GB2551386A

GB2551386A - Water craft jet pump heat exchanger with secondary pickup

Info

Publication number: GB2551386A
Application number: GB1610587.6A
Authority: GB
Inventors: Owen Hutchinson Thomas
Original assignee: BWM RIBS Ltd
Current assignee: BWM RIBS Ltd
Priority date: 2016-06-17
Filing date: 2016-06-17
Publication date: 2017-12-20
Also published as: GB201610587D0

Abstract

A water jet propelled water craft (600, fig.10) has a jet pump impeller (608) mounted in a jet tube (610) and driven by a water cooled engine (604). A heat exchanger 100 for the engine coolant is incorporated into the jet tube (610) and takes the form of a chamber 104 formed by an inner tube 106 disposed around, or forming part of, the jet tube (610); an outer tube (58, figs.6,8) surrounding, and spaced-apart from, the inner tube 106; and annular end walls 110, 112 closing off opposite ends of the chamber 104. The chamber 104 has an inlet 116 and an outlet 118 separated by a divider 120 causing the fluid to follow a generally circumferential flow path through the chamber 104. A fluid pickup 124 for a secondary coolant system comprises a through aperture 128 in the inner tube 106, a secondary outlet 132 and a conduit 130, which are disposed within the chamber 104 and are located wholly to one side 98 of the divider 120. The chamber 104 may contain baffles. The divider (220, fig.4) may be double-walled. The pick-up conduit may be formed by a U-shaped rib (332, fig.5).

Description

Title: Water craft jet pump heat exchanger with secondary pickup

Description:

The present invention relates to a to a heat exchanger with secondary pickup for a jet pump propelled water craft, such as Rigid Inflatable Boats (RIBS) and jet tenders, but without limitation to same, and to a jet pump incorporating a heat exchanger with secondary pickup and to a jet propelled water craft incorporating a heat exchanger with secondary pickup.

There is a large market for small powerboats for use as pleasure craft, tenders to larger boats, safety boats, etc. and many small powerboats are of the "rigid inflatable" type known as RIBS. A RIB generally comprises a hull having a rigid bilge portion and an inflatable bulwark defining an outer periphery of the vessel. RIBS are particularly suited for use as tenders, safety boats and for use in confined areas.

Many RIBS are powered by outboard engines which are normally mounted on brackets at the stern of the boat. However, these can be unsightly and noisy and higher specification RIBS therefore generally comprise an inboard engine which is concealed beneath a quarter deck or seat of the boat. Such engines are most usually water-cooled, and whilst most use an open-circuit cooling system to cool the engine, some use a closed-loop cooling system in which an engine-driven pump circulates a cooling liquid, such as a water/glycol mixture, through a cooling circuit which includes a heat exchanger. In such cases, the heat exchanger typically takes the form of a plate heat exchanger mounted on the underside of hull of the boat so as to be in contact with the water in which the boat is being operated (hereinafter, the "lake/sea water"); and having connected thereto heat conducting pipes through which the cooling liquid flows. By this means, as the boat moves through the water, heat transferred to the plate from the engine cooling water is subjected to the cooling action of the lake/sea water with which it is in contact. Some RIBS are of the "jet propulsion" type which utilise an inboard engine driving a "jet pump", rather than a conventional screw propeller. In a "jet pump" design the engine is connected to an impeller of a jet pump assembly by a prop shaft. The jet pump assembly is disposed in a "jet tube" which has a water inlet aperture though which lake/sea water can enter the jet pump and be pumped by the impeller and forced out though an outlet of the jet tube. The boat can thus be propelled forwards or backwards in the water, according to the direction of thrust of the water jet; and steered by vectoring the thrust from the jet pump using a movable nozzle or other vector adjusting means, such as a rudder placed downstream of the jet tube's outlet.

The present invention is particularly concerned with a heat exchanger for a watercraft that uses a jet pump propulsion system and which is driven by a water-cooled engine. Usually a plate type heat exchanger of the above described type is used. These have the disadvantage of requiring a large surface area to achieve the desired cooling efficiency and hence add to the weight of the boat. Furthermore, because these types of craft frequently use a planing hull, the heat exchanger has to be positioned towards the rear of the hull to ensure adequate water contact, in use. This can shift the centre of gravity of the boat aft and/or dictates positioning the jet amidships which can give impaired handling and weight distribution. It may also dictate using longer pipe runs in the cooling circuit with a consequential weight penalty and an increased likelihood of the pipes becoming blocked. Additionally, the cooling efficiency of the plate-type heat exchanger is reduced quite considerably when the vessel is stationary because, unless the lake/sea water is moving relative to the plate heat exchanger, the lake/sea with which it is in contact tends to heat up, and eventually stops providing a useful cooling effect.

The present invention seeks to address some of the problems associated with the use of a hull-mounted plate heat exchanger by dispensing with it.

