IMPROVEMENTS TO HEAT EXCHANGERS

Abstract: In a heat exchanger, the flowing fluid is caused to impinge vigorously against the heat conducting walls, for the purpose of improving heat transfer efficiency. Disclosed is an apparatus for and a method of making a twisted plastic ribbon that fits snugly in the tubes of the heat exchanger, and which imparts a helical swirling velocity to the fluid flowing through the tube. Efficiency is enhanced so economically that it becomes practical to make an automative radiator from round aluminum tubes, provided they are fitted with the twisted plastic ribbons; aluminum being a much cheaper material then the traditional copper. The apparatus includes two sets (55, 58) of traction rollers for conveying the ribbon from the extruder. One set of roller rotates to important the required helical twist. The ribbon is heated (56) to above its critical temperature so that the twist shaped becomes permanent. The ribbon is cooled (by quenching it in water (54, 57)) to give it the strength it needs to enter the respective sets of rollers. This apparatus allows the twisted ribbon to be produced on a continuous-production basis.

Description: IMPROVEMENTS TO HEAT EXCHANGERS This invention relates to heat exchangers. In a heat exchanger, two fluids are kept physically separated by ' interface walls, through which heat transfer between the two fluids can take place. It is known that the efficiency of heat transfer is improved if the fluids, or one of the fluids, is made to flow over the surfaces of the interface walls at high speed. It is known for the normal flow velocity of the fluid through the exchanger effectively to have superimposed upon it a swirling velocity, so that the fluid circulates over the surfaces with greater vigour than it would if the fluid were simply flowing normally. A swirling action may be generated by providing lengths of plastic ribbon that are twisted into helices. The twisted ribbons are inserted in the tubes of the heat exchanger. Fluid flowing along the tube therefore travels in a helical path, the fluid near the walls of the tube moving the fastest. There is no buffeting of the fluid, through: the fluid moves smoothly, and yet readily mixes with itself so that internal temperature gradients do not build up in the fluid. The twisted ribbon should substantially fill the tube; if the ribbon is too small, then the characteristic swirling effect will be lost\ If it were necessary to attach the plastic ribbon to the inside wall of a tube, then that would be too difficult and expensive. However, the ribbon need not be attached, but simply needs to be held against rotation relative to the tube, and that can easily be done from outside the tube. In fact, the twisted plastic ribbon simply needs to be made twice as long as one tube, the ribbon being doubled over and the ends inserted each in a respective tube: no other mounting or location is then necessary. Automotive engine radiators are a common form of heat exchanger. Conventionally, such radiators have had copper tubes, which are somewhat flattened to increase their surface area. Aluminum nearly approaches the thermal conductivity of copper, but it is rather difficult to produce flattened tubes in aluminum, since aluminum tends to crack easily: but if the tubes are circular so as to be free from cracking, then their surface area is too small. However, * round-cored aluminum radiators become economically feasible if some means is provided inexpensively to increase the flow velocity of the coolant over the inside surfaces of the aluminum tube walls; the twisted plastic ribbon just described is admirably suited to just that application. BRIEF DESCRIPTION OF THE INVENTION The invention provides an apparatus for, and a method of, making the twisted plastic ribbon in such a way as to render the use of the ribbon a viable proposition for automotive radiators, which in turn means that round-cored aluminum radiators become viable. The fact that the ribbon can be made of such a quality and at such a price as to be suitable for automotive use also leads to a wide variety of other kinds of heat exchanger that can advantageously use the ribbon. The ribbon is twisted after extrusion, according to the invention, in the following manner. Two sets of traction rollers are provided, the degree of traction on each being adjusted so that the ribbon in passing between the sets of rollers is under a slight tension. Between the two sets of rollers, the ribbon is first heated to above its critical temperature, and then subsequently cooled. The final set of rollers is caused to rotate, about an axis along the length of the ribbon, to impart the required helical twist to the ribbon. Above the critical temperature, the material "loses its memory"; that is to say that if the material is distorted and deformed when at the high temperature, its new shape becomes permanent after cooling, whereas the material gradually returns to its original shape if it is