EP0539437A1 - Heat exchange apparatus - Google Patents

Abstract

A heat exchanger device has a first annular cross-sectional circuit for a fluid, defined between the surface of an inner wall of circular cross-section of an outer wall element (10) and the surface of an outer wall of cross-section circular cross-section of an intermediate wall element (11), a second circuit of annular cross-section for a fluid, defined between the surface of an inner wall of circular cross-section of said intermediate wall element and the surface of an external wall having a cross-section of an inner wall element (15), said annular cross-section circuits effecting reciprocal heat exchange via said intermediate wall element (11), as well as means such as a motor drive which serves to produce relative rotation between said intermediate wall member and each of said outer and inner wall members. The two wall surfaces defining at least one of said annular cross-section circuits are practically smooth. High heating and cooling rates are obtained, the heat transfer uniformity of which is improved.

Description

HEAT EXCHANGE APPARATUS

The present invention relates to heat exchange apparatus and to apparatus for heating and/or cooling fluids incorporating such heat exchange apparatus.

Whilst the invention will be described principally in connection with the heating and cooling of liquids, it has general applicability to the heating and cooling of fluids including gases. Liquids may be heated either by contact with a heated surface or by release of heat throughout the volume of the liquid as in steam injection or ohmic heating. Where a liquid is to be first heated and then cooled, an advantage of some surface heating systems is that they permit the recovery of most of the heat supplied by using the heated material to preheat the incoming liquid. Generally, surface heating systems are limited in their maximum rate of heating by the rate of heat transfer from the heated surface to the liquid which in turn is limited by the maximum temperature differential which is permissible between the heated surface and the liquid. The rate of heat transfer can be increased by agitating the liquid to improve heat transfer.

In Russian Specification No. SU-428189A there is disclosed a heat exchange unit for compressed gas cooling in which two concentric annular cross-section flow channels are defined between an inner rotatable generally cylindrical member, an intermediate generally cylindrical rotatable member and a generally cylindrical outer casing. Each of the rotatable cylindrical members is provided with a set of turbine blades protruding into the adjacent annular cross- section flow path. Streams of gas are passed in counter current through respective annular cross-section flow paths. Interaction between the gas streams and the turbine blades causes the rotatable cylindrical members to spin and the rotation of the cylinders improves heat transfer by destruction of the boundary layers of gas around the cylinders.

This form of construction necessitates relatively large annular gaps to accommodate the turbine blades. The speed of rotation of the cylindrical members depends upon the rate of flow through the heat exchange apparatus of the gases being treated. The rotation of the cylindrical members will therefore make relatively little contribution to increasing the rate of heat transfer in the apparatus. Furthermore, we have now appreciated that the linkage between the rate of flow of gas through the apparatus and the speed of rotation of the cylindrical members will impose a lack of controllability on the system.

GB-A-692375 discloses pasteurisation apparatus in which a cylindrical drum bearing fins or ribs. rotates within an intermediate, stationary cylindrical wall member. A liquid to be pasteurised is passed through the annular space between the drum and the wall member as the drum is rotated. An outer cylindrical, stationarywall member surrounds the intermediate wall member and a heating fluid is passed through the annular space therebetween in heat exchange relationship with the liquid to be pasteurised. The radial width of the annular space containing the liquid to be pasteurised is about 6 mm.

Swiss Patent Specification No. 451679 discloses an apparatus for sterilising liquids in which liquid to be heated is passed through an annular space defined between an outer cylinder and a rotating hollow finger. The interior of the finger is heated by an electric element extending therein. The annular space containing the liquid is of substantial radial width.

It would be desirable to provide heat exchange apparatus capable of a higher rate of heat transfer uniformly to or from the whole of a stream of product fluid from a stream of heating or cooling fluid for a given temperature differential between the heat exchanging streams. The present invention provides heat exchange apparatus comprising a first annular cross-section flow path for fluid defined between a circular cross-section inner wall surface of an outer wall member and a circular cross-section outer wall surface of an intermediate wall member, a second annular cross-section flow path for fluid defined between a circular cross-section inner wall surface of said intermediate wall member and a circular cross-section outer wall surface of an inner wall member, and means independent of the flow of fluid through said annular cross-section flow paths for producing relative rotation between said intermediate wall member and each of said inner and outer wall members.

Said means for producing relative rotation may be means for driving rotation of said intermediate wall member, or means for driving rotation of sr _d inner and outer wall members.

Optionally, the intermediate wall member is fixed and both of the inner and outer wall members are rotatable.

Optionally, means is provided for rotating the intermediate wall member in a first sense and means is provided for rotating one or both of the inner and outer wall members in the opposite sense.

However, it is preferred that at least one and preferably both of said inner and outer wall members are fixed and said intermediate wall member is rotatable. The means for driving rotation then acts on the rotatable intermediate wall member.

