Abstract:
An apparatus according to various embodiments can be configured to use a cryogen for cooling articles, particularly having applications for chilling extrusions. The apparatus removes thermal energy from an article by conductive and convective heat transfer. The apparatus allows for heat transfer from an outer surface of an article, and from an inner surface of the article.

Description:
This application is a divisional of application Ser. No. 11/941,930, filed Nov. 17, 2007, now U.S. Pat. No. 8,287,786, which claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 60/889,139, filed Feb. 9, 2007; U.S. Provisional Patent Application Ser. No. 60/866,538, filed Nov. 20, 2006; and U.S. Provisional Patent Application Ser. No. 60/866,279, filed Nov. 17, 2006. 
    
    
     BACKGROUND 
     The present disclosure relates generally to a method and apparatus for cooling an extrudate or extruded articles. Specifically, the present disclosure relates to circulating a cryogen through a cooling chamber including sizing and/or calibration tools, through a hollow in the article itself or a combination of the aforementioned to cool an extrudate. Additionally, many other applications of the invention will become apparent to those skilled in the art upon a review of the following specification and drawings. 
     Extruded materials, for example products comprising plastics, rubbers, wood composites, etc., are formed by mixing raw materials under high temperature and pressure and passing the mixture through a die to form the final shape. The extruded material, or extrudate, is subject to deformation after leaving the extruder because of the plastic material properties at high temperature. The extrudate must be cooled to provide rigidity for further operation. If the extrudate is not quickly and effectively cooled, the extrudate may deform, leading to rejection of the extruded products. 
     Certain continuously extruded materials, e.g., rubber products, plastic products, metal products, wood composites, must be cooled after passing through the extrusion operation in order to prevent deformation. In conventional extrusion operations, the extruded materials, be it hose, pipe, rod, bar or any other shape may deform from its own weight if the temperature was not decreased rapidly after leaving the extruder. Cooling the product rapidly creates, at least, a minimum amount of rigidity in the extrudate such that the manufacturer can cut, stack or otherwise handle the extrudate without unwanted deformation. If the product is not cooled effectively, and quickly, the resultant deformation can lead to excessive rates of rejection of the manufactured or extruded product. Further, the rate at which the extrudate is cooled directly affects the rate at which product may be produced. In other words, the faster an extrudate is cooled, the faster the end product can be produced. 
     Historically, cool water systems have been utilized as the primary medium for cooling articles, including extrusions. For example, conventional extrusion chilling systems employ a “cooling” chamber downstream from the extruder. The extrusion is fed through the cooling chamber, wherein the extrusion can be sprayed with water, or partially/fully submerged in water in order to chill the extrusion. Various other components may also be included in such systems, such as a vacuum sizing chamber intermediate to the extruder and the cooling chamber. The vacuum sizing chamber can be used for both solid and hollow extrusions and employs an external vacuum pump to create a vacuum to assist the extrusion in maintaining its shape while it cools. Water can also be used in the vacuum chamber to cool the extrusion while the vacuum supports the shape. 
     However, cooling water systems have several drawbacks. Many products are adversely affected if contacted with water. Thus extra care must be taken to avoid such occurrences. Extrusion speeds are limited because the cooling water generally has a well defined heat transfer capability and thus can only cool the fresh extrudate accordance therewith. In practice, optimum cooling temperature of approximately 50° F. is achievable from a cost-effective standpoint, which limits the manufacturers ability to cool extrusions quickly. Additionally, cooling water systems require excessive floor space and also require treatments or special additive packages to prepare and maintain proper water chemistry, as well as to prevent scaling and bacterial growth, which add significantly to the cost thereof. 
     Coolant mediums other than water which have been used in cooling processes can be referred to collectively as refrigerants, including cryogens. Cryogens include liquid nitrogen, liquid carbon dioxide, liquid air and other refrigerants having normal boiling points substantially below minus 50° F. (−46° C.) Prior art methods of cooling articles using cryogens disclose the benefits of hilly vaporizing a cryogen into a gaseous refrigerant prior to contact with the articles to be cooled. Cryogens due to their extremely low boiling point, naturally and virtually instantaneously expand into gaseous form when dispersed into the air. This results in a radical consumption of heat. The ambient temperature can be reduced to hundreds of degrees below zero (Fahrenheit) in a relatively short time, and much quicker than may be realized with a conventional cooling water system. The extreme difference in vaporized cryogen and the extruded product allows the manufacturer to quickly cool an extrudate. 
     However, prior methods of cryogenic cooling fail to realize the advantages, both in increased efficiency and in improved system control, that can be achieved by utilizing forced gas convection in combination with cryogenic refrigerants, such as nitrogen and air. Some disadvantages of prior art cryogenic cooling systems include lower efficiency and limited options for controlling the cooling process. Such systems generally rely exclusively on the cooling effect of the refrigerant, to lower the ambient temperature and chill the article. Although prior art methods utilize forced convection to ensure complete vaporization of the cryogen, no methods use forced gas convection to control the rate of cooling of the article by controlling the wind chill temperature. Consequently, the only control variable in the prior art methods to adjust (lower) the temperature is the introduction of a liquid cryogen into the system. In contrast, utilization of forced gas convection adds a wide range of variable control to adjust the effective temperature, up or down, by controlling the velocity at which the refrigerant is circulated over/around the article to be cooled. Such a forced gas convection system is disclosed by Thomas et al. in U.S. Pat. No. 6,363,730, U.S. Pat. No. 6,389,828, and U.S. Patent Application Publication No. 2004/0216470 (now abandoned) incorporated herein in their entirety by reference thereto. 
