GB2241339A - Leak detection in a heat exchanger - Google Patents

Abstract

A heat exchanger 2 which serves to separate and to conduct heat between two heat exchanging fluids 3, 4 consists of at least two layers 5, 6, 7 in intimate physical and thermal contact wherein each layer 5, 7 which directly contacts a heat exchanging fluid is substantially resistant to corrosion by that fluid and at least one of the layers 6 is easily corrodible by at least one of the fluids, such that if a leak of at least one in particular of the heat exchanging fluids commences development, it would cause an ever-spreading area of corrosion in the easily corrodible layer(s) 6. This corrosion would encompass the fine leak detecting channels 8 provided between the interface between at least one of the layers 5, 7 and the corrodible layer 6. Channels 8 form an intersecting network and one formed by grooves in one or both of the contiguous layers 5, 6, or 6, 7 or by strips of unbonded interface between the layers 5, 6 or 6, 7. The heat exchanger may be plate-like instead of tubular. If only one fluid is corrosive or abrasive then the network of channels need be provided only at the interface of the corrosible layer and the layer in contact with that fluid. <IMAGE>

Description

}tEAT T EXCHANGER

This invention relates to a means of detecting leaks which might develop in a member which serves to separate two or more fluids. It relates particularly to applications where heat is transferred thorough the member from one fluid to the other.

It is an object of the present invention to provide improvements in, or modifications to, the invention disclosed in British patent application No 9004055.1.

In British Patent application No 9004055.1, there is described a standard, readily available form of 'leak detector' piping. The e operation of this is described, and terminology introduced such as 'partial leak', 'full leak', and 'directly separating member'. Amongst other things, all the terminology introduced therein is assumed in this document.

In British Patent application No 9004055.1, various arrangements, alternative to those of standard 'leak detector' pipe, are described. Each of the embodiments described therein possesses at least one or other major disadvantage however. An example of some of the disadvantages, which this present invention seeks to overcome, can be elucidated most clearly by reference to the first example of the fifth embodiment of the invention therein, as illustrated therein in Fig.12, Fig.13, Fig.14, and Fig. 15 The disadvantages are elucidated as follows:: Two variations of construction for the embodiment were described, (referring now in particular to Fig. 15 therein) where either the members forming the walls of the indicator path (ie the convolution and the annular plates) were bonded (perhaps metallically) together over a large fraction of their area of mutual contact, (which bonded area in fact largely equates to the area physically directly between the two heat-exchanging fluids), or the members were arranged simply to contact one another, to a greater or lesser extent, under a contact pressure.

In the former case, an intimate physical contact and excellent thermal conductivity was ensured between the members forming the walls of the indicator path, by the presence of the generalized bonding. However, this generalized bonding ensured that the indicator path was of limited extent, and existed, within the general confines of the bonded region, only in a small fraction of the generally bonded area, in particular in narrow channels where the surfaces were not bonded together. Thus a plurality of potential leak paths existed from one fluid to another via the bonded area, which leak paths would be isolated from the indicator path by the bonded area surrounding them.

It might be observed at this point that a theoretical plurality of full leak paths, isolated from the indicator path, from one -flutd to another existed in the standard leak detector design, as described in the aforementioned patent application. However, the geometry of the cross-section of this standard leak detector tube, with its channels deep in the radial sense, ensured the gross improbability of any leak path forming directly from one fluid to another, and not instead meandering on its path into a channel and causing a partial leak rather than a full leak. However, the section of the standard pipe is a factor which ensures its high manufacturing cost. A main aim of the inventions described therein and herein is to provide a lower cost method of leak detection.This in turn implies the use of thinner section materials, and the thinner section materials and the thin cross-section they imply tend to render it more probable that a leak path could develop straight through from one fluid to another without meandering in the process into the indicator path. This leads to the main disadvantage of this former (bonded) arrangement, (ie as described in British patent Application No 9004055.1 which is that the satisfactory operation of the leak detector function (ie its ability to detect the occurrence of leak paths) depends on the occurrence of a aeneral degradation of the surface of the directly separating member, such that a plurality of leak paths would commence development, rendering it most probable that the indicator path would intercept at least one such path, so forming a partial leak and effecting detection of the presence of the leaks.

