GB2446472A

GB2446472A - Gas heat exchanger

Info

Publication number: GB2446472A
Application number: GB0710922A
Authority: GB
Inventors: Charlie Penny
Original assignee: Senior UK Ltd
Current assignee: Senior UK Ltd
Priority date: 2007-06-07
Filing date: 2007-06-07
Publication date: 2008-08-13
Anticipated expiration: 2027-06-07
Also published as: GB0710922D0; GB2446472B

Abstract

A gas heat exchanger comprises a rigid gas bypass tube 209 forming a gas passage, a concentric heat transfer tube 206 surrounding the gas bypass tube 209 and transferring heat between a gas and a fluid coolant, a tubular outer casing 100 encasing at least partially the heat transfer tube 206 and at least a portion of the bypass tube 209, and a plurality of helical undulations extending along a length of a wall of the heat transfer tube 206. Bypass tube 209 may be rigid enough to form a main load bearing component of the heat exchanger that may be installed in an exhaust system of a vehicle. The helical undulations may be provided with a plurality of secondary forms (fig 4) such as corrugations that comprise of a plurality of parallel ribs on the sides and on a crest of the helical ridges or within a valley between a pair of the helical ridges. The corrugations may comprise of scallop protrusions protruding from the sides of helical ridges (fig 7). The heat transfer tube 206 may comprise of a number of helix starts. Bypass tube 209 may have a smooth or helically formed outer surface. Outer casing 100 may have a plurality of bellows formations 203 for absorbing expansion or contraction, and an inlet and outlet pipe 104 for coolant.

Description

IMPROVED GAS HEAT EXCHANGER

Field of the Invention

The present invention relates to gas heat exchangers, and particularly although not exclusively to gas heat exchangers for use in automotive applications.

Background to the Invention

There are many applications in which it is desirable to use gas heat exchangers to cool down a gas, for example in exhaust gas recirculation (EGA) coolers, and applications in which the heat extracted from the gas can be recovered, one example is to warm the cabin of a vehicle.

Under some circumstances, heat extraction may be required, but under other circumstances it may be undesirable. For example, when starting a vehicle, heat may be extracted from engine exhaust gas to assist in warming up the passenger cabin of a vehicle. In this case, heat extraction is required. However, once the cabin reaches the required temperature, further warming could be undesirable. Therefore under these circumstances, heat extraction from the exhaust gas is not required.

Another example is exhaust gas recirculation. Exhaust gas recirculation is a method of reducing noxious emissions from internal combustion engines. In particular, the presence of exhaust gas in the combustion mixture reduces the tendency to form NOx compounds.

In general it is advantageous to cool the re-circulated exhaust gas. Its reduced temperature helps to lower the combustion temperature within the cylinder of the engine. Recirculated exhaust gas is also more dense when cooled, and therefore for a given mass of gas, a lower volume of air is displaced from the combustion chamber. Cooling the recirculated gas is not desirable under all conditions however. When engine temperature is low and under low engine loads it is often preferable to recirculate exhaust gas without cooling.

Many applications requiring gas heat exchange, including EGR systems, therefore require a bypass channel to control whether the gas is cooled or not.

When heat losses from the gas are to be minimised, the gas is diverted through the bypass channel. When it is required for heat losses from the gas to be increased, that is to say, when the gas is being cooled, the efficiency of the cooler should be high, and the gas is not passed through the bypass channel.

A known exhaust gas re-circulation cooler typically comprises at least one gas cooling conduit configured to carry gas, at least one conduit configured to carry a coolant fluid, and a bypass conduit. The coolant conduit and the gas cooling conduit are in close proximity, such that gas that is transported through the gas cooling conduit is in proximity to coolant fluid and therefore is cooled down. When gas cooling is required, the gas is diverted through the gas cooling conduit. Under circumstances where gas cooling is not required, then the gas is diverted through the bypass conduit. A bypass valve controls whether the gas is carried in the gas cooling conduit or the bypass conduit. For EGR applications, the bypass valve is separate from an EGR valve, which controls whether recirculated exhaust gas is flowing at all.

When gas is being transported through the bypass conduit, it is undesirable for the gas to be cooled. To achieve this, there should be little or no contact between the bypass conduit and the coolant conduit, to avoid the coolant fluid in the coolant conduit cooling the gas that is transported through the bypass conduit.

Prior art solutions to minimise contact between the bypass conduit and the coolant conduit are already known.

In US2003/150434, and W003085252, it is known to use an external bypass channel. However, the external bypass channel takes up additional space, which is a disadvantage for applications where packaging in the engine bay space is restricted. However, because the bypass conduit is external to the exhaust gas cooling conduit and the coolant conduit, the bypass conduit avoids being cooled by the coolant conduit.

A solution to the extra space requirement of the external bypass system has been provided in US 6718,956 in which the bypass conduit is disposed within the main housing. The housing comprises a coolant conduit, in which a series of gas cooling conduits are disposed and also in which a bypass conduit is disposed.

However, in this type of cooler, the bypass conduit is in contact with the coolant, which is undesirable since there is residual cooling of the gas even when bypassed. Complicated modifications are required to minimise the degree of cooling between the coolant fluid and the bypass conduit when the exhaust gas is flowing through the bypass conduit. These include having a double -walled bypass conduit with a substantially stagnant gas layer between the two walls to reduce a heat exchange between the coolant fluid contained in the coolant conduit and the exhaust gas carried by the bypass conduit.

Another disadvantage of having a bypass conduit is that the material in the bypass conduit acts as a heat sink when hot EGR gas is diverted through it. A transient period occurs during which some heat is extracted from the gas by contact with the conduit wall.

An alternative to having a bypass conduit is to have a valve that controls whether the coolant fluid flows through the coolant conduit or not. In this instance, a bypass conduit for the gas is not required. If cooling of the exhaust gas is required, then coolant flows through the coolant conduit and the exhaust gas is cooled. If cooling of the exhaust gas is not required then coolant does not flow through the coolant conduit and therefore the exhaust gas is not cooled.

However, there are problems with this type of system. For example, when cooling is not required, residual coolant can be left in the coolant conduit.

Coolant fluid typically contains volatile additives, which may be given off when the conduit is heated. The formation of steam and volatile substances is not desirable.

A further development in which there is disclosed a gas heat exchanger having a thermal barrier between a coolant conduit and a gas bypass conduit is known from GB0417920.6 (GB2417067). In that system, there is a tn -concentric tube arrangement in which coolant flows in an outer cylindrical void, which surrounds an inner cylindrical channel which carries exhaust gas in a heat extraction gas cooling mode, and a central bypass tube is provided to bypass the exhaust gas when heat extraction is not required.

Due to the ongoing pressure on vehicle manufacturers to reduce greenhouse gas emissions, this manifests itself as a requirement for reduced size vehicles and consequently a requirement for increased efficiency in exhaust gas heat exchangers. Further there is always a need for smaller size heat exchanger components in automotive applications, and improvements in heat exchangers which can lead to a lower component size or lower installation volume.

Summary of the Invention

Specific embodiments disclosed herein aim to provide a tn -concentric heat exchanger with an improved heat transfer efficiency whilst maintaining a low gas pressure drop.

Specific embodiments herein further aim to provide heat exchanger which allows installation in a reduced sized space compared to conventional heat exchangers.

