Abstract:
A method and apparatus for using a cryogen for cooling articles, particularly having applications for chilling extrusions, food, and similar articles, utilizing dispersion of liquid cryogen into a feed chamber wherein it is substantially vaporized and then circulated through a cooling chamber containing the article to be cooled. A circulation device can circulate the vaporized cryogen through the cooling chamber, or through the article, at a variably controllable velocity to enhance the cooling efficiency using the principle of forced air convection and to provide improved temperature control in the system.

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/298,856 filed Jun. 15, 2001, U.S. Provisional Application Ser. No. 60/298,851 filed Jun. 15, 2001, U.S. Provisional Application Ser. No. 60/299,131 filed Jun. 15, 2001, U.S. Provisional Application Ser. No. 60/298,854 filed Jun. 15, 2001, and U.S. Provisional Application Ser. No. 60/298,852 filed Jun. 15, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a method and apparatus for cooling extrusion articles, and more specifically to substantially vaporizing a liquid cryogen and then circulating the vaporized cryogen through a cooling chamber, 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. The invention is particularly useful as an extrusion chiller, and may also be utilized for chilling foods. 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. 
     BACKGROUND OF THE INVENTION 
     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, cooling 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 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 in accordance therewith. In practice, an optimum cooling temperature of approximately 50° F. is achievable from a cost-effective standpoint, which limits the manufacturer&#39;s 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 fully 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 nitrogen or any other refrigerant. 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 in U.S. Pat. No. 6,389,828, incorporated herein in its 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, which is frequently reported on TV or radio by weather announcers. In that context, wind chill temperature is what the temperature outside “feels” like, 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 chill” 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 51% 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 cooling 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 vapor/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 may 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 invention, 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. Prior art cooling systems provide the manufacturer with only the ability to cool an 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. The present invention provides an apparatus and method for cooling an extrudate having at least one hollow by circulating a vaporized cryogen through the hollow, preferably in combination with exterior cooling techniques as disclosed in U.S. Pat. No. 6,389,828 and taught herein. This provides for increased cooling capacity and control, as well as reduced cooling system length requirements. 
     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, and a cooling system that does not require external vacuum pumps. 
     SUMMARY OF THE INVENTION 
     A method and apparatus for cooling articles are provided which can utilize the dispersion of a liquid cryogen into a feed chamber wherein the liquid cryogen is substantially vaporized and then circulated through a cooling chamber containing the article to be cooled. The vaporized cryogen can be further circulated though the cooling chamber at a controllable velocity, over/around the surface of the article to be cooled and/or tooling, in order to regulate the rate of cooling the article by controlling the wind chill temperature, based upon the principles of forced gas convection. 
     A presently preferred cryogen is liquid nitrogen. The liquid nitrogen can be dispersed into a feed chamber in a controlled manner using a valve, which can be operated by a controller, such as a microprocessor. Since the temperature in the feed chamber is much higher than the boiling point of the liquid nitrogen, a high BTU (British Thermal Unit) and expansion rate is captured thereby producing an extremely effective refrigerant. The feed chamber can be communicated with a cooling chamber into which the vaporized cryogen can be circulated by a fan, or other device for circulating a gas and/or vaporized cryogen. Either the feed chamber or the cooling chamber can be vented to dissipate pressure generated as the liquid nitrogen rapidly expands to gaseous form. The fan can preferably be a variable speed fan, or other variable speed circulation device, for circulating the vaporized cryogen through the system at a controllable velocity to take advantage of principles of forced gas convection. The fan can be located in the feed chamber to aid in substantially vaporizing the liquid cryogen. However, considering the relatively high temperature utilized in the cooling chamber compared to the boiling point of the cryogen, even without the fan, the liquid cryogen will virtually completely and instantaneously vaporize as it is injected into the feed chamber. The fan can be operated by the controller which can regulate the speed of the fan to provide improved temperature control over the system by controlling the wind chill temperature in the cooling chamber. The system can also include a temperature sensor, connected to the controller, for monitoring the temperature in the cooling chamber, and to calculate the wind chill temperature. An additional external temperature sensor is provided and connected to the controller. The external temperature sensor is adapted to monitor the temperature of an article after the article has exited the cooling chamber and relays the output signal to the controller, which can operate the fan and valve to provide improved temperature control over the system by controlling the wind chill temperature in the cooling chamber in relation to the article&#39;s exit temperature. A heating device can be provided to increase the temperature in the cooling chamber, if needed. The speed of the fan can be controlled by the microprocessor to circulate the refrigerant at a high volume (CFM) to maximize the cooling efficiency, thereby minimizing cryogen consumption. Essentially, the rate of cooling of the article can be increased for a given amount of cryogen dispersed into the feed chamber by increasing the speed of the fan. Another way to express this concept is to say that the “effective temperature” in the chamber can be reduced by increasing the speed of the fan. The articles to be cooled can be delivered into the cooling chamber by means of a conveyor belt, or various other ways of feeding articles, for example pulling extrusions, through the cooling chambers. 
