ZA200700345B - Flexible film heat exchanger - Google Patents

Description

FIELD OF THE INVENTION

The present invention lies in the field of flexible film heat exchangers; a preferred field of application is to counter-flow heat exchangers using thin plastic film heat exchange surfaces.

BACKGROUND

Thin plastic film heat exchangers hold out the promise of an economical, corrosion resistant heat exchanger with a fair to good heat transfer coefficient, low fouling and low flow resistance. The walls of the heat exchanger separating the flows from each other are of thin flexible film, through which the heat exchange takes place. They are further characterized by relatively low cost materials with high corrosion resistance and low fouling adherence. The heat exchanger is light and in some cases flexible or foldable, enabling compact storage and transport, and easy installation.

Large amounts of energy B both sensible heat and latent heat B are used in the drying of agricultural and forestry products. There is a need for a durable, corrosion and fouling resistant, economical heat exchanger capable of recovering some of this heat and transferring it to the incoming fresh air stream. Insofar as condensation may take place, water and other products such as essential oils may be recovered as condensate.

In industry energy is lost from moist flue gases. Latent and sensible heat may be recovered. Also, volatile acids or substances harmful to the environment may be removed from flue gases. In colder climates, energy may be economically recovered in the ventilation of buildings or small aircraft.

In high density aquaculture of warm water species such as tilapia and catfish a part of the water must be replaced with fresh water continuously, or at intervals, and heat may be recovered from the outgoing stream, and transferred to the incoming stream. Heat and moisture recovery may here also be effected from the CO, enriched air stream leaving the aquaculture system and its biological filters, and transferred to the oxygen-rich stream entering the system and its filters. Aquaculture may also use waste heat from a power station via a heat exchanger.

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Problems are caused by high investment costs, especially when corrosive substances are condensed ® from the gases, and also by the fact that structures generally available on the market operate in cross flow, which limits the heat that can be recovered. Heat exchange structures for large amounts of gas are rigid, bulky and heavy, and therefore difficult to transport and install.

Really efficient low cost structures are not available. The introduction of an efficient low cost corrosion resistant heat exchanger into the market will considerably increase the recovery of heat in many applications at the same time as it could be possible to take into account environmental requirements relating to airborne emissions.

The patent specification US4411310 discloses a method for manufacturing a heat exchanger from plastic films by honeycomb-like bonding of multiple parallel films. Due to flow resistance, in countercurrent flow in this heat exchanger the pressure difference between the two streams will change in the flow direction. This will drastically change the shapes and the cross sectional areas available to the two fluids between their inlets and outlets, and make long versions of this heat exchanger inefficient, decreasing the heat transfer coefficient while increasing the flow resistance.

It also does not readily lend itself to continuous changes in flow direction of both streams.

The patent US5634269 discloses a serpentine heat exchanger for absorption chillers, welded from 2 thin plastic films. At the 180 degree bends of such devices, wrinkles and pleats appear on inflating.

These Acreate sites for fatigue and potential failure@ due to pressure-induced creep. The patent discloses a method of ameliorating this, including immersing the bend areas of the inflated apparatus Ainto water of sufficiently high temperature to cause said plastic material to become thermoformablee@.

The patent WO 98/31529 discloses a heat exchange element for a film distiller, in which saline water evaporates on the outside of the heat transfer element, while vapor condenses inside it. The film elements are formed from pairs of plastic films which are heat sealed along lines to form channels. A permanent thermo-forming deformation is caused in the flexible film forming the heat exchange surfaces of the element by using hot pressurized air, or other heating and a pressure difference to stretch and permanently deform it against a wire net or perforated mould between the bonding lines. Then during the later application of pressure to the element, its interior can be expanded so that it avoids “wrinkles” and “sharp folds” which gradually result in “holes and tears” in the films.

