GB2188137A

GB2188137A - Heat storage material

Info

Publication number: GB2188137A
Application number: GB08706567A
Authority: GB
Inventors: Klaus Ernst Pohlmann
Original assignee: Rohm GmbH; Roehm GmbH Darmstadt
Current assignee: Rohm GmbH; Roehm GmbH Darmstadt
Priority date: 1986-03-20
Filing date: 1987-03-19
Publication date: 1987-09-23
Anticipated expiration: 2007-03-19
Also published as: NL8700261A; IT210484Z2; DE8607689U1; ATA310786A; GB8706567D0; GB2188137B; AT393902B; SE8701068L; SE8701068D0; IT8753085V0; FR2595981B1; FR2595981A1

Abstract

Heat storage material for a rotary thermal regenerator or a plate heat exchanger comprises plates of thermoplastic weldable plastics material, each having a plurality of projections 6, 7, 8 integrally formed with the plate, which are welded by ultrasound or friction welding or by hot wires to form a stack in which the surfaces of the plates are welded to the tips of the projections on the adjacent plate surfaces and held at a spacing. The plates are formed by extruding a flat sheet (22), which is then passed between rotating cooled rollers (23, 24), at least one of which has peripheral moulding recesses (26) for forming the projections, the recess and projections preferably being tapered to facilitate release of the projections. A hot wire may be moulded into a continuous surface projection from the sheet, by magnetically retaining the wire within a roller recess which moulds the continuous projection. <IMAGE>

Description

SPECIFICATION Heat storage material This invention relates to a heat storage material.

More particularly it relates to stacks of plastics plates or boards which are suitableforuse as storage material for the transfer of heat between gas currents in heat exchangers. One aspect of the invention relates to a simplified method of producing the weldable plates and joining them togethertoform stacks of plates.

WO 83/02997 describes a storage material made of plastics which is particularly suitable for reheating the boiler waste gases from a furnace (these gases having been cleaned in a wet cleaning operation) by heattransferfrom the unpurified boiler waste gases which are about to be subjected to wet purification. A plurality of stacks of plates are arranged in a rotor which projects into two flue gas currents in such a way that as it rotates the individual stacks of plates are passed alternatelythrough both flue gas currents. Thus, they absorb heat from the hotter flue gas current and release this heat to the colderflue gas current. Between passing through the flue gas currents, the stacks of plates are subjected to a flow of cleansing fluid, especially water, in orderto remove any solids which have been deposited on the surface of the plates from the flue gas.During such a process the stacks of plates are subjected to alternating stress from high temperature and the corrosive action of the flue gases and mechanical stress produced by the cleansing fluid flowing in at high speed and the corrosive action of the cleansing fluid. WO 83/02997 suggests polyphenylene oxide as being a suitable material for withstanding these stresses.

As a cheap and simple method of producing stacks of plates which serve as heat stores, the prior art proposes that flat plates of plastics material with spacers placed between them in layers should be welded using ultrasound. in this process, the spacers have to be individually placed on the flat surface of each plate. To avoid this procedure it has also been proposed that the spacing profiles be formed as areas ofthe plate-shaped storage elements. This requires an additional procedure. Moreover, the surface ofthe plates is interrupted by hollowed-out portions at the spacers.

The arrangement described in the above-mentioned prior art has numerous disadvantages. Thus, the mechanical strength ofthe plate surface is reduced by these interruptions. The spacers formed by reshaping part of the plate surface are necessarilythinnerthan the plate material and are hollow inside; they therefore have only limited strength. When identical plate elements are stacked, there is the further disadvantage that the spacers formed in one plate engage in the hollow reverse ofthe spacers in the next plate, thus making it difficu It to weld the plates together at this point.

Furthermore, if the spacers are offset relative to one another in the individual planes, the stack of plates as a whole is less strong than if all the corresponding spacers were located one above the other as if in a continuous column.

We have now devised a material and a method of manufacturing it which substantially avoids the disadvantages mentioned above. In particular, we have been able to simplify the production of weldable plate elements, to construct a stack of plates suitable for use as a heat storage material and to avoid the use of separately produced spacers. We havefurtherbeen ableto reduce the numberof operations required to assemble the individual plate elements into welded stacks of plates.