In the latter case, (ie the unbonded arrangement referred to in British patent Application No 9004055.1 ) a high integrity leak detector arrangement was provided, with the indicator path present (although in some cases in a physically very thin form) in all regions between the members enclosing it. Thus no potential leak paths existed directly between the heat exchanging fluids, which could avoid passing through the indicator path. This arrangement, however, leads to the disadvantage that the integrity of the physical and thermal contact between the surfaces enclosing the indicator path, across which heat is to be transmitted, is somewhat dubious.It may also be observed, that if for example a thermal grease were to be employed, to reduce the thermal contact resistance at the interface between two members forming part of the indicator path wall, then the presence of the thermal grease in the very thin gap between two members in very close proximity might serve, disadvantageously, and for a limited time at least, to physically block up the indicator path, and so possibly isolate a leak path from the indicator path.

According to the present invention, a member serving to separate and conduct heat between two heat-exchanging fluids is provided, where the member consists of at least two layers, with each layer in intimate physical contact with the layer(s) contiguous to it, and where each layer which directly contacts one of the separated heat-exchanging fluids is at least substantially resistant to corrosion by that heat-exchanging fluid, and where least one of the layers is easily corrodible by at least one in particular of the heat-exchanging fluids, such that if a leak of at least one in particular of the heat-exchanging fluids commences development, it would cause development of an ever-spreading area of corrosion in the layer(s) easily corrodible by it which it contacted, which area of corrosion could conduct a flow of the leaking fluid, and so form an indicator path.

Also in accordance with the present invention, a primary indicator path consisting of a fine channel or collectively of fine channels may be provided in the member, which channel(s) are at least partially in, against, or otherwise bounded by at least one of the easily corrodible layer(s), such that if a leak of at least one in particular of the heat-exchanging fluids commenced development, causing development of an ever-spreading area of (flow-conducting) corrosion in the layer(s) easily corrodible by it which it contacted, the area of corrosion, if it occurred in a layer or layers which bounded indicator path channels, would be sure, by virtue of the provision of a fine inter-channel spacing and fine channel width relative to the thickness of the corrodible layer(s), eventually to spread to encompass some or all of the channels and hence act as a secondary indicator path, connecting the leak path with the primary indicator path, and so facilitating indication of a partial leak, even if the leak path might not have connected with the primary indicator path without the presence of the area of corrosion. The channel(s) forming the primary indicator path may be provided at one or more interface between the contiguous layers, and the channels may originate as grooves in one or more of the surfaces meeting to form the respective interface.

Alternatively, the grooves may be as deep as the layer they are in, so that the layer concerned is effectively in strips. The layers may be metallic, and metallically bonded together.

The invention is now described by example only by reference to the accompanying drawings, in which: Figure 1. shows a perspective view of a first embodiment of the invention.

Figure 2. shows an enlarged view of the encircled area of Figure 1.

Figure 3. shows a view of a stepped sheet.

Figure 4. shows a perspective view of a second embodiment of the invention.

Figure 5. shows a perspective view of a third embodiment of the Invention.

In a first preferred embodiment (1) of the invention, referring now to figure 1, a member is provided, which member is in the form of a tube assembly (2). The tube assembly (2) serves to separate and to conduct heat between a first heat-exchanging fluid (3) inside the tube assembly (2), and a second heat- exchanging fluid (4) outside the tube assembly (2).

