GB2025599A

GB2025599A - Waste-heat recovery method and apparatus

Info

Publication number: GB2025599A
Application number: GB7927326A
Authority: GB
Original assignee: Long P W
Current assignee: Long P W
Priority date: 1978-05-04
Filing date: 1979-08-06
Publication date: 1980-01-23

Abstract

A method for the recovery of heat from exhaust gas discharging through the flue of a hot water boiler (e.g. in a central heating system, Figure 3) comprises circulating a fluid (e.g. the water passing to the boiler) through a heat exchange passage arranged in heat exchange relation with the flue gas. Said passage may comprise an annular space 6 in a concentric tube heat exchanger Figure 1 sealed at its ends into the flue. Or the passage may be formed in a box structure 21 attached alongside the flue, Figure 5. Fins 3, Figure 1 or 26, Figure 5, assist heat transfer. <IMAGE>

Description

SPECIFICATION A heat recovery unit The present invention relates to a heat exchanger and particularly, but not exclusively to a heat exchanger for use in a domestic hot water system.

.At the present time a considerable proportion of the heat produced in domestic hot water and central heating systems is discharged with the exhaust gases of the boiler to the boiler flue and thence out to atmosphere.

The overall efficiency of the system could be increased if the heat passed through the flue with the exhaust gases could be reduced. In this way fuel would be conserved and heating charges reduced.

According to one aspect of the invention there is provided a method of recovering utilisable heat from exhaust gases discharged through a flue of a boiler of a hot water and/or central heating system, comprising circulating heat exchange medium in heat exchange relationship with said gases in said flue. Preferably, the heat exchange medium is also curculated through a radiator of said central heating system.

According to a second aspect of the invention there is provided a heat exchanger for use in the above method comprising a body having a heat exchange structure and being adapted for attachment to said flue with said heat exchange structure exposed to exhaust gases flowing along said flue, a cavity in said body in heat exchange relationship with said heat exchange structure, and an inlet and an outlet to said cavity whereby heat exchange medium can be circulated through said cavity.

In one embodiment of the invention said body comprises inner and outer jackets between which said cavity is defined, and said inner jacket provides said heat exchange structure and defines a passage for said exhaust gases, the body being adapted for insertion into the flue.

Said embodiment of the invention may also comprise any one or more of the following advantageous features.

(a) The inner and outer jackets are cylindrical and coaxially arranged.

(b) Means for creating turbulence in the exhaust product flow through the passage are disposed in the passage.

(c) A plurality of substantially equally spaced radially extending fins are disposed in the exhaust product passages.

(d) There is heat exchange medium inlet and outlet at opposite axial ends respectively of the jacket.

(e) Clips are provided at opposite axial ends of the outer jacket for connecting the heat exchanger into an exhaust product flue.

(f) The inner and outer jackets are sealed together at opposite axial ends respectively.

(g) The means (b) for creating turbulence com prises a plate disposed at the inlet end of the passage for the exhaust products.

In a further embodiment of the heat exchanger of the invention the body thereof is adapted for attachment on a side of the flue, said heat exchange structure comprising one or more projections adapted to extend into the flue for contact with said exhaust gases. Preferably, said projections are in the form of pins although if desired other structures may be used.

The invention also comprises a hot water and/or central heating installation comprising a heat exchanger as defined above in the exhaust gas flue from the boiler of the installation.

In order that the invention may be more clearly understood, embodiments thereof wili now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a perspective view of one form of a heat exchanger according to the invention; Figure 2 shows one method of attachment of the heat exchanger of Figure 1 in the exhaust gas flue of a central heating boiler; Figure 3 shows diagrammatically the arrangement of the boiler of Figure 2 in a simplified central heating circuit comprising one radiator.

Figure 4 is a diagrammatic perspective view of an alternative form of heat exchanger applied to a flue; and Figure 5 is a sectional view through the arrangement of Figure 4.

Referring to Figure 1, the heat exchanger comprises an inner jacket 1 coaxially arranged with an outer jacket 2. Both jackets arecof circular cross-section and made of a corrosion resistant material, such as stainless steel, or of a material which is not inherently corrosion resistant provided with a corrosion resistant coating, such as vitreous enamel on its surface exposed to exhaust gases. Radially extending fins 3 (eight shown) extend inwardly from the interior surface of the inner jacket 2. These fins, which in use of the device increase the degree of heat exchange as between the exhaust gases and heat exchange medium, are of the same material or materials as the jacket 2 but may be of a different, corrosion resistant material if desired.A plate 9 at the lower end of the inner jacket 1 is provided to create turbulence in the exhaust gases and thus to assist heat exchange. This plate 9 is also of corrosion resistant material or coated with such material.

The inner and outer jackets are welded to each other at 4 and 5 to produce water tight seals at opposite axial ends respectively of the jackets. An annualar space 6 is thus produced into which an inlet 7 leads and from which an outlet 8 leads. The inlet 7 and outlet 8 are connected into the water circuit of the central heating installation shown in Figure 3 and the heat exchanger inner jacket 1 replaces a cylindrical section of the conventional exhaust gas flue 10 specially cut out for that purpose. The ends of the jacket 1 are sealed to the adjacent end of the flue 10 by a layer of fine heat resistant cement 11 and 12 (see Figure 2). The flue 10 leads from the conventional central heating boiler 13 which supplies hot water to a central heating radiator 14 (only one radiator has been shown for simplicity) and which may be gas, coal or oil fired.The water returns from the radiator, goes to the inlet 7 of the heat exchanger and the water serving as the heat exchange medium, after passing through the heat exchanger and absorbing heat from the boiler exhaust gases passing up the flue, then returns to the boiler 13 via the outlet 8.