     The basis of forced gas convection is the principle that increasing velocity of a refrigerant over a heated surface, such as by blowing, greatly enhances the transfer of heat from that surface. In the context of cold temperatures, this principle is probably better known indirectly from the commonly used phrase “wind chill” temperature. In that context, wind chill temperature is the apparent temperature to human flesh, taking into account the ambient temperature and the prevailing velocity of the wind. The stronger (higher velocity) the wind, the lower the temperature “feels,” compared to if there were no wind present. Forced gas convection cooling systems, as disclosed herein, take advantage of this “wind chill” affect in their ability to remove heat from an object faster with a constant temperature of a gas. 
     In other words, if a 400° F. object is placed in a constant 75° F. atmosphere without velocity of the surrounding atmosphere, the transfer of energy from the object to the surrounding atmosphere by convection is much slower than if the atmosphere has a velocity over/around the object. An increase in velocity will increase the rate of energy transfer, even though the temperature of the atmosphere is constant. The rate of cooling can be increased or decreased by manipulating the velocity of the cooling medium as the temperature of the medium remains constant. This principle is advantageously utilized to significantly enhance the cooling efficiency of the system by creating, and controlling, “wind chili” temperature during the cooling process. As a result, the efficiency of the process is increased while simultaneously reducing the size, which is typically the length, of the cooling system. 
     However, the previous method disclosed by Thomas utilizes only a measurement of the ambient temperature within the cooling chamber to adjust the velocity and discharge of cryogen. An extrudate leaving a cooling chamber does not necessarily need to be cooled to an even temperature throughout, but may rely on “equilibrium cooling.” This principle is advantageously utilized according to the invention to significantly enhance the cooling efficiency of the system by creating and controlling the “wind chill” temperature during the cooling process in relation to a measurement of the temperature of the product after leaving the cooling chamber. The basis for “equilibrium cooling” is that a mass having two different temperature zones, or a temperature gradient, will exchange energy between the two zones until an “equilibrium” temperature is reached. Thus, a manufacturer can reduce cooling time and cooling system length by super-cooling at least the surface of the extrudate mass to form a “skin” having sufficient rigidity such that the extrudate may be handled as needed and then allowing the “equilibrium cooling” effect to take place after the extrudate has left the cooling system. 
     Another type of prior art cooling system utilizes a device called a “calibrator,” and typically multiple such calibrators, to cool extrusions. A calibrator is a tool which generally has a central opening through which the extrusion is fed, the central opening having a surface which is generally in contact with the surface of the extrusion as it is fed through. As a result of contact with the surface of the extrusion, the calibrator acts as a heat sink and the heat is conducted to the calibrator and away from the extrusion thus cool the extrusion. Since cooling of the extrudate tends to make the material contract or change shape, a vacuum generated by external vacuum pumps is generally drawn through grooves in the calibrator inner surface making contact with the extrudate. This vacuum assists in maintaining the shape of the extrudate. 
     To enhance the heat transfer from the extrusion, internal passages or circuits are provided in the calibrator through which a coolant is circulated. Typically, the coolant is water, but liquid nitrogen is also known to have been used to some degree. However, circulating liquid nitrogen through the cooling circuits has met with some difficulties regarding contact of the liquid nitrogen with the calibrators. Additionally, cooling water systems include the inherent problems associated therewith as discussed above. The aforementioned U.S. Pat. No. 6,389,828 to Thomas discloses that it is preferable to first vaporize a liquid cryogen, such as liquid nitrogen, and then to circulate the super-cold vaporize refrigerant through the cooling circuits instead of the liquid cryogen, which thus requires a system for vaporizing the liquid cryogen prior to circulation through the cooling circuits of the calibrator. 
     Although such a method is an improvement over the prior art, the system my still require the use of external vacuum pumps as previously stated. The present invention provides for a calibration tooling chamber utilizing forced-gas convection of a cryogenic refrigerant in combination with a calibrator tooling or sizing template having a plurality of fins in an outer surface thereof to allow the extrudate to be cooled at an effective rate. This eliminates the need for internal passages, and thus the additional manufacturing costs associated with the required set-up/connection/break-down of the equipment between different product runs. Further, the present disclosure, by use of a forced gas convection cooling chamber, provides a means of generating an internally induced vacuum to assist the extrudate without the requirement of a separate external pump. External vacuum pumps are expensive, require continued maintenance and repair, are noisy and they must be replaced often. 
     Many extruded articles include at least one hollow, such as pipe, hose, etc., or may contain several hollow portions. Some cooling systems provide the manufacturer with only the ability to cool extrudate from an outer surface thereof by contact with a cooler medium (liquid, gas or solid depending on the system). Depending on the product geometry, however, a significant amount of an extrudate&#39;s mass may be positioned inward of the outer surface and between several hollow portions. Thus, it is difficult to quickly and effectively cool such an extrudate quickly because the cooling medium does not make contact with those portions. 
