Abstract:
A refrigeration system to be located at carbon dioxide using locations for providing cooled or sub-cooled liquid carbon dioxide at temperatures as low as minus 65° F. to various liquid carbon dioxide dispensing/using devices. The system is capable of being added to virtually every type of carbon dioxide storage vessel used at customer sites, and is especially useful where relatively short carbon dioxide use periods are involved, as the hybrid refrigeration cycle utilizes the liquid carbon dioxide in the storage vessel as a rechargeable refrigeration sink.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable 
     STATEMENT OF FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not applicable 
     BACKGROUND 
     1. Field of Invention 
     This invention relates to the apparatus and methods suitable for liquid carbon dioxide storage and process systems typically located at customer or user sites which supply liquid carbon dioxide (CO2) to devices which then utilize the liquid CO2 so as to provide various refrigeration effects. Such systems, while they may have many other beneficial uses, are especially useful as ground support/filling apparatus for trucks or rail cars expending liquid carbon dioxide for in-transit cooling, or for devices for food chilling or freezing or for making dry ice. 
     2. Description of Prior Art 
     Solid carbon dioxide (dry ice) has long been used as an expendable refrigerant for many cooling applications because of its ease of application, its non-toxity, its very large refrigeration effect when subliming, its direct change to the gas phase and its desirable low range of refrigeration temperatures. Dry ice, at atmospheric pressure, sublimes at minus 110° F. and has a heat of sublimation (refrigeration) of 244 btu/lb. Initially, liquid CO2 typically was made at central manufacturing plants, converted to dry ice in the form of blocks and then transported to the customer or using sites, stored, then placed or mixed when and where cooling was desired. If CO2 vapor was desired for carbonation, the blocks were placed inside high pressure converters (about 1,000 psig) and allowed to warm to ambient temperature. 
     However, the inconvenience of handling dry ice and the attendant weight loss after purchase, but before use (which typically averaged 50%), caused the CO2 industry to change to liquid CO2 distribution and customer storage. The standard for the U.S. CO2 industry became about 0° F. liquid CO2 with an equilibrium pressure of about 300 psig for distribution and customer storage. This temperature was selected as one that could be maintained readily by small single stage, air cooled freon type refrigeration units adjacent to an insulated customer storage vessel, with the coils for cooling the CO2 located in the ullage space of the customer storage vessel; and all so that the maximum allowable working pressure (MAWP) of the vessel was not exceeded. If vapor was desired for carbonation, etc., it was piped direct from the vessel&#39;s ullage volume or for large users, a liquid CO2 vaporizer was utilized. If the CO2 was to be used for cooling, the liquid CO2 was piped directly from the customer storage vessel to the using device. Subsequently, about 10,000 such vessels with internal coils and attendant refrigeration units of various sizes have been installed within the United States. In addition, many variations of this arrangement have been produced. A fleet of liquid CO2 trucks are also in place to distribute liquid CO2, and liquid CO2 production plants typically produced liquid CO2 suitable in temperature and pressure to support this system. However, one lb. of liquid CO2 at these conditions converts to only about 0.47 lb. of dry ice, thus providing only a heat of sublimation (refrigeration) of about 115 btu per lb. of liquid CO2 used. During the conversion about 0.53 lb. of CO2 is released as vapor. Thus while the change from a dry ice distribution system to a liquid CO2 distribution system greatly reduced the losses of dry ice CO2 and eliminated the inconvenience of dry ice handling; the use of liquid CO2 for cooling applications imposed an undesirable CO2 loss. The steel chosen to fabricate the insulated storage vessel was chosen to be safe at low ambient temperatures and various insulations were used, including foam glass. More recently, vertical storage vessels with vacuum insulation are available, which typically do not contain internal coils, and which are suitable for temperatures as low as about minus 40° F., and are replacing the older vessels. 
     It was well known that lower temperature liquid CO2 produced a higher percentage of dry ice/cooling when used, thus came a trend to production and distribution of lower temperature liquid CO2, so as to better support dry ice/cooling applications. Accordingly, in many geographic areas, a temperature of minus 20° F. and 225 psig for liquid CO2 delivery became feasible. Virtually no changes in existing equipment was required to accommodate this lowered distribution temperature, and any vessel&#39;s refrigeration unit, while less required, were left in place because of vacation and other low or non-use periods. However, principally because of metal safety concerns for the storage vessels, distribution equipment, etc., to further reduce the liquid CO2&#39;s temperature at the production plant would require replacing much of the existing distribution equipment and customer storage vessels. At about minus 70° F., CO2 begins to form a solid, and thus cannot be readily transported as a liquid, but a minus 65° F. liquid produces about 0.57 lb. of dry ice, a conversion improvement of about 20%. It has been estimated that about 4,000 tons per day of liquid CO2 is used for cooling applications in the U.S., thus 800 tons per day could be saved if all could be cooled to minus 65° F. before use. Accordingly, a number of refrigeration devices have been developed to cool liquid CO2 at the final use location for a wide variety of applications. Examples are: U.S. Pat. No. 4,888,955 issued December, 1989 to the present inventor, et al; U.S. Pat. Nos. 3,660,985 issued May, 1972, 3,672,181 issued June, 1972, 3,754,407 issued August, 1973, 4,100,759 issued July, 1978, 4,127,008 issued November, 1978, 4,211,085 issued July, 1980, 4,224,801 issued September, 1980, 4,693,737 issued September, 1987, 4,695,302 issued September, 1987, and 5,934,095 issued August, 1999, all to the present inventor. 
     While cooling liquid CO2 to low temperatures may seem to be a straightforward mechanical refrigeration problem; the highly unusual nature of CO2 (especially the triple point occurring at useful temperature and pressures) combined with the problems in moving a liquid that becomes a solid if allowed to de-pressurize (even momentarily) below the triple point pressure, combined to prevent a totally satisfactory solution. Some of the specific problems unique to CO2 and thus the industry include the facts that: (1) flowing liquid CO2 when de-pressurized even momentarily to about 60 psig (the triple point), almost instantly becomes a mixture of liquid and solid and only changes back to liquid with the relatively slow application of heat; and (2) in any subsequent flow, this mixture easily clogs lines, valves and use devices as additional solid/slush CO2 forms, and any subsequent pressure reduction will cause it to turn progressively solid. Accordingly, most prior art inventions did not move very cold liquid CO2 to a use point, without providing sub-cooling with a pump or by some type of gas pressurization. 
