Abstract:
The present invention relates to a plastic tube heat exchanger suitable for refrigerant use is provided by wrapping a layer of tubes around a curved surface, on a base which is generally perpendicular to the surface. Several to many tubes can be in each layer. Then, further layers can be wrapped continuously by overlaying the first layer. Heat exchangers, including refrigerant apparatus, condensers and evaporators, can be made in this configuration. Processes and articles of manufacture are parts of the invention.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority under 35 U.S.C. §119 from U.S. Provisional Application Ser. No. 60/238,507 (filed Oct. 6, 2000), which is incorporated by reference herein for all purposes as if fully set forth. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Plastic tube heat exchangers are known in the art, however, it has proven to be difficult to use existing technology to make practical refrigerant heat exchangers by the existing techniques.  
           [0003]    Some of the patents disclosing plastic tube heat exchangers include U.S. Pat. No. 4,484,624—Vleggaar et al., 1984, and U.S. Pat. No. 4,867,233—Gemeinhardt, 1989. The former discloses various ways of configuring tubes inside a heat exchanger, including winding overlapping spirals, and the latter includes ways to gather together the ends of a bundle of tubes and connect them to a common header.  
           [0004]    Refrigerant heat exchangers include condensers and evaporators. Typically an evaporator consists of a number of finned metal tubes, typically ¼ inch (6.35 mm) outer diameter with aluminum fins, the tubes having a greater internal diameter than the liquid refrigerant inlet tubes, to allow for expansion and cooling, and having a specified length to allow for complete evaporation to the gaseous phase. There are other limitations with metal heat exchangers with fins in other shapes, such as those wound in the shape of open bee hives, with a fan in the middle.  
           [0005]    Condensers are configured in an analogous manner, but usually must operate at higher pressures to effect conversion of the gaseous refrigerant to a liquid phase. When attempting to design a refrigerant-capable exchanger from plastic tubing, a number of factors must be considered:  
           [0006]    a) Refrigerant to air exchangers have a relatively low flow rate of refrigerant inside the tubes. It is therefore possible to design exchangers with fairly long tubes, as limited by pressure drop considerations. The use of long tubes allows for a reduction in the number of tube connections, for a given exchanger surface area.  
           [0007]    b) The need to handle high pressure further requires that the polymeric tubing for evaporators and condensers should be small in diameter.  
           [0008]    c) The use of small diameter (1.5 mm OD) further provides a large surface area for heat transfer, per unit of volume, with the potential of matching the heat duty of a metal exchanger, its ability to handle a certain amount of heat exchange in a period of time, in a smaller volume.  
           [0009]    By taking these factors into account, it has been determined that individual tube lengths of 10-30 ft (3.3-10 m) are suitable for the refrigerant-air exchangers. For practical reasons it is desirable to form tubing of these lengths in some sort of coil, in order to expose a large surface area to a relatively localized air stream.  
           [0010]    Earlier work on polymeric refrigerant-to-air exchangers involved coiling some woven tubing and blowing air through the face of the coil as a replacement for a finned-metal exchanger of rectangular shape. One of the drawbacks of this design is that the volume in the centre of the coil, inside the innermost winding, is unavailable for heat transfer and is mainly wasted space. To minimize the wasted space in the centre, it is desirable to use tubing with a very small coiling radius to avoid kinking the tubing, and this can impose limitations on the choice of materials or dimensions of the tubing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING(S)  
       [0011]    [0011]FIG. 1 is a plan view of the process of the invention, starting to lay down the tubes.  
         [0012]    [0012]FIG. 2 is a perspective schematic view of the process when the first layer of tubes has been laid down and the tubes are being elevated to start the second layer.  
         [0013]    [0013]FIG. 3 is a plan view of a plastic tube heat exchanger of the invention which could be a refrigerant heat exchanger, including a condenser and an evaporator.  