     Another problem encountered with cryogenic cooling systems, especially those that are separated-air gasses such as nitrogen, is the loss of cryogenic coolant. Cryogenic cooling systems are usually pressurized and cryogen is lost through system leaks or through cryogen being expelled into the hollow of an extruded article and exhausted to atmosphere. Such loss of cryogen must be replaced within the system and can be expensive. 
     Accordingly, there is a need for a method and apparatus for cooling articles which can provide improved efficiency, reduce the size of the cooling system. 
     SUMMARY 
     The present disclosure is for an apparatus and method for a cryogenic cooling system for removing thermal energy from an extruded article known as an extrudate. The cryogenic cooling system includes a calibration table having one or more calibrators. Each calibrator having body and an internal cooling conduit allowing for the flow of a coolant through the calibrator. The calibrator removes thermal energy from the extrudate through conduction and also provides mechanical support for the outer surface of the extrudate after the extrudate leaves the extruder die. Each calibrator is also connected to a refrigeration unit for removing thermal energy from the circulating coolant. 
     The cryogenic cooling system of the present disclosure also includes a high velocity gas chiller. The high velocity gas chiller circulates a coolant within a chamber removing thermal energy from the extrudate by convection. The high velocity gas chiller controls the velocity of the coolant across the extrudate to optimize cooling efficiency. 
     The cryogenic cooling system of the present disclosure may also include inner profile cooling for extruded articles formed having a hollow or void. The inner profile cool may be performed by introducing coolant into the hollow or void of the extrudate to remove thermal energy from an inner surface. Directly introducing coolant into an extrudate inner void removes heat by convection. Nozzles and/or an inner profile coolant plenum may be used to direct the flow of coolant along the inner profile. Inner profile cooling may also be performed by an inner profile conduction cooler having a plurality of heat transfer surfaces configured to contact the inner surface of a hollow extrudate. 
     Another aspect of the present disclosure includes a cascade refrigeration system using multiple refrigeration systems arranged in series to maximize heat removal from the coolant circulated throughout the components of the cryogenic cooling system. 
     Yet another aspect of the present disclosure is a cryogenic gas chiller. The cryogenic gas chiller utilizes a liquid cryogen as a heat transfer medium for cooling a gaseous coolant circulated through the cryogenic cooling system. 
     The system of the present disclosure allows the working coolant to be circulated in one or more closed loops, thus minimizing coolant loss. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will be described hereafter with reference to the attached drawings which are given as non-limiting examples only, in which: 
         FIG. 1  is a schematic representation of an embodiment of the cryogenic extrudate cooling system of the present disclosure, including an calibration table and a high velocity convection chiller; 
         FIG. 2  is a representative perspective view of a calibrator for conductively cooling an outer surface of an extrudate; 
         FIG. 3  is a representation of coolant properties as the coolant flows through the calibrator depicted in  FIG. 2 ; 
         FIG. 4  is a schematic representation of a high velocity convection chiller; 
         FIG. 5  shows a detailed view of an inlet or outlet seal for the high velocity convection chiller of  FIG. 4 ; 
         FIG. 6  is an end view showing a seal frame and seal skirt for the seal of  FIG. 5 ; 
         FIG. 7  is a schematic representation of a cascade refrigeration system that allows compressed and dried atmospheric air to be used as a coolant for the cryogenic extrudate cooling system of the present disclosure; 
         FIG. 8  is a representation of a cryogenic gas chiller that may be used to remove thermal energy from the coolant used in the cryogenic extrudate cooling system of the present disclosure; 
         FIG. 9  is a perspective view of an inner profile convection cooler; 
         FIG. 10  is a section view of the inner profile convection cooler of  FIG. 9 ; 
         FIG. 11  is a perspective view of an inner profile convection cooler including an inner profile coolant plenum; 
         FIG. 12  is a plan view of the inner profile coolant plenum of  FIG. 11 ; 
         FIG. 13  is a perspective view of an inner profile conduction cooler of the present disclosure; and 
         FIG. 14  is an exploded perspective view of the inner profile conduction cooler of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     While the present invention will be described fully hereinafter with reference to the accompanying drawings, in which a particular embodiment is shown, it is to be understood at the outset that persons skilled in the art may modify the invention herein described while still achieving the desired result of this invention. Accordingly, the description that follows is to be understood as abroad informative disclosure directed to persons skilled in the appropriate art and not as limitations of the present invention. 
     The present disclosure relates to a cryogenic extrudate cooling system  10  and method. Referring to  FIG. 1 , material, for example plastic resin, is heated above its melting temperature and forced by an extruder  12  through a die  14  to form a predetermined shape. The extruded material  2 , or extrudate, having an outer surface  2   a  and possibly an inner surface  2   b  defining avoid  2   c , must be cooled to a temperature below its melting point to gain rigidity and to maintain the desired shape. A calibration table  200 , including one or more calibration tools, or simply “calibrators”  202 , are provided to exchange heat by conduction away from the extrudate  2 . After being cooled initially by the calibration tools  202 , the extrudate is fed through a cooling chamber wherein dried air convectively cools the extrudate. The present disclosure may also include inner profile cooling for extrudate having a shape that includes an inner cavity or void. The inner profile cooling of the present disclosure includes an inner profile coolant spray system introducing cooled cryogen into the cavity to conduct heat away from the inner surface of the extrudate by convection. The inner profile cooler may also include an inner profile conductive cooler in contact with the inner surface  2   b  of extrudate  2 . 