deformed when below the temperature. When the ribbon is above its critical temperature, it lacks structural strength to the extent that all it can support is a very light tensile force. In the invention, the order in which the ribbon encounters the components of the apparatus is therefore important: the ribbon is not strong enough to be fed through rollers unless it is cool, and placing the heater and the cooler in sequence between the two sets of rollers allows this to be the case. In the invention, when the ribbon is hot it does not need to touch anything. The ribbon is also hot (and weak) at the point where it emerges from the extrusion die. Preferably therefore the ribbon is cooled before it is fed into the sets of the pairs of traction rollers. Water baths provide a convenient cooling means, both for the cooling between the sets of rollers, and for the cooling between the extrusion die and the first set of rollers. An exemplary embodiment of the invention will now be described, with reference to the accompanying drawings. " IN THE DRAWINGS Figure 1 is a pictorial view of a twisted plastic ribbon; Figure 2 is a section of an automotive radiator which includes a twisted plastic ribbon; Figure 3 is a diagrammatic view of an apparatus for making the ribbon. The radiator 11 shown in Figure 2 comprises a header tank 12, a bottom tank 13, and a core 14. The core 14 comprises a series of finned tubes 15. The walls 16 of the tubes 15 are circular, and are made of aluminum. A length of twisted plastic ribbon 17 is doubled over, and its two ends are inserted one each into respective tubes 15. All the tubes 15 contain twisted plastic ribbons, arranged in this manner. (In the construction of the radiator 11 the header tank 12. was not secured to the core 14 until after the ribbons 17 had all been inserted in the tubes 15.) The circle circumscribing the twisted ribbon 17 is a little less than the bore of the tube 15 (i.e., the inside. diameter of the walls 16). The ribbon . 17 can thus be easily inserted by hand all the way into the tube 15, yet the ribbon 17 is a snug fit within the tube 15: water substantially cannot pass along the tube without following the helical path created by the twisted ribbon 17. - ' . The ribbon 17 was formed to the helically twisted shape using the apparatus and method of the invention which will now be described. The ribbon is extruded in the conventional way, emerging from an extrusion die 50, as shown in Figure 3. The ribbon at this point is very hot (about 250 degrees C) and very delicate. Before passing through the first set of traction rollers, the ribbon has to be cooled t.o an extent that it can pass through the ° rollers without sticking to them, and without being damaged. This cooling is effected by extruding the ribbon straight into a preliminary cooling means, comprising a bath 54 of water, which is maintained at around 40 degrees C: after the ribbon has passed 5 through the first set of traction rollers, it will be heated again, so there would be no point in cooling the ribbon more than is necessary to give the ribbon just enough strength to pass through the rollers. Upon emerging from the bath 54, the ribbon enters 0 the first set 55 of traction rollers. The traction on the set 55 is arranged to give a slight tension to the ribbon so that the ribbon dips into the water in the bath 54, without touching the bottom of the bath. Next, the ribbon enters a heating coil 56. 5 Conveniently, the heater is electrically powered. Its purpose is to heat the ribbon up above its critical O PI ' temperature, so that when the ribbon is twisted it will retain its twisted shape. The heater should be set at a temperature of about 250 degrees C to achieve this. Once again, the ribbon becomes 5 extremely flimsy at such temperatures. From the heater 56, the ribbon enters a main cooling means, comprising a cold water quenching bath 57, to bring it down to room temperature, when it becomes strong and tough. 10 The now-robust ribbon that leaves the bath 57 is picked up by the final set 58 of traction rollers. The traction on the set 58 is set so as to maintain a • * slight tension on the ribbon between the first set 55 and the final set 58: only a little tension, however, 5 since the ribbon becomes very weak as it passes through the heater 56. In fact, the slight tension described does have the effect of elongating or drawing the.ribbon to some extent, and the extruded cross-section should be made slightly oversized to 0 compensate for the consequential reduction of section. The final set 58. of ' traction rollers is arranged , for rotation about an axis along the length of the ribbon, and it is this rotation that imparts the 5 required helical twist to the ribbon. Since the _O PI " ribbon is at its weakest when it is within the heater 56, most of the twisting takes place at that point. The traction drive to the rollers 58 may be provided by an electric motor: since the set of rollers has to rotate, the power