         [0014]    [0014]FIG. 4 is an elevation view of a plastic tube heat exchanger of the invention, showing what FIG. 3 would look like generally in elevation.  
         [0015]    [0015]FIG. 5 is a side view of a spacer used in the invention.  
         [0016]    [0016]FIG. 6 is a plan view of the spacer of FIG. 5. 
     
    
     BRIEF SUMMARY OF THE INVENTION  
       [0017]    The present invention provides a plastic tube heat exchanger utilizing for separation of one heat exchange fluid from another heat exchange fluid multilayered plastic tubes wherein at least one of the layers is at least 50% by weight of polyamide resin and at least one of the layers is at least 50% by weight thermotropic liquid crystal polymer (LCP) blended with other polymer to make it tough enough so that in the form of 1.5 mm outside-diameter tube it can be bent around a radius of 12.5 mm without cracking or delaminating. Preferably, the tubing can withstand an internal pressure of 10,300 kPascals without bursting. In preferred embodiments, the LCP is toughened by blending with a useful amount of rubber or other low-crystallinity polymer in amounts of about 2 to about 30 percent by weight of the total weight of the LCP and rubber or other polymer, preferably about 5 to about 20 percent by weight. Preferably the rubber or other polymer has on it reactive functional groups such as epoxide groups.  
         [0018]    A preferred embodiment of the heat exchanger of the invention provides a configuration made possible by the use of the materials described herein, with a set of at least three plastic tubes positioned around a surface having the shape of a closed curve and on a multiplicity of spaced-apart spacers which hold said tubes in a spatial relation to each other, said spacers providing a generally planar base, to form a layer of tubes, said base being generally perpendicular to said surface, wherein the set of tubes is positioned on said base in a generally planar relation to form a layer, with a first tube in said layer having an inward side adjacent said surface and an outward side opposite the inward side, a second tube in said layer proximate the first tube and on the outward side of said first tube opposite said surface, and each succeeding tube on the spacers with an inward side proximate the outward side of previous tube, with a first end of each of said tubes projecting off the base so that they can be joined together in a first header,  
         [0019]    where the layer of tubes is about to reach the part of said tubes that projects off of said base, the elevation of the set of tubes rises relative to the base so the next layer of tubes lies on the first layer of tubes with the first tube adjacent said surface, with a multiplicity of layers, each overlaying the previous layer to form a group of layers, the second end of each tube projecting away from said group, and the first end of said tubes joined together to form said first header and said second end of said tubes joined together to form a second header.  
         [0020]    Alternative embodiments provide for the spacers in succeeding layers being held together by columns at the inner ends of each spacer in a layer and preferably additional columns also hold together the outer ends of each spacer. Alternatively, instead of using one or both columns, a spacer could be attached to the one below it between the tubes in a layer, such as by snap-on connections.  
         [0021]    In various embodiments, the surface is rounded or generally circular or has a curvilinear shape other than rounded, such as a shape generally in the nature of a figure eight.  
         [0022]    Preferably at least three spaced-apart spacers are provided on top of each layer, configured so as to provide spacing both between tubes in a layer and between layers, with the spacing within a layer generally being less than the spacing between layers.  
         [0023]    In further preferred embodiments, the steps are repeated to form a multiplicity of tube groups, each with its own terminations, each row has from 3 to 100 tubes, preferably from 15 to 30 tubes, more preferably about 20 tubes, each tube group has from 4 to 10 layers of tubes, and the number of tube groups provided is from 3 to 10.  
         [0024]    The heat exchanger can be a refrigeration heat exchanger, such as a condenser or an evaporator.  