Patent WO 02/12815 discloses a heat exchanger in which a heat transfer element consists of two i [ plastic films and a stronger support film positioned between these films, with all three films joined together along lines so that parallel flow channels are formed. The said channels are thermo-formed during manufacture so that the element retains its longitudinal and transverse outer dimensions,

Several elements are connected to form a heat exchanger so that the free edges of the plastic films of adjacent elements are bonded together, whereby a closed space is formed also outside the elements. In order to expand the flow channels, the flow is directed to the interior of the flow channels with a higher pressure than to the exterior. The support film between the films through which heat transfer takes place does not separate the two streams between which heat transfer takes place, and therefore does not contribute heat transfer surface area between these streams. But jt does reduce the hydraulic diameter of the inside of the heat transfer elements, and thereby substantially increases the flow resistance. With this higher flow resistance, the flow speed and

Reynolds number will be smaller, and heat transfer less efficient.

The inlets and outlets shown in that patent have large sudden changes in cross sectional area. This will cause large sudden changes in flow speed, which will further increase the flow resistance without a commensurate increase in heat transfer. The arrangement of the flow tubes shown is in- line, not staggered.

The design shown, wherein the in-line flow tubes of adjacent elements touch, is ill-suited to heat recovery, as the cross sectional area between the elements is much less than the cross sectional area inside the elements. With this arrangement, the flow speeds of the counter-flowing fluids will differ drastically in most applications, leading to widely differing heat transfer coefficients and flow resistances B an inefficient arrangement.

W098/33029 and WO00/59598 disclose a thin film heat exchanger which has elements comprising extreme zig-zag patterns formed between two sheets, these elements multiplied inside a container; the role or effect of the patterns are not referred to.

US 6,758,261 discloses a thin film heat exchanger, for counter current heat exchange and which can be folded for transport.

THE INVENTION

According to the invention heat transfer between two gases or liquids with a small pressure difference can be improved by building a heat exchange apparatus from thin flexible plastic film operating with a gently undulating principle. Preferably, smoothly diverging inlets and smoothly converging outlets are also included, and the apparatus is for use principally in counterflow.

The present invention is a heat exchanger comprising heat transfer elements (herein simply referred to as “elements” formed by superposing two layers 1 of flexible film, and bonding (for example, welding, or heat sealing) these along parallel preferably gently undulating lines 2. Thereby parallel flow channels 5 are formed between these bond lines. These interior flow channels will be for a first fluid stream 17.

The heat exchanger is constructed by superposing several elements in planes parallel to each other, and then joining adjacent ones along their edges, or fixing said edges to the sides of a rigid enclosure. Thereby an enclosed flow path 6 is formed between each adjacent pair of elements.

Flow paths 6 may also be formed beyond the first and last elements of the heat exchanger. All of these paths, exterior to the elements, will be for the second fluid stream 18.

In use, the pressure inside the elements will be larger than the pressure in the enclosed spaces 6 outside the elements. Each element then has a gently undulating air mattress-like shape.

During or after the bonding along parallel preferably undulating lines the part of the film between the bond lines may be heated and stretched beyond the elastic limit B thermoformed B by air, gas or other pressure onto a die to help form smooth circular or other suitable channels on subsequent inflation after cooling.

In this specification the term “heat exchanger” denotes the entire apparatus for heat exchange between two fluid streams, while “heat transfer element” or “element” denotes any of the gently undulating air mattress-like pairs of flexible film 1 or 3 welded, bonded or joined together along parallel gently undulating lines 2 or 4.

In this specification the general, or mean flow direction of the cooled stream is denoted as “longitudinal”. A plane or section at right angles to the longitudinal will be referred to as

“transverse”. The direction in the general plane of the films, at right angles to the longitudinal will ® be denoted as “the transverse direction”. The term “parallel” in relation to the gently undulating lines denotes lines that have a substantially constant transverse distance between them (are equi- distant) while undulating.