     Referring to  FIG. 2 , a calibrator  202  includes a passage  204  defining an inner surface  206  that makes contact with, but also provides for the passage of an extrudate  2 . By making contact with the extrudate  2 , the calibrator  202  acts as a heat sink and removes energy from the extrudate  2  through conduction. The calibrator  202  also assists the extrudate in maintaining its extruded shape. 
     Each calibrator  202  includes a first section  208  and a second section  210 . Each calibrator  202  may also include an internal cooling conduit  212  having an inlet  214  and an outlet  216 . Cooling conduit  212  is configured to allow the flow of cryogen through each of the first and second sections  208 ,  210  of calibrator  202 . While one cooling conduit is shown in the exemplary embodiment for clarity, it should be apparent that multiple conduits may be employed depending on the cooling requirements of a particular application. Also, calibrator may have additional sections. Further, internal cooling conduit may be configured entirely within a single calibrator section, or across two or more sections. 
     The calibrator may have an outer surface including a plurality of fins extending outwardly therefrom and running substantially parallel to the center axis of the passage  220 . The plurality of fins define a plurality of channels there between. Inclusion of the plurality of fins  234  greatly increases the outer surface area of the calibrator  202 . By increasing the outer surface area of the calibrator  202 , greater amounts of energy can be dissipated to the vaporized cryogen circulated in the cooling system. The plurality of fins also increase the mass of the calibrator  202  which increases the amount of energy (heat) the calibrator can remove from the extrudate. Vacuum grooves may also be provided in the inner surface  206 , preferably spaced apart and extending the entire circumference of the product passage  204 . At least one pinhole (not shown) may be provided from within each vacuum groove and extending to the outer surface, such that the pressure realized outside of the calibrator  202  is also communicated to the vacuum groove. Preferably, a pinhole is provided at such that a single vacuum groove includes a plurality of pinholes communication with the atmosphere outside the calibrator  202 . 
     Referring again to  FIG. 1 , a throttling expansion valve  218  is configured to admit cryogen coolant into the cooling conduit  212  at a temperature and pressure such that the cryogen is near its saturation point but yet completely in the liquid phase. The cryogen coolant enters the cooling conduit  212  at a point represented by A on  FIG. 3 . As the cryogen flows through the conduits in the calibrator, the heat removed from the extrudate is transferred to the cryogen resulting in a mixed-phase cryogen. The cryogen exits the cooling conduit  212  at outlet  216  as a gas represented by B on  FIG. 3 . By using a cryogen coolant that undergoes a phase change, the system of the present disclosure takes advantage of the latent heat of vaporization of the cryogen coolant to increase the heat transfer efficiency of thermal energy from the extrudate  2 . 
     Throttling expansion valve  218  may be controlled by any suitable controller known in the art, such as a microprocessor, programmable logic controller and other such systems. 
     Cooling conduit  212  forms part of a cooling circuit that further includes refrigeration unit  220  as shown in  FIG. 1 . Refrigeration unit  220  may be of the vapor-compression type commonly known in the art, using commercially available refrigerants such as R404A. Refrigeration unit may also be of the cryogenic gas chiller type described below. 
     Refrigeration unit  220  removes the heat absorbed by the cryogen coolant from extrudate  2 , allowing the cryogen coolant to be circulated in a closed loop circuit as generally indicated by arrows  222 ,  224 . The exemplary embodiment shown in  FIG. 1  utilizes a number of refrigeration units for calibrator  202  cooling circuits. The number of refrigeration units may be varied depending on the thermal load, the number of calibrators, the desired control characteristics, etc. For example, a given calibrator  202  may be associated with a single refrigeration unit  220 . Alternatively, a calibrator  202  may have more than one refrigeration unit  220  if the thermal loading requires. Also, a single refrigeration unit may support more than one calibrator  202  for lower thermal loads. 
     Referring to  FIG. 1 , after passing through one or more calibrators  202  the extrudate  2  may also pass into a high velocity extrusion chiller  400 . The high velocity convection chiller  400  circulates a cryogenic gas about the outer surface of the extrudate to remove additional heat from the extrudate. 
     A schematic representation of a high velocity convection chiller  400  is shown in  FIG. 4 , depicting the internal duct work of the chiller  400  including a chamber  402 . The framework and insulation materials have been removed for ease of discussion. Forced gas convection cooling systems are described in U.S. Pat. No. 6,389,828, which is incorporated herein in its entirety. The chiller  400   10  includes a variable speed fan  404  or other suitable means for circulating a gas. The chiller  400  includes a back chamber  406 , referred to as a chamber, and a front chamber  408 , known as the cooling chamber, connected by end duct  410 . The end duct  410  includes an extrudate inlet throat,  412  including an extrudate inlet passage  414 , or other opening, through which an extrudate  2  may enter the chiller  400 , preferably traveling in a direction shown by arrow  416 . 
