PROTECTION OF PARTS EXPOSED TO FLUE GAS

Abstract: An item to be exposed to flue gas, especially a heat exchanger (10) to recover additional heat and in which condensation will form, is given a protective coating of thermosetting synthetic resin to prevent attack by acid in the condensation. The coating is preferably applied by dipping into a heat-curable paint, drying the paint under moderate heat while rotating about two axes to prevent "solvent wash" and to ensure that no localised thinning of the coating can occur, then stoving, preferably also under rotation, to produce a finished hard coating of synthetic resin.

Description: PROTECTION OF PARTS EXPOSED TO FLUE GAS FIELD OF THE INVENTION This invention is concerned with articles having surfaces to be exposed to flue gases or condensation therefrom. In particular, although by no means exclusively, this invention is applicable to heat exchange apparatus, also known as an economiser, connected so as to receive hot flue gas from fuelburning heating apparatus and transfer heat from the flue gas to a fluid medium so as to achieve an overall enhancement of thermal efficiency. Such an economiser is described in my European patent application 81.300296.1 published under no. 33229 Articles to which this invention is applicable also include flues and components thereof. BACKGROUND OF THE INVENTION The combustion of a number of fuels leads to flue-gas containing one or more sulphur oxides which can lead to the presence of corrosive sulphur acids in flue systems. In particular, oil when burnt gives rise to a substantial quantity of water vapour as one of the products of combustion. This may condense in an economiser unit as mentioned above or in a flue. The sulphur oxides which derive from the sulphur content of the oil dissolve in the condensation. Consequently the condensation contains corrosive sulphur oxyacids. It appears that in the past, the nature of this problem has not been fully appreciated. It appears that hitherto there has been no practical and effective way to avoid the corrosive effects of such acid condensation. For instance an ordinary paint treatment would be quite ineffective and would scarcely delay the onset of corrosion by any appreciable time. It may well be that this has prevented the development of the economiser units mentioned above. At the time this invention was made economiser units had not become generally available commercially even though there have been various proposals for such units. Moreover, at present it is not uncommon for flues to be lined by the insertion of flexible helically-wound stainless steel tubing despite the fact that stainless steel is not resistant to the corrosive condensation. In extreme cases such tubing is so eaten away after some years that what is left resembles a helical spring. SUMMARY OF tE INVENTION Broadly, in one aspect, the present invention provides articles having surfaces to be exposed to flue gases or condensation therefrom, provided with a coating of acid resistant material on surfaces which will be so exposed. Preferably the coating consists of thermosetting synthetic resin. However, another possible coating material is vitreous enamel, further possible materials are certain thermoplastics materials, which give coatings with sufficient heat resistance. It is preferred that synthetic resin is applied as a film forming composition, and cured by stoving. Preferably -the said surfaces are completely covered by the coating to a minimum coating thickness of 0.001 inch (25 microns). Articles which may be protected in accordance with the present invention include in particular heat exchange apparatus (an economiser) intended to receive hot flue gas from fuel-burning heating apparatus and transfer heat from the flue gas to a fluid medium. Such an economiser preferably comprises a casing having an inlet thereto and an outlet therefrom for flue gas and heat exchange means within the casing defining one or more spaces for the passage of a fluid medium, the heat exchange means having surfaces exposed to the flue gas for the transfer of heat from the gas to the said fluid medium, the lower part of the casing having at least one drainage outlet for any moisture which may condense out within the casing, and being shaped so that such moisture will drain to the or a said drainage outlet. It is very desirable that all of an articles surfaces which will be exposed to flue gas cooled below the dewpoint (so that condensation can form) have complete coverage by the coating, without breaks, pinholes or thin patches. It is also desirable to arrange that surfaces onto which condensation will run are similarly coated, and conversely to arrange that condensation does not run onto surfaces not protected by the coating. Preferably then the drainage of condensation is guided away from any uncoated surfaces. It may be arranged that all surfaces liable to be exposed to acid condensation have their operating temperature limited by the fluid medium. This can enable such surfaces to be satisfactorily protected by means of a coating material unable to withstand the temperature at which the hot flue gas leaves the fuel-burning heating apparatus while condensation is prevented from running from the cooler parts of the economiser onto surfaces which are heated by theflue,gas to a greater temperature than the coating material could withstand. Besides the aforementioned economiser units, articles