According to a first aspect there is provided a gas heat exchanger comprising: a rigid gas bypass tube forming a bypass gas passage; a concentric heat transfer tube surrounding said bypass tube said heat transfer tube capable of transferring heat between a gas and fluid coolant; and a tubular outer casing, encasing at least partially said heat transfer tube and encasing at least a portion of said bypass tube, wherein said heat transfer tube comprises a tubular wall formed into a plurality of parallel helical undulations extending along a length of said heat transfer tube.

Said gas bypass tube may either be a smooth cylindrical surface or alternatively have low parallel helical undulation extending along length of said bypass tube at the same pitch as the heat transfer tube Preferably, said bypass tube is rigid enough to form a main load bearing component of the heat exchanger, such as to be installable in an exhaust system of a vehicle without the need for additional supports or mounts.

Preferably, said heat transfer tube comprises surface wall having a plurality of helical ridges, each said helical ridge being provided with a plurality of secondary forms, of relatively reduced depth compared to a depth of said helical ridges.

Peferably, said outer casing comprises a plurality of annular bellows formations, capable of absorbing expansion or contraction in a direction axially of a main length of the outer casing.

Preferably, a chamber is formed between said outer casing and said heat transfer tube, for containing coolant fluid which contacts a wall of said heat transfer tube, passes through said chamber and thereby cools said wall of said heat transfer tube.

Preferably, said outer casing is provided with a coolant inlet pipe and a coolant outlet pipe for passing coolant through said outer casing.

Preferably, said heat transfer tube is arranged concentrically inside said outer casing; said bypass tube is arranged concentrically inside said heat transfer tube; and the bypass tube is arranged concentrically within said heat transfer tube and said outer casing.

The design of the heat transfer tube is driven by the heat exchange and gas pressure drop targets. The maximise heat exchange the surface area should be as large as possible, to minimise gas pressure drop the gas path cross sectional area should be maximised.

There may be a number n of parallel helices formed in a tube of outer diameter D, and the pitch P of an individual helix is determined by the expression P=KxnxD where K is in the range 0.1 to 2.

A ratio of the radial depth of said helical corrugations to the outer diameter of the tube may be in the range 0.1 to 0.35.

A ratio of the depth of the secondary form to a depth of a said helical ridge may be in the range 0 to 0.5.

Preferably, said secondary form on the heat transfer tube have a minimum depth of 0.2 mm.

An aspect ratio of the secondary form is in the range 0.1 to 0.4, wherein the aspect ratio is defined as the ratio of depth of a secondary corrugation to its pitch between forms.

Preferably, said secondary forms comprise a series of parallel ribs running transverse to a main tangential direction of a said helix.

Said secondary forms may comprise a plurality of parallel ribs superimposed on said helical ridges, said ribs having a pitch of between 2mm and 10 mm, and a depth of between 0.2mm and 2 mm.

Said secondary forms may comprise a plurality of parallel ribs formed on the sides and on a crest of said helical ridges.

Said secondary forms may comprise a plurality of parallel ribs formed within a valley between a pair of said helical ridges.

Said secondary forms may comprise a plurality of scallop like indents protruding into the gas path from the sides of said helical ridges.

Said secondary corrugations may comprise a plurality of scallop like indents protruding into the gas path from the sides of said helical ridges, said protrusions being arranged alternately opposite each other such that a gas flowing in a valley between two said helical ridges is directed through a serpentine like path along a length of said valley and between said oppositely and alternately positioned scallop shaped protrusions.

Said heat transfer tube may have a clear internal diameter of between 25 mm and 200 mm.

Said heat transfer tube may have a clear internal diameter in the range 40 mm to 75 mm.

Said heat transfer tube may have a maximum outer diameter in the range 28 mm to 300 mm.

Said heat transfer tube may have a maximum outer diameter in the range mm to 110 mm.

Said heat transfer tube may have a number of helix starts in the range 2 to 12.

Preferably, said heat transfer tube has a number of helix starts in the range 5tolO.

Said heat transfer tube may have a plurality of helices, each helix having a tangent along its main length direction which makes an angle of between 30 and degrees to a main length axis of said heat transfer tube.

Preferably, said bypass tubes has a wall material thickness in the range 0.5 mm to 3 mm.

Said heat transfer tube may have a wall material thickness in the range 0.2mm to 1.0 mm.

Said outer casing may have a wall material thickness in the range 0.2 mm to 1.0 mm.

According to a second aspect there is provided a gas heat exchanger comprising: a rigid gas bypass tube forming a bypass gas passage; a concentric heat transfer tube surrounding said bypass tube said heat transfer tube capable of transferring heat between a gas and fluid coolant; and a tubular outer casing, encasing at least partially said heat transfer tube and encasing at least a portion of said bypass tube, wherein said gas bypass tube is rigid enough to form a main load bearing component of the heat exchanger, such as to allow the heat exchanger to be installable in an exhaust system of a vehicle without the need for additional supports or mounts.

Preferably, said outer casing comprises a plurality of annular bellows formations, capable of absorbing expansion or contraction in a direction axially to the main length of the outer casing.

According to a third aspect there is provided a method of manufacturing a heat transfer tube for a heat exchanger, said heat transfer tube comprising: a substantially tubular annular metal wall having a plurality of helical undulations, and formed on said plurality of helical undulations, a plurality of secondary forms, said method comprising: clamping a cylindrical annular tube between first and second forming tools, wherein said first forming tool is placed internally of said tube and said second forming tube is placed externally of said tube, said first and second tubes having matching helically undulated surfaces; urging said internal and external former tools towards each other; rotating said tube between said internal and external former tools and drawing the tube in an axial direction through said internal and external former tools; and hydroforming a plurality of said secondary forms by the internal application of a pressurised fluid to the tube, such that a wall of said tube is forced against an external tubular forming tool having a plurality of corrugated shaped indents.

According to a fourth aspect there is provided a gas heat exchanger comprising: a coolant conduit; a gas cooling conduit; and a gas by-pass conduit, each of said conduits comprising respective inlet and outlet ends and at least one of said conduits comprising a heat exchange surface having a first set of surface perturbations to enhance mixing of a gas flowing there-through whilst minirnising the pressure difference between its respective inlet and outlet ends, said heat exchanger characterised in that: said at least one conduit comprising said heat exchange surface comprises a second set of perturbations, superimposed on said first set of perturbations, configured to disrupt a laminar flow of said gas at the interface of said surface and said gas.

Preferably, at least one conduit comprising said heat exchange surface is in the form of a tube.

Preferably, said tube is substantially cylindrical.

Preferably, a said first set of surface perturbations comprise a plurality of elongate corrugations that extend helically relative to the longitudinal axis of said substantially cylindrically shaped tube.

A said surface perturbation of said second set of surface perturbations may comprise a main longitudinal axis that is transverse to the main longitudinal axis associated with said perturbation of said first set on which it is superimposed.

A said surface perturbation of said second set of surface perturbations may comprise a main longitudinal axis that is substantially perpendicular to said longitudinal axis associated with said perturbation of said first set of surface perturbations on which it is superimposed.

Preferably, at least one conduit comprising said surface includes said gas cooling conduit.

Preferably, said first set of surface perturbations have a depth profile that is greater than the depth profile of said second set of perturbations.

Preferably, said depth profile of said first set of perturbations is greater than 5.0 mm.

Preferably, said depth profile of said second set of surface perturbations lies in the range 0.2 mm to 2.5 mm.

Preferably, at least a portion of said gas cooling conduit is disposed between at least a portion of said coolant conduit and at least a portion of said gas by-pass conduit, said gas cooling conduit configured to provide a thermal barrier between said coolant conduit and said gas by-pass conduit.

Preferably, said heat exchanger is configured for use in an automotive application.