     In operation, the fan  404  preferably circulates the gas contained in the system in a direction shown by arrows  418 , although circulation may be in the reverse direction if desired. Gas is drawn through fan  404  from return chamber  420 , from the front chamber  405  through return throat  430  and discharged from the fan  404  into the back chamber  406 . The gas enters the front chamber  408  from the end duct  410  inlet throat  412 , such that the gas travels through the front chamber  408  in the same direction as the extrudate  2 . This process can be repeated as the gas is continuously circulated through the chamber  402  in a closed loop to cool an extrudate. A convection coolant feed line  422  is in communication with a convection coolant source  424  and is adapted to deliver convection coolant, such as nitrogen or air chiller  400 . Preferably, the feed line  422  extends into the back chamber  406  and includes a spray bar  426  having a plurality of orifices to evenly inject and distribute liquid cryogen. Preferably, the feed line  422  is placed in communication with the back chamber  406  downstream from the fan  404  to inject or distribute convection coolant into the stream of circulated gas. 
     The fan  404  can be controlled by a controller (not shown) to circulate the vaporized cryogen at a variable velocity through the back chamber  406 , end duct  410 , and front chamber  408  where it cools the extrudate  2 . The cooling process continues, including the injection of additional liquid cryogen into the back chamber  406  as needed to obtain, or maintain, a desired temperature in the front chamber  408 . 
     In one embodiment of the present disclosure, the convection coolant may be liquid nitrogen, however, other cryogens such as liquid carbon dioxide, liquid air and other refrigerants having normal boiling points substantially below minus 50° F. (−46° C.) can also be used. The liquid nitrogen expands 700 times its volume in liquid state, capturing a high BTU as it transitions to gaseous form, creating a highly effective refrigerant and rapidly reducing the temperature in the convection chiller  400 . 
     In another embodiment of the present disclosure, the convection coolant may be dried, super-cooled atmospheric air. Compressed, low-dewpoint air can provide an economical alternative to separated liquid and/or gaseous cryogens. The cooling system and method of the present disclosure may employ a cascade refrigeration system, as described below, to cool compressed and dried atmospheric air for use as a convection coolant. 
     The extrudate  2  enters the chiller  400  through the extrudate inlet passage  414  and travels through the front chamber  408  where it is cooled by the circulating convection coolant. An extrudate outlet passage  428 , or other opening, is provided at an end of the front chamber  408  opposite the extrudate inlet passage  414  that allows the extrudate to exit die chiller  400 . 
     Preferably, both the extrudate inlet passage  414  and outlet passage  428  are equipped with seals to prevent and/or reduce the ingress of ambient air and egress of convection coolant to and from the system. Optionally, the seals can be selected or designed to permit excess pressure in the system to vent outside. In such a case, a separate vent may not be needed. 
     The chiller  400  can further include a number of other components for controlling, optimizing, and generally automating the cooling process. These other components can include a vent  432 , an internal temperature sensor  434 , and a heating unit  436 . A controller (not shown) can include a microprocessor, programmable logic controller, etc., for controlling the operation of the cooling system  10  and its various subparts, either automatically or under the control of an operator. 
     A vent  432  can be provided, for example in the back chamber  406  as shown, to release pressure build up which may be created by the pressure increase resulting from heat transfer from the extrudate  2  to the convection coolant. The vent can simply be a small orifice and is preferably placed upstream of the coolant feed line  422  and spray bar  426  and downstream of the front chamber  408  (with respect to gas flow as shown by arrows  418 ) to minimize the loss of cooling capacity. By venting after the coolant has been circulated over the hot extrudate  2  and before the spray bar  426  distributes fresh convection coolant, the vented coolant has removed energy from the product and is the warmest portion of coolant in the system and therefore does not waste newly delivered coolant. 
     Temperature sensor  434  can be provided in communication with the convection coolant stream generally at any point, but is preferably in the front chamber  408 , back chamber  406 , or end duct  410 , as shown, to monitor temperature of the coolant at a desired point. Alternatively, additional temperature sensors could be positioned at different locations to detect the temperature of the gas at several points in the chiller  400 . Output from the temperature sensor  434 , and other sensors, if more are used, can be provided to the controller for use in regulating the speed of the fan  404  and controlling a valve  438  provided in the coolant line  422  to introduce coolant into the back chamber  406 . 
     The temperature sensor  434  can be, for example, a thermocouple. The controller can be programmed with the wind chili equation and can also receive a signal from the fan  404  indicative of the fan&#39;s speed. This data can be used to determine the effective temperature in the front chamber  408 . The heating unit  436 , can be a simple heating element and can be located, for example, in the back chamber  406 , as shown in the figure. The heating element can be operated by the controller to increase the temperature in the chamber  402 , if necessary, to adjust and maintain the desired ambient temperature. Multiple such cooling systems may be placed in series and operated independently or together. 