One prior art proposal for doing this is described in GB2363453 in which a marine jet drive incorporates a heat exchanger. The heat exchanger comprises a stator in the jet tube which has several hollow vanes connecting an inlet manifold of the heat exchanger with an outlet manifold of the heat exchanger. Because the stator vanes lie in the jet tube there is a limit to the number of vanes that can be used if the vanes are not to have the disadvantage of reducing the jet tube crosssection. Thus to achieve the desired effective surface area of the vanes the jet tube size has to be increased or the vanes have to be extended along a greater length of the jet tube.

Another prior art proposal is described in PCT application No: W02014080218, which proposes, inter alia, providing a jet pump assembly comprising a housing having a through bore accommodating a pump impeller and a heat exchanger, the heat exchanger comprising a chamber disposed outwardly of the through bore and having an inlet and an outlet for fluid to pass through the heat exchange chamber in use, in conjunction with a series of staggered radial baffles in the chamber defining an oscillatory and circumferential flow path for the fluid between the inlet and outlet. This document also discloses providing a secondary pickup port, which communicates with the lake/sea water within the through bore. The secondary pickup collects lake/sea water under pressure (the water in the jet tube being pressurised by the impeller, in use), and can be used to cool another component of the engine, such as the exhaust system or intercooler. The location of the secondary pickup in W02014080218 is within the dividing wall between the hot and cold sides of the heat exchanger, but this can lead to a number of drawbacks including, but without limitation to: a thermal gradient, in use, across the dividing wall. This can lead to cyclic differential expansion and contraction, which may eventually lead to fatigue failure in, say, components made from aluminium. This can also lead to heat transfer across the dividing wall, thereby reducing the efficiency of the heat exchanger; and one "side" of the secondary pickup (as it is formed as part of the dividing wall) being in contact with the hot side of the heat exchanger. This can cause, in use, the lake/sea water in the secondary pickup to be pre-heated, which is generally considered to be a disadvantage in most cooling applications. A need therefore exists for a solution to one or more of the above problems and this invention aims to provide such a solution and/or an alternative type of heat exchanger, and in particular, an alternative type of heat exchange to that disclosed in W02014080218.

Various aspects of the invention are set forth in the appended claims.

According to a first aspect of the invention, there is provided a heat exchanger for use in association with a jet tube of jet propulsion water craft, the heat exchanger comprising: a chamber having an interior volume defined by: an inner tube which, in use, is disposed around the jet tube or forms a peripheral part of the jet tube; an outer tube surrounding, and spaced-apart from, the inner tube; and first and second annular end walls closing off opposite ends of the chamber; the chamber having an inlet and an outlet for fluid to pass through the chamber in use; a divider within the chamber separating the inlet from the outlet thereby causing the fluid to follow a generally circumferential flow path through the chamber between the inlet and the outlet; the heat exchanger being characterised by: a fluid pickup comprising a through aperture in the inner tube, a secondary outlet and a conduit extending between the through aperture and the secondary outlet, wherein the through aperture and the conduit are disposed within the chamber and are located wholly to one side of the dividing wall.

The through aperture and the conduit are disposed within the chamber and are located wholly to one side of the dividing wall, and suitably are separate from, i.e. do not form any part of, the dividing wall.

The dividing wall serves to separate the "hot side" from the "cold side" of the heat exchanger, as well as preventing endless circulation of a cooling fluid (e.g. water/glycol mixture) around the chamber.

By placing the through aperture and the conduit wholly to one side of the dividing wall, the problems mentioned above with locating the secondary pickup within, or forming part of, the dividing wall are overcome. In a preferred embodiment of the invention, the through aperture and the conduit wholly located on the "cold" side of the divider, that is to say, close to, or as near to as possible, the cold outlet of the heat exchanger. This has the effect of reducing, as much as possible, any pre-heating effect. Alternatively, however, where the lake/sea water flowing through the fluid pickup needs to be pre-heated, the through aperture and the conduit can be wholly located on the "hot" side of the divider, that is to say, close to, or as near to as possible, the hot inlet of the heat exchanger.

Generally speaking, to increase the cooling efficiency of the heat exchanger, the "latency" of the cooling fluid (that is to say, the time that cooling fluid remains within the chamber as it flows between the inlet and outlet) is maximised. In a preferred embodiment of the invention, this is accomplished by increasing the length of the fluid flow path through the chamber. Specifically, this can be accomplished by the heat exchanger's chamber further comprising one or more baffles, which serve to direct the flow of coolant through the chamber along a serpentine flow path.

Preferably, the chamber of the heat exchanger comprises a set of first baffles within the chamber extending from the first end wall, but terminating short of the second end wall, to form a corresponding set of first openings between the ends of each first baffle and the second end wall; and a set of second baffles within the chamber extending from the second end wall, but terminating short of the first end wall, to form a corresponding set of second openings between the ends of each second baffle and the first end wall. By such means, the divider and baffles together can cause the fluid to follow a generally circumferential and alternating axial flow path through the chamber between the inlet and the outlet. This increases the length of the flow path of coolant through the chamber, and thus increases its latency.

