An example could be the heating of water for district heating systems using exhaust flue gases from fossil fuels as an adjunct to power generation.

Plastics composite material for use in heat transfer is described in GB-A-1572680 and comprises a metal mesh layer embedded in a plastics layer so that the plastic closes the mesh openwork. This plastic mesh composite material can be provided as sheets or panels for use in H. E apparatus and such as regenerative equipment or adapted so as to form tubing. The metal mesh described is a woven mesh but the mesh can be of other form e. g. expanded metal. GB-A-1572680 discloses how the plastics should be applied so that the composite material can exhibit a satisfactory heat transfer performance. More especially in this connection the GB patent directs that nodes of the metal mesh should be located close to the outer surface of the plastics layer: it is describable however that the metal is covered as a particular benefit of the material lies in its ability to be useable in inhospitable gas environments such as furnace gases.

It is the principal object of the present invention to provide an improved plastics composite material for use in heat transfer.

Therefore according to one aspect of the present invention there is provided a composite material including a layer of plastics material wherein there is embedded particulate material.

The particulate material can comprise metal particles, but preferably the particulate material defines a filler of material such as powdered glass, ceramic material or mineral matter or like substances.

The precise proportion of the filler relative to the plastics can be chosen as desired. Other suitable additives can be included in the layer.

In a preferred embodiment, the composite comprises a metal base layer of openwork form embedded in a strata of plastics material. The arrangement is such that the plastics keys to the base layer via the through openings in the openwork layer. The plastics strata is preferably applied such that the through openings of the base layer are fully filled with plastics material and the presence of air voids in the strata substantially avoided. The base layer may comprise an expanded metal or a woven product, but other metal sheetings having an appropriate aperture array of different apertures geometry are possible.

The filler can be of a material substantially less expensive then the plastics material of the layer and hence by having a substantial proportion of this filler present the cost of the composite material can be substantially reduced. However, the plastics material does encourage flexibility of the layer, in contrast for example to glass, enamel, ceramic or like materials which generally are brittle or breakable and which may comprise the filler substance, so that the presence of the plastics will be beneficial especially when bending of the composite material is to occur. This is a facet which will require consideration when deciding on the

particular proportions of the plastics material and filler in the composite. The composite material can be used in pipes or tubes, or as a sheet element.

The plastics/particle combine can be used as a closing layer for a metal mesh, and the metal mesh can be a woven mesh or an expanded metal mesh for example.

The particles may be of graphite e. g. graphite platelet or of carbon fibres of a chopped form.

By an inventive technique per se, the strata can be applied by powder coating by means of a spraying method using opposing spray guns on either side of the base layer means being provided to impart movement of the guns in unison over the base layer for effective coating by the base layer and filling of the apertures in the layer.

The spraying can comprise electro-static spray coating.

Other coating techniques are possible for example dip coating of the base layer. The particulate material can be applied by a subsidiary filling step.

Where embedded metal particles are used, the metal particles preferably have a relatively high heat conductivity characteristic, and can be of copper or bronze for example. The quantity of metal particle present per unit volume in the plastics can be selected as required, especially with regard to the heat transfer performance demanded.

According to another aspect of the present invention there is provided a conventional solid-drawn or electric- resistance welded plain tube of mild steel or other metal which is adapted to have on the outside or the inside or both of these a metal thermally connected mesh plastic composite layer which imparts to the outside, the inside or both surfaces, heat transmitting non-fouling, non-

scaling, non-wetting properties. A composite material in accordance with the first aspect of the present invention can be used for the composite layer of this second inventive aspect.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings wherein Figs. 1,2 and 3 show pictorial views of heat exchange panels embodying the present invention, while Fig. 1A shows a detail of Fig. 1 to a larger scale, and Fig. 1B shows the section x-x of Fig. 1 ; Fig. 4 shows a further form of base metal layer for the panel; Fig. 5 shows a plan view of the mesh spiral winding process in accordance with a further embodiment of the present invention; and Fig. 6 shows a sectional side view of part of the tube formed by the process, while Fig., 6A shows a detail of the tube to a larger scale.

Fig. 7 is an elevational view illustrating apparatus and a technique for applying the plastics material to the base layer.