whose surfaces are protected in accordance with this invention may be component parts capable of assembly into chimney linings, or may be other ductwork for flue gas. They may, in particular, be intended for use in a domestic or small commercial installation, where the fuel-burning boiler, air heating or other heating apparatus has an output of up to say 300,000 B.Th U/hour ("90 kw) and probably of at least 50,000 B.Th U/hour (¯15 kw). The articles can be made of mild steel rather than stainless steel, relying on the coating to protect the metal from corrosion. At some stage prior to coating, the mild steel is preferably degreased, given an acid pickle to remove any rust and/or mill scale and provide a sound surface to coat, and then -phosphated. This phosphating gives a good key for a coating layer. In a second aspect this invention provides a method of protecting an article having surfaces to be exposed to flue gases or condensation therefrom, characterised by applying an impermeable coating of acid resistant material to the said surfaces. One method of applying the coating is by dip coating with a thermosetting synthetic resin paint and then stoving the article to cure the resin film. This technique is suitable for coating a fully assembled economiser, and therefore allows it to be made by welding its parts together, which is an economical form of construction. An important development of this invention is to turn the dip-coated article while the paint coating dries, before stoving it. This has been found to prevent localised thinning of the cooling fluid coating, such as when the fluid coating runs off edges and corners as the solvent evaporates (so called "solvent wash"). It is rather like keeping treacle or honey on a spoon by twisting the spoon. Conveniently the article is rotated around two perpendicular axes (but of course rotations about any two non-parallel axes will have components about perpendicular axes). Swinging to and fro through an arc could possibly be employed in place of rotation through a full circle; although the latter is simpler to arrange. A preferred synthetic resin for dip-coating is phenolic epoxy resin, formulated as a heat-curable paint. The paint can be a stoving modified epoxy paint containing pigment, paint extenders (finely ground powders such as barytes and talc), liquid synthetic resins such as epoxy alkyd and melamineformaldehyde, hydrocarbon and other solvent liquids. A supplier of such paints is Pinchin Johnson Paints, Packaging and Coil Coating Division, London, England. An epoxy phenolic enamel paint designated by them as PJ 2088 has been found suitable. Prior to this invention it had been used for lining steel drums, in which application it was not of course required to withstand heat. The finished, baked coating of synthetic resin may be between 0.001 and 0.003 ins (25-75 microns) thick. Another possible method of coating is to apply a powder coating which is then fused to an impermeable coating film by stoving. Vitreous enamel is applied as a powder. Alternatively, synthetic resin powder can be applied by means of electrostatic spraying equipment. It has been found that electrostatic spraying can be unsatisfactory for coating fully assembled economiser units because these contain Faraday cage areas which the coating cannot reach. However, electrostatic application does give good coverage on edges and right angle corners. It thus avoids the problem of 'solvent wasbn and localised thinning mentioned above and could be employed for coating parts of an economiser designed to be bolted together after coating. Suitable synthetic resins applicable by electrostatic powder coating are polyesters, epoxy resins and mixtures or copolymers thereof curable in each case to an acid resistant coating film. One suitable material is the Crodalux PF 50 powders supplied by Croda Paints Limited, Harefield, Middlesex, England. These are blends of polyester resins, hardeners and heat resisting pigments. Another satisfactory type is the Crodalux PF 40 polyester-epoxy powders which consist of polyester resin combined with epoxy resin together with flow agents and heat resistant pigments. These powders can be supplied by means of conventional electrostatic powder spraying equipment, onto surfaces which have previously been degreased and phosphated in conventional manner. The electrostatically applied powder coating is cured by stoving in accordance with the manufacturers recommendations. For PF 50 satisfactory conditions range from 5 minutes at 2200C to 20 minutes at 160 C. For PF 40 satisfactory conditions range from 5-10 minutes at 2200C to 20-25 minutes at 160 C. The coating obtained will generally be 0.001 to 0.0015 ins thick (25-38 microns). After stoving a second coating may also be applied electrostatically with preheating of the article. Such a second coat is stoved in a smilar manner and increases the coating thickness to 0.002 - 0.003 ins (50-75 microns). Another possible material is Duraguard E511 supplied by Plascoat Systems Limited, Farnham, Surrey, England. This is a coating powder formulated from epoxy resins. The standard cure version requires stoving for 10 minutes at 200 C. A coating of 0.002-0.003 ins (50-75 microns) can be obtained with it. DRAWINGS, Ai DESCRIPTION OF THEM An embodiment of this invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a vertical cross section of an economiser unit, provided with a coating in accordance