Abstract:
A method and apparatus is provided for the efficient and controllable delivery of cryogen liquid droplets into thin walled containers before they are sealed, the pressurization of the sealed container caused by the evaporation of the liquid cryogen causing the walls of the container to stiffen. Discharge of the droplets immediately upstream of the container sealing station is facilitated using a horizontal displacement assembly to transport metered droplets from a liquid dosing unit to the point of injection above the container. The horizontal displacement assembly may be provided with internal heaters to prevent freeze up, and a sensor to confirm droplet discharge. It may also be provided with a separate source of heated nitrogen gas, which can be used to back purge the dispensing unit should it become clogged, to melt any frozen liquid occlusions which may have formed in the cryogen supply line. In one embodiment, the solenoid used to actuate the piston regulating the opening and closing of the needle valve, which meters the dispensing of droplets, is mounted in thermal contact with said piston, this placement of the solenoid serving to cool the piston and thus prevent overheating in the case of rapid cycling.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit of U.S. provisional patent application Ser. No. 60/487,022, filed Jul. 14, 2003; U.S. provisional patent application Ser. No. 60/510,907, filed Oct. 14, 2004; and U.S. provisional patent application Ser. No. 60/538,565, filed Jan. 23, 2004, all of which are herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to cryogenic liquid delivery systems and more particularly to managed dosing systems for injecting metered droplets of liquid nitrogen into beverage, food or other product containers as they move along high-speed production lines before being sealed.  
         [0004]     2. Description of the Related Art  
         [0005]     With thin walled containers, especially thin walled metal cans and plastic bottles, it has been found useful to stiffen them after filling, but prior to further processing, such as before labeling, shipping and handling to prevent subsequent container damage. To achieve such stiffening, a cryogen such as liquid nitrogen may be injected just prior to sealing. Injected as droplets, the liquid cryogen undergoes phase change to a gas, increasing the pressure inside the container, the increased pressure acting to stiffen the container walls.  
         [0006]     Typically, the liquid cryogen drops or droplets, once injected, will coalesce as they sit on the container contents, the vaporization process taking anywhere from 5-15 seconds. Accordingly, the time between injection and container closure must be kept short. It is to be appreciated the exact time of vaporization may vary depending upon the size of the injected droplet, and the temperature of the container contents. The resulting pressure within the container will similarly be a function of the size of the injected drop, the free space to be filled, and the time between droplet injection and container closure.  
         [0007]     Since the liquid nitrogen begins to immediately vaporize upon being dispensed, it is desirable to cap or close the container as soon as possible. Preferably, injection should occur immediately upstream of the closure station. However, because of the physical layout and limitations of conveyor systems used to bring containers to a capping or closure station, the size of the liquid delivery system head, and the configuration of the closure station itself, it is presently necessary to inject the liquid nitrogen a distance upstream of the point of closure.  
         [0008]     Typical of liquid injection delivery systems developed for injection of small amounts of nitrogen into containers as they pass along an assembly line are those sold by VBS International, Inc. of Campbell, Calif., under the trade names LCI-300, 400, and 2000M. See also U.S. Pat. No. 6,182,715 to Alex R. Ziegler, et al, which patent is incorporated herein by reference in its entirety.  
         [0009]     In these systems, a stream of liquid cryogen droplets is dispensed vertically into a moving container. In so doing, the force of injection can cause the droplets to substantially penetrate the surface of the container contents. The force of impact can result in splash-back of the contents onto the dosing head, where the splashed liquid may accumulate and later interfere with the operation of the dosing head itself.  
         [0010]     Conveyer systems are run at fairly high speeds where containers pass by fixed stations at the rate of 500 units per minute or more. In fact, some processing conveyor lines run to speeds in excess of 1500 to 2000 containers per minute. At lower speeds, e.g. 500 units per minute, the liquid nitrogen feed systems of the referenced prior art perform well. However, at higher line speeds, the dispensing assemblies must operate at higher frequencies. Pneumatically driven valves such as those used in the dispensing systems of VBS to meter dose amounts produce heat proportional to their speed of operation, the pressure of the gas source, and frictional loses. As a result, heat tends to build up as the pneumatic valve is more rapidly cycled.  
         [0011]     To date, it has been problematic to operate at the higher conveyor speeds of 1000 to 2000 containers per minute. In fact at such operational speeds, the pneumatic system gets hot to the touch (140° F.-160° F. and above), seals may fail and the unit burn out over the course of a day. Further, these delivery systems frequently are installed in assembly line areas where ambient temperatures may easily exceed 40° C., reducing the potential for effective ambient cooling.  
         [0012]     With such high speed lines where containers pass a fill point at the rate of upwards of 1000 to 2000 units per minute, the residence time at the liquid injection station also becomes a factor, with the time allowed for fill becoming shorter than the time required for delivery of the dispensed liquid dose stream. This mismatch can result in a good portion of the injected dose missing the container opening, and thus lost to the atmosphere by vaporization. As a further result, maintenance of dose accuracy and repeatability can be lost.  
