Abstract:
A method of constructing a heat exchange system for a medium to be frozen, including the steps of: extruding a composition to form a reconfigurable tube; cooling the tube with the tube in a substantially straight configuration so that the tube is substantially set in the straight configuration; after cooling, reconfiguring the tube from the straight configuration; transporting the tube reconfigured from the straight configuration to a site at which the tube is to be used; and at the site placing the tube in the straight configuration and connecting the tube in a medium to be frozen so that a fluid within the tube is in heat exchange relationship with the medium to be frozen.

Description:
This application is a divisional of prior application Ser. No. 08/722,489, filed on Sep. 27, 1996, now U.S. Pat. No. 5,970,734. This application also claims the benefit of U.S. Provisional Application No. 60/004,599, filed on Sep. 29, 1995. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method of manufacturing a tube. This invention also relates to a system for creating and maintaining a frozen surface, for example, for recreational exhibitions and athletic competitions at an ice skating rink. In particular, this invention relates to a system for efficiently conveying a coolant through a medium to be frozen. This invention also relates to a system that lends itself to facilitate installation and maintenance. 
     BACKGROUND OF THE INVENTION 
     The earliest ice skating rinks were frozen ponds or lakes. Such ice sport venues had the sizeable limitation that their existence was entirely dependent upon the temperature of the environment. For a long time, the dependency upon naturally-formed ice restricted the enjoyment of ice sports in most countries to a limited seasonal period. 
     In the late nineteenth century, indoor ice skating rinks were designed to provide venues on which ice sports could be enjoyed in most countries year-round. These early indoor ice skating rinks used a system of steel or iron pipes to carry an artificially-cooled refrigerant, such as calcium chloride brine, under a tank of water to create a frozen surface capable of being skated upon. The steel or iron pipes were embedded in concrete or sand beneath the tank, and had an inner diameter of 1 to 1½ inches with 4 inches between the centers. 
     While capable of providing a frozen surface which could be skated upon indoors year-round, the steel or iron pipe construction had its drawbacks. Perhaps, one of the greatest limitations on the steel or iron constructions was the surface area that these systems provided for heat exchange with the medium to be frozen, also known as the dynamic surface area. In the steel or iron constructions, as structurally and dimensionally described above, the dynamic surface area was substantially less than the area of the skating surface available for heat exchange with the environment. The dynamic surface area of the steel or iron constructions is estimated to be at most 82% of the skating surface area. 
     More recently, ice skating rink systems have been constructed using smaller diameter plastic tubing, such as those systems described in U.S. Pat. Nos. 3,751,935; 3,893,507; and 3,910,059. In operation, a main supply pipe, or header, feeds into a plurality of supply subheaders, each of which in turn is attached to the proximal ends of a plurality of coolant tubes. The plurality of coolant tubes can be fastened at their distal ends to one end of a plurality of U-shaped connectors, which in turn are fastened to a second plurality of coolant tubes. The second plurality of coolant tubes is attached at their proximal ends to a plurality of return subheaders, which in turn feed into a main return header. The inner diameter of the coolant tubes used in these plastic constructions generally varies from ¼ to ½ inches. By using a smaller center spacing between smaller tubes, these plastic systems may provide a larger dynamic surface area than the steel or iron constructions. 
     However, the dynamic surface area is only one factor influencing the overall efficiency of a system designed to create and maintain a frozen surface. As important to the efficiency of the system as the dynamic surface area is the ability of the coolant to flow through the system without significant pressure loss or flow interruption. As a consequence, even though the plastic systems may have improved the dynamic surface area over the iron and steel constructions, the efficiency of these plastic systems is often significantly compromised in practice by unsatisfactory coolant flow characteristics at various points in the system. 
     For example, as shown in FIGS. 1 and 2 herein, one common area for flow restriction to occur is at the transfer point between a subheader  30  and a coolant tube  32 . In the conventional construction shown in FIGS. 1 and 2, the subheader  30  has an opening  34 , through which is disposed a connection fitting  36 . The connection fitting  36  is soldered into place with the proximate end of the fitting  36  occluding as much as 25 percent of the interior cross-sectional area of the subheader  30 . This occlusion can cause a layer  38  of coolant to build up against the fitting  36 , and seriously degrade the flow characteristics of the coolant in the area adjoining the transfer point. 