     The cooling system can also employ a plurality of cooling chambers, preferably adjacent, each of which can be individually controlled by one or more controllers. The controllers can manage the speed of the fan and the nitrogen injection for each individual cooling chamber, thereby providing for maximum heat exchange rates for efficiency and effectiveness. Each cooling chamber can be equipped with its own temperature sensor, nitrogen injection valve to control the introduction of nitrogen into the cooling chamber, and variable speed fan for circulating refrigerant through the cooling chamber. 
     In general operation, the temperature sensor detects the temperature in the cooling chamber, or of the circulated refrigerant, and the external temperature sensor detects the temperature of an article that has exited the cooling chamber and each feed the respective information to the controller. The controller can be programmed with a desired temperature to which the temperature inside the cooling chamber is to be regulated or to the desired temperature of the article as it exits the cooling chamber. The controller can also control the nitrogen injection valve and the speed of the fan to cause the temperature in the cooling chamber to correspond to the desired temperature or temperature calculated to cool the article to the desired article temperature. An equation for calculating the “effective temperature,” i.e. wind chill temperature, from the speed of the fan and the ambient temperature in the cooling chamber can be programmed into the microprocessor. The speed of the fan can thus be regulated to increase or decrease the rate of cooling of the article, by adjusting the effective temperature in the cooling chamber, in order to maximize the efficiency of the cooling system. Principles of forced air convection can thus be utilized to increase cooling efficiency while minimizing the consumption of nitrogen. Likewise, principles of forced gas convection can be utilized in combination with principles of “equilibrium” cooling to quickly cool surfaces of an article to produce a “skin” of sufficient rigidity for further handling. A “skin” may be super-cooled (cooled to a temperature below the desired article temperature), but the core remaining at a temperature higher than the desired article temperature. The warmer core regions continue to transfer energy to the cooler “skin” regions after exiting the cooling chamber until the two regions reach an “equilibrium” temperature. Thus, the cooling systems of the present invention can produce the required cooling with less line space. The fan additionally permits improved system control over the effective temperature in the cooling chamber. A method of cooling an article using “equilibrium” cooling according to the invention comprises the following steps: a) introducing liquid cryogen into a feed chamber wherein said liquid cryogen is substantially vaporized; b) circulating said vaporized cryogen from said feed chamber into a separate cooling chamber containing said article to be cooled; c) circulating said vaporized cryogen at a controllable velocity from said feed chamber into said cooling chamber and around said article to create a wind chill temperature in said cooling chamber to increase a rate of cooling of said article; d) sensing the temperature in at least one of said feed chamber and said cooling chamber; e) calculating said wind chill temperature in said cooling chamber, said wind chill temperature being a function of the temperature in said cooling chamber and the velocity at which said vaporized cryogen is circulated through said cooling chamber over said article; f) selecting a desired product temperature; g) sensing the temperature of the article prior to entering said cooling chamber and calculating a difference between said desired product temperature and said temperature of the article prior to entering said cooling chamber; h) calculating an amount of energy that must be removed from said article during the resonance time said article is in said cooling chamber necessary to cool greater than 50% of the mass of said article to a super-cool temperature below the desired product temperature, such that the difference between said super-cool temperature and said desired product temperature is greater than or equal to said difference between the sensed temperature of the article prior to entering the cooling chamber and the desired product temperature, said amount of energy being a function of the heat capacity, thermal conductivity, and resonance time of said article in said cooling chamber; i) calculating a wind chill temperature necessary to remove said amount of energy; and i) controlling said velocity to cause said wind chill temperature to correspond to said wind chill temperature necessary to remove said amount of energy. 
     Another embodiment of the invention is a cooling system which, utilizing wind chill temperatures, is particularly adapted to vaporize a liquid cryogen and circulate the refrigerant over/pass metal tools for an article within the tool. Specific examples of such tools are a calibrator and a sizing template, which are commonly used to cool extruded articles. The metal tools are provided with a plurality of fins extending from an outer surface thereof that provide for increased external surface area. The metal tools are enclosed within a cooling chamber, or chambers and the metal tools, such as calibrators, through which an extrusion is passed to be cooled, is itself, along with the extrusion, cooled within a cooling chamber. Advantageously, such a system can be vacuum assisted without the need for costly external vacuum pumps. The cooling chamber includes an outlet throat through which refrigerant enters the cooling chamber and an inlet throat through which the refrigerant exits the cooling chamber and is recirculated by a fan. By providing the outlet throat with a cross-sectional area less than the cross-sectional area of the inlet throat, the fan is thus “starved” and a vacuum is induced within the cooling chamber. Preferably, a restrictor plate or other suitable mechanism is provided that can be operated to vary the cross-sectional area of the outlet throat, inlet throat, or both. 