The configuration of the sets of first and second baffles can vary. However, various options are envisaged, including, but without limitation to:

The first and second sets of baffles being circumferentially staggered, which arrangement may serve, in use, to turn the flow direction through substantially 180 degrees at each opening;

The baffles of each set being substantially parallel to one another, and/or all of the baffles extending substantially parallel to an axis of the inner tube, which arrangement may increase the length of the flow path of coolant through the chamber, without the changes of direction of the coolant exceeding 180 degrees at each opening;

The baffles extending part-helically and non-parallel to an axis of the inner tube, which may reduce the resistance or back-pressure of the heat exchanger in certain embodiments;

The first set of baffles being substantially parallel to one another, the second set of baffles being substantially parallel to one another, but the first and second sets of baffles extending part-helically and non-parallel to an axis of the inner tube; or

Other baffle configurations.

In order to facilitate connection of the heat exchanger to an open-loop cooling circuit of an engine to which is it connected, the secondary outlet may be formed in the first or second end wall, and the conduit may extend substantially axially through the chamber. In such a configuration, the can conduit form, or be part of a baffle located within the chamber, or where provided, one of the first or second baffles. Alternatively, the secondary outlet may be formed in the outer tube and the conduit may thus extend substantially radially through the chamber.

In order to properly separate the inlet from the outlet (i.e. the hot and cold sides of the heat exchanger), the divider suitably comprises a wall extending between the inner and the outer tube, and between the first and second end walls. Preferably, and possibly to reduce the transmission (e.g. the conduction/convection) of heat from the hot side to the cold side of the heat exchanger, the divider may comprise a plurality of spaced-apart walls, each wall extending between the inner and the outer tube, and between the first and second end walls. The spaced-apart walls may form an air gap therebetween, or in certain embodiments of the invention, the gap between the spaced-apart walls may be at least partially filled with a thermally-insulating material.

To facilitate manufacture, the inner tube and any one of more of the group comprising: the first end wall, the second end wall, the baffles, the divider and the conduit may be integrally formed. Alternatively, the outer tube and any one of more of the group comprising: the first end wall, the second end wall, the baffles, the divider and the conduit may be integrally formed. Either configuration may facilitate forming/manufacturing the said parts integrally, say, as a unitary casting.

In order to make best use of the available surface area of the inner tube in contact with the lake/sea water, the chamber suitably extends circumferentially by up to 360°, and preferably through 360°.

Suitably, the fluid pickup forms a lake/sea water pick-up for a secondary coolant system, such as an exhaust or intercooler cooling system. Because the fluid pickup communicates with the inner tube, the pressure of the water within the inner tube, which forms part of a jet tube, is usually pressurised, in use. This usefully provides a pressurised flow of water, the pressure of which is usefully related, in many cases, to the work rate of the engine. Therefore, the higher the throttle setting of the engine, the higher the lake/sea water pressure within the Jet tube will be, and so the higher the pressure of lake/sea water at the fluid pickup will be. Therefore, when connected to a secondary cooling system of an engine, the greater its work rate (and hence the greater its cooling requirement), the greater may be the flow of cooling lake/sea water for the engine's secondary cooling system(s).

The interior of the inner tube may comprise one or more stator vanes, which may be moveable, for example, to effect thrust vectoring, in use, of a Jet propulsion system of which the heat exchanger forms part. Additionally or alternatively, one or more of the stator vanes may directly or indirectly support a bearing for the pump impeller of a Jet propulsion system of which the heat exchanger forms part. A second aspect of the invention provides a Jet propulsion system for a Jet propulsion water craft comprising an engine, a prop shaft connecting a rotary output of the engine to a pump impeller located within a Jet tube, a heat exchanger as herein-described forming part of the Jet tube, the engine having a closed-loop cooling system connected to the inlet and the outlet of the heat exchanger and adapted to circulate coolant fluid through the heat exchanger.

In the said Jet propulsion system, the closed-loop cooling system suitably comprises a pumped, primary cooling system for the engine. The engine may further comprise an open-loop cooling system, to whose inlet, the fluid pickup of the heat exchanger is operatively connected. The open-loop cooling system is suitably used to cool one or more of: an exhaust system; and an intercooler, of the engine. A third aspect of the invention provides a water jet propelled water craft comprising the Jet propulsion system as herein described. The water Jet propelled watercraft can be a Rigid Inflatable Boat ("RIB); or a Semi-Rigid Inflatable Boat ("S-RIB"), such as that described in published PCT application No: WO2015198026. The RIB or S-RIB suitably comprises a rigid hull portion and an inflatable tube extending around the rigid hull portion to form gunwales, as well as, for example, a central passenger area and a main deck upon which users can stand or upon which cargo can be placed. The Jet tube is suitably located below the main deck and comprises a sea water inlet and an outlet. The water Jet propelled watercraft may further comprise a steering nozzle located downstream of the outlet end of the jet tube. A (known/prior art) plate-type heat exchanger may have a size of say 200mm x 200mm giving a surface area of 0.02m^. However, the invention can provide a more compact arrangement by virtue of the possibility of it being integrated into the existing Jet tube of a water Jet propulsion system; and further, where the chamber is formed around a Jet tube of say 150mm diameter, the same surface area (0.02m^) can be achieved over a length of 85mm. Other dimensions are, of course, possible and within the scope of this disclosure.