Said heat exchanger may be configured for use in exhaust gas re-circulation to thereby reduce the emission of pollutants from the exhaust of a vehicle.

Said heat exchanger may be configured for use in extracting heat from a vehicle exhaust stream in order to heat a passenger cabin of said vehicle.

According to a fifth aspect there is provided a method of manufacturing a heat exchanger of the type described above, said method comprising the steps of: obtaining a blank work piece from which to make said at least one conduit comprising a heat exchange surface having a set of surface perturbations to enhance mixing of a gas flowing there -through; forming said first set of surface perturbations on said piece using a forming tool; and forming said second set of surface perturbations on said piece using a tool that is pre -formed into the shape of the second set of surface perturbations.

Preferably, said step of obtaining said piece comprises obtaining a tube shaped piece.

Preferably, said step of forming said first set of surface perturbations comprises clamping said conduit between an internal forming tool and an external forming tool.

Preferably, said internal forming tool and said external forming tool have interlocking helical corrugation forms.

Preferably, said step of forming said second set of surface perturbations on said piece comprises a hydro-forming process.

Preferably the crest diameter of the bypass tube helix is smaller than the diameter of the root of the heat exchange tube allowing the two tubes to be place Co -axially without the need of a screwing motion.

The improved heat exchanger may have an advantage of providing an increased efficiency of heat transfer by the use of a ridged heat transfer tube, which means that a relatively lower sized heat exchanger may be used for a particular heat exchange application requiring a particular rate of heat exchange.

The improved heat exchanger may have an advantage of requiring a relatively lower volume or space for its installation because it can be installed in line within an exhaust channel of a vehicle without the need for any additional brackets of supports tot he vehicle body.

The embodiments include a heat exchanger having an integral bypass tube/conduit having a helical gas flow path, in which a helical inner tube is provided with a textured surface.

The embodiments include a bypass tube having said helical form that urges the gas to flow in the spiral path provided by the relatively deeper helical form of the heat transfer tube.

Other aspects of the invention are as set out in the claims herein.

Brief Description of the Drawings

For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Figure 1 illustrates schematically in perspective view a heat exchanger according to a specific embodiment of the invention; Figure 2 illustrates schematically in cut away view from one side, the heat exchanger of figure 1 herein, showing a bypass tube, a heat transfer tube, and an outer casing, coaxially arranged; Figure 3 herein illustrates schematically in cut away view, the heat exchanger of figures 1 and 2, showing the helically wound heat transfer tube, and the triconcentric arrangement of the exchanger; Figure 4 illustrates schematically an outer surface of a heat transfer tube of the heat exchanger of figures 1 to 3 herein; Figure 5 illustrates schematically in tangential section view, along a ridge or valley, a single helix of the heat transfer tube of a heat exchanger described in figures 1 to4; Figure 6 illustrates schematically in perspective view from above and in a direction axiaUy along a main length of a heat exchanger, a set of secondary forms superimposed on a plurality of helical ridges; Figure 7 illustrates schematically a helical ridge in part cut away view according to a second specific embodiment, wherein a plurality of scallop shaped protrusions are provided on the helical ridge; Figure 8 illustrates schematically a gas path along a helix of a heat transfer tube according to the second embodiment as shown in figure 7 herein; Figure 9 illustrates schematically in view from one side, a third heat transfer tube according to a first specific embodiment; Figure 10 illustrates schematically in close up view, a portion of the third heat transfer tube of figure 9; Figure 11 illustrates schematically in view from one end, the third heat transfer tube of figures 9 and 10 herein; and Figure 12 illustrates schematically a cut away view of the heat transfer tube and bypass tube 121 wherein the bypass tube has a relatively shallow helical form of the same pitch as the heat transfer tube.

Detailed Description

There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to

unnecessarily obscure the description.

Throughout the description, the following terms are used: Gas cooling conduit' refers to a conduit through which gas is configured to pass when heat extraction from the gas is required.

Coolant conduit' refers to a conduit through which coolant medium such as coolant fluid is configured to pass. A coolant medium may be used to reduce the temperature of the gas in the gas cooling conduit, or may be used to extract heat from the gas in the gas cooling conduit to be used elsewhere.

Bypass conduit' refers to a conduit through which gas is configured to pass when heat extraction from the gas is not required.

Referring to Figure 1 herein, there is illustrated schematically in perspective view a gas heat exchanger having an internal bypass conduit.

The heat exchanger comprises a substantially cylindrical outer casing 100 having a gas inlet 101 and a gas outlet 102; a coolant inlet tube 104 for ingress of coolant fluid; a coolant fluid outlet tube 103 for outlet of coolant fluid; internally of the outer casing 100, is provided a concentric tubular heat transfer tube, being substantially cylindrical, and having a helically corrugated wall; and internally and arranged coaxiauy with the heat transfer tube and the outer casing, a substantially cylindrical smooth walled hotlow tubular bypass tube, for bypassing gas through the cooler. Alternatively the bypass tube may have a low height helical form.

Referring to Figure 2 herein, there is illustrated schematically in bisected cut away view the gas heat exchanger of Figure 1 herein. Outer casing 100 comprises a substantially cylindrical holiow metallic tube, having at a first end, a frustoconical portion 200 forming an outlet chamber 201; at a second end, an inlet manifold 101 forming an inlet chamber 202, the outer casing having positioned between the first and second ends, a corrugated annular beflows portion 203 comprising a plurality of annular corrugations disposed along a length of the outer casing 100. The bellows portions are provided to accommodate expansion and contraction of the outer casing caused by the thermal growth of the bypass tube to which the outer casing is attached via other components at both ends.

There is provided internally of the outer casing a first bulkhead 204 at the first end of the heat exchanger, and a second bulkhead 205 at a second end of the heat exchanger. The first and second bulkheads are formed from the heat transfer tube material. The first and second bulkheads are substantially annular in form and extend inside the casing 100 between an inner wall of the casing 100, and the main part of the heat transfer tube 206, so that a substantially cylindrical annular cavity 207 is formed between the heat transfer tube 206, and the outer casing 100, and through which coolant fluid may be passed via the inlet 103 and outlet 104. The deep helical form of the heat transfer tube is sufficiently flexible to accommodate expansion and contraction caused by the thermal growth of the bypass tube to which the heat transfer tube is attached via other components at both ends The internal heat transfer tube 206 is substantially cylindrical in form, and comprises a helically formed wall, which separates the coolant cavity 207 from a first generally annular cylindrical gas passage 208 inside the heat transfer tube.

Concentrically aligned within the heat transfer tube 206, is the bypass tube 209, which forms a second gas channel through the heat exchanger. The bypass tube comprises a hollow substantially cylindrically walled smooth-bore tube which is positioned concentrically within the heat transfer tube 206. Alternatively the outer surface of the bypass tube at least may have a helical form.

The innermost bypass tube 209, which contains the bypass gas in bypass mode is relatively thick-walled, and acts as the main load-bearing member of the heat exchanger. The bypass tube has at its gas outlet end, but still within the heat exchanger, a number of holes or cutouts, in order to allow gas at the outlet end of the heat exchange channel to be merged with the main exhaust channel via the bypass tube.

Operation of the heat exchanger is as follows.