     Another embodiment of the invention is a cooling system which, utilizing principles of forced gas convection, is particularly adapted to vaporize a liquid cryogen and circulate the vaporized through a hollow within an extrudate. The cooling system includes similar components as previously discussed, except the vaporized cryogen is communicated to the hollow through an inlet bore provided in an extruder die and mandrel. Preferably, the cooling system is “captive” and the vaporized cryogen is recirculated. For example, the vaporized cryogen can exit the hollow within a closed cutting chamber. The cutting chamber communicates with a fan via a return conduit. Operation of the system is the same as previously described. Optionally, the cooling system is used in combination with a cooling system to simultaneously cool the outer surface of the extrudate, such as a metal tool cooling system according to the invention. 
     Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying drawing figures of certain embodiments thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is perspective view of a simplified representation of a presently preferred embodiment of a forced gas convection cooling system. 
     FIG. 2 is a perspective view of another presently preferred embodiment of a forced gas convection cooling system  100  in combination with a conventional wet jacketed vacuum calibration cooling system  400 . 
     FIG. 3 is a perspective view of an embodiment of a forced gas convection cooling system  300  using sizing templates in combination with a forced gas convection calibration cooling system  200 . 
     FIG. 4 is a perspective view of a calibrator according to the invention. 
     FIG. 5 is a perspective view of a sizing template according to the invention. 
     FIG. 6 is a front perspective view of a sizing template assembly. 
     FIG. 7 is a front perspective view of the sizing template assembly shown in FIG.  6 . 
     FIG. 8 is schematic representation of the method of inducing an internal vacuum. 
     FIG. 9 is a perspective view of an extruder die having two mandrels to form an extrudate with two hollows. 
     FIG. 10 is a section view taken along line  571 — 571  of FIG.  9 . 
     FIG. 11 is a side view of a schematic representation of a presently preferred embodiment of a forced gas convection system for internally cooling an extrudate having a hollow. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 a broad informative disclosure directed to persons skilled in the appropriate art and not as limitations of the present invention. 
     A simplified perspective view of a forced gas convection cooling system  10  is shown in FIG. 1, depicting the internal duct work of the cooling system with an external “chamber”  11  shown in phantom lines. 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 cooling system  10  includes a variable speed fan  12  or other suitable means for circulating a gas. The fan  12  includes a motor housing  14  and a blade housing  16 , which encloses fan blades  18 . The cooling system  10  includes a back chamber  20 , referred to as a “feed” chamber, and a front chamber  22 , known as the “cooling” chamber, connected by end duct  30 . The end duct  30  includes an extrudate passage  32 , or other opening, through which an extrudate  25  (shown in FIG. 1 after passing through the cooling system for ease of illustration) may enter or exit the cooling system  10 , preferably traveling in a direction shown by arrow  15 . In operation, the fan  12  preferably circulates the gas contained in the system in a direction shown by arrows  13 , although circulation may be in the reverse direction if desired. Gas is drawn into the blade housing  16 , which acts as a return chamber, from the front chamber  22  through an inlet throat  26  and discharged from the fan  12  into the back chamber  20 . The gas enters the front chamber  22  from the end duct  30  through outlet throat  28 , such that the gas travels through the front chamber  22  in the same direction as the extrudate. This process can be repeated as the gas is continuously circulated through the cooling system  10  to cool an extrudate. A liquid cryogen feed line  36  is in communication with a liquid cryogen source (not shown) and is adapted to deliver liquid cryogen, such as nitrogen, to the system  10 . Preferably, the feed line  36  extends into the back chamber  20  and includes a spray bar  38  having a plurality of orifices to evenly inject and distribute liquid cryogen. Preferably, the feed line  36  is placed in communication with the back chamber  20  downstream from the fan  12  to inject or distribute liquid cryogen into the stream of circulated gas, which aids in the vaporization and distribution thereof before it reaches the front chamber  22  containing the extrudate. At the presently preferred operating temperatures of the cooling system  10 , substantially complete and instantaneous vaporization of the liquid cryogen occurs upon release or injection into the back chamber  20  or any other suitable point of entry. However, there may be alternative applications wherein a much lower operating temperature may be utilized, such that there is a greater probability of the liquid cryogen not totally vaporizing. In such applications, a larger feed chamber (not shown) in combination with the fan  12  can provide a region wherein substantially complete vaporization of the liquid cryogen is provided, thereby reducing the likelihood of any liquid cryogen being distributed onto the surface of the extrudate. The liquid cryogen is preferably 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 cooling system  10 . The fan  12  can be controlled by a controller  50  to circulate the vaporized cryogen at a variable