The ability to make the heat exchanger so compact allows the heat exchanger to be formed as part of the housing of the pump impeller. Alternatively, it may comprise one or more housing units that are positioned around the Jet tube or which themselves form a part of the Jet tube. The heat exchanger may comprise an annular housing having an inner bore that forms the or a part of the bore of the Jet tube. A series of housings may be used in combination to provide the required surface area in contact with the water in the Jet tube. An advantage of placing the heat exchanger in the Jet flow is that even when the pump impeller is at idle and the water craft stationary there will inevitably be some flow in the Jet tube, and therefore over the heat exchanger surface.

Optional, preferred and/or advantageous features of the heat exchanger are set out below. In one embodiment the heat exchange chamber forms a water jacket disposed radially outwardly of the jet tube bore. The water jacket at least partially surrounds the jet tube, but more preferably completely surrounds it. An annular heat exchange chamber is preferred. A particularly convenient and compact arrangement arises where the water jacket is part of the jet pump assembly and the water jacket has a plurality of baffles which serve to increase the length of the flow path for the cooling fluid between the inlet and outlet thereof. More particularly those baffles are disposed generally radially and spaced apart circumferentially. Preferably, adjacent baffles extend in from opposite axial ends of the housing and terminate short of the other end thereof to define a respective opening in the flow channel. The staggered baffles comprise an alternating series of first and second baffles. This staggered arrangement of the baffles serves to turn the flow direction through substantially 180 degrees at each opening thereby considerably increasing the length of the flow path and hence the cooling efficiency of the heat exchanger. The number of baffles and the axial and radial extent of the water jacket is determined by the cooling requirements. In one embodiment the baffles are aligned parallel to the axis of the jet tube and the axis of rotation of the pump impeller where provided. However, they could be disposed at other angles where this is found to improve the flow characteristics and/or avoid hot spots. The chamber extends by up to 360° around the jet tube.

The aforementioned heat exchange chamber preferably forms part of a closed loop cooling system and is usually used for cooling the coolant of the engine. However, it will be readily understood that the heat exchanger could be used for cooling other fluid systems. For example, it may be used as an oil cooler or an intercooler for induction air. It follows that there may be more than one heat exchanger with each serving a different purpose or that the heat exchanger may incorporate more than one cooling circuit. The other cooling circuits may be part of open or closed circuits.

The present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic side view of a prior art jet propulsion boat;

Figure 2 is a perspective view from one end and one side of the jet pump described in W02014080218;

Figure 3 is a perspective view from one end and the other side of a first embodiment of a heat exchanger in accordance with the invention, with its outer casing removed;

Figure 4 is a perspective view from one end and the other side of a second embodiment of a heat exchanger in accordance with the invention, also with its outer casing removed;

Figure 5 is a perspective view from one end and the other side of a third embodiment of a heat exchanger in accordance with the invention, also with its outer casing removed;

Figure 6 is a perspective view from one end and the other side of the first, second and third embodiments, but with its outer casing present;

Figure 7 is a perspective view from one end and the other side of a fourth embodiment of a heat exchanger in accordance with the invention, with its outer casing removed;

Figure 8 is a perspective view from one end and the other side of the fourth embodiment, but with its outer casing present;

Figure 9 is a schematic perspective view illustrating the circumferential and oscillating axial flow path of cooling fluid through a heat exchanger in accordance with the invention, such as those embodiments shown in Figures 4 and 5; and

Figure 10 is a diagrammatic side view of a jet propulsion boat comprising a heat exchanger in accordance with the invention.

The invention is described by way of example in relation to its application to cooling in a water craft provided with a jet propulsion tube. Referring firstly to Figure 1, a prior art jet propulsion Rigid Inflatable Boat (RIB) is illustrated which comprises a rigid hull portion 12 and an inflatable tube 14 extending around the rigid hull portion 12 to form gunwales. The RIB has a central passenger area (not illustrated) with main deck (not illustrated) upon which users can stand or upon which cargo can be placed. The hull is provided with a jet tube 18 which has a sea water inlet 20 and an outlet 22. A water-cooled internal combustion engine 16 is mounted in the hull and has a prop shaft 26 which drives a pump impeller 24. The pump impeller 24 is mounted for rotation in the jet tube 18. A steering nozzle 28 is provided at the outlet end of the jet tube 18. A heat exchanger plate 30 is mounted on the hull to towards the stern of the craft and aft of the jet outlet 22. Pipes 32, 34 extend from the water jacket of the engine 16 to the heat exchanger plate 30 to circulate the cooling water under the action of an engine driven water pump (not illustrated). It will be seen that this dictates positioning the jet tube amidships which can have an adverse effect on handling and weight distribution.