         [0013]     There thus remains a need to develop delivery systems which are less prone to clogging through splash-back, and able to more accurately and efficiently deliver a measured dose of cryogen to a container to be pressurized. There also remains the need to shorten the dispensing cycle time of existing liquid delivery systems so as to match the higher speeds of current conveyor systems. So too, there remains a need for these systems to be able to operate in harsher temperature environments, such that the surrounding ambient will have little to no effect on operations.  
       SUMMARY OF THE INVENTION  
       [0014]     By way of the present invention, a displacement assembly is provided which allows for the offset of the liquid injection point. In providing such an offset, the injection point can be placed proximate to a point immediately upstream of a closure station. As a secondary benefit, much of the vertical force of injection is dissipated as the delivery path of the cryogen is changed to first run horizontally for a select distance before being redirected vertically for injection. In so doing, the cryogen droplets hit the surface of the container contents with substantially less energy, thus significantly reducing, if not nearly eliminating, the tendency for liquid splash-back. As a still further benefit, by placing the injection head next to the container sealing position, the time lapse from injection to closure is greatly reduced, thus reducing the amount of pre-closure evaporation, which in turn permits the use of smaller amounts of cryogenic liquid per dose.  
         [0015]     The invention covers both an apparatus for horizontally displacing the injection point for cryogen liquid delivery and a method for affecting the delivery of a cryogenic to a container immediately before closure. The displacement assembly itself can be incorporated as part of the overall liquid delivery system, or can be provided as a retrofit for liquid delivery systems already in use, to allow for dispensing closer to the point of container closure than previously possible.  
         [0016]     The displacement method comprises the steps of metering a measured dose of liquid from the liquid delivery system, providing a substantially horizontally disposed pathway from the point of dosing to a remote dispensing point a measured distance from the first point. In one embodiment the pathway may be heated. In this embodiment, not only is sticking of cryogenic liquid onto the walls of the pathway prevented, but atomization of the liquid droplet stream occurs as well, which atomization serves to further reduce splash back and improve dose accuracy. In another embodiment, a gas can be introduced into the pathway at an upstream point to provide additional positive pressure behind the dispensed droplet stream to further promote travel along the horizontally disposed pathway to the point of injection.  
         [0017]     In another embodiment of the invention, a sensor is provided to monitor droplet injection. The sensor generally comprises a pair of opposing optical fibers which can be positioned along the displacement assembly. The one fiber is connected to a light source; light emitted from the first fiber directed to the second fiber, which itself is connected to a sensor for measuring the intensity of the received radiation. The sensor in turn in connected to a monitor, whereby when a droplet, is discharged from the dosing head and enters the transport pathway, its passage will interrupt the light beam passing from the first optical fiber to the second optical fiber. By detecting the drop in measured intensity of the transmitted light, and noting the time of signal interruption, one can correlate the passage of a droplet to a given opening/closing cycle of the needle valve of the dosing head, thus confirming for a given open/close cycle that a droplet was discharged. The system can be programmed such that failure to detect a beam interruption will trigger an error signal, which can be set to automatically shut down the system, or generate an alarm for notifying an operator, who can then initiate remedial action.  
         [0018]     Alternatively, the optical fibers of the sensor can be positioned orthogonal to the droplet discharge path, anywhere along the path. In this configuration, the interruption of transmitted light occurs only for that interval of time that a droplet stream passes between the fibers. By measuring the length of time of signal disruption, and knowing the diameter of the transport path, not only can droplet discharge be confirmed, but the volume of the droplet calculated as well. As with the first embodiment, the absence of a break in the detected beam in conjunction with the opening and closing of the needle valve of the dosing head creates an error signal, which alerts the operator to shut down the system.  
         [0019]     Common causes for failure to discharge include a freeze-up of the discharge head, or a blockage of the cryogenic supply line upstream of the dosing head. This problem can be addressed by back flushing or purging of the system with a heated gas to melt whatever frozen liquid occlusions may have formed in the delivery system. In this embodiment, the horizontal displacement assembly, provided with internal heating units, heats a reverse flow of pressurized nitrogen gas which can be introduced near the discharge end of the assembly. Introduced under a positive pressure relative to the pressure in the dosing head, the heated nitrogen will flow back to and through the dosing head, and the cryogen source lines, the heated gas serving to melt any upstream blockage. By monitoring system pressure, such as at an upstream vent, and observing the point at which the system pressure reaches steady state, the end point of the back-flushing process can be determined.  
         [0020]     In the cryogen dosing units employed with the present invention, a pneumatic actuator is used drive the needle valve, the actuator including a solenoid valve to regulate the flow of gas to a piston which in turn controls the opening and closing of the needle valve of the dosing head. In order to increase the operational speed of the liquid dosing assembly, it is possible to thermally manage the unit by positioning the solenoid valve in close proximity to the piston such that it makes thermal contact. The solenoid valve itself is cooled by the expansion of the gas used to drive the piston, as it is exhausted. This cooling effect is used to offset the heat generated by the rapid cycling. By utilizing the cooling effects generated by the solenoid valve in the operation of the needle valve, the needle valve can be operated much more rapidly without resultant overheating.  
