GB2130925A - Heat exchanger - Google Patents

Abstract

A method of producing a heat exchanger of the fin block type in which metal tubing is arranged to extend transversely through openings in an array of parallel metal fins and is expanded so as to fit tightly in the openings, is characterised in that the metal tubing 1 is provided internally with an axial metal core 2 connected to the inner surface of the tubing by wire spokes 3 extending radially from the core 2 and soldered or welded to said inner surface; and in that the tubing 1 is expanded by the internal application of a high pressure fluid. <IMAGE>

Description

SPECIFICATION Heat exchanger This invention relates to a method of producing a heat exchanger of the type comprising a block of fins around metal tubing.

Heat exchangers of the fin block type are well known. Metal tubing is disposed in a block of metal fins such that the fins fit tightly on the tubing. Heat is thereby conducted from a fluid in the tubing into a surrounding heat exchange fluid.

The tight fit required between the fins and the tubing is achieved by expanding the tubing after it has been "threaded" through the fins. This is effected by exerting an internal pressure on the tubing, generally by providing a steel ball or rod of a slightly larger diameter than the internal diameter of the tubing, which is pulled or pushed through the tubing to stretch the metal and thus expand it to fit. An alternative method is to inflate the tubing by the application of an internal high pressure fluid such as water, although this requires tubing of a consistent gauge and standard to prevent variation in diameter. For this reason it has been preferred to use the steel rod method.

One serious problem which is encountered in heat exchangers of this type is that the viscosity of the fluid in the tubing is often such that laminar flow conditions occur, thus preventing heat from the interior from reaching the surface. In order to provide the necessary turbulence to maximise the heat transfer to the tubing, and hence to the heat absorbing medium via the fins, it would be necessary to provide the tubing internally with means to break up the laminar flow. Internal projections or fitments in the tube are also desirable in that, like the external fins, they can also conduct heat and allow heat from the centre of the tube to be removed. To do this, it is vital that they are contiguous with the internal surface of the tubing; i.e. they are soldered or welded in place so that there is an uninterrupted heat pathway.Obviously, such a construction is impossible if an object has to be drawn or pushed through the tubing to expand it. Similarly, the fact that the internal means would have to be firmly bridged across the tubing leads one to suppose that expansion of the tube by any method, even by the use of high pressure water, would be virtually impossible.

Surprisingly, we have found that the use of an internal packing of a particular kind will permit expansion of the tubing by high pressure fluid.

According to the present invention, there is provided a method of producing a heat exchanger of the fin block type in which metal tubing is arranged to extend transversely through openings in an array of parallel metal fins and is expanded so as to fit tightly in the openings, characterised in that the metal tubing is provided internally with an axial metal core connected to the inner surface of the tubing by wire spokes extending radially from the core and soldered or welded to said inner surface; and in that the tubing is expanded by the internal application of a high pressure fluid.

The axial metal core is preferably a copper rod or stout wire of a gauge which will obviously vary with the internal diameter of the tubing, but may, in general, be about 520% of the diameter. For example a tube of internal diameter 1 3 mm might be fitted with a core of about 2 mm diameter. The wire spokes should preferably be of a relatively thin gauge copper wire, e.g. about 0.5 to 0.75 mm diameter. While the spokes may be separate, it is particularly convenient for them to form a continuous series of elongated loops, each having one end in contact with the core and the other end attached to the tubing. For effective turbulence production, the spokes or loops are preferably disposed in a helical array about the core, for example with the plane of each loop lying substantially within a transverse section of the tubing.The pitch of the helix and the number of loops can be chosen to provide the required degree of heat transfer and turbulence. For example, a helix pitch may be used which gives one complete turn per length equal to the internal diameter. The loops are connected to the core and one effective method of achieving this is by means of a helix of wire passing through each loop and tightly bound and soldered to the core. The loops must also be attached firmly to the inner surface of the tubing, e.g. by solder. Usualiy the helical array of loops or spokes will be formed on the core rod or wire and the whole assembly inserted into the tubing and soldered in place - for example by pre-finning the loops and fusing them onto the surface.Tubing of this type, with an internal coiled construction is available from suppliers in this field, such as Pyntech, Warlies Park House, Upshire, Waltham Abbey, Essex under the name Nol-Tube I-type. The tubing is then inserted into the fin block and expanded by pressure as described. Straight sections of tubing can then be joint by bends to produce a coil configuration.

