Method and apparatus for pressurizing containers
BACKGROUND OF THE INVENTION 
     This invention relates to a method for pressurizing containers by means of dispensing liquefied nitrogen into metal cans made of relatively thin aluminum or steel material, sealing the cans and then causing the liquefied nitrogen to gassify in the cans so as to preserve the contents of the cans and to create pressure therein to impart to them sufficient strength. The invention is suitable for preparing cans filled with liquid beverages or drinks not containing carbon dioxide. 
     A drawing and ironing method is now being widely used to manufacture what are referred to as two-piece cans made of steel or aluminum sheets having a wall thickness of about 0.15 mm. These two-piece cans as they are called are used for containing beverages because they are strong, efficient, lightweight and can be quickly heated or cooled. Aluminum cans also have the advantage that most liquids contained therein will not deteriorate or degrade in taste even though the inner surface of the cans is not subjected to any special treatment. In addition, since aluminum has an excellent workability, it is possible to integrally form its body or cylindrical portion and bottom portion, thereby reducing the manufacturing cost. 
     When the thin walled aluminum or steel cans are charged with such liquid beverages as beer or beverage containing carbon dioxide, the carbon dioxide liberated creates an internal pressure sufficient to increase the mechanical strength of the cans. However, when the cans are filled with liquid beverages not containing carbon dioxide, for example, coffee, fruit juice, plain water and wine, the internal pressure is not created. Accordingly, for use in such liquid beverages not containing carbon dioxide, one alternative has been to increase the wall thickness for the purpose of increasing the mechanical strength of the cans, and thin sheets manufactured by the drawing and ironing method cannot be used. 
     The general concept of adding liquid nitrogen to thin-walled containers has been broadly disclosed in U.K. Pat. No. 1,455,652; however, in that instance, the details of the apparatus are very sketchy. German OLS No. 3,141,465 published July 15, 1982, includes additional apparatus; however, this is concerned with specific details unrelated to the method herein. Japanese published applications Nos. 56-4521, 49-4389 and 145686/76, U.S. Pat. No. 2,978,336 and Canadian Pat. No. 1,062,671 also disclose use of liquid nitrogen in packaging but are not considered relevant to the present invention. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a method for injecting into and sealing liquefied nitrogen in containers having a thin wall thickness, for example less than 0.2 mm, and partially filled with various types of liquid beverages or plain water so as to create an internal pressure which increases the mechanical strength of the thin-walled containers. 
     According to one aspect of the invention there is provided a method for sealing nitrogen in a metal can wherein a liquefied nitrogen is injected into a metal can by means of a gun, the operation of the gun being controlled by a sensor detecting the metal can at a predetermined position and by an injection time regulator, and wherein the pressure of the liquefied nitrogen supplied to the gun is made constant, whereby the liquefied nitrogen is sealed off in the metal can while partially remaining in a liquid state. 
     The method is additionally characterized in that the liquefied nitrogen is prepared in a container with a top portion thereof being filled with a gassified nitrogen, a pressure detector is connected to the top portion of said container for detecting the pressure of the gassified nitrogen contained therein, a signal produced by said pressure detector is supplied to a pressure control mechanism which makes constant the pressure of the liquefied nitrogen contained in said container by means of shut-off arrangements connected thereto, the liquefied nitrogen of a constant pressure is supplied to said gun through a conduit, and the operation of said gun is controlled by a sensor detecting said metal can being in a predetermined position and by an injection time regulator, whereby the liquefied nitrogen is sealed off in said metal can while partially remaining in a liquid state. 
     A further feature of the method is that the liquefied nitrogen is supplied from a liquefied nitrogen cylinder to said gun through an intermediate tank, the operation of said gun is controlled by a sensor detecting said container being in a predetermined position and by an injection time regulator, a pressure of the liquefied nitrogen supplied to said gun is made constant, and the surplus liquefied nitrogen is returned back to said intermediate tank from said gun. 
     The conduit supplying the liquefied nitrogen of a constant pressure to said gun is provided with a gas/liquid separator so that only the liquefied nitrogen is supplied to said gun. According to another aspect of the invention, the liquid beverage not containing carbon dioxide contained in the metal cans is preheated to a temperature of 60°-95° C. for sterilization. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings; 
     FIG. 1 is a diagrammatic representation of apparatus utilized to carry out the invention; and 
     FIG. 2 is a diagrammatic representation showing a modification of the apparatus shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the embodiment shown in FIG. 1, a nitrogen gas cylinder 1 and a liquefied nitrogen cylinder 2 are provided. The nitrogen gas cylinder 1 is communicated with the liquefied nitrogen cylinder 2 through a conduit 22 including a pressure regulator 3 and a motor operated valve 4. At the top portion 2a of the cylinder 2 containing nitrogen gas are provided a motor operated gas release valve 5 and a pressure detector 6. A pressure level signal produced by the pressure detector 6 is supplied to a pressure control unit 7, which may take the form of a computer or a back pressure regulator, and the output signal of the pressure control unit 7 is applied to the motor operated valves 4 and 5 to actuate them to either respectively increase or decrease pressure so as to maintain the pressure of the liquefied nitrogen in cylinder 2 at a substantially constant pressure. 