         [0025]    Also, as a process for preparing such plastic tube heat exchangers, the invention provides a method of making a plastic heat exchanger involving winding a set of at least three plastic tubes in a helical manner around a surface having the shape of a closed curve defining an aperture, with said tubes on a multiplicity of spaced-apart spacers which hold said tubes in a spatial relation to each other, said spacers providing a generally planar base, to form a layer of tubes, said base being generally perpendicular to said surface, wherein the set of tubes is positioned on said base in a generally planar relation to form a layer, with a first tube in said layer having an inward side adjacent said surface and an outward side opposite the inward side, a second tube in said layer proximate the first tube and on the outward side of said first tube opposite said surface, and each succeeding tube on the spacers with an inward side proximate the outward side of previous tube, with a first end of each of said tubes projecting off the base so that they can be joined together in a first header, and with the steps of:  
         [0026]    arranging said tubes in the described configuration,  
         [0027]    positioning said tubes relative to each other so that the layer of tubes winds around the surface,  
         [0028]    when the layer of tubes is about to reach the part of said tubes that projects off of said base, adjusting the elevation of the set of tubes relative to the base if needed so the next layer of tubes lies on the first layer of tubes with the first tube adjacent said surface,  
         [0029]    repeating the previous step so that a group of layers having a multiplicity of layers is formed, each overlaying the previous layer,  
         [0030]    providing a termination of said set of tubes to provide a second end of each tube, and  
         [0031]    joining together the first end of said tubes to form said first header and joining together said second end of said tubes in a second header.  
         [0032]    Preferably the surface is round or has a curvilinear shape other than round, such as a shape generally in the nature of a figure eight.  
         [0033]    Additional preferred methods provide the apparatus of the invention, described above.  
         [0034]    Preferably the plastic of the tubes is thermoplastic, at least when it is being formed into the configuration of the heat exchanger. Normally it would not be crosslinked after forming, but in some circumstances it may be desirable to do so.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    The present invention provides heat exchangers having plastic coils constructed from materials which permit the construction of complex geometries wherein the preferred geometry of the coil would be difficult to produce from existing metal tube and fin constructions. It further provides for specific coils and methods of manufacture of these coils which take advantage of the properties of these materials. In a preferred embodiment air-flow is through the sidewalls of the coil rather than through the faces. One advantage of this geometry is that there is no longer any wasted space in the center, as the fan is now in the center.  
         [0036]    According to the present invention, instead of winding tube layers concentrically, they are wound helically, like a spring or spiral staircase, starting on a flat plate or ring. The tubes are wound in a loop and when they reach the starting point, they are elevated to the next layer, to begin another revolution, and so on, building a higher and higher stack of tubing. A commercial winding operation could involve building a stack on a rotating disk. Spacers are inserted between each layer at several locations around the circumference (generally at 4 to 12 locations) and provide the necessary spacing between tube layers.  
         [0037]    This design offers the further advantage that multiple circuits can be added more easily than with the concentric method of winding. Additional circuits can be added, one circuit at a time, to make the stack as high as needed. This method is expected to make it easier to wind multiple parallel circuits, thus facilitating the development of larger prototypes and scale-up to commercial manufacturing operations.  
         [0038]    The combination of having the fan inside the coil, the use of stackable spacers to hold the tubes in place, and the use of this configuration, in which the coil is wound in a helical fashion, can provide polymeric tubing exchangers which are compact, efficient and relatively easy to construct.  
         [0039]    Another potential feature is that the shape does not need to be circular, but can be, for instance, in a figure eight or racetrack shape or in other desired shapes. It appears that the new method offers much greater flexibility in design.  
         [0040]    A feature of some of the configurations of the invention may be that the tubes near the outer perimeter are longer than the inner tubes. This will mean the flow of refrigerant will be higher in the inner tubes than in the outer tubes in order to equalize the pressure drop. It is possible to equalize the tubing length by flipping over the tube array after half of the turns are completed, so that inside tubes then become outside tubes.  
         [0041]    Although this difference in tube length could sometimes be a disadvantage, it may also be an advantage in some applications when understood. For example, if warm air is flowing through an evaporator coil from inside to outside, then it will have the greatest temperature difference when it hits the inside tubes, so the inner tubes may be able to make good use of the higher refrigerant flow, thereby improving overall performance.  