     A related problem is due to the nature of use of most expendable refrigerants, of which CO2 is member, whether used in liquid form or in solid form (dry ice). Expendable refrigerants typically are used precisely when the cooling is desired and in the exact amount needed, thus the use rate can vary greatly. Low use rates can be followed by high use rates, varying quickly from no use to very high use. Patents &#39;985, &#39;407, &#39;759, &#39;085, &#39;737, &#39;302, &#39;955, and &#39;095 all solved the problem of when very cold liquid CO2 is being used, by incorporating a storage function of previously cooled liquid CO2 for supply to CO2 using/dispensing devices along with the storage of warmer liquid CO2; thus storing the cold liquid CO2 in the sub-cooled condition. However, none were versatile enough to find wide use. 
     While sought for years and despite all these efforts, a sufficiently versatile solution to have wide applicability has evaded the CO2 industry. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods and systems for safely receiving liquid CO2 at a range of temperature and pressures into either an existing or a new customer located storage vessel that, by temperature and pressure manipulation, is subsequently able to increase the liquid CO2&#39;s refrigeration potential to the extent possible by cooling the liquid CO2 to between about minus 65° F. and about minus 30° F. prior to final use; and to maintain this liquid CO2 in the cooled and/or sub-cooled condition so it is available for ready flow to the use point without fear of dry ice blockages inadvertently occurring as it is being used. In one aspect, this hybrid system is able to incorporate use of the existing vessel refrigeration unit and standard events associated with distribution and use of liquid CO2 to simplify and minimize the size of the refrigeration equipment, without imposing the burden of discarding the existing equipment. It is modular, thus one or more of the system&#39;s components (and in different sizes) can be installed, as best fits the individual users needs and equipment availability. Apparatus for maximizing the existing storage vessel&#39;s (and its contents) potential refrigeration effect storage (thermal storage) for future utility is also included. In addition, in another aspect, the system is able to utilize the frequently largely unused, but already installed vessel refrigeration equipment. Accordingly, the modular system is able to be readily adapted to meet virtually all the different user&#39;s sizes and pattern of liquid CO2 use requirements, but without the burden of custom designed and engineered systems or special customer station vessels. Thus a simple, add-on type modular and versatile system is provided that inter-reacts with most existing liquid CO2 production, distribution and customer storage and refrigeration equipment, so as to provide more efficient conversion of liquid CO2 to a colder or sub-cooled condition for those users who benefit from such additional cooling and reducing CO2 use by about 20%. Accordingly, one important aspect of the invention is incorporation in the process tank of a separate storage function for the high refrigeration potential liquid CO2, and the liquid CO2 stored in this separate process tank can be maintained in the sub-cooled condition, ready for instant use without fear of blockages. Another aspect is that the colder and/or sub-cooled liquid CO2 systems are able to recharge the storage simultaneously while the storage is being drawn upon by customer use. Still another aspect is that a storage vessel pressure control management system is included. One special advantage is that the size and of the storage vessel and the size of the sump or processing tank are independent of each other; and the size of both the deep cooling equipment and storage vessel refrigeration unit(s) are also independent. This allows selection of the receiving storage vessel&#39;s size to include distribution economies; and selection of the processing tank&#39;s size, and selection of both the deep cooling equipment and storage vessel&#39;s refrigeration units&#39; sizes to include individual user CO2 needs/use patterns. This added equipment can be located near the receiving vessel or in circumstances where the CO2 use point is elsewhere, located so as to minimize the distance the chilled CO2 is piped to use. In still another aspect, one refrigeration unit can be provided which alternately either acts as a chiller for the storage vessel, or acts as a hybrid or modified binary cascade low temperature chiller for the process vessel, having a thermal storage/flywheel feature associated with the CO2 portion of the hybrid cycle. If desired, the chilled CO2 can be maintained in the sub-cooled condition without the use of a pump, so the pressure drop associated with flow can be accomplished without the CO2 flashing to vapor and interfering with the flow of liquid, so to provide predictable flow characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagramatic/schematic view of a system embodying various features of the invention with portions broken away and with a number of components shown schematically and the system components grouped by function. While the CO2 storage vessel is shown as horizontal, as horizontal vessels typically contain large refrigeration coils, it can be vertical. It has connections for filling and for use; and a mechanical refrigeration section having a closed cycle freon type refrigerant compressor, an air cooled refrigerant condenser and common controls, connecting with the refrigeration coils within the vessel condensing CO2 vapor, thus maintaining the pressure/equilibrium temperature in the storage vessel below the vessel&#39;s maximum allowable working pressure (MAWP). A smaller combination low temperature process and storage tank is depicted as located near the storage vessel, but could be located elsewhere if desired. A separate CO2 type low temperature refrigeration cycle system direct deep cools the liquid CO2 removed from the process tank and returns it in a sub-cooled condition, (which also serves as a reservoir of deep cooled liquid CO2, which is in the sub-cooled condition). Depending upon the pressures and temperatures involved, this second refrigeration system rejects part of its heat to the atmosphere, and part of its heat as CO2 vapor to the storage vessel using a pressure management system. This vapor can then be reliquefied by the vessel refrigeration system. 
     FIG. 2 is the systems of FIG. 1, except that the single liquid CO2 expansion and vapor CO2 compression portion is replaced with a multiple expansion and vapor compression portion. 
     FIG. 3 is a similar system to either FIGS. 1 or  2 , except that the low temperature CO2 refrigeration, semi-open cycle system indirectly deep cools in a heat exchanger the liquid CO2 from the combination storage and process tank before returning it in the sub-cooled condition. 
     FIG. 4 is a similar system to that in FIG. 1, except that the low temperature CO2 refrigeration semi-open cycle system is replaced with an alternate low temperature system; a compound closed cycle mechanical refrigeration unit using a suitable low temperature refrigerant (such as R-502, R-404A or other), rejecting part of its heat to the atmosphere and part as CO2 vapor back to the storage vessel. 
     FIGS. 5 and 6 show identical refrigeration apparatus and liquid CO2 storage apparatus, but each show the system operating in a different node. This embodiment is particularly useful with either horizontal or vertical customer vessels with no or limited internal refrigeration coils. FIG. 5 is the system operating as a CO2 vapor condenser and chiller for the storage vessel. FIG. 6 is the same system, but operating as a low temperature chiller for liquid CO2 in the process tank. 
     FIG. 7 is a variation of FIG. 5, but with an added CO2 vapor compressor to remove vapor from the vessel, and with associated controls, useful for providing added refrigeration to the storage vessel when in that mode of operation; and able to further lower the temperature of the liquid CO2 stored there. 
    
