FIELD OF THE INVENTION This invention relates to a tube finning machine and to a tube finning method.

Tube finning machines are designed to mount a number of fins onto a number of tubes. The machines are commonly used to make heat exchangers and the present invention is expected to find its greatest utility in heat exchanger applications. The following description will therefore relate primary to those applications, but the use of the machine for other applications is not thereby excluded.

BACKGROUND TO THE INVENTION

Often it is necessary to cool a working fluid, and it is known for this purpose to use a heat exchanger. Heat exchangers are made in many different sizes, are used with many different working fluids, and utilise many different fluids as the coolant.

Heat exchangers usually comprise a number of tubes suspended between two tube plates, though it is known to use U-shaped tubes with each tube connected at opposite ends to a single tube plate. Typically, the working fluid flows through the tubes, whilst the coolant passes around and between the tubes, the working fluid giving up latent heat (by way of the tubes) to the coolant flowing around the tubes.

Each tube will typically carry a number of external fins, sometimes called "extended surface members", which are mechanically coupled to or integral with the respective tube. The fins increase the available surface area for heat transfer, but also cause an increase in the pressure drop as the coolant passes between and around the tubes. The heat exchanger designer will typically seek to increase the density of the fins so as to increase the heat exchange, without exceeding a maximum permissible pressure drop. Often, each fin will engage more than one tube, with the fins substantially filling the space between the tubes. During the manufacture of a heat exchanger the manufacturer will often make sub-assemblies comprising a chosen number of tubes fitted with a chosen number of fins. These sub-assemblies are referred to herein as heat exchanger blocks, and are sometimes called fin blocks. The heat exchanger is assembled by securing the desired number of heat exchanger blocks to the tube plates.

It is a requirement for industrial heat exchangers to minimise the cost of manufacture. The time taken to manufacture the heat exchangers, and in particular the time for which the manufacturing or assembly line is utilised for a particular heat exchanger, is a significant proportion of the cost of manufacture. Most heat exchanger manufacturers therefore wish to reduce the time taken to manufacture their heat exchangers, and also seek alternative materials and methods in order to reduce the manufacturing cost.

Another requirement for industrial heat exchangers is to minimise their size and weight without compromising their heat exchange performance. Whilst a larger heat exchanger will typically provide a greater rate of heat exchange from the working fluid to the coolant, heat exchanger designers typically seek to minimise the size and weight of the heat exchanger so as to make it easier to package the heat exchanger alongside its related componentry.

DESCRIPTION OF THE PRIOR ART

Heat exchangers are most often constructed from metallic materials, i.e. metallic fins fitted to metallic tubes. Metals are commonly used because of their good thermal transfer properties. To secure a fin to the tube it is known to provide an aperture in the fin and to weld or braze the fin onto the tube. This method of manufacture suffers from several significant disadvantages. Firstly, the materials which can be used for the tubes and the fins are limited to those which can be welded or brazed. Secondly, the grade of the materials used, such as for example the minimum wall thickness of the tube, is determined by the requirement to withstand the welding or brazing operation (so that a relatively thick tube may need to be used, whereas a thinner tube would enhance the heat exchange performance). Thirdly, the welding or brazing operation raises the temperature of the tubes and fins sufficiently to heat treat the materials, the final product being softer than the starting materials - the starting materials must therefore be chosen so that the final product meets the desired material requirements. Fourthly, the requirement for a welding or brazing operation adds time and cost to the manufacture of the heat exchanger. In an alternative known method of manufacture the fins are initially located as a substantially loose fit upon the tubes and the tubes are thereafter mechanically expanded by a specialised expanding machine into thermal engagement with the fins. This method of manufacture also has a number of significant disadvantages. The first disadvantage is shared with the first method stated above, namely that the material of the tube in particular is limited to those which can be mechanically expanded. The second disadvantage is also shared with the first method as stated above, namely that the minimum thickness of the tubes is determined by the requirement for expansion - very thin tubes, which might be particularly suitable for heat exchangers, cannot be used if there is a possibility that they would split during the expansion process, or be sufficiently weakened by the expansion process to fail in service. The third disadvantage is that the fins are sometimes pushed along the tubes during the expansion process, so that the resulting fin spacing or density is not always consistent along the length of the tubes - this can have a significant effect upon both the heat exchanger performance and the pressure drop of the coolant. One arrangement using this method utilises fins with integral spacer means. The integral spacer means avoids the third disadvantage, but the first and second disadvantages present a significant concern to heat exchanger manufacturers who might wish to use this method.

