GB2134233A

GB2134233A - Heat exchange apparatus

Info

Publication number: GB2134233A
Application number: GB08401962A
Authority: GB
Inventors: Archibald Watson Kidd
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-01-25
Filing date: 1984-01-25
Publication date: 1984-08-08
Also published as: GB2134233B; GB8401962D0

Abstract

Condensation from flue gases in a heat exchange apparatus (1) can lead to corrosive solid residue, notably on the base (43) of the casing (3) of the apparatus, thereby shortening the life of the casing (3). In this invention the base (43) slopes continually towards a drain (49) so that corrosive residue does not have time to form. The base (43) is separable from the casing (3) so that it may easily be replaced. Corrosive solid residue may also form if the condensate evaporates and this is discouraged by thermal insulation (57) between the base and the flue gas inlet. <IMAGE>

Description

SPECIFICATION Heat exchange apparatus This invention relates to heat exchange appa ratus in particular this invention relates to heat exchange apparatus known as an ''econom- iser" which serves to withdraw additional heat from flue gases. Primarily the flue gases are from domestic or small commercial heating apparatus used to heat a fluid medium, especially water. Such heating apparatus would typically have a heat output in the 60,000 to 300,000.Th.U/h (approximately 1 6 to 84 kilowatts) and heat a fluid medium such as water for central heating.

My pending European patent application published as EP 33229A shows an "economiser" heat exchanger used to recover additional heat from the flue gas leaving such domestic or small commercial heating apparatus thereby increasing the overall heating efficiency.

One problem which can arise with heat exchange apparatus of the type indicated is that if an overall efficiency of much over 80% is achieved the flue gases are cooled amost to the dew point. The combustion products of oil and natural gas include a lot of steam and such cooling entails a risk of condensation forming. This can form in the heat exchange apparatus or in the chimney especially during starting up from cold. The amount of this condensation can be substantial. It can have a damaging effect on the structure of the chimney as well as other problems, and can lead to corrosion of the boiler, thus shortening its life.

The problem is exacerbated in the case of fuel with a substantial sulphur content. Heating oil frequently does have a substantial sulphur content. The condensation tends to absorb sulphur-containing combustion products emanating from any sulphur content of the fuel and corrosive sulphur acids can be formed. It will be readily appreciated that the presence of such corrosive acids on cast iron or welded steel parts can greatly reduce the working life of the equipment. Even stainless steel is not resistant to these acids.

In the past, the corrosive nature of condensation in economiser devices did not appear to have been appreciated. My copending European applications published as EP 33229A and EP 57095A proposed the application of a protective epoxy resin paint coating, applied by dip-coating, so as to give completeness of coverage.

Although this paint treatment is generally effective, I have now appreciated that there is still a problem when hot flue gases pass close to the economiser casing surface on which condensate is collected, or pass close to surfaces which are in good contact with such condensate collecting surfaces. In this case the metal forming the collecting surface may reach such a high temperature that the epoxy resin coating is damaged or burnt off, revealing uncoated metal which is of course highly vulnerable to corrosion. I have also realised that condensate on such surfaces will tend to evaporate, concentrating the sulphur acids in it and possibly giving a solid, highly corrosive, residue. In my EP 33229A a drainage outlet for condensation is provided but there is a small flat area between this outlet and the hot flue gas inlet.I have now appreciated that this provides an area in which corrosive solid residues may tend to lodge.

Specifically the present invention is concerned with heat exchange apparatus for transferring heat from flue gas to a fluid medium, comprising a casing for flue gas to flow through, within which casing there are heat exchange means defining one or more spaces for the passage of the fluid medium and having surfaces exposed to the flue gas, the apparatus including a part onto which condensation will fall.In this invention such apparatus has at least one of the following features: i) the said condensation receiving part is constructed to be separable from the remainder of the casing; ii) the surface of the said condensation receiving part onto which condensation falls is shaped to provide a continuously dropping condensation drainage route from its condensation receiving areas nearest the path of the incoming flue gas; iii) thermal insulation is provided to insulate the said condensation receiving part from the incoming flue gas, so as to reduce the temperature to which it becomes heated.