In a gas heat exchange mode, for example for cooling exhaust gases, exhaust gases enter the inlet chamber 202, and are routed via a bypass valve (not shown in Figure 2) through the first (cooling) gas passage 208 formed between the heat transfer tube 206, and the concentrically aligned internal bypass tube 209. Gas flows outside the bypass tube and inside the heat transfer tube in the gas chamber 208 there between. Since the wall of the heat transfer tube is helically formed, gas experiences swirling turbulent flow, and encounters the helical wall of the heat transfer tube, which is cooled from the other side by the coolant flowing through the coolant chamber 207 formed between the outer casing 100 and the heat transfer tube 206. The gas experiences a gradual decrease in temperature from the inlet chamber 202 to the outlet chamber 201, as heat is transferred from the gas to the coolant which continually flows through the heat exchanger under force of an external coolant pump. A plurality of apertures in the bypass tube 209 are provided which

connect the outlet chamber 201 to the bypass tube, so that when in heat exchange mode, with the gas passing through the heat exchange chamber 208 in path A, the gas may be passed out of the heat exchanger through the end of the bypass tube, which is also load bearing and forms a main structural component of the heat exchanger.

In the heat exchange mode, gas to be cooled is diverted into the channel formed in the space between the bypass tube and the heat exchange tube. At the end of the heat exchange channel, the gas is then merged with the main exhaust channel through the outlet end of the bypass channel, via aperture openings 210 between the bypass channel and the heat exchange channel.

Using this arrangement, this allows the bypass tube to be a unitary load bearing component which is attachable to remainder of a vehicle exhaust system, and to act as the outlet in both the bypass mode and the heat exchange mode.

During heat exchange mode, the gas is directed in the annulus between the bypass tube and the heat transfer tube. The cooled gas returns into the main stream of the bypass tube via the holes or cutouts 210 in the bypass tube.

In a bypass mode, where the gas is to avoid being cooled, the gas bypass valve is switched to route the incoming gas through the central bypass tube 209, and out of the outlet chamber 201, avoiding contact with the heat transfer tube 206. The internal bypass tube 209 is internally cylindrical and has a substantially smooth wall so as to impede the flow of gas as little as possible. Further, since the internal bypass tube 209 is spaced apart from and concentric with the wall of the heat transfer tube 206, there is a gap between the bypass tube and the wall of the heat transfer tube, which may contain residual gas, and which acts to insulate the bypass tube from the cooled wall of the heat transfer tube 206.

Consequently, if hot gases are routed through the bypass valve, after a period of time, the bypass tube may raise in temperature. Since the tube is insulated from -20.

the coolant by the cavity of the first gas passage 208, the gas transfers through the bypass channel relatively uncooled, or cooled to an insignificant extent, compared to the other parts of the exhaust system.

During bypass mode, the annular chamber between the bypass tube and the heat transfer tube is filled with nominally stagnant gas. This may act as an insulator between the bypass tube and the coolant.

The material thickness of the bypass tube is preferably in the range 0.5 mm to3mm.

The material thickness of the heat transfer tube is preferably in the range 0.2 mm to 1 mm.

The material thickness of the outer tube is preferably in the range 0.2 mm to 1mm.

The bypass channel may be effectively a continuation of the external gas system to which the heat exchanger is attached. The aperture openings which allow cool gas to merge with the main external gas channel may be perforations or slots in the bypass tube. They may also be "separators" whereby the bypass tube could be joined to a larger diameter tube interface at the outlet of the heat exchanger.

In the preferred embodiment, the bypass tube is a straight cylindrical annular tube, but in other embodiments need not necessarily be so. The bypass tube is preferably sufficiently rigid so as to form a rigid load bearing component which supports the weight of the heat exchanger, and to which the heat transfer tube and outer casing can be attached to and supported by.

Since the bypass tube is rigid, there is a need for the annular corrugations 203 on the outer casing in order to allow for difterential thermal expansion between the rigid tube and the outer casing.

The heat transfer tube itself can accommodate some thermal expansion relative to the bypass tube and/or outer casing, since it has helical corrugations which may accommodate an amount of compression or expansion in a direction along a main length axis of the heat transfer tube.

Since the bypass tube is load bearing, it can form an integral part of an exhaust system, for example between an internal combustion engine and a conventional exhaust system, without the need for additional support of the exhaust system around the heat exchanger component.

Referring to Figure 3 of the accompanying drawings, there is illustrated schematically in cut away view the heat exchanger of Figures 1 and 2, showing the helically formed heat transfer tube surrounding the internal cylindrical tubular bypass tube.

Referring to Figure 4 herein, there is illustrated schematically in perspective view, a portion of the outside of the heat transfer tube 206, showing the helical ridged wall which contacts the coolant fluid.

Since the tube is formed of a single sheet of material, the interior surface of the heat transfer tube has a corresponding interior textured surface pattern.

The design of the heat transfer tube is such as to maximize heat exchange between the gas, and the coolant. There are three major factors in maximizing the heat exchange as follows: (a) The ensure that the majority of gas flows around the helixes, and only a small percentage of gas leaks between the inlets of the heat transfer tube and the bypass tube, the clearance between the inlet of the bypass tube and the inlets of the heat transfer tube is designed to be as small as possible, without contact. This is aided by having a smooth tubular surface 210, 211 of the heat transfer tube at the inlets and outlets. Alternatively the bypass tube can have a shallow helix matching the pitch of the heat transfer tube thus closing the annular gap between bypass tube and heat transfer roots.

(b) The heat transfer tube surface area is maximized by having relatively deep corrugations with a relatively small pitch.

(C) The heat transfer coefficient is designed to be as high as possible, by texturing the major part of the surface of the heat transfer tube with a plurality of secondary perturbations or corrugations.

In the best mode embodiment herein, the heat transfer tube has the following characteristics.

Ideally, a wall thickness of the heat transfer tube is between 0.2 mm and 1 mm.

A clear internal diameter of the heat transfer tube is between 40 mm and 75 mm, and preferably between 25 mm and 200 mm.

A maximum outer diameter of the heat transfer tube is between 70 mm and mm, and preferably between 28 mm and 300 mm.

A number of helix starts is between 2 and 12, and preferably in the range 5 to 10, meaning that there is anywhere between 2 and 12 parallel helix ridges running along the length of the heat transfer tube and preferably 5 to 10.

Referring to Figure 5 herein, there is illustrated schematically in cross sectional view as viewed along a main direction of a helix, one of the ribbed portions.

Referring to Figure 6 herein, there is shown in further detail, in cutaway perspective view a portion of one embodiment of a textured helical wall of a heat transfer tube, showing a plurality of helix ridges, each helix ridge having formed thereon a plurality of parallel ribs.

The heat transfer tube wall is formed into an overall helical shape having a plurality of ridges 601 and troughs 602 between the ridges. Superimposed on the troughs and ridges are a series of parallel ribs 603 running across the main direction of the helix, running along the axis of the tube, or at any angle in between. In a best mode, the ribs have a pitch of between 2 mm and 10 mm, and a depth of between 0.2 mm and 2 mm. The ribs are formed on both the inner and outer surface of the tube, thereby increasing the surface area of the heat transfer tube which contacts the coolant fluid and increasing the surface area of the tube which contacts the gas flowing through the heat exchanger in a heat exchange mode.

The ribs may be formed both on the sides and the crest of the helix. The root of the helix may be either left smooth, or may also have ribs formed thereon.

An outer surface of a heat transfer tube is shown having three helical ridges, each having formed thereon a plurality of parallel ribs as described hereinabove.

