EP0825406A2

EP0825406A2 - Heat exchangers

Info

Publication number: EP0825406A2
Application number: EP97306110A
Authority: EP
Inventors: Roy Bratley; Neville Lucas
Original assignee: Caradon Ideal Ltd
Current assignee: Caradon Ideal Ltd
Priority date: 1996-08-13
Filing date: 1997-08-12
Publication date: 1998-02-25
Also published as: GB9616956D0; EP0825406A3

Abstract

To reduce the migration of condensate formed on the fins (18) of a condensing gas boiler, the fins are inclined downwardly in a direction away from the combustion chamber (10) of an angle of between 2° and 20° to the horizontal. An excess inward migration of condensate can lead to a build up of corrosion products on the inner edges (32) of the fins which may fall into the combustion chamber and disrupt the operation of the boiler.

During manufacture, the heat exchange tubes 1 on which the fins are mounted are staggered relative to each other such that the alignment of the tubes inclines the fins at the desired angle.

Description

This invention relates to heat exchangers and is mainly concerned with heat exchangers for gas boilers, especially so-called condensing boilers.

Known in the prior art are gas boilers having a cylindrical burner surrounded by an annular heat exchanger enclosing a combustion chamber in which the burner is centrally located. The heat exchanger can comprise several conduits or tubes for conducting the water to be heated extending substantially parallel to the axis of the burner between headers or manifolds located at the opposite ends of the heat exchanger. The tubes are generally equipped with fins or the like to increase the surface area available for heat exchange with hot flue gases flowing from the combustion chamber between the tubes.

In recent times condensing boilers which operate with greater efficiency by extracting more heat from the flue gases in particular by lowering the temperature of the gases below their dew point, have become popular. However, with such boilers the condensation occurring within the heat exchanger calls for some means of draining the condensation away. Furthermore, a film of condensation forming on the surfaces of the heat exchanger can itself impede the heat exchange and thereby reduce the heat exchanger efficiency. These issues are addressed in our co-pending European Patent Application No 95303625.8. In accordance with one aspect of the invention of our co-pending application there is provided a heat exchanger comprising a plurality of generally parallel upright tubes, substantially horizontal fins attached to the tubes, and means connected to the fins and extending downwardly therefrom to define a predetermined drainage path for condensate formed on the fins.

The means providing for drainage from the fin surfaces and thereby discouraging creation of condensate films covering these surfaces may take various specific forms. Although this arrangement is generally effective in practising the invention, under certain conditions it has been found at the end of a firing period of the burner, a significant proportion of condensate formed at the radially outer areas of the heat exchanger migrates back to the inner edges of the fins instead of draining from them.

When the gas burner is re-ignited the condensate tends to be evaporated, and a small amount of corrosion products, e.g. corrosion salts, which are contained in the condensate, can be deposited at edges of the fins. A build up of corrosion products can occur over a prolonged period of operation, and it is not impossible for the deposits to fall from the fins onto the burner igniter electrodes and thus prevent burner ignition.

The present invention addresses this problem and accordingly provides a heat exchanger for a boiler comprising several substantially upright tubes for passage of liquid to be heated, and a plurality of fins attached to the tubes for hot gases to flow between the fins from a combustion chamber, wherein the fins are inclined downwardly in the direction away from the combustion chamber at an angle of between 2° and 20° to the horizontal.

In accordance with a preferred embodiment the fins are inclined to the horizontal at an angle of between 3° and 8°. By inclining the fins at an angle of between 3° and 8° to the horizontal the total number of fins mounted on the tubes need not be reduced significantly compared to horizontally mounted fins on the tubes of the same length, thereby minimising any loss in the efficiency of the heat exchanger. Further, the invention provides a simple mechanism for improving the overall reliability of the boiler.

Each fin may be provided with means to impede further the inward migration of condensate, such as an interruption in the upper surface of the fin. For example, a longitudinal slit defining an inner edge and an outer edge may be provided adjacent the outer sides of the innermost tubes. The inner edge of the slit can be disposed above the outer edge in the assembled heat exchanger to form a physical barrier to reduce or prevent the inward migration of condensate. Alternatively, a plurality of apertures may be provided in the area adjacent the outer sides of the innermost tubes. As with the slit, the apertures form a physical barrier against the migration of condensation.

The surface area of each fin may be reduced or minimised in the region of the innermost row of tubes to impair the efficiency of heat transfer in this region and thereby mitigate or avoid condensate collecting or forming adjacent the innermost edge of the fins.