     Moreover, at the distal end of the tube  32 , where the tube  32  attaches to a U-shaped connector  40 , the conventional methods of construction can cause additional flow restriction problems. One flow restriction problem commonly occurring in conventional constructions is illustrated in FIGS. 3 and 4. The U-shaped connector  40  shown is fabricated by bending a copper tube having an internal diameter similar to that of the coolant tube  32 . By using this method of fabrication, the resulting inner diameter at a bight  42  of the U-shaped connector  40  may be reduced to approximately half the diameter of the original copper tube. The dramatic decrease in the inner diameter of the U-shaped connector  40  at the bight  42  has a proportionally dramatic effect on the fluid flow throughout the system. 
     Additionally, loss of flow pressure can result from the present methods of system construction used to join the coolant tubes  32  with the U-shaped connectors  40 . The coolant tubes  32  are fastened directly to the U-shaped connectors  40  by means of glue and a circular clamp or an eyelet, as shown in FIGS. 3 and 4. As a consequence, the tubes  32  have a tendency to leak, or even pop off of the U-shaped connector  40 , spilling coolant directly into the medium to be frozen and underlying foundational material and decreasing the pressure and flow rate at which the coolant is being transported throughout the system. 
     Furthermore, these plastic systems are often constructed using a type of plastic coolant tube having unfavorable performance characteristics. Commonly, polyethylene or polypropylene tubing is used for the coolant tubes in plastic ice skating rink systems. During manufacture, the polyethylene or polypropylene tubing is usually extruded, and then passed through a standard length (10-14 foot) cooling tank before being machine-coiled on to spools for delivery. As a consequence of this method of fabrication, the polyethylene or polypropylene tubing thermally sets with a curved, rather than a straight, structure in the memory of the plastic. Therefore, when the tubing is uncoiled to be used in the plastic construction illustrated in the patents mentioned above, the tubing does not naturally lay straight and flat, but takes on a serpentine structure in at least one plane. 
     As a further consequence, when these polyethylene or polypropylene ice rink systems are installed, the coolant tubing will commonly force its way under pressure to the skating surface, and protrude from the surface of the ice, providing a substantial obstacle and hazard for persons, for example skaters, using the frozen surface. It is therefore necessary to resubmerge the tubing under the surface of the ice through a method known as “burning in”. The tubing is “burned” into the surface of the ice by melting the surrounding ice, and then holding the tube in place under pressure until the ice reforms around the problematic section of tubing. Because of the pressure of the coolant running through the tubing, as well as the thermally-set disposition of the tubing to return to the serpentine structure, it may be necessary to repeat the “burning in” process a number of times each season to maintain a skating surface free from obstructions and to prevent damage to the tubing. 
     However, polyethylene and polypropylene tubing is sensitive to repeated bending. Repeated bending of the polyethylene or polypropylene tubing has been known to cause permanent damage to the tubing, and can result in the cracking or rupture of the tubing with a concomitant loss of coolant pressure in the system. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a method of manufacturing a tube includes the steps of preparing a composition using ethylene vinyl acetate, extruding the composition to form a tube, and cooling the tube with the tube in a substantially straight configuration so that the tube is substantially set in a substantially straight configuration. 
     According to another aspect of the present invention, a system for creating a frozen surface on a medium includes a mechanism for exchanging thermal energy between a medium and a coolant, a mechanism for removing thermal energy from a coolant, and a mechanism for transporting a coolant between the mechanism for exchanging thermal energy between a medium and a coolant and the mechanism for removing thermal energy from a coolant. The mechanism for transporting a coolant includes first and second pipes and a mechanism for releasable connecting the first pipe to the second pipe so as to prevent the first pipe from moving axially relative to the second pipe in a first operational state, and to allow the first pipe to be moved axially relative to the second pipe in a second operational state. 
