Abstract:
A combination liquid or slush carbon dioxide system, which receives warm carbon dioxide and then cools it to −69° F. before use, making carbon dioxide slush. The percentage of solid carbon dioxide in the slush is controlled. Slush is useful when subsequent carbon dioxide snow is being sought for refrigeration purposes. The system is versatile enough to be used successively to deliver slush and then cold liquid, or vice versa.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Priority for the present invention is based upon prior filed Provisional Patent Application Ser. No. 60/106,898 of Lewis Tyree, Jr. entitled COMBINATION LOW TEMPERATURE CARBON DIOXIDE LIQUID OR SLUSH GROUND SUPPORT SYSTEM filed on Nov. 3, 1998. 
    
    
     STATEMENT OF FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE 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 very cold liquid or liquid and solid (slush) carbon dioxide to devices which then form dry ice snow (a form of solid carbon dioxide) and useful when creating refrigeration effects. Such systems, while they may have other beneficial uses, are especially useful as ground support/filling apparatus for trucks or rail cars utilizing carbon dioxide as an expendable refrigerant for cooling. 
     BACKGROUND-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-toxic nature, 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 −110° F. and has a heat of sublimation of 244 btu/lb. Dry ice typically has been made at central points 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. In some cases, the user was sufficiently large to have an on-site dry ice machine, usually making small extrusions, called nuggets, served by an on-site supply of liquid carbon dioxide. 
     One early use of carbon dioxide to cool rail cars or other transport was to place already formed dry ice blocks inside an insulated portion of the car or transport container, and optionally have a thermostat controlled fan to enhance circulation and control refrigeration provision. This practice has continued today, but more directed at smaller volume units, without fans. 
     Today liquid carbon dioxide is typically received and stored at customer sites in insulated storage vessels under about 300 psig pressure and at a temperature of about 0° F.; and then converted, when needed, to dry ice by the customer in a variety of machines, generally characterized within the carbon dioxide industry as “dispensing devices” or “dispensing equipment”. In many cooling applications, such as filling the dry ice bunker of a rail car or such, as shown in U.S. Pat. Nos. 4,704,876 (1987) to Hill, in 5,168,717 (1992) to Mowatt-Larssen and in 5,660,057 (1997) to the present inventor, the liquid carbon dioxide is piped to the rail car, then expanded inside the bunker to atmospheric pressure, where it partly turns to a solid, termed snow (a loose, non-compressed form of particulate dry ice), but with a substantial part of the liquid carbon dioxide flashing to vapor as it expands. This flash gas or vapor, at −110° F. can be used to cool the inside walls and the floor of the car as it exits the car, but its refrigeration is largely wasted. The amount of solid carbon dioxide (dry ice snow) needed to be provided in the car is determined by analysis of the intended trip, considering both en route time and ambient temperature anticipated; and the proper amount dry ice snow placed in the bunker, determined by measurement of liquid used (either by use of volumetric flow meters or timed injection into the bunker through orifices of known flow characteristics with liquid carbon dioxide), and with conversion of liquid carbon dioxide to snow calculations based upon the temperature of the liquid carbon dioxide. The dry ice snow deposited in the bunker provides the subsequent cooling needs of the rail car, subliming in the process. The use of liquid carbon dioxide at a temperature below the normal storage temperature of 0° F. is desirable in such applications because the use of such colder liquid carbon dioxide during the expansion process produces a larger percentage of solid carbon dioxide and a smaller percentage of vapor carbon dioxide, which is largely wasted; all resulting in reduced liquid carbon dioxide use and lower costs to the users. U.S. Pat. No. 3,660,985 (1972) to the present inventor represents an early method to achieve the convenience of liquid conveyance to the actual using device, but also provided improved dry ice conversion efficiency by reducing the temperature of the liquid carbon dioxide. U.S. Pat. No. 4,888,955 (1989) to the present inventor, et al, shows a different method of reducing the temperature of the liquid carbon dioxide before use. Reductions in carbon dioxide usage of up to about 20% are made possible by the use of very cold liquid carbon dioxide. In two early U.S. Pat., Nos. 3,810,365 (1974) to Hampton et al and 3,933,001 (1976) to Muska, a carbon dioxide slush (also termed a slurry or a multi-phase mixture) was created and then