     In a preferred embodiment of the present invention, an external temperature sensor  440 , such as an infrared temperature sensor, is provided at a desired point downstream from the extrudate outlet passage  428  to sense the temperature of the extrudate  2  after exiting the front chamber  408 . For example, the external temperature sensor  440  could be placed adjacent the extrudate outlet passage  428  or may be placed further downstream, such as adjacent a cutting assembly or puller. The external temperature sensor  440  senses the surface temperature of the extrudate  2  and relays the output to the controller. The controller utilizes the output from external temperature sensor  440  in addition to temperature sensor  434  (and additional temperatures if provided) in regulating the speed of the fan  404  and controlling the valve  438  provided in the coolant feed line  422  to introduce coolant into the back chamber  406 . 
     The cooling efficiency of the system can generally be optimized by using principles of forced air convection. Extraction of heat from an extrudate  2  can be increased by blowing coolant over a warm surface. The effective temperature inside the front chamber  408 , or cooling chamber can be calculated from the ambient temperature and the velocity that the coolant is blown over the surface of the extrudate  2  using the following equation for calculating wind chill temperature:
 
 T   wc =0.0817(3.71 V   0.5 +5.81−0.25 V )( T− 91.4)+91.4
 
     More specifically, the efficiency of the chiller  400  can be optimized, i.e., maximum cooling using a minimum amount of coolant by controlling the speed of the fan  404 . In particular, for a given amount of coolant introduced into the back chamber  406  or feed chamber, the speed of the fan  404  can be increased in order to increase the rate in cooling of the front chamber  408  without adding additional coolant. Only when the speed of the fan  404  is at its maximum, would it be necessary to inject additional coolant into the back chamber  406  to further reduce the temperature in the front chamber  408 . Moreover, the temperature in the front chamber  408  can also be regulated to a set point temperature by adjusting the speed of the fan  404 , faster or slower, instead of injecting more coolant. Output from the external temperature sensor allows the controller to manipulate the wind chill within the front chamber  406  to increase or decrease the cooling of the extrudate  2 . In this sense, the chiller  400  can be optimized based on the optimum product temperature. Thus, minimum necessary coating using a minimum amount of coolant is achieved. 
     Convection chiller  400  may also include one or more refrigeration systems  442 ,  444 , to provide additional cooling of the coolant circulating in chamber  402 . Refrigeration systems  442  and  444  may be connected to heat exchangers  446 ,  448 , respectively. As the convection coolant is blown by fan  404  through the back chamber  406 , heat is removed from the coolant as the coolant passes over heat exchanges  446 ,  448 , allowing the coolant to be recycled in a closed loop system. 
     Refrigeration units  442  and  444  may be of the mechanical vapor-compression type, or may be of the cryogenic gas chiller type described below. In one embodiment of the present disclosure, refrigeration units  442 ,  444  may utilize a vapor-compression refrigeration cycle wherein a refrigerant, such as R404A, may be circulated through a thermal expansion valve  450 ,  452 . 
     High velocity extrusion chiller  400  may also include seals  454 ,  456  positioned to prevent and/or reduce ingress of ambient air into chamber  402  through inlet passage  414  and outlet passage  428 . Because of the operating temperatures of the chiller  400 , ingress of ambient air can result in frost build up on heat exchangers  446 ,  448  which acts to reduce the efficiency of heat transfer and also can impede air flow. 
     Seals  454  and  456  are essentially the same configuration, and a detailed view of seal  454  is shown in  FIG. 5  and is discussed as an exemplary embodiment. Seal  454  includes a seal housing  456  disposed between seal frames  458 ,  460 . In the exemplary embodiment, seal  454  also includes additional seal housings  462  and  468  and additional seal frames  464  and  470 . Seal frames  458 ,  460 ,  464 , and  470  support a seal skirt  472 , as shown in  FIG. 6 . Seal skirt  472  is formed from a resilient material such as neoprene 5rubber, polyethylene, or other suitable material. Seal skirt  472  is formed to include an aperture  474  matching the exterior shape of extrudate  2 . 
     In the exemplary embodiment of  FIG. 5 , seal frames  458 ,  460 ,  464 , and  470 , and seal housings  456 ,  462 , and  468 , define a series of seal locks  476 ,  478 ,  480 . Seal skirts  472  are in contact with the outer surface of extrudate  2  as it passes through seal  454 , creating separate gas-tight zones prevent ingress of ambient air and egress of coolant. 
     Seals  454  and  456  may also include a coolant manifold  482  to introduce pressurized coolant into locks  476 ,  478 ,  480 . Coolant manifold may also include control valves  484  and pressure sensors  486 . Control valves  484  and pressure sensors  486  may be connected to a controller such as a microprocessor, programmable logic controller, or other device to maintain a predetermined pressure within each lock. Maintaining the seal pressure above the ambient pressure will thus prevent ingress of atmospheric air into chamber  402 . Likewise, maintaining the seal pressure above the coolant pressure inside chamber  402  will prevent loss of coolant. 
     Referring again to  FIG. 4 , the back chamber  406  of the high velocity extrusion chiller  400  may also include a diffuser duct  488  and a convergence duct  490 . 