         [0042]    In order to make functional refrigerant-to-air heat exchangers, some means of joining the plastic tubes and connecting them to the copper piping is desirable. This can achieved by sealing the ganged tubes into a copper pipe using a suitable epoxy resin available from Loctite or Ciba-Geigy, such as Loctite E90FL toughened epoxy resin, a two part product with an amine hardener; the copper pipe can then be joined to the expansion or compression device by conventional metal joining processes. Any number of tubes may be brought together in this manner, depending on the dimensions of the tubes and the number of tubes required to effect heat transfer with minimal pressure drop.  
         [0043]    The outer (surface) layer of the tubing may be the same as the bulk of the tube, preferably a polyamide, or may be a polyamide modified to improve bonding, coextruded on to the main structural layer of polyamide. Additional layers of thermoplastic can be incorporated into the tubing such as by coextrusion, including a layer of thermotropic liquid crystal polymer (LCP) to enhance the barrier of the structure. Barrier layers could also be formed in other ways from other materials. The tubing structure may also contain layers of other materials, including inorganics, which may include coatings applied by various methods, to improve barrier properties.  
         [0044]    The tubes can be of any diameter and wall thickness, consistent with the need to separate inner and outer heat transfer fluids and to transfer heat. Typical wall thicknesses are 0.005-0.015 in. (0.13-0.38 mm). In general, a minimum inner diameter of 0.030-0.060″ (0.76-1.5 mm) is desirable to avoid pluggage in use. The outer diameter is determined by the internal pressure needs of the tube, generally up to a maximum of 0.150-0.250 in. (3.8-6.4 mm).  
         [0045]    For practical sizes and configurations of refrigerant heat exchangers, it is desirable to use tubes which are quite flexible and able to bend to a defined small radius without fracture or delamination, yet which also provide good barrier properties to keep in contained refrigerant and to keep out air and moisture. Also, tubes which can be melt-bonded to the spacers after forming the heat exchangers can be desirable. The spacers can be made of a variety of materials, including nylon 6 or 66, or of the same or similar materials as the tubes.  
         [0046]    Other optional ingredients may be selected from flame retardants, anti-blocking agents, slip additives, pigments or dyes, processing aids, plasticizers and ultra-violet blocking agents. These may be used in suitable quantities as are well known to those skilled in the art.  
         [0047]    Liquid crystal polymers are preferably used in forming layers in the tubes, including as one of the materials an isotropic thermoplastic (ITP). It has been found that a layer of a thermotropic liquid crystalline polymer (LCP) used in the heat exchange surface material (HESM) often alleviates or eliminates a variety of potential problems. By an LCP is meant a polymer that is anisotropic when tested in the TOT Test described in U.S. Pat. No. 4,118,372. An HESM is a material which is used as part of a heat exchanger or a component thereof, and which is the material through which the major portion of the heat that is exchanged between the two fluids (gas or liquid) is meant to flow. It also performs the function of keeping apart the two fluids between which heat is being exchanged. Isotropic herein means that the polymer is isotropic when tested by the TOT Test described in U.S. Pat. No. 4,118,372, which is hereby included by reference. Any ITP may be used so long as it meets certain requirements. It must of course withstand the temperatures to which the HESM is exposed, and should throughout that temperature range provide sufficient strength (together with the LCP) to the HESM to reasonably maintain its shape and contain the fluids in the heat exchanger, as needed. If it is exposed to one or more of the fluids in the heat exchanger (or any other adventitious materials that may contact it) it should be preferably reasonably chemically stable to those fluids so as to maintain its integrity.  