The term “transverse spacing” or “P” denotes the transverse distance between adjacent bonding lines 2 or 4. The term “longitudinal period” or “wavelength” or “L” denotes a longitudinal distance after which the shape of the undulating line repeats itself. The term “swing” or “S” denotes the maximum deviation of a line in the transverse direction - in wave notation, this is twice the “amplitude” of the wave. Adjacent elements are separated by a space or channel 6 between them, in which the second fluid will flow, undergoing heat exchange with the first fluid flowing inside the channels 5 inside the elements.

By “gently undulating” is meant that after inflation of the elements the swing is not more than the transverse spacing of the welds, preferably half of that; in addition the wavelength is preferably at least 8 times the swing: L > 8S.

By side view is meant a view in the transverse direction, and by plan view a view at right angles to the plane of the elements. Neither of these terms imply a restriction on the orientation of the heat exchanger during use. Indeed, for the case of a condensing flow, the longitudinal direction of said flow is preferably downwards vertically, or at an angle.

According to the invention, a bonding configuration is that in which each bonding line 2 or 4 is preferably continuous and undulates with a given “wavelength”, and in which there are several such parallel bonding lines. By way of exception, small gaps may be left in the bonding lines 2 or 4 for the purpose of allowing condensate to drain to adjacent channels S inside the same element, when possible condensation inside elements is expected. By way of further exception, the bonding lines may be straight. This straight line exception may be used for ease of manufacture and/or to avoid dynamic forces associated with the bending flow.

An important preferred feature of the invention is that the “swing” S is half the transverse spacing

P. Preferably the elements are so superposed parallel to one another that in adjacent ones, the bonding (e g welding) patterns are mirror images of each other. Then, when the flow inside a given element turns to the left, inside the adjacent elements the flow will turn to the right. All even numbered elements will then have similar bond patterns; likewise all odd numbered elements — [] whose bond patterns will be mirror images of those of even numbered elements.

List of Items

I heat transfer films of odd numbered elements "2 bond lines of odd numbered elements 3 heat transfer films of even numbered elements 4 bond lines of even numbered elements channels formed inside a heat transfer element for a first flow 6 enclosed flow space between elements B for a second flow in heat exchange arrangement with the first flow inside the channels § 7 tube-plate forming pieces 8 battens for holding together tube-plate pieces 7 with the ends of heat transfer elements clamped or bonded between them 9 insert tubes or tube pieces for insertion into ends of channels 5 at positions of the pieces 7 10, I1and 12 spacing plates for tubes 13 inlet and outlet pieces for in- and outlets to spaces/flow channels 6 between heat transfer elements 14 & 15 no references 16 outer casing 17 the first stream for heat exchange B which flows inside channels 5 inside the elements 18 the second stream for heat exchange B which flows in the enclosed spaces 6 between the elements

THE DRAWINGS

The invention is more fully described by way of example, with reference to the drawings, in which:-

Fig la represents a particularly preferred form, in which the weld lines 2 and 4 turn smoothly between short straight sections in the longitudinal direction;

Figures 1b to 1d show similar weld patterns of varying wavelengths and undulating shapes;

In figures 1a - 1d, solid lines indicate bond lines 2 between films 1 in odd numbered elements, and dashed lines indicate the mirror image bond lines 4 between films 2 in adjacent elements;

® Figures 2a to 2d illustrate weld patterns that are excluded from the scope of this invention;

Figures 3a, 3b and 3c show cross sections of the bond pattern shown in figure 1a, at the positions A,

Band C;

Figures 3d and 3e show cross sections where the elements have been stretched in the transverse direction;

Figure 4a is a side view on exit passages,

Figures 4b, 4c, 4d and 4e are transverse cross sections on the exit passages shown in figure 4a, on sections QQ, RR, SS and TT respectively;

Figure 4f is a transverse cross section on section PP;

Figure 5a is a plan view of a heat exchanger;

Figure 5b shows in oblique projection one of the rows of connection formations 13 for the entrance into and exit from the enclosed spaces 6 between the elements; and

Figures Sc-h show alternative arrangements B plan views B of the entrance and exit of the elements;

Figure 6 is a side view of alternate exit passages;

Figure 7 shows in oblique projection one of the rows of connection formations 13 for the entrance into and exit from the enclosed spaces 6 between the elements, for use between extensions of the elements which taper towards an outlet, or away from an inlet;

Figure 8 is a transverse cross section on a section between sections PP and QQ for any of the embodiments where extensions of the elements converge towards an outlet, or diverge away from an inlet.