US patent application 3,482,999 discloses a tube finning machine and method of this type. The fins are located upon a sliding head which moves vertically down along the tubes carrying a chosen number of fins. The fins move along the tubes primarily under the influence of gravity, but it is disclosed that the tubes can be vibrated to assist the movement of the fins.

An alternative and improved method of mounting fins upon heat exchanger tubes (and thereby manufacturing a heat exchanger block) is described in WO 96/35093. That document discloses a tube finning machine in which fins have apertures which are sized to closely match the outside diameter of the tubes. A significant force is therefore required to push or press the fins along the tubes, this force being provided by a linear motor. The linear motor has the accuracy required to ensure that the fins are accurately and consistently spaced along the tubes. The close matching of the fin apertures to the tubes means that the required thermal engagement between the tubes and the fins is provided without any subsequent welding or brazing step, and without any subsequent expansion of the tube. The materials of the tubes and/or fins is therefore less limited than the earlier-described methods, and the machine and method can be used with a mixture of different materials for the tubes and/or fins in a single heat exchanger block.

For the avoidance of doubt, the reference herein to the size of the fin aperture(s) closely matching the size of the tube(s) means that the thermal engagement between a fitted fin and the tube is sufficient without any subsequent welding, brazing or expansion step. The invention does not, however, exclude the possibility of a subsequent brazing step (for example) if this is desired for a particular application.

The apertures in the fins in many embodiments described in WO 96/35093 are formed with collars which engage the tubes in use and enhance the heat exchange performance. Whilst it is primarily intended that the linear motor will determine the position of each of the fins, it is often desired that the collar of one fin will engage the collar of the adjacent fin, and it is disclosed that in some heat exchangers the fin spacing can be determined by the engagement between adjacent fins. Since the required fin spacing is usually predetermined by the heat exchange and pressure drop required, in such embodiments the length of the collars is designed to provide the desired fin spacing.

Another tube finning machine for manufacturing heat exchanger blocks is disclosed in WO 02/30591 . That document discloses the use of a cartridge mechanism into which a large number of fins can be loaded, and which can thereafter be pressed onto the tubes together. This machine and method can provide a considerable reduction in the time taken to manufacture, and therefore the manufacturing cost of, certain heat exchanger blocks.

Another tube finning machine is disclosed in WO 2012/107757. That application describes a machine in which one or more tubes are aligned substantially vertically with their free ends uppermost. One or more fins are placed onto the top ends of the tubes, the fins again having apertures of a size to closely match the outside of the tubes so that they must be pressed down into the desired location upon the tubes. The tubes are vibrated during the finning process, the vibration reducing the force required to move the fin(s) along the tube(s).

Whilst the tube finning machine of WO 2012/107757 has proved to be very successful, it is desired to provide a more automated tube finning machine which shares the vibratory action of the machine of WO 2012/107757.

SUMMARY OF THE INVENTION

According to the invention there is provided a tube finning machine for mounting a number of fins onto a number of tubes, the fins having an aperture for each of the tubes, the size of the aperture closely matching the size of the tube, the machine having:

a base,

a tube support mounted upon the base, the tube support having means to locate the ends of a chosen number of tubes, a pressing plate with a number of openings therethrough, the openings being adapted to allow the pressing plate to move along the chosen number of tubes in use,

a drive means for moving the pressing plate along the tubes in use, the pressing plate being movable in a driving direction towards the tube support and in a reversing direction away from the tube support,

