Abstract:
A process for making a multilayer plastic tube heat exchanger with a ribbon of molten polymer being poured over layers of tubes, and then another layer of tubes being added and another ribbon of polymer. Heat exchanger structures so formed are also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/324,004 filed Sep. 21, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to processes for the manufacture of plastic tube bundles useful for heat exchangers. More particularly, this invention relates to such processes incorporating polymer to affix such tubes into desired positions. 
     BACKGROUND OF THE INVENTION 
     Many heat exchangers are constructed from tubes and tube sheets. Tube sheets are essentially flat sheets provided with holes to admit the tubes. In assembly of the heat exchanger, tubes are inserted into these holes and are then bonded to the tube sheet, by brazing or other methods in the case of metal cores, or by thermal bonding processes involving melting in the case of plastic cores. The process of inserting small diameter, thin wall plastic tubes into the holes of the tube sheet, also known as threading, can be mechanically demanding and can limit the rate of heat exchanger production and economic attractiveness. 
     Alternatively, in the case of plastic cores, the tube sheet can be formed by stacking a number of saddle-like layers. Each element is bonded to the adjacent element(s) by a thermal process such as vibration or ultrasonic welding. Such a process is described in EP 0 673 496 B1. While this process avoids the need for a threading step, the mechanical strength of the resulting tube sheet is limited by the number and quality of the individual bonds required to build the tube sheet. 
     It is an object of the present invention to provide a process for the fabrication of plastic tube bundles for heat exchangers which is adaptable to incorporate a variety of shapes of tubes. A feature of the present invention is the improved speed of fabrication of the heat exchanger assembly versus conventional techniques. It is an advantage of the present invention to provide a process that does not require acute positioning of the tubes prior to securing them together. These and other objects, features and advantages will become apparent upon having reference to the detailed description herein. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for fabricating a plastic tube bundle for a heat exchanger structure comprising: 
     providing a layer of polymeric tubes having a plurality of tubes held in proximity and side by side to each other, 
     pouring molten polymer over the tubes sufficient to embed the tubes therein, 
     positioning an additional layer of such tubes on the polymer while said polymer is still soft, so that the tubes are embedded in the layer, 
     pouring additional molten polymer over the additional layer of tubes, 
     repeating the last two steps at least once to form a bundle, 
     cooling said bundle to harden the polymer, and 
     fashioning the tube bundles including the ends of the tubes encased in said polymer to communicate with one or more headers to collect heat exchange fluid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the heat exchanger fabrication sequence of the instant invention, including apparatus useful therefor; and 
     FIG. 2 shows a side view of the apparatus depicted in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Having reference to FIG. 1, the exchanger fabrication process is illustrated generally at  10  in a series of sequences. The key piece of equipment is the exchanger mould  12 . It is a metal box, the size of the finished exchanger, and has a movable inside plate  14 . The mould  12  can be water cooled if necessary. 
     Sequence (1) shows the polymer (in this case, formed as a ribbon  16 ) being extruded from the extrusion die  17  into the waste tray  18 , the exchanger mould  12  in position with the movable inside plate  14  at the top. 
     Sequence (2) shows the exchanger mould  12  traversing into the polymer ribbon  16  and a layer of polymer being put down on inside plate  14 . 
     Sequence (3) shows the tube transfer mechanism  20  positioning the tubes  22  over the mould  12 . 
     Sequence (4) shows the mould  12  traversing to the right to encase the tubes  22  while they are being held in position by the transfer mechanism  20 . 
     Sequence (5) shows the mould  12  traversing to the left while the inside plate  14  is indexed down one tube layer. The tube transfer mechanism  20  is being refilled. 
     Sequence (6) shows the next layer of tubes  22  being put into position ready to be encased by the polymer ribbon  16  as the exchanger mould  12  traverses to the right. 
     The process is repeated until the desired number of tube layers have been laid down and sufficient polymer ribbon  16  laid down to complete the exchanger. The exchanger mould  12  is then indexed out of the path of the polymer, the exchanger is allowed to solidify, and is then pushed up out of the mould  12  by the inside plate  14  and removed. In a production facility it would preferable to have two moulds  12  side by side that could be indexed so that as one exchanger is cooling, the other mould  12  is being filled. The process would be continuous. 
     FIG. 2 shows a side view of the process depicted in FIG.  1 . It is readily seen that as the Sequences (1)-(6) as above are repeated, a heat exchanger including multiple layers of tubes  22  encased in polymer ribbon  16  is formed. Cooling air (depicted by arrows  24 ) may be applied towards or through inside plate  14 . Moreover, a vacuum (depicted by arrow  26 ) may also be applied in the vicinity of the next layer of tubes  22  (eg those of Sequence (6)) using conventional means. 