Said .-riving means is preferably suitable to produce a relative rotation rate in the range from 50 rpm to 25,000 rpm, e.g. from 150 rpm to 10,000 rpm. K re preferably, the relative rotation rate is from 750 to 2,000 rpm, e.g. about

1,500 rpm.

Preferably, the surface of the intermediate wall mem; r bounding one of the annular flow paths, in practice that used for the product flow, is substantially smooth, i.e. is free of ribs, fins or stirrers. Turbulence is produced in the liquid in the adjacent annular flow pa-h by virtue of the speed of relative movement of the wall surfaces bounding the annular flow path and the radial width of the annular flow path. This is in contrast to what is shown in GB-A-692375 where fins or ribs are provided to produce stirring of the product flow. Whilst fins or ribs may increase the stirring of the bulk of the liquid in which they move they may also create small pockets in which some liquid tends to be trapped for a time. Thus, each stirrer blade has a leading face and a trailing face. A zone of reduced pressure is formed behind the trailing face into which product liquid can be drawn and for a time can be retained, out of contact with the heat transfer surface of the rotating wall member.

In a process of thermal processing, it is necessary not merely to heat every part of the liquid to at least a target temperature but also to maintain the target temperature for a certain period for each part of the liquid. This applies to processes such as sterilisation. The required process intensity is therefore determined by those parts of the liquid which are slowest to heat. The greater the uniformity of heating, the lower the overall heating intensity can be.

By creating sheltered zones, ribs or fins on the rotating wall member may increase the overall process intensity required.

Accordingly, it is preferred according to the invention to provide the intermediate wall member with a smooth surface defining at least one of the annular flow paths and to rely upon a small radial width in the annular flow path for the product liquid and a high rate of rotation to obtain intense uniform product turbulence. Preferably therefore, at least one surface of the intermediate wall member and the opposite surface of its respective annular flow path are smooth to the degree that they are free from surface projections of greater than 1 mm, more preferably 0.5 mm and most preferably 0.1 mm in height. The opposite surface of the intermediate wall member may however be provided with surface projections such as ribs for the purpose of strengthening so that the wall thickness of the intermediate wall member can be minimised to improve heat transfer.

In an alternative aspect the invention includes heat exchange apparatus comprising a first annular cross-section flow path for fluid defined between a circular cross-section inner wall surface of an outer wall member and circular cross- section outer wall surface of an intermediate wall member, a second annular cross-section flow path for fluid defined between a circular cross-section inner wall surface of said intermediate wall member and a circular cross-section outer wall surface of an inner wall member, said annular cross- section flow paths being in heat exchange relationship with one another via said intermediate wall member, and means for producing relative rotation between said intermediate wall member and each of said inner and outer wall members wherein the wall surfaces defining at least one of said annular cross- section flow paths are both substantially smooth.

The effect of the rotation at preferred rotation rates is to generate very substantial agitation, mixing, or turbulence in the broad sense of the word in the fluid in both of the annular cross-section flow paths, but especially that for product fluid flow, by virtue of subjecting the fluid to high rates of shear. This turbulence produces intense mixing and increases heat transfer between the fluid and the wall members substantially. Preferably Reynolds numbers of the order of 100,000 or more are produced in one or both liquid streams.

This is in further contrast to what is shown in GB-A- 692375 and similar teachings in which only the product liquid flow is agitated by rotation of the inner drum and the heating medium passes between stationary wall surfaces. Thus in the apparatus of GB-A-692375, there will be little tendency for mixing of the heating fluid within the heating jacket constituted by the outer annular space so that there will be substantial thermal gradients between the heating fluid in contact with the intermediate wall member and the hotter heating fluid which is radially further out.

The degree of turbulence generated will depend not only upon the rate of rotation but also upon the clearance between the wall members.

Preferably, the radial width of the or a said smooth walled flow path which may be the width of the first annular cross-section flow path measured as the clearance between the outer wall member and the intermediate wall member or the width of the second annular cross_^section flow path measured as the clearance between the intermediate wall member and the inner wall member is from 0.1 mm to 5 mm, more preferably no more than 3 mm, e.g. from 0.25 mm to 3 mm, still more preferably no more than 2mm, e.g. from 0.5 mm to 2 mm or 0.5 mm to 1 mm.

Whilst the radial width of one of the two annular flow paths is preferably as stated above, the other may be of a significantly greater width, e.g. from 5 mm to 10 mm, e.g. from 3 mm to 5 mm.

The diameter of the apparatus, e.g. as measured as the outer diameter of the intermediate wall member, may be chosen within wide limits and may vary along the length of the apparatus. Typically, however, it may be within the range of 5 to 20 cm, e.g. about 10 cm.