velocity through the back chamber  20 , end duct  30 , and front chamber  22  where it cools the extrudate. The cooling process continues, including the injection of additional liquid cryogen into the back chamber  20  as needed to obtain, or maintain, a desired temperature in the front chamber  22 . The extrudate enters the cooling system through the extrudate passage  32  and travels through the front chamber  22  where it is cooled by the circulating cryogen gas. An extrudate outlet passage  40 , or other opening, is provided at an end of the front chamber  22  opposite the extrudate passage  32  that allows the extrudate to exit the system  10 . Preferably, both the extrudate inlet passage and outlet passage  32  and  40  are equipped with a sealing means, such as an end template (shown in FIG.  3 ), neoprene gasket or other means known in the art, that prevents or reduces the ingress of air and egress of vaporized cryogen to and from the system. Optionally, the sealing means 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 cooling system  10  can further include a number of other components for controlling, optimizing, and generally automating the cooling process. These other components can include a vent  34 , an internal temperature sensor  42 , and a heating unit  44 . The controller  50  can include a microprocessor, for controlling the operation of the cooling system  10 , either automatically or under the control of an operator. The vent  34  can be provided, for example in the back chamber  20  as shown, to release pressure build up which may be created by the expansion of the liquid nitrogen as it is injected into the cooling system  10 . The vent can simply be a small orifice and is preferably placed upstream of the cryogen feed line  36  and spray bar  38  and downstream of the front chamber  22  (with respect to gas flow as shown by arrows  13 ) to minimize the loss of cooling capacity. By venting after the gas has been circulated over the hot extrudate and before the spray bar  38  distributes fresh liquid cryogen, the vented gas has removed energy from the product and is the warmest portion of gas in the system and therefore does not waste newly delivered liquid cryogen. The temperature sensor  42  can be provided in communication with the gas stream generally at any point, but is preferably in the front chamber  20 , back chamber  22 , or end duct  30 , as shown, to monitor temperature of the vaporized cryogen at a desired point. Alternatively, the temperature sensor could be positioned elsewhere, such as the blade housing  16  in order to detect the temperature of the gas stream coming into the fan  12 . Similarly, additional temperature sensors could be positioned at different locations to detect the temperature of the gas at several points in the cooling system  10 . Output from the temperature sensor  42 , and other sensors, if more are used, can be provided to the controller  50  for use in regulating the speed of the fan  12  and controlling a valve  46  provided in the cryogen feed line  36  to inject liquid cryogen into the back chamber  20 . The temperature sensor  42  can be, for example, a thermocouple. The controller  50  can be programmed with the wind chill equation and can also receive a signal from the fan  12  indicative of the fan&#39;s speed. This data can be used to determine the effective temperature in the front chamber  22 . The heating unit  44 , can be a simple heating element and can be located, for example, in the back chamber  20 , as shown in the figure. The heating element can be operated by the controller to increase the temperature in the cooling system  10 , 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  48 , such as an infrared temperature sensor, is provided at a desired point downstream from the extrudate outlet passage  40  to sense the temperature of the extrudate  25  after exiting the front chamber  22 . For example, the external temperature sensor  48  could be placed adjacent the extrudate outlet passage  40  or may be placed further downstream, such as adjacent a cutting assembly or puller. The external temperature sensor  48  senses the surface temperature of the extrudate  25  and relays the output to the controller  50 . The controller  50  utilizes the output from external temperature sensor  48  in addition to temperature sensor  42  (and additional temperatures if provided) in regulating the speed of the fan  12  and controlling the valve  46  provided in the cryogen feed line  36  to inject liquid cryogen into the back chamber  20 . 
     The controller  50  can control the speed of the fan  12 , the valve  46  to inject the cryogen  37  into the back chamber  20  and the heating unit  44 , and thereby closely regulate the wind chill temperature in the front chamber  22  to correspond to, and be maintained at a desired wind chill temperature to ensure that the extrudate exiting the front chamber  22  has reached an optimum product temperature. The optimum product temperature desired for the extrudate exiting the extrudate outlet passage  40  (or other point depending on where the external temperature sensor  48  is placed) can be input to the controller  50  by an operator. The controller  50  can monitor the speed of the fan  12  (and thus the velocity of the gas stream circulating through the front chamber  22 ) and feedback from the external temperature sensor  48  and temperature sensor  42  to cause the sensed temperature, or calculated wind chill temperature, to increase or decrease depending on the external temperature sensor  48  reading. Thus, the controller can efficiently control the cooling of the extrudate  25  to provide an optimum product temperature (rigidity) for further processing, such as cutting the extrudate  25 . 