an oscillator adapted to impart vibrations to the tubes,

a detector, and

a controller connected to the detector and to the drive means, the controller being adapted to actuate the drive means to drive the pressing plate in its driving direction, and to switch the drive means to drive the pressing plate in its reversing direction when the detector indicates that the pressing plate has reached a limit of movement towards the tube support. Whilst the detector could measure the position of the pressing plate directly (as is disclosed in WO 96/35093), preferably the limit of movement of the pressing plate is determined to be the cessation of movement towards the tube support. In other words the detector is able to detect when the pressing plate has stopped moving along the tubes. The pressing plate will stop moving when the fin(s) being pressed by the pressing plate either engage the support plate (for the leading fins to be placed onto the tubes) or engage fins which have previously been pressed onto the tubes.

Preferably, the detector is adapted to detect the position of the pressing plate or another related part of the machine. The detector must therefore be able to differentiate between movements of the pressing plate (or another related part of the machine) which are caused by the drive means, and those which are caused by the vibrations. In embodiments in which the limit of movement is the cessation of movement towards the pressing plate, the detector must be able to recognise that the pressing plate has stopped moving towards the tube support despite the fact that the tube support and pressing plate will be vibrating due to the oscillator. Preferably the tube support is mounted to the base by way of a number of resilient bushes, so that the vibrations imparted to the tube support are substantially isolated from the base. In such embodiments the tube support vibrates relative to the base. Also, however, the drive means can cause the tube support to move relative to the base. The controller and detector can therefore be adapted to determine that the pressing plate has reached its limit of movement indirectly by detecting movement of tube support.

The oscillator causes the tube support to vibrate and the maximum amplitude of those vibrations can be determined. It can be arranged that the drive means provides a force which is sufficient to move the tube support (against the bias of its resilient bushes) by a distance which is greater than the amplitude of the vibrations. It will be understood that when the pressing plate has pushed the fins into engagement with the tube support (or into engagement with previously fitted fins) the drive means continues to drive the pressing plate towards the tube support, thereby compressing the resilient bushes. The movement of the tube support caused by the drive means will be greater that that caused by the oscillator, and the controller can be set up to ignore movements of the tube support which are equal to or less than the maximum amplitude of the vibrations, but can cause the drive means to switch from the driving direction to the reversing direction when a greater magnitude movement is detected. Automated return movement of the pressing plate is therefore possible.

The detector is preferably a sensor mounted upon the base and responsive to the location of a position element carried by the tube support. The detector can be a linear transducer for example, movement of the tube support relative to the base being detected as an electric current.

Mounting the detector on the base reduces the mechanical vibrations which must be catered for, i.e. the presence of resilient bushes between the tube support and the base reduces the vibrations experienced by the base, and thereby reduces the structural robustness required of the detector mounted thereupon. Preferably, the drive means is pneumatic. Preferably also the oscillator is pneumatic. The same pressurised air supply can be used for the oscillator and the drive means so that the mechanical complexity of the machine is reduced. The use of a pneumatic drive means also has the advantage that the pneumatic supply to the vibrating parts of the machine can be made by way of suitable flexible hoses.

In an alternative embodiment the drive means is mechanical, ideally a ball screw, and it will be understood that any suitable drive means may be utilised with the invention.

Notwithstanding that the preferred machine has a detector adapted to determine the limit of movement of the pressing plate indirectly by way of the tube support, it would alternatively be possible to mount respective parts of the detector upon the tube support and the pressing plate, and to measure the separation between these components directly. The vibrations could then be substantially ignored as the tube support and the pressing plate will generally vibrate synchronously, and there will be no relative movement once the pressing plate has reached its limit of movement. In such embodiments the detector will also be vibrating, however, and must therefore be sufficiently robust to withstand the vibrations during use.