Thus, according to one aspect of the present invention, we provide a heat storage material comprising a plurality of plates ofthermoplastic weldable plastics material, each of said plates having a plurality of projections integrally forming part of or connected to the plate body and distributed overthe surface of each of said plates, the plates being arranged in spaced relationship to each other and the tips of the projections on each such plate being welded to the surface of an adjacent plate to form a stack.

The invention will now be illustrated byway of example with reference to the accompanying drawings in which: Figure 1 shows a cross section through a plate element useful in the invention with the projections arranged thereon; Figures 2 to 4 show perspective views of different embodiments of the plate elements; Figure5showsthe arrangementofa plurality of plate planes with the same direction offlowto form a parallel-flow heat exchanger; Figure 6shows the arrangement oftwo plate elements, offset crosswise, to form a cross-flow heat exchanger; Figures 7to 9 show crnss-sections through various embodiments of the projections which are suited to different welding processes;; Figure 10 shows a cross section through a finished weld joint; Figure 11 shows part of a cross section through a finished stack of plates; and Figure 12 diagrammatically shows an apparatus for producing the plate elements according to the invention.

Referring to Figure 1, characteristic of the plates from which the stack is made is a plate body 1 which is not interrupted at the points of attachment of the projections 2.

It is possible to provide projections for connecting to other plates on both sides ofthe plate, but it is preferable for all the projections which are provided for connection to other plates to be arranged on one side ofthe plate. In orderto arrange the plates parallel to one another in spaced relationshiptoform a stack of plates, it is preferable for all the projections which are used for the weld connection to have the same height h. In addition to the projections which are usedforwelding, other, less high projections may be provided on one or both sides of the plate, serving for example to deflect the flow of gases orto cause turbulence in the gases flowing through.The word "projections" used hereinafter refers solely to the projections which are used for welding to the adjacent plate.

These projections are preferably arranged at spacings of from 20 to 250mm, preferably 20 to 200 millimetresfrom one another. For practical manufacturing reasons and also in orderto increase strength, it is preferableforthe projections to be larger in diameter at their base, where they abut on the plate body itself, than at their points. They are preferably wedge-shaped in cross section, merging with the surface of the plate body in an arcuate shape. This transition preferably has a radius of curvaturerofatleasti, preferably 2to 10 millimetres.

The projections preferably have a substantially square orcircularbase surface and are in the form of pyramids or cones. They may also be in the form of substantially parallel strips. If point welds are sufficient, the plates may have individual projections in the form of truncated pyramids orcones 4, as shown in Figure 2. This arrangement produces a particularly low flow resistance. It may be advisable to provide, in the edge portion, a projection in the form of a continuous strip 5 by means of which the stack of plates can be sealed off at the sides.

The edge strips of all the individual plates may be arranged in the same direction orthe edge strips of successive plates may extend at right angles to one another.

The projections may also be in the form of shorter or longer strip portions 6, as shown in Figure 3.

Hydraulically, it is advantageous if the strips run parallel to one another and are pointed or rounded at at least one end face in orderto promote flow. To achieve greaterturbulence of flow, strips 7 or strips 8 of periodically fluctuating diameter in the longitudinal direction, corrugated in the same direction or preferably in the opposite direction, may be provided. The turbulence promotes the transfer of heat and helps to preventdustfrom being deposited.

Referring to Figure 4, maximum strength of the stack of plates is achieved by using strips 9 which continue overthe entire length of the plates.

The plate body is generally from 0.5 to 3 mm thick.

Apart from the projections and the transitional area at their base, the plate body is preferably of uniform thickness with onlythe variations caused by manufacture. If, exceptionally, this is not the case, the plate thickness at the thinnest points is used as a basisforthe dimensions which follow.

The invention makes it possible to produce projections which have a heightgreaterthanthe thickness of the plate. Their height h is preferably more than twice the thickness of the plate, and more preferably from 2 to 10 times the thickness ofthe plate. The projections may, for example, be from 1 to 10, preferably from 3to 6 mm high. The invention also makes it possible to provide projections of considerable thickness. The diameter d half way up the projection is generally greaterthan the thickness ofthe plate and may be,forexample, 1 Sto 3times the thickness of the plate.

