CO.sub.2 Snow Producer With Heat Exchanger

Liquid nitrogen and liquid CO.sub.2 are conventionally utilized to cool the interiors of insulated containers as well as materials stored therein for the purpose of maintaining low temperatures of the materials and the interior of the insulated containers, especially when the containers are to be used in shipment of the materials therein over extended periods of time without benefit of mechanical or absorption type refrigeration systems. In addition, insulated containers are also conventionally provided with cold holdover plates which may be refrigerated by suitable mechanical or absorption type refrigeration equipment at a starting point and thereafter disconnected from the refrigeration equipment and utilized to maintain the interior of a container in which they are disposed as well as other contents of the container refrigerated over extended periods of time.

The utilization of liquefied nitrogen and carbon dioxide as well as cold holdover plates is presently enjoying increased popularity, inasmuch as transit type containers refrigerated by mechanical or absorption type refrigeration systems often experience malfunction of the attendant refrigeration units and the contents of the refrigerated container is subject to spoilage.

The utilization of liquefied nitrogen for refrigerating the interiors of shipping containers is usually employed when very low freezing temperatures are desired within the container and cold holdover plates are usually used for refrigerating containers when cooling or mild freezing temperatures within the container are required. The use of liquefied carbon dioxide for refrigerating shipping containers is generally limited to containers in which cooling or moderately low freezing temperatures are required and the liquefied carbon dioxide is, in most cases, utilized to form carbon dioxide snow which is approximately minus 69.degree. F., at the triple point pressure of 75 lbs. per square inch absolute, but which drops from minus 69.degree. F. to minus 109.degree. F. at atmospheric pressure. Further, CO.sub.2 snow is conventionally formed by discharging chilled liquid carbon dioxide from spray nozzles. In such instance, the liquid carbon dioxide is supplied to the spray nozzles at approximately 0.degree. F. with the result that one lb. of liquid carbon dioxide will yield approximately 0.40 to 0.45 lbs. of carbon dioxide snow. However, if the temperature and pressure of the liquid carbon dioxide can be maintained at the triple point wherein the pressure thereof is approximately 75 lbs. per square inch absolute and the temperature is approximately minus 69.degree. F., one pound of liquid carbon dioxide will yield approximately 0.46 to 0.51 lbs. of carbon dioxide snow.

There are three distinct thermal cycles which may be used in the manufacture of solid carbon dioxide (carbon dioxide snow). These distinct thermal cycles include the non-regenerative, the regenerative and the complete re-expansion cycles. However, each of these cycles requires considerable machinery and are thus not practiced to any great extent in the production of carbon dioxide snow to be used in insulated transport containers.

Accordingly, a need exists for an improved, simple and efficient method of producing a maximum amount of carbon dioxide snow from a given amount of liquid carbon dioxide.

Examples of carbon dioxide snow producing machines including some of the general structural and operational features of the instant invention are disclosed in U.S. Pat. Nos. 2,247,850, 2,483,064, 2,731,807, 3,561,226, 3,695,056 and 3,815,377.

An intermediate portion of a liquid carbon dioxide supply line equipped with spray jet discharge structure on the outlet end thereof is passed through the interior of a tank containing an eutectic solution and the spray jet discharge structure is positioned to direct spray jets of liquid carbon dioxide onto the exterior of the eutectic solution tank for cooling the tank structure and the eutectic solution therein with the latter thereby cooling the intermediate portion of the liquid carbon dioxide supply line passing through the eutectic solution. The pressure of the liquid carbon dioxide within the discharge end of the supply line is maintained at approximately 75 psi absolute and the volume and type of the eutectic solution used, the thermal conductivity of the tank and the eutectic solution and the length of the intermediate portion of the liquid carbon dioxide gas supply line extending through the eutectic solution is predetermined such that the rate of flow of liquid carbon dioxide through the supply line will result in the temperature of the liquid carbon dioxide in the outlet end of the supply line being reduced to approximately minus 69.degree. F. In this manner, the weight of carbon dioxide snow produced by the spray head structure will be equal to approximately 50% of the weight of the liquid carbon dioxide consumed in the production of the carbon dioxide snow. Further, the eutectic solution tank utilized in this process is disposed within the insulated contained to be ultimately maintained cooled by the carbon dioxide snow produced and collected therein. In this manner, substantially all of the latent heat of fusion is transformed into heat absorbing capacity within the insulated container to be maintained cooled.

The main object of this invention is to provide an improved method of more efficiently producing carbon dioxide snow for cooling the interiors of insulated containers.

Another object of this invention is to provide a method and apparatus in accordance with the immediately preceding object and wherein the structure required to carry out the method is maintained extremely simple and thus quite inexpensive.

A still further object of this invention is to provide a method and apparatus for converting liquid carbon dioxide to carbon dioxide snow wherein the production of the desired amount of carbon dioxide snow may be accomplished at a rapid rate.

A final object of this invention to be specifically enumerated herein is to provide a method and apparatus in accordance with the preceding objects and which will conform to conventional forms of manufacture, be of simple construction and easy to use so as to provide a method and apparatus which will be economically feasible and extremely dependable.

These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.