The tube assembly (2), referring now in particular to Fig.l and to Fig.2, consists of three concentric tubes in intimate contact with each other over a substantial area of their mutually contacting inner and/or outer surfaces, These tubes are comprised of an inner tube (5), a middle tube (6) and an outer tube (7). The middle tube (6) is made of a material easily corrodible by the heat exchanging fluids which pass down the bore and around the outside respectively of the tube assembly (2). The inner and outer tubes (5) and (7) are made of a material at least substantially resistant to corrosion by the heat-exchanging fluid they are in contact with. A network of channels (8) is provided. ( These are shown in full only on the section, but the center-lines of the hidden channels of the outer interface (10) are shown dashed for clarity ).These channels may run along the interfaces (9) and (10) between the tubes (5), (6), and (7), which channels (8) provide a plurality of pathways between one part of the network and any other. This network of collective channels (8) provides the primary indicator path. The channels may be narrow and of fine pitch relative to the thickness of either or both the layers between them and the heat- exchanging fluids, so that they do not significantly impede the conduction of heat through the member from one fluid to another.

To ensure good thermal contact between the contiguous layers of the separating member, in this case the constituent tubes (5), (6) and (7) at their interfaces (9) and (10), the layers must be in intimate physical contact. This intimate contact may be achieved in a number of ways. One possible method would be to exploit the difference between the pressure on the external surfaces of the separating member (the heat- exchanging fluid pressure) and the pressure at the interface regions if the interface regions were to be evacuated.This method of ensuring physical contact would be particularly effective in tubular embodiments if one or more of the tubes forming the interface (s) concerned were able to deform radially to an exaggerated extent under the differential pressure, for example by virtue of being provided with axial corrugations to increase the flexibility in this radial sense.

A more certain way of achieving intimate contact between the layers at their interfaces would be by bonding the tubes together at their interfaces. In this case, for reasons of thermal conductivity, the bonding process employed may be metallic, particularly if, as is likely in heat exchanger applications, the layers are also metallic. The process by which the interfaces (9) and (10) of the layers are bonded together might be soldering or brazing. In particular, if the heat exchanging fluids are water or water-based, the middle tube (6) may be of iron- or steel- based material (easily corrodible by water), and the inner and outer tubes (5) and (7) of Copper or Copper alloy (substantially resistant to corrosion by water). In this case, the tubes could be easily soldered together at their interfaces (9) and (10).Alternatively, if the material of construction of a tube were incompatible with the particular joining process, then it could be coated by an electro-plating or hot-dipping process with a material that was compatible. The middle tube (6) wall may be of a substantially thicker gauge than the gauge of the inner (5) or outer (7) tube. The tube assembly may be formed initially from a tube, with any subsequent tubular contiguous layers formed from an initially flat sheet which is folded into a slitted tubular shape and subsequently joined to form a tube in situ against the initial tube. The initial tube may be easily corrodible and of thicker gauge than the gauge of the subsequent corrosion resistant layers.Referring now in particular to Fig. 3, the inner (5) and/or outer (7) tubes may be formed from an initially flat sheet (11), one edge of which may be formed into a step (12) to provide the required clearance for a subsequent lap joint. This might then be folded into a tubular shape and then jointed (perhaps lap-jointed) to form a tube. The joint might be formed in situ on the existing middle tube, perhaps simultaneously with the inter-tube bonding process, perhaps by bonding the overlapping leaves (13) and (14) and (15) and (16) of the sheets forming the tubes (5) and (7) by a process similar to that which bonds the inner and/or outer tube (5) and (7) to the middle tube (6). The channels (8) at the interfaces may be provided originally by grooves in the interfacing surfaces of either the middle tube (6) or the inner and outer tubes (5) and (7) or a combination of these.In particular, if the inner or outer tube were each formed from a sheet, then the sheet in each case could be provided economically with grooves by embossing the sheet in a pressing operation when it was flat. Alternatively, the channels (8) may be constitused by strips where a gap is present between the surfaces which are otherwise bonded together, the thickness of the gap in the dimension normal to the layers being of the order of the thickness of the layer of bonding agent, the lack of bonding in the strips being due to the action of of a masking agent, applied before bonding occurred, to the surfaces to be otherwise bonded.

Alternatively, the presence of masking agent in the grooves could help prevent the bonding agent from blocking any of the channels (8), by running into them.