     According to a further aspect of the present invention, a system for creating and maintaining a frozen surface on a medium includes a mechanism for exchanging thermal energy between a medium and a coolant, the mechanism for exchanging thermal energy between a medium and a coolant having a substantially uniform cross-sectional area for passing a coolant therethrough. The system also includes a mechanism for removing thermal energy from a coolant. The system further includes a mechanism for transporting a coolant between the mechanism for exchanging thermal energy between a medium and a coolant and the mechanism for removing thermal energy from a coolant. The mechanism for transporting a coolant is connected to the mechanism for exchanging thermal energy between a medium and a coolant so that substantially all of a coolant flowing from the mechanism for transporting a coolant to the mechanism for exchanging thermal energy between a medium and a coolant flows directly from the mechanism for transporting a coolant into the mechanism for exchanging thermal energy between a medium and a coolant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional view of a portion of a prior art subheader showing in detail the transfer point between the subheader and a coolant tube; 
     FIG. 2 is a partial cross-sectional view of the transfer point between the subheader and the coolant tube taken about line  2 — 2  in FIG. 1; 
     FIG. 3 is a partial cross-sectional view of a prior art U-shaped connector showing in detail the connection of the U-shaped connector and a coolant tube; 
     FIG. 4 is a partial cross-sectional view of the connection of the U-shaped connector and the coolant tube taken about line  4 — 4  in FIG. 3; 
     FIG. 5 is an overall plan view of an ice skating rink including an embodiment of the present invention for creating and maintaining a frozen surface; 
     FIG. 6 is an enlarged, partial cross-sectional view of an insulation blanket or layer which is useful for insulating below the system shown in FIG. 5; 
     FIG. 7 is an enlarged plan view showing in detail an embodiment of a panel for use in the embodiment shown in FIG. 5, and the interconnection of the panel with supply and return headers; 
     FIG. 8 is an enlarged plan view showing in detail another embodiment of a panel for use in the embodiment shown in FIG. 5 in particular at the curved ends of the ice skating rink, and the interconnection of the panel with supply and return headers; 
     FIG. 9 is an overall plan view of an ice skating rink including another embodiment the present invention for creating and maintaining a frozen surface with the spacers and spacing bars removed; 
     FIG. 10 is an enlarged plan view of an embodiment of a spline-connector used to connect two adjoining pipes in the header in the embodiment shown in FIG. 5, the spline-connector including a releasably attachable female coupling connected to a flexible hose element; 
     FIG. 11 is an enlarged plan view of another embodiment of a spline-connector for use in the embodiment shown in FIG. 5, the spline-connector including a releasably attachable coupling connected to a fixed coupling attached directly to the spline-connector; 
     FIG. 12 is an enlarged plan view of still another embodiment of a spline-connector for use in the embodiment shown in FIG. 5, the spline-connector including a valve connected between a releasably attachable coupling and a fixed coupling attached directly to the spline-connector; 
     FIG. 13 is an enlarged, partial cross-sectional view of a flexible hose used to connect a spline-connector with either a supply or a return subheader; 
     FIG. 14 is an enlarged, partial cross-sectional view of any of the embodiments of a spline-connector shown in FIGS. 10,  11 , and  12  showing in detail a first and a second locking mechanism used to prevent relative movement between the spline-connector and a header pipe; 
     FIG. 15 is a partial cross-sectional view of an embodiment of the present invention showing in detail a transfer point at the intersection of a subheader with a coolant tube; 
     FIG. 16 is a partial cross-sectional view of the transfer point at the intersection of the subheader and the coolant tube taken about the line  16 — 16  in FIG. 15; 
     FIG. 17 is a cross-sectional view of an embodiment of the present invention showing in detail a U-shaped connector; 
     FIG. 18 is a cross-sectional view of the U-shaped connector taken about line  18 — 18  in FIG. 17; 
     FIG. 19 is a partial cross-sectional view of the U-shaped connector of FIGS. 17 and 18, showing in detail the interconnection of the U-shaped connector and a coolant tube; and 
     FIG. 20 is a cross-sectional view of the U-shaped connector and the coolant tube taken about the line  20 — 20  in FIG.  19 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In general terms, the system of the present invention creates and maintains a frozen surface, such as ice, by removing thermal energy from a liquid medium, such as water, and exhausting the thermal energy at a location remote to the medium to be frozen. Specifically with reference to FIG. 5, pressurized, chilled coolant passes through a plurality of tubes spaced within a tank or container  46  holding the medium to be frozen. As the coolant passes through the plurality of tubes, thermal energy is transferred from the medium to the coolant through the walls of the tubes. The coolant then passes from the tubes to a pump  54 , and from the pump  54  to a refrigeration unit  70 . The refrigeration unit  70  extracts the thermal energy from the coolant and returns the chilled coolant to a collection tank  68 , whereupon the cycle is repeated. 