with this invention; Fig. 2 is a side view, partly cut away, of a rotary oven used in applying the coating to the economiser; and Fig. 3 is an end view of the oven, looking in the direction of the arrow III in Fig. 2, and with an end of the heated box removed and one hoop cut away. The economiser unit shown in Fig. 1 is the interior part of a thermally insulated unit described in detail in my European application 81.300296.1, published under number 33229 as mentioned above. The disclosure ofthat application is incorporated herein by reference. When completed, the economiser unit shown in Fig. 1 is clad with thermal insulation surrounded by an outer casing. It is then suitable to be mounted above a water heating boiler to extract extra heat from the flue gas leaving that boiler. The boiler may be as described in my earlier European application 80.302818.2 published under number 0024376 (whose disclosure is also incorporated herein by reference.) Briefly, the unit shown in Fig. 1 comprises a casing 10 containing three heat exchange vessels 12,14,16 extending across its width. In use flue gas enters up an inlet duct 20, flows around the path denoted by the arrows A, and then out through an outlet duct 22. The unit is of all welded construction. Each of the vessels 12,14,16 consists of two steel plates bent to an L-shaped cross section and welded together to form a box. The ends of these boxes are welded to plates 24 which are then welded together along the seams 26 to form the sides of the casing 10. The vessels 12, 14,16 are connected by ducts 28 on the outside of the casing andki use are filled with water. There is a cleaning aperture 30 in the front face of the casing and a condensation drain at 32. After fabrication the entire unit is given a protective coating as will be described below. In use of the economiser, the heat exchange vessels 12,14,16 and also the parts of the casing 34,36,38 beneath, in front of, and above these vessels all have their operating temperature limited by the water which flows through the heat exchange vessels. The incoming flue gas remains above the dew point unit it is at least part way up the space 40 between the vessel 12 and the back 42 of the casing. A baffle 44 at the base of the rear face of the heat exchange vessel 12 prevents condensation which runs down the back face of this vessel 12 from passing down into the inlet 20. This is desirable because although the inlet duct 20 would be coated in the same manner as the rest of the unit shown, the temperature in this inlet duct 20 might possibly rise to a greater temperature than the coating material can withstand. After the various parts of the unit have been welded together to the form sawn in Fig. 1, it is degreased and subJected to an acid pickle (immersion in a 10% solution of concentrated hydrochloric acid) to remove any incipient rust or mill scale. It is then washed, and phosphated in conventional manner. After this it is washed again, dried and straightaway coated with a thermosetting resin paint. Description will be given with reference to an example found satisfactory, the Pinchin Johnson epoxy phenolic enamel paint, designation PJ 2088. As supplied the paint contained 40-44N solids by weight. For application it was diluted by adding about 4 to 5 litres of Pinchin Johnson thinner number 0000 5 1059 to 100 litres of paint. The thinner comprises ethyl cellosolve (2-epoxy ethan-1-ol) blended with low boiling naptha. This dilution gave a cream like consistency a little more viscous than domestic gloss paint. The inlet 46 and outlet 48 of the system of water filled spaces are temporarily blocked off with suitable plugs. Then the unit - fully assembled as shown in Fig. 1 - is submerged in a suitably shaped tank filled with this paint, so that all interior, and exterior, surfaces of the casing 10 and the exterior surfaces of the heat exchanger vessels 12,14,16 are coated by the paint. The unit is lifted out of the tank and surplus paint allowed to drain back into the tank at room temperature for 15 to 30 minutes. After it has drained the unit is transferred to the apparatus shown in Figs. 2 and 3 to dry and then stove the paint. The rotary oven shown in Figs. 2 and 3 comprises ca cuboidal insulated box 100 mounted within a pair of steel hoops 102,103. These run on flanged rollers 104, 106 carried on a base frame108. The two rollers 104 are driven, for example from an electric motor 110 beneath the frame 108 and provided with reduction gearing so that the box 100 rotates at a slow speed such as 1-3 r.p.m. A yoke 112 is journalled in the sides of the box 100, to rotate within the box. The yoke 112 carries slides 113 to receive a unit as shown by Fig. 1. Clips latches or othermeans, not shown are provided for attaching the unit to the slides. The yoke 112 is driven by a gear wheel 114 secured by gussets to a shaft 116 fast with the yoke. This wheel 114 engages a fixed gear ring 118 which encircles the box 100, with a little clearance. It is supported in position by an angle iron frame (not shown) which is fast with the base frame 108 and is arranged so as not to interfere with movement of the gear 114. Thus as the driven rollers 104 rotate the box 100 about a horizontal axis, the wheel 114 engaging the fixed ring 118 turns the yoke 112 in the box about an axis perpendicular to that of the box, and it is of course turning about the axis of the box at the same time. Four electrical heating