         [0021]     In yet another embodiment of this invention, exhausted nitrogen gas is used to further cool the actuator assembly by passing it over the assembly before being exhausted from the system. Still further, the assembly can be enclosed by a walled container such as a cylindrical housing, the interior of which is open to atmosphere, i.e., maintained at atmospheric pressure. The cooled nitrogen exhaust gases are passed through the enclosed space to cool the ambient immediately surrounding the actuator, thus providing further, more distributed cooling of the actuator assembly.  
         [0022]     As an advantage of this arrangement, a small, compact dispensing head may be provided. As a further advantage, by cooling the actuator using the cooling effect of the expanding exhaust gas from the solenoid valve, the actuator is able to run at much higher cycles. In fact, it can be operated at up to 1000 to 2000 cycles per minute, without overheating, or burning out over long periods of operation. As a still further advantage of the assembly of this invention, the dispensing head may be operated in warm environments, such as may be encountered on a factory floor, the actuator thermally insulated from the higher ambient by the cooled housing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0024]      FIG. 1  is a cutaway view of a cryogen dosing system manufactured by VBS for use with the displacement assembly of this invention.  
         [0025]      FIG. 2  is a front 3D view of the dosing assembly of  FIG. 1  further incorporating a displacement assembly of this invention.  
         [0026]      FIG. 3  is a rear 3D view of the displacement assembly of  FIG. 2 .  
         [0027]      FIG. 4A  is a 3D view of a heated collar which can be used with the dosing system of  FIG. 1 ,  FIGS. 4B-4D  3D views of various configurations of the displacement assembly of this invention.  
         [0028]      FIGS. 5B and 5D  are top and side views of the displacement assembly of  FIG. 4D ,  FIGS. 5A  and D being sectional views of the same assembly taken along the section lines illustrated.  
         [0029]      FIG. 6  is a sectioned top and side view of the displacement assembly of  FIG. 4C  incorporating the optical sensor system of this invention.  
         [0030]      FIG. 7  is a sectioned top and side view of the displacement assembly of  FIG. 4C  illustrating an alternative positioning of the optical sensor of the invention.  
         [0031]      FIG. 8  is a schematic view of the dosing system employing a back purge feature of this invention.  
         [0032]      FIG. 9  is a schematic view of the dosing system including the back purge feature, employing an alternative means for sealing the end of the displacement assembly.  
         [0033]      FIG. 10  is a cutaway view of an alternative arrangement of the dosing unit, in which the solenoid used to drive the pneumatically actuated valve is shown in thermal contact with the actuator.  
     
    
     DETAILED DESCRIPTION  
       [heading-0034]     The Horizontal Displacement Assembly  
         [0035]     A typical dose assembly  101  sold by VBS is illustrated in  FIG. 1 , whereby droplets of liquid nitrogen are metered from a dosing head  102 . The dosing head  102  includes a needle valve system for dispensing of the liquid nitrogen, the needle valve including a valve stem  104 , with valve head  106  at its distal end, the valve head  106  sized for sealable engagement with valve seat  108 . Reservoir  110  defined by valve body  112  acts as a local liquid cryogen supply chamber for holding liquid cryogen, inundating the seating area of the needle valve. Liquid nitrogen is fed to reservoir  110  through source conduit  114 , extending from flexible dosing arm  132 . It is contained in chamber  110  at slightly elevated pressure, e.g. 1 PSI above atmospheric. In a passive system, the pressure is created by the hydrostatic head of a larger cryogen source reservoir (not shown) placed above and supplying conduit  114 . This liquid nitrogen supply may be pressurized, if desired. Typical pressures can range from near zero to 10 psi above atmosphere, with 6 psi being a customary upper limit. With the valve open, liquid nitrogen will flow through the metering orifice in valve seat  108 , the flow interrupted when the valve is closed.  
         [0036]     In order to precisely meter the amount of nitrogen dispensed into each container, it is important to be able to quickly open and close the dosing valve. This is achieved with a pneumatic actuator of the type shown in  FIG. 1 . Therein, and by way of illustration, valve stem  104  is secured at its proximate end to the end of a pneumatically actuated piston  116 . The piston includes a piston head  118 , a stem  120 , upper and lower chambers  122  and  124 , and ports for sequentially injecting and exhausting a gas such as nitrogen into both the upper and lower chambers to cause movement of the piston either upwardly or downwardly, in turn moving the needle valve to either the open or closed position.  
         [0037]     The actuator may be spring loaded to bias the valve to the closed position. With the valve open as shown in  FIG. 1 , the lower chamber  124  of the pneumatic piston is pressurized, the upper chamber exhausted to atmosphere via vent  131 . To lower valve head  106  and thus close the valve, upper chamber  122  is pressurized by flowing gas into that chamber, while the lower chamber is exhausted to atmosphere.  