     According to an embodiment of the present invention, a system  44  for creating and maintaining a frozen surface is shown in FIG.  5 . The system  44  in FIG. 5 is shown fitted in a tank or rink  46 . The rink system  44  includes a main supply header  48 , a main return header  50 , and a plurality of panels  52 . Unlike the constructions discussed above, the panels  52  used in the embodiments of the present invention discussed herein are placed within the medium to be frozen, rather than being embedded in or placed underneath inches of sand or concrete beneath the rink  46 , although such a configuration is possible using the present invention. As a consequence of the direct thermal energy exchange relationship between the coolant in the panels  52  and the medium to be frozen, the efficiency of the system  44  is improved as a whole as it is unnecessary to first cool the floor of the tank  46  prior to cooling the medium to be frozen. 
     To preserve the advantages of this direct thermal energy exchange relationship by preventing thermal energy from entering the tank from surface below the tank  46 , an insulation layer or blanket  53 , as shown in FIG. 6, is placed beneath the panels  52 . The insulation layer  53  is fabricated in a sandwich construction in which two layers of bubble packaging material  53   a  are laid face to face such that the bubbles of one layer fit within the dimples of the other layer. The two layers  53   a  are then covered on the externally facing surfaces  53   b ,  53   c  with a layer  53   d  of foil on the surface  53   b , and a layer  53   e  of foil, or polyethylene, on the surface  53   c . During installation, the layer  53   d  is placed against the surface below the tank  46 , while the layer  53   e  faces and is covered by the medium to be frozen. 
     A pump  54  is connected at an outlet  56  to the main supply header  48  via the refrigeration system  70  and the collection tank  68 , and forces a coolant, for example, a mixture of either ethylene glycol or propylene gylcol and water, into the main supply header  48  under pressure. Under most conditions, the coolant is, for example, a mixture of either ethylene glycol or propylene glycol and water in a ratio of 45:55. If the system  44  is intended for use in a environment where the temperature of the surrounding environment is less than −20 degrees F., the coolant is, for example, a mixture of either ethylene glycol or propylene glycol and water in a ratio of 55:45. The coolant passes from the main supply header  48  and into the individual panels  52 . 
     Each panel  52 , generally indicated in FIG.  5  and shown in greater detail in FIGS. 7 and 8, is four feet wide and 100 feet long, and includes a supply subheader  58 , a return subheader  60 , a first and second plurality of tubes  62 ,  64 , and a plurality of U-shaped connectors  66 . The pressurized coolant flows from the main header  48  into the supply subheader  58 , which feeds into the first plurality of tubes  62 . As the coolant flows through the medium, thermal energy is transferred from the medium to the coolant through the walls of the tubes  62 . The coolant then passes through the plurality of U-shaped connectors  66  and into the second plurality of tubes  64 . As the coolant flows through the medium for a second time, additional thermal energy is transferred from the medium to the coolant. 
     The coolant feeds from the plurality of tubes  64  to the return subheaders  60 , which are connected to the return header  50 . The coolant is transported along the return header  50  to the pump  54 , from which the coolant returns to the refrigeration system  70 . The refrigeration system  70  extracts the thermal energy from the coolant, and exhausts the thermal energy to the environment. The chilled coolant is then returned to the collection tank  68 , for example a 15 gallon tank, to be re-introduced into the main header  48 . 