Referring now to Figure 2, there is illustrated the Jet pump assembly described W02014080218, which comprises a body 50 having a through bore 52 defined by inner wall element 53. The diameter of bore 52 corresponds substantially to the diameter of the Jet tube e.g.18 of the vessel in which the jet pump is to be used. A pump impeller (not illustrated) is mounted for rotation in the bore 52 and supported relative thereto by radial members 54 (similar to that shown in Figures 6 and 8). A bearing housing for the impeller is shown at 56. An outer cylindrical casing 58 (similar to that shown in Figures 6 and 8) which is larger in diameter than the inner wall element 53 is fitted to the body 50 and forms the outer wall of a fluid coolant chamber hereinafter referred to as a 'water' Jacket for ease of reference as the coolant fluid for the particular example described is likely to be a water/glycol mixture. However, it will be understood that the coolant may be another type of fluid as mentioned elsewhere in this description. The body has axially spaced annular flanges 60, 62 which, in practice, carry circumferential sealing elements, say in the form of 0-ring seals, disposed in seal grooves 64, to make a fluid tight seal with the outer casing 58. A flow channel is defined between the inner wall element 53, the end flanges 60, 62 and the outer casing 58 by the further provision of a plurality of baffles 70, 72. Baffles 70 extend in an axial direction from flange 60 toward flange 62 but terminate before reaching it to define openings 74. On the other hand, baffles 72 extend from flange 62 towards flange 60 but terminate before reaching it to define openings 75. This configuration defines a flow path with numerous reverses in direction (see arrows 77) and thereby increases the length of the flow path significantly compared with basic circumference of the jacket. End flange 60 incorporates an inlet connection (see arrow 79) via which the hot fluid coolant enters the jacket of the heat exchanger; and an outlet connection (see arrow 81) via which the cooled engine coolant fluid returns to the engine.

The known heat exchanger also has a pick-up point for a secondary coolant circuit which feeds coolant (e.g. lake or sea water) from the jet tube to an intercooler and exhaust outlet. The pick-up point is formed by a bore 90 in a boss 92. The bore 90 opens into bore 52 which in use forms part of the jet tube of the craft and merges with a cross-bore not visible, with an outlet connection in flange 60 (see arrow 83 showing the exit flow direction). Also illustrated is a wall 94 extending between the end flanges of the housing and separating the hot inlet side of heat exchange chamber from the cold outlet side. It will be noted that the boss 92 and wall 94 are united, or integrally formed, such that the boss 92 forms part of the dividing wall between the hot and cold sides of the heat exchanger. Thus, lake/sea water in the bore 90 or cross-bore (not shown) is partially heated by the hot side 96 of the heat exchanger (that is to say, the right-hand side in the drawing).

Various embodiments of the invention are shown in Figures 3 to 10 of the drawings.

Referring to Figure 3, a first embodiment of a heat exchanger 100 in accordance with the invention comprises a main body 102 formed as an aluminium casting, which forms part of a chamber 104. The chamber 104 is defined by an inner tube portion 106 whose inner surface 108 forms part of the wall of a jet tube, and which is in contact, in use, with lake/sea water for providing a cooling effect. The inner surface 108 is substantially cylindrical, which means that the impeller (not shown) of a jet pump can be easily accommodated inside it, if required.

The main body also has integrally formed end walls 110, 112, formed as flanges extending radially outwardly from the inner tube portion 106, and the outer surfaces of the end walls have grooves 114 in them for receiving 0-ring seals (not shown). The 0-ring seals (not shown) sealingly seat against the inner surface of an outer tube 58, which is shown for completeness in Figures 6 and 8, this forming a hollow interior volume, which is the chamber 104.

The chamber 104 has an inlet 116 and an outlet 118 for cooling fluid to pass through the chamber 104 in use. The inlet 116 and outlet 118 have ports formed in the first end wall 110 into which coolant fluid pipes (not shown) to and from a connected engine (not shown) can be screwed: and the ports communicate with the interior of the chamber 104 via conduits extending through the first end wall 112. A divider 120 is provided within the chamber 104, which separating the inlet 116 from the outlet 118; thus separating the hot side 96 of the heat exchanger 100 from its cold side 98. The divider 120 thereby causes cooling fluid to follow a generally circumferential flow path through the chamber 104, between the inlet 116 and the outlet 118, as indicated generally by arrows 122.