         [0038]     To effectuate such rapid opening and closing, the piston is driven by a 4-way solenoid valve  130  which controls the flow of nitrogen gas to the chambers above and below the piston head. As shown in the  FIG. 1 , this valve is separately mounted on dosing arm  132 , some distance from the liquid nitrogen dispensing valve. In the mode illustrated, a pressurized source of nitrogen (or other inert gas) is supplied via supply line  126 , the 4-way valve  130  biased in the closed position. When opened, the gas flows through the solenoid actuated valve to one of the piston chambers, to cause either opening or closing of the needle valve. The operation of the solenoid is controlled by a controller, not shown, which can be programmed to adjust valve cycle time, and thus control dose settings. Using the dosing assembly as above described, it is possible to rapidly introduce a cryogen liquid close to, but upstream of the container capping or sealing station.  
         [0039]     Typically the containers are capped in a rotary capping station which receives the individual containers, and moves them in a circular path during the capping process. With the device of this invention, it is possible to bring the point of liquid injection within the footprint of the closure equipment. As previously noted, by facilitating the injection of the cryogen droplets immediately upstream of the sealing station, where closure is more immediate after injection, less of the cryogenic liquid will evaporate before container closure, thus allowing for the use of smaller amounts of liquid cryogen to obtain the same container pressures after sealing.  
         [0040]     Referring now to  FIGS. 2 and 3 , a typical VBS micro dose dispensing head is illustrated in which a solenoid valve  130  is used to control the flow of liquid nitrogen to piston head  116 , which in turn is used to drive and thus open and close the dispensing needle valve (not shown) which is contained within the housing of dosing head  102 .  
         [0041]     To move the injection point closer to the point of container sealing, the displacement assembly of this invention, as shown in  FIGS. 2-5 , can be used. The displacement assembly  200 , affixed to the base of dosing assembly  101  comprises a generally elongate body  205  such as a rectangular block into which a hollow transport channel  209  has been bored there-through from a fist end  218  to a second end  220 , for directing liquid cryogen horizontally along the bore to an injection port  207 . At the injection port, the injection path is reoriented to the vertical for controlled droplet delivery into the container to be filled. In this arrangement, most of the downward velocity of the droplet introduced at metered dosing point  136  is dissipated as the droplet travels along horizontal path  209  to the injection port. By reducing droplet velocity into the container, the opportunity for splash-back is reduced, thus diminishing the likelihood splashed-back liquid will reach and freeze on the injection head, accumulating and eventually causing unit clogging.  
         [0042]     Various versions of the displacement unit are depicted in FIGS.  4 B-C. With reference to the unit of  FIG. 4D , block  205  is machined along its length to culminate in a smaller injection head  206 , the reduced size allowing for placement of injection port  207  nearer to the point of container closure.  
         [0043]     The illustrated assembly is configured to attach to the end of the metering unit, with connection collar  210  sized to engage extension  134  at the base of the needle valve. “O” rings  221 , see  FIGS. 4 and 5 , are positioned within spaced grooves on the inner wall of connection collar  210 . With the collar engaged with dosing head extension  134 , the O rings serve to provide sufficient pressure against the peripheral wall of the extension to both prevent vibration and secure the horizontal displacement unit in place.  
         [0044]     With reference to  FIG. 4A , a prior art heated collar  210 ′ designed for attachment to the end  134  of the dosing assembly is depicted. In this application, heating elements associated with the collar elevate the temperature of the collar so as to prevent freezing of splashed back liquid. In one application, the heating elements comprise an external heater affixed to the collar, in combination with a press fit bronze insert which receives and distributes heat across the collar.  
         [0045]     In the present case, the heater is replaced by a displacement assembly mounted to the liquid delivery system using the same connection collar arrangement. Being configured with the same mounting system as the heater assembly, the displacement unit can easily be affixed to existing equipment already deployed in the field. It is to be noted that though the horizontal displacement unit may be retrofitted to dosing units such as that of  FIG. 1 , the displacement assembly may be designed to be integral to the dosing head, thus eliminating the need for connecting collar  210 .  
         [0046]     Over time, the cycling of liquid nitrogen at −196° C. through the block causes cool down of the block to the point where sticking of droplets (i.e., sticking to the walls of the bore) may occur during transit. To address this situation, in another feature of the invention, body  205  may be heated. Here, the body itself will be made of a material selected from any number of thermally conductive materials, and preferably those of relatively high thermal conductivity. Such materials include aluminum, bronze, copper, and brass. The use of these materials facilitates rapid response to increases or decreases in the amount of heat inputted to the block by the block heaters, thus facilitating tighter control of block temperature. The faster the response, the easier it is to fine tune block temperature.  
         [0047]     A suitable heat source for the displacement assembly can include one or more resistive heaters  211  running a substantial the length of block  205 . The temperature of block  205  is monitored by a thermocouple  212 , which provides a signal representative of block temperature to a controller, which in turn is programmable to maintain a temperature set point by appropriately adjusting power to the heaters.  