The fins are conveniently formed of thin aluminium sheet, e.g. about 0.25 mm, and are regulariy spaced apart, e.g. at about 3-5.5 per cm. The spacing may be provided by the use of preformed spacing collars which may be integral with the fins. In the usual way in plate fin heat exchanger the array of fins may be contained within steel or alloy end plates, also tightly fitting on the tubing, which may form the walls of a container for the heat-absorbing fluid, e.g. cooling water.

The tubing itself is preferably a copper alloy. It must be sufficiently ductile to be expanded by pressure without splitting or holing and must be solderable or weldable to the spokes. Typically a wall thickness of 0.3 to 0.6 mm is suitable for an external diameter of 10 to 25 mm. For such a size of tubing, water at a pressure of about 140-250 kg.cm-2 is required for suitable expansion.

A particular embodiment of the invention will now be described by way of Example, and with reference to the accompanying drawings in which: Figure 1 represents a cut-away perspective view of a portion of tube, showing the internal construction; and Figure 2 represents a schematic sectional view of a fin block array with tubing in place, respectively before and after expansion.

Referring to Figure 1, copper tubes 1 of 15.75 mm external diameter and 0.5 mm wall thickness are provided with copper axial core rods 2 of 2.5 mm diameter. A helical coil of wire loops 3 surrounds each rod 2 and is anchored thereto by means of a helical binder wire 4. The binder wire 4 and the inner ends of the wire loops 3 are soldered in place on the rod 2. The assembly of rod 2, loops 3 and binder wire 4 is inserted into the tube 1 and soldered in place by the application of heat to the outside of the tube 1. The tubes 1 are then passed through aligned holes 5 in a fin block 6. The block 6, as shown in Figure 2(a), comprises an array of aluminium fin platens 7, each provided with an integral spacer collar 8 to give a spacing of about 5 mm. At each face of the array is a mild steel enclosure plate 9 (one shown) of 1.6 mm thickness.Free ends of the tubes are then connected to a supply of water at a pressure of about 1 55 kg.cm-2 (2200 p.s.i.) and the water admitted. Before expansion, the clearance in the holes 5 is about 0.56 mm in the aluminium fins 7 and about 0.38 mm in the plates 9. The internal pressure expands the external diameter of the tubes 1 by about 0.76 mm so as to fit tightly in the collars 8 and the end plates 9 as shown in Figure 2 at (b), completely filling the holes 5. At the same tirne the wire loops 3 are stretched but remain anchored to the core 2 and the inner walls of the tube 1. Free ends of the tubes 1 are then joined by curved bends (without internal wire turbulisers to form a multilayer coil of pitch spacing about 40 mm and spacing between layers about 35 mm, passing and re-passing through the fin block 6.

in this way a heat exchanger is produced in which the tubing is tightly located in the fin block but, contains within it an internal "turbuliser" and heat conductor. It is estimated that the increased heat transfer capability of the exchanger, compared with a plain tube system is about 20%.

Claims (6)

1. A method of producing a heat exchanger of the fin block type in which metal tubing is arranged to extend traversely through openings in an array of parallel metal fins and is expanded so as to fit tightly in the openings, characterised in that the metal tubing is provided internally with an axial metal core connected to the inner surface of the tubing by wire spokes extending radiaily from the core and soldered or welded to said inner surface; and in that the tubing is expanded by the internal application of a high pressure fluid.

2. A method according to claim 1, in which the wire spokes form a continuous series of loops.

3. A method according to claim 2, in which the loops are anchored to the core by a helically wound wire or strip and/or solder.

4. A method according to any of claims 1 to 3, in which the high pressure fluid is water at 140--250 kg.cm-2.

5. A method according to any of claims 1 to 4, substantially as herein described.

6. A heat exchanger produced by a method according to any of claims 1 to 5.