The larger the diameter, the lower the rotational speed that will be needed to establish a desired differential in surface speed between the intermediate wall member and the other wall member or members. Suitable surface speed differentials will generally lie in the range of at least 25 cm/sec, e.g. from 25 to 13,000 cm/sec, more preferably at least 100 cm/sec, e.g. 100 to 5000 cm/sec, and most preferably at least 400 cm/sec, e.g. 400 to 1000 cm/sec. The means for driving rotation may be chosen with these speeds in view. The smaller the width of the product flow path, the lower will be the rate of rotation which may be found sufficient.

The flow of the product fluid through such an apparatus is preferably from 5 to 20 litres/minute, e.g. 10 to 15 litres/minute where the diameter of the intermediate wall member is about 10 cm, and from 20 to 80, e.g. 40 to 60 litres/minute where said diameter is about 20 cm.

The intermediate wall member is desirably formed from a material of high thermal conductivity. Desirably also, the intermediate wall member is of a material having a highly inert surface. Preferred materials when the apparatus is to be used in the heat treatment of foodstuffs, including drinks, are copper plated with nickel, e.g. electroless nickel plated copper, anodised aluminium or nickel. For less potentially corrosive liquids, e.g. water, plain copper may be preferred. As is conventional in electroless plating, a sealer coat, e.g. of Needox SF2R, may be used to prevent exposure of the copper when electroless nickel plated copper is used.

Typically, the intermediate wall member will be of thin wall thickness so as to enhance heat transfer, e.g. from 0.5 mm to 5 mm wall thickness, more preferably 1 to 2 mm excluding any strengthening ribs present.

It may be desirable that fluid tight rotationally sliding seals are provided (a) between portions of said intermediate wall member and said inner wall member, and optionally (b) between portions of said outer wall member and said intermediate wall member, the said portions where said seals are formed being of a reduced diameter compared to a main portion of each wall member where in use heat exchange is predominantly to take place.

Tapered connecting portions ma' ~>e employed between the reduced diameter portions and the mcin portions of the wall members.

Preferably also said fluid tight seals are each made between a fixed wall member and the outer wall surface or an axially directed annular face of a rotatable wall member. Preferably, the fluid flow path between the inner wall member and the intermediate wall member has a portion which passes over, in thermal contact with, a seal for said flow path. This flow path may pass over the outer surface of a rotary resilient sleeve member such as a spring bellows pressing a rotary seal member axially against a stationary sealing surface, so as to absorb heat generated in said seal. It has been found that it is easier to obtain a satisfactory seal to the outer surface or an axially directed annular face of a rotating tubular wall member than to its inner surface. It has also been found that it is more difficult to make a fluid tight rotary seal when the sliding velocity is high. Accordingly, it may be preferred to reduce the diameter of the rotatable wall member in the region of each seal so as to reduce the sliding velocity at the seal whilst maintaining a high shear rate in the heat exchange region of the apparatus.

Heat exchange apparatus according to the invention may be combined with other heating apparatus or two or more examples of heat exchange apparatus according to the invention may be combined together to achieve various effects.

In particular, the invention includes heating and cooling apparatus for receiving a flow of fluid to be treated and, in either order, heating said fluid and cooling said fluid, comprising and first and second heat exchange apparatus of the invention as described above arranged such that one of said first and second flow paths of a first one of said heat exchange apparatus is connected to one of said first and second flow paths of second one of said heat exchange apparatus, wherein the other of said flow-paths of said first one of said heat exchange apparatus is connected to a source of relatively hot fluid for passage therethrough to heat said fluid to be treated and the other of said flow paths of said second one of said heat exchange apparatus is connected to a source of relatively cold fluid for passage therethrough to cool said fluid to be treated. Heating and cooling apparatus of this kind, examples of which are illustrated in Figure 1 and Figure 3 of the accompanying drawings, can be used to receive a fluid to be treated at a starting temperature such as ambient, heat the fluid to a desired maximum temperature and recool the fluid to for instance the starting temperature. In such a configuration, such apparatus can for instance be used in processes such as the heat treatment for pasteurisation or sterilisation of consumable liquids such as milk, fruit juices and beers. The invention further includes heating and cooling apparatus for receiving a flow of fluid to be treated, and in either order, heating said fluid and cooling said fluid, comprising heat exchange apparatus according to the invention as described above, wherein the downstream end of one of said first and second flow paths is cr nected to the upstream end of the other said flow paths via m :ns for heating said fluid to be treated or via means of cooling said fluid to be treated. An example of apparatus of this kind is shown in Figure 2 of the accompanying drawings. Such an arrangement allows a fluid flow at a starting temperature such as ambient to be heated by flowing in counter current heat exchange relationship with the same fluid after it has passed through a separate heating means. The fluid to be treated is thus first heated by heat exchange, has its heat content topped up by the separate heating means and is then cooled by heat exchange with its own incoming flow to produce a product at a finishing temperature which may be substantially the same as the starting temperature. An advantage of such a system is that it can operate at very high thermal efficiency, a large proportion of the heat imparted to the fluid during the heating phase being recovered during the cooling phase.