     The cooling efficiency of the system can generally be optimized by using principles of forced air convection. Extraction of heat from an extrudate  25  can be increased by blowing cooler air over a warm surface. The “effective” temperature inside the front chamber  22 , or “cooling” chamber can be calculated from the ambient temperature and the velocity that the gas (cryogen  37 ) is blown over the surface of the article  16  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 cooling system  10  can be optimized, i.e., maximum cooling using a minimum amount of liquid cryogen  37 , by controlling the speed of the fan  12 . In particular, for a given amount of liquid cryogen  37  injected into the back chamber  20  or “feed” chamber, the speed of the fan  12  can be increased in order to increase the rate in cooling of the front chamber  22  without adding more liquid cryogen  37 . Only when the speed of the fan  12  is at its maximum, would it be necessary to inject additional liquid cryogen  37  into the back chamber  20  to further reduce the temperature in the front chamber  22 . Moreover, the temperature in the front chamber  22  can also be regulated to a set point temperature by adjusting the speed of the fan  12 , faster or slower, instead of injecting more liquid cryogen  37 . Output from the external temperature sensor allows the controller  50  to manipulate the “wind chill” within the front chamber  22  to increase or decrease the cooling of the extrudate  25 . In this sense, the cooling system  10  can be optimized based on the optimum product temperature. Thus, minimum necessary cooling using a minimum amount of liquid cryogen  37  is achieved. In contrast, prior art cryogenic cooling systems typically control the temperature solely by controlling the amount of liquid cryogen injected into the system or only monitor the “wind chill.” The efficiency of the system can be further optimized if it becomes necessary to increase the temperature in the cooling chamber by using the heating unit  44 . Prior to expending energy to operate the heating unit, the speed of the fan  12  can be reduced to lower the wind chill temperature, and thus decrease the rate of cooling. If reducing the speed of the fan  12  alone is insufficient, then the heating unit  44  can be operated. By reducing the speed of the fan  12  first, energy can be conserved, thus increasing the efficiency of the cooling system  10 . It should therefore be appreciated that “rate of cooling,” is dependent both on the sensed temperature and the wind chill, i.e., “effective,” temperature. To summarize, increasing the speed of the fan  12  results in lowering the effective temperature in the front chamber  22 , which results in an increase in the rate of cooling of the extrudate  25 . Conversely, reducing the speed of the fan  12  results in an increase in the effective temperature in the front chamber  22 , which results in a decrease in the rate of cooling of the extrudate  25 . Accordingly, it can be appreciated that controlling the speed of the fan  12  and cryogen injection in relation to the extrudate temperature after exiting the “cooling” chamber  22  can be advantageously utilized to control the “effective” temperature in the “cooling” chamber  22 , and thus the rate of cooling of the extrudate  25 . This prevents ineffective or unnecessary “overcooling” of the extrudate, when only the optimum product temperature must be reached. 
     It also should be understood that the configuration and number of passageways provided to circulate the gas through the cryogenic cooling system, and around the article to be cooled, can be varied to suit different applications and conditions. Consequently, the embodiments illustrated are by way of example only, and are in no way intended to be an exhaustive representation of every possible configuration. 
     Instead of or in addition to cooling the outer surface of an article, vaporized cryogen can also be used to cool tooling, or articles held therein, by circulating cooling water or vaporized cryogen (as disclosed in U.S. Pat. No. 6,389,828) through internal cooling passageways, e.g., cooling circuits, provided in the tooling. One example applicable to cooling extrusions is tools called calibrators. A prior art type calibrator based cooling system  400 , often referred to as a wet, vacuum-jacketed calibration tooling is shown in FIG. 2 in combination with a downstream cooling system  100  configured similarly to the cooling system  10  shown in FIG.  1  and including a sizing template assembly  180  positioned in front chamber  122 , discussed in more detail below. Cooling system  100  is shown with an external chamber  111  having a top cover  124  in an open position that surrounds the front chamber  122 , back chamber (not shown), end duct (not shown), etc. that is depicted in FIG. 1 with respect to cooling system  10 . A fan  118  is shown positioned near a front end  120  of cooling system  100 , however, the cooling system fan is preferably positioned near the rear end (not shown) as detailed in cooling system  10  illustrated in FIG.  1 . The cooling system  400  includes a calibrator  112 , and such a system can typically utilize several, such as calibrators  112   a-g,  positioned at spaced apart locations through which an extrudate  125  is fed and thereby cooled. Water and vacuum conduits (not shown) are connected to a water manifold  114  and vacuum manifold  116  respectively, such that cooling water (or vaporized cryogen) may be circulated through the internal cooling circuits and a vacuum may be applied to the outer surface of the extrudate  125  to assist in maintaining its shape. The extrudate enters system  400  through a calibrator inlet passage  122 , seen in calibrator  112   g.  A vacuum is drawn through grooves in the calibrator  112  to maintain contact between the extrudate  125  and an inner face of the calibrator extrudate passage. However, these prior art calibrator-based cooling systems require costly external vacuum pumps to create an assist vacuum and often also come with the disadvantages of using cooling water. The present invention eliminates the need for the external vacuum pumps and the associated vacuum/water conduits associated with the prior art systems. 