elements 122 extend along corners of the box. Power for these is led in through slip rings 124. A fan 126 is provided to blow air through the holes 128 in the ends of the box. The end 130 of the box is removable. After an economiser unit as shown in Fig. 1 has been dip coated with paint and allowed to drain, it is loaded into the box 100 which is then rotated while heat is supplied from the elements 122. Air is blown through the box to entrain solvent evaporating from the paint. In this way the paint on the unit is dried while the unit is rotating about two axes and so the evaporating solvent does not wash paint away from edges and corners. The time and temperature to effect drying can be found by experiment. Drying should obviously not be so fast that the evaporating solvent causes blow holes in the paint. A suitable temperature for drying the paint is about 200'F (about 90 Sic). When the paint is dry the temperature in the oven is raised to a suitable level for stoving the paint, by supplying more power to the heaters 122 and perhaps turning off the fan 126. For the paint specified stoving can be effected by a temperature of 403'F (2065C) for 7 minutes. Rotation can be continued during stoving, but is not essential. The paint film applied by the dip-coating process described above is much thicker than normal spray.paint coatings. It has been found to give an eventual baked coat about 0.002 inch (50 microns) thick. The thickness obtained depends on the viscosity of the paint into which the economiser unit is dipped. Only a single coat would normally be applied but if appropriate to meet extremely difficult conditions a further coat could be applied. This would be put on after the first coat had been baked and the article allowed to cool back to room temperature. It would be applied by dip coating as above, and dried in the oven as before followed by stoving at the same stoving temperature but for 15 minutes. The coating of thermosetting resin obtained in the manner described is an impermeable coating of an acid resistant material. It is sufficiently hard to permit the vessesl 12,14,16 and the interior of the casing 10 to be cleaned by brushing.

Claims: CLAIMS 1) An article having surfaces to be exposed to flue gases or condensation therefrom characterised in that the said surfaces have an impermeable coating of acid resistant material. 2) An article according to claim 1 wherein the coating comprises thermosetting synthetic resin. 3) An article according to claim 2 wherein the coating is applied as a film forming composition and cured by stoving. 4) An article according to claim 1, claim 2 or claim 3 which is a heat exchange apparatus for transferring heat from flue gases to a fluid medium, the apparatus comprising a casing having an inlet thereto and an outlet therefrom for flue gas and heat exchange means within the casing defining one or more spaces for the passage of a fluid medium, the heat exchange means having surfaces exposed to the flue gas for the transfer of heat from the gas to the said fluid medium, the lower part of the casing having at least one drainage outlet for any moisture which may condense out within the casing, and being shaped so that such moisture will drain to the or a said drainage outlet. 5) A method of protecting an article having surfaces to be exposed to flue gases or condensation therefrom, characterized by applying an impermeable coating of acid resistant material to the said surfaces. 6) A method according to claim 5, wherein the coating is applied by dip coating with a thermosetting synthetic resin paint and then stoving the article to cure the resin film. 7) A method according to claim 6 wherein the dip coated article is turned while the paint film dries, prior to the stoving of the article 8) A method according to claim 7 wherein the said surfaces of the article are acid treated to remove any rust and/or mill scale, and phosphated prior to the said dip coating, and the dip coated article is turned about two non-parallel axes while heat is applied to dry the paint film, whereafter the said stoving is effected at a higher temperature. 9) A method according to any one of claims 5 to 8 wherein the article is a heat exchange apparatus for transferring heat from flue gases to a fluid medium, the apparatus comprising a casing having an inlet thereto and an outlet therefrom for flue gas and heat exchange means within the casing defining one or more spaces for the passage of a fluid medium, the heat exchange means having surfaces exposed to the flue gas for the transfer of heat from the gas to the said fluid medium, the lower part of the casing having at least one drainage outlet for any moisture which may condense out within the casing, and being shaped so that such moisture will drain to the or a said drainage outlet, the said coating having been applied after assembly of the said casing and heat exchange means therein, by dip coating with thermosetting synthetic resin paint, turning the article while heating to dry the paint and thereafter stoving to cure the resin film. 10) A method according to claim 5 wherein the coating is synthetic resin applied electrostatically as a powder, after which the article is stoved to fuse the powder to a coating film and cure the same.

Inventors: KIDD, ARCHIBALD WATSON
Applicants: KIDD, ARCHIBALD WATSON
Application year: 1982
Main classification: B05D5
IPC codes: B05D3, B05D1, F28F19, B05D7, F28D21, B05D5