transported to a customer location for use. In U. S. Pat. No. 3,817,045 (1974) to Muska, a method of using slush of up to 85% solid is revealed in the manufacture of dry ice pellets (nuggets). In another early U.S. Pat., No. 3,984,993 (1976) also to Muska, a method a method of making high solid concentration carbon dioxide slush is revealed. However, the inherent problems of moving the slush mixture to many actual using devices (where the slush expands to atmospheric pressure) and the slush&#39;s use within the using device itself, were so severe and unsolved that these patents found no use. For some applications, such as shown in U.S. Pat. No. 4,695,302 (1987) to the present inventor-liquid carbon dioxide is converted to a triple point mixture and with the liquid and solid phase mixing so as to form a slush. This slush is then used to cool the liquid carbon dioxide used for snow making/bunker filling and for filling each car&#39;s individual small tank with liquid carbon dioxide. This results in the near 20% reduction stated above, with the reduction being in the amount of vapor formed. However, slush was not used in the &#39;302 U.S. patent identified above to expand to snow in the bunker, only aiding in the production of cold liquid carbon dioxide. 
     While cooling carbon dioxide to low temperatures, or to the stage where slush is created may seem to be straightforward mechanical refrigeration problems and then moving the slush to a use point similar in nature to moving a water slush mixture; the highly unusual nature of carbon dioxide, and especially the problems in moving a slush mixture that instantly becomes a solid if allowed to depressurize even slightly below the triple point pressure, were such that no satisfactory solution was found. Some of the contributing problems unique to carbon dioxide usage include: 1) the fact that liquid carbon dioxide when depressurized to 75 psia (the triple point), it initially becomes a mixture of liquid and vapor; 2) as additional vapor is removed and the pressure drops, a layer of particulate solid carbon dioxide is created on the upper surface of the liquid; 3) the particulate solid carbon dioxide is heavier than the liquid, thus tends to sink to the bottom of the liquid; 4) the fact that slush carbon dioxide, when being moved, easily clogs lines at piping anomalies and at valves, etc.; and 5) subsequent pressure reduction to below the triple point due to flow induced pressure drop can cause carbon dioxide slush to turn entirely solid and block the conduit. Accordingly, most prior art inventions did not move the slush to a use point and then expand it directly to solid. Much the same type problems arose if an attempt was made to intermittently move or use liquid carbon dioxide whose condition was near the triple point. 
     A related problem is due to the nature of use of most expendable refrigerants, of which carbon dioxide is a member, whether used in liquid form or in solid form (dry ice). This problem is that expendable refrigerants are used precisely when the cooling is desired (or the need commences), thus the use rate can vary greatly. Low use rates can be followed by high use rates, varying quickly from no use to high use. U.S. Pat. Nos. 4,888,955 (1989) and 5,934,095 (1999) to the present inventor, et al, were directed at solving this problem when very cold liquid carbon dioxide is being used, by incorporating a storage function of previously cooled liquid carbon dioxide. 
     U.S. Pat. No. 5,255,523 (1993) to Burgers et al discloses a method of determining the % solid content in a slush mixture of liquid and solid carbon dioxide by adding a trace substance to the mixture. However, the carbon dioxide is not used in a dispensing device, but remains in the slush chamber. In addition, many carbon dioxide uses require high purity, and trace substances would be most objectionable. 
     Although some of these systems have worked well for individual applications involving chilled liquid carbon dioxide, there are none involving slush carbon dioxide (offering the greatest carbon dioxide use efficiency). Accordingly, they have not solved the most needed problems, and consequently improvements in this area are sought. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods and systems for safely receiving liquid carbon dioxide at a range of temperature/pressures into a storage system that, by pressure and temperature manipulation is subsequently able, prior to further use, to increase the liquid carbon dioxide&#39;s refrigeration potential to the extent desired by reducing its temperature and/or causing part to become solid and form a slush mixture of desired solid-liquid proportions, and to store this product so it is available for ready use. As such, its principal objective is to increase the efficiency of liquid carbon dioxide conversion to snow and thereby reduce the cost of carbon dioxide cooling, but also retain the convenience of liquid carbon dioxide supply to the dispensing device or equipment. 