Referring to Figs. 1,1A and 1B, a mesh plastics composite (MPC) heat exchange panel 1 comprises a base metal layer 2 in the form of expanded metal having diamond form perforations 3. The metal material of the base layer 2 is fully embedded in a strata 4 formed generally of plastics material, so that the layer 2 is fully coated. The coating strata 4 may be fluoro carbon or an epoxy, or indeed any other plastics material,

compatible with the chemical and operating temperature requirements to be met by the panel 1. The strata 4 is applied to the layer 2 by a forming technique ensuring that a total single phase of the plastics material fills the perforations 3 in the layer and seals around the panel edges e. g. at edges 5,6 of Fig. 1, while forming layer portions L on the surfaces of the layer 2 and especially such that no air (or gas) cavities are present in the strata 4. Examples of suitable forming techniques are dip-coating, electro-static coating and electro- static spray coating. The thickness T of the plastic layer portions L can be for example 100 p m.

A preferred technique for the application of the plastics strata 4 is by an electro-static spray coating technique (as illustrated in Fig. 7) wherein two opposing spray guns G1 and G2 move in unison over the base metal layer 2. Thus, during the coating application there is always two directly facing essentially identical spray jets J from the guns, applying coatings on respective sides of the layer 2 and a particular benefit of this is that the perforations or apertures 3 can be effectively filled with plastics material as the guns G1, G2 move (in unison) over the layer 2 and over the perforations 3.

Suitable particles or additives 7 (Fig. 1A) are embedded in the strata 4 and can serve to improve the heat transfer (conducting) property of the strata 4, and subsequently also that of the total panel 1. These particles 7 may take the form of graphite platelets, for example having an arial dimension of about 50 p M, or alternatively may comprise chopped carbon fibres with dimensions for example 10 p m-30 p m diameter and length 50 p m approximately. The particles 7 may of course be of other relatively high heat conductivity materials such as copper, bronze, aluminium or nickel, although these materials may not be so easily handled or applied as graphite or carbon materials, and variations

in size of the particles 7 is possible and could for example be down to about dust size.

The particles or additives 7 may be applied conveniently as a subsidiary filling step during the application of the plastics strata 4 to the layer 2 by electro-static techniques, and steps will be taken to see that the particles 7 are substantially uniformly present in the strata 4. Thus Fig. 7 shows an arrangement where the platelets/fibres 7 are dispensed from hopper 20 moving with the guns GI, G2- The plastics strata 4 is preferably such as to exhibit low or non fouling, non-scaling and non-stick characteristics as is achievable by the use of fluorocarbon material for example polyvinyldene fluoride (PVDF).

In the Fig. 2 embodiment a mesh plastics composite heat exchange panel 1 is shown generally in accordance with GB patent 1572680. Thus a woven mesh 8 having warp and weft strands 9A, 9B is embedded in a layer 10 generally of plastics material, then covering sheets/films 11A possibly being applied at the outer surface to ensure the mesh 8 is covered and possibly also chosen of a material to mitigate against fouling of the panel. In one example, the layer 10 is a plastics layer and embedded within the plastics layer 10 are particles 12 of a foam and material as above described are again deposited so as to be reasonably evenly distributed throughout the layer 4.

The presence of the particles 12 greatly improves the heat transfer/conducting performance of the panel over the basic panel of GB-A-1572680, and a further possible advantage is that the close spacing of the nodes of the strands 9A, 9B from the outer surfaces of the plastics layer may not now be so critical.

In an alternative arrangement, the layer 10 comprises a combination of a plastics material and a filler (but possibly also the above metal particles 12 could be present). The filler comprises a relatively inexpensive material such as powdered glass, or other ceramic or mineral or like material. The precise proportions of the plastics material and filler in the layer 10 will be chosen as desired but it is envisaged that the proportion of the filler could be as much as 40% of the total volume content of the layer 10 or possibly even greater.

Generally no less than 10% or possibly 5% of filler will be present by volume. Additional additives could also be present in the layer 10. The plastics material used in the layer 10 may for example be PVDF fluorocarbon, and the filler material could be a substantially less costly material (e. g. powdered glass) so that the filler may be only 10% of the plastics cost on a volume basis. Therefore by having a substantial proportion of filler in the layer the overall cost of the composite material can be substantially reduced.

