EP0033229B1

EP0033229B1 - Heat exchange apparatus

Info

Publication number: EP0033229B1
Application number: EP81300296A
Authority: EP
Inventors: Archibald Watson Kidd
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-01-29
Filing date: 1981-01-22
Publication date: 1984-12-12
Also published as: EP0033229A3; DE3167669D1; EP0033229A2

Description

This invention relates to heat exchange apparatus, also known as an "economiser", serving to withdraw heat from flue gases. It is primarily concerned with apparatus which would receive the flue_ gas 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 range of approximately 17 to 87 kilowatts (60 000 to 300 000 BTU/h) and heat a fluid medium such as water for central heating or air for a ducted warm air central heating system. It may be oil or gas fired apparatus.
It is often the case with domestic or small commercial heating apparatus that the flue gas leaving the apparatus still contains a certain amount of recoverable heat and the apparatus of this invention can be employed for recovering further heat from the flue gas and thereby increasing the overall heating efficiency. The apparatus of the present invention serves to transfer heat from the flue gas to a fluid medium and this fluid medium will generally be the same as that which is heated in the main heating apparatus; if so the heated fluid medium which flows out from the heat exchange apparatus of this invention passes on to the main heating apparatus where it is heated further.
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 almost 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. Hitherto it has frequently been considered necessary to keep the temperature above the dew point throughout the system, which entails substantial waste of heat.
The problem is exacerbated in the case of fuel with a substantial sulphur content. 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 cor- rossive 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. Heretofore, only limited attention appears to have been paid to the problem of acid condensation from flue gas.
FR-A-2321094 and equivalent GB-A-1502746 propose a heat exchanger arrangement mounted above a central heating boiler, in which the flue gases are caused to pass upwardly around finned tubes through which the central heating water passes. Provision is made for the drainage of condensation dripping off these tubes. With this arrangement the tubes extend between manifolds set slightly inwardly from the side walls of a casing through which the flue gas passes and the tubes are provided with fins which increase the heat exchange area but make it possible for dirt to accumulate on and between the fins. This prior proposal indicates that the condensation may contain "aggressive substances" and recommends construction from a resistant material. However, the material particularly suggested is stainless steel which is not in fact resistant to sulphur oxy- acids.
DE-A-2720397 also proposes a heat exchanger for recovering additional heat from flue gas. In this proposal a finned structure is disposed in the path of the flue gases and some of the central heating water is made to pass through a pipe which coils around this finned structure. Again provision is made for the drainage of condensation forming within the heat exchanger unit.
DE-A-2758181 also provides an enclosure through which pass hot gases from a central heating burner. Within the enclosure there are so-called "guide panels" to remove heat from the flue gas. they project from and are supported by the top or bottom of the enclosure. The guide panels are not illustrated in detail but are said to be constructed as registers or panels of tubes comprising a closed area of tube convolutions abutting on one another. Presumably dirt could become lodged in the interstices between adjoining tubes. The casing is provided with a cleaning flap in its bottom and this flap is provided with a drain orifice.