         [0048]    Although various types of heat exchangers made simply of ITPs have been described, ITPs sometimes have serious drawbacks when the are the only materials in HESMs. Sometimes an ITP may not be chemically stable to one or more of the fluids in the heat exchanger, for instance, many polyesters hydrolyze or otherwise degrade in the presence of water, water-alcohol, or water-glycol mixtures, especially at higher than ambient temperatures. Many ITPs are relatively permeable to many liquids and/or gases, and therefore allow losses and/or migration of these materials in or from the heat exchanger. Some ITPs may be swollen by one or more of the fluids used in the heat exchanger thereby changing their dimensions and/or physical properties. All of the above are of course problems in plastic heat exchangers.  
         [0049]    If the LCP layer is placed between a fluid and any particular ITP in the HESM it usually protects that ITP from chemical degradation by the fluid, and/or also often protects the ITP from being swollen by that fluid. In addition, even if the ITP is swollen, the LCP because of its high relative stiffness, and the fact that it is not swollen by many fluids, help the overall HESM maintain its shape and dimensions. Also, the LCP acts as an excellent barrier layer to many fluids. For instance, in automotive heat exchangers which help cool the engine, the commonly used internal coolant is a mixture of a glycol and water, and the external coolant is air. With many ITPs diffusion of water and/or glycol is so rapid that frequent replenishment of the water/glycol mixture is needed. If an LCP layer is included, the diffusion is greatly decreased.  
         [0050]    In order to obtain rapid heat transfer through the HESM, thickness through the material between the heat transfer fluids should be a small as possible. This would be true with any material used for an HESM, but is especially important with plastics since their heat transfer coefficients are usually relatively low when compared to metals. Since the LCP is usually the more expensive of the polymers present in the HESM, it is economically preferable to limit its use. Therefore, in most constructions it is preferred that the LCP is present in relatively thin layer(s) and that layer(s) of the ITP be relatively thick so as to carry much of the structural load of the HESM (i.e., pressure of the fluid(s), maintain structural shape and dimensions, etc.).  
         [0051]    The HESM is made up of one or more LCP layers and one or more layers of ITP. If more than one layer of LCP or ITP is present, more than one type of LCP or ITP, respectively, can be used. In addition other layers may be present. For example, so called tie layers, also called adhesive layers, may be used to increase the adhesion between various LCP and ITP layers, or between ITP layers or between LCP layers. The number and placement of the various layers in the HESM will vary depending on the particular polymers chosen, the fluids used in or by the heat exchanger, temperature requirements, environmental needs, etc.  
         [0052]    Most commonly, tie layers and LCP layers will be relatively thin compared to the ITP layer(s). Typical constructions are given below, wherein Fluids 1 and 2 represent the fluids involved in the heat transfer:  
         [0053]    (a) Fluid 1/LCP/ITP/Fluid 2  
         [0054]    (b) Fluid 1/ITP-1/LCP/ITP-2/Fluid 2  
         [0055]    (c) Fluid 1/LCP-1/ITP/LCP-2/Fluid 2  
         [0056]    (d) Fluid 1/ITP-1/LCP-1/ITP-2/LCP-2/Fluid 2  
         [0057]    (e) Fluid 1/ITP-1/ITP-2/LCP/Fluid 2  
         [0058]    (f) Fluid 1/LCP-1/ITP-1/ITP-2/LCP-2/Fluid 2  
         [0059]    In all of the above constructions, tie layers may be present between all, some or none of the various polymer layers.  
         [0060]    Some of the above constructions may be particularly useful in certain situations. If Fluid 1 but not Fluid 2 chemically attacked the ITP, construction (a) may be particularly useful, but (c) and (f) may also be utilized. If both Fluids 1 and 2 attacked the ITP present construction (c) or (f) may be particularly useful. If one wanted to minimize diffusion of one fluid to another, a construction having two LCP layers, such as (c), (d) or (f) could be chosen. If a special surface is required to reduce abrasive damage on the Fluid 1 side, but great stiffness is also required from the ITP, a construction such as (e) could be chosen wherein ITP-1 and ITP-2 have the requisite properties. These and other combinations of layers having the correct properties for various applications will be obvious to the artisan.  