In operation, the leak-detection function is provided primarily by the channels, which constitute the primary indicator path, so called because the channels provide the main path for a partially-leaking fluid. If a heat exchanging fluid passes through either the inner (5) or outer (7) tube wall and interface bond, it may pass directly into one or more of the channels (8), ie directly into the primary indicator path. Alternatively, the leak in either the inner (5) or outer (7) tube wall may lead the leaking fluid to contact the corrodible middle tube (6) in between the path of the channels (8) near to the leak path. In the latter case, corrosion of the middle tube (6) would ensue, with the development of an ever-spreading area of corrosion (ie that increased in size with time).The products of corrosion tend to show a swelling of volume relative to that of the parent material, and tend to be porous. They would therefore conduct a flow of leaking fluid. It could be arranged therefore, that by providing a channel spacing of fine pitch relative to the corrodible (middle) tube (6) wall thickness, that the area of corrosion in the middle tube (6) caused by the developing leak would be very likely to spread to encompass some or all of the channels (8) and hence act as a secondary indicator path by connecting the leak path to the primary indicator path, and so facilitating indication of a partial leak, even if the leak path might not have connected with the primary indicator path without presence of the area of corrosion. Thus, indirectly, a sure indicator path arrangement is provided, whilst providing a very high integrity thermal contact throughout the tube assembly (2).

It may be the case that one surface of the tube assembly is to perform a much harsher duty than the other. For example, the inner surface might be subjected to a high velocity flow of dirty fluid, with a consequent risk of abrasion and wear, whereas the other (outside) surface might be subjected to a slow flow of pure fluid with no solid content and low abrasiveness. A second embodiment described hereafter exploits this situation by reducing the provision of the channel network, whilst providing possible savings in production cost over the first embodiment. In the second embodinent, a network of channels is provided in one interface only. The channels, (which form an indicator path) are provided in the interface formed partly by whichever of the inner or outer layers is deemed most likely to suffer degradation.

In a second embodiment (17) of the invention, referring now in particular to Fig.4, an assembly (18) of two main tubes (19) and (20) in intimate contact with each other over their mutually contacting surfaces is provided. The assembly serves to separate and to conduct heat between two heat-exchanging fluids (not shown) which pass through the bore and over the outside respectively of the assembly (18). The tube subjected to the harsher duty, here the inner tube (19) (shown solid), is of a thin gauge, corrosion resistant material. The outer tube (20) is of a material corrodible by both heat-exchanging fluids, protected against corrosion by being wrapped in a very thin sheet (not shown for clarity) of material resistant to corrosion by the heat-exchanging fluid it contacts, perhaps metallically bonded to its outer surface (21).Alternatively, the outer tube (20) may be protected (perhaps by a process of electro-deposition or hot-dipping) on its outer surface (21) by a coating of a corrosion resistant material. A network of interconnecting channels (22) (hidden detail not shown) at the interface (22A) between the tubes (19) and (20) provides an indicator path arrangement similar to that described in the first embodiment. The construction of the inner tube (19) is of possible arrangements similar to those described in the first embodiment. The possible methods of achieving intimate contact between the tubes, of providing channels at the interface, and of forming a tube from an initially flat sheet, as described earlier in this document, apply equally to this embodiment.

In operation, it is assumed that the outer surface (21) of the outer tube (20) is protected against corrosion by its wrapping or coating. If however this protection breaks down, the ensuing corroded area will progress through the outer tube (20) in an ever-spreading area, and inevitably intersect the path of the channel (22) network, which will facilitate indication, (if somewhat belatedly), of a partial leak. If the inner tube (19) is abraded away or fails for any other reason, then a partial leak will be indicated via a similar possible sequence of events as described in the first embodiment.

In an alternative application, the two heat-exchanging fluids may differ in nature. For example, the heat-exchanging fluid in the bore may be water based, the other heat-exchanging fluid a hydrocarbon, such u mineral oil. Again, if it were expected that one surface of the tube assembly (18) were to be subject to a harsher duty than the other, then the two-saln-tube construction would be beneficial. The wrapping or coating of the outer tube surface (21) might be omitted, because the outer tube, say of steel, itself might be resistant to corrosion by this hydrocarbon. The inner tube might be of Copper, ie substantially resistant to corrosion by water. Thus one only of the layers (the steel layer) is easily corrodible by a heat-exchanging fluid, and that layer is easily corrodible by one only of the fluids (the water).