     Alternatively, the system  44  may be configured to accommodate placement of the refrigeration system  70  and pump  54  at the center of the rink  46 . As shown in FIG. 9, with like numbers used for like elements, a central supply header  72  is connected through the refrigeration system  70  and a collection tank  68  to the pump  54 , branching off at a first T-first  74  to form two main supply headers  48 , one for each half of the rink  46 . The supply headers  48  each feed into a plurality of subheaders  58 , which in turn feed into a plurality of panels  52  in a direct thermal energy transfer relationship with the medium to be frozen. The coolant returns to the refrigeration system  70  via a system of return subheaders  60  and return headers  50 . The return headers  50  are connected at a second T-joint  76  to form a main return header  78 , which feeds directly into the pump  54 . 
     Because the system  44  can be assembled to accommodate rinks of different widths and lengths by adding additional panels  52 , the requirements for the pumpsize and the pressure and flow rate of coolant (expressed as gallons per unit of time) will necessarily differ according to the exact dimensions of the assembled system  44 . The coolant has an inlet temperature (as measured at the inlet of the supply header  48 ) of 18-20 degrees F., and an outlet temperature (as measured at the inlet of the pump  54 ) of 20-24 degrees F. It has been found experimentally that to provide a uniform thermal energy transfer, or thermal energy extraction, from the medium to be frozen, the velocity of the coolant in the system  44  should be at least 1 foot/second. 
     In an embodiment of the present invention, wherein the rink system  44  may be assembled and disassembled, for example at the end of a seasonal period or after an athletic competition or exhibition, the supply header  48  and the return header  50  are made from lenghts of pipe  80 , for example, enhanced PVC pipe (type 1, grade 1, 2000 psi hydrostatic stress material, in accordance with ASTM D1784) with an inner diameter of between 2 to 6 inches, for example 4 inches, joined together at spaced intervals by connectors  82 ,  84 , also fabricated from enhanced PVC schedule  80  pipe. The lengths of pipe  80  are joined together at four foot intervals to coincide with the four foot width of the panels  52 . 
     The connector  82 , as shown in FIGS. 10,  11  and  12 , is used in the main supply header  48  and the first section of the main return header  50  upstream to the U-shaped joint  86  in the system  44  shown in FIG. 5, and U-shaped joints  88  and  90  in the system  44  shown in FIG.  9 . The connector  82  is also designed to connect the main supply header  48  and the main return header  50  to the supply subheaders  58  and the return subheaders  60 . 
     The connector  82  may include a pipe section  92 , a flexible hose  94 , a fixed coupling  96  and either a male or female coupling  98 . An opening  100  is machined in the pipe section  92  at half the distance from the ends. The opening  100  is then tapped to accept the threads of the fixed coupling  96 . The pipe section  92  and the fixed coupling  96  are screwed together until the pipe section  92  and the fixed coupling  96  mate securely. 
     A first, proximate end of the flexible hose  94 , which has an inner diameter of one inch and is manufactured as shown in FIG. 13 with a helical steel spring  102  embedded within the wall of the hose  94 , is then placed over a portion of the distal end of the fixed coupling  96  and secured using a circular clamp, for example, a stainless steel clamp. The second, distal end of the flexible hose  94  is then placed over a portion of the proximate end of the attachable coupling  98  and secured using a circular clamp, also a stainless steel clamp. The attachable coupling  98  allows the connector  82  to be connected to a mating male or female coupling  99  attached at the ends of the subheaders  58 ,  60 . 
     Alternatively, the attachable coupling  98  is attached directly to the fixed coupling  96  of the supply header  48 , while a mating male or female coupling  99  is attached via a flexible hose  94  to the supply subheader  58  and return subheader  60  corresponding to the given panel  52 , as shown in FIG.  8 . The mating couplings  99  are alternated between the supply and return subheaders  58 ,  60  for a given panel  52 , i.e., each of the supply subheaders  58  may have a male coupling  99 , while the return subheaders  60  may have a female coupling  99 . In this fashion, when the system  44  is to be disassembled to be transported or stored, the coolant in the panel  52  can be isolated in the panel  52  by attaching the male coupling  99  of the supply subheader  58  to the female coupling  99  of the return subheader  60 . 