    
     These elements in concert provide systems with an unusual ability to provide the various cooling/sub-cooling loads desired, and the use of modularity allows the ready provision of a system that meets the different needs of individual users. To better meet variable CO2 demands, the cooling/sub-cooling cycle incorporates a reservoir and storage tank, which accumulates a supply of cooled/sub-cooled liquid CO2, and which can be replenished concurrently with usage from it without warming the cooled/sub-cooled liquid CO2 already within the reservoir or storage tank. The size of this reservoir is independent of the other components of the system, therefore as one example, a relatively small process tank could be provided for refilling the about 1,000 lb. capacity liquid CO2 tank carried on each truck of a fleet of 15 trucks refilled over an 2 hour span, and a larger tank provided for filling all trucks or filling one railroad car bunker with about 10,000 lbs. of dry ice snow both within 20 minutes, with all the other system components the same. 
     Note: In all drawings where CO2 flow is shown, a single headed arrow → indicates CO2 vapor flowing; a two headed arrow →→ indicates CO2 liquid flowing; 
     and a three headed arrow →→→ indicates very cold sub-cooled CO2 liquid flowing. A circle following the arrow -•-→ indicates a freon type refrigerant is flowing, with the other arrow designations similar. 
     Where the identical part appears in different Figures, or in variations of related embodiments (as FIGS. 1,  2 ,  3 , &amp;  4  as well as FIGS. 5, &amp;  7 ) or the same embodiment but operating in a different mode (as FIGS.  5  and  6 ), the same reference character is used. Where the same part appears in different embodiments, a single primed reference character is used. 
     For the purpose of simplifying the Figures, some lines/connections to the vessels or tanks standardly provided in the CO2 refrigeration industry, as well as those used in freon type closed cycle refrigeration systems have been omitted, such as fill or transfer lines, auxiliary liquid or vapor lines, surge tanks, safety relief valves and burst discs, level/contents devices, pressure gauges, clean-out connections, and others. System monitoring devices, controls and programmers are included as desired. Valves can be electric, pneumatic, other, remotely controlled or manual. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Illustrated in FIGS. 1,  2 ,  3 ,  4 ,  5 ,  6 , and  7  are embodiments and variations thereof of a system to be located at a liquid CO2 users site for delivering very cold, sub-cooled if desired, liquid CO2 to various types of CO2 dispensing devices; at selected temperatures (between about minus 65° F. and about minus 300° F.), and at selected pressures (between about 65 psig and about 500 psig). It is useful with both horizontal liquid CO2 storage vessels, typically having large internal refrigeration coils; and also with vertical vessels, typically having small or nonexistent internal refrigeration coils. It includes methods of using any liquid CO2 in the storage vessel as a means of providing thermal storage to be subsequently utilized to help create very cold and/or sub-cooled liquid CO2 during periods of heavy use; and also provides a separate reservoir of very cold and/or sub-cooled liquid CO2 so as to further assist the system in meeting periods of heavy use. 
     Illustrated in FIGS. 1,  2  and  3  is the system with a CO2 lower stage refrigeration system, rejecting its heat to an upper stage freon type mechanical system which condenses CO2 vapor within the storage vessel and is able to cool its CO2 contents, enabling the contents to act as an inter-stage thermal flywheel. This allows the CO2 lower stage to reject heat (cool) at a very high rate. This binary refrigeration combination is referred to by some as a hybrid system. The cycle illustrated in FIG. 2 is commonly referred to as a CO2 bleeder cycle, which greatly reduces the demand on the CO2 compression system by utilizing multi-staging and split flows of compressors. Moreover, to best utilize the thermal storage potential of the storage vessel, its liquid CO2 contents and any companion refrigeration system; a return CO2 vapor pressure management system can be provided. Normally, there will little or no use of liquid CO2 from the system at night. Accordingly, the vessel refrigeration system(s) can progressively reduce the temperature of the liquid CO2 within the storage vessel and thereby both increase the thermal storage potential and reduce the cooling needed when the liquid CO2 is being deep chilled. One feature of the pressure management system is the CO2 vapor being returned to the vessel by the CO2 refrigeration cycle is returned to the vessel&#39;s ullage volume, thereby not warming the liquid CO2 previously cooled within the vessel. Another feature of the pressure management system is provision so that the condensing coils operate more efficiently by having the CO2 vapor they are condensing saturated (or near saturated), so that de-superheating of the CO2 vapor does not have to occur prior to condensation by the coils. Still another feature of the pressure management system is that is the vessel pressure approaches the MAWP, the vapor being returned passes through the cooled liquid, thereby decreasing the refrigeration load on the vessel refrigeration system. All is especially useful for coil-in-vessel systems. 
     Turning first to FIG. 1, depicting five semi-independent groupings of apparatus connected so as to form one variation of this system/invention. Storage vessel system  10  contains an inner vessel  11 , which is filled with liquid CO2  12 A from a delivery vehicle (not shown) through liquid fill line  14 , with a fill-vapor return line  15  relieving excess pressure occurring during filling back to the delivery vehicle, and thus returning CO2 vapor  12 B from the top/ullage volume of the inner vessel  11  to the vehicle. Vapor CO2  12 B will then return to the shipping point via the vehicle for disposal or re-liquefication. This returned vapor represents a refrigeration load removed from system  10 . Fill line  14  can be divided into sub lines as desired, i.e. one to the top and one to the bottom of inner vessel  11  as well as one or more intermediate entry lines if provided on the vessel (not shown), so as to provide for ease of filling and control of temperature/pressure of the liquid CO2  12 A in the inner vessel  11  during filling operations. Refrigeration coil  16  is located within the ullage volume of vessel  11  and connected to the second apparatus grouping, appropriately sized refrigeration unit  17 . If two coils  16  are provided within the vessel (or provision for such), then two units  17  can be utilized. System  10  is supported upon legs  18 . A liquid withdrawal line  19  with valve  20  and branch line  22  is provided for filling the third apparatus grouping, containing the low temperature combination storage/process tank  24  through its upper portion. Tank  24  can be located near system  10 , so as to simplify and minimize the piping between system  10  and itself and promote the ready flow of liquid CO2  12 A from vessel  11  to and through tank  24  (where it becomes cooled/sub-cooled liquid CO2  12 C) to the using device  25 . Alternately, tank  24  can be located