     A conduit 17 extending into the body 2b of the cylinder 2 is in communication with a motor operated gun or injector 8 through a gas/liquid separator 11. The gun 8 injects liquified nitrogen into a metal can 20 successively brought to a position beneath the gun 8. Adjacent the can 20 is disposed a can sensor 10, and the output signal thereof is supplied to the motor operated gun 8 through a timer 9 for ejecting the liquefied nitrogen into the can 20 for a predetermined interval of time. The method of controlling the feed speed of cans and the method of sealing cans are well known in the art. Usually, the cans are continuously fed at a speed of from 600 to 1800 cans per minute and on average, at a speed of 1200 cans per minute. After injection of the liquified nitrogen the opening at the top of the can is immediately sealed off. 
     In the modification shown in FIG. 2, the nitrogen gas cylinder 1 is omitted and an intermediate liquefied nitrogen tank 12 is connected to the liquefied nitrogen cylinder 2 via a motor operated valve 15 which is controlled by a liquid level controller 16 to maintain the level of the liquefied nitrogen in the intermediate tank 12 at a constant level. The liquefied nitrogen in the intermediate tank 12 is sent to the motor operated gun 8 via a pump 13 and a pressure regulator 14. Nitrogen gas generated while the liquefied nitrogen is conveyed to the gun 8 and surplus liquefied nitrogen are returned to the intermediate tank 12 via conduit 18. 
     A pressurized air operated gun may be substituted for the motor operated gun. However, since liquefied nitrogen should be maintained at an extremely low temperature a motor operated gun is preferred in view of temperature difference and response speed. 
     Following examples are given for explaining the embodiments of the method of this invention where apparatus shown in FIGS. 1 and 2 are used. 
     EXAMPLE 1 
     This example uses the apparatus shown in FIG. 1. As is well known in the art, nitrogen is maintained in a liquid state when the temperature is below -196° C., and when the temperature rises above this temperature the liquefied nitrogen gassifies so that cylinders 1 and 2 and conduit 22 interconnecting them and the conduit 17 interconnecting the cylinder 2 and the gun 8 are coated with suitable heat insulating material. Under these conditions, liquefied nitrogen was injected into aluminum cans 20 each having a capacity of 267 cc and a wall thickness of 0.1 mm and manufactured by the drawn and iron method. Each can was filled beforehand with 256-258 cc of warm water preheated to 85° C. After injecting the liquefied nitrogen the opening at the top of the can is sealed off so that the liquefied nitrogen is filled in the can with a space therein of about 15-17 cc. The cans 20 were fed at a speed of 1200 cans per minute. The time of opening the gun 8 controlled by the can sensor 10 was set to 0.015 sec., and the gauge pressure at the top portion 2a in the cylinder 2 as 0.5 kg/cm 2 . Under these conditions, 0.2 cc of liquefied nitrogen was injected into each can 20 at a speed of 1200 cans/min. The opening into which the liquid nitrogen was injected was sealed off by applying a lid thereto in a time of about 1 second after injection of the liquefied nitrogen. Sealing was effected by double seaming the lid on. After sealing, the liquefied nitrogen gassified at a room temperature of 20° C. to increase the pressure in the sealed can to a gauge pressure of 0.8-1.2 kg/cm 2  which is sufficient to increase the mechanical strength of the can. 
     EXAMPLE 2 
     In this example, the apparatus shown in FIG. 2 was used and the same aluminum can as used in Example 1 was used. The quantity of the liquefied nitrogen sent to the pump 13 was 5000 cc/min. The injection gun 8 had an inner diameter of 2.4 mm and the liquefied nitrogen was supplied at a pressure of 0.3 kg/cm 2  by the action of the pressure regulator 14. Surplus liquefied nitrogen was returned to the intermediate tank 12 via conduit 18. Each can 20 made of an aluminum sheet having a thickness of 0.1 mm had a capacity of 267 ml and was prefilled with 256 -258 ml of warm water preheated to about 85° C. Immediately thereafter, 0.25 cc of the liquefied nitrogen was injected into each can, and less than two seconds later a lid was applied to the can which sealed the same. After sealing, the internal pressure of the can was found to be at a gauge pressure of 0.8-1.2 kg/cm 2  at a temperature of 20° C. 
     The results of Examples 1 and 2 show that the method of this invention are suitable for producing cans containing coffee, fruit juice, various drinks, etc. 
     As above described, according to this invention, it is possible to manufacture strong cans filled with liquid beverages not containing carbon dioxide, and to inject liquefied nitrogen at high efficiencies. 
     Although the teachings of the invention have been discussed with reference to certain specific disclosured embodiments, it is to be understood that these are by way of illustration only and that variations may be made in the methods and apparatus without departing from the spirit and scope of the invention, as defined by the appended claims.