Satisfactory leak detection in this particular application would then rely on the assumption of the overwhelming probability that any leak path forming would initiate through from the inner side, (ie the side in contact with the fluid which can cause corrosion in at least one of the layers), hence causing corrosion in the layer corrodible by it which it contacted.

Advantages of the embodiments of the invention described above are that the assemblies (2) and (18) may be mechanically strong due to the possible use of a thick walled layer of corrodible material. The assemblies are however corrosion resistant because of the protection imparted to the corrodible layer by its coating and/or other layers. The material costs of the assemblies may be reduced however by virtue of the fact that the strong, thick walled tube may be of a corrodible material, which material is nearly always intrinsically cheaper than the corrosion resistant material of the inner (and in the first embodiment) the outer tube, which corrosion resistant material, although intrinsically expensive, is used in thin gauge material (and hence in small quantities). Another advantage of the arrangements described is that the thin gauge tube material may originate in sheet form.

It is usually much cheaper to reduce a material to a thin gauge by rolling it in sheet form than by drawing it in tubular form. Also, the formation of the inner and/or outer tube from folded sheet in situ against an existing tube, as well as aiding assembly, means that a requirement for very tight limits on the tube diameters, which would have to apply if pre-formed tubes were used, does not arise. The channels which provide the indicator path are easily formed as they originate as grooves or as a masking pattern on a surface, which grooves or pattern are easier to form than long thin holes. Because the primary indicator path may be of fine channels compared to the thickness of the layers, these channels do not significantly reduce the effectiveness of the assembly at conducting heat in the through direction.

An important limitation with the above designs Le that the bore of the tube assemblies would have to be above a certain size to allow practical manufacture of the assemblies.

In a third preferred embodiment (23) of the invention, referring now in particular now to fig 5., the separating member is provided in the form of a substantially flat, plate assembly (24), which serves to separate and to conduct heat between two heat-exchanging fluids on either side of it.

A first heat-exchanging fluid is against one face of the plate assembly (24), a second heat-exchanging fluid is against the other face (neither fluids shown). The plate assembly (24) would normally be provided with casings (not shown) positioned against both of its sides, to prevent the fluids from flowing around the edges of the plate and mixing. (NB the plate/casing arrangement may be similar to the arrangement shown in Fig.8, British Patent Application No 9004055.1) In the simplest descriptive terms, the plate assembly (24) can be considered to be an arrangement similar to that of the tube assembly (2) in the first embodiment, but where an imaginary tube assembly has been slit axially, and then unfolded. The plate assembly therefore consists of three layers, (25), (26) and (27), with the outer layers (25) and (27) each made from a corrosion resistant material.The middle layer (26) consists of a material which is readily corrodible by the heat-exchanging fluids. For reasons of cost saving, the middle layer may be relatively thick compared with the outer layers. The layers may be metallic and metallically bonded together at their interfaces (28) and (29), in arrangements similar to those described in the first embodiment. By arrangements similar to those described in the first embodiment, a channel or network of channels (30) may be provided at the interfaces (28) and (29) between the layers, to form a plurality of possible paths for leaking fluid which collectively form the primary indicator path. (Note that the channels may originate as grooves in either the middle layer, outer layers, or both). The channels (30), because they originate as surface grooves in the layers, are cheap to produce compared to the drilling of long, thin holes of similar dimensions, and in any case can be arranged to be curved and interconnecting as drilled holes could not be.

In operation, the plate assembly (24) serves to provide leak detection in a manner similar to that of the tube assembly (2) in the first embodiment. This, to recant, is as follows. If a heat exchanging fluid penetrates an outer layer, it will either pass directly into the indicator path and trigger the leak detection function, or it will pass into the indicator path indirectly via a region of the middle tube which it will subsequently corrode to form a secondary indicator path, to the same triggering effect.