The optimum shape of the top of the projections dependsontheparticularwelding method. For ultrasonicwelding a pointed top 10 as shown in Figure 7 is advantageous, and the point angle a is preferably between 70 and 110 . On a strip-shaped projection the top is in the form of a continuous blade.

For friction welding, a rounded top 11 as shown in Figure 8 is more suitable. It is particularly advantageous if the friction weld is produced between the top 11 and a flat depression 3 in the underside of the body 1 which is to be joined. Since, identical plates are generally welded together in spaced relationship to form a stack, the top 11 of a projection on one plate makes contact with the underside of the base surface ofthe corresponding projection in the next plate. Generally, a flat depression is located here. This depression is automatically formed ifthe underside 12 ofthe plate runs over a conventional cooling or smoothing roller without any recesses and the projection is of considerable height and/or thickness. In this case, a flat depression 3 is formed by thermal oscillation during cooling.

Referring to Figure 9, if the projections consist of strips 13 which run from one edge of the plate to the opposite edge, the hot wire welding method may be used. Forthis purpose, the hot wire 14 is inserted in the top ofthe projection 13. i.e. the strips preferably have a hot wire at the weld seam with the adjacent plate.

The stack of plates according to the invention or the plates used to construct it are conveniently made ofthermoplasticweldable plastics. The plastics must be workable by extrusion and must fuse, when heated to the melting temperature, optionally under slight pressure, in such a way that after cooling there is a solid joint of material. In other respects, the material properties of the plastics depend on the conditions of use. For heat exchange with air at temperatures in the rangefrom -20to800C, it is possible to use a variety of industrially used thermoplastic plastics such as polyvinyl chloride, polystyrene, acrylic glass or polycarbonate.Forthe preferred use in flue gas cleansing plants, plastics with a non-deformability attemperatures over 1 50 C, preferably over 1 200C may be used, such as polyphenylene oxide, polyether sulphone or polyether imide. Polyphenylene oxide is preferred.

According to a further aspect of the present invention we provide a method of forming a heat storage material which comprises welding together a plurality of plates of thermoplastic weldable plastics material, each of said plates having a plurality of projections integrallyforming part of or connected to the plate body and distributed overthe surface of each of said plates,the plates being arranged in spaced relationship to each other, the tips of the projections of one plate being in contact with the surface of another plate priorto welding.

In orderto produce the new plates economically, it is essential that they be produced in a single operation. The apparatus diagrammatically shown in Figure 12 is suitable forthis. Plasticised moulding composition isfed from an extruder 20 into a slotted die 21 and extruded from it in the form of a flat strip 22. While still plastic, the strip is passed into the calendergapformed by the rollers 23 and 24. The extrusion speed and the rotational speed of the calender rolls are adapted to each other so that a bead 25 of molten moulding composition isformed in front of the calender gap. At least one calender roll 23 has, in its surface, a plurality of recesses 26for producing the projections in the extruded sheet.

Underthe mass pressure prevailing inthecalender gap, plastic moulding composition is pressed into the recesses 26, thus forming the projections 2. A conical shape for the recesses 26 and the projections 2 formed therein makes it easier to remove the projections from the mould. The calender rollsare cooled to below the melting point of the plastics, so that after removal from the mould the projections 2 retain the shape which they have taken on in the recesses 26. The calender rolls 23,24are cooled by means of coolant lines 27 carrying coolant which flows through the hollow roller spindles, in known manner.

In order to produce conical projections 4, the surface of the roller 23 is provided with suitable conical recesses 26. Edge strips 5 (see Figure 2) can be produced by means of an annular groove cut into the periphery. Accordingly, recesses 26 may be let into the roller surface in orderto produce strip-like projections 6,7 (see Figure 3). A continuous strip 5,9 (See Figures 2 and 4 respectively) may be produced either by means of an annulargrooveformed in the periphery or by means of a groove which is cut concentrically in the surface. In the former case, an endless strip is produced in the direction of extrusion whereas in the latter case a strip is formed at right angles to the direction of extrusion over the entire width of the plate.In both cases, a hot wire 14can be incorporated in the top of the projection by placing a hot wire in the groove ofthe calender roll before entering the roller gap, this hot wire being located on the basis of the groove if possible and if necessary held in place by means of magnets. When the groove fills with moulding composition the hotwire is embedded therein.