         [0048]     The heating arrangement is best shown in cross section  5 D taken along line A-A of  FIG. 5C , which itself is a top view of the displacement unit of  FIG. 4D . There, each of the resistive heaters is electrically energized, internal wires  213  to the heaters covered by cap  214 , and electrically joined to external connector  215 . The amount of power to the heaters is regulated by a controller (not shown). Almost any type of elongate resistance heater may be used. Exemplary of commercially available heaters are cartridge component heaters available from Chromalox of Pittsburg, Pa. under the trade name CIR (Incoloy).  
         [0049]     While two heaters are illustrated, it should be appreciated that the unit may be operated using a single heater. Also, other heating means may be used such as a heating blanket, or channels bored in the block through which a heating medium such as hot water can be flowed. However, with the electrical units described, a faster response to changes in temperature is possible and thus better control of block temperature achievable.  
         [0050]     As the liquid nitrogen leaves the dosing head, a measured amount of nitrogen is dispensed in the form of a string of liquid droplets. While vaporization begins immediately, in the case of a heated unit, there is a spike in the vaporization rate as the droplets reach the heated inner walls of the transport pathway of the displacement assembly. This rapid increase in vaporization rate results in a sharp rise in pressure in the transport pathway, greatly accelerating the transport of the liquid droplets to the second end of the pathway for injection into a container. As a consequence, the injection period is greatly compressed, such that all of the dosed droplets are injected into the container during that interval of time the container opening is in residence below injection port  207 . It has been observed that the time compression of the dispensing period can be as much as 80%. In the past, with the systems of the prior art, the dosing period was much longer, such that much of the liquid to be injected arrived at the container opening either before or after the opening was in position to receive the liquid. By compressing the period of injection, almost all of the dispensed dose is injected into the container, thus increasing dosing accuracy, efficiency and repeatability.  
         [0051]     It is to be noted that operated in the manner described above, both ends of the transport passageway are initially open during dispensing of a given dose. That is, the needle valve is in the open position with the valve head  106  displaced from valve seat  108  for a limited period of time to allow for flow of the desired amount of cryogen from the cryogen reservoir through the opening in the valve seat to the displacement assembly. During the time of dose transport, the needle valve head  106  engages with valve seat  108  to close off the receiving end of the transport path. By so closing the receiving end, while leaving the dispensing end open, the rapid buildup of pressure caused by the vaporization of the heated cryogen acts on the metered dose to accelerate it in the one available direction, toward the open, discharge end of the passageway. By proper scheduling of the opening and closing of the needle valve, the receiving end of transport pathway can be kept closed during the entire time of dose transport.  
         [0052]     For a unit as depicted in  FIG. 5 , given a displacement length of approximately 4,″ it has been found that sticking of the dispensed droplets in bore, i.e. transport channel  209  can essentially be prevented by maintaining block temperature between a few degrees above room temperature to about 140° F. Preferably large temperature swings should be avoided, and temperatures maintained in a narrower range, such as for example between 90° F. to 100° F.  
         [0053]     For the displacement assemblies of  FIGS. 4C and 4D , a dose capture guide  208  is provided to collect the cryogen droplets as they are dispensed from dosing head  102  of dispensing assembly  101 , to capture the droplets and direct them through dose receiving port  222  to transport pathway  209 . Bore  209  is most commonly of circular cross section, and is disposed substantially horizontally. Preferably, as illustrated in  FIG. 5A , it can be angled slightly downward, such as for example by about 3 degrees±1 degree from the horizontal to assist in the flow of cryogen droplets through the transport passageway to dispensing port  207 . Typically the slope angle can be varied a few degrees, for example between 0° and 10°. At higher slope angles, a thicker block is required to define a channel of equal horizontal displacement, the thicker block potentially impeding placement of the injection head  206  adjacent the point of container sealing, due to dimensional constraints of the conveyor and/or the sealing unit. At shallower angles, e.g. 0 degrees, the dispensed droplet will not as easily transport along pathway  209  without the application of an alternative displacing force. In one alternative, this force can be applied by simply turning the flexible dosing arm to thus tilt the dispensing assembly a few degrees. In another, the displacing force can be provided by introduction of a pressurized gas to the system, or vaporization of the metered cryogen by application of heat to induce a pressure spike.  
         [0054]     In one embodiment, a constant positive pressure can be created within the displacement assembly, wherein a gas feed line (not show) is provided which connects to a gas inject port  216 , the port introducing gas into channel  209  at a point upstream of where dispensed droplets enter the channel. It should be understood that a variety of gases can be used, but preferably one which does not form a reaction product with the cryogen liquid, nor constitutes a contaminant to the container contents. In one embodiment the gas of choice is nitrogen, though other inert gases such as argon can be used.  
         [0055]     By way of example, for a system such as that of  FIG. 5 , in which the horizontal displacement is about 4 inches, and the bore diameter is about 0.150 inches, a gas flow rate of 1-5 standard cubic feet per hour has been found to be effective. The primary requirement is that the pressure applied be sufficient to further sweep the dispensed droplets forward along transport passageway  209  to the dispensing port. The temperature of the injected gas is not critical, and may be injected at room temperature. In one embodiment of the invention, the gas pressure applied to bore  209  is applied continuously. It may, however, be applied intermittently, the gas flow timed to sequence with the dispensing of a metered droplet into the transport passageway.  