     Moreover, the panels  52  may be isolated in operation as well as in storage by disposing a valve  104 , for example, a brass or stainless steel ball valve, between the fixed coupling  96  and the attachable coupling  98  on the spline-connector  82 , as shown in FIGS. 7 and 12. By connecting the valves  104  to the supply and return header connectors  82 , the coolant in a panel  52  may be isolated by closing the valves  104 . 
     By way of example only, isolation of the panel  52  could be advantageous should one of the coolant tubes  62 ,  64  of a panel  52  rupture. Isolation could prevent loss of the coolant into the medium to be frozen and the underlying foundational material, prevent loss of pressure throughout the system  44 , and otherwise allow the repair of the panel  52  with the ruptured tube  62  or  64  to be performed while maintaining the frozen surface on the portions of the medium unaffected by the loss of coolant flow through the isolated panel  52 . 
     Additionally, again by way of example only, isolation of the panels  52  could be advantageous during the freezing of the medium. Specifically, the panels  52  could be isolated so that the medium is frozen in stages, panel by panel, until all of the medium in the rink  46  is frozen. Such a staged process could be especially advantageous when attempting to freeze a medium when the temperature of the surrounding environment is substantially greater than the temperature at which the medium will freeze. 
     FIG. 14 shows the locking mechanisms used in any of the embodiments of the connectors  82  shown in FIGS. 10,  11  and  12 . Particularly, each end of the connector  82  is machined to include a shoulder  110 , an interior o-ring groove  112  and an interior spline groove  114 . Similarly, each end of the pipe  80  is machined to have an exterior spline groove  116 , which corresponds axially with the interior spline groove  114  of the connector  82  when the end  118  of the pipe  80  abuts the shoulder  110  of the connector  82 . 
     In operation, an O-ring  108  is first placed in the interior O-ring groove  112 . The pipe  80  is then placed into the connector  82  until the end  118  abuts the shoulder  110 . The o-ring  108  and the exterior surface of the pipe  80  thus forms a first sealing and locking mechanism  120  preventing relative movement of the pipe  80  and the connector  82  in the axial direction. A second locking mechanism  122  is formed when the spline  106  is placed through a hole  124 , the hole  124  being connected through the wall of the connector  82  to the interior spline groove  114 . The spline  106  fills the channel formed by the corresponding interior and exterior spline grooves  114 ,  116 , also preventing the relative movement of the pipe  80  and the connector  82  in the axial direction. 
     A further embodiment of the spline-connector, designated  84  in FIGS. 5,  7 ,  8 , and  9 , is used to couple the pipes  80  used in the second section of the main return header  50 . Because the connectors  84  are not intended to be connected to the return subheaders  60 , the connectors  84  are not manufactured with the opening  100  into which the fixed coupling  96  can be screwed. The connectors  84 , like the connectors  82 , however, do feature both the first and second locking mechanisms  120 ,  122 . 
     As shown in FIGS. 7 and 8, the panel  52  is defined by of the supply subheader  58 , the return subheader  60 , the first and second plurality of tubes  62 ,  64  and the plurality of U-shaped sections  66 . As further illustrated in FIGS. 15 and 16, the supply and return subheaders  58 ,  60  fabricated from copper pipe, are machined with plurality of openings  126 . A barbed saddle fitting  128 , for example a copper fitting, is soldered over each opening  126 , using a silver based solder. Use of the saddle fitting  128  is advantageous in that there is limited obstruction of the fluid flowing from the subheader  58 ,  60  into the tubes  62 ,  64  and the subheaders  58 ,  60  have a substantially uniform cross-sectional area. One end of one of the tubes  62 ,  64  is fitted over the barbed end  130  of saddle fitting  128  and fastened with a circular clamp. The use of barbed ends allows a secure attachment between the tubes  62 ,  64  and the subheader  58 ,  60  to be formed. 