THE PREFERRED EMBODIMENTS

® Figures la-d exemplify preferred weld patterns. The odd numbered element bond lines 2 are shown in solid lines, whilst the preferred mirror image bond lines 4 of adjacent elements are shown in dashed lines.

Figures 3a-e exemplify transverse cross sections through the heat exchanger. That is, cross sections at right angles to the general flow direction. Figures 3a-c are at the respective sections AA, BB and

CC in figure la. Figures 3d-e are for elements stretched in the transverse direction.

Figure 4a exemplifies a side view of a smoothly converging outlet. A diverging inlet would be a mirror image about a transverse plane of this. Figures 4b-e exemplify transverse cross sections (at right angles to the general direction of the flow) at the positions QQ, RR, SS and TT of the tube spacer plates 10, 11 and 12 in figure 4a. Figures 4f and 4b exemplify the assembled tube plate forming spacer-clamp pieces 7 used between planes PP and QQ in figure 4a B at position PP the insert tubes 9 are absent, but at position QQ they are present. The film elements also follow the lines shown, as they are clamped between the spacer-clamp pieces 7 and the insert tubes (or tube pieces) 9.

Until now the weld pattern has been described. When the elements are inflated by air, gas, water or another fluid being directed at greater pressure to the insides 5 than to the outsides 6 of the elements, the transverse dimensions change. Except for the width of the weld lines, the change of these dimensions is substantially by a factor f= 2/n = 2/3.14159 = 0.6366. Experience shows that when including the width of the lines, the overall transverse shrinkage factor on inflating may be in the region of F = 2/3. (For transversely stretched elements, this factor is larger B between 2/5 and 1 - and depends on the degree of stretching.)

According to the invention, when the elements are inflated, preferably the wavelength is about 8 to times the swing S and the swing is equal to or less than the transverse spacing. When the wavelength exceeds 8 times the swing S (L > 8 S), the undulations will be termed “gentle”, and will not much influence the flow resistance at moderate flow speeds. Yet they will induce continuous or continual changes in flow direction both for the flow inside the gently undulating flow channels 5 or tubes in the elements, and for the counter-flow 6 between the elements. Thus they will increase the heat transfer coefficient. Should the need arise for a larger flow speed, a heat exchanger with elements with a larger ratio L/S - gentler undulations - may be used.

® When, as described above, the swing is approximately half the transverse spacing, and the bonding (welding) lines in adjacent elements are mirror images, then, as exemplified in figures la to 1d, : adjacent flow channels or tubes will cross twice in each wavelength B at points B, D,F,H,J,L..in fig la. At each such crossing, the space between the adjacent elements will be in-line, and somewhat pinched. However, for the greater part the flow channels are staggered, and not in line.

Figures 2a, 2b, 2c and 2d are excluded from the scope of this invention. They do not at all satisfy the requirement that the undulations are gentle, as defined herein above, Indeed, the swing S in their cases is many times the transverse spacing P. According to the invention, swings S up to P may be used, but the preferred value of S/P is near one half. Figures 2a and ¢ exemplify alternative, excluded weld patterns characterized by substantially parallel weld lines. F igures 2b and d also include the mirror image patterns of adjacent elements,

Considering the dynamic forces on the thin flexible walls, these forces will of course be larger, the higher the density and the speed of the fluids. This is one reason for using gentle undulations where higher flow speeds are anticipated. For the sake of definiteness, we assume flow in the channels 5 inside the elements from left to right in figures la. At points A, E, I and M the flow channel is at its rightmost position. From A to B (and Eto F, . .) the flow turns left, and from B to C to D it turns right. From D to E to F it turns left again. The counter-flow in channels 6 outside the elements will be opposed to the flow. :

Where the flow inside the channels or tubes 5 (see e.g. figs. 3) turns towards the upper end of the diagram (as from A to B, and from D to E to F) the fluid inside accelerates and exerts a force in the opposite direction on the thin flexible walls of the parallel tubes. This tends to distort the tubes.