         [0061]    Useful LCPs include those described in U.S. Pat. Nos. 3,991,013, 3,991,014 4,011,199, 4,048,148, 4,075,262, 4,083,829, 4,118,372, 4,122,070, 4,130,545, 4,153,779, 4,159,365, 4,161,470, 4,169,933, 4,184,996, 4,189,549, 4,219,461, 4,232,143, 4,232,144, 4,245,082, 4,256,624, 4,269,965, 4,272,625, 4,370,466, 4,383,105, 4,447,592, 4,522,974, 4,617,369, 4,664,972, 4,684,712, 4,727,129, 4,727,131, 4,728,714, 4,749,769, 4,762,907, 4,778,927, 4,816,555, 4,849,499, 4,851,496, 4,851,497, 4,857,626, 4,864,013, 4,868,278, 4,882,410, 4,923,947, 4,999,416, 5,015,721, 5,015,722, 5,025,082, 5,086,158, 5,102,935, 5,110,896, and 5,143,956, and European Patent Application 356,226. Useful thermotropic LCPs include polyesters, poly(ester-amides), poly(esterimides), and polyazomethines. Especially useful are LCPs that are polyesters or poly(ester-amides). It is also preferred in these polyesters or poly(esteramides) that at least about 50 percent, more preferably at least about 75 percent, of the bonds to ester or amide groups, i.e., the free bonds of —C(O)O— and C(O)NR 1 — wherein R 1  is hydrogen or hydrocarbyl, be to carbon atoms which are part of aromatic rings. Included within the definition herein of an LCP is a blend of 2 or more LCPs or a blend of an LCP with one or more ITPs wherein the LCP is the continuous phase.  
         [0062]    Useful ITPs are those that have the requisite properties as described above, and include: polyolefins such as polyethylene and polypropylene; polyesters such as poly(ethylene terephthalate, poly(butylene terephthalate), poly(ethylene 2,6-napthalate), and a polyester from 2,2-bis(4-hydroxyphenyl)propane and a combination of isophthalic and terephthalic acids; styrenics such as polystyrene and copolymers of styrene with (meth)acrylic esters; acrylonitrile-butadiene-styrene thermoplastics; (meth)acrylic polymers including homo- and copolymers of the parent acids, and/or their esters and/or amides; polyacetals such as polymethylene oxide; fully and partially fluoropolymers such as polytetrafluoroethylene, polychlorotrifluoroethylene, poly(tetrafluoroethylene/hexafluoropropylene) copolymers, poly[tetrafluoroethylene/perfluoro(propyl vinyl ether)] copolymers, poly(vinyl fluoride), poly(vinylidene fluoride), and poly(vinyl fluoride/ethylene) copolymers; ionomers such as an ionomer of an ethylene-acrylic acid copolymer; polycarbonates; poly(amide-imides); poly(estercarbonates); poly(imide-ethers); polymethylpentene; linear polyolefins such as polypropylene; poly(etherketoneketone); polyimides; poly(phenylene sulfide); polymers of cyclic olefins; poly(vinylidene chloride); polysulfones; poly(ether-sulfones); and polyamides such as nylon-6,6 nylon-6, nylon-6,12, nylon-6,12, nylon 4,6, and the polyamides from terephthalic acid and/or isophthalic acid and 1,6-hexanediamine and/or 2-methyl-1,5-pentanediamine. Polyamides are preferred ITPs and preferred amides are nylon-6,6, nylon-6, and a copolymer of terephthalic acid with 1,6-hexandiamine and 2-methyl-1,5-pentanediamine wherein 1,6-hexanediamine is about 30 to about 70 mole percent of the total diamine used to prepare the polymer. Especially preferred polyamides are nylon-6,6, nylon-6 and a copolymer of terephthalic acid with 1,6-hexandiamine and 2-methyl-1,5-pentanediamine wherein 1,6-hexanediamine is about 50 mole percent of the total diamine used to prepare the polymer. Included within the definition of ITP herein are blends of 2 or more ITPs or blends of one or more ITPs with an LCP provided that the ITP(s) is the continuous phase.  