The material required to fill the recesses is taken from the bead 25 which must consequently be of sufficient size. When strips are produced in the direction of travel of the extruded plate it is advantageous to make the extruded material 22 somewhat thicker at those points where a strip isto be formed, by suitably widening the exit slot ofthe die 21, so as to ensure that the quantity of material needed to form the strip is always available. The continuously produced length of material provided with projections is divided up into individual sheets in known manner as required.

An example of a stack of plates according to the invention is shown in cross section in Figure 11. A plurality of plate elements (31 a, 31 b, 31c...) comprising projections are arranged parallel to one another, the upper ends of the projections being welded to the underside of the plate above. Atypical cross section through a weld joint is shown in Figure 10. The moulding composition melted during the welding operation has formed a bead 15which integrally connects the top of the projection to the surface of the next plate. Preferably, all the individual plates 31a, bc,... are the same size and arewelded together in such a way that corresponding edges are located in the same plane. The base surface ofthe stack of plates is generally rectangular.If each plate contained in the stack has a continuous edge strip 5 (see Figure 2) along two opposite edges, the entire stack of plates is closed off along these sides for the medium flowing through.

The size ofthe individual plates depends on the particular application envisaged and is limited in practice only by the width of the extrusion die.

Therefore, the width is generally between 0.1 and 2 m and the length in the direction of extrusion is conveniently between 0.1 and 10 m.Thestackof plates may,forexample, be between 0.1 and 2 m high, corresponding to between 10 and 200 individual plates stacked one above the other. When determining the number of plates required for a specific stack height it should be remembered that the height of the projections may decrease by upto about one millimetre during the welding process.

When welding by the ultrasonic method, high frequency oscillation energy is applied, under pressure, to the point of contact between the tip of a projection and the smooth reverse of a plate. If the oscillation transmitter is suitably shaped, a whole line of point welds or a linearweld seam may be produced along a continuous strip in a single operation. By moving the welding apparatus along line by line, all the projections in one plane of a plate can be welded to the adjacent plate step by step. In this way a stack of plates can be added to, layer by layer, by the addition of one plate each time.

In the friction welding process, an entire plate is set oscillating parallel to its base surface, so that all the projections are simultaneously melted by the heat of friction produced and thereby welded to the reverse ofthe abutting plate. This process is therefore considerably faster than the ultrasonic welding process. An even higher operating speed can be achieved using the hot wire welding method in which the hot wires 14 embedded as in Figure9 in the tips of the continuous strips 13 are heated by the application of a precisely measured electrical voltage, thus simultaneously melting the tip of the projection and the adjacent surface of the next plate and so welding them together. In this process an entire stack of plates can be welded in a single operation if all the hotwiresaresimultaneously connected to the heating voltage. In this process, also, the individual plates are held together under slight pressure until the welding process is complete.

The stacks of plates of paral lel-flow construction (e.g. as in Figure 5) produced according to the invention are preferably used as heat storage material in heat exchangers used in exhaust gas cleansing plants. Other applications include air conditioning plants for buildings, greenhouses, stables and the like,which may be heated orcooled as necessary. Heat exchange is carried out between incoming fresh air and out going stale air. As well as the regeneration principle in which the stack of plates alternately absorbs and releases heat, it is also possible to use the recuperation principle in which the heat-exchanging currents flow simultaneously through alternate layers of the stack of plates; this purpose may be served by the cross-flow recouperators constructed according to Figure 6.

Although the heat exchange ofairorflue gases is of predominant importance, heat exchangers ofthis kind may also be used for other gaseous or liquid media such as water,

Claims

1. A heat storage material comprising a plurality of plates of thermoplastic weldable plastics material, each of said plates having a plurality of projections integrallyforming part of orconnected to the plate body and distributed over the surface of each of said plates, the plates being arranged in spaced relationship to each other and the tips of the projections on each such plate being welded to the surface of an adjacent plate to form a stack.