near device  25 . Tank  24  can be of any desired size, as it also serves as a storage reservoir, in use supplementing the low temperature refrigeration system&#39;s output. Branch line  22  connects the top of tank  24  to the top of vessel  11  in such a manner that when valve  20  is opened, liquid CO2  12 A flows from the lower liquid space of inner vessel  11  through line  19  into the upper volume of tank  24 , and any vapor  12 B flows into the upper or ullage volume of vessel  11 . A safety relief line, having a number of safety related functions connects to the top of inner vessel  11  and a similar function line connects to the top of tank  24  (not shown). An automatic blow-back system, of the type common in the cryogenic industry (for returning any liquid CO2 trapped in tank  24  to vessel  11  when valve  20  is closed), can be provided (not shown). All vessels, tanks, valves, and lines etc. that operate at below ambient temperatures have suitable insulation  26 . If desired, anti-mixing devices  28  are located inside tank  24  so as to promote stratification. Temperature sensor  30  is inserted into tank  24 . 
     The fourth apparatus grouping consists of the low temperature portion of the hybrid refrigeration system. Vapor withdrawal line  32  connects the upper volume of tank  24  to evaporative cooling tank  34  and includes pressure regulator  36 , valve  38  and insulation  26 . Tank  34  includes a two position level control  40  and a pressure switch  42 . Line  44  connects the top of tank  34  to compressor  46  which discharges to and through receiver  47  via line  48 . Line  50  connects receiver  47  with the top of tank  34  and contains valve  52 . Cooled liquid CO2  12 C transfer line  54  connects the bottom of tank  34  to the bottom of process tank  24  and contains pump  56  and check valve  58 . Any NPSH required by the pump, if and when needed, can be provided by opening valve  52  to the extent required, thus admitting CO2 vapor through line  50 . Liquid CO2  12 A can thus be removed from the upper portion of tank  24 , moved to tank  34 , deep cooled to condition  12 C, at a temperature between about minus 30° F. and about minus 65° F. in tank  34  and returned to the bottom of tank  24  in batch cycles, as controlled by level control  40 , switch  42 , and sensor  30 , after passing through condenseable contaminants separator  59 . Non-condenseable contaminants can be purged or used for pneumatic valve operation or vented (not shown). Should they be required, the optional anti-mixing devices  28  (located at different levels in tank  24 ) or low velocity entrance arrangements (not shown) maintain the separation between the colder liquid CO2  12 C and the warmer liquid CO2  12 A wherever the thermocline occurs in tank  24  when liquid CO2  12 A or liquid CO2  12 C enter tank  24  during use. Valve  60  located in line  63  controls the flow of cold/sub-cooled CO2  12 C from tank  24  to using device  25 . Line  63  can have a pressure sensing and purge control system to prevent formation of dry ice therein or within device  25 , when valve  60  is opened, as used within the CO2 industry (not shown). 
     The fifth apparatus grouping, comprises the pressure management system  64 , especially useful when system  10  includes one or more internal coil  16  and large refrigeration unit  17 , but whose use is optional, and whose function will be described later in detail. By the use of this arrangement, CO2 vapor can be withdrawn from the process tank  34 , raised in pressure by compressor  46 , and then returned to the inner vessel  11 , all as determined by the logic of the process controls  66 . 
     In addition, controls  66  monitors and controls the various elements of the entire system, in a manner compatible with the needs of device  25 , the anticipated use cycle, and the capabilities of the individual elements of the entire system. 
     While compressor  46  has been depicted as a non-lubricated (oil-less) rotary vane compressor, any suitable type can be utilized; and all control devices and sensors could be replaced with other types, such as electronic. Filters, vents, purge valves, clean-out arrangements, and other details surge tanks and many other items normal to the CO2 industry, the CO2 refrigeration industry, and to the freon refrigeration industry can also be included as desired. 
     Illustrated in FIG. 1 (as well as in FIGS. 2,  3 ,  4 ,  5 ,  6  and  7 ) are systems to be installed at a user&#39;s site for delivering very cold/sub-cooled liquid carbon dioxide to various types of dispensing/using devices, at selected temperatures (usually between about minus 30° F. and about minus 65° F.) and at selected pressures (usually between about 65 psig and about 500 psig). They are useful with both horizontal and with vertical liquid carbon dioxide storage vessels, those with large, medium, small or non-existent refrigeration coils. All include a method of using the liquid carbon dioxide within the storage vessel as a means of providing thermal storage (or equivalent) to be utilized to create very cold/sub-cooled liquid carbon dioxide during periods of heavy use; and also provides a reservoir of very cold/sub-cooled liquid carbon dioxide so as to further assist in meeting heavy use demands. These embodiments provide a system with an unusual ability to follow the various cooling loads required, and the use of modular elements allow the provision of systems that can be sized to meet use demands from small to very large. 
     Referring to FIGS. 1,  2 ,  3  &amp;  4 , and specifically to line  48  of FIG. 1 containing carbon dioxide vapor  12 C which is to be returned to vessel  11 , system  64  provides versatility as to the various flow paths/entrances of this vapor into vessel  11 . This allows maximizing the benefits of the specific equipment, while allowing and providing for the differences in optimum characteristics of individual dispensing/using devices  25 . 
     The function of the vessel pressure management system  64  is best understood if an example is given. This system maximizes all the refrigeration capabilities related to vessel system  10 , including control of vessel 11&#39;s pressure to secure desirable liquid carbon dioxide pressure being supplied to dispensing/using device  25  just prior to and during on-use periods; and control (lower) the temperature of the liquid carbon oxide  12 A stored in vessel  11  during off-use periods, so as to both reduce the amount of cooling subsequently required to produce the desired sub-cooled carbon dioxide, and increase the thermal storage potential of liquid CO2  12 A within vessel  11 , all as explained later. For this example, it is assumed that the Maximum Allowable Working Pressure (MAWP) of the vessel  11  is 350 psig and the minimum safe temperature at that pressure is minus 20° F. Tank  24  is constructed so as to be safe at least about minus 70° F. and about 350 psig (lower or higher pressures are possible if provided for). Deliveries of liquid carbon dioxide into system  10  typically can range between about 225 psig and 300 psig equilibrium pressure. The equilibrium pressure-temperature relationship of liquid carbon dioxide at various intermediate conditions are as follows: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 pressure, psig 
                   