The heat exchanger 100 further comprises a fluid pickup 124. The fluid pickup 124 is formed as a boss 126 extending radially outwardly from the inner tube's surface 106, which has a through aperture 128 in it that extends all the way through the wall of the inner tube 106. A cross-bore (not visible), within a tubular axial conduit 130 formed integrally with the inner tube 106, intersects the through aperture 128 and feeds into a secondary outlet 132. The secondary outlet 132 has port formed in the first end wall 110 into which coolant fluid pipe (not shown) leading to an open-loop cooling system (not shown) can be screwed. Lake/sea water can thus be forced into the open-loop cooling system using the pressure of lake/sea water within the jet tube, via the through aperture 128, conduit and port 132.

It will be noted that, in contrast to the known heat exchanger shown in Figure 2 of the drawings, that the fluid pickup 124, that is to say, the boss 126, through aperture 128, cross-bore and axial conduit 130, are all located to one side of the divider 120. In the illustrated embodiment, this places the fluid pickup 124 entirely on the cold side 98 of the heat exchanger 100. Flowever, it will be readily appreciated that if the inlet 116 and outlet 118 were to be reversed (i.e. with the inlet being port 118 and the outlet being port 116), then the hot 96 and cold 98 sides of the heat exchanger 100 would be reversed, thus putting the fluid pickup 124 entirely on the hot side 96 of the heat exchanger 100.

The second embodiment of the invention 200, as shown in Figure 4, is broadly similar to that described above in relation to Figure 3, but with the following two modifications:

First, the divider 220 in the second embodiment of the heat exchanger 200 is formed as a pair of slightly spaced apart walls 222, 224, thus forming a small gap 226 between the two. This gap 226 can, in certain embodiments, be packed with a thermally insulative material, such as a thermal grout. The provision to two dividing walls 222, 224 and the gap 226 and/or thermally insulative material serves to better thermally separate the hot 96 and cold 98 sides of the heat exchanger 200 from one another. The result is a reduced transference of heat from the relatively hotter coolant fluid in the hot side 96 of the heat exchanger 200 (where it enters the chamber 104 via the inlet 116) to the cold side 98 of the heat exchanger 200 (where it leaves the chamber 104 via the outlet 118).

Second, the chamber 104 is provided with two sets of baffles 230, 240. The first set of baffles 230 extend from the first end wall 110 but terminate short of the second end wall 112 to leave gaps forming a first respective set of openings 232. The second set of baffles 240 extend from the second end wall 112 but terminate short of the first end wall 110 to leave gaps forming a second respective set of openings 242. This configuration of baffles gives rise to a generally circumferential flow path of coolant between the inlet 116 and outlet 118, as well as an alternating axial flow path of coolant. This is shown schematically by the arrows 250 in Figure 4, which illustrate how the flow direction of the coolant fluid reverses as it passes through each successive opening 232, 242. It will also be noted that the fluid pickup 124, that is to say, the boss 126, through aperture 128, cross-bore and axial conduit 130 form one of the first baffles 230, in the illustrated embodiment, thus diverting the flow of coolant 250' around the boss 126 and axial conduit 130.

Turning now to Figure 5 of the drawings, a third embodiment of the invention 300 is shown, which is broadly similar to that described above in relation to Figure 4, but with the following modification:

The axial conduit 330 of the fluid pickup 124, in this embodiment, is formed by a generally elongated U-shaped rib 332 extending radially outwardly from the inner tube 106. The through aperture 128 is drilled through the inner tube at the end of the "U" and thus the hydrodynamic resistance of the axial conduit 130 is reduced somewhat, compared to the configuration shown in Figures 3 and 4 of the drawings. Further, the fluid pickup 124 is effectively just a "wider" first rib 230, and this diverts the flow of coolant 250' around the fluid pickup 124 more smoothly than the configuration shown in Figures 3 and 4 of the drawings.

Figure 6 is a perspective view of the embodiments of the heat exchanger 100, 200, 300 shown in Figures 3, 4 and 5, but within the outer tube 58 fitted thereto.

Turning now to Figures 7 and 8 of the drawings, a fourth embodiment of a heat exchanger 400 in accordance with the invention is shown, which is broadly similar to that described above in relation to Figure 4, but with the following modification:

In this embodiment 400, the fluid pickup 424 is formed as a boss 426 extending radially outwardly from the inner tube's surface 106, which has a through aperture 428 in it that extends all the way through the wall of the inner tube 106, and which registers with the through bore 420 of an outwardly-radially-extending spigot 422 formed in the outside of the outer tube 58. An 0-ring seal 430 seats in a groove (not visible) formed in the upper edge of the boss 426, and that seals against the inner surface of the outer tube 58 in the region surrounding the through bore 420.