     Diffuser duct  488  expands in cross sectional area of back chamber  406  thus reducing the velocity of the coolant flow as the coolant flows from fan  404  to heat exchangers  446  and  448 . Slowing the velocity of the coolant across heat exchangers  446 ,  448 , produces a more efficient heat transfer. After the coolant passes through heat exchangers  446  and  448 , it is desirable to increase the velocity to take advantage of convective cooling to remove heat from extrudate  2 . It is desirable that the velocity of the coolant across heat exchangers  446 ,  448  be approximately 100 to 1000 ft/min. After the coolant passes across heat exchangers  446  and  448  convergence duct  490  reduces the cross section of back chamber  406  and consequently increases the coolant velocity for introduction into front chamber  408 . 
     Referring again to  FIG. 1 , coolant source  424  is in fluid communication with chamber  402  and may also provide pressurizing coolant to seals  454  and  456 . In embodiments of the present disclosure, coolant may be any cryogen, such as nitrogen, carbon dioxide, or air. Commercially available cryogens in their pure form require them to be separated from air and transported to the end user. An embodiment of the present disclosure includes a cascade refrigeration system  600  as part of coolant source  424  that allows for compressed, dehumidified atmospheric air to be used as a coolant in the system of the present disclosure. 
     Cascade refrigeration system  600 , as shown in  FIG. 7  allows dried, compressed atmospheric air to be used as a coolant in the cryogenic cooling system  10  of the present disclosure. Cascade refrigeration system  600  includes an air cooler  602  for removing thermal energy from dried compressed air. Dried compressed air is provided by an air compressor  604 . After being compressed the air may pass through an after cooler  606 , a storage tank  608 , an air/air heat exchanger  610 , and an air dryer  612 . After the air is dried, it is passed through air cooler  602  and cooled to a temperature between −50° F. and −150° F. 
     Cascade refrigeration system  600  uses a series of vapor-compression refrigeration systems. The exemplary embodiment uses a first refrigeration system  614  including a compressor  616 , a condenser  618 , a thermal expansion valve  620 , and a heat exchanger  622  acting as an evaporator. A blower  624  is provided to remove heat from the working fluid of first refrigeration system  614  in condenser  618 . A second refrigeration system  626  includes a compressor  628 , heat exchanger  622  acting as a condenser, a thermal expansion valve  630 , and air cooler  602 . First refrigeration system  614  cools the working fluid of second refrigeration system  626 , making possible removal of a large amount of heat from the air in air cooler  602 . 
     An aspect of the present disclosure may also include a cryogenic gas chiller  700  for cooling a cryogenic fluid delivered to the extrudate cooling system  10  and may generally be used in place of mechanical vapor-compression chillers. Referring to  FIG. 8  the cryogenic gas chiller includes a housing  702  defining a chamber. The chamber  704  is configured receive a liquid cryogen  706 , such as liquid nitrogen. Within the chamber is a heat exchanger  708 . In the exemplary embodiment, the heat exchanger  708  comprises an inlet manifold  710  and an outlet manifold  712 . Heat exchange conduits are coiled between the inlet and outlet manifolds. In the exemplary embodiment, copper tubing is used as the inlet and outlet manifolds  710 ,  712  and the heat exchange conduits. As should be apparent, brass, aluminum, and an array of other thermally conductive materials and alloys are equally acceptable. 
     The heat exchanger coil  708  is positioned in the chamber such that it is partially submerged in the liquid cryogen. The cryogenic gas chiller also includes a plunger  714  within the chamber  702 . The plunger  714  is coupled to a positioner  716  configured to move the plunger  714  up and down within the chamber  704 . In exemplary embodiments, the positioner may be a hydraulic or pneumatic cylinder, servo motor, or other suitable mechanism. Cryogenic gas cooler  700  may also include insulation  718  for chamber  702  to reduce heat transfer from the ambient environment to the liquid cryogen within chamber  704 . 
     Raising and towering the plunger  714  results in a displacement of the liquid cryogen  706  contained in the chamber  704 . Consequently, lowering the plunger causes the level of liquid cryogen to rise, contacting a greater surface area of the heat exchanger. Alternatively, raising the plunger results in a drop in the level of liquid cryogen, wherein the liquid cryogen in contacts less of the surface area of the heat exchanger. 
     The rate of heat transfer by the heat exchanger  708  may be controlled by adjusting the level of the liquid cryogen  706 . The greater the surface area of the heat exchanger in contact with the liquid cryogen, the greater the rate of heat transfer. 
     A cryogenic gas is introduced through the tube-side of the heat exchanger. The cryogenic gas may then be used for other cooling applications such as inner profile cooling, forced convection extrudate cooling, and other similar applications. 
     The cooling system  10  of the present disclosure may also include cryogenic inner profile cooling for removal of thermal energy from the inner surface  2   b  of extrudate  2  as shown in  FIGS. 9-14 . In one aspect of inner profile cooling of the present disclosure, a cryogenic gas is introduced directly into an internal cavity or void of the extrudate as illustrated in  FIGS. 9-12 . The cryogenic gas transfers heat by convection from the extrudate and is exhausted to atmosphere. 
     Inner profile cooling system  800  includes a coolant source delivered through coolant line  802 , preferably a cryogen such as nitrogen, carbon dioxide, or air, the injection of which into an inner cavity or void of an extrudate  2  can be controlled by a feed valve (not shown) placed in coolant line  802 , which itself can be operated by a controller such as a microprocessor, programmable logic controller, etc. 
