GB2204117A

GB2204117A - Heat exchanger with flow redistribution means

Info

Publication number: GB2204117A
Application number: GB08709905A
Authority: GB
Inventors: Owen Hayden
Original assignee: National Nuclear Corp Ltd
Current assignee: National Nuclear Corp Ltd
Priority date: 1987-04-27
Filing date: 1987-04-27
Publication date: 1988-11-02
Also published as: GB8709905D0

Abstract

In tube-in-shell heat exchangers, it is often necessary to plug defective tubes. The invention resides in using the plugged tubes to aid mixing of the heat exchange medium flowing through the shell so as to reduce the risk of thermal striping problems in for example sodium-water/steam heat exchangers for fast reactor plant. Slots are punched in the wall of each plugged tube (10a) to form tabs (22) which are deflected outwardly to displace streams of hot heat exchange medium radially away from the plugged tube to mix with streams passing externally of and lengthwise of adjacent unplugged tubes. <IMAGE>

Description

Heat exchanger with flow redistribution means This invention relates to heat exchangers of the type in which one of the fluids between which heat exchange is to take place passes through an array of tubes which extend through a chamber or shell containing a second fluid. The invention is particularly, but not exclusively, concerned with heat exchangers for use in the nuclear power industry. Thus, in one application of the invention, the heat exchanger may take the form a steam generating unit (SGU) in which said one fluid is water/steam and the second fluid is a liquid metal such as sodium, such SGUs being used in liquid metal cooled fast fission reactor plant.

In such SGUs, it is important to avoid or at least minimise interaction between the fluids, ie sodium and water, because of the violent nature of the interaction.

Such interactions may occur if leaks develop from the water side to the sodium side. The conventional practice adopted in the event of leaks developing is to plug the offending tube or tubes so that water can no longer pass along tubes having leaks. There is a limit to the number of tubes that can be plugged in this way since heat extraction efficiency is impaired in the immediate vicinity of each plugged tube and localised hot zones may develop in the regions of clusters of tubes which have to be plugged thus leading to streams of sodium flow through the SGU which differ markedly in temperature and thereby give rise to differential thermal stressing of SGU components, a phenomonen known in the art as thermal striping.

According to a first aspect of the present invention there is provided a heat exchanger of the type initially referred to having at least one plugged tube, characterised in that the or each plugged tube, as well as being plugged to prevent flow of said one fluid therethrough, is also formed with at least one integral projection pressed out of the tube wall to act as flow deflecting means for said second fluid.

According to a second aspect of the invention there is provided a method of repairing a heat exchanger of the type referred to in which at least one tube has developed a leak, characterised in that, in addition to plugging the tube to prevent flow of said one fluid therethrough, the tube is also deformed locally to press out at least one integral projection from the tube wall so that the projection(s) so produced act as flow deflector(s) for said second fluid.

The projections are conveniently pressed out so that axially adjacent projections are angularly offset relative to one another, preferably forming a helical arrangement of projections around each plugged tube.

Thus, in accordance with the invention, while the tubes that have been plugged become redundant in terms of conducting flow of one of the heat exchange media, they may still be usefully employed as flow mixing elements by virtue of said integral projections thereby reducing thermal striping problems in steam generators for liquid metal cooled fast fission reactors.

To promote further understanding of the invention, one embodiment of the mention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a heat exchanger; Figure 2 is a diagrammatic longitudinal sectional view showing parts of adjacent plugged and unplugged views; Figure 3 is a plan view corresponding to that of Figure 2.

The invention may be applied to various tyes of heat exchanger. Figure 1 illustrates in diagrammatic form a simplified heat exchanger in which one heat exchange medium, for example water/steam, is passed through a series of tubes 10 extending between inlet and outlet tube plates 12, 14 enclosed within a shell 16. A second heat exchange medium, eg liquid sodium, is circulated through the shell between inlet 18 and outlet 19.

In practice, if a leak develops the faulty tube may be rendered redundant by plugging its ends to prevent water/steam flow through it. In Figure 2 a plugged tube is depicted by reference numeral 10a, the plug at one end of the redundant tube being depicted by reference numeral 20.

Because sodium flow around the plugged tube 10a is no longer subject to cooling by that particular tube, it will be understood that the sodium flow stream in this region will tend to remain at a higher temperature than in regions where there are no plugged tubes present.

This reduced cooling effect in certain regions can be accentuated if a localised cluster of leaky tubes has to be plugged.

To promote intermixing of hotter sodium flow streams around plugged tubes or plugged clusters of tubes with cooler sodium flow streams, the leaky tubes 10a (prior to being plugged) are formed with integral radial projections 22 (see Figures 2 and 3) which may be punched out from the tube wall by inserting a tool along each tube 10a. The tubes iOa are thereafter plugged. The projections 22 serve to deflect the hot sodium streams flowing adjacent the plugged tubes outwardly towards the surrounding cooler sodium streams thereby promoting mixing in the region of the plugged tube or tube cluster to reduce the damage potential of such hot sodium streams.

The projections 22 may be located at suitable points along the length of each plugged tube, preferably in a generally helical arrangement. The projections 22 need not be located uniformly along the entire length of the plugged tube; they may for example be arranged in localised groups immediately adjacent grid plates for spacing the tubes 10 apart (for example on the upstream side of the grid plates as considered in the direction of flow of the heat exchange medium, eg sodium, circulating through the shell). Although as shown in Figures 2 and 3, the projections 22 extend towards the adjacent unplugged tube, the projections may project towards the intertube spaces between the adjacent tubes.

Claims

1. A heat exchanger of the type referred to having at least one plugged tube, characterised in that the or each plugged tube, as well as being plugged to prevent flow of said one fluid therethrough, is also formed with at least one integral projection pressed out of the tube wall to act as flow deflecting means for said second fluid.

2. A heat exchanger as claimed in Claim 1 in which the projections are pressed out so that axially adjacent projections are angularly offset relative to one another.

3. A heat exchanger as claimed in Claim 2 in which the projections are pressed out to produce a helical arrangement of projections around each plugged tube.

4. A method of repairing a heat exchanger of the type referred to in which at least one tube has developed a leak, characterised in that, in addition to plugging the tube to prevent flow of said one fluid therethrough, the tube is also deformed locally to press out at least one integral projection from the tube wall so that the projection(s) so produced act as flow deflector(s) for said second fluid.

5. A method as claimed in Claim 4 in which the projections are pressed out so that axially adjacent projections are angularly offset relative to one another.

6. A method as claimed in Claim 5 in which the projections are pressed out to produce a helical arrangement of projections around each plugged tube.

7. A heat exchanger of the type referred to having at least one plugged tube, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

8. A method of repairing a heat exchanger of the type referred to, substantially as hereinbefore described with reference to the accompanying drawings.