The main advantage of the configuration shown in Figures 7 and 8 of the drawings is that there are fewer changes of direction of the lake/sea water as it flows through the pickup 424, with a resultantly greater flow rate and/or pressure at the outlet port 132. A manifold 450 can thus be screwed into the spigot 422 to provide a plurality of open-loop cooling water feeds 132', 132" for a corresponding plurality of open-loop cooling systems.

Figure 9 of the drawings illustrates, schematically, the circumferential and axially oscillating coolant fluid flow path 500 for coolant fluid in the second, third and fourth embodiments of the invention. As described above, the chamber 104 is formed by the interior space between the inner tube 106, the outer tube 58 and the two end walls 110, 112. The inlet 116 feeds coolant into the chamber 104 on one side of the divider 120, 220, and the outlet 118 is located on the opposite side of the divider 120, 122. Thus, overall, the coolant fluid follows a generally circumferential flow path 502. However, because of the baffles (not shown in Figure 9 for clarity), the coolant fluid flow path 500 also goes from front to back in an oscillatory fashion 504, turning through 180 degrees at each end of the chamber 104. Thus, the length of the coolant fluid flow path 500 is considerably greater than the circumference of the chamber 104 and the length of the chamber 104, thereby increasing the time that the coolant fluid is in contact with the inner tube 106, thereby increasing the cooling efficiency of the heat exchanger.

Finally, Figure 10 of the drawings is a schematic illustration of a RIB 600 fitted with a heat exchanger in accordance with the invention.

The RIB 600 has hull 602 formed by a rigid hull portion 604 and an inflatable gunwale 606. The RIB 600 is powered by an inboard petrol or diesel engine 604, whose output is connected to a prop shaft 606, which drives an impeller 608 located within a jet tube 610 formed in the underside of the rigid hull portion 604. The jet tube 601 has an inlet 612, which draws in lake/sea water 614, and an outlet nozzle 616. Downstream of the outlet nozzle 616, there is provided thrust vectoring tube 618, which enables the RIB 600 to be steered by vectoring the jet of water produced by the jet pump. A heat exchanger 400 in accordance with the invention forms part of the jet tube 610 surrounding the impeller 608. Its inlet 116 and outlets 118 are connected to a liquid cooling jacket 620 of the engine 604, via feed and return pipes 622, and coolant is circulated through the liquid cooling jacket 620 and the heat exchanger by a pump 624. Meanwhile, the fluid pickup 424 of the heart exchanger 400 is connected 626 to an intercooler 628 of the engine 604, and the lake/sea water from the intercooler 628 is discharged into a "wet" exhaust system 630 of the engine 604. The wet exhaust system 630 also comprises a silencer 632, whose muffling effect is improved by a constant flow of water through it. This constant flow of lake/sea water 614 is provided by a further feed pipe 634, which is also connected to the fluid pickup 424 of the heat exchanger 400.

All of the foregoing is housed within the space located between the rigid hull portion 604 and a foredeck 636, quarter deck 638 and/or seat/cockpit 640 of the RIB 600.

The foregoing embodiments are merely exemplary of several possible embodiments of the invention, and are not intended to be strictly restrictive of the scope of the invention. For example, it will be understood that the water craft does not have to be a RIB or an S-RIB. The heat exchanger comprises a chamber or water jacket which surrounds the Jet tube and more preferably still is disposed in close proximity to the Jet pump impeller, although that is not to say that it cannot be disposed at any convenient position along the Jet tube, for example fore and/or aft thereof when not forming part of the pump impeller assembly itself. Indeed, several heat exchanger modules may be provided at different positions. Engine coolant pipes connect with appropriately positioned ports on the heat exchanger, for example, as described herein; but the position in which they are shown in the drawings is merely diagrammatic. Further, whilst several specific embodiments of the invention have been described, each having certain "modifications" compared with the others, this does not mean that certain features, or combinations of features, from one embodiment could not equally be applied in or to a different embodiment. Further, the shapes, dimensions, materials etc., whether mentioned explicitly, or whether they can be implied from the foregoing description are not necessarily restrictive of the scope of the invention, but are merely stated to illustrate certain embodiments of the invention.

Claims

Claims:

1. A heat exchanger for use in association with a jet tube of jet propulsion water craft, the heat exchanger comprising: a chamber having an interior volume defined by: an inner tube which, in use, is disposed around the jet tube or forms a peripheral part of the jet tube; an outer tube surrounding, and spaced-apart from, the inner tube; and first and second annular end walls closing off opposite ends of the chamber; the chamber having an inlet and an outlet for fluid to pass through the chamber in use; a divider within the chamber separating the inlet from the outlet thereby causing the fluid to follow a generally circumferential flow path through the chamber between the inlet and the outlet; the heat exchanger being characterised by: a fluid pickup comprising a through aperture in the inner tube, a secondary outlet and a conduit extending between the through aperture and the secondary outlet, wherein the through aperture and the conduit are disposed within the chamber and are located wholly to one side of the dividing wall.