However, in these same areas the counter-flow 6 on both sides outside the element will be accelerated in the opposite direction to the fluid inside, and will partly balance the said force on the flexible walls.

Hydrostatic forces may be considered, when for example heat transfer occurs between aqueous flows, as in aquaculture. As water (or other liquid) is in equilibrium in a rigid tank, a more or less rigid outer container, or else a strong thick outer plastic film can according to the invention be used to remove hydrostatic stresses due to height differences from the thin flexible elements, and from the inter-connections between elements. Alternatively, the flexible film heat exchanger elements may be immersed in the relevant liquid B for example, in a protected part of an aquaculture tank or { C pond. As described above, the fluid directed to the interior § of the heat transfer elements must be at a higher pressure than the fluid directed to the space 6 between elements. Also, if liquids rather than gases are used, the first and second flows 17 and 18 B in channels 5 and 6 must be liquids with the same or closely similar density.

In the case of a rigid container, the inflated elements may be stretched sideways by attaching them to the walls of the container. When inflated, such stretched elements will have flow channels 5 that have a flattened cross section. According to the invention this flattened channel shape comprises two circular arcs attached by the weld lines. See fig 3d - 3e. According to the invention, the elements may be attached to the walls by clamps.

The spacing 6 between elements may be constructed according to the use to which the heat exchanger will be put. For energy recovery in ventilation of buildings, many drying and industrial flue processes and aquaculture the outward and inward counter-flows B and thermal capacitance flows B are approximately equal. In such cases, a spacing is preferred in which the transverse cross sectional area inside an element, and the transverse cross sectional area between adjacent elements are roughly equal - to have roughly equal flow speeds and similar heat transfer coefficients on both sides.

For such cases fig 3a shows the transverse cross section shown in figure 1a at AA, fig 3b the same at the crossing points BB and DD, and fig 3c at CC. With the counter-undulating (mirror image) flow patterns in adjacent elements, fig 3c is a mirror image of fig 3a.

Figures 3d and 3e show (rotated through 90 degrees) the cross section at AA, for heat exchangers with elements stretched sideways by different amounts. The stretched versions have better resistance to dynamic forces - which act in the sideways, i.e. transverse direction. They also allow more heat transfer surface to be included into the same overall volume - at the cost of an increased flow resistance. (They also have less curvature, which means less pressure difference capability).

Thus stretched elements with non-circular flow channels 5 inside them have significant advantages for the thin film elements in withstanding sideways dynamical forces in undulating flow.

In most heat exchangers, the cross sectional area in the pipes or ducts to and from the heat ® exchanger is much smaller than the total cross section for flow of either stream inside the heat ! exchanger. To avoid sudden changes in flow cross section in either circuit, this predicates a smooth flow convergence towards the outlet, and a smooth flow divergence from the inlet of the heat exchanger in each flow circuit. Large sudden changes in these areas will imply sudden changes in flow speeds, stagnant areas and an unwanted increase in flow resistance.

Figure 4a exemplifies in side view a preferred outlet from the elements. Solid horizontal lines (eventually converging to the right) are those of (relatively thick-walled polymer) insert tubes 9 inserted into the ends of film tubes nearest to the viewer; similar dashed lines are those of insert tubes 9 staggered with respect to the first. The outermost hatched ridge between planes PP and QQ are battens 8 to aid in clamping everything together.