These three features are all considered inventive and could conveivably be employed independently of each other. However, it is preferred to employ them in combination.

One benefit of feature (i), making the said condensation receiving part separable is that a particularly vulnerable part becomes a component which can be replaced without major work on the whole economiser structure. In order to make it separable it is preferably attached by means of releasable fastenings such as bolts, or possibly by renewable fastenings such as pop rivets. Alternatively or additionally renewal sealant material may be employed.

Referring to feature (ii) the said lower part may have a continuously dropping drainage route from its hottest condensation receiving area(s) to a condensation drainage outlet.

Where the part is separable it is desirable that the drainage outlet should be incorporated in this separable part. An alternative possibility, however, would be for the drainage outlet to be provided in another part qf the economiser casing and for the drainage route on the separable part to drop continuously from the condensation receiving area(s) nearest the incoming flue gas to an edge over which the condensation ran off the separable part.

As to feature (iii), thermal insulation, suitably consisting of asbestos or another refractory material, may be provided as a lining to the path of incoming flue gas where it flows nearest to the said condensation-receiving part of the economiser casing.

This invention is applicable in particular to an economiser construction as shown in my European application EP 33229A in which flue gas enters at a lower region of the economiser casing and passes upwardly over a heat exchange surface, the lower part of the casing being shaped to catch condensation running down this said heat exchange surface, with a small aperture between this heat exchange surface and the lower part of the casing through which aperture the condensation passes together with a bleed flow of flue gas.

It is desirable that the drainage route slopes downwardly from the horizontal by at least 1+ degrees at all points.

Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings in which: Figure 1 shows a vertical cross section through a heat exchange apparatus taken on the line I-I of Fig. 2; Figure 2 is a perspective view of the base of the inner casing; and Figures 3 and 4 are partial cross sections, analogous to Fig. 1 and showing alternative possibilities.

Referring to Figs. 1 and 2 of the drawings, the economiser 1 (i.e. heat exchange apparatus) is a modified version of that shown in my EP 33229A. It comprises a casing 3 through which flue gas passes and within which there are heat exchange vessels 5,7,9 which in use are filled with water to be heated and which are exposed to the flue gas. If desired these vessels 5,7,9 could be provided with a gently undulating corrugating surface to enhance their heat exchange efficiency.

The casing 3 is contained within an outer casing 11, whose front face is designated 1 3.

The space between the two casings is packed with thermal insulation such as glass wool 1 5.

As described in my EP 33229A the economiser 1 is mounted above an oil-fired water-heating boiler 1 7. Hot flue gases from the boiler 1 7 pass up a duct 27, which extends across substantially the full width of the boiler and leads to the inlet 29 to the casing 3. The duct 27 is formed by an extension of the casing 3 and it is contained within an outer casing 31 integral with the casing 11. Heat insulation 1 5 is provided between the duct 27 and this outer casing at the front and rear and also at each side. The inlet 29 to the casing 3 extends across the full width of that casing.

As shown by Fig. 1, within the casing 3 the flue gases are constrained by baffles 33, 35 to flow first upwardly over the rear surface 37 of the heat exchange vessel 5 then downwardly over the facing surfaces of the vessels 5 and 7 and therafter round and up over the front surface 39 of the vessel 7 and both surfaces of the vessel 9. The flue gases finally flow out of the casing 3 through an upper outlet 41.

Both the main boiler 1 7 and the economiser 1 are employed to heat water, for a central heating system for instance. This water flows first through the vessels in the economiser 1 generally in countercurrent to the flue gas and then into the tank 21 of the boiler 17.

The vessels 5,7,9, and the casing 3, are constructed from sheet metal which is bent and welded together as necessary.