                 temperature ° F. 
               
               
                   
               
             
             
               
                 350 
                   
                  +8 
               
               
                 300 
                   
                  +2 
               
               
                 250 
                   
                  −8 
               
               
                 125 
                   
                 −42 
               
               
                  80 
                   
                 −60 
               
               
                  60 
                 Triple Point 
                 −70 
               
               
                   
               
             
          
         
       
     
     Control panel  66  monitors the pressure in vessel  11  and at the appropriate times cause the respective elements of the vessel pressure management system  64  to function. For the purposes of this example, it is assumed that either the bunker of a very small rail car, container or truck is being filled with snow (with a desired filling time of 1 hour), minimum pressure of 300 psig is desired during such use; or a number of small liquid tanks carried on trucks for later use in providing cooling. It is also assumed that liquid carbon dioxide use by these examples only occurs between about 8 am and about 6 pm. Also normal liquid carbon dioxide truck/rail delivery pressure into system  10  is 250 psig. 
     Accordingly at about 7:30 am, the pressure in vessel  11  could be about 250 psig, either from a delivery or from the action of refrigeration unit(s)  17 . Initially, controls  66  cause the low temperature refrigeration system to operate so tank  24  becomes full of sub-cooled liquid carbon dioxide  12 C. Compressor  46  begins to operate so as to remove the evolving carbon dioxide vapor, and compressed carbon dioxide vapor begins to flow through line  48 . Controls  66  determines that the vapor should flow through line  70  directly to the top/ullage volume of vessel  11  so as to raise the vessel pressure to the desired about 300 psig as rapidly as possible and accordingly opens valve  72  so that the carbon dioxide vapor  12 B flows directly to the ullage volume of vessel  11  through line  70 , until at least the desired minimum pressure of about 300 psig is reached. At the same time, refrigeration unit(s)  17  are not allowed to operate until the pressure of vessel  11  reaches about 300 psig, all so unit(s)  17  operate in a more efficient range. If the pressure of vessel  11  rises to about 320 psig, valve  72  is closed and valve  74  is opened and the carbon dioxide vapor now flows through line  76  into saturater/de-superheater  78 . As the vapor flows into saturater/de-superheater  78 , injector  80  causes it to bubble through liquid carbon dioxide  12 A admitted from vessel  11  by valve  82  opening line  84 . After bubbling through liquid carbon dioxide  12 A, the CO2 vapor  12 B becomes cooled and de-superheated, and then passes (along with the vapor evolving from the liquid carbon dioxide  12 A vaporized in the process) to vessel  11  through line  86  where it can be condensed by coil(s)  16  and refrigeration unit(s)  17 . However, by these means, the bulk temperature of liquid carbon dioxide  12 A in vessel  11  remains essentially unchanged. The capacity of coil(s)  16  is greater if the carbon dioxide vapor  12 B they are condensing is already saturated, effectively raising the capacity of refrigeration unit(s)  17 . As the pressure in vessel  11  raises, the capacity of refrigeration unit(s)  17  progressively increases, due to the coils condensing at a warmer temperature and the suction pressure of refrigeration unit(s)  17  becoming correspondingly higher. In addition, as the carbon dioxide vapor  12 B flows into the ullage volume of vessel  11  at increasing pressures, that volume accepts and stores that vapor for later condensation, effectively adding to the thermal storage potentials of the system. 
     If the pressure of vessel  11  drops below 310 psig, the refrigeration unit(s)  17  can be stopped. Cycling of these different elements continues as required. Should the pressure in vessel  11  raise to about 325 psig (about 5° F. equilibrium temperature), valves  74  and  82  will be closed and valve  88  opened, which then allows the vapor to flow directly into the vessel  11  through line  90  to optional sparger  92 . Since the body of liquid carbon dioxide  12 A in vessel  11  could be at as low a temperature as about minus 8° F., the amount of vapor  12 B now reaching the ullage volume will be reduced by the amount of condensation taking place in the liquid  12 A. This vapor is also de-superheated. This method uses the sub-cooled condition of the liquid carbon dioxide  12 A as a thermal storage medium, so as to reduce the refrigeration load on unit(s)  17 . As the pressure in vessel  11  changes due to the use circumstances and other events effecting system  10 , control  66  opens or closes valves  72 ,  74 , and  88  appropriately. The system typically is able to follow the use pattern of liquid carbon dioxide  12 C supplied to the dispensing/using device  25  without venting of carbon dioxide vapor by maximizing the refrigeration capacity of refrigeration unit(s)  17  and coil(s)  16 , and the thermal storage capabilities of the liquid carbon dioxide  12 A in vessel  11  and the equivalent thermal storage capability of vessel  11 &#39;s ullage volume. 