Figure 4b shows the tube plate B built up of tube plate forming pieces 7 B in transverse cross section QQ (or any in between this and PP), fig 4c the tube spacer plate 10 at transverse Cross section RR, fig 4d the tube spacer plate 11 at transverse section SS and fig 4e the tube spacer plate 12 at transverse cross section TT. In each of these, the area between the concentric circles represents the walls of the insert tubes 9. In the region between sections PP and QQ specially machined or moulded tube plate forming pieces 7 (exemplified in fig 4f) clamp the film elements between themselves (at the positions of the film weld or bond lines), and also between themselves and tight-fitting precision extruded (or drawn, or machined) tubes 9 inserted into the ends of the channels or “tubes” 5 formed between the film weld lines on inflating the elements by the said higher pressure inside the elements.

The film elements extend from the bulk of the heat exchanger up to the plane QQ - their joint lines are straight, parallel and staggered between planes PP and QQ. However, the inserted tubes 9 smoothly bend just after plane QQ to converge toward the outlet, as seen from figures 4a to 4e. The flow through the enclosed spaces between the elements starts near plane PP (where it enters - see fig 5a), and proceeds to the left in F ig 4a to just before the corresponding plane PP, where it leaves - see fig Sa.

The inlet to the inside of the heat transfer elements is (apart from the flow direction) similar to the mirror image of fig 4a. Figures 4f and 4b shows the tube-plate forming end pieces or spacer-clamps 7 in transverse cross section.

® Fig 5a exemplifies, in plan view, fluid inlets and outlets for the second stream to the spaces 6 between the elements. By plan view we mean: normal to the general plane of the elements. By side view we mean: looking in a transverse direction (at right angles to the general flow direction, but in the general plane of an element). Neither of these terms imply any restriction on the orientation of the heat exchanger.

Fig 5b shows a short piece 13 of extrusion or a moulding - to be used berween adjacent elements in the position of either of the cuts UU - to keep open the space between said adjacent elements to let fluid into or out of the flow spaces 6 between the elements. According to the invention these pieces 13 may also taper sideways to aid smoothly and steadily diverging inlets and smoothly and steadily converging outlets for the second stream as shown in fig 7.

The inlets and outlets for the second stream — to these spaces 6 between the elements — also preferably smoothly diverge from and converge towards the external ducts or tubes connected to them.

According to the invention, smoothly diverging inlets and converging outlets for the first stream may also be made with end sections of the heat transfer elements, as will be clear to a reader skilled in the art. These end sections preferably have straight or smoothly curving bond lines instead of undulating ones. Also, such inlets or outlets, when viewed in plan may, in the region between the planes MM and NN in figures Sc-f bend gradually and smoothly in the transverse direction while converging in side view as shown in fig 6. These end sections preferably smoothly converge in plan view as shown in figures Se-h. They also preferably converge in side view as shown in fig 6.

Such variants will be understood to be included in the invention.

Despite the use of terms like “plan view” and “side view” it is understood that in use the heat exchanger can be oriented as needed for its particular application. Thus for the case of a condensing flow, the (longitudinal) direction of said flow is preferably downwards vertically, or at an angle.

Figure 5a exemplifies, in plan view, a heat exchanger with in- and outlets according to figure 4a-f.

The flow 17 inside the elements is from left to right, entering and leaving tubes 9 at planes TT with tubes closely spaced in the spacer plate 12, progressing (at the inlet side) through (drilled or punched) tube support plates 11 and 10 at SS and RR to a transition to heat transfer elements at the ® tube plates formed by clamped pieces 7 between planes QQ and PP. At the outlet side the progress is from a transition from heat transfer elements to tubes 9 at tube plates between planes PP and QQ to tube support plates 10 and 11 at sections RR and SS, to a final tube support plate 12 at section

TT, whence the outlet preferably further converges as shown if fig 4a.

For the second fluid stream 18, which flows in the enclosed spaces 6 between the elements, flow enters and leaves at planes UU, through inlet and outlet spacer pieces 13 as exemplified in fig 5b for a rectangular profile (or in fig 7 for a tapered profile) between adjacent elements.

Figure 5a also shows the undulating bond lines in the main (heat exchange) section of the apparatus.

