GB1576378A

GB1576378A - Heat exchanger and apparatus including same particurarly for use in heating fluids

Info

Publication number: GB1576378A
Application number: GB1336377A
Authority: GB
Original assignee: Jarrett A H; Sims E F
Current assignee: Jarrett A H; Sims E F
Priority date: 1978-03-30
Filing date: 1978-03-30
Publication date: 1980-10-08

Description

(54) HEAT EXCHANGER AND APPARATUS INCLUDING SAME, PARTICULARLY FOR USE IN HEATING FLUIDS (71) We, ALLAN HARGRAVE JARRETT of 60 Camrose Avenue, Edgware, Middlesex, and ERSKINE FREDERICK SIMS of 18 Aldermans Hill, Hockley, Essex, each a British subject, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to heat exchangers and apparatus including same, particularly for use in heating fluids, and particularly but not exclusively to such apparatus as boilers, e.g.

gas-fired boilers, for domestic central heating installations.

A conventional gas-fired boiler comprises a burner for a gas/air mixture and a heat exchanger including a heat exchange matrix composed of at least one conduit through which flows liquid to be heated by heat transference from the gas flame and/or from the hot gaseous products of combustion of the gas/air mixture flowing through passages of the matrix. UK Patent No. 1246581 and Dutch Patent No. 6901052 (published in the UK on 8 August 1969) as well as UK Patent No. 1314099 (havingaUK filing date of 16 October 1970) each disclose a heat exchanger whereof the matrix is of rigid cylindrical form and the or each conduit has a plurality of small spherical balls bonded to its external surface to contact that surface and thereby provide the heat transference to the liquid flowing in the conduit from hot gases flowing over the spherical balls.Although the heat transfer efficiency of such a matrix is high, problems have occurred in practice (for example when in use there is no liquid in the conduit(s) - i.e. it is run dry) when the bonding between the balls and the conduit(s) fails so that the balls fall off and cause a decrease in the heat transfer efficiency of the matrix or even its total failure as the gap(s) in the matrix left by the missing balls enable the gas flame to burn through the conduits and/or an outer casing for the matrix.

This invention relates to the solution of such problems.

According to this invention there is provided a heat exchanger comprising a heat exchange matrix of cylindrical form and composed of two axially spaced header chambers and a plurality of mutually parallel pipes extending in angularly spaced relation between the two header chambers, each pipe extending through and in contact with a plurality of annular discs each of which provides a venturi-like passage between its outer periphery and the external surface of each of its two neighbouring pipes, the annular discs of neighbouring pipes interdigitating with one another.

Preferably the external surface of each pipe is bonded to the annular discs through which it extends.

Advantageously each of the annular discs is in contact with the annular discs to each side of it with which it interdigitates, and preferably is bonded thereto.

Conveniently said bonding is derived by the brazing at an elevated temperature (e.g.

1100 C) of a copper layer, preferably in an inert gas atmosphere. Alternatively said bonding is derived by tinning (e.g. wherein excess tin is spun off).

Preferably said annular discs are in the form of washers having a thickness preventing their being readily deformed. Conveniently said thickness is of the order of 0.06 inches to provide approximately 8 . 4 washers per inch length of pipe.

By way of example, one embodiment of this invention will now be described with reference to the accompanying drawings of which: Figure 1 is a schematic, part-sectional side elevation of a heat exchanger according to this invention; Figure 2 is a schematic plan view of the heat exchange matrix of Fig. 1; Figure 3 is an enlarged schematic view of part of Fig. 2; and Figure 4 is an enlarged schematic side elevation in the direction of arrow IV of Fig. 3.