In another embodiment the controller could be adapted to ignore movements of the tube support (or pressing plate) above a predetermined frequency (the predetermined frequency being similar to that of the oscillator). Such embodiments reflect the fact that movements of the tube support which are caused by the pressing plate and drive means will not be oscillations, and can be detected as lower frequency movements than the vibrations caused by the oscillator. Despite the inherent difficulty in detecting the limit of movement of the pressing plate whilst the pressing plate is being vibrated, the inventor has conceived several methods of doing so, which methods are sufficiently reliable and robust to be usable in a tube finning machine. The inventor can therefore provide a tube finning machine in which the movement of the pressing plate can be further automated, thereby increasing the efficiency of the tube finning operation.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described in more detail, by way of example, with reference to the accompanying drawing which shows a side view of part of the tube finning machine according to the invention.

DETAILED DESCRIPTION

The tube finning machine 10, the major components of which are shown in the drawing, comprises a base 12 which is substantially rigid. The machine may be of a size for the base 12 to be mounted upon a workbench, or the base may be secured directly to the floor of the workspace.

The base has three resilient bushes 14 on which is mounted a tube support 16. The tube support 16 comprises a first plate 18, a block 20 and a second plate 22. The multi-part construction of the tube support 16 enables the machine 10 to be readily adapted to suit different sizes and arrays of tubes so that different specification heat exchanger blocks can be made on the same machine. Specifically, the tubes 24 (only four of which are shown in the drawing) are securely mounted to the block 20 by way of suitable openings and clamps in the block 20. The precise method of mounting the tubes can vary widely and is not part of the present invention; any suitable method of mounting the tubes can be chosen. The block 20 can mount an array of tubes, and it will be understood that in most heat exchanger blocks there will be significantly more tubes 24 than the four shown in the drawing. The size of the tubes, the number of tubes, and the arrangement of the tubes, are predetermined by the heat exchanger designer and the block 20 is formed to accommodate the tubes in the chosen array. The block 20 is releasably secured to the first plate 18 and may be removed and replaced by a different block when it is desired to use the machine to make a different form of heat exchanger block.

The second plate 22 is optional and removable and is used in this embodiment to increase the spacing of the leading fin 26 from the ends of the tubes 24.

An oscillator 28 is mounted upon the first plate 18. In this embodiment the oscillator is pneumatically actuated and is supplied with pressurised air through flexible hose 30. The oscillator contains an eccentric weight which is caused to rotate by the flow of pressurised air, the rotating eccentric weight causing vibrations of the tube support 16 and the tubes 24 mounted thereupon. In this embodiment the tubes 24 are mounted with their longitudinal axes substantially vertical, and this is the preferred arrangement as the force of gravity assists the movement of the fins 32 onto the tubes. However, this orientation is not essential and the machine can be used with the longitudinal axes of the tubes substantially horizontal, or at some intermediate angle, as desired. Less preferably, the tubes can be oriented with their free ends below their clamped ends.

The drawing shows a large number of fins 32 mounted upon the tubes 24, so that the machine 10 has already undertaken several cycles of operation. The number of fins 32 which must be added to the tubes 24 and their spacing along the tubes 24 will be predetermined by the heat exchanger designer.

A pressing plate 34 is used to drive the fins 34 along the tubes 24. The pressing plate 34 is connected to a piston 36 of a pneumatic ram (the cylinder of which is not shown), the piston 36 therefore providing part of the drive means of the machine. It will be understood that the pressing plate 34 has a number of openings therethrough which are somewhat larger than the tubes 24. The pressing plate 34 can therefore pass along the tubes 24 with little or no frictional resistance from the tubes. In this embodiment the openings in the pressing plate completely surround the tubes as they are moved therealong. In other embodiments the openings in the pressing plate are open-sided and do not completely surround the tubes.

The base 12 carries a detector 40 which can determine the location of a position element 42 carried by the tube support 16. As shown schematically in the drawing, the detector 40 is connected to a controller 44, the controller also being connected to the drive means.

In the present embodiment the operation of the machine is partially automated, and in particular the machine is able to undergo cycles of operation in which one or more fins 32 are driven along the tubes 24 by the pressing plate 34 and then the pressing plate automatically returns to a start position, without operator involvement. In its start position (which is not shown in the drawing) the pressing plate 34 lies above the free ends of the tubes 24. The gap between the pressing 34 plate and the free ends of the tubes 24 is sufficient to allow one or more fins to be placed onto the free ends of the tubes. The number of fins 32 which can be pressed onto the tubes 24 in each cycle will be determined in advance, and can be one or more as desired for a particular heat exchanger block.