Referring to Figure 7 herein, there is illustrated schematically an alternative form of texturing which can be applied to the ridges and valley walls of the helically formed heat transfer tube. In this embodiment, a series of oval or scallop shaped indents are pressed into the valley sides of the helical ridges, On each ridge, scallops on one valley side are alternated along the length of the ridge with scallops on the other valley side, so that a gap between first and second scallops on one side of the helix ridge lie opposite to a scallop indent on the other side of the helix ridge. This has the effect that gas flowing along the valley floor is experiences alternately on alternate sides of the valley, scallops on one side spaced apart from each other, and in the spaces between the scallops on one side of the valley, there is interspersed scallops protruding on the other side of the valley, so that the gas has to pass between the scallops, changing direction and causing turbulent flow immediately adjacent the heat transfer tube wall as shown in Figure 7 herein.

Referring to Figure 8 herein, there is illustrated schematically in plan view along a length of a trough of a helix, a gas flow path. As a general movement, gas may flow between the ridges of a helix. It would be understood that a ridge protruding into the gas channel 208, when viewed from inside the coolant chamber 207 would be a trough in the helix. For gas travelling in a path along the trough of the helix, which would be in an overall spiral path along the length of the tube, as a subsidiary motion, the gas travels in a serpentine fashion along the direction of the helix trough, as it weaves in and out of the scallops between adjacent valley walls of the helix. This has the effect of further interrupting slowing down the gas movement following that path, as well as increasing the relative surface area of heat transfer tube valley wall which is in contact with passing gas, and thereby relatively increasing the cooling effect of the heat transfer tube of the gas, leading to an overall relatively more efficient cooling, compared to a smooth valley walled helix.

In the heat exchangers disclosed herein, there is usually a competing requirement between efficiency of heat exchange, and minimizing the pressure differences between the ends of the heat exchanger to achieve adequate gas flow without back pressure. A non-annular corrugated form is beneficial, since it occludes the flow of gas to a minimum extent, whilst optimizing heat exchange.

In particular, a helical path with corrugations may allow gas to flow down the heat exchanger, encountering efficient heat transfer, but with minimized back pressure.

On a finer scale, when gas flows in a conduit under turbulent conditions, a layer of gas forms near the interface which flows under laminar conditions. This is known as the "boundary layer' and if disrupted at a position along the conduit, it will reestablish itself further along the flowpath if allowed to do so. Heat exchange at the conduit wall is improved when a laminar boundary layer condition is not permitted to establish, since transport of gas from the bulk of the fluid gas flow to the interface with the heat transfer tube is easier. Disrupting the boundary layer is therefore beneficial to heat transfer. In the ridged helical heat transfer tube shown in Figure 9, the ridges disrupt laminar flow at the metal-gas interface, therefore promoting better heat exchange between the gas and the heat transfer tube.

If the gas layer is disrupted too much however, turbulent eddies are set up, which are detrimental to both heat exchange and pressure drop. For this reason, perturbations of the basic helical surface which are too deep are not optimum, since although they disrupt the boundary layer, they result in poor heat transfer at the conduit wall immediately after the disruption.

The heat transfer tube is corrugated or perturbed, with two or more patterns of different depth. That is, one corrugation profile is relatively deep, in this case the helix pattern, with additional shallow corrugations (ripples, undulations, indents, ridges or perturbations) superimposed on the first corrugated surface.

The main deeper helical profile winds around a helical path following the main length axis of the conduit. In selecting the angle of the helix relative to the main length axis of the heat transfer tube, consideration needs to be given to occlusion of the gas flow, since many of the benefits of heat transfer will not be achieved if the gas flow is significantly impeded. The secondary corrugations must be at an angle to the main profile path, that is, ideally they should not be ridges which lie parallel to the lengths of the main helical corrugations.

In the embodiments disclosed herein, the ratio of the depth of the main corrugations to the outer diameter of the tube is preferably in the range 0.1 to 0.35. A ratio of the depth of the secondary form to a depth of a said helical ridge may be in the range 0 to 0.5.

An aspect ratio of the secondary form is in the range 0.1 to 0.4, wherein the aspect ratio is defined as the ratio of depth of a secondary corrugation to its pitch between forms Where a helical main profile is used, this may consist of one or more helices on the heat transfer tube. The number of individual starts, that is the number of individual helices which lie parallel to each other and are intertwined with each other can be anywhere in the range 2 to 12, and preferably 5 to 10.

There may be a number n of paraUel helices formed in a tube of outer diameter D, and the pitch P of an individual helix is determined by the expression P=KxnxD where K is in the range 0.1 to 2.

A preferred arrangement would be a helical corrugation for the main profile with multiple parallel helices, for example 6. The orientation of the ripples superimposed on the helix may be at any angle to the path of the helix, but preferably the ridges are arranged with their main length at an angle of 900 to a tangent of the ridge of the helix on which they are formed.

A method of manufacture of a helically corrugated heat transfer tube with a textured surface may be as follows: Helical corrugations are formed in an annular tube by forming the tube into a mandrel itself with a helical form with either a forming tool with a helical form or a number of roller discs. These tools have matching interlocking helical corrugation forms.

The external tool is designed so that it can be gradually tightened Onto the internal former. The outer tool is rotated around the mandrel and tube, which acts as a screw thread, drawing the outer tool along the tube axis and drawing in the tube material.

In this way, the corrugation depth is progressively increased from one end of the tube into the main corrugations. This helps to smooth the transition of the gas into its helical path. Similarly, the formers are gradually separated towards the distal end of the tube where the tube is to merge back into a cylindrical annular tube.

Once the main corrugation depth is achieved, the finer "ripples" or ridges are applied by hydro forming. That is by internal application of pressuring water or hydraulic fluid. The corrugated tube is forced into a tool which is preformed into the rippled shape. Alternatively the secondary form may be pressed or rolled in.

In a best mode heat transfer tube, the main corrugation may have dimensions as follows: outer diameter 80 mm corrugation depth 11 mm corrugation path -6 parallel helices helix pitch 135 mm (per helix) crest separation 22.5 mm The secondary corrugations, or perturbations superimposed on the main helix may have dimensions as follows: depth 1.6 mm pitch 5.3 mm This results in the following ratios: (main corrugation depth) / (outer diameter) = 0.137 (secondary corrugation depth) / (main corrugation depth) = 0.1455 secondary corrugation aspect ratio = 0.3 The aspect ratio (D/L) is the ratio of the depth of a perturbation to its pitch.

Referring to Figure 9 herein, there is illustrated schematically a further heat exchange tube suitable for the heat exchanger shown in figures 1 to 3 herein, having a secondary corrugation profile superimposed on a general helical profile.

In this example, ridges of the secondary corrugations are at an angle of 90 to the main direction of the helix.

Referring to figure 10 herein, there is illustrated schematically in close up view, one end of the heat transfer tube of figure 10, showing a helix start.

At one end of the heat transfer tube, there is a substantially smooth annular cylindrical end portion 1000, which is provided so as to connect the heat transfer tube to other components (outer casing) within the heat exchanger. Each of the helix starts commences in a gradually increasing trough depth as shown in figure 10. On the internal surface of the heat transfer tube, the corresponding inner helical ridge profile allows for gas travelling along the valley between two helices to exit the valley in a smooth unrestricted manner, thereby reducing back pressure effects.

Referring to figure 11 herein, there is illustrated schematically the heat transfer tube in view from one end. The outer end collar portion 10 is shown in end view, along with a profile of spirally wound helices. In the center of the tube, there is a clear unrestricted gas passage 1100.