to that motor is conveniently supplied by slip-rings. By having the traction drive independently adjustable from the rotational speed of the set of rollers, the correct degree of ribbon tension for a particular pitch of convolutions may be finely tuned. Once the correct balance is found, the rotational speed and the traction speed can be fixed proportionately by having the one geared from the other. Similarly, the traction speed of the first set 55 of rollers can be geared to that of the final set 58. With the apparatus arranged as described, the ribbon can be manufactured as a continuous operation. There is no need to cut the ribbon into lengths before heating it and twisting it. The above described economies and usefulness of the invention all derive basically from the fact that the twisted ribbon can be made in a continuous process. Of course, the ribbon has to be cut into lengths eventually, but this can be done when the rib " bon has emerged from the final set 58 of rollers, whilst the ribbon is still rotating. A rapid chopping action - cuts the ribbon without causing undue wind-up of the • rotating ribbon. The twisted ribbon is ready for use as soon as it is cut. The material of the ribbon used in the radiator was a general-purpose extrusion-grade homopolymer polypropylene, which has inherently high melt strength, good chemical resistance and good heat stability. For other applications, other materials might be used such as BS, rigid PVC, nylon, or other structural polymers. The benefits that arise from the economical production of twisted ribbon are not only applicable to aluminum round-cored automotive engine radiators. The potentially more efficient heat transfer is always useful, though the twisted ribbon only achieves its full performance when the heat exchanger tubes are round. Also, if high temperature fluids are in use, the ribbon m.ight be unsuitable as the plastic described would be damaged by a temperature of more than about- 160 degrees C. Choice of appropriate material is therefore indicated, depending on the fluids in which the ribbon will work. WI

Claims: CLAIMS 1. Apparatus for producing twisted ribbon, characterized by the following structure:- a first set of traction rollers (55); a heating means (56) for heating the material of the ribbon above its critical temperature; a main cooling means (57) for co ' oling the ribbon; a final set of traction rollers (58); wherein the ribbon is passed through the first set of rollers, the heating means, the main cooling means and the final set of rollers, in that consecutive order; and wherein the final set of rollers is mounted' for rotation about an axis along the length of the ribbon , * to twist the ribbon. 2. Apparatus of claim 1, wherein the traction drives of the respective sets (55,58) of rollers are adjusted so as to maintain the ribbon between the two sets in a state of slight tension. 3. ' Apparatus of claim 2 wherein the apparatus includes a preliminary cooling means (54) for cooling the ribbon; and wherein the ribbon is passed through the -preliminary cooling means before entering the first set (55) of traction rollers. 4. Apparatus of claim 3, wherein the ribbon passes into the preliminary cooling means (54) immediately after it is extruded. 5. Apparatus of claim 4, wherein the main cooling means (57) and the preliminary cooling means (54) comprise respective water baths. 6. Method of producing a twisted ribbon characterized, .by comprising the steps, in sequence, of: passing untwisted ribbon through a first set (55) of traction rollers; heating the ribbon to a temperature above its critical temperature; cooling the ribbon; passing the ribbon through a final set (58) of traction rollers, and rotating the final set of rollers about an axis along the length of the ribbon to twist the ribbon. OMPI "7. Method of claim 6, wherein the ribbon is maintained in a state of slight tension between the sets (55,58) of rollers. 8. Method of claim 7, wherein the ribbon is extruded, and is cooled prior to being fed to the first set (55) of traction rollers. * 9. Method of claim 5 wherein the ribbon is cut into lengths after having passed through the final set (58) of rollers, by chopping the ribbon, whilst the final set of rollers (58) is rotating about the said axis. 10. A length of twisted ribbon that has been made on the apparatus of claim 5. 11. A length of twisted ribbon that has been made according to the method of claim 9. 12. Ribbon of claim 10,- wherein the material of the ribbon is an extrusion grade homo-polymer polypropylene, and wherein the ribbon is inserted into round-cored aluminum tubes of an automotive radiator. O q H 13. Ribbon of claim 10, wherein the ribbon is inserted into the tubes of a heat exchanger. O PI

Inventors: BLOOMFIELD GEORGE ALBERT, BLOOMFIELD, GEORGE, ALBERT
Applicants: ASQUITH ANTHONY, PV TRIM LTD, ASQUITH, ANTHONY, P.V. TRIM LIMITED
Application year: 1984
Main classification: B29C53
IPC codes: B29C53, B29L31, F28F1, F28F13