FR-A-2293674 shows two different forms of heat exchanger for removing heat from flue gas. In one form the flue gas is guided to flow over a convoluted water filled tube. In a second form of heat exchanger, the flue gas is made to flow around vessels each of which is formed from two pressed sheets (so as to be hollow over part of its surface area). These vessels rest on the bottom of the casing. In the heat exchanger of this prior proposal no provision is made for the drainage of condensation out of the enclosing casing. The specification suggests that the parts in contact with the flue gas can be provided with a coating of heat resisting paint, but no details are given as to how this might be applied. The nature of the corrosion is possibly not understood, or alternatively it is intended only that the corrosion should be slowed down, because galvanising is also put forward as an alternative protective coating.
Another problem which can arise with heating apparatus in which fuel is burnt is that fly ash particles i.e. particles of solid material entrained in the flue gas, can accumulate and tend to block the chimney, particularly at its base or at a point in the chimney at which there is a change in the direction of flow. Such fly ash can cake into hard material. in the presence of the condensation referred to above.
Of the four prior proposals mentioned above, only DE-A-2758181 makes any mention of dealing with dirt such as fly ash. This specification provides a cleaning flap as mentioned above but this seems to be provided for removing dirt from where it may happen to lodge without any attempt to control the point of deposition. As will be explained below, the present invention employs reversal of the flue gas flow direction to encourage deposition of fly ash. In DE-A-2758181 there is indeed reversal of flow from a downward to an upward direction but this is accompanied by an apparent narrowing of the cross-section available for flow which would tend to increase the speed of flow and offset the effect of the flow reversal. In the second form of heat exchanger shown in FR 2293674 there is again a reversal in the direction of flue gas flow from downwardly to upwardly but again the cross section for flow appears to diminish where the reversal occurs and in addition there is no apparent provision for access to the interior of the heat exchanger casing to clean it.
One aspect of this invention is concerned with providing a simple and advantageous form of heat exchange apparatus for withdrawing heat from flue gases to enable an improvement in overall efficiency as compared to heating apparatus where further heat is not withdrawn from flue gases without undue difficulty being caused by condensation. The overall efficiency may for example reach 90-95%. In this aspect the invention provides that hollow vessels within the heat exchange apparatus extend fully across a casing between its side walls, and are supported by those side walls, while also providing a drainage outlet from the lower part of the casing, this lower part of the casing being shaped so that moisture will drain through the drainage outlet(s). Preferably condensation forming on any cooled part of the apparatus, or dripping back from the chimney, is intercepted and drained out, so that none of the condensation returns to the boiler.
In a second aspect the present invention seeks to overcome the problem of fly ash deposition mentioned above by inducing the deposition of entrained solid particles within the heat exchange apparatus and moreover at a place where this deposition can be tolerated and from which the deposited particles can reasonably easily be removed. Deposition is induced by constraining the flue gas to reverse its direction from downwardly to upwardly, accompanied by an increase in the cross sectional area available for flow, and with provision of a closable aperture for removing deposited solid material.
Preferably, in order to provide resistance to the corrosive effects of sulphur acids contained in any condensation, surfaces of the casing and of the heat exchange means which are exposed to the flue gas have a coating of a thermosetting synthetic resin, applied by dip coating the casing with the heat exchange means already fixed therein, with an organic solvent-based heat curable paint. Preferably it is an epoxy phenolic paint.
The economiser can be mounted, for example on a wall, above an existing boiler or other fuel burning heating apparatus. Alternatively, where the economiser and fuel burning apparatus are being designed to go together, they can be made to form a single unit with the economiser mounted above and supported by the fuel-burning apparatus.
An example of heat exchange apparatus (to be referred to as an "economiser") embodying this invention will now be described with reference to the accompanying drawings in which:

