Refrigeration System With Carbon Dioxide Injector

Reference is made to copending applications Ser. No. 818,503 filed July 25, 1977 and entitled Method and Apparatus For Producing Refrigeration, and to Ser. No. 838,204 filed Sept. 30, 1977 and entitled Process and System for Producing Refrigeration.

This invention relates in general to refrigeration, and in particular relates to refrigeration systems employing liquified carbon dioxide as the coolant.

A number of refrigeration systems have heretofore been developed employing liquified carbon dioxide (LCO.sub.2) to produce refrigeration. The LCO.sub.2 typically is metered through a nozzle or snowhorn into a volume which is to be refrigerated. In another system the LCO.sub.2 is injected through an air amplifier device of the type sold under the name Transvector and which is disclosed in the above-referenced applications Ser. Nos. 818,503 and 838,204.

Among the disadvantages and limitations of previous LCO.sub.2 refrigeration systems is that injection of the coolant through a nozzle or snowhorn causes some of the liquid to flash to solid CO.sub.2, known as dry ice or snow. The resulting plume of gas and snow is at a relatively cold temperature on the order of -110.degree. F. so that it is difficult to control the temperature of the refrigerated volume. Also, deposit of the snow on certain food products can create localized freezing or "freezer-burn", which is highly objectionable from the standpoint of product degradation and spoilage. Certain systems have employed exhaust fans in an attempt to remove the snow and prevent the freezer-burn, but this has not been completely successful and moreover increases equipment and operating cost.

Typically LCO.sub.2 refrigeration systems heretofore have required the use of a gas purge system to ensure that the nozzles or snowhorns do not become blocked by buildup of solid CO.sub.2. The requirement of a gas supply and piping for the purge system increases the equipment and operating costs, and in addition is a source of malfunction and increases maintenance requirements.

Refrigeration systems employing Transvectors also have limitations and shortcomings. While the use of Transvectors for transforming the LCO.sub.2 into a gas reduces the problem of dry ice buildup in certain cases, it is not completely successful with the result that over a period of time there can be an objectionable accumulation of dry ice in the refrigeration zone and on the product. In addition the Transvector devices employed in such systems are relatively complicated, bulky and expensive.

It is a general object of the invention to provide a new and improved carbon dioxide refrigeration system.

Another object is to provide a liquid carbon dioxide injector for a refrigeration system which is relatively simple in design and inexpensive in cost.

Another object is to provide a method and apparatus for producing refrigeration by injecting liquid carbon dioxide into a stream so that buildup of dry ice is minimized or eliminated, and in which the requirement for a gas purge system is obviated.

The invention in summary includes method and apparatus in which pressurized liquid carbon dioxide coolant is injected through a nozzle along a primary stream while the pressure is reduced to cause expansion of the liquid into a gas. The stream is directed through an open-ended hollow enclosure so that an annular stream of ambient gas is drawn into the enclosure about the primary stream. The ambient gas turbulently intermixes with the primary stream so that solid carbon dioxide is sublimed into a gas. The mixture of coolant and ambient gas is discharged through the outlet of the enclosure to produce refrigeration.

FIG. 1 is a schematic diagram of a refrigeration system according to the invention.

FIG. 2 is a vertical section view of the liquid carbon dioxide injector apparatus which is a component of the system of FIG. 1.

FIG. 3 is a cross-sectional view taken along the line 3--3 of FIG. 2.

In the drawings FIG. 1 illustrates generally at 10 a system for producing refrigeration with liquified carbon dioxide. A source of liquified carbon dioxide 12 is stored within an insulated pressure tank 14 at the desired temperature and pressure levels. A supply pipe 16 and on-off valve 18 direct liquid coolant from the tank to injector apparatus 20 provided within refrigeration zone 22. The refrigeration zone is enclosed by the walls of a compartment 24, which in the illustrated embodiment is shown as a refrigerated room containing a typical food product 26. The refrigeration zone could also be within a mobile reefer trailer, freezer tunnel or the like.

