the desuperheating zone and subcooling zone, here form the cOnrp.ej=rt

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and the bottom of the chamber 23. They are X~se ed with the simple metal sheet 24, whereby a closed space not subjected to steam is created. Corresponding to the outer contour of the plates 7' and this metal sheet 24 can have an almost hemicylindrical shape or a polygonal shape.

It is to be understood that, for example in the case of a multi-baffled desuperheating zone, the |I support plates 7' and 7" might equally well abut against the outer shell 25 of the desuperheater, with corresponding adaptation of the metal sheet 24 in the hemicylindrical interior of the desuperheating zone.

The mode of operation of the three-zone equipa.q* ment is explained below: Superheated steam enters the desuperheating zone 4 through the steam inlet orifice 2, flows around the steam baffle 7 arranged therein and around the chamber 23 and passes directly into the steam distribution shaft 8, through which it flows up- S o wards. From the mutually opposite steam outlet orifices 11 in the cover sheets 12, the steam directly enters the annular space of the condensation zone 5 and distributes itself therein. At a comparatively low velocity it impinges on the condensation surface of the tube bundles 9, 10 in accordance with the direction of the arrow in Figure 2 and flows radially inwards through the bundles. After heat release and condensation of the heating steam, the condensate flows downwards and collects above the subcooling zone, the level G gof the condensate flooding the subcooler being controlled at the value desired at the time. Air and incondensable gases are taken off, after the end of con- -7densation, through the vent pipes 14 inside the tube bundles 9 and The f eedwater is preheated in the tubes in a three-stage process first in the subcooling zone 18, 9 then in the condensation zone 5 and finally in the desuperheating zone 4.

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Claims

1. A regenerative high-pressure feedwater preheater of upright construction for heating feedwater with steam, said preheater comprising a U-shaped tube bundle to carry water through a subcooling zone flooded with condensed steam, a condensation zone in which steam condenses on the tube bundle and a desuperheating zone in which steam is first contacted with said tube bundle, a pair of water chambers being provided below the U-shaped tube bundle to feed unheated water to said tube bundle and remove the heated feedwater from said tube bundle, wherein said preheater further comprises a chamber with no free steam flow therein, said chamber being arranged around the tube e* bundle in the desuperheating zone and sized to prevent 15 condensation nuclei forming in the desuperheating zone.

2. Regenerative high-pressure feedwater preheater according to claim 1, characterised in that both the top and the bottom of the chamber are formea by tube support plates which are connected by a metal sheet. 20

3. A regenerative high-pressure feedwater preheater "g substantially as hereinbefore described with reference to the accompanying drawings. ee* DATED this 14th day of April 1994 ASEA BROWN BOVERI LTD Patent Attorneys for the Applicant: F.B. RICE CO.