In a preferred embodiment, drainage means are provided, the drainage means being connected to the fins and extending downwardly therefrom to define a predetermined drainage path for condensate formed on the fins.

Also provided in accordance with the present invention is a method of manufacturing a heat exchanger for a boiler, the heat exchanger comprising several tubes, and a plurality of substantially parallel fins having corresponding apertures proportioned for an interference fit with the tubes, the method comprising the steps of:

holding the fins in a parallel array with their apertures in alignment;

inserting the tubes into the apertures in a direction orthogonal to the plane of the fins;

stopping the insertion of the tubes with the ends of the tubes lying substantially in a plane inclined at an angle of between 2° and 20° to the plane of the fins;

releasing the fins; and

longitudinally displacing the tubes to bring the ends of the tubes into alignment and thereby to incline the fins relative to the tubes.

In a preferred method, each fin has a slit or an aperture defining an inner edge and an outer edge, and the fins are deformed during alignment of the tubes such that the inner edge of the slit or aperture is disposed above the outer edge.

A clear understanding of the invention in its different aspects will be gained from the following more detailed description, reference being made to the accompanying drawings, in which:-

Figure 1 is a front elevation of a known heat exchanger for a gas boiler;

Figure 2 is a side elevation of the heat exchanger shown in Figure 1 with the gas burner assembled therewith;

Figure 3 is a horizontal cross section through the heat exchanger of Figure 2;

Figures 4A and 4B are side and plan views of one particular form of heat exchanger segment;

Figures 5A and 5B are side and plan views of the second form of heat exchanger segment;

Figures 6A and 6B are side and plan views of a third form of heat exchanger segment;

Figure 6C is a plan view showing a modified form of the fin plate of Fig. 6B;

Figure 6D is an enlarged plan view showing the region of one of the water tube holes in the fin plate of Fig. 6C;

Figure 6E is an enlarged plan view showing the configuration of one of the fin edge tabs of the fin plate of Fig. 6C;

Figure 6F is an enlarged partial cross-section showing the positioning of the additional tabs in relation to the water tubes in the assembled heat exchanger.

Figure 7 is a side elevation of a heat exchanger in accordance with the invention with a gas burner assembled therewith;

Figure 8 is a side elevation of a heat exchanger according to the invention during manufacture;

Figure 9 is an enlarged partial side view of the heat exchanger shown in Figure 8.

Figure 10 is a plan view of a heat exchanger fin plate according to one embodiment of the invention;

Figure 11 is an illustrative cross sectional side view along the line X-X' of Figure 10; and

Figure 12 is a plan view of a heat exchanger fin plate according to a further embodiment of the invention.

Illustrated in Figures 1 to 6 is a known heat exchanger and gas burner assembly suitable for a domestic gas boiler. The heat exchanger includes several parallel metal tubes 1 arranged in an annular array and fastened between upper and lower

tube end plates

2,3. The tube end plates are respectively secured and sealed to upper and

lower headers

4,5 to confine within the headers chambers for conducting water to and from the tubes as explained in more detail below. The lower header includes a water inlet 6 for entry of water to be heated into the heat exchanger, and the upper header includes a water outlet 7 for hot water having flowed through the heat exchanger and been heated. The tubes are fitted with fins 8 which in well known manner serve to increase the surface area for heat exchange with hot flue gases passing between the tubes 1. The fins are mounted over the full height of the tubes 1 and are only partially shown in Figures 1 and 2. For convenience the assembly of tubes, end plates and fins is manufactured as four equal segments. Each heat exchanger segment includes nine tubes with their axes disposed in two arcuate rows, as best seen in Figure 3, there being five tubes in the inner row and four tubes in the outer row. Each fin plate in each segment surrounds all the tubes of this segment, these fin plates being substantially horizontal in the assembled heat exchanger. The fin plates may be fitted frictionally to the tubes or may be fixed thereon such as by soldering. The upper and lower headers are shaped to form chambers communicating with the tube ends to provide a predetermined flow path for water through the heat exchanger. In particular, the incoming cold water first flows upwardly through the four outer tubes (.) of each of the two lower segments as viewed in Figure 3, it is then transferred by the upper header and flows downwardly through the four outer tubes (x) of each of the two upper segments of Figure 3. Upon reaching the lower header again the water is transferred to flow upwardly through the neighbouring adjacent two of the inner tubes (..) of the two upper segments in Figure 3, following which the water is transferred by the upper header to flow downwardly through the next adjacent pairs of inner tubes (xx) of the two upper segments. The water is then directed by the lower header to flow upwardly through the adjacent inner tubes (...) of the upper and lower segments as viewed in Figure 3, and is subsequently directed to flow downwardly through intermediate adjacent tubes (xxx) of each lower segment, and finally is directed by the lower header to pass upwardly through the neighbouring adjacent pairs of inner tubes (....) of the two lower segments, whereafter the water passes out through the outlet of the upper header. Thus it will be appreciated that each drop of water is constrained to make seven passes along the heat exchanger tubes in flowing from the inlet 6 to the outlet 7. Of course a greater or smaller number of tubes could be used with the chambers in the headers arranged accordingly, and if desired the water may be constrained to make more or less than seven passes along the tubes in flowing through the heat exchanger.