Referring to FIGS. 2 and 3 injector apparatus 20 is illustrated in greater detail. The outlet end of LCO.sub.2 supply pipe 16 is threaded into the inlet 28 of a metering valve 30. Metering valve 30 comprises a body 32 formed with a flow passage or channel 34 across which a valve plate 36 is mounted for vertical sliding movement. The valve plate is formed with an aperture 37 which is moved down by the plate into register with the channel for metering LCO.sub.2 flow. A piston 38 mounted at the upper end of the plate is slidable within a chamber 40 formed by a cylinder 42 mounted above the valve body. A compression spring 44 carried within a housing 46 mounted below the valve body seats against a plate 48 which is attached to the lower end of the valve plate.

A pneumatic control system is provided for operating valve 30, and the control system includes a suitable temperature controller 50 which delivers a gas pressure signal through piping 52 into chamber 40 responsive to a temperature signal received from a capillary-type sensor 54 mounted within compartment 22. The gas source for the pneumatic signal in the controller can be provided from a suitable pressure-builder coil and/or a line, not shown, leading from LCO.sub.2 tank 14. Conventional means is provided in controller 50 for regulating the gas pressure signal in piping 52 when a temperature above or below a pre-set level is sensed within the compartment. A buildup of the pressure signal within chamber 40 urges piston 38 toward the position indicated at 38' so that the valve plate is moved down against the force of spring 44 to carry valve aperture 37 into alignment with flow passage 34 for metering coolant through the valve.

A nozzle 56 is threadably mounted at the end of valve body 32. A central bore 58 in the nozzle forms a continuation of the flow channel 34. A replaceable nozzle tip 60 is threadably mounted in the distal end of the nozzle. The tip is also formed with a center bore 62 as a continuation of the flow channel. A circular orifice 64 in the tip communicates with the flow channel for injecting the coolant into an outwardly diverging primary stream 66. The bore or flow channel in the tip 60 is formed with a frusto-conical end wall 68 which converges toward and merges with orifice 64. The frusto-conical shape of end wall 68 serves to prevent buildup of solid coolant within the tip because the flow of coolant in the passage will carry along and expel any solid coolant along the converging end wall and through the orifice. This configuration is particularly effective to prevent blockage due to solid coolant which may build up when the injector is shut down. Immediately upstream of valve plate 36 another forwardly converging frusto-conical wall 70 is formed in the flow channel of valve body 32. The configuration of this wall also serves to prevent blockage due to buildup of solid coolant in a manner similar to that explained for tip end wall 68.

An elongate hollow enclosure 72 is positioned coaxially about primary stream 66 immediately downstream of the injector tip. The enclosure comprises a cylindrical shell having a circular inlet 74 and circular outlet 76. The shell is carried below the compartment ceiling by means of a bracket 78. A plurality of circumferentially spaced struts 80-86 are secured at their inner ends about injector 56 and diverge outwardly for connection at their outer ends to inlet 74 of the enclosure.

Enclosure inlet 74 is radially spaced about nozzle tip 60 so as to define an annular opening 88 (FIG. 3) through which an annular stream of ambient gas or air from within compartment 22 is drawn or entrained by primary stream 66. The size and positioning of the enclosure inlet is predetermined with respect to the size and positioning of nozzle orifice 64 to achieve an optimum relationship between the volume of entrained ambient gas and the volume of injected coolant. The predetermined relationship between the inlet and the nozzle is established so that the primary stream is injected in a pattern which diverges outwardly from the orifice in a direction which intersects the annular stream of ambient gas so that the intersecting streams cause optimum mixing of gas and coolant. This mixing causes substantially all solid coolant or dry ice in the primary stream to sublime into a gas by ensuring that a sufficient volume of relatively warmer ambient gas contacts the coolant solids in the primary stream. The turbulent intermixing is also enhanced due to the positioning of struts 80-86 which act as spoilers in the path of the ambient gas stream.