The separate means for heating referred to above may itself be constituted by a further heat exchange apparatus according to the invention. The invention includes heat treatment apparatus in which heat exchange apparatus according to the invention is preceded by or followed by a holding region within which in use a fluid flow which is to be or has been heated or cooled in said apparatus is kept at an approximately constant temperature whilst being subjected to shear in the space between continuations of the relatively rotating wall members of said apparatus between which said fluid is to be passed or from which said fluid has emerged.

By subjecting the fluid to the shearing forces experienced between relatively rotating wall members, the uniformity of the temperature within the fluid during the holding period is improved. The benefits of uniformity in heat treatment are described in detail below.

The invention includes a method of effecting heat exchange between a first fluid flow and a second fluid flow comprising passing said first and second fluid flows respectively through said first and second flow paths of heat exchange apparatus according to the invention as described above to allow heat transfer from the initially hotter of said flows to the initially cooler of said flows whilst effecting said relative rotation.

The invention also includes a method of heating and subsequently cooling a fluid flow comprising passing the fluid flow through the first or the second of the flow paths of heat exchange apparatus described above, thereafter heating said fluid flow and passing said heated fluid flow through the other of said first and second flow paths of heat exchange apparatus whereby said fluid flow is heated initially by heat exchange, is then further heated and is recooled by heat exchange, whilst effecting said relative rotation. The invention includes an analogous method of cooling and subsequently heating fluid flow comprising passing the fluid flow through the first or the second of the flow paths of the heat exchange apparatus as described above, thereafter cooling said fluid flow and passing said fluid flow through the other of said first and second flow paths of said heat exchange apparatus, whereby said fluid flow is cooled initially by heat exchange, is then further cooled and is then reheated by heat exchange, whilst effecting said relative rotation.

Rates of heating or cooling of from 50 to 500^βC per second, e.g. from 100 to 200*C per second may be achieved. The invention includes fluids heated or cooled by the use of heat exchange apparatus as described above.

In particular, the invention includes liquids for human or animal consumption, especially those which are heat- degradable at pasteurisation or sterilisation temperatures, which have been pasteurised or sterilised by heating and subsequent cooling using heat exchange apparatus as described above.

The invention will be illustrated by the following description of a preferred embodiments thereof with reference to the accompanying drawings in which:-

Figure 1 is a s. iβmatic longitudinal cross-sectional view through heating and cooling apparatus according to the invention;

Figure 2 is a schematic longitudinal cross-section through a second embodiment of heating and cooling apparatus according to the invention;

Figures 3 to 3' ' ' show a longitudinal cross-section through a third embodiment of heating and cooling apparatus according to the invention; Figure 4 shows in greater detail an end region of a modified version of the apparatus of Figure 3; and

Figure 5 shows schematically a temperature control system for apparatus as shown in Figures 1, 3 and 4.

As shown in Figure 1, heating and cooling apparatus according to the invention comprises a first and a second heat exchange apparatus of the invention arranged to work together sequentially to treat a fluid flc .

The apparatus comprises a circular cross-section outer wall member 10 surrounding a circular cross-section intermediate wall member 11 which is mounted for rotation in bearings (not shown). The outer wall member 10 is fixed. End wall portions 12 of the outer wall member 10 form a liquid tight seal against the rotatable intermediate wall member 11. The interior of the cylinder defined by the intermediate wall member 11 is divided into three by a pair of bulkheads 13 and 14 which rotate with the intermediate wall member. To the left of bulkhead 13 in the drawing, a circular cross- section inner wall member 15 is positioned coaxially within the intermediate wall member 11. Inner wall member 15 is provided with a circular end closure 16 through which opens the end of a central feed tube 17. An end cap 18 is positioned over the ends of the intermediate wall member 11 and the inner wall member 15 and forms a liquid tight seal 19 with the inner surface of the inner wall member and a liquid tight seal 20 with the outer surface of the intermediate wall member 11 beyond the end of the outer wall member 10. A manifold cavity 21 defined within the end cap 18 communicates with an outlet pipe 22. Feed tube 17 extends through the end cap 18 to provide an inlet for fluid to pass through the feed tube 17, into the annular cross-section flow path defined between the outer surface of the inner wall member 15 and the inner surface of the intermediate wall member 11 and out through the outlet pipe 22.

An inlet 23 and an outlet 24 are provided for the annular cross-section flow path defined between the outer surface of the intermediate wall member 11 and the inner surface of the outer wall member 10.

To the right of the bulkhead 14 in the drawing, the arrangement is similar to that described with respect to the zone to the left of bulkhead 13. A feed tube 25 runs through an end cap 26 provided at one end of a further inner wall member 27 and opens through an annular end closure 28 for the inner wall member 27. A collection duct 29 forms an external seal over the rotatable intermediate wall member 11. A flow path for fluid is therefore provided through the feed tube 25, into the annular cross-section flow path between the outer surface of the inner wall member 27 and the inner wall surface of the rotatable intermediate wall member 11 and out through the collection duct 29.