     In one aspect, it can create a mixture of solid and liquid carbon dioxide, to the pre-determined desired proportions of each, and to store this product so it is available for ready use, and to maintain it during storage until used in the pre-determined desired proportions of liquid to solid. It also manages the mixture during its creation and its subsequent movement, so that it remains sufficiently fluid to be conveyed by piping to the desired use point and through the dispensing device as and when needed. Since the precise geometry and flow characteristics of each carbon dioxide expansion valving arrangement and conduit contained in dispensing devices, and the piping between it and the slush chamber are different, not only can the proportion of solid in the slush be varied as needed by such equipment and the arrangement and conduit specifics, but if an inadvertent dry ice blockage occurs, the present invention has the ability to quickly clear the blockage by melting the dry ice, using either vapor or liquid. The invention thus provides a method of creating desired proportions of solid to liquid in the slush mixture and also a method of creating and maintaining the homogeneity of a slush mixture during and after it has been created. It is most desirable to be able to produce the desired proportions of solid and liquid in the slush mixture so as to be able to also accurately predict the amount of snow produced from a given amount (weight and/or volume) of slush. If not, the usage of the slush mixture can not be accurate, and either too much or too little can be used, thus not providing the needed cooling or wasting cooling and thus carbon dioxide. 
     In another aspect, lower temperature/pressure liquid carbon dioxide is available to fill small on-board, transportable tanks for subsequent truck or rail car or container cooling. 
     In still another aspect, very low temperature (near −70° F.) liquid carbon dioxide can be available for filling dry ice bunkers in rail cars or containers, such as in U.S. Pat. Nos. &#39;876 and &#39;717, earlier identified, where rail cars or containers cannot accept slush carbon dioxide because of their arrangement/conduit configuration. 
     In still another aspect, slush carbon dioxide can be available for accurately filling dry ice bunkers in rail cars, containers or trucks, such as shown in U.S. Pat. No. 4,186,562 (1980) to the present inventor. 
     In a different application of these aspects, lower temperature/pressure liquid or slush carbon dioxide is available to food mixers and like cooling devices. 
     Various and different use rates and amounts are needed for these different applications. As an example, filling a rail car bunker with about 9,400 lbs. of dry ice can require about 20,000 lbs. of liquid carbon dioxide at about 0° F. within a 30 minute period (releasing about 11,600 lbs. of vapor); or about 16,000 lbs. of liquid carbon dioxide at about −65° F. within the same 30 minute period (releasing about 6,500 lbs. of vapor), and a reduction in carbon dioxide usage of about 20%; or about 12,200 lbs. of a 50% liquid/50% solid mixture by weight slush carbon dioxide within the same 30 minute period (releasing about 2,800 lbs. of vapor), and a reduction in carbon dioxide usage of about 39%; or about 11,000 lbs. of a 25% liquid/75% solid mixture by weight slush carbon dioxide within the same 30 minute period (or different if desired) and releasing about 1,600 lbs. of vapor and a reduction in carbon dioxide usage of about 45%. A mixer for blending meats can require 500 lbs. of dry ice in 5 minutes (and with the same proportions of carbon dioxide at the different conditions as for the rail car above); but there can be a number of purposeful interruptions/delays during the use of the carbon dioxide, as these often are batch processes controlling the temperature of an active mixture as different temperature ingredients are added. Accordingly, an important aspect of the invention is the ability to intermittently cycle small amounts of carbon dioxide liquid or slush to the mixer (dispensing device). 
     While the utility of the invention has been described with respect to certain applications, the variety of it&#39;s capabilities is such that many liquid carbon dioxide applications, where carbon dioxide snow/dry ice is involved as the final carbon dioxide condition, could be well served by a variation or combination of these aspects. One special advantage is that the size of the storage vessel and the size of the processing vessel are independent of each other and the size of the compressor and/or refrigeration unit(s) are also independent. This allows selection of the receiving storage vessel&#39;s size to include distribution economies; selection of the processing vessel&#39;s size and to include use patterns, and selection of the compressor and/or refrigeration units&#39; size to include individual user needs. 