The preferred relationship between the enclosure inlet and nozzle orifice which produces the results described above are achieved in the invention by a configuration in which the annular opening cross-sectional area has a large ratio to the cross-sectional area of the nozzle opening, preferably on the order of 300:1 or greater. As an example, one specific configuration which has been found suitable for this purpose comprises a nozzle tip diameter of 1/2" and an enclosure inlet diameter of 21/2" so that the annular opening 88 has a cross-sectional area of 4.72 in.sup.2. Also in this example the nozzle orifice has a diameter of 1/8" with a resulting cross-sectional area of 0.0123 in.sup.2 so that the area ratio is substantially 380:1.

The desired interaction between the primary coolant stream and surrounding ambient gas stream is enhanced in the invention by locating enclosure inlet 74 so that it lies in a plane substantially perpendicular to the direction of the primary stream and with the nozzle orifice positioned substantially adjacent such plane. The example illustrated in the embodiment of FIG. 2 provides for the nozzle orifice being spaced upstream of the plane of the inlet opening substantially 1/8". This permits the stream of ambient gas to completely surround the injector tip to optimize entrainment with the primary stream.

The use and operation of the invention will be described in connection with the following example. A supply of liquid carbon dioxide at a temperature of -10.degree. F. and a pressure of 275 psig is loaded within tank 14. Where the product 26 to be refrigerated within the compartment comprises a food such as meat and milk, controller 50 is set at a temperature of 38.degree. F. Valve 18 is turned on and the controller senses a low temperature condition within the compartment by means of bulb 54. The controller directs a pneumatic signal through piping 52 into chamber 40 and the pneumatic pressure acts against piston 38 to move valve plate 36 down. As valve aperture 37 moves across flow channel LCO.sub.2 coolant is metered through the valve and injected out through nozzle orifice 64, with any dry ice lodged within the channel being swept out by the flow of coolant. The coolant is injected from the orifice in an outwardly diverging primary stream 66 concentric with enclosure 72. The injected coolant undergoes a rapid drop in pressure as it expands into a gas. The concomitant cooling effect causes formation of some solid CO.sub.2 which is carried in the stream. The relatively high velocity primary stream causes entrainment of ambient gas surrounding the injector tip. A large volume of the entrained ambient gas flows in an annular stream through the enclosure inlet about and intersecting with the primary stream. Turbulent intermixing is created by the intersecting streams as well as by the spoiler effect of the struts. The thermal energy of the relatively warmer ambient gas causes dry ice in the primary stream to rapidly sublime into a gas. The mixture of ambient gas and expanded coolant continues along the enclosure and discharges from outlet 76 to produce refrigeration within compartment 24. The force of the discharging gas together with the suction effect of the ambient gas entrained into the injector causes return circulation of the gas through the compartment and around the food products so that uniform cooling is established throughout the compartment without the need for circulating fans or blowers. When the compartment temperature drops below the level pre-set in the controller, bulb 54 triggers the controller to reduce the penumatic signal in piping 52 so that valve plate 36 is urged upwardly by spring 44 to reduce the flow of coolant being injected.

The invention makes it possible to modify the injector for use in varying refrigeration applications or for calibration purposes. The injector can be modified by unscrewing nozzle tip 60 and replacing it with another tip having an orifice of greater or lesser size so that a greater or lesser cooling rate is produced. Replacement of the nozzle tip can be easily accomplished in the field without disassembling the entire injector.

From the foregoing it is apparent that there has been provided herein a refrigeration system which provides important advantages and results over existing systems. The LCO.sub.2 injector is of relatively simple design and is inexpensive to manufacture, assembly and calibrate, especially as compared to systems employing the previously described air amplifier devices. The system produces a refrigerating stream of coolant which is substantially free of dry ice, which otherwise could produce undesirable results such as freezer-burn on certain food products. The injector acts in a manner which enhances interaction between the streams of injected coolant and ambient gas, and turbulent intermixing of the streams is induced. The straight-through flow channel of the injector and the frusto-conical walls in the channel act in a manner to expel solid coolant and minimize blockage of the injector, thereby eliminating the requirement for a gas purge system.

While the foregoing embodiments are at present considered to be preferred it is understood that numerous variations and modifications may be made therein by those skilled in the art and it is intended to cover in the appended claims all such variations and modifications as fall within the true spirit and scope of the invention.