     It will be readily appreciated by those having skill in the art that any number of variations and adaptations on the process techniques described herein may be introduced, and these are contemplated as within the scope and purview of this invention. For example, a supply hopper may be added to the assembly, filled with precut tubing. This hopper may be vibrated if necessary to assist in feeding. The tubing exits the hopper into the tubing transfer mechanism  20 , which has grooves to receive the tubes  22 . The aforementioned vacuum is applied to the mechanism so that the tubes  22  are held into grooves set into the transfer mechanism  20 . The transfer mechanism  20  is traversed under the hopper until all the grooves are filled, then retracted and inverted, the tubes  22  being held in position by the vacuum. The end sections of the tubes  22  may pass through a treatment zone of either flame or a mixed gas plasma discharge (such equipment is made by “Enercon”) which is not shown. 
     A preferred technique for laydown of the polymer onto the tubes  22  is to fashion the polymer as a ribbon  16 . For example, the polymeric ribbon  16  may generally be sized to cover all or a significant portion or a desirable portion of the tubes  22 . The ribbon bonding of tubing presents the opportunity to simplify heat exchanger core manufacture by elimination of the need for injection moulding tube sheets and the need for threading and bonding large numbers of small tubes into prefabricated holes. It creates a system that can form an exchanger from non-round tubes with profiles such as oval or triangular. 
     The key to this process would be the choice of materials and the control of temperatures. The polymer of the ribbon  16  has to be of sufficient fluidity to flow around the tubes  22  but have sufficient viscosity as to not flow too much latterly before solidifying. The latteral flow can be controlled by the ribbon extrusion temperature, the mould temperature, and the cooling air rate and temperature. The temperature of the ribbon  16  can be lower than the melting point of the tubes  22  but the closer to that point the bonding is better. The flame or plasma treatment of the tubes  22  is very important in order to get a good bond. 
     If the tubes  22  are nylon 66, or nylon 66 coated, then nylon 6 is a good candidate for the material of the ribbon  16 . 
     The invention will become better understood upon having reference to the following example. 
     EXAMPLE 
     In this example a simple apparatus was constructed, generally as depicted in FIGS. 1 and 2. The mould has a platform or bottom, which is movable downward. The mould was traversed back and forth under the molten polymer ribbons building up a layer of polymer. Then a layer of tubes was added and the mould was traversed under the ribbons so the tubes were encapsulated in polymer, the platform was indexed down one tube layer thickness, and an other tube layer placed on top. This layer was encapsulated in polymer by traversing the mould under the ribbons. This was repeated until all the tube layers had been laid down, then the mould was traversed several more times to build the polymer layer up. The mould was withdrawn from under the ribbons, and after a short cooling period the tube bundle was removed, and quenched further in cold water. The water was blown off. Spacer bars which had held the tube layers, were removed. The ends were cut off on a band saw, then smoothed using a high-speed edger or planer. 
     The resulting heat exhanger included 7 layers of 24 tubes each, for at total of 168 tubes (each having an outer diameter 3.7 mm and a wall thickness 0.2 mm). This assembly had the following dimensions: 160 mm in width and 50 mm in thickness, with an average width of each encapsulating layer (meaning the width of the ribbon itself) of 10 mm. Each tube was 180 mm in length. The tubes were made of Zytel® 42A polyamide 6—6 (available from E. I. DuPont de Nemours &amp; Co.) and the encapsulating polymer was nylon 6 available from BASF as Ultramid BS700-A. 
     The polymer supply was from a conventional 30 mm extruder equipped with a 229 mm flat film die which was deckled off so as to give 12.7 mm wide ribbons at each end. 
     Apparatus used in the process described herein should preferably incorporate a mechanical traversing device whose stroke length and rate are controllable and adjustable. Additional equipment considerations include a positive indexing device for the mould platform, a tube placement device, a tube end positioning device, and a top mould plate which is positioned to shape the molten polymer after the encapsulation is completed. 
     The system also preferably incorporates a treatment zone where the ends of the tubing in the region where the ribbon is to adhere are treated by a mixed gas plasma discharge. Flame treatment can be more difficult to control. 
     Instead of a conventional extrusion die to make the ribbon, one can use a couple of heated tubes to convey the polymer. The exit shape may not be that important, i.e. a round hole may be adequate. 
     If it is desirable to provide support to the tubing between the ends (eg for prevention of tube vibration or flutter), three, four or any number could be used. For example, molten polymer may be put down at selected intermediate locations in addition to locations near the tube ends to provide additional support to the tube bundle.