     Accordingly, the system is modular, and able to be readily adapted to meet virtually all the different user&#39;s requirements, but without the burden of a custom engineered system and design. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For purposes of simplifying the figures, some lines/connections to the storage vessel standardly provided in the carbon dioxide industry have been omitted, such as fill lines, auxiliary liquid and vapor lines, safety relief valves, level/contents device, pressure gauge, clean-out and others. 
     FIG. 1A 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, with the invention as used to deliver slush carbon dioxide to the dry ice i.e. snow making device(s) inside the bunker of an insulated rail car. 
     FIG. 1B is a view of the system of FIG. 1A, but connected to some other dispensing device such as a mixer, and delivering slush carbon dioxide as needed. 
     FIG. 2A is a view of the system of FIG. 1A, but delivering very cold liquid carbon dioxide to the dry ice making device(s) inside the bunker of a rail car 
     FIG. 2B is a variation of FIG. 2A but delivering very cold liquid carbon dioxide to some other dispensing device, such as a mixer. 
     FIG. 2C is a variation of FIG. 2A, but delivering cold liquid carbon dioxide (about −45° F.) to a tank carried on a truck for subsequent cargo cooling, the carbon dioxide cooling the truck either with an indirect or direct process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Note: In all drawings where carbon dioxide flow is shown, a single headed arrow→indicates vapor flowing; a two headed arrowindicates liquid flowing. Where slush is shown in section, a wavy liquid line above triangles is used                           
     Illustrated in FIG. 1A is a system to be located at a user&#39;s site capable of delivering either very cold liquid or slush carbon dioxide to various types of dispensing devices at various equilibrium conditions, generally between about 60 psig and −69° F. and about 300 psig and 0° F. (depending upon the type device and the pressures desired when using each), but shown delivering slush. The system preferably includes either a vertically oriented vessel system  10 , with an inner vessel  11  having a height greater than its interior width and being sized to hold a reservoir of liquid carbon dioxide sufficient for the customer needs, such as those using liquid carbon dioxide for truck or rail car cooling or users benefiting from the use of very cold liquid or slush carbon dioxide. Vessel  11  is provided with suitable insulation  12  so as to maintain the temperatures therewithin at temperatures below 0° F. Vessel  11  is made from metals, or other materials suitable for both the temperatures and pressures anticipated. While vertical vessels are generally preferred because of a smaller footprint, a horizontal vessel can be substituted (and a great number exist). 
     In use, liquid carbon dioxide is typically supplied to vessel  11  by truck or rail, so as to create a reservoir of liquid carbon dioxide therewithin. Following the initial filling of vessel  11 , this reservoir of liquid carbon dioxide will generally be at about equilibrium temperature and pressure conditions throughout, for example about 0° F. and 300 psig or about −20° F. and 225 psig or intermediate conditions. Past practice has been to maintain these conditions by the provision of a standard freon type refrigeration unit  14  providing its refrigeration output to built-in coils  15  located in the upper vapor space of vessel  11 . More current practice for vertical vessels (and favored for use with this invention) would be to provide both the coils and refrigeration unit outside the vessel  11  (not shown), as in U. S. Pat. No. 5,934,095 (1999) to the present inventor. Refrigeration unit  14  typically contains a freon type compressor, a condenser, which is cooled by ambient air forced through it by a fan, expansion valve and control panel, which turns on the refrigeration unit when the carbon dioxide pressure in vessel  11  becomes too high and turns off when it becomes low. Other normal devices normal to such refrigeration units are used but not specifically identified. While a freon type refrigerant is stated as the refrigerant of choice, there are other alternatives that may be preferred and would operate satisfactory. 
     Still referring to FIG. 1, vessel  11  is filled with liquid carbon dioxide from a delivery vehicle (not shown) through liquid fill system  16 , with a fill-vapor return system  17  relieving excess pressure occurring in vessel  11  during fill, and in the process also scavenging air and non-condenseables that may have collected from the top of vessel  11  through a vapor scavenger (not shown). 