Referring to figure 12 herein, there is illustrated an embodiment wherein the bypass tube also has a helical form on its outer surface. The inner surface may or may not have a helical form. Preferably the crest diameter of the bypass tube helix is smaller than the diameter of the root of the heat exchange tube allowing the two tubes to be place coaxially without the need of a screwing motion. The crest diameter of the bypass tube should however be as close to the diameter of the heat transfer tube root to oppose gas flowing between helical gas paths and to urge the gas to maintain a helical flow.

The crest diameter of the bypass tube helix 122 is preferably just smaller than the diameter of the root of the heat transfer tube 123, allowing assembly of the one tube to the other without the need to screw them together.

The helical ridge corrugations of the heat transfer tube are particularly suitable for use in the heat exchanger, because the gas flows in an annulus between the bypass tube and the heat transfer tube surrounding it. The fact that the gas can follow a continuous path along the helical corrugation provides less resistance to gas flow, and therefore avoids pressure build-up and areas of very low gas velocity, compared to an equivalent annularly corrugated heat transfer tube, which relatively would create more back pressure, which may affect the power output and/or efficiency of an internal combustion engine to which the exhaust heat exchanger may be fitted.

Specific embodiments disclosed herein may have advantages as follows: The different corrugation profiles between the basic helical corrugation and the ridged, scalloped or ribbed corrugation superimposed onto the helices are of different scale and therefore have different, but complimentary physical effects on the gas flow and heat transfer.

The advantage of shallow ripples or perturbations on the surface of the main helical profile is to disrupt the formation of the gas boundary layer. By optimizing the pitch and depth of the ripple, the heat exchange characteristics can be optimized so as not to set up detrimental recirculation eddies.

Because the gas in the main helical or ridged corrugation is not occluded, the depth of corrugation can be much greater than would be the case for a corresponding annular corrugation. A disadvantage of an annular corrugation would be that gas may become stagnant within the corrugations. By provision of a helical corrugation, this allows the surface area for heat transfer to be relatively increased.

The concept of increasing the surface area within a given volume is in contrast to the usual well established method of increasing surface area by simply increasing the number of gas carrying tubes in a gas heat exchanger.

Using several parallel helices rather than one, allows the pitch of each helix to be increased, and allows the helix angle to become more parallel to the tube main axis. This improves the flow of gas in the corrugations of the helix in comparison to a single helix, thereby minimizing pressure drop.

The helical corrugations encourages turbulence and the mixing of gas on a scale comparable to the corrugation depth, but without forming regions where gas can stagnate. This enhances heat transfer by bringing gas from the main core of gas flow to the metal -gas interface at the heat transfer tube surface.

Claims

Claims: 1. A gas heat exchanger comprising: a rigid gas bypass tube

forming a bypass gas passage; a concentric heat transfer tube surrounding said bypass tube said heat transfer tube capable of transferring heat between a gas and fluid coolant; and a tubular outer casing, encasing at least partially said heat transfer tube and encasing at least a portion of said bypass tube, wherein said heat transfer tube comprises a tubular wall formed into a plurality of parallel helical undulations extending along a length of said heat transfer tube.
2. The heat exchanger as claimed in claim 1, wherein said bypass tube is rigid enough to form a main load bearing component of the heat exchanger, such as to be installable in an exhaust system of a vehicle without the need for additional supports or mounts.
3. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube comprises surface wall having a plurality of helical ridges, each said helical ridge being provided with a plurality of secondary corrugations, of relatively reduced depth compared to a depth of said helical ridges.
4. The heat exchanger as claimed in any one of the preceding claims, wherein said bypass tube has a substantially smooth outer surface.
The heat exchanger as claimed in any one of claims 1 to 3, wherein said bypass tube has a helically formed outer surface.
6. The heat exchanger as claimed in any one of the preceding claims, wherein said outer casing comprises a plurality of annular bellows formations, capable of absorbing expansion or contraction in a direction axially of a main length of the outer casing.
7. The heat exchanger as claimed in any one of the preceding claims, wherein a chamber is formed between said outer casing and said heat transfer tube, for containing coolant fluid which contacts a wall of said heat transfer tube, passes through said chamber and thereby cools said wall of said heat transfer tube.
8. The heat exchanger as claimed in any one of the preceding claims, wherein said outer casing is provided with a coolant inlet pipe and a coolant outlet pipe for passing coolant through said outer casing.
9. The heat exchanger as claimed in any one of the preceding claims, wherein: said heat transfer tube is arranged concentrically inside said outer casing; said bypass tube is arranged concentrically inside said heat transfer tube; and the bypass tube is arranged concentrically within said heat transfer tube and said outer casing.
10. The heat exchanger as claimed in claim 9, wherein there are a number n of parallel helices formed in a tube of outer diameter D, and the pitch P of an individual helix is determined by the expression P=KxnxD where K is in the range 0.1 to 2.
11. The heat exchanger as claimed in claim 9 or 101 wherein the ratio of the radial depth of said helical corrugations to the outer diameter of the tube is in the range 0.1 to 0.35.
12. The heat exchanger as claimed in any one of claims 9 to 11, wherein the ratio of the depth of the secondary corrugations to a depth of a said helical ridge is in the range 0 to 0.5.
13. The heat exchanger as claimed in any one of claims 9 to 12, wherein said secondary corrugations on the heat transfer tube have a minimum depthof 0.2 mm.
14. The heat exchanger as claimed in any one of claims 9 to 13, wherein the aspect ratio of the secondary corrugations is in the range 0. 1 to 0.4, wherein the aspect ratio is defined as the ratio of depth of a secondary corrugation to its pitch between forms.
15. The heat exchanger as claimed in any one of claims 9 to 14, herein said secondary corrugations comprise a series of parallel ribs running at angle angle from transverse to a main tangential direction of a said helix to 45 degrees to the main tangential direction of a said helix.
16. The heat exchanger as claimed in any one of claims 9 to 15, wherein said secondary corrugations comprise a plurality of parallel ribs superimposed on said helical ridges, said ribs having a pitch of between 2mm and 10 mm, and a depth of between 0.2mm and 2 mm.
17. The heat exchanger as claimed in any one of claims 9 to 16, wherein said secondary corrugations comprise a plurality of parallel ribs formed on the sides and on a crest of said helical ridges.
18. The heat exchanger as claimed in any one of claims 9 to 17, wherein said secondary corrugations comprise a plurality of parallel ribs formed within a valley between a pair of said helical ridges.
19. The heat exchanger as claimed in any one of claims 9 to 18, wherein said secondary corrugations comprise a plurality of scallop like protrusions protruding from the sides of said helical ridges into the gas path.
20. The heat exchanger as claimed in any one of claims 9 to 19, wherein said secondary corrugations comprise a plurality of scallop shaped protrusions protruding from the walls of said helical ridges, said protrusions being arranged alternately opposite each other such that a gas flowing in a valley between two said helical ridges is directed through a serpentine like path along a length of said valley and between said oppositely and alternately positioned scallop shaped protrusions.
21. The heat exchanger as claimed in any one of the preceding claims wherein said heat transfer tube has a clear internal diameter of between 25 mm and 200 mm.
22. The heat exchanger as claimed in any one of claims 1 to 14,, wherein said heat transfer tube has a clear internal diameter in the range 40 mm to 75 mm.
23. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube has a maximum outer diameter in the range 28 mm to 300 mm.
24. The heat exchanger as claimed in any one of claims 1 to 16, wherein said heat transfer tube has a maximum outer diameter in the range 70 mm to 110 mm.
25. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube has a number of helix starts in the range 2 to 12.
26. The heat exchanger as claimed in any one of claims 1 to 19, herein said heat transfer tube has a number of helix starts in the range 5 to 10.