2. The heat exchanger of claim 1, further comprising: a set of first baffles within the chamber extending from the first end wall, but terminating short of the second end wall, to form a corresponding set of first openings between the ends of each first baffle and the second end wall; a set of second baffles within the chamber extending from the second end wall, but terminating short of the first end wall, to form a corresponding set of second openings between the ends of each second baffle and the first end wall; the divider and baffles together causing the fluid to follow a generally circumferential and alternating axial flow path through the chamber between the inlet and the outlet.

3. The heat exchanger of claim 1 or claim 2, wherein the secondary outlet is formed in the first or second end wall, and the conduit extends substantially axially through the chamber.

4. The heat exchanger of claim 4, wherein the conduit forms, or is part of, one of the first or second baffles.

5. The heat exchanger of claim 1 or claim 2, wherein the secondary outlet is formed in the outer tube and the conduit extends substantially radially through the chamber.

6. The heat exchanger of any preceding claim, wherein the divider comprises a wall extending between the inner and the outer tube, and between the first and second end walls.

7. The heat exchanger of claim 6, wherein the divider comprises a plurality of spaced-apart walls, each wall extending between the inner and the outer tube, and between the first and second end walls.

8. The heat exchanger of claim 7, wherein the gap between the spaced-apart walls is at least partially filled with a thermally-insulating material.

9. The heat exchanger of any of claims 2 to 8, wherein the first and second sets of baffles are circumferentially staggered.

10. The heat exchanger of any of claims 2 to 9, wherein the arrangement of the baffles serves, in use, to turn the flow direction through substantially 180 degrees at each opening.

11. The heat exchanger of any of claims 2 to 10, wherein the baffles are substantially parallel to one another.

12. The heat exchanger of claim 11, wherein the baffles extend substantially parallel to an axis of the inner tube.

13. The heat exchanger of claim 11, wherein the baffles extend part-helically and non-parallel to an axis of the inner tube.

14. The heat exchanger of any of claims 2 to 9, wherein the first set of baffles are substantially parallel to one another, wherein the second set of baffles are substantially parallel to one another, but wherein the first and second sets of baffles extend part-helically and nonparallel to an axis of the inner tube.

15. The heat exchanger of any preceding claim, wherein the inner tube and any one of more of the group comprising: the first end wall, the second end wall, the baffles, the divider and the conduit are integrally formed.

16. The heat exchanger of any of claims 1 to 14, wherein the outer tube and any one of more of the group comprising: the first end wall, the second end wall, the baffles, the divider and the conduit are integrally formed.

17. The heat exchanger of claim 15 or 16, wherein the said parts are integrally formed as a unitary casting.

18. The heat exchanger of any preceding claim, wherein the chamber extends circumferentially by up to 360°.

19. The heat exchanger of any preceding claim, wherein the fluid pickup forms a lake/sea water pick up for a secondary coolant system.

20. The heat exchanger of any preceding claim, wherein the interior of the inner tube comprises one or more stator vanes.

21. The heat exchanger of claim 20, wherein a stator vane is moveable to effect thrust vectoring, in use, of a jet propulsion system of which the heat exchanger forms part.

22. The heat exchanger of claim 20 or claim 21, wherein a stator vane directly or indirectly supports a bearing for a pump impeller of a jet propulsion system of which the heat exchanger forms part.

23. A jet propulsion system for a jet propulsion water craft comprising an engine, a prop shaft connecting a rotary output of the engine to a pump impeller located within a jet tube, a heat exchanger according to any preceding claim forming part of the jet tube, the engine having a closed-loop cooling system connected to the inlet and the outlet of the heat exchanger and adapted to circulate coolant fluid through the heat exchanger.

24. The jet propulsion system of claim 23, wherein the closed-loop cooling system comprises a pumped, primary cooling system for the engine.

25. The jet propulsion system of claim 23 or claim 24, wherein the engine further comprises an open-loop cooling system, wherein the inlet of the open loop cooling system is connected to the fluid pickup of the heat exchanger.

26. The jet propulsion system of claim 25, wherein the open-loop cooling system is used to cool one or more of: an exhaust system; and an intercooler, of the engine.

27. A water jet propelled water craft comprising the Jet propulsion system of any of claims 23 to 26.

28. The water jet propelled watercraft of claim 27, wherein the water craft is a RIB or an S-RIB, comprising a rigid hull portion and an inflatable tube extending around the rigid hull portion to form gunwales.

29. The water jet propelled watercraft of 28, wherein the RIB or S-RIB comprises a central passenger area and a main deck upon which users can stand or upon which cargo can be placed, and wherein the jet tube is located below the main deck and comprises a sea water inlet and an outlet.

30. The water jet propelled watercraft of any of claims 27 to 29, further comprising a steering nozzle located downstream of the outlet end of the jet tube.

31. A heat exchanger substantially as hereinbefore described, with reference to, and as illustrated in Figures 3 to 10 of the accompanying drawings.