The heat exchanger is of annular form enclosing a central combustion chamber wherein a cylindrical burner 10 is disposed. The outer contour of the heat exchanger is defined by the fins and end plates (see Figure 3) is generally circular with flat sides allowing the heat exchanger to be housed in a substantially square casing whereby corner spaces are defined within the casing to form flue ducts extending along the heat exchanger and into which the flue gas passes by flowing between the fins 8 from the combustion chamber. On passing through the fin interspaces, the flue gases give up their heat and become cooled. The heat transfer efficiency is such that the flue gases can at least under some operating conditions be reduced in temperature to below the dew point and moisture then condenses out on the surfaces of the fins. As the temperature of the fins will be lowest adjacent their outer edges, due to the outer tubes carrying the coolest water, condensate is most likely to be formed on the outer portions of the fins and around the outer tubes 1. To prevent the condensate accumulating on the fin surfaces and forming a film or layer of water thereon, which would act as a thermal insulator and hence reduce the efficiency of the heat exchange with the flue gases, drainage means are provided in association with the fins to facilitate rapid removal of condensate to the lower region of the heat exchanger below the inter-fin flue gas passages. This means may take different specific forms. In Figures 4A and 4B there is shown an embodiment of a heat exchanger segment in which several rods 12 are arranged to extend vertically through aligned apertures in the fins 8. The rods 12 extend continuously at least from the uppermost fin to below the lowermost fin. The rods contact the fins but do not fill the cross-section of the holes in the fins so that gaps are left through which water drops can pass. Under normal surface tension effects, the condensate collecting on the upper surface of a fin will be drawn towards a rod adjacent to which a liquid drop will form until it is large enough to run down along the rod under its own weight due to gravity. As clearly shown in Figure 4B, the rods are disposed in clusters of four around the outer water tubes 1, and three additional rods are located at intermediate positions spaced at greater distances from the water tubes. All the rods are positioned between the inner water tubes and the outer edge of the fins. (In the interest of clarity only two water tubes have been included in Figure 4A, which also applies to Figures 5A and 6A).

In the embodiment shown in Figures 5A and 5B, instead of solid rods elongate wicks 13 are arranged to extend longitudinally of the heat exchanger. Although the wicks could be located as described above in relation to the rods, there are shown two wicks positioned at the outer peripheral edge of the fins. The wicks are inserted into slots 14 which may be conveniently provided at the time of pressing the metal fins. The wicks, due to their natural absorbency and ability to conduct liquid provide an effective means of removing condensate from the fin surfaces. Of course, it is not necessary for a single pair of adjacent wicks to be provided in each heat exchanger segment and additional wicks could for example be inserted in any one or more of the further slots 14 shown formed in the fins in Figure 5B.