It can be seen that the apparatus described provides a first heat exchange apparatus according to the invention in a zone A-A and second heat exchange apparatus according to the invention in a zone B-B. The apparatus may be used for heating and cooling a fluid flow to be treated in the following manner. The fluid flow to be treated is introduced through the inlet 23 to the annular cross-section flow path between the outer surface of the intermediate wall member 11 and the inner surface of the outer wall member 10 and leaves through the outlet 24. Heating liquid is supplied through the feed tube 17 to the annular cross-section flow path defined between the outer surface of the inner wall member 15 and the inner surface of the intermediate wall member 11. Heat exchange takes place between the hot liquid and the fluid flow to be treated, heat transfer occurring through the intermediate wall member 11. By means not shown such as a pulley mounted on the exterior surface of the intermediate wall member 11 adjacent the collection duct 29, the intermediate wall member 11 is rotated at for instance 1,500 rpm. This generates vigorous turbulence of the liquid to be treated and of the hot liquid on either side of the intermediate wall member 11 promoting intense heat transfer and hence rapid heating of the liquid to be treated. In that region of the flow path for the liquid to be treated which lies immediately outside the area between the bulkheads 13 and 14, neither heating or cooling of the liquid to be treated takes place. This holding section may be of any desired length so that the liquid to be treated is held at its maximum temperature for a desired period, for instance to allow sterilisation or pasteurisation to take place.

Cold liquid may be supplied to the feed tube 25 and may exist from the collection duct 29. In the region B-B, heat exchange will therefore take place across the intermediate wall member to cool the liquid to be treated prior to its exit at outlet 24.

The drawing is not to scale and in general the clearances between the coaxial wall members are greatly exaggerated. By the use of clearances in the region of 0.5 to 2 mm, in combination with high rotational speeds of the intermediate wall member, uniform heating rates and cooling rates greatly in excess of those obtainable using apparatus according to the prior art may be obtained. The walls of the outer, intermediate and inner wall members are smooth but the intermediate wall member could be provided with strengthening members on its inner surface.

The apparatus shown in Figure 1 is suitable for providing maximum heating and cooling rates without attempting to obtain maximum conservation of heat energy. The apparatus illustrated in Figure 2 is suited to providing the sequential heating and cooling of a liquid flow at slightly less rapid heating and cooling rates whilst achieving good thermal efficiency. The apparatus shown in Figure 2 comprises an outer wall member 210 having a circular cross-section and coaxially surrounding an intermediate wall member 211 of circular cross-section defining therebetween an annular gap. Intermediate wall member 211 protrudes at its lower end through an end cap member 212 of the outer wall member 210 against which it makes a liquid tight seal whilst remaining free to rotate. An inlet 223 to the annular space between the outer wall member 210 and the intermediate wall member 211 is provided in end cap member 212. Intermediate wall member 211 is supported for rotation on bearings 250 mounted to an external support 251 and is provided with a pulley 252 by which the intermediate wall member 211 may be rotated.

An inner cylindrical wall member 215 is provided extending coaxially within the intermediate wall member 211 leaving an annular gap and is provided with an end cap 218 including a seal 220 to the external surface of intermediate wall member 211 defining an outlet 222 for liquid connecting with the annular section flow path defined between the outer surface of the wall member 215 and the inner surface of the intermediate wall member 211. In the upper part of the drawing, part of the thickness of the intermediate wall member 211 is provided by an inset sleeve 253 of heat insulative material such as a high temperature food grade plastics material. At its upper end, the intermediate wall member is supported for rotation by a set of bearings 254 set to support the intermediate wall member 211 between the outer wall member 210 and the inner wall member 215. An end cap 219 closes the annular gap between the outer wall member 210 and inner wall member 215 beyond the free end of the intermediate wall member 211.

A heating jacket 255 is positioned surrounding the upper part of the outer wall member 210 and is provided with an inlet 256 and an outlet 257 for a heating fluid such as steam.

Liquid to be treated is introduced to the annular cross- section flow path defined between the outer surface of the intermediate wall member 211 and the inner wall surface of the outer wall member 210 at the inlet 223. At the upper end of the apparatus the liquid to be treated passes into the inner annular cross-section gap defined between the inner surface of the intermediate wall member 211 and the outer wall surface of the inner wall member 215. During a start up phase, liquid in the upper part of the apparatus is heated by heat exchange with a heating fluid passed through the heating jacket 255. Normal flow of liquid to be treated through the inlet 223 is then established and the liquid to be treated is heated intensely by heat exchange with the flow of the same liquid passing through the inner annular gap causing that liquid to be cooled and to exit from the outlet 222 at a desired relatively cool outlet temperature. During normal running, substantially all of the heating of the liquid to be treated takes place in the long section of the apparatus marked A-A. Some heat loss from the apparatus is inevitable and this is topped up by operation of t.e heating jacket 255 in the section of the apparatus marked B-B. The zone of the inner flow path in the region B-B constitutes a holding stage at which the liquid to be treated is kept at a substantially constant temperature by virtue of the presence of insulative material 253 before being cooled by heat exchange with the incoming cool liquid to be treated.