     A separate arrangement (not shown) would be to have the deep cooling systems reduce the temperature of the liquid carbon dioxide  12 A in the vessel  11  at night, to the extent safely allowed by the materials of construction of vessel  11  (construction and materials of vessels can differ and about a minus 40° F. capability can be found) by providing a branch line from pump  56  back to the lower portion of vessel  11 , and the use of appropriate control settings. This would have the result of increasing the capacity of the system for providing sub-cooled liquid carbon dioxide when later needed by reducing the amount of cooling required and also increasing the thermal storage potential of the liquid carbon dioxide  12 A within vessel  11 , as will be explained later. Should a lower pressure for liquid carbon dioxide  12 C be desired at device  25 , optional pressure regulator  94  can be located in line  63 . Conversely, should a higher pressure be desired, optional pump  96  can be located in line  63 . Should a lower pressure be desired in tank  24 , an optional pressure regulator can be located in line  19 , downstream of line  22  (not shown). 
     Turning next to FIG. 2, we turn to the operation of tank  24  and tank  34  when a bleeder type expansion and vapor carbon dioxide refrigeration cycle is employed, utilizing a second compressor  100  which is in series with the first compressor  46 . While this arrangement is shown with two compressors, additional compressors (and additional stages) could be employed. Following regulator  36  in line  32 , vapor-liquid separator  102  is added along with regulator  103  in line  104  (which connects the liquid outlet of separator  102  and tank  34 ). Line  106  connects the vapor outlet of separator  102  with interstage receiver  108 . Line  110  connects the discharge of compressor  46  with receiver  108 . Line  112  connects receiver  108  with the inlet of compressor  100 . Motor  113 , (with variable speed control  114 , responding to the intermediate stage pressure(s) caused by changes in flash gas amounts if the temperature of the liquid carbon dioxide  12 A changes, and pressures sensor  115 , speeding up or slowing down motor  113  appropriately, if desired), drives compressor  100 . Line  116  connects the discharge of compressor  100  with receiver  47  and line  48 , and with the remainder of the system, all as in FIG.  1 . The action of this variation is similar to that in FIG. 1, and continues operation until temperature sensor  30  senses that tank  24  is full of cold liquid carbon dioxide  12 C. This use of two or more compressors greatly increases the deep cooling capacity of this embodiment. 
     The approximate cooling capacity of these three refrigeration elements (refrigeration unit(s)  17 , thermal storage of liquid  12 A in vessel  11 , and vapor  12 B acceptance into the ullage volume of vessel  11 ) for a standard  30  ton customer vessel with provision for two (2) refrigeration units is as follows: 
     (a) for eight (8) horsepower freon refrigeration unit(s)  17 , as used in the CO2 industry, in lbs./hour of sub-cooled (about minus 60° F.) liquid  12 C; ten (10) horsepower units  17  have approx. ⅓ more cooling ability); 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 one (1) unit 
                 two (2) units 
               
               
                   
                   
               
             
             
               
                   
                 1,500. 
                 3,000. 
               
               
                   
                   
               
             
          
         
       
     
     (b) for a standard 30 ton capacity horizontal customer vessel, as used in the CO2 industry, and depending upon the amount of liquid  12 A in the vessel at time of use to supply liquid  12 C, in lbs./day cycles: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 vessel 11 
                 liquid 12A 
                 vapor 12B 
                   
               
               
                   
                 contents-12A 
                 warming 
                 acceptance 
                 total 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 ¼ full 
                 1,700. 
                 1,700. 
                 3,400. 
               
               
                   
                 ½ full 
                 3,400. 
                 1,100. 
                 4,500. 
               
               
                   
                 ¾  full 
                 5,000. 
                 500. 
                 5,500. 
               
               
                   
                   
               
             
          
         
       