The fins in the embodiment of Figures 6A and 6B are provided with tabs 15 at the outer edges, these tabs being conveniently formed by slots 14 produced at the time of manufacture of the fins by pressing from metal sheet. The corresponding tabs of all the fins are bent downwardly, so that the deflected tabs define a flow path for water condensing on the fin surfaces. The water is drawn by surface tension towards the tabs and runs down the continuous flow path defined by the tabs. Each tab may be deflected so that it contacts the next tab below, but this is not essential since if gaps are left it merely means that drops of water will collect at the extremities of the tabs and then fall away when they have grown large enough for their weight to overcome the surface tension forces. The fins are shown provided with additional tabs 16 located adjacent those radially outer tubes 1 which carry the cooler water during its passage through the heat exchanger. Illustrated in Figure 6C is a modified fin plate 108 formed with a plurality of tabs 115 at its outer edge. At least one and preferably all of these tabs are deflected downwardly in the assembled heat exchanger so that the aligned deflected tabs define a drainage path for conducting condensate to the lower part of the heat exchanger. As shown in Figure 6E, each tab 115 is defined by a pair of adjacent notches 114 shaped as a circular hole connected to the fin edge by a slot of width less than the hole diameter so that the tab has a waisted neck portion, which may assist in drawing water onto the tab. Around the periphery of each water tube hole 143 are small protrusions 140 for providing a loose interference fit between the fin plate and the tube. Furthermore, additional condensate drainage tabs 145 are provided at the periphery of at least some of the holes. As shown all the holes for the outer tubes, and the holes for the three medial inner tubes are equipped with additional tabs 145. Each tab is defined by a pair of parallel slots 144 and is formed with such a length that its free end projects into the tube hole so that when the tube 1 is inserted, the tab is deflected downwardly and rests with its free end abutting the tube surface as shown in Figure 6F. Consequently, these downwardly deflected tabs 145 combine with the tubes 1 to define drainage paths for conducting condensate from the fins to the lower region of the heat exchanger. It will be noted that the tabs 145 associated with the outer tubes are positioned so that they are distributed over an outer portion of the fin plate, and the tabs 145 associated with the inner tubes are located at the outer, i.e. cooler, side of these tubes.

It should be understood that the embodiments of Figures 4 to 6 could be combined in the same heat exchanger. For example, the fins illustrated in Figure 6B or 6C could have one or more rods inserted through the holes shown, and/or one or more wicks could be inserted in the

slots

14 or 114 flanking

tabs

15 or 115 which have not been downwardly deflected. Of course, additional ones of the tabs not shown bent downwardly in Figure 6B could be deflected to form flow paths for condensate removal and preferably all the tabs are bent down to achieve maximum drainage.

A heat exchanger in accordance with the invention is shown in Figure 7. The general arrangement of the tubes 1 and the fins 8 is as described with reference to the known heat exchanger illustrated in Figures 1 to 6. However, the fins are inclined downwardly in a direction away from the gas burner 10 at an angle between 3° and 8° to the horizontal, and thus condensate which collects on the fins after a firing period flows downwardly and away from the gas burner under its own weight due to the force of gravity. Thereafter the drainage means associated with the fins, and which may be as described above in relation to Figures 1-6, facilitate rapid removal of the condensate from the fins, as hereinbefore described.

With reference to Figures 8 and 9, in the preferred method of manufacturing a heat exchanger according to this invention, the fins 8 for a heat exchanger segment are mounted on a jig (not shown) in parallel with one another and the tubes 1 are placed on mandrels prior to being driven by a pneumatic ram through the aligned apertures in a direction orthogonal to the fins 8. The apertures in the fins 8 are proportioned to ensure an interference fit is obtained between the tubes 1 and the fins 8 such that the fins 8 are held in position when they are removed from the jig. The initial stop positions of the tubes 1 in the heat exchanger segment assembly are varied so that tubes positioned at different radial distances from the inner edges of the fins have their ends located in an imaginary plane 30 which is inclined at an angle of between 3° and 8° to the planes of the fins. After the fins have been released from the jig, the ends of the tubes 1 are forced into the

end plates

2, 3 of the heat exchanger, thereby bringing the tubes 1 into alignment so that the fins 8 which are firmly attached to the tubes are pivoted on the tubes to an angle approximately equal and opposite to the angle of the plane 30. To secure the fins in their inclined positions, they can be fixed to the tubes, such as by brazing. In order to accommodate the sloping fins on the tubes, it is necessary to reduce the number of fins by one. However, in view of the large number of fins, this reduction does not significantly affect the efficiency of the heat exchange between the flue gases and the fluid passed through the tubes. The four segments of the complete heat exchanger are manufactured by the same method.

In an alternative embodiment of a heat exchanger according to the invention, each fin 8a has an arcuate slit or cut 20 extending longitudinally between the inner and

outer rows

30,31 of tubes 1. when the tubes 1 are aligned the fin plate 8 is deformed such that the inner edge 21 of the slit 20 is disposed above the outer edge 22 when the tubes are oriented vertically as shown in Figure 11. The vertical spacing between the edges increases to maximum at the longitudinal centre of the slit, at which point the spacing is approximately half the pitch between adjacent fins. The vertical gap formed by the slit acts as a physical barrier against the migration of condensate towards the innermost edge 32 of the fin plates 8a, thus enhancing the advantageous effects of the inclination of the fin plates 8a.

The slit 20 can be formed by any appropriate cutting or stamping operation, and it may take the form of a continuous arcuate or linear opening. Alternatively, each fin plate could have a series of discrete openings defining a plurality of slits.