         [0056]      FIG. 4B  is a depiction of yet another variation of the displacement assembly, having a shortened block  205 . In this embodiment, foreshortened for less horizontal displacement, the primary purpose in using the displacement block is to reduce splash-back in conveyor systems where the insertion point is neither critical nor particularly constrained. Generally the length of the displacement unit and the size of the injection head can be varied and tailored to meet the dimensional requirements of the system in to which it will be placed for container filling. The only functional requirement is that the length of the displacement assembly be no longer than needed to bring the injection point to a location immediate to the station where container closure occurs.  
         [0057]     With the filling process using the heated assembly of this invention, it has been observed that droplets from the dosing head tend to become atomized due to the rapid increase in pressure within the transport channel. This break up of the droplets into finer droplets in the displacement assembly results in a droplet stream which, when injected, causes far less splash back of container contents. It is believed in the process of injecting a finer stream of droplets, the net force of injection remains the same, though dispersed over the multiple droplets, such that the force per droplet is much less, resulting in far less penetration of individual droplets into the container&#39;s liquid contents. With direct, vertical injection, where splash back is of concern, the splash back material can also include some of the dispensed liquid nitrogen. Such loss of liquid nitrogen can result in variance of cryogen dose from container to container, leading to a variance of pressure within individual containers after capping. By carefully controlling dose delivery, and eliminating splash back, more repeatable dosing is achieved with each of the containers to be filled.  
         [0058]     In another application for the invention, the displacement assembly may be used for injection of liquid nitrogen droplets into containers such as gas tight packages before they are sealed to provide an inert atmosphere within the sealed package. Typical packages for the use of inerting atmospheres include potato chip bags, foil coverings for individual tea bags, and the like. By displacing the air/oxygen before sealing, freshness of the contents is preserved over a longer period of time.  
         [0059]     In the foregoing application, the small liquid droplets can be injected horizontally either in a straight-forward path, or at right angles (side to side) to the initial direction of travel of the droplets, vertically, both up and down or in several directions at once. In fact the injection head can be designed to redirect the liquid flow path from one which is coincident to the horizontal central axis of the displacement unit to any path that is neither concentric nor coincident with the horizontal central axis.  
         [0060]     For ease of manufacturing, injection head  206  can be machined as a separate component, the pathways first formed in head  206 , and the head then attached to elongate body  205  of the displacement assembly. Head  206  can be secured by a variety of attachment means, such as by welding, gluing, screws or other mechanical fastening devices.  
         [heading-0061]     Fluid Dispensing Sensors  
         [0062]     The fluid dispensing sensor of the invention will next be described with reference to  FIGS. 6 , and  7 . In  FIG. 6 , a sensor is shown having a fiber optic cable  300  attached to the first end  218  of the horizontal displacement assembly, a second fiber optic cable  302  attached to the end of discharge pathway  209  of displacement unit  205 , in alignment with the face of the first fiber optic cable. The optical fibers are can be made from glass, plastic or other optically clear materials, and are commercially available from such companies as Banner Engineering.  
         [0063]     A light source  304  is connected to the first fiber to provide a light beam, the presence of which will be detected by the second optical fiber. Suitable light sources include an LED. The optical fibers are positioned such that as soon as a liquid droplet from the dosing unit reaches displacement pathway  209 , the light from the first fiber will be interrupted, and will remain so until the droplet is discharged via injection port  207 . A commercially available fiber optic sensor  306 , such as one sold by Banner Engineering under the designation Omni-beam, attached to the end of the second optical fiber converts the detected beam of light into an electrical signal, the strength of the signal proportional to the intensity of the transmitted beam. By detecting the drop in signal strength, the sensor signals the system&#39;s computer controller  308  that a discharge event has occurred. This signal should follow in sequence a signal from the computer controller to open the needle valve by activation of the solenoid. By confirming after each open valve command, that a drop in transmitted light intensity has occurred, liquid discharge from the displacement assembly is confirmed. That is, for each open/close cycle of the dispensing needle valve, either a droplet was determined to be “present” or “not present” in the displacement pathway. So long as the presence of a droplet is detected, the dispensing cycle will continue to the next open/close cycle of the dispensing needle valve.  
         [0064]     At such time as there is no loss in signal strength, and thus a “not present” condition encountered, controller  308  will issue an error signal. The unit can be programmed to either shut down further processing, or generate an alarm to alert an operator that a “not present” event has occurred. At this point, the operator can shut down the system, and investigate the cause, taking remedial action as appropriate. In the event that the heaters in the displacement assembly were to fail, and droplets freeze up in transport pathway  209 , the opposite condition would occur. That is a continuous “present” condition would exist, and the controller can be programmed to flag such a continuous condition and similarly issue an error signal.  