The hot flue gases coming into the economiser 1 from the boiler 1 7 yield up a large proportion of their heat to the incoming return water flowing through the vessels 5,7,9. This cools the flue gases sufficiently that condensation can occur within the economiser (where it initially forms on the vessels 5,7,9) and also within the chimney into which the flue gas from the outlet 41 passes. Any condensation which forms on the front surface of the rearmost heat exchange vessel 5, or on the vessels 7 or 9, or any which drops back into the casing 3 from the chimney will fall onto the base 43 of the casing 3. The baffle 33 is shaped so that any condensation running down the rear surface of the heat exchange vessel 5 will be diverted through the small gap 45 between the vessel 7 and the baffle, rather than dripping back into the boiler.A bleed flow of flue gas through this gap assists this. The reduction in efficiency caused by the gas bleed through this aperture 45 is sufficiently small as to be acceptable.

As shown by Fig. 1 the whole base of the casing 3 is a separate part 43 secured to the casing by self tapping screws or other fastenings 44. This base 43 provides the surface onto which condensation from the vessels 5, 7, 9 drips. As shown by Fig. 2, the base 43 is provided with four sloping surfaces which lead any condensation away from the baffle 33 towards the drainage outlet 49. The base 43 has slopes in two directions. As shown by Fig. 2 the surface 47 slopes forwardly towards the drainage outlet 49 away from the baffle 33 but the surfaces 51 and 53 slope from side to side across the casing down to the drainage outlet 49. These various slopes have been somewhat accentuated in the drawings.

The shape is such that there is a continuously sloping drainage route from the upper region 56 of the baffle 33 down that baffle and down the surface 47 or the surfaces 47, 51, 53 to the drainage outlet. In consequence condensation runs down quickly without collecting as a puddle at any point. This tends to wash off any deposits which might otherwise form. Condensation is led from other areas down the surfaces 51, 53, 55 to the drainage outlet 49.

In order largely to prevent fly ash from entering the outlet 49, a guard 61 is placed around this. It consists of a small metal strip bent into a U-shape and positioned around the outlet 49 so that the opening between the arms of the U is at the rear. Alternatively the guard could completely encircle the outlet 49, but have a serrated bottom edge standing on the base 43. Condensate would pass between the serrations but these would act as a coarse filter, holding back the fly ash.

Apart from arranging for them to be washed away, the formation of corrosive deposits through the flash evaporation of condensation falling into the baffle 33 is inhibited by reason of an insulating slab 57. This slab is formed of a refractory thermally insulating material such as asbestos. It is secured to the rear side of the baffle 33 by clips 59 secured to the wall of the duct 27. In the absence of this slab, flue gas in the duct 27 would come into direct contact with the baffle 33 and the region 60 of the front wall of the inlet 29, heating these to a relatively high temperature such that any condensation falling onto the baffle 33 or the upper region of the surface 47 would tend to flash evaporate. The slab prevents these regions from being heated at such a high temperature and so reduces the evaporation.

Because the base 43 is a separate part it can be removed from the remainder of the economiser for replacement if that should ever become necessary. When it is fitted, heat resistant silicone rubber sealant is applied to the joints between the base 43 and the casing 3, particularly at each side. Access for this purpose is gained through the fly ash removal door 63. A preferred such sealant is "Silmate" (Registered Trade Mark) made by General Electric Company, Silicone Products Department, Waterford, New York 12188, U.S.A.

After the casing 3 of the economiser has been assembled with the vessels 5, 7, 9 incorporated inside, but before fitting the base 43, it is given a protective coating. This consists of a fairly thick film of a phenolic epoxy resin paint which is baked to cure it and provide a hard impermeable coating. The application of such coating is described in my earlier European applications EP 33229A and EP 57095A.