It shows the (generally left-to-right) flow direction of the first fluid stream 17 inside the elements (and inside the in- and outlet tubes 9), and the opposing second flow 18 entering through inlet pieces 13 at a plane UU, bending towards the lef, flowing generally in an opposing direction to the first flow, and finally bending towards the exit through outlet pieces 13 at a second UU plane.

Figures 5c to Sh exemplify, in plan view, heat exchangers where the smoothly diverging inlets and converging outlets are extensions of the heat transfer elements. In these extensions the weld lines preferably do not undulate, but bend gradually and smoothly in the transverse direction- According to the invention, the flow paths 5 are preferably staggered in these extensions, as also shown in drawings 4b to 4f B and not aligned as in fig 3b.

The in- and outlets 13 at UU may be retained in positions as shown in fig 5a, where the diverging and converging parts of the flow are beyond the outlets and inlets UU to the flow spaces/channels 6.

But as shown in figures Sc to Sh, the in- and outlets 13 at UU are preferably moved to the extensions of the heat transfer elements. Here the diverging and converging parts of the first flow 5 or 17 are in the same general regions as the in- and outlets for the second flow 6 or 18.

Consequently, the inlet and outlet pieces 13 will be tapered (and may have smaller passages nearer to the ends of the heat exchanger) as exemplified in fig 7.

Figure 6 shows in side view an outlet with extension of the elements. As in fig 4a, the flow paths 5 of odd numbered elements are shown in solid lines, and those of even numbered elements in broken lines. In this side view, due to the staggering of the flow paths 5, these paths appear to be larger

! than the flow paths 6 between elements, but are generally not larger in (longitudinal) cross sectional ® area.

The channels 5 for the first fluid as well as the channels 6 for the second fluid initially both gradually taper down in size towards the exit. From about the section NN (which is also shown in plan view in figures Sc to 5h) the channels 5 inside the elements gradually cease tapering down, but the flow spaces 6 between the elements continue tapering down to near zero at the tube plate forming end pieces 7 in the region between sections PP and QQ. In this last region figure § is a cross sectional view showing the converged films 1 and 3 with their enclosed flow channels 5, the tube-plate forming end pieces 7 which close the flow channels 6 at the ends and the insert tube pieces 9 that hold the ends of the flexible channels open against the tube plate pieces 7.

In figure 6, as in figure 4a, an inlet may typically be a mirror image about a longitudinal plane of such an outlet B only the flow directions (arrows) are not mirrored.

In all of the cases illustrated in figures 5c to Sh, the convergence may start near the section MM.

The tube spacing plates 10, 11 and 12 (at planes RR, SS and TT in figures 4a-f, and figure Sa) are now preferably omitted, and the tapering of the elements as seen in a side view (for example, in figure 6) are preferably determined by corresponding tapering of the element fixing lines against a rigid outer enclosure or casing 16. The diameter of the flow channels 5 in the element extensions may now gradually taper down to a smaller value towards the section NN. As indicated above, the spacing of the channels 6 between the elements may taper from around the sections MM past the sections NN to at or near where they terminate at the tube-plates between QQ and PP. Therefore the insert tube pieces 9 between the planes PP and QQ will now be of a smaller outer diameter than the inside diameter of the main part of the elements.

The inlets and/or outlets 13 for the second stream 6 or 18 at positions UU are now preferably moved to the diverging and/or converging parts of the first stream 5 or 17, and the inlet and outlet pieces 13 are preferably tapered as shown in fig 7. Tapering may then be determined by corresponding tapering of the fixing lines against a rigid outer enclosure or casing.