Nevertheless, the properties of the composite need not be adversely affected to any substantial degree, in particular with regard to the non fouling and non scaling effect of the composite material. The plastics material however can impart a fair degree of flexibility to the composite-in contrast to glass, enamel or other ceramic or the like which are essentially brittle and breakable where flexing occurs, so that presence of the plastics material has definite benefits. The composite panel/sheeting 1 including the filler (or metal particles) can be used in the formation of pipes or tubes, or in panel constructions.

Fig. 3 shows a heat transfer ducting 1A wherein woven mesh 8 is forming a core layer having its nodes 13

embedded in spaced plastics layer 10A, 10B so that a fluid duct 14 containing the openwork mesh 2 is located between the layer 10A, 10B.

The plastics layers 10A, 10B again substantially include embedded metal particles 12 as above, again improving the heat transfer performance of the ducting 1A. It will be appreciated of course that invention could be applied in other forms of plastics composites than that in Figs. 1 and 2.

Fig. 4 shows a panel 1B wherein the perforations 3 of the metal layer 2 are of a circular form although other geometry of the perforations 3 is possible.

Referring now to Figs. 5 and 6, a tube according to an example of the second aspect of the invention is shown which is adapted to operate with a flow of relatively pure water internally which is heated by the extraction heat of boiler flue exhaust gases passing over the outside which gases might contain condensable gaseous oxides of sulphur or nitrogen, gaseous oxides of carbon nitrogen gas, hydrochloric acid gas, etc. The tube might be subject to the impact of fly-ash particles carried by the flue gas also sundry other larger objects of mineral, metal and other"tramp"material borne by such exhaust gases from time to time.

It is the object of the invention that the tube supplied in accordance with it, is capable of remaining relatively scale and dust free on its outer surface such that the heat transfer performance remains high while the tube surface itself is relatively unaffected by the contact with condensation product of such flue exhaust gases above and below the dew point at which aqueous condensation takes place. Where such a tube is used to re-heat cleaned desulphurised flue gas prior to discharge to a chimney-vent the tube would not suffer from the

build up of calcium sulphate (gypsum) deposit arising from the drying out or wet chemical residues carried over after a desulphurisation process as is common environmental practice at the present time.

The production of one example of a tube according to the invention will now be described. This will be a tube processed according to this invention on the external surface only.

A tube 5 of the length required of carbon steel which may be up to 8 metres in length is obtained after any conventionally boiler/heat exchange tube production method as for example solid drawing, seamless cold or hot finished, electric resistance welded or thermally welded seamed etc. The tube will be of a gauge thickness suitable for the duty when reinforced by the added external strips.

It is necessary to clean, de-grease and de-scale the tube surface to be adapted before proceeding.

A continuous strip of expanded metal mesh 16, wire or other interlocked or woven construction of metal filaments is prepared, de-greased and otherwise cleaned.

This mesh 16 is secured to the outside of one end of the aforesaid tube 15 by tack, stitch, thermal resistance or other welding and when secured, helically wound in a spiral with the strip edges butting over the whole length of the tube while subjected to a winding-on tension at least 25% of the equivalent ultimate tensile strength of the wire strip in a longitudinal direction. It is convenient during this stage for the tube to be rotated axially between centres and for the mesh strip to be fed from a coil D on a drum 17 tranversing with the above tensioned feed at the appropriated rate axially and parallel to the tube.

During the process of winding the mesh onto the tube the mesh itself is secured to the tube outside the surface by any known welding method such that surfaces of the mesh in contact with the tube are welded 18 and secured. Methods which may be used for this welding process are thermal contact welding at red heat, electric resistance band, roller or spot welding or pulse spot welding to give the number and area of contacted weld spots as the specification load requires.

When the mesh weld bond is finally secured and the ends trimmed the surfaces are grit blasted and a layer of extruded or hog spray electrostatically powder coated plastic 19 e. g. PVDF is applied to embed the mesh especially totally to the required thickness of coating for the duty required. In addition a filler as above described, could be used in the plastic 19 and hence assist in reducing the overall cost of the product.

The material of the present invention can also have the benefit of preventing or reducing algae and other marine growths, especially by virtue of the copper fill present.