     
     From the above, it is clear that the different factors can change in relationship, but that each is important. This example, while specifically relating to FIG. 2, applies to the concept of all the other embodiments. 
     Turning next to FIG. 3, the low temperature refrigeration apparatus is changed in that the carbon dioxide deep cool cycle creates the sub-cooled liquid carbon dioxide by using a heat exchanger, rather than by the direct self-cooling of FIGS. 1 and 2 (de-pressurizing the liquid carbon dioxide, then re-pressurizing it). A standard type evaporator-cooler  117  replaces cooling tank  34  of FIGS. 1 &amp; 2. Line  32 , connected to tank  24 , branches just after valve  38 , with one branch containing expansion valve  118 , which provides low pressure liquid carbon dioxide which cools the liquid carbon dioxide in the second branch line  119  as it passes through the heat exchanger portion of cooler  117  (and separator  59 ) as influenced by pump  56 . Inasmuch as the liquid carbon dioxide is sub-cooled when it reaches pump  56 , line  50  and valve  52  of FIG. 2 are not required. The resultant carbon dioxide vapor leaves exchanger cooler  117  by line  44 , and is compressed and returned to vessel  11  in the same manner as in FIG. 1 (or if a bleeder expansion arrangement is used, in the same manner as in FIG.  2 ). A variety of controls are suitable; one such shown as having expansion valve  118  controlled by a liquid level sensor (not shown) located in exchanger  117  (flooded type), compressor  46  controlled by pressure switch  42  (not shown), and pump  56  controlled by temperature sensor  30 . A blow-down line 120 is provided so as to periodically discharge from the system any accumulated non-condensable impurities, such as air etc. One advantage of this embodiment is any impurities, condensable or non-condensable, which are in the liquid carbon dioxide when it is delivered to system  10  are eliminated, when and where they are most likely to be formed. 
     Turning next to FIG. 4, the low temperature refrigeration apparatus of FIGS. 1,  2 , &amp;  3  is replaced with a different type and pressure management system  64  is removed, as the carbon dioxide vapor returning to vessel  11  is not under sufficient pressure to manipulate and can be arranged to already be saturated (not shown). A combination or hybrid freon (a low temperature freon such as R-502, R-404A or other suitable refrigerant) and carbon dioxide refrigeration system  121  consists of a freon compressor  122 , a freon condenser  124  (illustrated as operating against ambient air, although water or any other condensing agent could be used), a freon sub-cooler  126  (utilizing liquid carbon dioxide for sub-cooling), freon expansion valve  128  and freon evaporator/chiller  130  (also deep cooling liquid carbon dioxide). As compressor  122  must handle intake pressures of the refrigerant at the equilibrium pressure in the range of about minus 60° F. and discharge pressures at the-equilibrium pressure in the range of about 100° F.; compressor  122  must be capable of at least about 10 compression ratios and capable of operating at vacuum or near vacuum intake conditions. Very low temperatures are possible in the evaporator as the sub-cooler  126  cools the freon (or other refrigerant) to about 0° F. (the nominal temperature of liquid carbon dioxide  12 A in vessel  11 ) before freon expansion in valve  128 . Without this sub-cooling the deep cooling capacity of system  121  would be small. Line  19  and line  131  supply the liquid carbon dioxide  12 A from vessel  11 , with the flow controlled by valve  132  (carbon dioxide vapor from sub-cooler  126  returns to the ullage volume of vessel  11  through line  133  and line  22 ) responding to level control  134 , making certain the freon sub-cooler  126  functions properly (other arrangements are possible, but not shown). Line  32  brings liquid carbon dioxide  12 A from tank  24  to evaporator  130 , as circulated by pump  138 , being sub-cooled as it passes through evaporator  130 . Pump  138  is driven by motor (not shown) which operates when control  30  calls for cooling. Capacity (temperature of exiting deep cooled liquid carbon dioxide) of pump  138  is matched to the capacity of heat exchanger  130  to deep cool liquid carbon dioxide from tank  24  by changing the speed of motor (not shown) or other means. This arrangement is as capable of producing about as low a temperature sub-cooled liquid carbon dioxide  12 C as is those in FIGS. 1,  2  &amp;  3 , but it rejects part of its heat to the atmosphere directly (condenser  124 ) and part back to vessel  11 . Accordingly, one desirable use is where the size or number of unit(s)  17  are limited. One such frequent case is with vertical vessels, having only limited sized coils  16  and resultant refrigeration unit(s)  17 . While refrigeration system  121  is shown as a compound freon w/carbon dioxide sub-cooling, other types of low temperature refrigeration systems can be used. 
     Illustrated in FIGS. 5,  6 , and  7  is a different embodiment of the invention, comprising a modified freon type closed cycle which operates at two different temperature levels; one about minus 10° F. and the other about minus 65° F. It achieves very low temperatures by first rejecting its heat of condensation to the atmosphere and then sub-cooling the now liquid freon with liquid carbon dioxide before expansion. 
     These embodiments depict a vertical storage vessel without an internal refrigeration coil(s) or associated refrigeration unit(s); as this embodiment is especially useful in such circumstances (although useful with horizontal storage vessels). The modified hybrid refrigeration system is able to serve as a vessel refrigeration unit or alternately as a deep chiller, depending upon the method desired at the time. FIG. 5 depicts the system when operating as a vessel chiller, with the flows of both CO2 and freon shown by appropriate use of arrow symbols. FIG. 6 depicts the identical system when operating as a deep chiller with very cold/sub-cooled liquid CO2 being supplied to a using device, and with the flows of both CO2 and freon shown by appropriate use of arrows and symbols. FIG. 7 depicts a CO2 vapor compressor added so as to enhance the performance of the system when operating as a vessel chiller. 
     The operation of the invention as depicted in FIG. 5 is where by removal of vapor  12 B′ from vessel  11 ′, liquefying this vapor and returning it to vessel  11 ′, both the temperature and pressure of liquid  12 A′ is reduced. This feature has various benefits: one being to maintain the pressure in vessel  11 ′ during periods of non-use so as to prevent venting; another by reducing the temperature of liquid  12 A′ to increase the thermal storage potential of that liquid; and still another to reduce the amount of cooling required to cool liquid  12 A′ to the lower temperature of liquid  12 C′. System  10 ′ is depicted as vertical and without internal refrigeration coils ( 16  in FIG.  1 ), but could contain such. Inner vessel  11 ′ contains liquid CO2  12 A′ and vapor CO2  12 B′ in the ullage volume. Liquid  12 A′ is withdrawn for use through line  19 ′, as shown in FIG.  6 . Insulation  22 ′ suitably surrounds various elements of the invention. 