Once again, the drawing is not to scale. The thickness of the intermediate wall member 211 will normally be chosen to be as small as possible to facilitate good heat transfer in the region A-A of the apparatus. The annular gaps between either side of the intermediate wall member 211 in this region will preferably be in the region of 0.5 to 2 mm. The wall members of the apparatus need not as shown be of a constant diameter but can vary along the length of the apparatus as may be desired. In the lower region around the bearing 250, the diameter of the wall members may be reduced to facilitate supporting the intermediate wall member 211 for rotation and to facilitate making an exterior seal 220 to the intermediate wall member 211. In the region A-A, the diameter of the wall members may be increased so as to provide a greater surface area for heat exchange. In the region B-B, the gap between the intermediate wall member 211 and the inner wall member 215 may be locally increased to provide a greater time of passage for the liquid to be treated through this region in which it is held at a high temperature. Rather than the insulating material 253 being set into the thermally conductive material of the intermediate wall member 211, it may be attached to the exterior thereof forming an increased wall thickness. Instead of being provided on the inner surface of the intermediate wall member 211, the insulative material 253 may be provided on the outer surface.

A further embodiment according to the invention is illustrated in Figures 3 to 3^■ ' ' . A stationary inner wall member is provided by a cylinder 315 comprising an elongate hollow cylindrical member 361 supported at each end on a solid end member having a reduced diameter portion 360. The cylindrical member 361 has an external surface of inert material and extends over most of the overall length of the cylinder 315. Cylinder 315 has at each end a threaded bore 362. An end cap 363 is fastened by a bolt 364 to the right hand end of reduced diameter portion 360 and the joint between the two is sealed by a sealing gasket 365.

A support post 366 has a horizontal bore therethrough receiving a tubular sleeve 367 which at its right hand end terminates flush with the right hand end face of the post 366 and at its left hand end protrudes from the post 366. The junction between the end cap 363 and the slee - 367 is sealed by a gasket 368. An annular gap 369 is left between the sleeve 367 and the reduced diameter portion 360 of the cylinder 315.

An intermediate cylindrical wall member 311 is provided having a narrow annular clearance from the larger diameter portion of the cylinder 315. The intermediate wall member 311 is of constant internal diameter. Externally, it has an enlarged diameter portion at each end and in a central region but over most of its length is of lesser external diameter but has a helically running strengthening rib 309 running around its exterior surface.

At its right hand end, the intermediate wall member 311 is inserted into a mounting jap assembly'370 which at its left hand end grips the external surface of the intermediate wall member through the use of an annular wedge and at its right hand end is supported by a twin row self aligning bearing 354 on the external surface of the sleeve 367 to the left of its exit from the post 366. A pulley 352 is mounted to the mounting cap assembly over the bearing 354.

A port 324 for the outlet of product liquid is provided in the end cap 363 which communicates with the annular space between the sleeve 367 and the reduced diameter portion 360 of the cylinder 315. That annular space in turn communicates with the narrow annular clearance between the cylinder 315 and the intermediate wall member 311.

An outer cylindrical wall member 310 surrounds somewhat less than half the length of the intermediate wall member 311. The annular clearance between the outer wall member 310 and the intermediate wall member 311 is substantially greater than that between the intermediate wall member 311 and the inner wall member constituted by cylinder 315. At each end, a seal 312 is provided between the outer wall member 310 and the underlying enlarged external diameter portion of the intermediate wall member 311. The outer wall member 310 is mounted to suitable external supports.

At the left hand end of the apparatus, the reduced diameter portion 360 of the cylinder 315 is bolted to an end cap 318 which has an inlet port 323 for liquid to be treated. End cap 318 is in turn bolted to a large diameter sleeve 371 received in a support post 351. A smaller diameter sleeve 372 passes through a horizontal bore in sleeve 371 and generally corresponds to sleeve 367 at the right hand end of the apparatus. A mounting cap assembly 373 corresponding to mounting cap assembly 370 receives the left hand end of the intermediate wall member 311 and supports it for rotation over sleeve 372 by bearings 350.

At its left hand end, sleeve 372 is connected by a bellows 375 to the end cap 318.

A second outer wall member 310' overlies the left hand half of the intermediate wall member 311 and is provided with an inlet port 322 and an outlet port 317 for heating fluid. It is sealed to adjacent enlarged diameter portions of the intermediate wall member by further seals 312 as described previously.