2. A material as claimed in claim 1, wherein all the projections of each individual plate are arranged on one side of the plate.

3. A material as claimed in either one of claims 1 and 2, wherein many or all of the projections of each individual plate are ofthe same height.

4. A material as claimed in any one of claims 1 to 3, wherein the projections of each individual plate areatspacingsoffrom 20 to 200 mm from one another.

5. A material as claimed in any one of claims 1 to 4, wherein the projections have a larger diameter at the base than at the tip.

6. A material as claimed in claim 5,wherein the projections are wedge-shaped in cross section.

7. A material as claimed in either of claims 5 and 6, wherein the plate surfaces merge into the surface of the projections with a radius of curvature of at least 1.

8. A material as claimed in claim 7 wherein the plate surfaces merge into the surface of the projectionswith a radius of curvatu re of 2 to 10 mm.

9. A material as claimed in any one of claims 1 to 8, wherein the plates have depressions in their other surfaces at the pointwhere the projections of the adjacent plate are to be welded on.

10. A material as claimed in any one of claims 1 to 9, wherein the projections have a height which is more than twice the thickness of the plate.

11. A material as claimed in claim l0wherein the projections have a height which is 2 to 10 times the thickness of the plate.

12. A material as claimed in either of claims 10 and 11, wherein the projections are from 1 to 10 mm high.

13. A material as claimed in claim 12, wherein the projections are from 3 to 6 mm high.

14. A material as claimed in any one of claims 1 to 13, wherein the projections have a diameter at half their heightwhich is greater than the thickness of the plate.

15. A material as claimed in claim 14, wherein the projections have a diameter at halftheir height of from 1.5 to 3 times the thickness ofthe plate.

16. Amaterial as claimed in anyoneofclaims 1 to 15, wherein the projections have a substantially square or circular base surface and are in the form of pyramids or cones.

17. A material as claimed in any one of claims 1 to 16, wherein the projections are in the form of substantially parallel strips.

18. A material as claimed in claim 17, wherein the strips have a periodically fluctuating diameter in the longitudinal direction.

19. A material as claimed in claim 17, wherein the strips are corrugated in the longitudinal direction.

20. A material as claimed in any one of claims 17 to 19, wherein the strips are at spacings of from 20 to 250 mm.

21. A material as claimed in any one of claims 17 to 20, wherein the strips are pointed or rounded at at least one end face in order to promote flow.

22. A material as claimed in anyone of claims 17 to 21, wherein the strips extend overthe entire length ofthe plate.

23. A material as claimed in claim 20,wherein the strips have a hot wire at the weld seam with the adjacent plate.

24. A material as claimed in any one of claims 1 to 23, comprising a plastics material which is non-deforma ble attemperatures above 1 50"C.

25. A material as claimed in claim 24, wherein the plates are formed of polyphenylene oxide, polyether sulphone or polyether imide.

26. A material as claimed in claim 25, comprising individual plates ofthe same size, the corresponding edges of which are located in the same plane.

27. A material as claimed in claim 26, wherein each plate in the stack has a continuous edge strip along two opposite edges.

28. A material as claimed in claim 27, wherein the edge strips of all the individual plates are arranged in the same direction.

29. A material as claimed in claim 27, wherein the edge strips of successive plates extend at right angles to one another.

30. A material substantially as herein described with reference to the accompanying drawings.

31. A method of forming a heat storage material which comprises welding together a plurality of plates ofthermoplasticweldable plastics material, each of said plates having a plurality of projections integrallyforming part of or connected to the plate body and distributed overthe surface of each of said plates, the plates being arranged in spaced relationship to each other, andthetips ofthe projection on each such plate being welded to the surface of an adjacent plate to form a stack, the tips of the projections of one plate being in contact with the surface of another plate prior to welding.

32. A method as claimed in claim 31 substantially as hereinbefore described and with reference to the accompanying drawings.