     In operation, panel  66 ′ causes compressor  122 ′ to circulate a suitable freon type refrigerant, where it is condensed by condenser  124 ′ and thence by line  145  to suction heat exchanger  146  and to three way valve  148 , set in this mode to connect to line  150 . Line  150  connects to suction heat exchanger  152 , where after further cooling, the refrigerant flows by line  154  to three way valve  156  and thence to expansion valve  158 A, set for operation at about minus 100° F. The now cooled refrigerant flows through line  160  to evaporator  161  and returns to compressor  122 , passing through exchangers  152  and  146  enroute. During this time, vapor CO2  12 B ′ flows through line  142  to be condensed in evaporator  161 . After condensation to liquid CO2  18 A′, it flows through line  162  to pump  164  and thence to three way valve  166 , set in this mode to connect to line  168 , which in turn connects to three way valve  169 , set in this case to return liquid CO2  12 A′ to the lower portion of vessel  11 ′ by line  170 . (optionally it could be returned by line  171  to the upper part of vessel  11 ′, by reversing the setting of valve  169 ). All as controlled by panel  66 ′, so that the refrigeration system operates as a storage vessel refrigeration unit. 
     The operation of the invention as depicted in FIG. 6 is an alternate use of the refrigeration system of FIG. 5, and where liquid CO2  12 A′ is being supplied as deep cooled and/or sub-cooled liquid CO2  12 C′ to dispensing device  25 ′; having passed through tank  24 ′, which both stores liquid CO2  12 C′ and acts as a process tank for cooling liquid CO2  12 A′ to the temperature of liquid  12 C′. The operation of the low temperature refrigeration system is changed so that the temperature level achieved is much lower, about minus 65° F.; by use of a refrigerant sub-cooler  174  and pump  176 , which takes liquid CO2  12 ′ from vessel  11 ′ and circulates it back to vessel  11 ′ as at least partly vapor  12 B′ (a compressor system could alternately be used, but not shown). This method uses the thermal storage potential of the liquid CO2  12 A′ in vessel  11 ′ for sub-cooling the freon. For this example, we will assume that device  25 ′ is in use (filling a small CO2 tank-not shown) and liquid CO2  12 C′ is being supplied in a cooled/sub-cooled condition of about minus 65° F. and about 125 psig. As liquid CO2  12 C′ leaves tank  24 ′, warmer replacement liquid CO2  12 A′ is drawn in through line  19 ′ from vessel  11 ′. Temperature sensor  31 ′ causes the refrigeration system to operate (or continue to operate) so as to bring liquid CO2  12 A′ then within tank  24 ′ to the desired low temperature. Pressure regulator  178  can be installed in line  19 ′, should the MAWP of tank  24 ′ be less than that of vessel  11 ′, and can also be installed in line  63 ′ to limit the pressure of liquid CO2  12 C′ being supplied to device  25 ′ (not shown). In the operation of the low temperature refrigeration system, panel  66 ′ causes compressor  122 ′ to circulate a suitable freon type refrigerant, where it is first condensed by condenser  124 ′ and thence by line  145  to suction heat exchanger  146  for cooling and thence to three way valve  148 , set in this mode of operation to connect to line  180  and to sub-cooler  174 . The sub-cooled freon type refrigerant next flows through line  150  to suction heat exchanger  152 , where after further cooling, the refrigerant flows by line  154  to three-way  156  valve set in this mode, thence to expansion valve  158 B, set for operation at about minus 65° F. The now expansion cooled refrigerant (a vapor-liquid mixture) flows through line  160  to evaporator  161  and then returns as vapor to compressor  122 ′ after passing through exchangers  152  and  146  enroute. During this time, as controlled by sensor  181 , pump  164  removes cold liquid CO2 (about minus 60° F.) from condenser  161  and thence to three way valve  166 , set in this mode to return this cold liquid CO2  12 C′ to line  54 ′ and thence to the lower portion of tank  24 ′. This arrangement allows the provision of liquid CO2  12 C′ from both that stored in tank  24 ′ and that cooled by the low temperature refrigeration system. Should the amount needed increase, either the size of tank  24 ′ can be increased (either by replacing with a larger unit or by adding another tank); or the size of the refrigeration unit increased (with the same type options). Again all as controlled by panel  66 ′ so that the system may be operated as a low temperature process cooler and with the thermal storage capabilities of vessel  11 ′ as previously described. 
     The operation of the invention as depicted in FIG. 7 is similar to that in FIG.  5  and with system  10 ′ identical, except; a vapor compressor  182  with pressure control  183  has been added into line  142 ′, line  168 ′ connects to evaporator  161 ′ instead of valve  166  (which is eliminated), a back pressure regulator  184  added in line  168 ′ and level sensor  186  added to evaporator  161 ′, all so that vapor 12B′ can be removed from vessel  11 ′, raised in pressure, condensed to liquid  12 A′ in evaporator  161 ′ and returned to vessel  11 ′ to either the upper portion (using line  171 ′) or lower portion (using line  170 ′) of vessel  11 ′, as selected by the setting of three way valve  169 ′ and all as controlled by panel  66 ′. This compressor arrangement provides both a means of increasing the refrigeration system&#39;s capacity by raising its evaporator temperature and/or lowering the pressure (and thus temperature) of the liquid  12 A′ in vessel  11 ′, thereby increasing the thermal storage potential of liquid  12 A′ and decreasing the amount of cooling required for it to be cooled to the temperature of liquid  12 C′ (thereby increasing the entire capacity of system  10 ′, beyond that previously), and very useful for those vessels  11 ′ which are suitable for temperatures of about minus 40° F. 
     It should be understood that where the term “ground support” is used in the following claims it includes, but is not limited to, systems for filling small tanks with liquid CO2 carried on trucks, rail cars later or containers using CO2 for cooling, or filling dry ice bunkers on the same. The term “using device” (or substantially equal), is used, that term includes small tanks being filled with liquid CO2 for later use, as well as food freezers, food mixers, dry ice makers or systems for any CO2 using apparatus that perform better or more efficiently as to it&#39;s use of CO2, when supplied with deep cooled (below about minus 30° F.) or sub-cooled liquid CO2. The term “conduit” used in the following claims is to be interpreted broadly to include pipe, tube, valve, pump and other devices used for the transfer of fluid or vapor. Likewise, the term “vessel” is to include tanks and other containers for liquids under pressure. In addition, the term “freon” is to include any low temperature freon, R-502, R-404A or other suitable low temperature refrigerant. 
     Although the invention has been described with regard to what is believed to be the preferred embodiment, changes and modifications as would be obvious to one having ordinary skill in both refrigeration and CO2 art can be made without departing from its scope. Particular features are emphasized in the claims that follow.