Each mounting cap assembly 370, 373 comprises a mounting cap 376 having a smaller diameter tubular portion containing its respective bearing and a larger diameter portion having an annular end face. An inner wedge ring 377 having an inwardly directed annular wedge face and an outer wedge ring 378 having an outwardly directed wedge face are bolted to the annular end face of mounting cap 376 with a respective end of the intermediate wall member 311 being received within the inner anc outer wedge rings. The interior of the smaller diameter tubular portion of the mounting cap 376 is in each case sealed to the underlying stationary sleeve 367, 372 by a metal bellows cartridge seal

379 of a kind known to those skilled in the art.

In use, a heating medium is flowed through inlet 322, through the annular space outside the intermediate wall member 311 and out of the outlet 317. A cooling medium is flowed through inlet 325, through the corresponding annular space between outer wall member 310 and the intermediate wall member 311 and out of the outlet 329. A product liquid is passed through inlet 323 into the annular space between the intermediate wail member 311 and the inner wall member constituted by cylinder 315 and then out through the outlet 324. The cylinder 325 is rotated by means of the pulley 352 and intense shearing action between the intermediate wall member 311 and the inner wall member 315 produces turbulence in the product liquid. Similarly, turbulence is produced in the heating and cooling fluids by virtue of the relative rotation of the intermediate wall member 311 and the outer wall members 310 and 310' . The product liquid is first heated and is then cooled. For maximum heating and cooling rates, separate supplies of heating and cooling fluids are preferably employed. For maximum recovery of heat, it is preferable to employ a single fluid for heating and cooling. The cooling fluid coming from exit 329 (now heated through heat exchange with the product flow) may be further heated to replace heat lost from the apparatus as a whole and may then be introduced into inlet 322. The heating/cooling fluid exiting from outlet 317 (now cooled by heat exchange with the incoming product flow) may then be returned via a pump to inlet 325. The apparatus illustrated in Figures 3 to 3' ' ' therefore comprises a first heat exchanger according to the invention located in the zone marked A-A and a second heat exchanger according to the invention in the zone marked B-B, the two being integrally joined end to end.

Figure 4 shows a modification of the seal and bearing arrangements for the intermediate wall member 311. A similarly modified arrangement may be employed at the opposite end of the apparatus. In Figure 4, the inlet 323 is positioned inboard of the support post 351. Sleeve 372 is mounted in an annular wedge assembly 401 similar to that which holds the intermediate wall member. To accommodate thermal expansion bearing 350 is positioned over a small sliding bearing 402. The detail of the bellows cartridge seal 379 is most clearly seen in this figure. It comprises a static ceramic ring 403 against which the rotating metal bellow 379 presses a rotating graphic ring 404 which has an axially directed annular face which seals against and slides on ceramic ring 403.

The product flow path in Figure 4 is modified so that it passes over the rotating bellows 379. In both Figure 3 and Figure ^"4, the product fluid is in contact with the outside of bellows 379 to conduct away heat generated by its rotation, but in Figure 3, this area is a branch off the actual flow path and is not swept through by the product fluid.

Apparatus as described with reference to the drawings will be particularly suitable for the sterilisation or pasteurisation of liquids for human or animal consumption such as milk, fruit juices and beer. However, heat exchange apparatus according to the invention may be used for the heating of liquids and gases generally. In particular, heat exchangers of the kind described above are likely to be of particular value in contexts where rapid heating is required, for instance for the heating of materials which are likely to be damaged by prolonged exposure to heat such as foodstuff ingredients and pharmaceuticals. A particular benefit of the apparatus as described herein is that the intense mixing action to which the fluid is subjected produces improved uniformity in the heating of the subject fluid. In a pasteurisation or sterilising operation all of the fluid must be heat treated to at least a specific minimum degree in terms of the time for which it must be held above a minimum heat treatment temperature. In conventional apparatus, non-uniformity of heating means that to ensure adequate heating for all the fluid, some of the fluid must be heated more than is desired. The overheating of a small proportion of the fluid may contribute substantially to the development of undesirable flavours in foodstuff products or to heat degradation generally. More uniform heating means that the most heated portion of the fluid is less overheated and also that the overall intensity of the heat treatment can be reduced whilst still ensuring that even the least heated portion of the fluid is heat treated adequately.

An optional method of controlling the heating intenε-ity in apparatus as shown in Figure 3 or Figure 4 is illustrated in Figure 5. A temperature sensor 501 at the end of the heating section A-A is positioned to sense the temperature of the product fluid. Sensor 501 is read by a controller 502 which compares the measured temperature with a desired temperature set by an operator and produces an output signal to control the speed of rotation of the intermediate wall member 311. By regulating the rotation speed, the rate of heat transfer in the heating stage, and hence the maximum product temperature may be regulated.