As shown in Fig. 1, the heat exchanger 10 comprises a generally cylindrical casing 11 provided by a cup-shaped metal pressing connected to an outlet 12, the casing's open radial end being connected by a clamping ring 13 of U-section to a heat exchange matrix 15 disposed virtually completely within the casing 11.The heat exchange matrix 15 is of cylindrical form and comprises two radially disposed header chambers 16, 17 (represented schematically in Fig. 1 as of plate-like form) spaced apart along the axis 26 (Fig. 2) of the cylindrical matrix 15, and a number of pipes 18 extending between, and communicating with, the header chambers 16, 17. Each pipe 18 extends through and is in contact with a plurality of annular discs or washers 22 each of which provides a venturi-like passage 24 between the disc's outer periphery 23 and the external surface 25 of each of the two neighbouring pipes 18. For clarity of illustration in Fig. 1 the washers 22 are illustrated in vertically spaced relation.The pipes 18 are arranged, parallel to one another and to the axis 26 of the matrix cylinder, the axes of the pipes 18 being equi-angularly spaced apart by an angle a (Fig. 3) and lying on a pitch circle of diameter D. Each pipe is a steel tube having its external surface 25 tinned or plated with a copper layer.

The header chambers 16, 17 are each formed by metal pressing to an annnular or toroidal shape and are internally provided with partitions (not shown) so that water or other fluid to be heated can flow through the matrix 15 from an inlet 30 to header chamber 16, down one or more (but not all) of the pipes 18 and up the remainder of the pipes 18 to an outlet 32 from header chamber 16.Where for example (and as illustrated for this embodiment) the number of pipes 18 provided in the cylindrical array is twenty, they may be divided by the header chambers' partitions into four groups (of five pipes per group) extending in quadrants around the cylinder from the inlet 30 to the outlet 32 and such that the flow is initially from inlet 30 downwardly through the first group of five pipes, then upwardly through the second group of five pipes, then downwardly again through the third group of five pipes, and finally upwardly again through the fourth group of five pipes to emanate from outlet 32.

The upper header chamber 16, which is of larger diameter than the lower header chamber 17, carries beneath it the remainder of the matrix 15 and, when the latter is disposed in the casing 11, the header chamber 16 forms a lid-like closure for the casing secured thereto by the U-section clamping ring 13. The annular header chamber 16 has a plate-like extension (not shown in Fig. 1) directed towards the matrix axis 26, and this plate-like extension is centrally apertured at 34 and is provided with an adjacent slot-like opening 36.A burner in the form of a perforated metal cylinder for burning a gas/air mixture (e.g. a burner such as that disclosed in UK Patent Specification No. 1286356) is mounted in aperture 34 to extend inwardly of the cylindrical matrix 15 coaxially therewith, and an ignition device, e.g. a spark ignition device, is mounted in the slot-like opening 36 to extend inwardly of the cylindrical matrix 15 (between the matrix and the burner) for igniting the gas/air mixture emanating from the burner's perforations. Heat given off by the gaseous products of combustion and/or by the flame(s) of the burner is transferred to the pipes 18, and the fluid flowing through them, by the annular discs or washers 22 encompassing the pipes 18.

The washers 22 are of steel, either tinned or plated with a copper layer, and are sufficiently thick. e.g. at least 0.025 inch and preferably approximately 0.05 or 0.06 inch thick, to prevent their being readily deformed manually. During assembly of the matrix 15, a first level of washers 22 is provided by sliding or forcing a washer along the external surface 25 of each alternate one of the pipes 18. e.g. all the odd-numbered pipes 18 of the circular array, then providing a second level of washers 22 by sliding or forcing a washer 22 along the external surface 25 of each of the remaining pipes 18, e.g. all the even-numbered pipes 18 of the circular array, then a third level of washers 22 for the (odd-numbered) pipes extending through the washers of the first level, a fourth level for the (even-numbered) pipes extending through the washers of the second level, and so on until the washers extend along the whole length of each pipe between the headers 16, 17 (see Fig. 4).

To ensure that the washers of neighbouring pipes 18 interdigitate with one another, i.e.

that the washers in each level overlap and/or are overlapped by the washers in the immediately preceding and/or succeeding level, and to ensure that the washers 22 on one pipe do not contact either of the two neighbouring pipes 18, each washer 22 has an outer diameter d1 governed by the relationship 2D sin q - dz > d1 > D sin, (I) where D is the diameter of the pitch circule of the axes of the pipes 18 of the cylindrical array, a is the angle between radii of the pitch circle through the axes of adjacent pipes, and d2 is the outer diameter of each pipe 18 pthe inner diameter of each washer).