         [0065]     An alternative sensor arrangement is shown in  FIG. 7 . Here, the optical fibers  300  and  302  are positioned along the displacement path  209 , orthogonally to the direction of fluid flow, preferably near the tip of the displacement assembly, at injection head  206 , as illustrated. In this alternative, the time period of decreased light intensity is directly proportional to the size of the droplet stream moving across the displacement path. The longer the period of light beam interference, the larger the droplet stream, and vice versa. Knowing the diameter of the displacement path, the period of beam drop off, the displacement assembly temperature, and the dispensing pressure, one can calculate the volume of the dispensed droplets using the associated system controller  308 .  
         [heading-0066]     System Purging  
         [0067]     Typical droplet dispensing failure causes can include clogged lines, disruption of the supply of liquid nitrogen, exhaustion of the liquid nitrogen supply or loss of pressure within the dispensing system. In the case of clogged lines, one remediation technique is to back flush or purge the needle value and liquid nitrogen supply (i.e. source conduit) lines with heated nitrogen gas. Such can be accomplished using an assembly such as illustrated in  FIG. 8  or  9 . In  FIG. 8 , an external pressurized source of nitrogen  400  is plumbed to the displacement assembly, via conduit  402 , which is connected to displacement path  209  near the discharge end of displacement assembly  205 . Flow to the displacement path is controlled by cut-off valve  404 .  
         [0068]     Before beginning to flow nitrogen from source  400 , the end of the displacement path must first be closed off. This can be done in any number of ways known to the prior art. In the embodiment depicted in  FIG. 8 , a stopcock type valve  408  is positioned in-line within displacement path  209 . In operation of the dispensing assembly, the valve as shown at  FIG. 8B  is in the open position. When back-flushing the system, the valve as shown in  FIG. 8C  is rotated 90° to close the pathway, and thus contain the flow of nitrogen gas within the system.  
         [0069]     While nitrogen source  400  may be externally heated, it is preferred to use the embedded heaters  211  of the displacement assembly to heat the gas. By introducing the same near the discharge point, the maximum residence time for gas heating in the assembly is afforded. The heaters can be run at the same temperature used during dosing operations, e.g. about 130° F., or adjusted upwardly to temperatures as high as 250° F. At temperatures above 212° F., any water that may be in the system will also be dissipated.  
         [0070]     To affect a back purge of the system, the remote source of liquid nitrogen is first turned off. Valve  408  is closed, and valve  402  opened, to start the flow of gaseous nitrogen, the heaters set to the desired temperature. Once the displacement assembly is pressurize, the needle valve can then be opened. The gaseous nitrogen need be pressurized only to the point of providing a positive back pressure such that the heated gas will flow past the needle valve, into the cryogen source conduit  114 . A system vent positioned along the flexible support arm, downstream of the system&#39;s cryogen source reservoir can be monitored for gas discharge. This vent line is valved such that when the valve is closed, cryogen flows from the cryogen source to the dosing head, and when opened, the source conduit vents to atmosphere, flow form the cryogen source now stopped. If there is an obstruction in the line, the heated nitrogen gas will not vent. At such time as heated gas melts whatever occlusions may exist, gas will start to flow and the measured gas pressure at the vent increase. When the increase in gas pressure reaches steady state, the back flush operation can be stopped. Alternatively, one can monitor the pressure downstream of the metered dosing point  136 . In this embodiment, one can locate a pressure tap in the transport passageway anywhere along the pathway length. When back flush is first begun, an increase in pressure will initially be observed if the occlusion is upstream of the sampling point. On the other hand, one will initially observe no increase in pressure if the occlusion is downstream. At such time as the occlusion is removed, the measured pressure will reach a steady state, indicating that the end of the purging process has been reached.  
         [0071]     In an equivalent, but alterative set up to that shown in  FIG. 8  is illustrated in  FIG. 9 , in which a moveable cylinder  600  is fitted in the end of the injection head  206 , with it&#39;s central axis parallel and concentric to the discharge end outlet. The cylinder is partially sealed in a cavity  602  that is connected to a controlled nitrogen source  400  through internal conduit  608 . In addition, the cylinder has a conduit  604  beginning central to its axis, boring to half the total length, and then exits perpendicular thereto. Post  610  serves to space cylinder  600  from the end of the cavity, such that gas from conduit  608  can flow into said cavity with the cylinder in the open or dispensing position as shown in  FIG. 9B . Spring  611 , surrounding post  610  and affixed at one end to the wall of cavity  602 , is affixed at its other end to cylinder  600 . The spring serves to bias the cylinder in the closed position, limits its downward movement in the cavity under the influence of the pressure from the introduced nitrogen gas, and causes the cylinder to retract when gas flow is discontinued. A protrusion (not shown) can also be provided on the wall, near the lower end of cavity  602  to further limit the downward movement of cylinder  600 . When a sufficient supply of gas has been introduced into the cavity, the position of cylinder  600  will shift to the closed position illustrated in  FIG. 9C , blocking off the normal outlet and allowing gas conduit  604  to move into fluid engagement with displacement passageway  209 , thus allowing for the initiation of back purge flow. The needle valve of the pneumatic actuator must be open for any flow to occur. In addition, a tap (not shown) measures the pressure in delivery conduit  608 . This allows for detection of an occlusion by means of a pressure switch which compares occluded versus clear, that is unblocked, flow. (If the pressure is higher than a particular set point, an occlusion is indicated. When the occlusion is eliminated, the pressure will drop).  