     These non-condenseables will then return to the shipping point via the delivery vehicle for proper disposition. Fill system  16  can be divided into sub lines as desired, i.e. one to the top and one to the bottom of vessel  11  as well as one or more intermediate entry lines (not shown), so as to provide ease of filling and control of the temperature/pressure of the liquid carbon dioxide in vessel  11  during filling operations. A liquid withdrawal line  18  is provided for supplying process vessel  19  with liquid carbon dioxide or for other customer use. Vessel  19  is located as near as possible to the final carbon dioxide dispensing point, so as to simplify the piping between the final use point and itself and thus minimize the opportunity for slush blockages, and thus vessel  19  may be some distance from system  10 . Branch line  20  connects liquid line  18  to the bottom of vessel  19  and contains control valve  21 . Branch line  22  connects liquid line  18  to the top of vessel  19  and contains control valve  23 . A safety relief line, having a number of safety related functions connects to the top of vessel  11  and a similar relief line connects to the top of the vessel  19  (not shown). Vessels  11  and  19  are surrounded by insulation  12 , and each is supported on legs  24 . As stated previously, not shown for clarity&#39;s sake are the standard additional lines and devices provided on such pressurized liquid carbon dioxide storage vessels, for instance safety items, level/contents/pressure indicators, pressure building connections, duplicate liquid and vapor lines, vacuum readout-if appropriate and other similar items. 
     Vapor line  25  connects the ullage volumes of vessels  11  and  19 . Vapor may flow in both directions in line  25 , and in some cases it may be desirable to provide separate lines. Control valve  26  and downstream regulator  28  are located in line  25   a . Vapor withdrawal line  30  connects the upper (ullage) volume of vessel  19  to booster compressor (and motor)  31 . Compressor  31  discharges to three way valve  32 . In one position, valve  32  connects back to line  25  via line  25   b ; in the other position it connects to alternate discharge line  34  which is connected to the bottom of vessel  19 . In line  34  are receiver  36  and control valve  38 . Level monitors/switches  46 ,  48 ,  50  &amp;  52  are used with vessel  19  at points that allow the known reduction in volume of liquid carbon dioxide as it becomes a mixture of liquid and solid (slush) to make an accurate determination as to the percent solid and percent liquid carbon dioxide in the slush, as will be explained later (as well as alternate methods of such determination). Pressure switch  54  senses and monitors the pressure in vessel  19 . The legs  24  of vessel  19  rest upon weight cells  56 , allowing determination of the weight of the process vessel and its carbon dioxide contents as desired, as will be explained later, and so that an accurate determination of the amount of liquid or slush carbon dioxide delivered to the dispensing device can be made. Slush discharge line  58  connects to the bottom of vessel  19  and to chamber  65  and line  58  includes control valve  62 . Branch line  64  connects line  18  with chamber  63  and contains control valve  65 . Line  66 , containing check valve  67 , control valve  68  and pressure switch  70 , connects the ullage volume of vessel  19  to chamber  63 . Loading connection hose  72  connects chamber  63  with rail car connection  60 . Car  74  includes conduit/manifold  75 , and includes shut off valve  76  and terminates with automatic expansion valves  78 . Expansion valves  78  can be the pressure responsive automatic shut-off type widely used in the carbon dioxide industry, i.e. PRASO valves. Valves  78  are located in the dry ice (snow) bunker  80  of the carbon dioxide dispensing device, car  74 . For frozen foods, typically vents  82  connect bunker  80  with cargo volume  84 , so the cooling available from the flash carbon dioxide vapor is usefully employed in cooling before venting to the atmosphere. A car vent for this purpose (not shown) is typically included in car  74 . 
     Process control panel  86  monitors and controls the various elements of the entire process as selected by the user. By use of this arrangement, carbon dioxide vapor can be withdrawn from the vessel  19 , raised in pressure by compressor  31 , and then returned either to the vessel  11  or returned to the bottom of the vessel  19 , all as determined by the logic of the control panel  86 . While for ease in depiction, compressor  31  has been depicted as a non-lubricated (oiless) rotary vane compressor, any suitable type can be used; and all control devices could be replaced with other types, such as electronic. Filters etc. can also be included as desired. 