         [0063]    One or more of the LCPs and ITPs may be toughened. Toughening is known in the art, and may be accomplished by adding one or more or a rubber, functionalized rubber, resin which reacts with the LCP or ITP such as an epoxy resin, or other materials. Toughened polyamides are preferred.  
         [0064]    The polymers may contain other materials conventionally found in polymers, such as fillers, reinforcing agents, antioxidants, antiozonants, dyes, pigments, etc. An especially useful material is a filler with high heat conductivity, which may increase the efficiency of the heat exchanger.  
         [0065]    Between the layers of tubing made by coextrrusion, tie layers can be used to minimize the likelihood of delamination. The composition of a tie layer will depend on which two polymers are on either side of it. For instance the tie layer may be an ITP functionalized or grafted to provide adhesion between the ITP and LCP layers, or may be a blend of one or more ITPs and one or more LCPs.  
         [0066]    Preferably LCP used in the invention will be toughened, especially if it is to be used in tubes which will be wound to a fairly tight radius, such as down to 12.5 mm. Such a small radius may be found either in the coils themselves, such as in a condenser without a fan in the center, or it may be found in a transition form the coil to the outside connection. LCPs can be toughened as is known in the art in various ways, for example by melt blending an LCP with a rubber or other polymer having low crystallinity. In the melt blending it is preferred that the rubber or other polymer be dispersed into the LCP so that the LCP is the continuous phase and rubber or other polymer is present in relatively small particles. It is often preferred that the rubber or other polymer have reactive functional groups on it such as epoxide groups. It is known that this often improves the toughening of the LCP and also may improve the adhesion of the LCP layer to other polymer layers in the tubing. When toughening an LCP a useful amount of rubber or other polymer is about 2 to about 30 percent by weight of the total weight of the LCP and rubber or other polymer, preferably about 5 to about 20 percent by weight. Relevant teachings may be found in U.S. Pat. No. 5,997,765—Furuta and Yamaguchi (1999), EP Appl. 0 380 112 A2—Izumi et al. (1990) and PCT Publication WO 93/24574—Heino et al. (1993) which are all hereby included by reference.  
         [0067]    Typical thicknesses for ITP layers will range from about 0.025 to about 0.25 mm. Typical thicknesses for LCP layers will be about 0.01 to about 0.1 mm. Tie layers will usually be as thin as possible, consistent with their providing adhesion between polymer layers. This is usually about 0.01 to about 0.1 mm. The total thickness of the structure is preferably less than about 0.7 mm, more preferably about 0.12 to about 0.5 mm, and especially preferably about 0.15 mm to about 0.4 mm.  
         [0068]    With reference to the drawings, FIG. 1 illustrates the beginning of a process of making a plastic heat exchanger of the invention. A set of tubes  10  is laid on spacers  29 A (—C) on a base plate  11 . A first end of tubes  21 ,  22 ,  23 ,  24 ,  25  and  26  extend out from base  10  where they can be gathered together in a header  20 , such as a copper tube. The second end of tubes  21 - 26  can be fed through a guide  30  from drums  31 - 36 , respectively. Base  11  is provided with additional spacers,  29  B and C. Preferably the spacers are attached to a column  27 A at their inner ends, and preferably they are also attached to another column  28 A (—C) at their outer ends, to hold them in place. In the drawings, the heat exchangers illustrated are in a cylindrical shape, being formed around a surface  12  incorporating an aperture  13 . However, it will be apparent that heat exchangers of the invention can be formed in other curvilinear shapes, depending on where the columns are situated on the base. Also, surface  12  can extend above the plane of base  11  in the form of a mandrel or other form, or it can simply be a surface shaped in space without any physical embodiment. If a mandrel is used, it can be removed after forming the heat exchanger, or it could be made of a porous material, such as a mesh or a perforated sheet, so that coolant can flow through it. Removal of a mandrel could be done physically or chemically, by etching it away. Furthermore, the tubes can be laid down on the spacers by hand or with whatever degree of automation is desired, with reels  31 - 36  supplying the tubes, or even with the tubes previously cut to the desired lengths and fed in by hand.  