Many modifications and variations of the invention as described above are possible within the scope of the invention. For instance, the cross-sectional area of the annular flow path of the apparatus through which the product fluid passes need not be of constant cross-section between the start of the heating section A-A and the end of the cooling section B-B as shown in Figure 3. The cross-sectional area of this flow path could be increased in the holding section between the heating and cooling sections so that the overall length of the apparatus could be reduced by shortening the holding section whilst keeping the time of passage of the fluid through this section constant.

Also the radial width of the product flow path could be greater where the temperature differential between the product and the heating medium is larger (i.e. at the start of the heating section in Figure 3) and could be made smaller where the temperature differential is smaller (i.e. at the end of Section A-A in Figure 3), so as to maximise heat transfer in this latter region while reducing the overall length of the apparatus and minimising the pumping power required to obtain a satisfactory flow rate through the apparatus. Thus for instance, the radial width of the product flow path could be 1 mm over the first 30 to 70% of the length of the heating section and 0.5 mm over the last 70 to 30%, e.g. 40% of its length.

Claims

1. Heat exchange apparatus comprising a first annular cross- section flow path for fluid defined between a circular cross- section inner wall surface of an outer wall member (10) and circular cross-section outer wall surface of an intermediate wall member (11), a second annular cross-section flow path for fluid defined between a circular cross-section inner wall surface of said intermediate wall member and a circular cross- section outer wall surface of an inner wall member (15), said annular cross-section flow paths being in heat exchange relationship with one another via said intermediate wall member (11), and means independent of the flow of fluid through said annular cross-section flow paths for producing relative rotation between said intermediate wall member and each of said inner and outer wall members.

2. Apparatus as claimed in Claim 1, wherein the wall surfaces defining at least one of saic? annular cross-section flow paths are both substantially smooth.

3. Apparatus as claimed in Claim 2, wherein the width of the or a said smooth walled flow path measured as the clearance between the wall members (10, 11) defining the flow path is no more than 3 mm.

4. Apparatus as claimed in Claim 3, wherein said width of said flow path is no more than 2 mm.

5. Apparatus as claimed in any one of Claims 1 to 4, further comprising a fluid tight seal (19, 20) provided between portions of said intermediate wall member (11) and said inner wall member (15) , the said portions where said seals are formed being of a reduced diameter compared to a main portion of each wall member where in use heat exchange is predominantly to take place.

6. Apparatus as claimed in any one of Claims 1 to 4, further comprising a fluid tight seal (19, 20) provided between portions of said intermediate wall member (11) and said inner wall member (15), said fluid tight seal being made between a fixed said wall member (IS) and the outer wall surface of, or an axially directed annular face of a rotatable said wall member (11).

7. Heating and cooling apparatus for receiving a flow of fluid to be treated and, in either order, heating said fluid and cooling said fluid, comprising a first (A-A) and a second (B-B) heat exchange apparatus as claimed in any one of Claims 1 to 6 arranged such that one of said first and second flow paths of a first one of said heat exchange apparatus is connected to one of said first and second flow paths of the second one of said heat exchange apparatus, wherein the other of said flow paths of said first one of said heat exchange apparatus is connected to a source (17) of relatively hot fluid for passage therethrough to heat said fluid to be treated and the other of said flow paths of said second one of said heat exchange apparatus is connected to a source (25) of relatively cold fluid for passage therethrough to cool said fluid to be treated.

8. Heating and cooling apparatus for receiving a flow of fluid to be treated and, in either order, heating said fluid and cooling said fluid, comprising heat exchange apparatus as claimed in any one of Claims 1 to 6, wherein the downstream end of one of said first and second flow paths is connected to the upstream end of the other of said flow paths via means (255) for heating said fluid to be treated or via means for cooling said fluid to be treated.

9. Heat treatment apparatus in which heat exchange apparatus as claimed in any one of Claims 1 to 6 is preceded by or followed by a holding region within which in use a fluid flow which is to be or has been heated or cooled in said apparatus is kept at an approximately constant temperature whilst being subjected to shear in an annular space defined between continuations of the relatively rotating wall members of said apparatus between which said fluid is to be passed or from which said fluid has emerged.

10. A method of effecting heat exchange between a first fluid flow and second fluid flow in heat exchange apparatus comprising a first annular cross-section flow path for fluid defined between a circular cross-section inner wall surface of an outer wall member (10), and a circular cross-section outer wall surface of an intermediate wall member (11), a second annular cross-section flow path for fluid defined between a circular cross-section inner wall surface of said intermediate wall member (11) and a circular cross-section outer wall surface of an inner wall member (15), said annular cross-section flow paths being in heat exchange relationship with one another via said intermediate wall member (11), and means (29) independent of said fluid flows for producing relative rotation between said intermediate wall member and each of said inner and outer wall members, comprising passing said first and second fluid flows respectively through said first and second flow paths of said heat exchange apparatus, to allow heat transfer from the initially hotter of said flows