FIG. 1 is a fragmentary perspective view illustrating the rear of an insulated container comprising a truck load body and with the structure of the instant invention operatively mounted within the container;

FIG. 2 is an enlarged vertical sectional view of the carbon dioxide snow producing structure of the instant invention; and

FIG. 3 is a sectional view taken substantially upon the plane indicated by the section line 3--3 of FIG. 2.

Referring now more specifically to the drawings, the numeral 10 generally designates a truck-type vehicle including a load box referred to in general by the reference numeral 12 and comprising an insulated container such as that conventionally used on truck chassis for transport of chilled or frozen materials over extended periods of time.

Conventionally, the container 12 may be provided with liquid nitrogen jet discharge structure for chilling the interior thereof, cooled holdover plates which may be mechanically refrigerated at a shipping starting point or chilling cabinets or containers having carbon dioxide snow therein.

The utilization of jets of liquid nitrogen within the container 12 after its closure requires venting of the container before workmen may enter therein at each stop, cold holdover plates are quite heavy and expensive and the use of carbon dioxide snow filled chilling cabinets has in the past resulted in excessive use of liquid carbon dioxide in order to form the desired amounts of carbon dioxide snow.

The instant invention utilizes a cabinet or container 14 mounted within the load body or container 12 and with the lower portion of the container 14 defining an upwarly opening receptacle in which to receive gravity falling carbon dioxide snow. The upper portion of the interior of the container 14 contains a tank 16 of eutectic solution 18, such as Freon 11, or a propylene glycol. The tank 16 includes a fill neck 20 opening downwardly through its top wall 22 removably closeable by a removable cap 24 and the top wall 22 also includes an excess pressure relief valve 26 opening therethrough.

A supply line 28 extends from a suitable source 30 of chilled (0.degree. F.) liquid carbon dioxide and includes an inlet end portion 32 extending from the source 30 toward the tank 16, an outlet end portion 34 supported beneath the tank 16 and an intermediate portion 36 which passes through the tank 16 and the eutectic solution 18 disposed therein. The supply line 28 includes a pressure regulator 38 for maintaining the pressure within the supply line 28 at a predetermined adjusted pressure and the outlet end portion 34 of the supply line 28 includes upwardly opening spray jet heads 40 spaced therealong positioned to direct upward spray jets 42 of liquid carbon dioxide onto the undersurface of the bottom wall 44 of the tank 16.

In operation, liquid carbon dioxide is allowed to flow through the supply line 28 at a pressure of to approximately 75 lbs. per square inch absolute and to be discharged from the spray jet heads 40 at that pressure onto the undersurface of the bottom wall 44 of the tank 16. The discharge of the spray jets 42 onto the bottom wall 44 of the tank 16 will not only cause an initial amount of carbon dioxide snow 46 to be formed and to fall by gravity downwardly into the receptacle defining upwardly opening lower portion of the cabinet or container 14, but the bottom wall 44 of the tank 16 will be cooled to a temperature considerably lower than minus 70.degree. F. As a result of the bottom wall 44 being cooled, the eutectic solution 18 will be cooled and the intermediate portion 36 of the supply line 28 will also be cooled. The rate of flow of liquid carbon dioxide through the supply line 28 is controlled by the valve 38 such that the cooling action of the eutectic solution 18 on the intermediate portion 36 of the supply line 28 will lower the temperature of the liquid carbon dioxide flowing through the intermediate portion 36 to a temperature approaching minus 69.degree. F. Accordingly, inasmuch as the triple point pressure of 75 lbs. per square inch absolute and the corresponding temperature approaching minus 69.degree. F. is maintained, the liquid carbon dioxide being discharged from the spray jet heads 40 in the form of the spray jets 42 will be transformed substantially directly into carbon dioxide snow with little loss as carbon dioxide gas. It has been found that if the above triple point temperature and pressure are substantially realized, the weight of carbon dioxide snow produced will be approximately 50% of the weight of the liquid carbon dioxide used to produce the carbon dioxide snow. Accordingly, considerable additional production of carbon dioxide snow is realized than is generally accomplished by merely discharging liquid carbon dioxide through snow producing heads substantially independent of temperature and pressure controls of the liquid carbon dioxide being discharged through the spray heads. In addition, inasmuch as the cabinet or container 14 itself is enclosed within the insulated load box or container 12, the additional heat absorbing capacity of the liquid carbon dioxide utilized to refrigerate the tank 16 and the eutectic solution 18 therein is retained and suitable blowers or other structures capable of circulating the air within the container 12 through and about the container 14 during transit of the container 12 may be utilized and controlled by thermostatic control in order to provide the necessary cooling of the interior of the container 12.

It is to be appreciated that while the weight of carbon dioxide snow produced is only 50% of the weight of the liquid carbon dioxide used to produce the carbon dioxide snow, the cooling capacity of the carbon dioxide snow produced is augmented by the cooling capacity of the chilled tank 16 and the eutectic solution 18 therein to the extent that the over-all cooling capacity of the carbon dioxide snow produced and the chilled eutectic solution resonably closely approaches the latent heat of sublimation, not including the heat of superheat at 0.degree. F. which is approximately 20 B.t.u. per pound.

Although the structure and method of the instant invention has been specifically disclosed hereinabove utilizing liquid carbon dioxide, it is to be noted that the invention may also incorporate the utilization of a suitable liquid refrigerant other than liquid carbon dioxide.

The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.