The base 43 can be provided with a hard ceramic glaze, such as by the well-known stove enamelling process. This is facilitated by the relatively small size of the part 43, which could be sent away to a specialist stove enamelling firm for treatment. Moreover, the expensive stove enamelled coating is only applied to a relatively small area of the economiser structure, so that the cost is acceptable. The advantage of a stove enamelled coating on this base 43 is that it will resist the high temperature to which the baffle 33 and the adjoining region of the surface 47 could be heated by reason of their proximity to the incoming hot flue gas, whereas an epoxy resin could be burnt off from these regions. However, the refractory slab 57 insulates these vulnerable areas from the incoming flue gases and it may therefore be found adequate to apply an epoxy resin paint coat to the base 43, just as is done with the remainder of the casing 3, Fig. 3 shows a slightly modified arrangement in which part 71 of the base of the casing 3 is integral with it. A separable part 73 provides the baffle 33 and parts of the surfaces 47, 51 and 53. Condensation runs off this part 73 onto the part 71 over its edge 75 which is sealed to the part 73 with silicone rubber sealant.

Fig. 4 shows another possibility. The casing 3 has an integral complete base 81. A separate part 83 is mounted above the hotter region of the base 81, and carries condensation dripping from the vessel 5 down a continuously sloping drainage route to drip over edge 85 onto the base 81, where it then runs on down to the outlet 49.

Claims

CLAIMS:

1. Heat exchange apparatus for transferring heat from flue gas to a fluid medium, comprising a casing for flue gas to flow through, within which casing there are heat exchange means defining one or more spaces for the passage of the fluid medium and having surfaces exposed to the flue gas, the casing have a drainage outlet and the apparatus including as part of, or within, the casing a part onto which in use condensation will fall and thereafter drain via the drainage outlet, wherein the said condensation receiving part onto which condensation falls is shaped to provide a continuously dropping condensation drainage route from its condensation receiving areas nearest the path of the incoming flue gas.

2. Heat exchange apparatus according to claim 1 having the said condensation receiving part onto which the condensation falls, or this part and the base of the casing, shaped to provide a said drainage route which drops continuously to a region spaced away from the path of the incoming flue gas, and at least proximate the drainage outlet.

3. A heat exchange apparatus according to claim 1 or claim 2 wherein the said condensation receiving part forms at least a portion of the base of the casing and incorporates the drainage outlet.

4. A heat exchange apparatus according to claim 3 wherein the condensation receiving part is separable from the remainder of the casing.

5. A heat exchange apparatus according to claim 1 wherein the condensation receiving part is separable from a portion of the base of the casing which incorporates the drainage outlet, the drainage route on the separable condensation receiving part dropping continuously from its condensation receiving area(s) nearest the incoming flue gas to an edge over which the condensation runs off the separable part.

6. Heat exchange apparatus for transferring heat from flue gas to a fluid medium, comprising a casing for flue gas to flow through, within which casing there are heat exchange means defining one or more spaces for for the passage of the fluid medium and having surfaces exposed to the flue gas, the casing having a drainage outlet and the apparatus including as part of, or within, the casing a part onto which in use condensation will fall and thereafter drain via the drainage outlet, wherein the said condensation receiving part is constructed to be separable from the remainder of the casing.

7. A heat exchange apparatus according to any one of the preceding claims wherein thermal insulation is provided to insulate the condensation receiving part from the incoming flue gas.

8. A heat exchange apparatus according to claim 7 wherein the thermal insulation is provided by a slab of refractory material.

9. A heat exchange apparatus according to any one of the preceding claims having baffle desposed to prevent condensate from running back through the flue-gas inlet to the casing.

10. A heat exchange apparatus according to claim 9 wherein condensate collected by the baffle is urged towards the drainage outlet by a bleed flow of flue gas.

11. A heat exchange apparatus according to any one of the preceding claims wherein the said condensation receiving part is separable from the remainder of the casing and is provided with a protective coating of a ceramic glaze.

1 2. A heat exchange apparatus according to claim 11 wherein the glaze is stove enamel.

1 3. A heat exchange apparatus substantially as herein described with reference to and as illustrated in Figs. 1 and 2, Fig. 3 or Fig. 4 of the accompanying drawings.