Fig. 1 is a perspective view of the economiser showing its mounting as a common unit with fuel-burning heating apparatus, and
Fig. 2 is a section through the economiser taken on the line II-II of Fig. 1.

Referring to the drawings, the economiser 8 (i.e. heat exchange apparatus) broadly comprises a casing 10 through which flue gas passes and within which there are heat exchange vessels 12, 14, 16 which in use are filled with water to be heated and which are exposed to the flue gas. If desired these vessels 12, 14, 16 could be corrugated to enhance their heat exchange efficiency, although as shown they have simple plane surfaces.
The casing 10 is contained within an outer casing 18, whose front face is designated 19. The space between the two casings is packed with thermal insulation such as glass wool 20.
As shown by Fig. 1, the economiser 8 is mounted above an oil-fired water-heating boiler 22. The two pieces of apparatus are constructed as a single unit with the weight of the economiser taken by the boiler 22 beneath. It will be seen that the sides 24 of the outer casing 18 of the economiser lie flush with the sides 26 of the outer casing of the boiler.
Within the boiler 22 oil is burnt as fuel (although gas could be used as fuel) and the hot flue gases produced rise up through an array of tubes 30 extending through a tank 32 containing water to be heated. The flue gases then collect in an upper manifold 34 and leave by an exit 35 which extends across substantially the full width of the boiler 22 between the layer of heat insulation which the boiler has at each side. From the exit 35 a duct 36, which also extends across substantially the full width of the boiler, carries the flue gas up to the inlet 38 to the casing 10. The duct 36 is formed by an extension of the casing 10 and it is contained within an outer casing 39 integral with the casing 18. Heat insulation 20 is provided between the duct 36 and this outer casing at the front and rear (as shown by Fig. 2) and also at each side. The inlet 38 to the casing 10 extends across the full width of that casing.
Within the tubes 30 are spiral metal retarders (not shown). The spacing of the economiser 8 above the boiler 22, together with a forward tilt to the rearmost tubes 30 allows these retarders to be pulled out for cleaning.
As shown by Fig. 2, within the casing 10 the flue gases are constrained by baffles 40, 42 to flow first upwardly over the rear surface 44 of the heat exchange vessel 12 then downwardly over the facing surfaces of the vessels 12 and 14 and thereafter round and up over the front surface 46 of the vessel 14 and both surfaces of the vessel 16. The flue gases finally flow out of the casing 10 through an upper outlet 48.
Both the main boiler 22 and the economiser 8 are employed to heat water, for a central heating system for instance. This water flows first through the vessels in the economiser 8 generally in countercurrent to the flue gas and then into the tank 34 of the boiler 22. In more detail, the cold return of water from the central heating system is connected so as to flow into the heat exchange vessel 16 through its inlet 50 (Fig. 1). Water leaves this vessel through an upper outlet hole 52 and is carried by duct 66 to an inlet hole 54 of the vessel 14. The water flows out of vessel 14 through a hole 56 into duct 68 leading to an upper inlet hole 58 of the vessel 12 which has a lower, outlet hole 60 connected by a pipe 62 to an inlet 64 of the tank 34. An outlet, not shown, from the tank 34 provides the hot flow to the central heating system.
The ducts 66, 68 are cuboidal boxes welded to the side wall of the casing 10. Each of these boxes is open on its side welded against the wall of the casing, which thus closes the boxes to form ducts between the holes 54 and 56 and between the holes 58 and 60.
In order to allow venting of air when the apparatus is initially filled, a small tube 70 is provided connecting the ducts 66 and 68 and on the outer side of the duct 66 a small bleed valve, of the type used for central heating radiators, is provided through which air trapped in the apparatus can be vented.
Each of the vessels, 12, 14, 16 is constructed from two pieces of sheet steel which are bent into an L shape (see Fig. 2) and the two pieces then joined by welds 74 to form a hollow box section. This box section is then welded at each end to a plate 76 forming a part of a sidewall of the casing 10. When the economiser is assembled the three plates 76 at each side butt edge to edge and are welded together at the butt joins 78. Sufficient gas-tightness is achieved without welding down the full length of each butt join 78 but welding must be provided where the ducts 66, 68 cross butt joins, in order to achieve water-tightness.
The hot flue gases coming into the economiser 8 from the boiler 22 yield up a large proportion of their heat to the incoming return water flowing through the vessels 12, 14, 16 and which is consequently warmed by 4-6°C (7-10°F) before returning to the boiler 22 itself. The unit formed by the boiler 22 and the economiser 8 can achieve an overall water heating efficiency of around 90-95%. This cools the flue gases sufficiently that condensation can occur within the economiser (where it initially forms on the vessels 12, 14 16) and also within the chimney into which the flue gas from the outlet 48 passes. Any condensation which forms on the front surface of the rearmost heat exchange vessel 12, or on the vessels 14 or 16, or any which drops back into the casing 10 from the chimney will fall onto the bottom surface 80 of the casing 10. Also the baffle 42 is shaped so that any condensation running down the rear surface of the heat exchange vessel 12 will be diverted through the small gap 82 between the vessel 12 and the baffle, rather than dripping back into the boiler. The reduction in efficiency caused by gas leakage through this aperture 82 is sufficiently small as to be acceptable.