     Referring to FIG. 3, a forced gas convection calibration tooling cooling system  200  is shown in combination with a downstream forced gas convection sizing template cooling system  300 . Cooling system  200  includes a fan  212  and external chamber  211  and top cover  224  that surrounds the remaining elements discussed in reference to cooling system  10  and shown in FIG. 1, including a front chamber  222 . Similarly, cooling system  300  includes a fan (not shown) and external chamber  311  and top cover  324  that surrounds the remaining elements discussed in reference to cooling system  10  and shown in FIG. 1, including a front chamber  322 . An end template  214  is provided on external chamber  211  that includes an extrudate inlet passage  232  and provides a means of sealing against the extrudate (not shown) as previously discussed. Optionally, fan  212  may be used to circulate vaporized cryogen through both cooling system  200  and  300 , however, it is preferred that each cooling system  200  and  300  have an independent fan such that the systems may be controlled separately or separated altogether for different operations. A calibrator assembly  216  is positioned within front chamber  222 . The calibrator assembly  216  includes individual calibrators  218   a-e  coupled to guide rail  230 . The number of calibrators used in a calibrator assembly can vary from one to any number, and depending on the requirements of the product. Likewise, the size and shape of the calibrator(s) may vary depending on the specific product to be produced. The vaporized cryogen is circulated thorough front chamber  222  over the extrudate outer surface and the calibrators  218   a-e.    
     A calibrator  218  for use with cooling system is illustrated in FIG.  4 . The calibrator  218  includes a product passage  220  defining an inner surface  226  that makes contact with, but also provides for the passage of an extrudate. By making contact with the extrudate, the calibrator  218  acts as a heat sink and removes energy from the extrudate through conduction. The calibrator  218  also assists the extrudate in maintaining its extruded shape. The calibrator has an outer surface  232  including a plurality of fins  234  extending outwardly therefrom and running substantially parallel to the center axis of the product passage  220 . The plurality of fins  234  define a plurality of channels  236  there between. Inclusion of the plurality of fins  234  greatly increases the outer surface area of the calibrator  218 . By increasing the outer surface area of the calibrator  218 , greater amounts of energy can be dissipated to the vaporized cryogen circulated in the cooling system  200 . The vaporized cryogen flows over the outer surface of the calibrator removes energy therefrom by forced gas convection. The greater the outer surface area of the calibrator means greater contact with the circulated cryogen and more heat transfer. The plurality of fins  234  also increase the mass of the calibrator  218  which increases the amount of energy (heat) the calibrator can remove from the extrudate. Preferably, vacuum grooves  228  are provided in the inner surface  226 , preferably spaced apart and extending the entire circumference of the product passage  220 . At least one pinhole (not shown) is provided from within each vacuum groove  228  and extending to the outer surface, such that the pressure realized outside of the calibrator  218  is also communicated to the vacuum groove  228 . Preferably, a pinhole is provided at the bottom of each channel  236  such that a single vacuum groove includes a plurality of pinholes in communication with the atmosphere outside the calibrator  218 . Therefore, production of a vacuum within the front chamber  222  is transferred to the vacuum grooves  228 . A vacuum within the vacuum grooves  228  assists in maintaining the extrudate in contact with the calibrator, which in turns ensures a proper shape and advantageous conductive heat transfer. Preferably, the calibrator includes at least one guide slot  238  adapted to provide passage of a guide rail  230  (see FIG. 3) such that the calibrator  218  may be secured in a cooling system. A setscrew  240  allows the calibrator  218  to be tightly secured to the guide rail  230 . 
     FIG. 5 illustrates a sizing template  318 , another type of tooling that may be used with the present invention, that is similar to the calibrator  218  shown in FIG.  4 . The sizing template  318  includes a product passage  320  defining an inner surface  326  that makes contact with, but also provides for the passage of an extrudate. The sizing template  318  has an outer surface  332  including a plurality of fins  334  extending outwardly therefrom and running substantially parallel to the center axis of the product passage  320 . The plurality of fins  334  define a plurality of channels  336  there between. As previously discussed, inclusion of the plurality of fins  334  greatly increases the outer surface area of the sizing template  318 . Optionally, a circumferential rib  328  is provided in the inner surface  326 . Several such ribs may be incorporated, preferably spaced apart and extending the entire circumference of the product passage  320 . Preferably, the sizing template  318  includes at least one guide slot  338  adapted to provide passage of a guide rail  130  (see FIG. 2) such that the sizing template  318  may be secured in a cooling system (see FIG.  2 ). A setscrew  340  allows the sizing template  318  to be tightly secured to the guide rail  130 . 
     FIGS. 6 and 7 depict a front perspective and rear perspective, respectively, of a sizing template assembly  182  including an extrudate  225  passing through the product passages in the direction of arrow  186 . Although, the foregoing description is made with respect to a sizing template assembly, a calibrator assembly for use with the present invention may be structure in the same general way. The assembly  182  includes a plurality of sizing templates  318  positioned on four guide rails  184 . Preferably each sizing template  318  is positioned adjacent to a complimentary deflector plate  340 . As best seen in FIG. 6, each deflector plate  340  includes gas flow passages  342  that are adapted to guide the flow of vaporized cryogen over/through the plurality of fins  334  extending from the outer surface of the sizing template  318 . The deflector plate preferably includes a spoiler  344  (FIG. 7) extending from a backside  346  of the deflector plate in a generally downward direction. The spoilers  344  operate to direct the gas flow along the outer surface of the extrudate  225 . The assembly  182  is adapted to be placed within the front or “cooling” chamber of a forced gas convection cooling system. 