         [0069]    [0069]FIG. 2 illustrates a further step in the process of the invention, with the set of tubes  21 - 26  having been wound around surface  13  and columns  27  A and B, then rising above the first layer of tubes at  40  to overlay the first layer in forming the second layer. Outer columns  28  A-C are not shown in FIG. 2 to indicate that they are optional.  
         [0070]    [0070]FIG. 3 shows a plastic tube heat exchanger of the invention after the winding of tubes has been completed and headers  44  and  45  have been put in place on the bundles of tubes  46  and  47  coming away from the exchanger. At the center of the exchanger is a fan  41 , to draw air or other gas in from the ends and cause it to flow out through the sides, as shown at  42 . Spacers  29  and columns  27  and  28  are indicated, holding apart the tubes which can be 20 or more, or less, in each layer.  
         [0071]    [0071]FIG. 4 shows an elevation of the heat exchanger of FIG. 3, illustrating tubes at  10 , base  11 , spacers  43 , bundles of tubes  46 , and headers  44 . The spacers shown could be for inlet or outlet of coolant which will flow through the tubes.  
         [0072]    [0072]FIG. 5 shows a spacer  29  with grooves  51  and  52  on its top and bottom, respectively, for holding tubes apart.  
         [0073]    [0073]FIG. 6 is a side view of the spacer, also showing holes  53  and  54  for fitting on the columns.  
         [0074]    The selection of polymeric tubing for heat exchangers containing refrigerants must satisfy certain criteria:  
         [0075]    a) it must withstand internal pressures appropriate to containment of refrigerants, including normal operating pressures, test pressures, and safety mandated overpressure capabilities  
         [0076]    b) it must prevent or minimize egress of refrigerant and ingress of air or water  
         [0077]    c) it must be amenable to coiling tightly without kinking or fracturing the LCP layer  
         [0078]    d) the walls of the tubing must be thin enough so as not to substantially impede heat transfer  
         [0079]    These properties are affected by the tubing diameter, the thicknesses of individual layers, and the modification of the LCP with ITP.  
       EXAMPLE  
       [0080]    As an example, the tube structure used to construct a refrigeration evaporator was as follows;  
         [0081]    Outer diameter 0.059 inch (1.5 mm)  
         [0082]    Inner Layer—3 mils (76 micron) of a compounded blend of the LCP of example 6 of U.S. Pat. No. 5,525,700 Samuels and Waggoner (1996) (90%w/w), ethylene butylacrylate (5% w/w)(such as is available from Chevron as grade 1802 (18% BA) and Elvaloy 3934-4 (5% w/w)(ethylene/butyl acrylate/glycidyl methacrylate copolymer). The LCP is a copolymer of biphenol, hydroquinone, terephthalic acid, 2,6-naphthalenedicarboxylic acid, p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid in a molar ratio of 50/50/70/30/270/50).  
         [0083]    Middle Layer—1 mil (25 microns) of Bynel 4206 (LDPE grafted with maleic anhydride)  
         [0084]    Outer Layer—5 mil (127 microns) of Zytel 42A NC010 (nylon 6,6), containing minor additives including carbon black pigment, heat stabilizer and lubricant. Elvaloy, Bynel and Zytel are trademarks of DuPont Company, from whom the products are available.  
         [0085]    The tubing of this example has a burst pressure in excess of 1500 psig (10,300 kpascals) and can be coiled to a radius of curvature of 0.5 inches (12.3 mm)without kinking (all at 50%RH) and is therefore suitable for constructing coiled heat exchangers to handle refrigerants such as chlorodifluoromethane.”