-
27. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube has a plurality of helices, each helix having a tangent along its main length direction which makes an angle of between 30 and degrees to a main length axis of said heat transfer tube.
28. The heat exchanger as claimed in any one of the preceding claims, wherein said bypass tubes has a wall material thickness in the range 0.5mm to 3 mm.
29. The heat exchanger as claimed in any one of the preceding claims, where said outer casing has a wall material thickness in the range 0.2 mm to 1.0 mm. * S. *I * * S. S... * I S.... *5 S * . . * SS * S..

S I... * S I I. S ** *

S S

S IS

29. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube has a wall material thickness in the range 0.2mm to 1.0 mm.

30. The heat exchanger as claimed in any one of the preceding claims, where said outer casing has a wall material thickness in the range 0.2 mm to 1.0 mm.

31. A gas heat exchanger comprising: a rigid gas bypass tube forming a bypass gas passage; a concentric heat transfer tube surrounding said bypass tube said heat transfer tube capable of transferring heat between a gas and fluid coolant; and a tubular outer casing, encasing at least partially said heat transfer tube and encasing at least a portion of said bypass tube, wherein said gas bypass tube is rigid enough to form a main load bearing component of the heat exchanger, such as to allow the heat exchanger to be installable in an exhaust system of a vehicle without the need for additional supports or mounts.

32. The heat exchanger as claimed in claim 31, wherein said outer casing comprises a plurality of annular bellows formations, capable of absorbing expansion or contraction in a direction axially of a main length of the outer casing.

33. The heat exchanger as claimed in claim 31 or 32, wherein a chamber is formed between said outer casing and said heat transfer tube, for containing coolant fluid which contacts a wall of said heat transfer tube, passes through said chamber and thereby cools said wall of said heat transfer tube.

34. The heat exchanger as claimed in any one of claims 31 to 33, wherein said outer casing is provided with a coolant inlet pipe and a coolant outlet pipe for passing coolant through said outer casing.

35. The heat exchanger as claimed in any one of claims 31 to 34, wherein: said heat transfer tube is arranged concentrically inside said outer casing; said bypass tube is arranged concentrically inside said heat transfer tube; and the bypass tube is arranged concentrically within said heat transfer tube and said outer casing.

36. The heat exchanger as claimed in any one of claims 1 to 35, in which there are a number n of parallel helices formed in a tube of outer diameter D, and the pitch P of an individual helix is determined by the expression P=KxnxD where K is in the range 0.1 to 2 37. The heat exchanger as claimed in any one of claims 31 to 36, wherein said heat transfer tube has a clear internal diameter of between 25 mm and 200 mm.

38. The heat exchanger as claimed in any one of claims 31 to 37, wherein said heat transfer tube has a clear internal diameter in the range 40 mm to 75 mm.

39. The heat exchanger as claimed in any one of claims 31 to 38, wherein said heat transfer tube has a maximum outer diameter in the range 28 mm to 300 mm.

40. The heat exchanger as claimed in any one of claims 31 to 39, wherein said heat transfer tube has a maximum outer diameter in the range 70 mm to 110 mm.

41. The heat exchanger as claimed in any one of claims 31 to 40, wherein said bypass tube has a wall material thickness in the range 0.5 mm to 3 mm.

42. The heat exchanger as claimed in any one of claims 31 to 41, wherein said outer casing has a wall material thickness in the range 0.2 mm to 1.0mm.

43. A method of manufacturing the main helical corrugations of a heat transfer tube for a heat exchanger, said heat transfer tube comprising: a substantially tubular annular metal wall having a plurality of helical undulations, and formed on said plurality of helical undulations, a plurality of secondary forms corrugations, said method comprising: clamping a cylindrical annular tube between first and second forming tools, wherein said first forming tool is placed internally of said tube and said second forming tube is placed externally of said tube, said first and second tubes having matching helically undulated surfaces; urging said internal and external former tools towards each other; rotating said tube between said internal and external former tools and drawing the tube in an axial direction through said internal and external former tools.

43. A method of manufacturing the main helical corrugations of a heat transfer tube for a heat exchanger, said heat transfer tube comprising: a substantially tubular annular metal wall having a plurality of helical undulations, and formed on said plurality of helical undulations, a plurality of secondary forms, said method comprising: clamping a cylindrical annular tube between first and second forming tools, wherein said first forming tool is placed internally of said tube and said second forming tube is placed externally of said tube, said first and second tubes having matching helically undulated surfaces; urging said internal and external former tools towards each other; rotating said external forming toot around said internal tool and tube and drawing the tube in an axial direction into said internal former tool.

44. A method of manufacturing the secondary forms of a heat transfer tube for a heat exchanger hydroforming a plurality of said secondary forms by the internal application of a pressurised fluid to the tube, such that a wall of said tube is forced against an external tubular forming tool having a plurality of corrugated shaped indents.

45. Another method of manufacturing the secondary forms of a heat transfer tube for a heat exchanger pressing a plurality of said secondary forms corrugatione by the pressing of the tube, such that a wall of said tube is forced against an internal section forming tool having a plurality of shaped indents by an external section forming tool having a plurality of shaped indents.

46. Another method of manufacturing the secondary forms of a heat transfer tube for a heat exchanger pressing a plurality of said secondary forms by rollers, such that a wall of said tube has a secondary form rolled into it.

47. A gas heat exchanger comprising: a coolant conduit; a gas cooling conduit; and a gas by-pass conduit, each of said conduits comprising respective inlet and outlet ends and at least one of said conduits comprising a heat exchange surface having a first set of surface perturbations to enhance mixing of a gas flowing there-through whilst minimising the pressure difference between its respective inlet and outlet ends, said heat exchanger characterised in that: said at least one conduit comprising said heat exchange surface comprises a second set of perturbations, superimposed on said first set of perturbations, configured to disrupt a laminar flow of said gas at the interface of said surface and said gas.

48. A heat exchanger as claimed in claim 47, wherein said at least one conduit comprising said heat exchange surface is in the form of a tube.

49. A heat exchanger as claimed in claim 47 or 48, wherein said tube is substantially cylindrical.

50. A heat exchanger as claimed in any one of claims 47 to 49, wherein said first set of surface perturbations comprises a plurality of elongate corrugations that extend helically relative to the longitudinal axis of said substantially cylindrically shaped tube.

51. A heat exchanger as claimed in any one of claims 47 to 50, wherein a said surface perturbation of said second set of surface perturbations comprises a main longitudinal axis that is transverse to the main longitudinal axis associated with said perturbation of said first set on which it is superimposed.

52. A heat exchanger as claimed in any of any of claims 47 to 51, wherein a said surface perturbation of said second set of surface perturbations comprises a main longitudinal axis that is substantially perpendicular to said longitudinal axis associated with said perturbation of said first set of surface perturbations on which it is superimposed.

53. A heat exchanger as claimed in any one of claims 47 to 52, wherein said at least one conduit comprising said surface includes said gas cooling conduit.

54. A heat exchanger as claimed in any one of claims 47 to 53, wherein said first set of surface perturbations have a depth profile that is greater than the depth profile of said second set of perturbations.

55. A heat exchanger as claimed in any one of claims 47 to 54, wherein said depth profile of said first set of perturbations is greater than 5.0 mm.

56. A heat exchanger as claimed in any one of claims 47 to 55, wherein said depth profile of said second set of surface perturbations lies in the range 0.2 mm to 2.5 mm.