In a further embodiment of the invention, apertures or slots 25 are cut or stamped in the fins 8b adjacent the outer sides of the inner row 30 of tubes 1. Also segments 26 of the fins 8b are removed from the region adjacent the innermost edge 32 of the fin 8b to reduce or minimise the surface area of the fin 8b in this region. The slots 25 impede the migration of condensate toward the front edge 32 of the fins 8b. The reduction of the surface area of the fin 8b in the region adjacent the innermost row 30 of tubes impairs the efficiency of heat transfer from the flue gases to the fins 8b in this area, thereby reducing or eliminating the formation of condensate on the area of the fin adjacent the inner rows of tubes 30. The slots 25 and segments 26 are generally vee-shaped and they may be formed to leave rings or annuli 27 of substantially uniform radial width for engagement with the tubes. The front edges of the fins 8b project forwardly a substantially uniform distance from the innermost row of tubes 30.

The fin 8b illustrated in Figure 12 may be deformed by the preferred process of inclining the fins relative to the tubes such that the inner edges 33 the slots 25 are above the outer edges 34 (as described previously in relation to fin 8a). whilst the deformation of the fin 8b may further enhance the ability of the fin 8b to restrict migration and formation of condensate to the inner edge of the fin, bridging portions 28 of the fin are preferably left between adjacent slots 25 to limit the deformation of the fin 8b and hence to facilitate accurate control of the inclination of the fins 8b during heat exchanger manufacture.

Fins

8a,8b with one or more slots 20 or slots 25 can also be used in heat exchangers with substantially horizontal fins.

From the foregoing description it will be understood that the heat exchanger assembly of the invention provides an improved means for removing condensate water which collects on the heat exchanger, and thus improves the reliability of the boiler. Further the invention provides a simple and effective method of manufacturing the improved heat exchanger.

Claims

A heat exchanger for a boiler comprising several substantially upright tubes (1) for passage of liquid to be heated, and a plurality of fins (8) attached to the tubes for hot gases to flow between the fins from a combustion chamber, wherein the fins are inclined downwardly in the direction away from the combustion chamber at an angle of between 2° and 20° to the horizontal.
A heat exchanger according to claim 1, wherein the fins (8) are inclined to the horizontal at an angle of between 3° and 8°.
A heat exchanger according to claim 1 or 2, wherein each fin (8a;8b) is provided with means (20;25) for impeding the migration of condensate towards the inner edges of the fins.
A heat exchanger according to any one of claims 1 to 3, wherein drainage means (12,13,15) are provided, the drainage means being connected to the fins (8) and extending downwardly therefrom to define a predetermined drainage path for condensate formed on the fins.
A heat exchanger according to claim 4, wherein the drainage means comprise one or more continuous elongate members (12,13) extending generally in the direction of the tubes.
A heat exchanger according to claim 4, wherein the drainage means (15,16,115,145) comprise tabs defined by the fins (8) and deflected out of the planes thereof for aligned tabs of adjacent fins to define a drainage path.
A method of manufacturing a heat exchanger for a boiler, the heat exchanger comprising several tubes (1), and a plurality of substantially parallel fin(s) having corresponding apertures proportioned for an interference fit with the tubes, the method comprising the steps of:

holding the fins in a parallel array with their apertures in alignment;

inserting the tubes into the apertures in a direction orthogonal to the plane of the fins; stopping the insertion of the tubes with the ends of the tubes lying substantially in a plane inclined at an angle of between 2° and 20° to the plane of the fins;

releasing the fins; and

longitudinally displacing the tubes to bring the ends of the tubes into alignment and thereby to incline the fins relative to the tubes.
A method according to claim 7 wherein the insertion of the tubes is terminated with ends of the tubes in a plane at an angle of between 3° and 8° to the plane of the fins.
A method according to claim 9 or 10, wherein each fin (8a;8b) has a longitudinal opening (20;25) defining an inner edge (21;33) and an outer edge (22;34) and the fin is deformed by the longitudinal displacement of the tubes such that the inner edge is disposed above the outer edge when the tubes are oriented vertically.
A method according to any one of claims 7 or 9, wherein the fins are secured to the tubes by brazing after the ends of the tubes have been aligned.
A method according to any one of claims 7 to 10, wherein the tubes are inserted into the apertures by means of a fluid actuated ram.
A method according to any one of claims 7 to 11, wherein the ends of the tubes are brought into alignment during insertion of the tube ends into an end plate of the heat exchanger.