The two convex surfaces provided by the periphery 23 of a washer 22 and the external surface 25 of an adjacent pipe 18 form a venturi-like passage 24 between them, the neck of this venturi-like passage lying slightly inwardly of the pitch circle on a chord through two adjacent pipe axes. Using the above notation, the widths of this neck is governed by the relationship s = D sin 2-1(dl + d2) (Il) and is preferably arranged to be of the order of 0.025 in. This neck is of rectangular cross-sectional area given by s times the washer thickness, and is preferably approximately 0.0015 sq.in. Alternatively s is arranged to be of the order of 0.060 in. to provide a neck area of approximately 0.0036 sq. in.

In a specific example, twenty pipes 18 are arranged, equiangularly-spaced, in a cylindrical array of 5.75 inches pitch circle diameter (i.e. a = 18 and D = 5.75), each ipe being a length of 16 S.W.G. cold drawn steel tube nominally of 5/8 inch outer diameter d2 = 0.625) having a tinned or plated copper layer 0.001 inch thick on its outer surface. Thus from Equation (I) above, dl must exceed approximately 0.9 inches, and be less than approximately 1.175 inches. The washers 22 are each formed from 17 S.W.G. plate thickness provided with a tinned or plated copper layer 0.001 inch thick and, to reduce s to the preferred value of 0.025 inch, are each provided with an outer diameter of 1.125 inches (dl = 1.125), i.e. about 1.1/4 times the minimum of approximately 0.9 inch.To cater for the 0.001 inch tinned or plated copper layer on the pipes 18 and the washers 22, and to ensure ease of assembly yet good contact for efficient heat transfer between the washers and the pipes, the inner diameter of each washer 22 is in fact 0.629 inch, i.e. fractionally more than the nominal outer diameter d2 of the pipes 18. For an axial spacing between header chambers 16, 17 of respectively 3 inches, 3.87 inches, 4.75 inches and 9.5 inches, the total number of washers 22 required is respectively 500,640, 800 and 1600. Thus the number of washers 22 per unit length of each pipe 18 is approximately 8.4.

The final stages of assembly of the matrix 15 are concerned with bonding together the header chambers 16 and 17 to the ends of the pipes 18, the columns of washers 22 to the pipes 18 extending through the respective columns, and the washers 22 about each pipe to the interdigitating washers 22 about each adjacent pipe. Where the pipes and washers have been tinned, the bonding process involves dipping the assembled matrix in a bath of molten tin, spinning off the surplus tin and then solidifying the tin coating on the matrix.Where the pipes and washers have been provided with a copper layer, the bonding process involves applying a copper paste to appropriate locations of the assembled matrix and then brazing the assem bled matrix in an inert gas furnace at an elevated temperature of about 1100 C. With either bonding process, the matrix thus takes the form of a unitary, rigid, self-supporting structure to which a protective coating of aluminium can, if desired, be applied (e.g. by spraying) which is inserted bodily into the cylindrical casing 11 and its header chamber 16 clamped, in a gas-tight manner, to the open radial end of the casing 11 by means of the U-section clamping ring 13.

In use, e.g. in a boiler for a domestic central heating system, the central heating water is pumped through the inlet 30, the pipes 18, the outlet 32 and on through the system's radiators for return, via the pump to the inlet 30, and the water is heated during its passage through the pipes 18 by heat transfer through the pipe walls and via the washers 22 in bonded contact therewith from the heat source provided by the burner disposed centrally of the cylindrical heat exchange matrix 15. The hot gaseous products of combustion of the gas/air mixture supplied to the burner (by the action of a fan) flow through the venturi-like passages 24 and, in being constricted by each passage neck, transfer a very high proportion of their heat content to the washers 22 and hence to the water in the pipes 18.Tests with the abovementioned specific example have shown this proportion to be as high as 85% so that the gaseous products of combustion leaving the heat exchange matrix 15 do not heat the casing 11 to a temperature such that it cannot be painlessly touched by the human hand. The relatively low temperature gaseous products of combustion leaving the matrix 15 are passed to atmosphere via an exhaust flue connected to the casing outlet 12 of the heat exchanger 10.