57. A heat exchanger as claimed in any one of claims 47 to 56, wherein at least a portion of said gas cooling conduit is disposed between at least a portion of said coolant conduit and at least a portion of said gas by-pass conduit, said gas cooling conduit configured to provide a thermal barrier between said coolant conduit and said gas by-pass conduit.

58. A heat exchanger as claimed in any one of claims 47 to 57, wherein said heat exchanger is configured for use in an automotive application.

59, A heat exchanger as claimed in any one of claims 47 to 58, wherein said heat exchanger is configured for use in exhaust gas re-circulation to thereby reduce the emission of pollutants from the exhaust of a vehicle.

60. A heat exchanger as claimed in any one of claims 47 to 59, wherein said heat exchanger is configured for use in extracting heat from a vehicle exhaust stream in order to heat a passenger cabin of said vehicle.

61. A method of manufacturing a heat exchanger of the type claimed in claim 47, said method comprising the steps of: obtaining a blank work piece from which to make said at least one conduit comprising a heat exchange surface having a set of surface perturbations to enhance mixing of a gas flowing there -through; forming said first set of surface perturbations on said piece using a forming tool; and forming said second set of surface perturbations on said piece using a tool that is preformed into the shape of the second set of surface perturbations.

62. The method as claimed in claim 61, wherein said step of obtaining said piece comprises obtaining a tube shaped piece.

63. The method as claimed in claim 61 or 62, wherein said step of forming said first set of surface perturbations comprises clamping said conduit between an internal forming tool and an external forming tool.

64. The method as claimed in claim 61, wherein said internal forming tool and said external forming tool have interlocking helical corrugation forms.

65. The method as claimed in claim 61, wherein said step of forming said second set of surface perturbations on said piece comprises a hydra-forming process.

Amendments to the Claims have been filed as follows Claims: 1. A gas heat exchanger comprising: a rigid gas bypass tube forming a bypass gas passage; a concentric heat transfer tube surrounding said bypass tube said heat transfer tube capable of transferring heat between a gas and fluid coolant; and a tubular outer casing, encasing at least partially said heat transfer tube and encasing at least a portion of said bypass tube, wherein said heat transfer tube comprises a tubular wall formed into a plurality of parallel helical undulations extending along a length of said heat transfer tube, and wherein said bypass tube is rigid enough to form a main load bearing component of the heat exchanger, such as to be installable in an exhaust system of a vehicle without the need for additional supports or mounts.

2. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube comprises surface wall having a plurality of helical ridges, each said helical ridge being provided with a plurality of secondary corrugations, of relatively reduced depth compared to a depth of said helical ridges.

3. The heat exchanger as claimed in any one of the preceding claims, wherein said bypass tube has a substantially smooth outer surface. * .*

4. The heat exchanger as claimed in any one of claims 1 to 2, wherein S...

said bypass tube has a helically formed outer surface.

*:*. 30 5. The heat exchanger as claimed in any one of the preceding claims, wherein said outer casing comprises a plurality of annular bellows formations, S... * S S * . S. S S * * S.

capable of absorbing expansion or contraction in a direction axially of a main length of the outer casing.

6. The heat exchanger as claimed in any one of the preceding claims, wherein a chamber is formed between said outer casing and said heat transfer tube, for containing coolant fluid which contacts a wall of said heat transfer tube, passes through said chamber and thereby cools said wall of said heat transfer tube.

7. The heat exchanger as claimed in any one of the preceding claims, wherein said outer casing is provided with a coolant inlet tube and a coolant outlet tube for passing coolant through said outer casing.

8. The heat exchanger as claimed in any one of the preceding claims, wherein: said heat transfer tube is arranged concentrically inside said outer casing; said bypass tube is arranged concentrically inside said heat transfer tube; and the bypass tube is arranged concentrically within said heat transfer tube and said outer casing.

9. The heat exchanger as claimed in claim 8, wherein there are a number n of parallel helices formed in a tube of outer diameter D, and the pitch P of an individual helix is determined by the expression PKxnxD ** S * * * * ** where K is in the range 0.1 to 2. S... * . . S. * S S.

10. The heat exchanger as claimed in claim 8 or 9, wherein the ratio of the radial depth of said helical corrugations to the outer diameter of the tube is in the range 0.1 to 0.35.

11. The heat exchanger as claimed in any one of claims 8 to 10, wherein the ratio of the depth of the secondary corrugations to a depth of a said helical ridge is in the range 0 to 0.5.

12. The heat exchanger as claimed in any one of claims 8 to 11, wherein said secondary corrugations on the heat transfer tube have a minimum depth of 0.2 mm.

13. The heat exchanger as claimed in any one of claims 8 to 12, wherein the aspect ratio of the secondary corrugations is in the range 0. 1 to 0.4, wherein the aspect ratio is defined as the ratio of depth of a secondary corrugation to its pitch between forms.

14. The heat exchanger as claimed in any one of claims 8 to 13, herein said secondary corrugations comprise a series of parallel ribs running at angle from transverse to a main tangential direction of a said helix to 45 degrees to the main tangential direction of a said helix.

15. The heat exchanger as claimed in any one of claims 8 to 14, wherein said secondary corrugations comprise a plurality of parallel ribs superimposed on said helical ridges, said ribs having a pitch of between 2mm and 10 mm, and a depth of between 0.2mm and 2 mm. * **

16. The heat exchanger as claimed in any one of claims 8 to 15, **** wherein said secondary corrugations comprise a plurality of parallel ribs formed *:*. 30 on the sides and on a crest of said helical ridges. * S.. *..* * SS S. S * . * S S * S

17. The heat exchanger as claimed in any one of claims 8 to 16, wherein said secondary corrugations comprise a plurality of parallel ribs formed within a valley between a pair of said helical ridges.

18. The heat exchanger as claimed in any one of claims 8 to 17, wherein said secondary corrugations comprise a plurality of scallop like protrusions protruding from the sides of said helical ridges into the gas path.

19. The heat exchanger as claimed in any one of claims 8 to 18, wherein said secondary corrugations comprise a plurality of scallop shaped protrusions protruding from the walls of said helical ridges, said protrusions being arranged alternately opposite each other such that a gas flowing in a valley between two said helical ridges is directed through a serpentine like path along a length of said valley and between said oppositely and alternately positioned scallop shaped protrusions.

20. The heat exchanger as claimed in any one of the preceding claims wherein said heat transfer tube has a clear internal diameter of between 25 mm and 200 mm.

21. The heat exchanger as claimed in any one of claims 1 to 13, wherein said heat transfer tube has a clear internal diameter in the range 40 mm to 75 mm.

22. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube has a maximum outer diameter in the range 28 mm to 300 mm. * ** * S S * ** ****

23. The heat exchanger as claimed in any one of claims 1 to 15, wherein said heat transfer tube has a maximum outer diameter in the range 70 * mmtoll0mm. *** * *SSS * S * ** * ** S * S * * *S

24. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube has a number of helix starts in the range 2 to 12.

25. The heat exchanger as claimed in any one of claims 1 to 18, herein said heat transfer tube has a number of helix starts in the range 5 to 10.

26. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube has a plurality of helices, each helix having a tangent along its main length direction which makes an angle of between 30 and 80 degrees to a main length axis of said heat transfer tube.

27. The heat exchanger as claimed in any one of the preceding claims, wherein said bypass tubes has a wall material thickness in the range 0.5mm to 3 mm.

28. The heat exchanger as claimed in any one of the preceding claims, wherein said heat transfer tube has a wall material thickness in the range 0.2mm to 1.0 mm.