It will be understood that the fins 32 are a tight fit onto the tubes 24 so that no subsequent expansion of the tubes, or brazing of the fins, is required after the fins have been pressed into the tubes. The engagement between the fins 32 and tubes 24 is therefore sufficiently tight to provide the heat exchange required without any subsequent operations. The fins 32 may carry collars to enhance the heat exchange between the tubes 24 and the fitted fins 32.

It will also be understood that the fins 32 either have integral spacers so that each fin is automatically separated from its neighbours by a spacing determined by the height of the spacers. Alternatively, a removable temporary spacer is used to separate the fins during their location upon the tubes, the spacer being removed once the fins are in their desired positions. Both of these methods of providing the predetermined spacing between adjacent fins can be used with the machine 10.

Because of the tight fit of the fins 32 onto the tubes 24, the free ends of the tubes will preferably be fitted with tapered bullets (not shown) which facilitate the initial location of the fins. The fins 32 can be located at the free ends of the tubes 24 manually by the operator, or by an automated fin delivery apparatus, which is beyond the scope of the present invention. As shown, the fins 32 are common fins in that each fin engages all of the tubes 24, and whilst this is preferred it is not essential to the present invention.

When the chosen number of fins 32 has been located at the free end of the tubes 24, the drive means is actuated to drive the pressing plate 34 downwardly as viewed, towards the tube support 16. The vibration of the tubes 24 caused by the oscillator 28 reduces the force required to drive the fins 32 along the tubes 24.

The pressing plate 34 will continue to move towards the tube support 16 until the limit of movement is reached. For the first operating cycle the limit of movement is reached when the leading fin 26 engages the second plate 22. For all subsequent cycles the limit is reached when the fin(s) 32 being pressed engage previously fitted fins.

The advantage of using a pneumatic ram for the drive means is that the drive means will automatically stall when the limit of movement is reached, without damage being caused to the fins 32 or any part of the machine 10. It is, however, arranged that the force imparted by the drive means is sufficient to compress the resilient bushes 14 by a greater amount than is caused by the vibrations. Thus, whilst the resilient bushes 14 will be caused to compress and extend due to the vibrations caused by the oscillator 28, the compression caused by the drive means is significantly greater.

In embodiments using an alternative drive means such as the ball screw previously mentioned, it can be arranged that the force provided by the drive means is sufficient to compress the resilient bushes by an amount greater than that caused by the vibrations, but is insufficient to damage or deform the fins. Thus, a mechanical drive means can similarly be arranged to stall before damaging the fins.

The detector 40 measures the compression of the resilient bushes 14 by measuring the position of the first plate 18 relative to the base 12. The controller 44 can distinguish between the vibration-induced movements of the tube support 16 and the movements caused by the drive means when the pressing plate 34 has reached its limit of movement. When it is determined that the limit of movement of the pressing plate 34 has been reached, the controller 44 automatically switches the drive means so that the pressing plate 34 is moved back to the start position, and the cycle can be repeated. In this embodiment the detector 40 directly measures the position of the tube support 16 relative to the base 12, and the detector 40 can therefore be a linear transducer. Alternatively, the controller 44 could respond to the frequency of the movements of the tube support 16, i.e. it could distinguish between the vibrations of the tube support caused by the oscillator 28 (and which have a known frequency) and the slower movement caused by the drive means.

Clearly, other forms of detector could alternatively be used if desired. For example, an optical sensor could detect a light beam, the light beam being interrupted by the tube support when the tube support is driven to a certain position by the drive means. A light beam could alternatively be reflected from the tube support, or from the pressing plate.

A suitable frequency of operation for the oscillator 28 has been found to be 73Hz, a suitable pneumatic oscillator operating at this frequency being readily available. It is nevertheless expected that a wide range of frequencies could be used, perhaps suited to different material combinations of the fins and tubes.