     The forced gas convection calibration cooling system  200  and other forced gas convection cooling systems according to the invention do not require separate external vacuum pumps to provide vacuum assistance to the calibrators and other tools. Advantageously, the cooling system  200  may be operated to internally induce a vacuum within the front chamber  222  or “cooling”/calibration chamber. Referring back to FIG.  1  and cooling system  10 , which illustrates the internal duct-work and system components included in the forced gas convection cooling systems according to the present invention, gas flow enters the front chamber  22  from the end duct  30  via outlet throat  28  and exits the front chamber  22  to the blade housing  16  of fan  12  via inlet throat  26 . A vacuum is generated in the front chamber by operating the fan  12  and restricting the flow of gas into the front chamber  22 . Preferably, this is accomplished by ensuring that the cross-sectional area of the outlet throat  28  is less than the cross-sectional area of the inlet throat  26 . In this manner, the fan  12  is “starved” and produces a vacuum in the front chamber. The vacuum produced in the front chamber can easily reach 15 inches of water, but varies depending on the strength of the fan  12 . Such an internally induced vacuum can be produced with any forced gas convection system having a substantially “captive” system meaning that the gas circulation is a closed loop. Preferably, the outlet throat  28  is of a similar cross-sectional area as the inlet throat  26  but is affixed with a restrictor plate (not shown) which can be mechanically operated (manually or by a solenoid actuator driven by the controller  50 ) to vary the cross-sectional area of the outlet throat  28 . Thus, the controller  50  can manipulate and control the pressure within the front chamber  22 . A pressure sensor may be provided to sense the pressure within the front chamber  22  and send feedback to the controller  50  which then adjusts the cross-sectional area of the outlet throat  28  and hence the pressure. In a reverse scenario, if a positive pressure is required within the front chamber  22 , then the cross-sectional area of the outlet throat  28  should be larger than the cross 0 sectional area of the inlet throat  26 . In this instance, the inlet throat  26  can also be provided with a similar restrictor plate and control or simply designing the outlet throat  28  and restrictor plate such that a cross-sectional area of the outlet throat  28  can vary from an area less than to an area greater than the cross-sectional area of the inlet passage  26 . Referring to FIGS. 1-3, operation of the cooling systems  10 ,  100 ,  200  and  300  accordingly can provide a reduced pressure or “vacuum” within front chambers  22 ,  122 ,  222  and  322  respectively. FIG. 8 depicts a schematic representation of the method of creating an internally induced vacuum within the “cooling” chamber of a forced gas convection cooling system. Operation of the fan  3  and maintaining a cross-sectional area of inlet  2  into front chamber  5  less than the cross-sectional area of outlet  4  produces a vacuum in the front chamber  5 . 
     Another preferred embodiment of the present invention is illustrated by FIGS. 9-11. FIG. 11 shows a simplified version of a forced gas convection cooling system  500  for internally cooling an extrusion having a hollow profile. The components and operation of the cooling system  500  are generally the same as for the cooling systems  10 ,  200  and  300  illustrated in FIGS. 1-3, except that an outlet conduit  520  and the extrudate  525  essentially replace the front and back chambers. In particular, a source  509  of liquid cryogen  537 , preferably liquid nitrogen, the injection of which into the cooling system through spray bar  538  can be controlled by a feed valve  546  placed in feed line  536 , which itself can be operated by a controller  550 . As previously discussed, the liquid cryogen  537  substantially instantaneously vaporizes and cools the gas stream circulated by the fan  512 , preferably in a direction shown by arrows  513 . The vaporized cryogen stream is communicated to an extruder die  570  via outlet conduit  520 . Extruder die  570  is shown in more detail in FIGS. 9 and 10. Extruder die  570  includes an inlet bore  572  extending from an outer surface  574  of the extruder die  570  through a mandrel  576  that is adapted to form an extrudate hollow  578  within the extrudate  525 . The inlet bore  572  is adapted to be placed in fluid communication with the outlet conduit  520  and thereby pass vaporized cryogen through the extruder die  570  and mandrel  576  and into the extrudate hollow  578 . Preferably, the inlet bore  572  and outlet conduit are separably coupled such that different dies can be interchanged for different product configurations. Inlet bore  572  terminates at a mandrel outlet  586  where vaporized cryogen may enter the extrudate hollow  578 . Optionally, an outlet extension  580  is provided to ensure that the pressure exerted by the vaporized cryogen as it is introduced into the extrudate hollow  578  is spaced from a leading edge  582  of the die  570 . Optionally, the cooling system  500  is used in combination with an external forced gas convection cooling system, such as described in systems  10 ,  100 ,  200  and  300  (shown in phantom in FIGS.  10  and  11 ), that are placed substantially adjacent the die  570 , but a small separation  584  may exist. If the outlet extension  580  is not used, then a positive pressure within the extrudate hollow  578  may cause a bubble or distortion within the small separation that is undesirable. Preferably, a forced gas convection calibration cooling system, such as cooling system  200 , is used immediately adjacent the extruder and in combination with cooling system  500 . In this scenario, the outlet extension is selected to have a length such that the vaporized cryogen is released at a point within the length of a calibrator and the distortion problem is thus minimized. Preferably, mandrel outlet  586  and outlet extension  580  are separably coupled, such as with threads  588 , so that different length extensions may be used. The outlet extension  580  includes a nozzle  590  or other means for directing the flow of vaporized cryogen onto an inner surface of the extrudate  525 , as shown by arrows  592 . 