The bottom surface 80 of the casing 10 is inclined so that condensation falling onto it drains rearwardly, and flows out through an outlet aperture 84 from which a duct 86 leads first downwardly and then sideways (backwards from the plane of the paper as seen in Fig. 2) leading out through the side of the economiser. A flexible plastic tube 88 is connected onto the duct 86 and this is used to carry any condensation away to some convenient drain. The position of the economiser 8 above the boiler 22 gives some hydrostatic head, and enables the tube 88 to be run along a wall for some distance if this is required in order to reach a drain. A guard 89, to be further mentioned below, partially surrounds the outlet 84.
The economiser 8 also has provision for causing the deposition of fly ash at a point from which it can be removed reasonably easily. The vessels 12 and 14 together with the baffles 40, 42 constrain the flue gas to reverse its direction, as indicated by arrow 90, from downwardly to upwardly beneath the vessels 14, 16. The reversal of direction induces deposition of any fly ash from the flue gas stream. Moreover, the large void space at this point means that the cross section available for flow of the flue gas increases rapidly as the gas debouches from the passage between the vessels 12 and 14, so causing the speed of flow to reduce. This slowing further induces deposition of any entrained fly ash.
In the front face 19 of the economiser an access door 92 is provided enabling removal of any fly ash which as accumulated in the void space beneath the vessels 14, 16 (where the space available is such that some build up of ash is tolerable. Provision for promoting deposition of fly ash at a place from which it can be removed obviates the formation of blockages elsewhere. When the ash is removed the surfaces of the vessels 12, 14, 16 can be lightly brushed to maintain their heat exchange efficiency.
In order largely to prevent fly ash from entering the condensation drainage outlet 84, a guard 89 is placed around this. It consists of a small metal strip bent into a U-shape and positioned around the outlet 84 so that the opening between the arms of the U is at the rear. One arm only of the U-shape can be seen in Fig. 2. Alternatively the guard could completely encircle the outlet 84, but have a serrated bottom edge standing on the bottom surface of the casing 10. Condensate would pass between the serrations but these would act as a coarse filter, holding back the fly ash.
The parts of the economiser are made of mild steel plate. In order to protect the parts which are exposed to the sulphur acids contained in any condensation which forms, a theremset- ting synthetic resin coating is applied to all of the interior surfaces which in use are exposed to flue gas. The coating is provided by applying a fairly thick film of a phenolic epoxy resin paint curable by heating, and then baking to effect the curing and provide a hard impermeable coating. The paint is applied by dip coating to the whole of the inner casing 10, with the vessels 12, 14, 16 and the ducts 66, 68 in place and with the inlet 50 and outlet 60 temporarily blocked to close off the system of spaces which in use are filled with water.
To apply the paint the assembled casing 10 is submerged in a suitably shaped tank filled with the paint, so that (inter alia) all interior surfaces of the casing and the exterior surfaces of the heat exchanger vessels (which are the surfaces exposed to flue gas, in use) are coated by the paint. The casing is then lifted out and surplus paint allowed to drain back into the tank. After it has drained the casing is stoved to cure the coating.
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 melamine-formaldehye, hydrocarbon and other solvent liquids such as ethyl cellosolve (2-ethoxy- ethan-1-₀1).
An epoxy phenolic enamel paint has been successfully used. This paint has hitherto been used for coating steel drums, an application where it is not, of course, subject to heat in use. As supplied it contained 40-44% solids by weight. For application it was diluted by adding thinner. The thinner comprises ethyl cellosolve blended with low boiling naptha. About 4 to 5 litres of this were added to 100 litres of the paint. This dilution gave a creamy consistency slightly more viscous than domestic gloss paint. After dipping the casing, surplus paint was allowed to drain back into the tank at room temperature for approximately 30 minutes. After it had drained the casing was stoved at 206°C (403 °F) for 7 minutes to cure the coating. The paint film which remained after the casing had been allowed to drain was rather thick and gave an eventual baked coat about 50 ,um (0.002 inch) thick. 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 casing allowed to cool back to room temperature. It would be applied by dip coating as above, with stoving at the same temperature but for 15 minutes.