Claims (22)

1. i 16 cL

   I. A heat exchanger for transferring heat between gas or liquid flows in which the walls through ® which the heat transfer takes place are of flexible film, the heat exchanger comprising heat exchange elements each made by bonding together two layers of said film along equidistant gently undulating lines thereby producing parallel flow channels inside each such element, several such elements being superposed in spaced parallel planes, with the flow channels inside the elements carrying one fluid stream, and the spaces between the elements carrying the other fluid stream.
2. 2. A heat exchanger as claimed in claim 1, in which adjacent elements are superposed in mirror image bond pattern relative to each other.
3. 3. A heat exchanger as claimed in either one of claims 1 or 2, in which said bond lines are continuous except for possible gaps for draining condensate, where appropriate.
4. 4. A heat exchanger as claimed in any one of claims 1 to 3, in which said bond lines are made by welding or heat sealing of polymer film layers.
5. 5. A heat exchanger as claimed in any one of claims I to 4 in which, during or after bonding along said lines, the films are heated and the part of the film between the bond lines stretched between said bond lines, thereby being thermo-formed into circular or other suitable channels on subsequent inflation.
6. 6. A heat exchanger as claimed in any one of claims 1 to 5 in which the ratio S/P of the swing S of the undulations to the transverse spacing P between the lines when the elements are inflated is in the range 0.5 + 0.1.
7. 7. A heat exchanger as claimed in any one of claims 1 to 5 in which the ratio S/P of the swing S of the undulations to the transverse spacing P between the lines when the elements are inflated is in the range 0.5 + 0.3.
8. 8. A heat exchanger as claimed in any one of the previous claims in which the ratio of the “wavelength” L to the swing S is in the range above 8.
9. 9. A heat exchanger for transferring heat between gas or liquid flows in which the walls through ® which the heat transfer takes place are of flexible film, the heat exchanger comprising heat ! exchange elements each made by bonding together two layers of said film along equidistant lines thereby producing parallel flow channels inside each such element, several such elements being ! superposed in spaced parallel planes, with the flow channels inside the elements carrying one fluid stream, and the spaces between the elements carrying the other fluid stream.
10. 10. A heat exchanger as claimed in claim 10 in which the flow channels are staggered relative to those in adjacent elements .
11. I1. A heat exchanger as claimed in any one of the previous claims, provided with smoothly diverging inlet and/or converging outlet for the first fluid stream.
12. 12. A heat exchanger as claimed in any one of the previous claims, provided with smoothly diverging inlet and/or converging outlet for the second fluid stream.
13. 13. A heat exchanger as claimed in any one of the previous claims, in which the edges of each clement is connected to the inside of a rigid enclosure in such a way that the elements, on inflation, are stretched sidewise or transversely into shapes with non-circular channels.
14. 14. A heat exchanger as claimed in any one of the previous claims, in which the inlet and outlet sections for the first stream comprise extensions of the elements themselves B either in line with the main parts, or else smoothly bending from the main flow direction.
15. 15. A heat exchanger as claimed in claim 14, in which the bond lines in said extensions do not undulate.
16. 16. A heat exchanger as claimed in claim 14 or claim 15, in which the bond lines in said extensions smoothly and gradually converge towards the outlet, and smoothly and gradually diverge from the inlet.
17. 17. A heat exchanger as claimed in any of claims 14 to claim 16, in which the said extensions of the elements gradually and smoothly converge in side view towards the outlet, and smoothly and gradually diverge away from the inlet.
18. ® 18. A heat exchanger as claimed in any of claims 14 to claim 17, in which the said extensions of the elements have bond lines that are staggered relative to the bond lines of adjacent elements. i
19. 19. A heat exchanger as claimed in any one of the previous claims, in which the inlet and outlet sections for the second stream converge in the longitudinal direction towards the outlet, and diverge in the longitudinal direction away from the inlet. '
20. 20. A heat exchanger as claimed in any one of the previous claims, in which the inlet and outlet sections for the second stream converge in the transverse direction towards the outlet, and diverge in the transverse direction from the inlet.
21. 21. A heat exchanger as claimed in any of the preceding claims, in which the streams between which heat exchange takes place are in counter-flow arrangement.
22. 22. A heat exchanger substantially as described herein, with reference to any one or more of the features described in the drawings. DATED THIS 12th DAY OF JANUARY 2007. a ! a iM % iffan a [| NE APPLICANTS Adi]