The composite structure may be strong, (due to the thick corrodible layer), corrosion resistant, (due to the thin outer layers), and relatively cheap to produce because the expensive materials are used in thin sheet form.

Alternatively, a plate assembly (not shown) may be provided which is the 'unfolded' equivalent of the second embodiment. This would be of a function and of possible arrangements and construction similar to those of the tubular second embodiment (18), the major actual difference in function being that the plate assembly has the heat exchanging fluids against either face, whereas of course the tubular embodiment has the heat-exchanging fluids against its bore and its outer diameter respectively.

It is an important observation that the composite structures described in the above embodiments need not be tubular or flat, but that the same essential features as described above could be produced and employed in a variety of shaped structures, but one example of an alternative shape being that of a conically dished plate.

Claims (12)

CLAIMS.

1. A member serving to separate and conduct heat between two heat-exchanging fluids, where the member consists of at least two layers, with each layer in intimate physical contact with the layer(s) contiguous to it, and where each layer which directly contacts one of the separated heat-exchanging fluids is at least substantially resistant to corrosion by that heat- exchanging fluid and where at least one of the layers is easily corrodible by at least one in particular of the heat-exchanging fluids, such that if a leak of at least one in particular of the heat-exchanging fluids commences development, it would cause development of an ever-spreading area of corrosion in the layer(s) easily corrodible by it which it contacted, which area of corrosion could conduct a flow of the leaking fluid, and so form an indicator path.

2. A member as claimed in claim 1, where a primary indicator path consisting of a fine channel or collectively of fine channels is provided in the member, which channels are at least partially in, against, or otherwise bounded by at least one of the easily corrodible layer(s), such that if a leak of at least one in particular of the heat-exchanging fluids commenced development, causing development of an ever-spreading area of corrosion in the layer(s) easily corrodible by it which it contacted, the area of corrosion, if it occurred in a layer or layers which bounded indicator path channels, would be sure, by virtue of the provision of a fine inter-channel spacing relative to the thickness of the corrodible layer(s), eventually to spread to encompass some or all of the channels and hence act as a secondary indicator path, connecting the leak path with the primary indicator path , and so facilitating indication of a partial leak, even if the leak path might not have connected with the primary indicator path without the presence of the area of corrosion.

3. A member as claimed in claim 2, where the channels forming the primary indicator path are provided at one or more interface between the contiguous layers, and the channels originate as grooves in one or more of the surfaces meeting to form the respective interface.

4. A member as claimed in claim 1, claim 2 or claim 3 where the layers are metallic and are metallically bonded together.

5. A member as in claim 2, claim 3 or claim 4, where the member consists of three contiguous layers, where an indicator path consisting of channels is provided at both the interfaces between the layers.

6. A member as claimed in claim 2, claim 3 or claim 4, where the member consists of two contiguous layers, with the easily corrodible layer provided with a coating of material resistant to corrosion by the heat-exchanging fluid it contacts.

7. A member as claimed in claim 1, claim 2, claim 3, claim 4, claim 5 or claim 6, where the member is in the form of a tube.

8. A member as claimed in claim 7, where the member is formed initially from a tube, with any subsequent tubular contiguous layer formed from an initially flat sheet which is folded into a slitted tubular shape and subsequently formed into a tube in situ against the initial tube.

9. A member as claimed in claim 8, where the original tube is easily corrodible and of thicker gauge than the gauge of any subsequent corrosion -resistant or other layers.

10. A member as claimed in claim 8 or claim 9, where the initial tube is of iron- or steel- based material, and the additional layers are of Copper or Copper alloy, and the bonding agent is a solder.

11. A member as claimed in claim 3, where the presence of a masking agent in the grooves helps prevent the bonding agent from blocking any of the channels, by running into them.

12. A member as claimed in claim 2, where the channels are constituted by strips where a gap is present between the surfaces which are otherwise bonded together, the thickness of the gap in the dimension normal to the sheet being of the order of the thickness of the layer of bonding agent, the lack of bonding in the strips being due to the action of a masking agent applied to the surfaces to be bonded before the bonding occurred.