         [heading-0072]     Cooled Pneumatic Actuator  
         [0073]     In an embodiment of the invention, a thermally managed actuator assembly can be provided as shown in  FIG. 9 , the dosing assembly similar to that of  FIG. 1 . Herein, piston  516  is connected to a needle valve assembly including a needle valve stem  504  having at its distal end a valve head  506  configured to mate with valve seat  508  when in the extended or closed position, as shown. Valve seat  508  includes a metering orifice which is in fluid communication with the cryogen reservoir  510 . With the valve in the open position, a small amount of cryogen can pass from the reservoir through the orifice to be dosed to a container to be pressurized.  
         [0074]     The displacement of the piston is generally kept to a minimum to facilitate more rapid cycling. All that is required for operation of the system is that the valve head be retracted a distance from the valve seat sufficient to clear the metering orifice, and allow for the free passage of the liquid nitrogen through the said orifice. Generally, in the VBS system described this is accomplished by limiting piston travel to about 0.200 inches.  
         [0075]     With reference to  FIG. 9 , a standard 4-way valve  530  controlled by a solenoid  529  commercially available MAC of Wixom, Mich. is shown. The valve is situated immediately atop and in direct thermal contact with piston  516 . The system is biased to the closed position by spring  523 . At start up, a pressurized gas (e.g. at 60 psi) such as nitrogen, is fed through input line  526  to 4-way valve  530 , which directs the gas flow through line B to the space above the piston head, with line A open to exhaust port C of the valve assembly. To open the metering valve, solenoid  529  is activated to change the flow paths within the 4-way valve, redirecting the gas flow, such that nitrogen gas now flows through line A to the lower chamber of the piston, acting against the force of the spring  523 . Simultaneously line B connects to exhaust C, thus allowing for the vertical displacement of the needle valve relative to the valve seat. This open/close cycle is rapidly repeated to effect metered liquid dispensing.  
         [0076]     The solenoid valve, mounted in thermal contact with the piston&#39;s cylindrical casing, can be mounted by direct physical contact, or to a seat (not shown) which is itself mounted directly the casing of the cylinder, the seat made of any thermally conductive material such as aluminum or copper. Notwithstanding its relocated position, the solenoid actuated valve operates in the same fashion to move the piston, and thus open and close the dispensing needle valve.  
         [0077]     As the gas used to actuate the piston is exhausted from 4-way valve  530 , it expands at the exhaust outlet  531 . In expanding, it thermodynamically cools, thus cooling the solenoid 4-way valve assembly itself. It has been found that the cooling effect is quite substantial, the expanding exhaust gases cooling the solenoid valve from essentially room temperature to 5° C. or more below room temperature. By placing the cooled solenoid valve in thermal contact with the actuator assembly, the heat generated by its operation can effectively be balanced by the cooling or heat absorption capabilities of the solenoid assembly. It has been found that even at operational speeds of 2000 cycles per minute, the actuator can be operated without significant heat build up, thus facilitating its use with high speed conveyor systems. In the configuration of this invention it has been observed that running at 2000 cycles per minute in an ambient of 25° C., the actuator can still be maintained at a temperature as low as 20° C.  
         [0078]     In an embodiment of the invention, the actuator assembly, including the solenoid valve, is placed within a housing  534 , which has a restricted opening at one end to permit exhaust of gas to ambient. In the embodiment shown, the exhaust gas streams  536  pass over piston assembly  516  to provide further cooling of that assembly, and further acts as a thermal gas insulator between housing  534  and the outside ambient. In this way, the assembly can be operated in a warm or hot room, with lesser impact due to the surrounding ambient, thus further facilitating rapid, high speed operation without heat buildup. In fact, with the delivery assembly of this invention, speeds of up to 2000 cycles per minute can be achieved in elevated temperature environments, up to and including 80° C., without significant overheating.  
         [0079]     In describing the invention herein, particular reference has been made to the use of nitrogen as the liquid cryogen, and as the feed gas for operation of the actuator. However, it should be appreciated that other gases can be used such as argon, or other inert gases. As the cryogen used to pressurize the containers, it should be appreciated that nitrogen is preferred due to its safety and efficacy for foods, beverages and cosmetic products. Other cryogenic gases may be suitable for pressurizing containers where it is intended they contain materials not intended for human consumption or application. Generally a wide range of the materials may be used for the construction for the pneumatic piston and solenoid valve assembly. Most important is that they include thermally conductive materials, especially at the surface of thermal contact to insure a thermal pathway for cooling of the actuator body.  
         [0080]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.