     The tubes  62 ,  64  are made with a ½ inch inner diameter from a composition prepared using ethylene vinyl acetate (EVA), for example, from a composition prepared using 18% by weight of EVA combined with 82% by weight of polyethylene. The percentage of EVA may vary from between 15-25% by weight, while the polyethylene may vary from between 75-85% by weight. During manufacture, the composition is extruded to form the tubes and is passed through a cooling tank at a rate of 1 foot per second. Unlike the conventional methods for manufacturing the polyethylene or polypropylene tubing described above, the EVA/polyethylene tubes are passed through a cooling tank or tanks for a distance of between 25 and 36 feet with the tubes in a substantially straight configuration. The tubes may be cooled by spraying the tubes with water in the cooling tank or tanks, or by passing the tubes through a water bath maintained in the cooling tank or tanks. It is thought that the time spent by the tubes in the cooling tank or tanks allows the EVA/polyethylene tubes to thermally-set in a substantially straight configuration. The extruded, cooled product, having a final inner diameter of ½ inch, is then hand-coiled with the effective diameter of the coil being no less than 2.5 feet, and placed into a gaylord container for shipping. The tubes are fabricated in lengths of between 515 to 520 feet. 
     The tubes  62 ,  64  are joined in pairs, the proximate end of the tube  62  attached to the supply subheader  58  and the proximate end of the tube  64  to the return subheader  60 . Similarly, the distal ends of the pair of tubes  62 ,  64  are connected to one of the ends of the plurality of U-shaped connectors  66 . 
     As illustrated in FIGS. 17 and 18, each U-shaped connector  66  has a U-shaped section  132  and a pair of barbed fittings  134 . The U-shaped section  132  and the barbed fittings  134  are made of copper. The distal ends  136  of the barbed fittings  134  are placed inside of ends  138  of the U-shaped section  132  and soldered in place using a silver based solder. As shown in FIGS.  19  and  20 , one of the distal ends of tubes  62 ,  64  is then placed over each of the barbed, proximate ends  140  of the barbed fitting  134 , and fastened into place using a circular clamp  139 . 
     The U-shaped section  132  is of a constant inner diameter, for example, of nearly equal diameter to the tubes  62 ,  64  and thus provides a substantially continuous and substantially uniform cross-sectional area through which the coolant medium can pass. Furthermore, the barbed ends  140  of the fitting  134  provide for a secure attachment site to attach the ends of the tubes  62 ,  64  to the U-shaped connector  66 . 
     A uniform spacing between the centers of the tubes  62 ,  64  is achieved in part by welding a bar  142 , for example, a brass bar of hexagonal or rectangular cross-section, to the U-shaped bend in each of the U-shaped connectors  66  that make up the panel  52 . As shown in FIGS. 7 and 8, the bar  142  can be straight or curved to keep the proper spacing between tubes  62 ,  64  even in the rounded corners of the ice rink  46 . In addition, spacers  144 , for example, made of polyethylene, are placed at intervals along the tubes  62 ,  64  to maintain the spacing between the tubes  62 ,  64  and the spacing between the tubes  62 ,  64  and the surface over which the system  44  is installed. The spacing between the centers of the tubes  62 ,  64  is between 1 and 1-½ inches, while the spacing between the spacers  144  is approximately 14 inches. 
     The spacers  144  may either be removable or non-removable. If the spacers  144  are non-removable, i.e. enclose the entire circumference of the tubes  62 ,  64 , then it is preferable to place the tubes  62 ,  64  through the spacers  144  before attaching the tubes  62 ,  64  to the barbed saddle fittings  128  of the supply and return subheaders  58 ,  60 . If the spacers are removable, i.e. may be snapped around the tubes  62 ,  64 , the spacers may be attached to the tubes  62 ,  64  after the tubes  62 ,  64 , are connected to the respective supply and return subheaders  58 ,  60 . 
     Still other aspects, objects, and advantages of the present invention can be obtained from a study of the specification, the appended claims.