It will be appreciated that due to the dense packing of the washers 22, the flame(s) from the central burner cannot reach the actual pipes 18 and thus damage of the pipes through overheating is unlikely. Furthermore, since the washers 22 encompass the pipes 18, they cannot fall off into the casing 11 in the event that their bonding to the pipes should fail, and moreover, in view of the interdigitation of the washers 22, the spacing between the washers along each pipe 18 cannot vary. Also it will be noted that the use of the clamping ring 13 enables the matrix 15 to be readily removed from the casing 11 for inspection and any necessary cleaning.

By way of example, another embodiment of this invention will now be described with reference to Figures 5 and 6 of the accompanying drawings, Figure 5 being a schematic plan view thereof and Figure 6 being a cross-sectional view taken along the line VI-VI of Fig. 5.

The embodiment of Figures 5 and 6 is generally similar to that of Figs. 1 to 4 and like parts are provided with like reference numerals. Fig. 6 shows the header chambers 16, 17 formed by securing a metal pressing to a disc-like plate. The upper header chamber 16 has its metal pressing 16a of annular or toroidal form secured to a disc-like plate 16b that is centrally apertured at 34. The lower header chamber 17 has its metal pressing 17a of cup-shaped form secured to a non-apertured disc-like plate 17b. The positions of the partitions provided internally of the header chambers 16, 17 are shown schematically in Fig. 5 and referenced 16c, 17e respectively.

WHAT WE CLAIM IS: 1. A heat exchanger comprising a heat exchange matrix of cylindrical form and composed of two axially spaced header chambers and a plurality of mutually parallel pipes extending in angularly spaced relation between the two header chambers, each pipe extending through and in contact with a plurality of annular discs each of which provides a venturi-like passage between its outer periphery and the external surface of each of its two neighbouring pipes, the annular discs of neighbouring pipes interdigitating with one another.

2. A heat exchanger according to Claim 1, wherein said annular discs each have a thickness preventing their being readily deformed.

3. A heat exchanger according to Claim 1 or Claim 2, wherein said annular discs each have a thickness of the order of 0.06 inches.

4. A heat exchanger according to any preceding Claim, wherein said annular discs are in the form of metal washers.

5. A heat exchanger according to any preceding Claim, wherein said pipes extend parallel to the axis of the matrix.

6. A heat exchanger according to any preceding Claim, wherein the external surface of each pipe is bonded to the annular discs through which it extends.

7. A heat exchanger according to any preceding Claim, wherein each of the annular discs is in contact with the annular discs to each side of it with which it interdigitates.

8. A heat exchanger according to any preceding Claim, wherein each of the annular discs is bonded to the annular discs to each side of it with which it interdigitates.

9. A heat exchanger according to Claim 6 or Claim 8, wherein said bonding is derived by the brazing at an elevated temperature of a copper layer.

10. A heat exchanger according to Claim 9, wherein said brazing is performed in an inert atmosphere at approximately 1100 C.

11. A heat exchanger according to Claim 6 or Claim 8, wherein said bonding is derived by a tinning operation.

12. A heat exchanger according to Claim 11, wherein said tinning operation includes a step of spinning off excess tin.

13. A heat exchanger substantially as herein described with reference to and/or as illustrated in Figs. 1 to 4 or 5 and 6 of the accompanying drawings.

14. A gas-fired boiler for a domestic central heating installation, the boiler including a heat exchanger according to any preceding Claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. are provided with like reference numerals. Fig. 6 shows the header chambers 16, 17 formed by securing a metal pressing to a disc-like plate. The upper header chamber 16 has its metal pressing 16a of annular or toroidal form secured to a disc-like plate 16b that is centrally apertured at 34. The lower header chamber 17 has its metal pressing 17a of cup-shaped form secured to a non-apertured disc-like plate 17b. The positions of the partitions provided internally of the header chambers 16, 17 are shown schematically in Fig. 5 and referenced 16c, 17e respectively. WHAT WE CLAIM IS:

1. A heat exchanger comprising a heat exchange matrix of cylindrical form and composed of two axially spaced header chambers and a plurality of mutually parallel pipes extending in angularly spaced relation between the two header chambers, each pipe extending through and in contact with a plurality of annular discs each of which provides a venturi-like passage between its outer periphery and the external surface of each of its two neighbouring pipes, the annular discs of neighbouring pipes interdigitating with one another.