     The coolant is communicated to an extruder die  14  via outlet conduit  804 . Extruder die  14  is shown in more detail in  FIGS. 9 and 10 . Extruder die  14  includes an inlet bore  806  extending from an outer surface  18  of the extruder die  14  through a mandrel  16  that is adapted to form an extrudate hollow  2   c  within the extrudate  2 . The inlet bore  806  is adapted to be placed in fluid communication with the outlet conduit  804  and thereby pass coolant through the extruder die  14  and mandrel  16  and into the extrudate hollow  2   c . Inlet bore  806  and outlet conduit  804  may be separably coupled such that different dies can be interchanged for different product configurations. Coolant may enter the extrudate hollow  2   c  to transfer heat away from inner surface  2   b  of extrudate  2 . 
     In another aspect of the present disclosure includes, an outlet extension  808  is provided to ensure that the pressure exerted by the vaporized cryogen as it is introduced into the extrudate hollow  2   c  is spaced from a leading edge  20  of the die  14 . Outlet extension  808  may further include an inner profile coolant plenum  810 , as illustrated in  FIGS. 11 and 12 . Coolant plenum  810  is in fluid communication with coolant source  802  and is configured to have the shape of the hollow  2   c  of extrudate  2 . Coolant plenum includes a number of orifices  812  arranged to distribute coolant across inner surface  2   h  extrudate  2 . In the exemplary embodiment, coolant plenum  810  is coupled to outlet extension  808 . Preferably, coolant plenum is removable coupled to coolant source  802  to accommodate various shapes of extrudate  2 . 
     The outlet extension  808  may also include a nozzle  814  or other means for directing the flow of coolant onto an inner surface  2   b  of the extrudate  2 . Additionally, orifices  812  may also include nozzles to direct coolant flow from coolant plenum  810 . 
     Referring to  FIGS. 13 and 14 , the cryogenic cooling system  10  of the present disclosure may also include an inner profile conduction cooler  900 . The inner profile conduction cooler  900  includes heat transfer surfaces  902  having cooling conduits  904  through which coolant is circulated. In the exemplary embodiment shown in  FIGS. 13 and 14 , inner profile conduction cooler is configured for an extruded 4 inch by 4 inch hollow fence post. As should be apparent, inner profile conduction cooler  900  may be configured to cooperate with any inner profile shape of extrudate  2 . 
     Inner profile conduction includes a coolant source, preferably a cryogen such as nitrogen, carbon dioxide, or air, delivered through the mandrel  16  of die  14 . Coolant flow can be controlled by a feed valve (not shown) placed in coolant line, which itself can be operated by a controller such as a microprocessor, programmable logic controller, etc. Coolant inlets  906  located in mandrel  16  are connected to a coolant source. Coolant inlets  906  are in fluid communication with conduits  904  disposed within heat transfer surfaces  902 . An actuator  908  may be attached to the face  20  of die  14 . Heat transfer surfaces  902  are pivotally attached to support arms  910  at a lug  920 . Support arms are also pivotally coupled to abuse  912 , which is attached to the face  20  of die  14 . Actuator  908  includes a positioner  914  having a number of faces  916 , each configured to cooperate with corresponding surface  918  on each support arm  910 . 
     Actuator  908  moves positioner  912  to adjust support arms  910  such that heat transfer surfaces  902  maintain contact with inner surface  2   b  of extrudate  2 . As the extrudate  2  leaves die  14  and passes over heat transfer surfaces  902 , thermal energy is removed from inner surface  2   b  causing the extrudate to “skin” resulting in rigidity to maintain the desired inner profile shape. Additionally, inner profile conduction cooler  900  also provides mechanical support for extrudate  2  immediately as extrudate  2  exits die  14 . This additional mechanical support allows for greater control of the inner profile shape of the extrudate  2 , resulting in a higher quality product. 
     Aspects of the present disclosure may be connected to any of various controllers, such as microprocessor based controllers, programmable logic controllers, data acquisition systems, etc. allowing for automated control of the entire extrudate cooling system  10  or the various parts thereof. As should be apparent to one skilled in the art, data, such as temperature, pressure, velocity, flow rate, and other parameters may be provided to the controller by any of the various sensors available in the art. The controller may include one or more algorithms for analyzing the input data to automatically adjust system parameters to optimize system operation. 
     Advantageously, the present invention allows extrudate with a hollow profile to be cooled from the outside and from within. The internal and external surfaces of the extrusion can be cooled at equal or variable rates, which allows for extensive process control heretofore unseen. The present invention, by providing cooling from within the extrusion, provides for quicker cooling and shorter cooling chamber lengths. Also, the internal coolant flow provides a positive pressure against the internal surfaces of the extrusion, which in turn reduces or eliminates the need for an external vacuum on the outer surface of the extrudate to provide a quality product. Since less external vacuum is required, the amount of drag between the product and tooling is reduced, which provides for increased rates of production and smaller downstream, equipment such as pullers. 
     Various features of the invention have been particularly shown and described in connection with the illustrated embodiments of the invention, however, it most be understood that these particular embodiments merely illustrate and that the invention is to be given its fullest interpretation within the terms of the appended claims.