     FIG. 9 depicts a die  570   a  configuration including two mandrels  576   a  and  576   b  that form extrudate hollows  578   a  and  578   b , but do not include outlet extensions. An outlet conduit manifold (not shown) can be provided to provide more than one vaporized cryogen streams to two separate outlet conduits  520   a  and  520   b  and inlet bores, or an inlet bore manifold (not shown) may be provided to split a single vaporized cryogen stream into any number of inlet bores to provide vaporized cryogen to extrudate hollows. Splitting a single stream ensures that the temperature of the vaporized cryogen streams entering different hollows is substantially the same. However, depending on the profile of an extrudate, it may be desirable to provide each hollow with streams of a different temperature. In this case, each hollow that requires a separate temperature is placed in communication with a separate forced gas convection cooling system as herein disclosed. 
     Referring again to FIG. 11, temperature sensors  542   a  and  542   b  can be provided for detecting the ambient temperature in the outlet conduit  520 , preferably at a point downstream from liquid cryogen spray bar  538 , or within cutting chamber  560  and outputting that information to the controller  550 . Additionally, an external temperature sensor  548 , such as an infrared sensor, can be provided that outputs a product temperature reading to the controller  550  as discussed with respect to cooling system  10  illustrated in FIG.  1 . An outlet conduit valve  562  can similarly be operated by the controller  550 . A heating unit  564  can be provided that is operable by the controller to input heat to the system if necessary. A conveyor system  558  can similarly be used to support the extrudate  525  between the extruder and any downstream equipment. The controller  550  can regulate the temperature in the outlet conduit by controlling the fan  512  and the feed valve  546  based upon feedback from the temperature sensor  542   a,  the temperature sensor  542   b,  the external temperature sensor  548  or all three sensors. The controller  550  is programmed to operate system  500  in a similar manner as disclosed for system  10  to optimize the system&#39;s efficiency using principles of forced gas convection. The controller can regulate the speed of the fan  512 , operate feed valve  546  to control release of liquid cryogen  537  into outlet conduit  520  and the heating unit  564  to closely regulate the “wind chill” temperature within the extrudate hollow  578  to correspond to, and be maintained at the desired wind chill temperature which can be input by an operator. Optionally, the controller  550  can also act as the controller for additional cooling systems, such as systems  10 ,  100 ,  200  and  300  discussed herein, used in combination with cooling system  500 . 
     Preferably, the cooling system  500  is captive, i.e., closed, such that substantially no outside air enters the vaporized cryogen and the vaporized cryogen is recirculated. The extrudate  525  enters the closed cutting chamber  560  through an inlet portion (not shown) and exits through a similar outlet portion (not shown) provided with appropriate sealing portions as known to those in the art. Cutting chamber  560  includes a means for severing the extrudate  525  into desired lengths for further processing or as the final product. The extrudate  525  enters the cutting chamber  560  through a cutting chamber inlet (not shown) provided with appropriate sealing portions as known to those in the art. A saw (not shown) or other suitable cutting means is housed in the cutting chamber  560  and is operated to periodically cut the extrudate  525  into predetermined lengths. Care should be taken such that during the cutting stroke, the vaporized cryogen is allowed to escape from within the extrudate hollow  578 , such as through a saw blade (not shown) provided with slots. The slots prevent a positive cryogen pressure build-up within the extrudate  525  during the cutting stroke. If a continuous blade is used, even the brief amount of time required for the cutting stroke may cause a blockage of the flow of cryogen through the extrudate hollow  578 , and thus cause bellowing and distortion of the product as well as increased drag on tooling equipment. Return conduit  566  channels the vaporized cryogen back to the variable speed fan  512 . A vent  568  and vent valve  569  are provided to allow pressure in the system to be controlled by the controller  550 . Pressure sensor  567  can give feedback to the controller  550  which then operates the vent valve  569 , fan  512 , feed valve  546 , and outlet conduit valve  562  to vary the pressure within the system. Additional pressure sensors may be included at other points within the system to give feedback to the controller  550 . Optionally, a heat exchanger  568 , e.g., a shell and tube exchanger, is provided to pre-cool the recirculated cryogen and thus reduce the consumption of liquid cryogen  537 . A heating element  50  may be provided in communication with the circulated cryogen  24 , such as in the return conduit  42  as shown, such that heat may be added to the system if necessary. 
     Advantageously, the present invention allows an 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 gas flow of cryogen 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 must 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.