Claims

1. Heat exchange apparatus for transferring heat from flue gases to a fluid medium and comprising:

a casing (10) comprising a pair of side walls and a plurality of further walls extending therebetween,

an inlet (38) to the casing for entry of the flue gas, and an outlet (48) for exit of the cooled flue gas therefrom, at least the said inlet being disposed in a said further wall of the casing,

a plurality of heat exchange units (12, 14, 16) within the casing, each comprising a hollow vessel defining a space for the passage of a fluid medium, and having exterior surfaces exposed to the ftue gas for the transfer of heat from the gas to the fluid medium, and

at least one drainage outlet (84) from a lower part (80) of the casing for exit of any moisture which may condense out within the casing, the lower part being shaped so that such moisture will drain to the drainage outlet(s);

characterised in that the hollow vessels (12, 14, 16) are spaced apart, each extending fully across the casing (10) between its said side walls with its weight supported by the side walls.

2. Apparatus according to Claim 1, wherein condensation draining off any surface of the said heat exchange units (12, 14, 16) will drain to the or a drainage outlet (84).

3. Apparatus according to Claim 1, or Claim 2, wherein the flue gas outlet (48) is positioned so that any condensation draining back through it into the casing will drain to the or a drainage outlet (84).

4. Apparatus according to any one of the preceding claims, wherein the heat exchange vessels (12, 14, 16) and or one or more baffles (40, 42) are arranged to constrain the flue gas to reverse its direction of flow at least once as it flows through the casing (10).

5. Heat exchange apparatus for transferring heat from flue gases to a fluid medium comprising a casing (10) having an inlet (38) thereto and an outlet (48) therefrom for flue gas and heat exchange means (12, 14, 16) 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 heat exchange means (12, 14, 16) and/or one or more baffles (40, 42) being arranged within the casing (10) so as to constrain the flue gas to reverse its direction of flow from downwardly to upwardly, the casing having a closable aperture (92) giving access to the region of the reversal of direction, characterised in that the cross section available for flow of flue gas increases at the region of the reversal of flow, inducing a retardation of the speed of flow, thereby to induce deposition of any solid particles entrained in the flue gas as it reverses its direction, and also slows down;

the closable aperture (92) enabling periodic removal of solid material thereby deposited from flue gas in the region of the reversal of direction.

6. Apparatus according to any one of the preceding claims wherein surfaces of the casing (10) and of the heat exchange means (12, 14, 16) which are exposed to the flue gas have a coating of a thermosetting synthetic resin, applied by dip coating the casing (10) with the heat exchange means (12, 14, 16) already fixed therein, with an organic solvent-based heat curable paint.

7. Apparatus according to Claim 6, wherein the resin is an epoxy phenolic paint.

8. Apparatus according to Claim 6 or Claim 7 formed of mild steel, wherein the steel surfaces are phosphated prior to the application of the resin thereto.

9. Apparatus according to any one of the preceding claims, wherein the heat exchange means comprises a plurality of heat exchange units (12, 14, 16) each defining a space for the fluid medium and having surfaces exposed to the flue gas, the flue gas which passes through the casing being constrained to flow first upwardly over one surface of one heat exchange unit (12) thereafter downwardly between that unit (12) and a second (14), then to reverse its direction of flow from downwardly to upwardly and flow over a second surface of the second unit (14) and at least one surface of a third unit (16), the heat exchange units (12, 14, 16) being connected together, so that the flow of fluid medium through them is generally countercurrent to the flow of flue gas.

10. Apparatus according to any one of the preceding claims, wherein the apparatus is combined with a fuel burning apparatus (22), the two pieces of apparatus being of similar width and arranged as a single unit with the heat exchange apparatus (8) mounted above and supported by the fuel-burning apparatus (22), the flue gas passing through a connection (36) between them which extends across a major proportion of the width of the apparatus, and a heated fluid medium outlet (60) from the heat exchange apparatus being connected to a fluid medium inlet (64) of the fuel burning apparatus (22).