     FIG. 1B substitutes a mixer  90  for rail car  74  of FIG. 1A, containing meat  92 , or the like, which it is desired to cool as the mixing occurs. A snow horn  94  is positioned above the meat  92 . Slush compatible valves  96  (such as PRASO valves) are supplied with slush by conduit/manifold  97  and supplied with liquid, slush or vapor carbon dioxide by line  98 , containing control valve  99 . Line  98  then connects to chamber  60  so as to receive carbon dioxide for use in mixer  90 . 
     While not utilized in the production or transfer of slush, screen  100  is inserted inside the lower portion of vessel  19 , and line  101 , containing control valve  102 , connects the inside of screen  100  with chamber  63 , so that very cold liquid carbon dioxide may be removed from vessel  19  without removing slush, as will be explained later. 
     FIG. 2A is the same system as FIG. 1A, except that the expansion valves  78  in car  74  have been replaced with expansion devices or orifices  104 , which are unable to handle slush carbon dioxide, and thus must be supplied with only liquid, but cold liquid is an advantage. A number of cars  74  were constructed in this fashion. 
     FIG. 2B is the same system as FIG. 2A, but substitutes mixer  90  for the rail car  74 . However, slush compatible expansion valves  96  have been replaced with expansion devices or orifices  104  unable to handle slush carbon dioxide, and thus must be supplied with only liquid, but cold liquid is an advantage. Line  98  connects to chamber  63  (not shown). A number of mixers  90  are constructed in this fashion. 
     FIG. 2C is the same system as FIG. 2A, but substitutes refrigerated trailer/truck  108  for the rail car  74 . In addition, this arrangement supplies liquid carbon dioxide at about 125 psig to a small tank  109  carried in the truck expendable liquid carbon dioxide cooling system  110 , and later utilized to cool the cargo space of truck (not shown). Line  112 , containing fill valve  114 , connects tank  109  to hose  72  when filling is desired. 
     Turning next to the operation of the systems of FIGS. 1A and 1B, process vessel  19  has being filled with warm liquid carbon dioxide from vessel  11  up to level switch  46  by line  20 , as controlled by valve  21 , or alternately by line  22 , as controlled by valve  23 ; and influenced by compressor  31  operating and returning vapor through valve  32  and lines  25   b  and  25  to vessel  11 . Compressor  31  then continues to operated, with the vapor passing through valve  32  and lines  25   b  and  25  to vessel  11 . As the liquid level drops (and the pressure) in vessel  19 , additional liquid is added until the triple point pressure is reached (about  60  psig) so that vessel  19  contains triple point liquid at level  46 . If vapor continues to be removed, slush forms, and the density increases. If a slush mixture of about 25% solid is desired, vapor removal ceases when the level reaches switch  48 , located appropriately. As compressor  31  operates, particles of dry ice form on the upper surface of the liquid carbon dioxide within vessel  19 . If the rate of vapor withdrawal is slow enough, the dry ice particles with gradually sink to the bottom of vessel  19 . If not, flow from compressor  31  can be momentarily stopped; or alternately, rapidly cycling valves  34  and  38  on and off, causing the slush to be agitated by intermittent vapor injection. In addition, a mechanical mixing device (paddle or other type) can be used (not shown). Since the solid is denser than the liquid, as vapor is removed and ice is formed, the liquid level drops by more than the volume of liquid removed as vapor. This difference is utilized in this aspect to determine the percentage solid in the slush mixture and to appropriately locate switches  48 ,  50  and  52 . Different dispensing devices can tolerate different slush percentage solids. In this case, (as an example), it has been determined that when the slush level has dropped to level switch  52 , the desired percentage solid ( 75 ) is present, compressor  31  is stopped; and transfer of the slush can begin. 