Clips 15 and 16 serve to connect the heat exchanger to the exhaust gas flue.

It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention. For best results the heat exchanger is connected in the flue as close to the boiler as possible but other positions in the flue may be chosen if desired. Where the main central heating system comprises a circulating pump the gap between inner and outer jackets will be reduced to a minimum to increase the flow of heat exchange medium (water) therethrough. In such an arrangement, the position of the heat exchanger in the water circuit is not so important and it may be positioned other than on the return side of the boiler as it shown. The gap between jackets is widened where no pump is provided as heat transfer is then mainly be convection.

Further, the fins may be serrated and/or otherwise formed to give increased turbulence and therefore improved heat exchange in the vicinity of same.

Still further, the spaced jackets between which the water is circulated need not be separate, coaxial sleeves but may be integral walls whereby, for example, a spirally wound tube or the like may be used for the circulated water, such tube being coiled around the flue internally or externally thereof. It is also to be understood that the flue need not be of circular cross-section but may be a flue or duct of square or other cross-section.

Figures 4 and 5 show a further embodiment in which a heat exchanger is applied to a side of a boiler flue 20. The exchanger has a hollow box structure body 21 which has one side wall 22 shaped to conform to the curved side of the flue 20. The body 21 is secured to the flue 20 in any suitable manner.

The box structure 21 has an internal partition 23 and inlet and outlet connections 24,25 are provided on opposite sides of the partition 23 in the lower region of the box structure. Heat exchange medium circulated through the box structure 21 flows from the inlet 24to the outlet 25 around the partition.

Further baffles may be provided if desired.

The heat exchanger also has heat exchange pins 26 (not shown in Figure 4) formed from a metal of good thermal conductivity. The pins 26 are secured to and project through the curved side wall 22 of the box structure.

As can be seen from Figure 5, in order to install the heat exchanger it is only necessary to cut an opening or slot 27 in the side of the flue 20. The box structure 21 can then be secured in position with the pins projecting through the slot 27 into the flue.

Claims

1. A method of recovering utilisable heat from exhaust gases discharged through a flue of a boiler of a hot water and/or central heating system, comprising circulating heat exchange medium in heat exchange relationship with said gases in said flue.

2. A method according to claim 1 wherein said heat exchange medium is also circulated through a radiator of said central heating system.

3. A method according to claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

4. A heat exchanger for use in the method of claim 1 comprising a body having a heat exchange structure and being adapted for attachment to said flue with said heat exchange structure exposed to exhaust gases flowing along said flue, a cavity in said body in heat exchange relationship with said heat exchange structure and an inlet and an outlet to said cavity whereby heat exchange medium can be circulated through said cavity.

5. A heat exchanger according to claim 4 wherein said body comprises inner and outer jackets between which said cavity is defined and said inner jacket provides said heat exchange structure and defines a passage for said exhaust gases, the body being adapted for insertion into the flue.

6. A heat exchanger according to claim 5 where in the inner and outer jackets are cylindrical and coaxially arranged.

7. A heat exchanger according to claim 5 or 6 wherein means for creating turbulence in the ex haust product flow through the passage are dis posed in the passage.

8. A heat exchanger according to claim 7 where in a plurality of substantially equally spaced radially extending fins are disposed in the exhaust products passages.

9. A heat exchanger according to any one of claims 5 to 8 wherein there is a heat exchange medium inlet and outlet at opposite axial ends respectively of the jacket.

10. A heat exchanger according to any one of claims 5 to 9 wherein clips are provided at opposite axial ends of the outer jacket for connecting the heat exchanger into an exhaust product flue.

11. A heat exchanger according to any one of claims 5 to 10 wherein the inner and outer jackets are sealed together at opposite axial ends respectively.

12. A heat exchanger according to claim 7 or any claim dependent on claim 7 wherein the means (b) for creating turbulence comprises a plate disposed at the inlet end of the passage for the exhaust products.

13. A heat exchanger according to claim 4 wherein the body is adapted for attachment on a; side of the flue, said heat exchange structure comprising one or more projections adapted to extend into the flue for contact with said exhaust gases.

14. A heat exchanger according to claim 13 wherein projections in the form of pins are provided.

15. A heat exchanger according to claim 13 or 14 wherein said body comprises a box structure and said projection or projections also extend into the box for contact with said heat exchange medium circulated therethrough.

16. A heat exchanger substantially as hereinbe fore described with reference to and as illustrated in the accompanying drawings.

17. A hotwater and/or central heating system having a boiler and a flue to receive exhaust gases from the boiler, characterised in that said flue is provided with a heat exchanger according to any one of claims 4 to 16, said outlet of said heat exchanger is connected to a radiator, and circulation means is provided for effecting curculation of said heat exchange medium from said outlet through said radiator.