     Since the density of the slush mixture is a measure of the percentage solid, the apparatus of FIG. 1A can be utilized in an alternate method to determine that percentage by maintaining the slush mixture at a given level, as vapor is being removed from vessel  19 , and monitoring the weight of the slush in vessel  19  by means of cells  56 . The density of triple point liquid is about 73 lbs./cu. ft.; of 25% solid in slush, about 77 lbs./cu. ft.; of 50% solid in slush, about 82 lbs./cu. ft.; of 75% solid in slush, about 87 lbs./cu. ft. With the known volume and weight of the slush, an accurate determination and control of the solid percentage can be made by panel  86 , as selected by the operator. 
     After vessel  19  contains the selected percentage solid of slush, it will be injected into car  74 . Compressor  31  will be stopped and valve  26  opened, allowing vapor from vessel  11  to enter vessel  19 , at a pressure set by regulator  28 . While it is desired to have sufficient counter pressure to eject the slush from vessl  19 , too high a pressure warms the top layer of slush. Valve  68  is also opened allowing vapor at slightly less than the pressure of regulator  28  to pressurize line  66  and manifold  75 . The spring resistence of check valve  67  determines how much less the pressure in line  66  is than that in the vessel  19 . Pressure switch  70  monitors this pressure and does not allow valve  62  to open to begin flow until the pressure in line  66  is at least about 2 psi above the triple point pressure. Again, these exact figures are determined by the geometry of the piping connecting vessel  19  and expansion valves  78 . Long radius elbows,straight runs, full opening valves such as ball valves, etc. are preferable for slush flow. Once a suitable pressure has been reached and valve  62  opened, flow of slush begins, and valves  78  open and snow making in the bunker  80  begins. In this type car, this flash vapor passes through vents  82  in the floor of bunker  80  , around the cargo and then to vents to the atmosphere (not shown). If desired, a pump (not shown) can be utilized in line  58  to aid in flow of the slush. 
     In order to determine the amount of snow placed in bunker  80 , the percentage solid in the slush mixture (as determined by one of the methods above) and the weight of the delivered amount (as determined by cells  56 ) and the known conversion factor of that percentage solid in the slush, to snow are integrated together, and flow stopped by closing valves  62  and  68 , as controlled by panel  86 . 
     Turning next to the operation of FIGS. 2A,  2 B, and  2 C, vessel  19  is assumed to have been filled with an appropriate amount of slush, but of the 25 percentage solid type. Inasmuch as the purpose of this mode is to deliver cold liquid, slush does not leave vessel  19 . When filling of bunker  80  of FIG. 2A is to begin, valve  26  is opened, allowing the pressure in vessel  19  to increase over the triple point pressure. Next, valve  68  is opened, maintaining a pressure at least about 2 psi above the triple point pressure throughout the conduit. Next, valve  102  is opened and flow of liquid from vessel  19  commences (solid cannot pass through screen  100 . Once the desired amount has been delivered to car  74  (determined in the same fashion as with slush), valves  102  and  68  are closed. 
     Operation of FIG. 2B is identical to that of FIG.  2 A. 
     Operation of FIG. 2C is different, as it is desired to deliver liquid carbon dioxide at about −45° F. and about 125 psig. First, vessel  19  must be brought to a pressure slightly above 125 psig. Next, a temperature sensor in chamber  63  (not shown) causes panel  86  to adjust the flow of very cold liquid through line  101  by modulating valve  102  and adjust the flow of warm liquid through line  64  by modulating valve  65 , so as to achieve the desired temperature liquid carbon dioxide for filling tank  109 . If desired, unit  110  can be operated to assist in filling tank  109 . 
     The configuration of the system is such that an operator can switch back and forth from delivering cold liquid to delivering slush, an advantage for frozen food shipping points utilizing different types of transport equipment. 
     In all cases where valves or switches are said to be operated, this function would be controlled by panel  86 . Liquid or vapor carbon dioxide lines are shown in the manner simplest to illustrate. In actual practice, lines may be combined or separated 
     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 carbon dioxide art can be made to the invention without departing from its scope Particular features are emphasized in the claims that follow The term conduit used in the following claims is to be interpreted broadly to include pipe, tube, valve, pump and other devices used in the transfer of fluid, vapor or slush. The term slush used in the following claims is to be interpreted as a Mixture of solid and liquid carbon dioxide.